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Advanced Electrical
Installation Work
Fourth Edition
TREVOR LINSLEY
Senior Lecturer
Blackpool and The Fylde College
AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD
PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
Newnes is an imprint of Elsevier
Newnes
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
30 Corporate Drive, Burlington, MA 01803
First published by Arnold 1998
Reprinted by Butterworth-Heinemann 2001, 2002, 2003 (twice), 2004 (twice)
Fourth edition 2005
Copyright © 2005, Trevor Linsley
...
All rights reserved
The right of Trevor Linsley to be identified as the author of this work has been
asserted in accordance with the Copyright, Designs and Patents Act 1988
No part of this publication may be reproduced in any material form (including
photocopying or storing in any medium by electronic means and whether or not
transiently or incidentally to some other use of this publication) without the written
permission of the copyright holder except in accordance with the provisions of the
Copyright, Designs and Patents Act 1988 or under the terms of a licence issued
by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London,
England W1T 4LP
...
Permissions may be sought directly from Elsevier’s Science and Technology Rights
Department in Oxford, UK; phone: (ϩ44) (0) 1865 843830; fax: (ϩ44) (0) 1865 853333;
e-mail: permissions@elsevier
...
uk
...
elsevier
...
Whilst the advice and information in this book is believed to be true and accurate
at the date of going to press, neither the author nor the publisher can accept any legal
responsibility of liability for any errors or omissions that may be made
...
newnespress
...
Ltd, Chennai, India
www
...
com
Printed and bound in Great Britain
CONTENTS
Preface
vii
1 Health and safety and electrical principles 1
Introduction
1
Health and safety at work
1
The Health and Safety At Work Act 1974
2
The Management of Health and Safety at
Work Regulations 1999
4
Provision and Use of Work Equipment
Regulations 1998
4
The Control of Substances Hazardous to
Health Regulations 1988
4
Personal Protective Equipment (PPE) at
Work Regulations 1992
5
RIDDOR
7
The Control of Major Accidents and Hazards
(COMAH) Regulations 1999
8
Dangerous Substances and Explosive
Atmospheres Regulations 2002 (DSEAR)
8
The Construction (Design and Management)
Regulations 1994 (CDM)
8
The Construction (Health, Safety and Welfare)
Regulations 1996
9
Building Regulations – Part P 2005
9
The Electricity Safety, Quality and Continuity
Regulations 2002
10
The Electricity at Work Regulations 1989
(EWR)
11
The IEE Wiring Regulations to
BS 7671:2001
11
Environmental laws and regulations
12
Environmental Protection Act 1990
12
Pollution Prevention and Control Regulations
2000
13
Clean Air Act 1993
13
Controlled Waste Regulations 1998
Waste Electrical and Electronic Equipment
(WEEE) EU Directive 2005
Radioactive Substances Act 1993 (RSA)
Dangerous Substances and Preparations and
Chemical Regulations 2000
Noise Regulations
The Noise and Statutory Nuisance Act 1993
Noise Act 1996
Noise at Work Regulations 1989
The Environmental Health Officer
Laws protecting people
Employment Rights Act 1996
Health and Safety (First Aid) Regulations
1981
Data Protection Act 1998
Prejudice and discrimination
The Race Relations Act 1976 (RRA) and
Amendment Act 2000
Sex Discrimination Act 1975 (SDA)
Disability Discrimination Act 1995 (DDA)
The Human Rights Act 1998 (HRA)
Enforcement Law Inspectors
In-house Safety Representatives
Safe and effective working practices
Quality systems
Investors in People (IiP)
Health and safety applications
VDU operation hazards
Manual handling
Personal Protective Equipment
Safety signs
Accidents at work
Fire control
First aid
13
14
15
15
15
16
16
16
17
18
18
20
20
21
22
22
22
23
23
24
25
25
27
27
32
35
36
38
39
40
42
iv
CONTENTS
Accident reports
Secure electrical isolation
Permit-to-work system
The world of work
Good customer relationships
Changing work patterns
Electrical principles
Ohm’s law
Resistivity
Temperature coefficient
Resistors
Power and energy
Alternating waveforms
Magnetism
Electromagnetism
Electrostatics
Alternating current theory
Electrical machines
Direct current motors
Three-phase a
...
motors
Single-phase a
...
motors
Motor starters
Remote control of motors
Installation of motors
Motor maintenance
Power factor correction
Transformers
Transformer losses
Transformer efficiency
Transformer construction
Instrument transformers
Voltage transformers
Current transformers
Electricity supply systems
Low-voltage supply system
Industrial wiring systems
Cables
Installing cables
Protection of structures against lightning
(BS 6651)
Exercises
45
46
49
50
50
51
52
52
53
54
54
58
60
61
62
67
73
86
86
90
94
95
97
97
99
100
102
103
103
104
106
106
106
108
112
117
119
122
132
136
2 Inspection, testing and commissioning
The construction industry
The building team
The electrical team
Designing an electrical installation
Legal contracts
Management systems
146
146
146
147
148
148
150
On-site communications
Construction site – safe working practice
Electrical safety systems
Direct contact protection
Earth fault protection
Residual current protection
Isolation and switching
Overcurrent protection
Semi-enclosed fuses (BS 3036)
Cartridge fuses (BS 1361)
Miniature circuit breakers (BS 3871)
Fusing factor
Position of protective devices
Disconnection time calculations
Time/current characteristics of protective
devices
Discrimination
Earth fault loop impedance Zs
Size of protective conductor
Cable selection/calculation
Electrical testing
Moving coil instruments
Moving iron instruments
Damping
Making measurements
Range extension
Dynamometer wattmeter
Measurement of power in a three-phase
circuit
Energy meter
Tong tester
Phase sequence testers
Test equipment used by electricians
Test procedures
Live testing
Isolation of supply
Inspection and testing techniques
Certification and reporting
Safe working procedures when testing
Portable appliance testing (PAT)
Commissioning electrical systems
Exercises
3 Fault diagnosis and rectification
Symptoms of an electrical fault
Causes of electrical faults
Fault diagnosis
Wiring circuits
Designing out faults
155
164
165
165
167
168
168
169
170
171
172
173
173
174
175
175
176
178
179
182
182
184
185
185
187
189
189
190
191
191
191
193
194
194
194
206
206
207
211
211
217
217
217
219
219
227
CONTENTS
Finding the electrical fault
Safe working procedures
Live testing
Secure isolation of electrical supply
Faulty equipment – to repair or replace?
Selecting test equipment
Special situations
Antistatic precautions
Avoiding shutdown of IT equipment
Damage to electronic devices by ‘overvoltage’
Risks associated with high frequency or
large capacitive circuits
Presence of storage batteries
Safe removal of waste
Exercises
4 Electronic components
Electronic circuit symbols
Resistors
Capacitors
Inductors and transformers
Electromagnetic relays
Overcurrent protection
Packaging electronic components
Obtaining information and components
Semiconductor devices
Voltage divider
Rectification of a
...
Smoothing
Stabilized power supplies
Electronic circuit assembly
Safety precautions
Hand-tools
Soldering irons and guns
227
227
227
227
229
229
230
231
232
233
234
235
236
238
239
239
239
243
247
248
248
248
249
249
261
263
264
266
266
267
267
267
v
Soldering
Desoldering
Circuit boards
Breadboards
Interconnection methods
Fault finding
Electronic test equipment
Test instruments
Analogue and digital displays
The multimeter
The ohmmeter
The cathode ray oscilloscope
Signal generators
Power supply unit
Mains electricity supply
Insulation tester
Logic gates and digital electronics
The AND logic gate
The OR gate
The exclusive-OR gate
The NOT gate
The NOR gate
The NAND gate
Buffers
Logic networks
Logic families
Working with logic
British Standard symbols
Exercises
269
272
272
275
275
277
277
277
279
279
279
281
283
283
284
284
284
285
285
286
286
287
287
287
287
290
291
292
292
Solutions to exercises
Appendices
Glossary
Index
294
296
315
325
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PREFACE
The fourth edition of Advanced Electrical Installation
Work has been written as a complete textbook for the
City and Guilds 2330 Level 3 Certificate in Electrotechnical Technology and the City and Guilds 2356 Level
3 NVQ in Installing Electrotechnical Systems
...
The book will also assist students taking the
SCOTVEC and BTEC Electrical and Utilization units
at levels II and III and many taking engineering NVQ
and Modern Apprentiship courses
...
Modern regulations place a greater responsibility
upon the installing electrician for safety and the design
of an installation
...
The fourth edition of this book has been revised and updated to incorporate the requirements and amendments of the 16th
edition of the IEE Wiring Regulations BS7671:2001
...
For this reason
multiple-choice questions can be found at the end of
each chapter
...
I would like to acknowledge the assistance given by
the following manufacturers and organizations in the
preparation of this book:
Crabtree Electrical Industries Limited
Wylex Ltd
RS Components Ltd
The Institution of Electrical Engineers
The British Standards Institution
The City & Guilds of London Institute
I would also like to thank my colleagues and students at Blackpool and The Fylde College for their
suggestions and assistance during the preparation of
this book
...
Trevor Linsley
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1
HEALTH AND SAFETY AND
ELECTRICAL PRINCIPLES
Introduction
This first chapter of Advanced Electrical Installation
work covers all of the core skills required by the City &
Guilds Level 3 Certificate in Electrotechnical Technology
...
HEALTH AND SAFETY AT WORK
Let me begin by looking at the background to the
modern Health and Safety Regulations and the electricity supply and wiring regulations
...
This experiment, together with
many other experiments of the time, made it possible
for Lord Kelvin and Sebastian de Ferranti to patent
in 1882 the designs for an electrical machine called
the Ferranti–Thompson dynamo, which enabled the
generation of electricity on a commercial scale
...
This was
one of many privately owned electricity generating
stations supplying the electrical needs of the UK
...
In 1926 the Electricity Supply Act placed the
responsibility for generation in the hands of the
Central Electricity Board
...
In Scotland, generation was the joint responsibility of the North of
Scotland Hydro-Electricity Board and the South of
Scotland Electricity Board
...
In 1988 Cecil Parkinson, the Secretary of State for
Energy in the Conservative government, proposed
the denationalization of the electricity supply industry; this became law in March 1991, thereby returning the responsibility for generation, transmission and
distribution to the private sector
...
During the period of development of the electricity
services, particularly in the early days, poor design
and installation led to many buildings being damaged
by fire and the electrocution of human beings and
livestock
...
The first rules were made by
the American Board of Fire Underwriters and were
quickly followed by the Phoenix Rules of 1882
...
The current edition of these regulations is called the
Requirements for Electrical Installations, IEE Wiring
Regulations (BS 7671:2001), and since January 1993
we have been using the 16th edition
...
The laws and regulations affecting the electrotechnical industry have steadily increased over the years
...
These laws and regulations will permeate each and
every sector of the electrotechnical industry and reform
and modify our future work patterns and behaviour
...
First of all I want to look at the laws concerned with
health and safety at work, making the working environment safe
...
The Health and Safety at Work
Act 1974
Many governments have passed laws aimed at
improving safety at work but the most important
recent legislation has been the Health and Safety at
Work Act 1974
...
The Health and Safety at Work Act is an ‘Enabling
Act’ that allows the Secretary of State to make further
laws, known as regulations, without the need to pass
another Act of Parliament
...
This applies to regulations based on European directives as well as new UK
regulations
...
The Health and Safety
Commission has equal representation from employers,
trade unions and special interest groups
...
They
describe what must be achieved in the interests of
safety, but not how it must be done
...
To do this he
must ensure that:
■
■
■
■
the working conditions and standard of hygiene are
appropriate;
the plant, tools and equipment are properly maintained;
the necessary safety equipment – such as personal
protective equipment, dust and fume extractors and
machine guards – are available and properly used;
the workers are trained to use equipment and plant
safely
...
To do this they
must:
■
■
■
take reasonable care to avoid injury to themselves
or others as a result of their work activity;
co-operate with their employer, helping him or her
to comply with the requirements of the Act;
not interfere with or misuse anything provided to
protect their health and safety
...
ENFORCEMENT
Laws and rules must be enforced if they are to be effective
...
The HSE is divided into a number of
specialist inspectorates or sections which operate from
local offices throughout the UK
...
The HSE inspectors have been given wide-ranging
powers to assist them in the enforcement of the law
...
SAFETY DOCUMENTATION
Under the Health and Safety at Work Act, the
employer is responsible for ensuring that adequate
instruction and information is given to employees to
make them safety-conscious
...
Your employer must let you
know who your safety representatives are and the new
health and safety poster shown in Fig
...
1 has a blank
section into which the names and contact information of your specific representatives can be added
...
All workplaces employing five or more people must
display the type of poster shown in Fig
...
1 after 30th
June 2000
...
1
...
Source: HSE © Crown
copyright material is reproduced with the permission of the Controller
of HMSO and Her Majesty’s Stationery Office, Norwich
...
In companies which employ more
than 20 people this is normally undertaken by forming a safety committee which is made up of a safety
officer and employee representatives, usually nominated by a trade union
...
Small companies
might employ a safety supervisor, who will have other
duties within the company, or alternatively they could
join a ‘safety group’
...
An employee who
identifies a dangerous situation should initially report
to his site safety representative
...
This may mean changing company
policy or procedures or making modifications to
equipment
...
4
ADVANCED ELECTRICAL INSTALLATION WORK
The Management of Health and
Safety at Work Regulations
1999
The Health and Safety at Work Act 1974 places
responsibilities on employers to have robust health
and safety systems and procedures in the workplace
...
The Management of Health and Safety at Work
Regulations 1999 tell us that employers must systematically examine the workplace, the work activity and
the management of safety in the establishment
through a process of ‘risk assessments’
...
Information based on these findings must
be communicated to relevant staff and if changes in
work behaviour patterns are recommended in the
interests of safety, then they must be put in place
...
Risks, which may require a formal assessment in
the electrotechnical industry, might be:
■
■
■
■
■
■
■
■
■
working at heights;
using electrical power tools;
falling objects;
working in confined places;
electrocution and personal injury;
working with ‘live’ equipment;
using hire equipment;
manual handling: pushing, pulling, lifting;
site conditions: falling objects, dust, weather, water,
accidents and injuries
...
Provision and Use of Work
Equipment Regulations 1998
These regulations tidy up a number of existing
requirements already in place under other regulations
such as the Health and Safety at Work Act 1974, the
Factories Act 1961 and the Offices, Shops and Railway
Premises Act 1963
...
If an employer has purchased good quality
plant and equipment, which is well maintained, there
is little else to do
...
The Control of Substances
Hazardous to Health Regulations
1988
The original COSHH Regulations were published in
1988 and came into force in October 1989
...
The COSHH Regulations control people’s exposure
to hazardous substances in the workplace
...
Employees
should also know what cleaning, storage and disposal
procedures are required and what emergency procedures to follow
...
Hazardous substances include:
1 any substance which gives off fumes causing
headaches or respiratory irritation;
2 man-made fibres which might cause skin or eye
irritation (e
...
loft insulation);
3 acids causing skin burns and breathing irritation (e
...
car batteries, which contain dilute sulphuric acid);
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
4 solvents causing skin and respiratory irritation
(strong solvents are used to cement together PVC
conduit fittings and tube);
5 fumes and gases causing asphyxiation (burning
PVC gives off toxic fumes);
6 cement and wood dust causing breathing problems
and eye irritation;
7 exposure to asbestos – although the supply and use
of the most hazardous asbestos material is now prohibited, huge amounts were installed between 1950
and 1980 in the construction industry and much of
it is still in place today
...
These can be
found in Guidance Notes EH 71
...
Personal Protective Equipment
(PPE) at Work Regulations 1992
PPE is defined as all equipment designed to be worn,
or held, to protect against a risk to health and safety
...
1
...
5
This includes most types of protective clothing, and
equipment such as eye, foot and head protection, safety
harnesses, life jackets and high-visibility clothing
...
Safety signs such as those shown at
Fig
...
2 are useful reminders of the type of PPE to be
used in a particular area
...
Objects falling from a height present the major hazard
against which head protection is provided
...
Typical
methods of protection include helmets, light duty scalp
protectors called ‘bump caps’ and hairnets
...
Types of eye protectors include
safety spectacles, safety goggles and face shields
...
Working with VDUs (visual display units) can
cause eye strain and fatigue and, therefore, this hazard
is the subject of a separate section later in this chapter
headed VDU operation hazards
...
The Health and Safety Executive have
published a ‘Code of Practice’ and ‘Guidance Notes’
HSG 56 for reducing the exposure of employed persons to noise
...
Noise may be defined as any disagreeable or undesirable sound or sounds, generally of a random nature,
which do not have clearly defined frequencies
...
Whether noise of a particular level is
harmful or not also depends upon the length of exposure to it
...
A peak sound pressure of above 200 pascals or
about 120 dB is considered unacceptable and 130 dB
is the threshold of pain for humans
...
If the distance is only 1 m, the noise
level is about 90 dB
...
It may be possible to engineer out some of the
noise, for example, by placing a generator in a separate
sound-proofed building
...
Where individuals must be subjected to some noise
at work it may be reduced by ear protectors
...
The chosen ear protector must be suited to the
user and suitable for the type of noise and individual
personnel should be trained in its correct use
...
Some industrial
processes produce dust which may present a potentially serious hazard
...
Some people may prove to be allergic to quite innocent products such as flour dust in the food industry
or wood dust in the construction industry
...
1
...
effect of inhaling dust is a measurable impairment of
lung function
...
1
...
A worker’s body may need protection against heat
or cold, bad weather, chemical or metal splash, impact
or penetration and contaminated dust
...
Appropriate clothing will be recommended
in the company’s health and safety policy
...
Figure
1
...
Hands and feet may need protection from abrasion, temperature extremes, cuts and punctures, impact
or skin infection
...
Care in selecting the appropriate protective device is required; for example, barrier creams provide only a limited protection against
infection
...
Special slip resistant soles can also be
provided for employees working in wet areas
...
Where there is a
need for PPE to protect against personal injury and to
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
7
This is a food
production area
Protective clothing
must be worn
Wash your
hands before
commencing work
Wear hairnets
NO
smoking
Fig
...
4 PPE and safety signs to be found in the food industry
...
RIDDOR
RIDDOR stands for Reporting of Injuries, Diseases
and Dangerous Occurrences Regulation 1995, which
is sometimes referred to as RIDDOR 95, or just RIDDOR for short
...
What needs reporting? Every work related death,
major injury, dangerous occurrence, disease or any
injury which results in an absence from work of over
3 days
...
Within 10 days this must be followed up by a complete accident report form (Form No
...
Major
injuries sustained as a result of an accident at work
include amputations, loss of sight (temporary or permanent), fractures to the body other than to fingers,
thumbs or toes and any other serious injury
...
Dangerous occurrences are listed in the regulations and include the collapse of a lift, an explosion
or injury caused by an explosion, the collapse of a
scaffold over five metres high, the collision of a train
with any vehicle, the unintended collapse of a building and the failure of fairground equipment
...
However, the incident must be reported
within 10 days by completing Form F2508
...
Reportable diseases include certain poisonings, skin diseases, lung
disease, infections and occupational cancer
...
An accident at work resulting in an over 3 day
injury, that is, an employee being absent from work
for over three days as a result of an accident at work,
requires that accident report form F2508 be sent to
the local authority within 10 days
...
Who are the reports sent to? They are sent to
the Environmental Health Department of the local
authority or the area HSE offices (See the Appendix L
of this book for area office addresses)
...
For most businesses, a reportable accident, dangerous occurrence or disease is a very rare event
...
The easiest way to do this
would probably be to file a photocopy of the completed accident report form F2508, but a record may
be kept in any form which is convenient
...
Their main aim is to prevent any major
accidents involving dangerous substances such as
chlorine, liquefied petroleum gas (LPG), explosives
and arsenic pentoxide that would cause serious harm
to people or damage the environment
...
These regulations apply mainly to the chemical
industry but also apply to some storage facilities and
nuclear sites
...
This sets high standards of control
but by requiring operators to put in place measures for
both prevention and mitigation, which means to make
less serious, there is the recognition that all risks cannot
be completely eliminated
...
The COMAH Regulations are enforced by the
Health and Safety Executive (HSE) and the Environment Agency
...
They are
designed to implement the safety requirements of the
Chemical Agents and Explosive Atmospheres Directive
...
Dangerous substances include petrol, liquefied
petroleum gas (LPG), paint, solvents and combustible
or explosive dust produced in machining and sanding
operations, flour mills and distilleries
...
However, DSEAR does not address the health
risk, only the fire and explosion risk
...
The DSEAR Regulations follow the modern risk
assessment based approach
...
There is a
requirement to provide equipment and procedures
to deal with accidents and emergencies and also to
provide information and training for employees
...
Typical industries might be those concerned with the storage of
petrol as a fuel for vehicles, agricultural and horticultural storage and the movement of bulk powders for
the food industry, the storing of waste dust in a range of
manufacturing industries, dust produced in the mining
of coal, storage and transportation of paint and LPG
...
The person requesting that construction work commence, the client, must first of all appoint a ‘duty
holder’, someone who has a duty of care for health,
safety and welfare matters on site
...
The planning supervisor must produce a ‘pre-tender’ health and safety plan
and co-ordinate and manage this plan during the early
stages of construction
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
The degree of detail in the health and safety plan
should be in proportion to the size of the construction
project and recognize the health and safety risks involved
on that particular project
...
The
CDM Regulations will apply to most large construction
projects but they do not apply to the following:
■
■
■
■
Construction work, other than demolition work,
that does not last longer than 30 days and does not
involve more than four people
...
Construction work carried out for a domestic
client
...
The Construction (Health, Safety
and Welfare) Regulations 1996
An electrical contractor is a part of the construction
team, usually as a subcontractor, and therefore the regulations particularly aimed at the construction industry
also influence the daily work procedures and environment of an electrician
...
The temporary nature of construction sites makes
them one of the most dangerous places to work
...
Construction work is defined as any building or
civil engineering work, including construction, assembly, alterations, conversions, repairs, upkeep, maintenance or dismantling of a structure
...
Schedules
specify the requirements for guardrails, working platforms, ladders, emergency procedures, lighting and
welfare facilities
...
There
9
is now a duty for all those working on construction
sites to wear head protection, and this includes
electricians working on site as subcontractors
...
The scope of each
Approved Document is given below:
Part A
Part B
Part C
Part D
Part E
Part F
Part G
Part H
Part J
structure
fire safety
site preparation and resistance to moisture
toxic substances
resistance to the passage of sound
ventilation
hygiene
drainage and waste disposal
combustion appliances and fuel storage
systems
Part K protection from falling, collision and
impact
...
This means that anyone carrying
out domestic electrical installation work from 1st
January 2005 must comply with Part P of the
Building Regulations
...
What is going to change under
Part P is this new concept of ‘notification’ to carry out
electrical work
...
That is, work that involves:
■
■
■
■
the provision of at least one new circuit,
work carried out in kitchens,
work carried out in bathrooms,
work carried out in special locations such as swimming pools and hot air saunas
...
NON-NOTIFIABLE ELECTRICAL WORK
Work carried out by a person or firm registered under
an authorized Competent Persons Self-Certification
Scheme or electrical installation work that does not
include the provision of a new circuit
...
All replacement work is non-notifiable even when
carried out in kitchens, bathrooms and special locations, but certain work carried out in kitchens, bathrooms and special locations may be notifiable, even
when carried out by an authorized competent person
...
In specific
cases the Local Authority Building Control Officer or
an approved Inspector will be able to confirm whether
Building Regulations apply
...
Electrical work carried out by DIY home-owners
will still be permitted after the introduction of Part P
...
The work must then be carried out to
the standards set by the IEE Wiring Regulations
BS 7671 and a building control fee paid for such work
to be inspected and tested by the Local Authority
...
The
Government has approved schemes to be operated by
BRE Certification Ltd
...
, NICEIC Certification Services Ltd
...
All the different
bodies will operate the scheme to the same criteria
and will be monitored by the Office of the Deputy
Prime Minister
...
The work of members will then be inspected
at least once each year
...
The Electricity Safety, Quality
and Continuity Regulations 2002
The Electricity Safety, Quality and Continuity
Regulations replaces the Electricity Supply Regulations 1988
...
They are designed
to ensure a proper and safe supply of electrical energy
up to the consumer’s terminals
...
The
regulations are administered by the Engineering
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Inspectorate of the Electricity Division of the
Department of Energy and will not normally concern
the electrical contractor except that it is these regulations which lay down the earthing requirement of the
electrical supply at the meter position
...
The government agreed on 1 January 1995 that the
electricity supplies in the United Kingdom would be
harmonized with those of the rest of Europe
...
The permitted tolerances to the nominal voltage have also
been changed from Ϯ6% to ϩ10% and Ϫ6%
...
The next change will come in 2005, when the tolerance levels will be adjusted to Ϯ10% of the declared
nominal voltage
...
Regulation 29 gives the area boards the power to
refuse to connect a supply to an installation which in
their opinion is not constructed, installed and protected to an appropriately high standard
...
11
properly maintained
...
… In the case of work of an electrical nature,
it is preferable that the conductors be made dead before
work commences’
...
An electrical contractor could reasonably be expected
to have ‘exercised all diligence’ if the installation was
wired according to the IEE Wiring Regulations (see
below)
...
The Court accepted that an electrician
had caused the death of another man as a result of his
shoddy work in wiring up a central heating system
...
This
case has set an important legal precedent, and in future
any tradesman or professional who causes death
through negligence or poor workmanship risks
prosecution and possible imprisonment
...
The Regulations are
made under the Health and Safety at Work Act 1974,
and enforced by the Health and Safety Executive
...
Section 4 of the EWR tells us that ‘all systems must
be constructed so as to prevent danger …, and be
The Institution of Electrical Engineers Requirements
for Electrical Installations (the IEE Regulations) are
non-statutory regulations
...
Paragraph 7 of the
introduction to the Electricity at Work Regulations
(EWR) says: ‘the IEE Wiring Regulations is a code of
practice which is widely recognized and accepted in
the United Kingdom and compliance with them is
likely to achieve compliance with all relevant aspects of
the Electricity at Work Regulations’
...
c
...
The current edition of the IEE Wiring Regulations,
is the 16th edition incorporating amendment number
1:2002 and 2:2004
...
To assist electricians in their understanding of the
Regulations a number of guidance notes have been
published
...
Seven other guidance notes booklets are also currently available
...
These guidance notes are intended to be read in conjunction with the Regulations
...
ENVIRONMENTAL LAWS AND
REGULATIONS
Environmental laws protect the environment in
which we live by setting standards for the control of
pollution to land, air and water
...
1 An offence in common law which means damage
to property, nuisance or negligence leading to a claim
for damages
...
These
offences are nearly always ‘crimes’ and punished by
fines or imprisonment rather than by compensating
any individual
...
Environmental Protection Act
1990
In the context of Environmental Law, the Environmental Protection Act 1990 was a major piece of legislation
...
The Royal Commission of 1976 identified that a
reduction of pollutant to one medium, air, water or
land, then lead to an increase of pollutant to another
...
The processes subject
to an integrated pollution control are:
■
■
■
air emissions;
processes which give rise to significant quantities of
special waste, that is, waste defined in law in terms
of its toxicity or flammability;
processes giving rise to emissions to sewers or ‘Red
List’ substances
...
Where a process is under integrated control the
Inspectorate is empowered to set conditions to ensure
that the best practicable environmental option
(BPEO) is employed to control pollution
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Pollution Prevention and Control
Regulations 2000
The system of Pollution Prevention and Control is
replacing that of Integrated Pollution Control established by the Environmental Protection Act 1990, thus
bringing Environmental Law into the new millennium
and implementing the European Directive (EC/96/61)
on integrated pollution prevention and control
...
Pollution Prevention and Control is a regime for
controlling pollution from certain industrial activities
...
Industrial activities are graded according to their
potential to pollute the environment:
■
■
■
A(1) installations are regulated by the Environment
Agency
...
Part B installations are also regulated by the Local
Authority
...
Once an operator has submitted a permit application,
the regulator then decides whether to issue a permit
...
A(1) installations are generally perceived as having the greatest potential to pollute the environment
...
The industries affected by these regulations are those
dealing with petrol vapour recovery, incineration of
waste, mercury emissions from crematoria, animal
rendering, non-ferrous foundry processes, surface treating of metals and plastic materials by powder coating,
galvanizing of metals and the manufacture of certain
specified composite wood based boards
...
13
The smoke from these fires created air pollution and
the atmosphere in large towns and cities was of poor
quality
...
Smog was a very dense
fog in which you could barely see more than a metre
in front of you and which created serious breathing
difficulties
...
Using well engineered combustion equipment together with the efficient arrestment of small particles in commercial
chimneys of sufficient height, air pollution has been
much reduced
...
The Clean Air Act applied to all small and medium
sized companies operating furnaces, boilers, or incinerators
...
In general the
emission of dark smoke from any chimney is unacceptable
...
This might be caused by,
for example, the burning of old tyres or old cable
...
Simply
the evidence of burned materials, which potentially
give rise to dark smoke when burned, is sufficient
...
A public nuisance is ‘an act unwarranted by law or
an omission to discharge a legal duty which materially
affects the life, health, property, morals or reasonable
comfort or convenience of Her Majesty’s subjects’
...
Controlled Waste Regulations
1998
Under these Regulations we have a ‘Duty of Care to
handle, recover and dispose of all waste responsibly’
...
The Environmental Protection (Duty of Care)
Regulations 1991 state that as a business you have a
duty to ensure that any waste you produce is handled
safely and in accordance with the law
...
You are responsible for the waste that you produce,
even after you have passed it on to another party such
as a Skip Hire company, a Scrap Metal merchant,
recycling company or local council
...
So what does this mean for your company?
■
■
■
■
■
Make sure that waste is only transferred to an
authorized company
...
Segregate the different types of waste that your
work creates
...
Minimize the waste that you produce and do not
leave waste behind for someone else to clear away
...
Occupiers of domestic properties are exempt from
the Duty of Care for the household waste that they
produce
...
Special waste is covered by the Special Waste
Regulations 1996 and is waste that is potentially hazardous or dangerous and which may, therefore, require
special precautions during handling, storage, treatment
or disposal
...
The disposal
of special waste must be carried out by a competent
person, with special equipment and a licence
...
Most companies buy in the expertise of
specialist waste companies these days and build these
costs into the contract
...
The Regulation will come into effect in August
2005 and from that date any company which makes,
distributes or trades in electrical or electronic goods
such as household appliances, sports equipment and
even torches and toothbrushes will have to make
arrangements for recycling these goods at the end of
their useful life
...
Producers of electrical and electronic equipment
must have their plans in place by the first quarter of
2005 for dealing with the return and recycling of
equipment after August 2005
...
Some sectors are better prepared for the new regulations than others
...
In Holland
the price of a new car now includes a charge for the
recycling costs
...
Information is also available
on the DTI and DEFRA website under WEEE
...
The radioactive source may be sealed or unsealed
...
Sealed
radioactive sources are used in radiography for the
non-destructive testing of materials or in liquid level
and density gauges
...
These regulations are enforced by the Health and Safety Executive,
while the Radioactive Substances Act is enforced by
the Environmental Agency
...
The requirements of
RSA 1993 are in addition to and separate from IRR
1999 for any industry using radioactive sources
...
Dangerous Substances and
Preparations and Chemicals
Regulations 2000
Chemical substances that are classified as carcinogenic, mutagenic or toxic, or preparations which contain those substances, constitute a risk to the general
public because they may cause cancer, genetic
disorders and birth defects, respectively
...
The Regulations require that new labels be attached
to the containers of these drugs which identify the
potential dangers and indicate that they are restricted
to professional users only
...
The Regulations will be enforced by the Local
Authority Trading Standards Department
...
In contrast, today there are
many statutes, Government circulars, British Standards
and European Union Directives dealing with noise
matters
...
During the eighteenth century, in the vicinity of some London hospitals, straw
was put on the roads to deaden the sound of horses’
hooves and the wheels of carriages
...
In the context of the Environmental Protection Act
1990, noise or vibration is a statutory nuisance if it is
prejudicial to health or is a nuisance
...
There is a wealth of case law but a good
working definition might be ‘A statutory nuisance
must materially interfere with the enjoyment of one’s
dwelling
...
The line that separates nuisance from no nuisance
is very fine and non-specific
...
m
...
Similarly,
an all night party with speakers in the garden would
be a nuisance, whereas an occasional party finishing at
say midnight would not be a statutory nuisance
...
In the first
judgment of its kind, the judge told the Park’s owners
16
ADVANCED ELECTRICAL INSTALLATION WORK
that neighbouring residents must not be interrupted
by noise from rides such as Nemesis, Air, Corkscrew,
Oblivion or from loudspeakers or fireworks
...
, were fined the maximum sum of £5000 and
served with a Noise Abatement Order for being guilty
of breaching the 1990 Environmental Protection Act
...
Many nuisance complaints under the Act are domestic and are difficult to assess and investigate
...
Similarly, sources of commercial noise complaints are also varied and include deliveries of goods during the night, general factory noises,
refrigeration units, noise from public houses and clubs
are all common complaints
...
Industrial noise assessment is aided by BS 4142 but
no guidance exists for other noise nuisance
...
The Noise and Statutory
Nuisance Act 1993
This Act extended the statutory nuisance provision of
the Environmental Protection Act 1990 to cover noise
from vehicles, machinery or equipment in the streets
...
In the case of a car alarm a notice is fixed to the vehicle and an officer from the Local Authority spends
1 hour trying to trace the owner with help from the
Police and their National Computer system
...
The vehicle must be left as secure as possible
but if this cannot be done then it can be removed to a
safe compound after the Police have been notified
...
Home intruder alarms that have been sounding for
1 hour can result in a ‘Notice’ being served on the
occupier of the property, even if he or she is absent
from the property at the time of the offence
...
A Local Authority Officer can then immediately silence
the alarm without going into the property
...
The two key points
of the Code are the installation of a 20 minute cut-off
of the external sounder and the notification to the
Police and Local Authority of two key holders who
can silence the alarm
...
It provides a mechanism for permanent deprivation,
return of seized equipment and charges for storage
...
If a notice is
not complied with, a fixed penalty may be paid instead
of going to court
...
The 1989 Regulations
came into force on the 1st January 1990 implementing in the United Kingdom the EC Directive 86/188/
EEC ‘The Protection of Workers from Noise’
...
2 The second action level is a daily personal noise
exposure of 90 dB(A)
...
This
action level is likely to be most important where
workers are subjected to a small number of loud
impulses during an otherwise quiet day
...
So,
what is a loud noise? If you cannot hear what someone is saying when they are 2 m away from you or if
they have to shout to make themselves heard, then the
noise level is probably above 85 dB and should be
measured by a competent person
...
At the second action level the employer must
reduce, so far as is reasonably practicable, other than
by providing ear protection, the exposure to noise of
that employee
...
Young people, in
particular, should get into the routine of avoiding
noise exposure before their hearing is permanently
damaged
...
Vibration is also associated with noise
...
A condition known as ‘vibration white finger’
is caused by an impaired blood supply to the fingers,
associated with vibrating hand tools
...
In practice, they reduce noise
exposure far less than is often claimed, because they
may be uncomfortable or inconvenient to wear
...
If left off for even a short time,
the best protectors cannot reduce noise exposure
effectively
...
Wearing ear protection must be a last
resort
...
Ask the supplier to specify
noise levels at the operators working position;
enclose noisy machines in sound absorbing panels;
fit silencers on exhaust systems;
install motor drives in a separate room away from
the operator;
inform workers of the noise hazard and get them to
wear ear protection;
reduce a worker’s exposure to noise by job rotation
or provide a noise refuge
...
Every employer
must make a ‘noise’ assessment and provide workers
with information about the risks to hearing if the
noise level approaches the first action level
...
Employees must wear personal ear
protection whilst in such a zone
...
They are responsible
for monitoring and ensuring the maintenance of standards of environmental and public health, including
food safety, workplace health and safety, housing,
noise, odour, industrial waste, pollution control and
communicable diseases in accordance with the law
...
The majority of Environmental Health Officers are
employed by Local Authorities, who are the agencies
18
ADVANCED ELECTRICAL INSTALLATION WORK
concerned with the protection of public health
...
Your Local Authority Environmental Health Officer
would typically have the responsibility of enforcing
the environmental laws discussed above
...
In carrying out these duties, Officers have the right
to enter any workplace without giving notice, although
notice may be given if they think it appropriate
...
LAWS PROTECTING PEOPLE
We have now looked at some of the major pieces of
legislation that affect our working environment and
some of the main pieces of environmental law
...
Employment Rights Act 1996
If you work for a company you are an employee and
you will have a number of legal rights under the
Employment Rights Act 1996
...
There are strict guidelines regarding those
who are employed and those who are self-employed
...
You do not risk your own money
...
You do not employ anyone
...
Someone else has the right to control what you do
at work even if such control is rarely practised
...
You give a price for doing a job and will bear the
consequences if your price is too low or something
goes wrong
...
You may be paid an agreed amount for a job regardless of how long it takes or be paid according to some
formula, for example, a fee to ‘first fix’ a row of
houses
...
The titles ‘employed’ or ‘self-employed’ are not
defined by statute but have emerged through cases
coming before the courts
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Home working is a growing trend which prompts
the question as to whether home workers are employed
or self-employed
...
The Inland Revenue look with concern at those
people who claim to be self-employed but do all or most
of their work for one company
...
If you are an employee you have a special relationship in law with your employer which entitles you to
the following benefits:
■
■
■
■
■
■
■
■
■
■
■
■
A written statement of the particulars of your
employment
...
The
legal relationship between employer and employee
is one of contract
...
The date your employment started
...
Continuous
service is normally with one employer but there are
exceptions, for example, if a business is transferred
or taken over or there is a change of partners or
trustees
...
The job title
...
A brief description of your work
...
Holiday entitlement and holiday pay
...
Pension scheme arrangements
...
Where the employment is not intended to be permanent, the period for which it is expected to continue and the date when it is to end
...
The rate of pay and frequency, weekly or monthly
...
(ii) the amounts of any deductions and the purpose for which they have been made
...
(iii) The net amount of salary being paid
...
Even if the responsibilities are not written down in
the contract of employment, they are implied by law
...
These responsibilities include:
■
■
■
■
The obligation to pay an employee for work done
...
The obligation to take reasonable care of an
employee’s health and safety
...
An employee also has responsibilities to his
employer
...
Conducting yourself in such way as would best
serve your employer’s interests
...
An employee is not expected to carry out any order
that is plainly illegal or unreasonable
...
‘Unreasonable’ is more difficult to define, what is reasonable
to one person may be quite unreasonable to another
person
...
Employees are also under a general duty
not to assist a competitor of their employer
...
Even when an employee has left an employer,
confidential information is not to be disclosed
...
It does not matter whether the injury or illness
is caused by the work they do or not, what is important is that they are able to receive immediate attention
by a competent person or that an ambulance is called
in serious cases
...
It can save lives and prevent a minor incident
becoming a major one
...
What is adequate and
appropriate will depend upon the type of work being
carried out by the employer
...
Employers must consider:
■
■
■
■
■
■
■
■
■
How many people are employed and, therefore,
how many first aid boxes will be required
...
How many trained ‘first aiders’ will be required
...
Do employees travel frequently or work alone
...
How hazardous is the work being done – what are
the risks
...
What has been the accident or sickness record of
staff in the past
...
We will look at the first aid box contents and the
number of first aid boxes required a little later in this
chapter when we look at the application of first aid in
relation to particular injuries
...
Most of us,
for instance, would not want our medical records
freely circulated, and many people are sensitive about
revealing their age, religious beliefs, family circumstances or academic qualifications
...
With the advent of large computerized databases it is
now possible for sensitive personal information to be
stored without the individual’s knowledge and accessed
by, say, a prospective employer, credit card company or
insurance company in order to assess somebody’s suitability for employment, credit or insurance
...
The Act covers ‘personal data’ which is ‘automatically processed’
...
The Data Protection Act 1998 was passed in order
to implement a European Data Protection Directive
...
The Act gives the following useful definitions:
Data subjects: the individuals to whom the personal
data relate – we are all data subjects
...
They can be any type
of company or organization, large or small, within
the public or private sector
...
Personal data does not have to be particularly sensitive information and can be as little as
a name and address
...
The Act does not currently cover
information which is held on manual records, for
example, in ordinary paper files
...
Broadly these
state that data must be:
1 obtained and processed fairly and lawfully;
2 held for the lawful purposes described in the data
users’ register entry;
3 used for the purposes and disclosed only to those
people described in the register entry;
4 adequate, relevant and not excessive in relation to
the purposes for which they are held;
5 accurate and, where necessary, kept up to date;
6 held no longer than is necessary for the registered
purpose;
7 accessible to the individual concerned who, where
appropriate, has the right to have information
about themselves corrected or erased;
8 surrounded by proper security
...
Registration may not be necessary if the data is for
personal, family, household or recreational use
...
Data can be disclosed to the data subject’s agent
(e
...
lawyer or accountant), to persons working for
the data user, and in response to urgent need to
prevent injury or damage to health
...
THE RIGHTS OF DATA SUBJECTS
The Data Protection Act allows individuals to have
access to information held about themselves on computer and where appropriate to have it corrected or
deleted
...
The data
user may charge a fee of up to £10 for each register
21
entry for supplying this information but in some cases
it is supplied free
...
If not, you are entitled to complain to the
Registrar or apply to the courts for correction or deletion of the data
...
These are:
■
■
■
■
Right to compensation for unauthorized disclosure
of data
...
Right of access to data and to apply for rectification
or erasure where data is inaccurate
...
For more information see www
...
gov
...
Our culture is about the way
of life that we have, the customs, ideas and experiences
that we share and the things that we find acceptable
and unacceptable
...
When people have a certain attitude
towards you, or the group of people to which you
belong, or a belief about you that is based upon lack of
knowledge, understanding or myth, this is prejudice
...
Regardless of our age, ability, sex, religion, race or sexuality we should all be treated equally
and with respect
...
If you are being discriminated against or you see it
happening to someone else, you do not have to put up
with it
...
If you are a member of a Trade Union you may be able to get help from
them if it is an employment related matter
...
In the next
few years the law will change to make it unlawful to
22
ADVANCED ELECTRICAL INSTALLATION WORK
discriminate in the training or workplace on the
grounds of sexual orientation, religious belief and age
...
However, police
officers are office holders, not employees, and, therefore, Chief Officers of the police were not liable under
the 1976 Act for acts of racial discrimination
...
It is illegal to discriminate against someone because
of their race, colour, nationality, citizenship or ethnic
origin
...
It may mean
that the organization or institution does, or does not
do something, or that someone is treated less favourably
...
There are some exceptions in the RRA
...
These are known as ‘genuine occupational qualifications’ and might apply to actors and restaurants
...
cre
...
uk
Sex Discrimination Act 1975
(SDA)
‘Sexism’ takes place every time a person, usually a
woman, is discriminated against because of their sex
...
Job
advertisements must not discriminate in their language
but they can make it clear that they are looking for
people of a particular sex
...
There are some exceptions in the SDA known as
‘genuine occupational qualifications’ that might
apply to artists, models, actors and some parts of the
priesthood in the church
...
, and
women are not allowed to work underground
...
It includes
sexual harassment and unfavourable treatment
because a woman is pregnant
...
This is known as ‘direct sex
discrimination’
...
For example, an unnecessary height
requirement of 180 cm (5Ј 10Љ) would discriminate
against women because less women would be able to
meet this requirement
...
Further information can be found on
the SDA website at www
...
org
...
5 million disabled people in
the United Kingdom
...
It is now unlawful for employers of more than 15
people to discriminate against employees or job applicants on the grounds of disability
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Under Part 111 of the DDA, from 1st October
2004, service providers will have to take reasonable
steps to remove, alter or provide reasonable means of
avoiding physical features that make it impossible or
unreasonably difficult for disabled people to use their
services
...
If this is not possible then the
service should be provided by means of a reasonable
alternative such as bringing goods to the disabled
person or helping the person to find items
...
Some service providers will need to incur significant capital expenditure in order to comply with
the DDA
...
A subjective standard will apply when determining
what is reasonable under the circumstances at a given
location
...
Further information can be found on the DDA
website at www
...
gov
...
It means that if you
think your human rights have been violated, you can
take action through the British court system, rather
than taking it to the European Court of Human
Rights
...
The basic human rights in the Human Rights
Act are:
■
■
■
the right to life,
the right to a fair trial,
the right to respect for your private and family life,
■
■
■
■
■
■
■
■
■
■
23
the right to marry,
the right to liberty and security,
prohibition of torture,
prohibition of slavery and forced labour,
prohibition of discrimination,
prohibition of the abuse of rights,
freedom of thought, conscience and religion,
freedom of expression,
freedom of assembly and association,
no punishment without law
...
Rights
under the Act can only be used against a public
authority such as the Police or a Local Authority
...
For
more information see www
...
gov
...
The
system of control under the Health and Safety at Work
Act comes from the Health and Safety Executive (HSE)
or the Local Authority
...
The HSE are
responsible for all other work premises including the
Local Authorities themselves
...
They are allowed to:
■
■
■
■
■
■
enter premises, accompanied by a police officer if
necessary;
examine, investigate and require the premises to be
left undisturbed;
take samples and photographs as necessary, dismantle and remove equipment;
require the production of books or documents and
information;
seize, destroy or render harmless any substance or
article;
issue enforcement notices and initiate prosecutions
...
An improvement notice identifies a contravention
of the law and specifies a date by which the situation
is to be put right
...
24
ADVANCED ELECTRICAL INSTALLATION WORK
A prohibition notice is used to stop an activity
which the inspector feels may lead to serious injury
...
An appeal may be made to the Employment Tribunal but the notice remains in place and
work is stopped during the appeal process
...
Magistrates Court (Summary Offences) for Health
and Safety offences, employers may be fined up to
£20 000 and employees or individuals up to £5000
...
Crown Court (Indictable Offences) for failure to
comply with an enforcement notice or a court order,
fines are unlimited in the Crown Court and may
result in imprisonment for up to 2 years
...
A follow up visit will be
required to ensure that the matter has been corrected;
serve a prohibition notice – an activity has been identified which may lead to serious injury
...
On any visit one or more of the above actions may be
taken by the inspector
...
The day that one of these inspectors arrives
to look at the Health and Safety systems and procedures
that your company has in place is a scary day! Most
companies are very conscientious about their health and
safety responsibilities and want to comply with the law
...
To meet the requirements there
is a need for ‘competent persons’ to be appointed to
the various roles within the company structure to support the company directors in their management of
the Health and Safety Policy
...
To say that
‘everyone is responsible for health and safety’ is very
misleading and would definitely not impress a visiting
HSE inspector
...
Company directors and employers
have substantially more responsibilities than employees as far as the Health and Safety at Work Act is concerned
...
At the top of the Health and Safety structure there
will need to be a senior manager
...
Senior managers must take proactive
responsibility for controlling issues that could lead to
ill health or injury
...
Health and Safety responsibilities must then be
assigned to line managers and health and safety expertise must be available to them to help them achieve the
requirements of the Health and Safety at Work Act
and the regulations made under the Act
...
The prevention of accidents and ill health through management systems of control then becomes the focus
rather than looking for individuals to blame after an
accident has happened
...
The Safety Officer will be the specialist member of
staff, having responsibility for Health and Safety within
the company
...
The Safety Officer will probably hold a Health and
Safety qualification such as NEBOSH (National
Examination Board in Occupational Safety and
Health) and will:
■
■
■
■
■
monitor the internal Health and Safety systems,
carry out risk assessments,
maintain accident reports and records,
arrange or carry out in-house training,
update systems as Regulations change
...
The Safety Representative will be the person who
represents a small section of the workforce on the
Safety Committee
...
The office of Safety Representative is often held by the
Trade Union representative, since it is a similar role,
representing colleagues on management committees
...
The Safety Officer and Safety Representative hold
important positions within a company, informing both
employers and employees on health and safety matters
and helping the company meet its obligation to ‘consult with employees’ under the Health and Safety
Regulations
...
Similarly, monitoring and evaluating systems
systematically is an essential part of many quality
management systems
...
25
SAFE AND EFFECTIVE WORKING
PRACTICES
Quality systems
When purchasing goods and services these days the
customer is increasingly looking for good performance and reliability
...
Poor product reliability
has been identified as one of the chief causes of
customer dissatisfaction
...
Durability is closely linked
to reliability and is a measure of the amount of use
a customer receives from the product before it
deteriorates
...
Quality is also a customer’s subjective
impression of a product or service which has been
formed by images, advertisements, brand names or
reputation
...
Marks & Spencer, for example, have
built up a formidable reputation from providing
good-quality products and services
...
Today the
opposite is true
...
After watching the Japanese capture the major share
of these world markets, European and American companies have finally responded to the challenge and
introduced the quality standards used so successfully
by Japanese industry
...
There are four fundamental
approaches to managing quality: quality control, quality assurance, total quality control and total quality
management
...
It was introduced in the 1920s to
improve the quality of mass-produced goods
...
If the
sample was found to be faulty then all 100 parts would
be scrapped
...
The
problem with quality control is that the focus of attention is on the finished product rather than on the manufacturing process
...
Also, any scrapped
products become built-in costs which reduce company
profits and increase the product price in the shops
...
QUALITY ASSURANCE
Unlike quality control, which focuses upon postproduction inspection, quality assurance emphasizes
defect prevention through statistical quality control
and by monitoring processes to eliminate the production of bad parts
...
The
procedures and standard forms of documentation are
followed implicitly to ensure product conformity
...
TOTAL QUALITY CONTROL
Total quality control attempts to expand the quality
assurance philosophy to encompass all company
activities
...
It was slow to be adopted by Western companies
because it did not easily fit the organizational structures
...
Also, managers themselves
tended to work independently of each other and, as a
result, efforts to address company-wide issues such as
quality were often met with indifference or resistance
by the individuals involved
...
They embraced the word ‘total’ and
introduced quality assurance throughout their organizations
...
In the West, this company-wide
quality management philosophy is known as ‘total
quality management’
...
It is no longer ‘inspected in’, ‘built in’ or even ‘organized in’: quality is ‘managed in’ at all levels
...
Meeting the customer’s requirements is the simple
driving force behind total quality management
...
Striving to do error-free work means providing a
quality product or service first time, every time
...
Managing by prevention means that workers at all
levels must be encouraged to anticipate problems and
be given the power to make permanent changes to
procedures to prevent future errors
...
The cost of non-quality is the money a company
would otherwise spend on detecting, correcting and
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
preventing errors
...
BRITISH STANDARD QUALITY
British Standard 5750 (published in 1979) and the
ISO 9000 series, the world standard for quality assurance (published in 1987), have become synonymous
with quality assurance and are at the heart of most
quality management systems in Europe
...
A BS 5750/ISO 9000 certificate provides a framework for a company to establish quality procedures
and identify ways of improving its particular product
or service
...
Many electrotechnical companies are now accredited to ISO 9001 which means all of the company’s
standard systems and procedures have been documented into an approved quality management system
...
Once a year an external audit of
the systems takes place by an inspector nominated by
ISO 9001
...
This says to
potential customers ‘we are a serious professional
company working to the best standards of our industry and providing a quality service’
...
Investors in People (IiP)
Most people would agree that the people an organization employs are the most valuable asset of the business
...
For any business to succeed, everyone must
perform to the best of their ability from the youngest
trainee to the Managing Director
...
It recognizes that a company or business is investing
some of its profits in its workforce in order to improve
the efficiency and performance of the organization
...
The IiP standard lays down a set of ‘principles’ and
‘indicators’ of good practice which the participating
organization must meet
...
The IiP was started in 1990 and was driven by a
partnership between leading businesses and organizations such as the Confederation of British Industry
(CBI), Trade Union Councils (TUC), the Institute
of Personnel and Development and the National
Training Task Force
...
For more information on IiP go to www
...
co
...
They make our workplace safer, but
despite all this legislation, workers continue to be
injured and killed at work
...
The number of people who died in 2004 was
increased by the unfortunate accident in February of
that year, when 21 cockle pickers died in one single
night in the Morecambe Bay tragedy
...
The
number of deaths has consistently averaged about 200
each year for the past 8 years
...
If you include the self-employed and
members of the public killed in work related
accidents, the numbers almost double
...
It is a mistake to believe that these things only happen in dangerous occupations such as deep sea diving,
mining and quarrying, fishing industry, tunnelling
and fire-fighting or that it only happens in exceptional
circumstances such as would never happen in your
workplace
...
Some basic thinking
and acting beforehand, could have prevented most of
these accident statistics, from happening
...
To control the risk of an accident we usually:
■
■
■
■
■
■
■
eliminate the cause;
substitute a procedure or product with less risk;
enclose the dangerous situation;
put guards around the hazard;
use safe systems of work;
supervise, train and give information to staff;
if the hazard cannot be removed or minimized then
provide PPE
...
HAZARD AND RISK
A hazard is something with the ‘potential’ to cause
harm, for example, chemicals, electricity or working
above ground
...
Competent persons are often referred to in the
Health and Safety at Work Regulations, but who is
‘competent’? For the purposes of the Act, a competent
person is anyone who has the necessary technical
skills, training and expertise to safely carry out the
particular activity
...
Think about your workplace and at each stage of
what you do, think about what might go wrong
...
Here are
some typical activities where accidents might happen
...
Might this also include people who do not
work for your company?
Employers of more than five people must document
the risks at work and the process is known as Hazard
Risk Assessment
...
A record of all significant
risk assessment findings must be kept in a safe place
and be made available to an HSE Inspector if required
...
So risk assessment must form a part of any employer’s
robust policy of health and safety
...
He is not expected to assess the trivial and minor
types of household risks
...
An assessment of risk is nothing more than a careful examination of what, in your work, could cause
harm to people
...
The HSE recommends five steps to any risk
assessment
...
Ignore the trivial and concentrate only on significant hazards that could result in serious harm or injury
...
Step 2
Decide who might be harmed and how
...
Include members of the public or people who
share the workplace
...
Step 3
Evaluate what is the risk arising from an identified
hazard
...
What you have to decide, for
each significant hazard, is whether this remaining risk
is low, medium or high
...
For example, there are legal requirements on
the prevention of access to dangerous machinery
...
Your real aim is to make all risks small
by adding precautions, if necessary
...
29
If the work that you do varies a lot, or if there is
movement between one site and another, select those
hazards which you can reasonably foresee, the ones
that apply to most jobs and assess the risks for them
...
Step 4
Record your findings and say what you are going to
do about risks that are not adequately controlled
...
This means writing down the
more significant hazards and assessing if they are
adequately controlled and recording your most important conclusions
...
1
...
The important thing
is to make a record
...
Risk assessments need to be suitable and sufficient, not
perfect
...
It can help if an HSE
Inspector questions the company’s precautions or if the
company becomes involved in any legal action
...
Step 5
Review the assessments from time to time and revise
them if necessary
...
1
...
In most firms
in the commercial, service and light industrial sector,
30
ADVANCED ELECTRICAL INSTALLATION WORK
HAZARD RISK
ASSESSMENT
FLASH-BANG
ELECTRICAL CO
...
1
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
31
the hazards are few and simple
...
■
Step 1
List only hazards which you could reasonably expect
to result in significant harm under the conditions prevailing in your workplace
...
For example,
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
Slipping or tripping hazards (e
...
from poorly
maintained or partly installed floors and stairs)
...
g
...
Chemicals (e
...
from battery acid)
...
g
...
Rotating parts of handtools (e
...
drills)
...
High pressure air from airlines (e
...
air powered
tools)
...
g
...
Vehicles (e
...
fork lift trucks)
...
g
...
Dust (e
...
from grinding operations or thermal
insulation)
...
g
...
Manual handling (e
...
lifting, moving or supporting loads)
...
g
...
Poor lighting levels (e
...
working in temporary or
enclosed spaces)
...
g
...
High temperatures (e
...
working in boiler rooms or
furnaces)
...
Just think about groups of people doing similar work or who might be affected by your work:
■
■
■
■
■
■
■
Office staff
Electricians
Maintenance personnel
Other contractors on site
Operators of equipment
Cleaners
Members of the public
...
have you provided adequate information to staff,
have you provided training or instruction
...
If you can answer ‘yes’ to the above points then the
risks are adequately controlled, but you need to state
the precautions you have put in place
...
, in giving this information
...
If
the stated company procedure is to use battery drills
whenever possible, or 110 V drills when this is not
possible, and to never use 230 V drills, then this again
will reduce the risk
...
(Note: PAT testing is described in Chapter 2 of
Advanced Electrical Installation Work
...
Senior managers should apply the
principles below when taking action, if possible in the
following order:
1 Remove the risk completely
...
3 Prevent access to the hazard (e
...
by guarding)
...
5 Issue PPE
...
g
...
Any hazard identified by a risk assessment as high
risk must be brought to the attention of the person
responsible for health and safety within the company
...
The risks
are under control and identified as low risk’
...
1
...
Upon completion they should
be stapled together or placed in a plastic wallet and
stored in the dedicated file
...
1
...
Alternatively you might like to complete the
VDU workstation risk assessment checklist given in
the next section
...
into force on the 1st January 1993, and employers
who use standard office VDUs must show that they
have taken steps to comply with the regulations
...
To comply with the regulations an employer must:
■
■
■
■
■
■
VDU operation hazards
Those who work at Supermarket checkouts, assemble
equipment or components, or work for long periods
at a visual display unit (VDU) and keyboard can be at
risk because of the repetitive nature of the work
...
The term covers
a number of related medical conditions
...
The Regulations came
use a VDU more or less continuously on most days;
use a VDU more or less continuously for periods of
an hour or more each day;
need to transfer information quickly to or from the
screen;
need to apply high levels of attention or concentration to information displayed on a screen;
are very dependent upon VDUs or have little
choice about using them
...
User training
Good user training will normally cover the following
topics:
■
■
■
■
■
■
■
■
■
the operating hazards and risks as describe above;
the importance of good posture and changing position as shown in Fig
...
6;
how to adjust furniture to avoid risks;
how to organize the workstation to avoid awkward
or repeated stretching movements;
how to avoid reflections and glare on the monitor
screen;
how to adjust and clean the monitor screen;
how to organize working routines so that there is a
change of activity or a break;
how a user might contribute to a workstation risk
assessment;
who to contact if problems arise
...
1
...
Fig
...
7 A document holder typically used by a word processing VDU operator
...
user cannot solve
...
1
...
1
...
Workstation risk assessment
A simple way to carry out a workstation risk assessment is to use a checklist such as that shown later in
this section
...
They know what the problems at their
workstation are and whether they are comfortable or
not
...
Where possible encourage VDU users to carry out other tasks
such as taking telephone calls, filing and photocopying
...
The length of break
34
ADVANCED ELECTRICAL INSTALLATION WORK
required is not fixed by the law; the time will vary
depending upon the work being done
...
To be comfortable the operator should adjust the
chair and equipment so that:
■
■
Eye and eyesight testing
VDU users and those who are to become users of
VDU equipment can request an eye and eyesight test
that is free of charge to them
...
Users are also entitled to further
tests at regular intervals but if the user’s normal glasses
are suitable for VDU work, then the employer is not
required to pay for them
...
Hands can rest on the work surface in front of the
keyboard with fingers outstretched over the keys
...
The small of the back is supported by the chair
...
The arms on the chair or obstructions under the desk
must not prevent the user from getting close enough
to the keyboard comfortably
...
Users, trainers and assessors should focus on those
aspects which have changed
...
Users working from a number of source documents
will need more desk space than users who are word
processing
...
Some questions cannot be answered until
a user has had an opportunity to try the workstation
...
Users should be given
information on:
■
■
■
■
the health and safety relating to their particular
workstations;
the risk assessments carried out and the steps taken
to reduce risks;
the recommended break times and changes in
activity to reduce risks;
the company procedures for obtaining eye and eyesight tests
...
VDU WORKSTATION RISK ASSESSMENT CHECKLIST
Using a checklist such as that shown below or the
more extensive checklist shown in the HSE book
‘VDUs, an Easy Guide to the Regulations’ is one way to
assess workstation risks
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Risk factors are grouped under five headings and to
each question the user should initially give a simple
yes/no response
...
1
...
1 Are the characters readable?
1
...
3 Are brightness and contrast adjustable?
1
...
5 Is the screen free from glare and
reflections?
2
...
1 Is the keyboard tiltable?
2
...
3 Is there enough space to rest your hands
in front of the keyboard?
2
...
3
...
2
3
...
4
3
...
Is the surrounding environment risk free?
4
...
2 Are levels of light, heat and noise
comfortable?
Fig
...
8 Correct manual lifting and carrying procedure
...
3 Does the air feel comfortable in terms
of temperature and humidity?
Y/N
5
...
1
5
...
3
Y/N
Y/N
Y/N
Is the software user friendly?
Can you comfortably use the software?
Is the software suitable for the work task?
Have you had enough training?
A copy of all risk assessments carried out should be
placed in a dedicated file which can then be held by
the trainer/assessor or other responsible person
...
Other relevant publications include ‘Display Screen
Equipment Work and Guidance on Regulations L26 ’
and ‘Industry Advisory (General) Leaflet IND(G) 36(L)
1993 Working with VDUs’
...
Y/N
Y/N
Manual handling
Y/N
Manual handling is lifting, transporting or supporting loads by hand or by bodily force
...
Whatever the heavy object
is, it must be moved thoughtfully and carefully, using
appropriate lifting techniques if personal pain and
injury are to be avoided
...
When lifting heavy loads, correct lifting procedures
must be adopted to avoid back injuries
...
8
demonstrates the technique
...
Always lift
with the back straight and the legs bent so that the
powerful leg muscles do the lifting work
...
Grasp the object firmly and, keeping the back straight and the head erect, use the leg muscles to raise in a smooth movement
...
When putting the object down,
keep the back straight and bend at the hips and knees,
reversing the lifting procedure
...
There
have been too many injuries over the years resulting
from bad manual handling techniques
...
Publications
such as Getting to Grips with Manual Handling can be
obtained from HSE Books; the address and Infoline
are given in Appendix L
...
Consider some ‘good practice’ when lifting loads:
■
■
■
■
■
■
■
■
■
■
■
Do not lift the load manually if it is more appropriate to use a mechanical aid
...
Always use a trolley, wheelbarrow or truck such as
those shown in Fig
...
9 when these are available
...
Take account of the centre of gravity of the load
when lifting – the weight acts through the centre of
gravity
...
Always lift and lower loads gently
...
Use the manual lifting techniques described above
and avoid sudden or jerky movements
...
Take special care when moving loads wrapped in
grease or bubble-wrap
...
Fig
...
9 Always use a mechanical aid to transport a load when
available
...
This
includes most types of protective clothing, and equipment such as eye, foot and head protection, safety
harnesses, life jackets and high visibility clothing
...
Safety signs such as those shown at
Fig
...
10 are useful reminders of the type of PPE to be
used in a particular area
...
Objects falling from a height present the major hazard
against which head protection is provided
...
1
...
hazards include striking the head against projections
and hair becoming entangled in machinery
...
The eyes are very vulnerable to liquid splashes, flying particles and light emissions such as ultraviolet
light, electric arcs and lasers
...
Screen based workstations are being used increasingly
in industrial and commercial locations by all types of
personnel
...
Noise is accepted as a problem in most industries
and we looked in some detail at the Noise Regulations
a little earlier in this chapter under the Environmental
Laws section
...
The
usual basis for measuring noise or sound level is the decibel scale
...
This is the basis of the widely accepted limit of 85 dB of
continuous exposure to noise for 8 hours per day
...
These
may be disposable ear plugs, re-usable ear plugs or ear
Masks
must be worn
in this area
Respirators
must be worn
in this area
Fig
...
11 Breathing protection signs
...
The chosen ear protector must be suited to the
user and suitable for the type of noise and individual
personnel should be trained in its correct use
...
Some industrial
processes produce dust which may present a potentially serious hazard
...
Some people may prove to be allergic to quite innocent products such as flour dust in the food industry
or wood dust in the construction industry
...
This can be avoided by wearing an
appropriate mask, respirator or breathing apparatus as
recommended by the company’s health and safety
policy and indicated by local safety signs such as those
shown in Fig
...
11
...
Alternatively,
there may be a risk of the worker’s own clothes causing contamination of the product, as in the food
industry
...
Ordinary
working clothes and clothing provided for food
hygiene purposes are not included in the Personal
Protective Equipment at Work Regulations
...
Gloves or gauntlets provide protection
from most industrial processes but should not be worn
when operating machinery because they may become
entangled in it
...
Boots or shoes with in-built toe caps can give protection against impact or falling objects and, when fitted with a mild steel sole plate, can also provide
protection from sharp objects penetrating through
the sole
...
Whatever the hazard to health and safety at work,
the employer must be able to demonstrate that he or
she has carried out a risk analysis, made recommendations which will reduce that risk and communicated
these recommendations to the workforce
...
1
...
Fig
...
13 Prohibition signs
...
Safety signs
The rules and regulations of the working environment
are communicated to employees by written instructions, signs and symbols
...
They should give
warning of possible dangers and must be obeyed
...
The purpose of the regulations is to establish an internationally
understood system of safety signs and colours which
draw attention to equipment and situations that do, or
could, affect health and safety
...
From that
date, all safety signs have had to contain a pictogram or
symbol such as those shown in Fig
...
12
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
39
PROHIBITION SIGNS
SAFE CONDITION SIGNS
These are circular white signs with a red border and
red cross bar, and are given in Fig
...
13
...
These are square or rectangular green signs with a
white symbol, and are given in Fig
...
16
...
WARNING SIGNS
These are triangular yellow signs with a black border
and symbol, and are given in Fig
...
14
...
MANDATORY SIGNS
These are circular blue signs with a white symbol, and
are given in Fig
...
15
...
Accidents at work
Despite new legislation, improved information, education and training, accidents at work do still happen
...
An accident can nearly always be avoided if correct
procedures and methods of working are followed
...
1
...
Fig
...
15 Mandatory signs
...
1
...
First aid
post
40
ADVANCED ELECTRICAL INSTALLATION WORK
is notifiable to the HSE
...
To avoid
having an accident you should:
1 follow all safety procedures (e
...
fit safety signs
when isolating supplies and screen off work areas
from the general public);
2 not misuse or interfere with equipment provided
for health and safety;
3 dress appropriately and use personal protective
equipment (PPE) when appropriate;
4 behave appropriately and with care;
5 avoid over-enthusiasm and foolishness;
6 stay alert and avoid fatigue;
7 not use alcohol or drugs at work;
8 work within your level of competence;
9 attend safety courses and read safety literature;
10 take a positive decision to act and work safely
...
Fire control
A fire is a chemical reaction which will continue if
fuel, oxygen and heat are present
...
This is
often expressed by means of the fire triangle shown in
Fig
...
17; all three corners of the triangle must be
present for a fire to burn
...
Most
solvents are flammable
...
To eliminate fuel as a source of fire, all flammable
liquids and gases should be stored correctly, usually in
an outside locked store
...
1
...
with a lid
...
OXYGEN
Oxygen is all around us in the air we breathe, but can
be eliminated from a small fire by smothering with a
fire blanket, sand or foam
...
Most substances will burn if they are at a high
enough temperature and have a supply of oxygen
...
However, a danger does
exist from portable heaters, blow torches and hot air
guns which provide heat and can cause a fire by raising the temperature of materials placed in their path
above the minimum ignition temperature
...
HEAT
Heat can be removed from a fire by dousing with
water, but water must not be used on burning liquids
since the water will spread the liquid and the fire
...
Fires in industry damage property and materials,
injure people and sometimes cause loss of life
...
1
...
The base colour of all fire extinguishers is red, with a
different coloured flash to indicate the type
...
In the event of fire you should:
■
■
■
■
■
raise the alarm;
turn off machinery, gas and electricity supplies in
the area of the fire;
close doors and windows but without locking or
bolting them;
remove combustible materials and fuels away from
the path of the fire, if the fire is small, and if this
can be done safely;
attack small fires with the correct extinguisher
...
Always leave your
own exit from the danger zone clear
...
Fires are divided into four classes or categories:
■
■
■
■
Class A are wood, paper and textile fires
...
Class C are fires involving gas or spilled liquefied gas
...
Electrical fires do not have a special category because,
once started, they can be identified as one of the four
above types
...
Using the wrong type of
extinguisher could make matters worse
...
The normal procedure when dealing with electrical
fires is to cut off the electrical supply and use an extinguisher which is appropriate to whatever is burning
...
18 shows the correct type of extinguisher to
42
ADVANCED ELECTRICAL INSTALLATION WORK
be used on the various categories of fire
...
First aid
Despite all the safety precautions taken in the workplace to prevent injury to the workforce, accidents do
happen and you may be the only other person able to
take action to assist a workmate
...
The Health and Safety (First Aid)
Regulations 1981 and relevant approved codes of
practice and guidance notes place a duty of care on all
employers to provide adequate first aid facilities
appropriate to the type of work being undertaken
...
For this purpose a person shall not be
suitable unless he or she has undergone such training and has such qualifications as the Health and
Safety Executive may approve
...
The regulations and
codes of practice do not specify numbers, but set out
guidelines in respect of the number of first aiders
needed, dependent upon the type of company, the
hazards present and the number of people employed
...
‘First aid is
the treatment of minor injuries which would otherwise receive no treatment or do not need treatment
by a doctor or nurse’ or ‘In cases where a person will
require help from a doctor or nurse, first aid is treatment for the purpose of preserving life and minimizing the consequences of an injury or illness until such
help is obtained
...
Now having defined first aid, who might become a
first aider? A first aider is someone who has undergone
a training course to administer first aid at work and
holds a current first aid certificate
...
The
aims of a first aider are to preserve life, to limit the
worsening of the injury or illness and to promote
recovery
...
An appointed person is someone who
is nominated to take charge when someone is injured
or becomes ill, including calling an ambulance if
required
...
Appointed persons should not attempt to give first
aid for which they have not been trained but should
limit their help to obvious commonsense assistance and
summon professional assistance as required
...
1
...
First aid
personnel must be available at all times when people
are at work, taking into account shift working patterns
and providing cover for sickness absences
...
The size and contents of the kit
will depend upon the nature of the risks involved in
the particular working environment and the number
of employees
...
2 gives a list of the contents of
any first aid box to comply with the HSE Regulations
...
1 Suggested numbers of first aid personnel
Category of risk
Numbers employed at any location
Fewer than 50
50–100
More than 100
Medium risk
e
...
light engineering and assembly
work, food processing, warehousing
Higher risk
e
...
most construction, slaughterhouses,
chemical manufacture, extensive work
with dangerous machinery or sharp
instruments
At least one appointed person
At least one first aider
One additional first aider for every 100 employed
Fewer than 20
20–100
More than 100
Lower risk
e
...
shops and offices, libraries
Suggested number of first aid personnel
At least one appointed person
At least one first aider for every 50 employed (or part thereof)
One additional first aider for every 100 employed
Fewer than five
5–50
More than 50
At least one appointed person
At least one first aider
One additional first aider for every 50 employed
Table 1
...
16 BPC)
Triangular bandages
Sterile covering for serious wounds (where applicable)
Safety pins
Medium sized sterile unmedicated dressings
(Standard Dressings No
...
14
and the Ambulance Dressing No
...
9 and No
...
1)
Extra large sterile unmedicated dressings
(Ambulance Dressing No
...
The following minimum quantities should be kept:
Number of employees
Quantity of sterile water
1–10
1 ϫ 300 ml
11–50
3 ϫ 300 ml
There now follows a description of some first aid
procedures which should be practised under expert
guidance before they are required in an emergency
...
Clean the skin around the wound and apply a
plaster, pulling the skin together
...
Apply a sterile dressing or pad and bandage firmly
before obtaining professional advice
...
place it on the injured part
...
Burns
Breathing stopped
Remove heat from the burn to relieve the pain by
placing the injured part under clean cold water
...
Do
not apply lotions or ointments
...
Cover the injured
area with a clean dry dressing
...
Loosen tight
clothing around the neck, chest and waist
...
Tilt the head
backwards and open the mouth
...
However, if the casualty does not begin to breathe,
open your mouth wide and take a deep breath, close
the casualty’s nose by pinching with your fingers, and,
sealing your lips around his mouth, blow into his
lungs until the chest rises
...
Continue this procedure at your natural breathing rate
...
Give artificial respiration
until natural breathing is restored or until professional
help arrives
...
Do not move the casualty unless by remaining in that position he is likely to suffer further injury
...
Contact with chemicals
Wash the affected area very thoroughly with clean cold
water
...
Cover the
affected area with a clean sterile dressing and seek
expert advice
...
When handling dangerous substances it is also good practice to have a
neutralizing agent to hand
...
Exposure to toxic fumes
Get the casualty into fresh air quickly and encourage
deep breathing if conscious
...
Obtain expert medical advice as fumes
may cause irritation of the lungs
...
Soak a towel or cloth in cold water, squeeze it out and
Heart stopped beating
This sometimes happens following a severe electric
shock
...
Act
quickly and lay the casualty on his back
...
Cover this hand with your other hand
and interlace the fingers
...
Continue to do
this 15 times at the rate of one push per second
...
If none is felt, give two
breaths of artificial respiration and then a further 15
chest compressions
...
Pay close attention to
the condition of the casualty while giving heart massage
...
Look carefully at the rate of breathing
...
Treat the casualty for shock, place him in the
recovery position and obtain professional help
...
The severity of the shock depends upon the nature
and extent of the injury
...
He may feel faint, have blurred vision,
feel sick and complain of thirst
...
Loosen tight clothing and keep him warm and dry
until help arrives
...
Accident reports
Every accident must be reported to an employer and
minor accidents reported to a supervisor, safety officer
or first aider and the details of the accident and treatment given suitably documented
...
1
...
Failure to do so
may influence the payment of compensation at a later
date if an injury leads to permanent disability
...
If the accident results in death, serious injury or an
injury that leads to an absence from work of more
than 3 days, then your employer must report the accident to the local office of the HSE (Health and Safety
Executive)
...
They
will require the following information:
■
■
■
■
■
■
■
The name of the person injured
...
A summary of events prior to the accident
...
Details of witnesses
...
Name of the person reporting the incident
...
However, good practice
would recommend an employer or his representative
make an extensive report of any serious accident that
occurs in the workplace
...
1
...
45
Sketch diagrams of how the accident occurred,
where objects were before and after the accident,
where the victim fell etc
...
Collect statements from witnesses
...
Record the circumstances surrounding the accident
...
The area should be
made safe and the senior management informed so
that any actions to prevent a similar occurrence can be
put in place
...
46
ADVANCED ELECTRICAL INSTALLATION WORK
Secure electrical isolation
Electric shock occurs when a person becomes part of
the electrical circuit
...
The lethal level is approximately 50 mA, above which
muscles contract, the heart flutters and breathing
stops
...
Below 50 mA only an unpleasant tingling sensation
may be experienced or you may be thrown across a
room or shocked enough to fall from a roof or ladder,
but the resulting fall may lead to serious injury
...
All
exposed metal is earthed, fuses and miniature circuit
breakers (MCBs) are designed to trip under fault
conditions and residual current devices (RCDs) are
designed to trip below the fatal level as described in
Chapter 2
...
As an electrician
working on electrical equipment you must always
make sure that the equipment is switched off or electrically isolated before commencing work
...
When working on portable
equipment or desktop units it is often simply a matter
of unplugging the equipment from the adjacent supply
...
To deter anyone from re-connecting
the supply while work is being carried out on equipment, a sign ‘Danger – Electrician at Work’ should be
displayed on the isolator and the isolation ‘secured’
with a small padlock or the fuses removed so that no
one can re-connect whilst work is being carried out on
that piece of equipment
...
Where
a test instrument or voltage indicator is used to prove
the supply dead, Regulation 4(3) of the Electricity at
Work Regulations 1989 recommends that the following procedure is adopted
...
1
...
The
test device should indicate mains voltage
...
1
...
2 Next, isolate the supply and observe that the test
device now reads zero volts
...
1
...
4 Finally secure the isolation and place warning
signs; only then should work commence
...
These leads should incorporate barriers to prevent the user touching live terminals when testing and incorporating a protective fuse
and be well insulated and robust, such as those shown
in Fig
...
22
...
1
...
Fig
...
22 Recommended type of test probe and leads
...
1
...
with all the relevant regulations, we must follow a
procedure such as that given by the flow diagram of
Fig
...
23
...
When the heavy outlined boxes are reached,
pause and ask yourself whether everything is satisfactory up to this point
...
If the answer is ‘no’, go back as indicated by the
diagram
...
However, it
does acknowledge that some work, such as fault finding and testing, may require the electrical equipment
to remain energized
...
While live testing may be required by workers in
the electrotechnical industries in order to find the
fault, live repair work must not be carried out
...
Permit-to-work system
The permit-to-work procedure is a type of ‘safe
system to work’ procedure used in specialized and potentially dangerous plant process situations
...
For example:
■
■
■
■
■
■
■
■
Working on part of an assembly line process where
goods move through a complex, continuous process from one machine to another (e
...
the food
industry)
...
Working in confined spaces (e
...
vats and storage
containers)
...
Working underground or in deep trenches
...
Working near live equipment or unguarded
machinery
...
■
■
49
Working in hazardous atmospheres (e
...
the petroleum industry)
...
All the above situations are high risk working situations that should be avoided unless you have received
special training and will probably require the completion of a permit-to-work
...
2 The Site Manager has overall responsibility for the
permit to work even though he may delegate the
responsibility for its issue
...
4 The permit applies to everyone on site, other trades
and sub-contractors
...
6 The permit remains in force until the work is completed and is cancelled by the person who issued it
...
If the planned work
must be changed, the existing permit must be
cancelled and a new one issued
...
The people doing the work, the people to whom
the permit is given, take on the responsibility of following and maintaining the safeguards set out in the
permit, which will define what is to be done (no other
work is permitted) and the time scale in which it is to
be carried out
...
Employers must train staff in the use of such
permits and ideally, training should be designed by the
company issuing the permit, so that sufficient emphasis
can be given to particular hazards present and the precautions which will be required to be taken
...
hse
...
uk
50
ADVANCED ELECTRICAL INSTALLATION WORK
The world of work
TEAM WORKING
Team working is about working with other people,
probably with other employees from the company
you work for
...
All the separate parts of the job have to be finished
and eventually brought together at completion
...
We are often
dependent on other trades completing their work
before we can start ours
...
So, in this case, two different trades are
interdependent, working as a team to complete a
suspended ceiling job
...
One
such model is called ‘Forming, Storming, Norming
and Performing’
...
They
behave as individuals, their responsibility to the team
is unclear and they feel confused about what they
should be doing
...
Storming is the stage where people begin to see a role
for themselves within the team
...
Norming is the stage where team members have
generally reached an agreement upon their individual
roles and responsibilities to the group
...
The team may share the leadership role
...
Everyone knows what they are doing and
how their input fits into everyone else’s work in the
team
...
There is no requirement for instruction or assistance
because everyone knows what they have to do to be
successful
...
They have a shared vision and goal
...
You should always be polite and listen carefully to
their wishes
...
In a domestic situation, the playing of loud music on a radio
may not be approved of
...
When
working in houses, shops and offices use dust sheets to
protect floor coverings and furnishings
...
Dress appropriately: an unkempt or untidy appearance will encourage the customer to think that your
work will be of poor quality
...
It makes good sense to
help other trades where possible and to develop good
working relationships with other employees
...
Finally, remember that the customer will probably
see more of the electrician and the electrical trainee
than of the managing director of your firm and,
therefore, the image presented by you will be assumed
to reflect the policy of the company
...
Always give the impression of being capable and in
command of the situation, because this gives customers confidence in the company’s ability to
meet their needs
...
It is not unreasonable for a young
member of the company’s team to seek help and guidance from those employees with more experience
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Changing work patterns
The elctrotechnical industries cover a large range of
activities and occupations from panel building, instrumentation, maintenance, cable jointing, highway
electrical systems to motor re-winding, alarm and
security systems, building management systems and
computer installations
...
Electricians are also employed
directly by factories, local councils, large commercial
organizations, hospitals and the armed services where
their skills are in demand
...
There
were 21 000 electrical contracting companies registered in the United Kingdom in 2004
...
Then there
are the small self-employed electrical businesses and
those who work for the local authority, hospitals or
armed forces who do not get counted as electrical
personnel but as blue collar workers or soldiers
...
New editions of the Regulations create work opportunities in domestic and public buildings bringing them
up to the latest safety requirements
...
New technologies present new opportunities to
build on these core skills
...
The
acquisition of new skills gives the opportunity to
transfer these new skills to new employers
...
Flexible workers
are an attractive proposition to a prospective employer
...
For those employed in the maintenance of fluid
systems, this may present opportunities of further
responsibility or an increase in salary or status within
a company
...
If an electric motor was found to be faulty, then to replace it
would require mechanical engineering and fitting skills
as well as electrical skills and the one man who can do
that job has multiple skills and can demand more pay
...
The overuse and misuse of equipment
means that it breaks down more frequently
...
They are at work so why isn’t the electrician!
I live close to a seaside resort
...
Everything then closes down and they then get
on with their planned maintenance work
...
This leads to a demand for flexible working hours
or a flexible working week
...
If the rota is shared out, then
each individual only need cover, say one in four weekends, and as a result receives extra pay
...
New technologies will require that we continue to learn new
skills and new ideas will create new business opportunities for electrical companies
...
Employees will also need to
be flexible, not only in relation to what they can do
but when they can do it
...
52
ADVANCED ELECTRICAL INSTALLATION WORK
ELECTRICAL PRINCIPLES
All matter is made up of atoms which arrange themselves in a regular framework within the material
...
The electrical properties of a material depend largely
upon how tightly these electrons are bound to the
central nucleus
...
1
...
Materials
which are good conductors include copper, brass,
aluminium and silver
...
Good
insulating materials are PVC, rubber, glass and wood
...
1
...
The free electrons close to the positive plate of the battery are attracted to it since unlike
charges attract, and the free electrons near the negative plate will be repelled from it
...
This drift of electrons within a conductor is known
as an electric current, measured in amperes and given
the symbol I
...
If the circuit is broken by opening a switch,
for example, the electron flow and therefore the
current will stop immediately
...
This driving force is the electromotive force
(abbreviated to emf )
...
An emf is always associated with energy conversion,
such as chemical to electrical in batteries and mechanical to electrical in generators
...
The potential difference (abbreviated to p
...
) is the change in energy levels
measured across the load terminals
...
d
...
Every circuit offers some opposition to current flow, which we call the circuit resistance,
measured in ohms (symbol ⍀), to commemorate the
famous German physicist George Simon Ohm, who
was responsible for the analysis of electrical circuits
...
As a result
of this experiment, he arrived at a law, now known as
Ohm’s law, which says that the current passing through
a conductor under constant temperature conditions
is proportional to the potential difference across the
conductor
...
1
...
I ϭ
V
(A)
R
and
R ϭ
V
(⍀ )
I
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
EXAMPLE 1
An electric heater, when connected to a 230 V supply, was found to take
a current of 4 A
...
Rϭ
І Rϭ
V
I
230 V
ϭ 57
...
Table 1
...
4 ϫ
10Ϫ9 ⍀m, which means that a sample of silver 1 m
long and 1 m in cross-section will have a resistance of
16
...
Table 1
...
Calculate the leakage current
...
4 ϫ 10Ϫ9
17
...
5 ϫ 10Ϫ9
75
...
0 ϫ 10Ϫ9
V
R
400 V
ϭ 200 ϫ 10Ϫ6 A ϭ 200 A
2 ϫ 106 ⍀
EXAMPLE 3
When a 4 ⍀ resistor was connected across the terminals of an unknown
d
...
supply, a current of 3 A flowed
...
V ϭ I ϫR
EXAMPLE 1
Calculate the resistance of 100 m of copper cable of 1
...
5 ϫ 10Ϫ9 ⍀m
...
5 ϫ 10Ϫ9 ⍀ ϫ 100 m
ϭ 1
...
5 ϫ 10Ϫ6 m2
І V ϭ 3 A ϫ 4 ⍀ ϭ 12 V
EXAMPLE 2
Resistivity
The resistance or opposition to current flow varies for
different materials, each having a particular constant
value
...
The resistivity (symbol – the Greek letter ‘rho’) of
a material is defined as the resistance of a sample of
unit length and unit cross-section
...
3
...
R ϭ
rl
(⍀ )
a
where
ϭ the resistivity constant for the material (⍀ m)
Calculate the resistance of 100 m of aluminium cable of 1
...
5 ϫ
10Ϫ9 ⍀m
...
5 ϫ 10Ϫ9 ⍀m ϫ 100 m
ϭ 1
...
5 ϫ 10Ϫ6 m2
The above examples show that the resistance of an
aluminium cable is some 60% greater than a copper
conductor of the same length and cross-section
...
The other factor which affects the resistance of
a material is the temperature, and we will consider
this next
...
In general, conductors increase their resistance as the temperature increases and insulators
decrease their resistance with a temperature increase
...
Each material responds to temperature change in a
different way, and scientists have calculated constants
for each material which are called the temperature
coefficient of resistance (symbol ␣ – the Greek letter
‘alpha’)
...
4 gives some typical values
...
4 Temperature coefficient values
Material
Temperature coefficient (⍀/⍀°C)
Silver
Copper
Aluminium
Brass
Iron
0
...
004
0
...
001
0
...
For a temperature increase from 0°C
Rt ϭ R0(1 ϩ ␣t) (⍀)
where
Rt ϭ the resistance at the new temperature t°C
R0 ϭ the resistance at 0°C
␣ ϭ the temperature coefficient for the particular
material
...
If we take a 1 ⍀ resistor of, say, copper, and raise its
temperature by 1°C, the resistance will increase
by 0
...
004 ⍀
...
004 ⍀ is the
temperature coefficient of the material
...
c
...
Determine the resistance of the coil at 20°C if the temperature coefficient is 0
...
Rt
І Rt
Rt
Rt
ϭ R 0 (1ϩ at ) (⍀)
ϭ 100 ⍀ (1ϩ 0
...
08)
ϭ 108 ⍀
EXAMPLE 2
The field winding of a shunt generator has a resistance of 150 ⍀ at an
ambient temperature of 20°C
...
Calculate the resistance of
the winding at the higher temperature if the temperature coefficient of
resistance is 0
...
R1 (1ϩ at 1 )
ϭ
R 2 (1ϩ at 2 )
150 ⍀ 1ϩ 0
...
004 ⍀ /⍀ЊC ϫ 45ЊC
R2
150 ⍀ 1
...
18
R2
І R2 ϭ
150 ⍀ ϫ 1
...
08
It is clear from the last two sections that the resistance of a cable is affected by length, thickness, temperature and type of material
...
The tables of current ratings in
Appendix 4 of the IEE Regulations and Appendix 6
of the On Site Guide contain correction factors so that
current ratings may be accurately determined under
defined installation conditions
...
Resistors
In an electrical circuit resistors may be connected in
series, in parallel, or in various combinations of series
and parallel connections
...
Therefore, we say that
in a series circuit the current is common throughout
that circuit
...
1
...
The sum of the individual voltage drops, V1, V2 and V3 for example in Fig
...
25, will
be equal to the total voltage VT
...
Craft students need only
state the expression RT ϭ R1 ϩ R2 ϩ R3 for series
connections
...
1
...
The total
current will divide when it reaches a resistor junction,
part of it flowing in each resistor
...
1
...
We can summarize these statements as follows
...
V
From Ohm’s law we know, that I ϭ , and
R
therefore
Fig
...
25 A series circuit
...
For
any series circuit, I is common throughout the circuit
and
(1)
VT ϭ V1 ϩ V2 ϩ V3
the total current I T ϭ
total voltage VT ϭ I ϫ RT
the current through R1 is I 1 ϭ
V
R1
the current through R2 is I 2 ϭ
V
R2
the current through R3 is I 3 ϭ
Let us call the total circuit resistance RT
...
This
will cancel out I to leave us with an expression for the
circuit resistance:
RT ϭ R1 ϩ R2 ϩ R3
V
RT
Fig
...
26 A parallel circuit
...
67 A ϫ 6 ⍀ ϭ 4 V
The voltage drop across R2 is
V2 ϭ I ϫ R 2
І V2 ϭ 0
...
67 A ϫ 6 ⍀ ϭ 4 V
Note that the derivation of this formula is given for
information only
...
EXAMPLE
Three 6 ⍀ resistors are connected (a) in series (see Fig
...
27), and (b)
in parallel (see Fig
...
28), across a 12 V battery
...
Fig
...
28 Resistors in parallel
...
1
...
For any series connection
R T ϭ R1 ϩ R 2 ϩ R 3
І R1 ϭ 6 ⍀ ϩ 6 ⍀ ϩ 6 ⍀ ϭ 18 ⍀
Total current I T ϭ
І IT ϭ
V
RT
12 V
ϭ 0
...
We will label this group RP
...
29 may now be represented by the more simple equivalent
shown in Fig
...
30
RP ϭ
V
R3
12 V
ϭ2A
6⍀
SERIES AND PARALLEL COMBINATIONS
The most complex arrangement of series and parallel
resistors can be simplified into a single equivalent
resistor by combining the separate rules for series and
parallel resistors
...
1
...
Since all resistors are now in series,
EXAMPLE 1
Resolve the circuit shown in Fig
...
29 into a single resistor and calculate
the potential difference across each resistor
...
1
...
The total current flowing in the circuit may be found by using Ohm’s law
IT ϭ
VT
10 V
ϩ
ϭ1 A
R T 10 ⍀
Fig
...
29 A series/parallel circuit
...
Fig
...
31 Single equivalent resistor for Fig
...
29
...
33 may now be represented by a more simple equivalent
circuit, as in Fig
...
34
...
d
...
EXAMPLE 2
Determine the total resistance and the current flowing through each resistor for the circuit shown in Fig
...
32
...
1
...
Since the resistors are now in parallel, the equivalent resistance may
be found from
1
1
1
ϭ ϩ
R T R S R3
І
1 1ϩ 1
2
ϭ
ϭ
6⍀ 6⍀
RT
Fig
...
32 A series/parallel circuit for Example 2
...
The circuit may be
more easily understood if we redraw it as in Fig
...
33
...
І I3 ϭ
V
12 V
ϭ
ϭ2A
R3 6 ⍀
Let us call the current flowing through both resistors R1 and R2, as shown
in Fig
...
33, lS
...
1
...
For the series branch, the equivalent resistor can be found from
R S ϭ R1 ϩ R 2
І RS ϭ 3 ⍀ ϩ 3 ⍀ ϭ 6 ⍀
Power and energy
POWER
Power is the rate of doing work and is measured in watts
...
Substituting Equation (6) into Equation (5), we have
EXAMPLE 2
Power ϭ (I ϫ R) ϫ Current ϭ I ϫ R (W)
Two 50 ⍀ resistors may be connected to a 200 V supply
...
For (a), the equivalent resistance when resistors are connected in
series is given by
2
and substituting Equation (7) into Equation (5), we have
Power ϭ Voltage ϫ
V
V2
ϭ
(W)
R
R
We can find the power of a circuit by using any of the
three formulae
P ϭ V ϫ I,
P ϭ I 2 ϫ R,
P ϭ
V2
R
ENERGY
R T ϭ R1 ϩ R 2
І R T ϭ 50 ⍀ ϩ 50 ⍀ ϭ 100 ⍀
Power ϭ
І Power ϭ
Energy is a concept which engineers and scientists use
to describe the ability to do work in a circuit or system
Energy ϭ Power ϫ Time
but, since Power ϭ Voltage ϫ Current
then Energy ϭ Voltage ϫ Current ϫ Time
The SI unit of energy is the joule, where time is
measured in seconds
...
Electricity Board meters measure ‘units’ of
electrical energy, where each ‘unit’ is 1 kWh
...
Calculate the energy used during this time
...
Power ϭ
І Power ϭ
V2
(W)
R
200 V ϫ 200 V
ϭ 800 W
50 ⍀
For (c), the equivalent resistance when resistors are connected in parallel is given by
1
1
1
ϭ ϩ
R T R1 R 2
І
1
1
1
ϭ
ϩ
R T 50 ⍀ 50 ⍀
1
1ϩ 1
2
ϭ
ϭ
R T 50 ⍀ 50 ⍀
RT ϭ
50 ⍀
ϭ 25 ⍀
2
60
ADVANCED ELECTRICAL INSTALLATION WORK
Power ϭ
І Power ϭ
V2
(W)
RT
200 V ϫ 200 V
ϭ 1600 W
25 ⍀
This example shows that by connecting resistors together in different combinations of series and parallel connections, we can obtain various power
outputs: in this example, 400, 800 and 1600 W
...
T ϭ
ІT ϭ
1
f
1
ϭ 0
...
c
...
c
...
1
...
Most electrical equipment makes use of alternating
current supplies, and for this reason a knowledge of
alternating waveforms and their effect upon resistive,
capacitive and inductive loads is necessary for all practising electricians
...
The induced voltage follows a mathematical law known as the sinusoidal
law and, therefore, we can say that a sine wave has been
generated
...
1
...
In the UK we generate electricity at a frequency of
50 Hz and the time taken to complete each cycle is
given by
Fig
...
36 Characteristics of a sine wave
...
1
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
in the opposite direction, and so the instantaneous
values of the generated voltage are always changing
...
c
...
The maximum or peak value is the greatest instantaneous value reached by the generated waveform
...
The average value is the average over one half-cycle
of the instantaneous values as they change from zero
to a maximum and can be found from the following
formula applied to the sinusoidal waveform shown in
Fig
...
37:
Vav ϭ
EXAMPLE
The sinusoidal waveform applied to a particular circuit has a maximum
value of 325
...
Calculate the average and rms value of the waveform
...
637 ϫVmax
І Vav ϭ 0
...
3 ϭ 207
...
7071ϫVmax
Vrms ϭ 0
...
3 ϭ 230 V
When we say that the main supply to a domestic property is 230 V we
really mean 230 V rms
...
2 V and a maximum value of almost 325
...
c
...
V1 ϩ V2 ϩ V3 ϩ V4 ϩ V5 ϩ V6
ϭ 0
...
637 of the maximum value
...
c
...
c
...
The value can be found from the following
formula applied to the sinusoidal waveform shown in
Fig
...
37
...
7071 Vmax
6
For any sinusoidal waveform the rms value is equal to
0
...
Magnetism
The Greeks knew as early as 600 BC that a certain form
of iron ore, now known as magnetite or lodestone, had
the property of attracting small pieces of iron
...
Small pieces of lodestone attached to wooden splints
floating in a bowl of water always came to rest pointing in a north–south direction
...
Iron, nickel and cobalt are the only elements which
are attracted strongly by a magnet
...
Copper, brass, wood,
PVC and glass are not attracted by a magnet and are,
therefore, described as non-magnetic
...
1
...
62
ADVANCED ELECTRICAL INSTALLATION WORK
SOME BASIC RULES OF MAGNETISM
1 Lines of magnetic flux have no physical existence,
but they were introduced by Michael Faraday
(1791–1867) as a way of explaining the magnetic
energy existing in space or in a material
...
The symbol used for magnetic flux is the Greek
letter ⌽ (phi) and the unit of magnetic flux is the
weber (symbol Wb), pronounced ‘veber’, to commemorate the work of the German physicist
Wilhelm Weber (1804–1891)
...
3 Lines of magnetic flux behave like stretched elastic
bands, always trying to shorten themselves
...
5 Lines of magnetic flux travel along a magnetic
material and always emerge out of the ‘north pole’
end of the magnet
...
7 The region of space through which the influence
of a magnet can be detected is called the magnetic
field of that magnet
...
Strong magnetic fields have a high flux density
...
9 The places on a magnetic material where the lines of
flux are concentrated are called the magnetic poles
...
These two
statements are sometimes called the ‘first laws of
magnetism’ and are shown in Fig
...
39
...
Calculate the
flux density in the core
...
2 T
Fig
...
38 Magnetic fields around a permanent magnet
...
Gently tapping the
paper then causes the filings to take up the shape of the
magnetic field surrounding the permanent magnet
...
38 and 1
...
Electromagnetism
Electricity and magnetism have been inseparably connected since the experiments by Oersted and Faraday in
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
63
Fig
...
39 The first laws of magnetism
...
An electric current flowing
in a conductor produces a magnetic field ‘around’ the
conductor which is proportional to the current
...
The
magnetic field ‘spirals’ around the conductor, as shown
in Fig
...
40 and its direction can be determined by the
‘dot’ or ‘cross’ notation and the ‘screw rule’
...
The dot represents current
coming towards us when we would see the point of the
arrow or dart inside the conductor
...
Imagine a corkscrew or
screw being turned so that it will move in the direction
of the current
...
1
...
Fig
...
40 Magnetic fields around a current carrying conductor
...
1
...
A current flowing in a coil of wire or solenoid establishes a magnetic field which is very similar to that of
64
ADVANCED ELECTRICAL INSTALLATION WORK
a bar magnet
...
The advantage of the electromagnet when compared
with the permanent magnet is that the magnetism of
the electromagnet can be switched on and off by a
functional switch controlling the coil current
...
Figure 1
...
A current carrying conductor maintains a magnetic
field around the conductor which is proportional to
the current flowing
...
This is further discussed later in this chapter and
demonstrated by the diagrams shown in Fig
...
70
...
He
stated that ‘When a conductor cuts or is cut by a magnetic field an emf is induced in that conductor
...
This basic principle laid down the laws of present-day
electricity generation where a strong magnetic field is
rotated inside a coil of wire to generate electricity
...
EXAMPLE
A 15 cm length of conductor is moved at 20 m/s through a magnetic
field of flux density 2 T
...
emf ϭ Blv (V)
І emf ϭ 2 T ϫ 0
...
1
...
When the switch is opened the current stops flowing
and, therefore, the magnetic flux collapses
...
1
...
Fig
...
42 An inductive coil or choke
...
The effect is
known as self-inductance, or just inductance, and is one
property of any coil
...
Fluorescent light fittings contain a choke or inductive
coil in series with the tube and starter lamp
...
When two separate coils are placed close together, as
they are in a transformer, a current in one coil produces
a magnetic flux which links with the second coil
...
The two coils in this case are said
to possess mutual inductance, as shown by Fig
...
43
...
The emf induced in the right hand coil of Fig
...
43
is dependent upon the rate of change of magnetic flux
and the number of turns on the coil
...
The minus sign indicates
that the emf is a back emf opposing the rate of change
of current as described later by Lenz’s law
...
6 to 0
...
Calculate the average value of the
induced emf
...
6 Ϫ 0
...
1
...
The
arc is produced by the stored magnetic energy being
discharged across the switch contacts
...
EXAMPLE
The field windings of a motor have an inductance of 3 H and carry a current of 2 A
...
W ϭ 1⁄2 LI 2 (J)
W ϭ 1⁄2 ϫ 3 H ϫ (2 A)2
W ϭ 6J
MAGNETIC HYSTERESIS
Fig
...
43 Mutual induction between two coils
...
66
ADVANCED ELECTRICAL INSTALLATION WORK
Some materials magnetize easily, and some are difficult
to magnetize
...
The properties of a magnetic sample may be examined in detail if we measure the flux
density (B) of the material with increasing and then
decreasing values of magnetic field strength (H)
...
1
...
The hysteresis effect causes the magnetic material
to retain some of its magnetism after the magnetic field
strength has been removed
...
This residual flux density can be reduced to zero by
applying a negative magnetic field strength (ϪH)
...
When the material has been worked to magnetic
saturation – that is, Bmax and Hmax the residual flux density is called remanence and the coercive force is called
the coercivity
...
Materials from which permanent magnets are
made should have a high value of residual flux density
and coercive force and, therefore, display a wide hysteresis loop, as shown by loop (b) in Fig
...
44
...
Suitable materials will, therefore,
have a low value of residual flux density and coercive
force and, therefore, display a narrow hysteresis loop,
as shown by loop (a) in Fig
...
44
...
This energy loss is converted to heat in the iron
...
When an iron core is subjected to alternating magnetization, as in a transformer, the energy loss occurs at
every cycle and so constitutes a continuous power loss,
and, therefore, for applications such as transformers, a
material with a narrow hysteresis loop is required
...
1
...
We have seen that electricity and magnetism are connected together by Faraday’s laws of electromagnetic
induction, which say that forces are exerted on current carrying conductors placed within a magnetic
field and that conductors moving in a magnetic field
have an emf induced in them
...
When carrying out his experiments many years later, it occurred
to Fleming that the thumb and first two fingers of the
right hand could be used to predict the direction of
the induced emf and so he formulated the following
rule
...
If the first finger is pointed in the direction
of the magnetic field (north to south) and the thumb
in the direction of the motion, then the second finger
will point in the direction of the induced emf and
current flow
...
1
...
67
Electrostatics
If a battery is connected between two insulated plates,
the emf of the battery forces electrons from one plate
to another until the p
...
between the plates is equal to
the battery emf
...
The plates are then said to be charged
...
1
...
LENZ’S LAW
After the publication of Faraday’s work on the production of electricity from magnetism in 1831, scientists in other countries repeated the experiments and
built upon the basic principles adding to the total
knowledge
...
This states
that the direction of the induced emf always sets up a
current opposing the motion which induced the emf
...
1
...
Fig
...
46 The charge on a capacitor’s plates
...
46 shows the charges on a capacitor’s plates
...
The
property of a pair of plates to store an electric charge
is called its capacitance
...
d
...
It has the ability
68
ADVANCED ELECTRICAL INSTALLATION WORK
of storing a quantity of electricity as an excess of electrons on one plate and a deficiency on the other
...
Figure 1
...
EXAMPLE
A 100 F capacitor is charged by a steady current of 2 mA flowing for
5 seconds
...
d
...
Q ϭ lt (C)
І Q ϭ 2 ϫ 10Ϫ3 A ϫ 5 s ϭ 10 mC
Q ϭ CV
Q
І V ϭ (V)
C
Vϭ
10 ϫ 10Ϫ3 C
ϭ 100 V
100 ϫ 10Ϫ6 F
The p
...
which may be maintained across the
plates of a capacitor is determined by the type and
thickness of the dielectric medium
...
Fig
...
47 Construction and appearance of capacitors
...
They are often used
for radio tuning circuits
...
They are
expensive, but this dielectric is very stable and has low
dielectric loss
...
Paper-dielectric capacitors
Paper-dielectric capacitors usually consist of thin aluminium foils separated by a layer of waxed paper
...
These capacitors are
used in fluorescent lighting fittings and motor circuits
...
Electrolytic capacitors
The construction of these is similar to that of the
paper-dielectric capacitors, but the dielectric material
in this case is an oxide skin formed electrolytically by
the manufacturers
...
For this reason electrolytic capacitors must be connected to the correct voltage polarity
...
EXAMPLE
Capacitors of 10 and 20 F are connected first in series, and then in parallel, as shown in Figs 1
...
50
...
For connection in series,
Fig
...
49 Series capacitors
...
1
...
The equivalent capacitance, CT, of a
number of capacitors is found by the application of
similar formulae to those used for resistors and discussed earlier in this chapter
...
Fig
...
50 Parallel capacitors
...
66 F
3
For connection in parallel,
CT ϭ C1 ϩ C2
CT ϭ 10 F ϩ 20 F ϭ 30 F
Fig
...
48 Connection of and formulae for series and parallel
capacitors
...
66 F
...
The practical considerations of capacitors and the
use of colour codes to determine capacitor values are
dealt within Chapter 4
...
The energy
stored (symbol W ) in a capacitor is expressed in joules
and given by the formula
Energy ϭ W ϭ 1⁄2CV 2 (J)
where C is the capacitance of the capacitor and V is
the applied voltage
...
Calculate the energy stored in the capacitor when it is connected to the 230 V mains supply
...
17 J
CR CIRCUITS
As we have discussed earlier in this chapter, connecting a voltage to the plates of a capacitor causes it to
charge up to the potential of the supply
...
Such circuits are called capacitor–resistor (CR)
circuits, and have many applications in electronics as
timers and triggers and for controlling the time base
sweeps of a cathode ray oscilloscope
...
51 shows the circuit diagram for a simple
CR circuit and the graphs drawn from the meter readings
...
The mathematical name for the shape of these curves
is an exponential curve and, therefore, we say that the
capacitor voltage is growing exponentially while the
current is decaying exponentially during the charging
period
...
EXAMPLE 2
The energy stored in a certain capacitor when connected across a 400 V
supply is 0
...
Calculate (a) the capacitance and (b) the charge on the
capacitor
...
3 J
(400 V)2
C ϭ 3
...
75 ϫ 10Ϫ6 F ϫ 400 V
Q ϭ 1500 C
Fig
...
51 A CR circuit
...
The time taken to charge
a capacitor by a constant current is given by the time
constant of the circuit which is expressed mathematically as T ϭ CR, where T is the time in seconds
...
Determine the time constant of this circuit
...
2 s
We have already seen that in practice the capacitor is
not charged by a constant current but, in fact, charges
exponentially
...
Therefore, in 1
...
Graphical derivation of CR circuit
The exponential charging and discharging curves of
the CR circuit described in Example 1 may also be
Fig
...
52 Graphical derivation of CR growth curve
...
1
...
1 We have calculated the time constant for the circuit (T ) and found it to be 1
...
2 We know that the maximum voltage of the fully
charged capacitor will be 12 V because the supply
voltage is 12 V
...
4 Next draw a horizontal dotted line along the point
of maximum voltage, 12 V in this example
...
1
...
This corresponds
to 1
...
2 seconds
...
7 Next, draw a full line OB; this is the start of the
charging curve
...
9 Draw a horizontal line CD equal to the length of
the time constant (T )
...
11 Draw in the line CE, the second line of our charging curve
...
1
...
13 Finally, join together with a smooth curving line
the points OCGJ etc
...
Switching off the supply and discharging the capacitor through the 20 k⍀ resistor will produce the exponential decay of the voltage across the capacitor which
will be a mirror image of the growth curve
...
1
...
SELECTING A CAPACITOR
There are two broad categories of capacitor, the nonpolarized and the polarized type
...
A paper
dielectric capacitor is non-polarized and can be connected either way round
...
Electrolytic capacitors are polarized and are used where
a large value of capacitance is required in a relatively
small package
...
Fig
...
53 Graphical derivation of CR decay curve
...
The unit of capacitance is the farad (symbol F),
to commemorate the name of the English scientist
Michael Faraday
...
f
...
In an electronic
filter circuit a typical capacitor value might be 100 pF
at 63 V
...
It may be useful to remember that
1000 pF ϭ 1 nF, and 1000 nF ϭ 1 F
The working voltage of a capacitor is the maximum
voltage that can be applied between the plates of the
capacitor without breaking down the dielectric insulating material
...
c
...
c
...
c
...
c
...
When connecting a
capacitor to the 230 V mains supply we must choose
a working voltage of about 400 V because 230 V rms
is approximately 325 V maximum
...
An ideal capacitor which is isolated will remain
charged for ever, but in practice no dielectric insulating material is perfect, and the charge will slowly leak
between the plates, gradually discharging the capacitor
...
However, the capacitors used in electrical installation work for powerfactor correction are often fitted with a high-value
discharge resistor to encourage the charge to leak away
safely when not in use
...
36 and 1
...
c
...
In this chapter we will first of
all consider the theoretical circuits of pure resistance,
73
inductance and capacitance acting alone in an a
...
circuit before going on to consider the practical circuits of
resistance, inductance and capacitance acting together
...
RESISTANCE
In any circuit, resistance is defined as opposition to
current flow
...
c
...
The inductance and
capacitance of an a
...
circuit also cause an opposition
to current flow, which we call reactance
...
c
...
It causes the current in
the circuit to lag behind the applied voltage, as shown
in Fig
...
54
...
142 a constant
f ϭ the frequency of the supply
L ϭ the inductance of the circuit
Fig
...
54 Voltage and current relationships in resistive, capacitive and inductive circuits
...
c
...
It causes the current in
the circuit to lead ahead of the voltage, as shown in
Fig
...
54
...
1
(⍀ )
2fC
where and f are defined as before and C is the capacitance of the circuit
...
05 H inductor if
they were separately connected to the 50 Hz mains supply
...
І XC ϭ
1
ϭ 21
...
142 ϫ 50 Hz ϫ 150 ϫ 10Ϫ6
For inductive reactance
XL ϭ 2 fL
where f ϭ 50 Hz and L ϭ 0
...
І XL ϭ 2 ϫ 3
...
05 H ϭ 15
...
c
...
Thus
impedance is the combined opposition to current
flow of the resistance, inductive reactance and capacitive reactance of the circuit and can be calculated
from the formula
Z ϭ
R 2 ϩ X 2 (⍀ )
Calculate the impedance when a 48 ⍀ resistor is connected in series
with a 55 ⍀ capacitive reactance
...
c
...
c
...
We say that the
waveforms are in phase
...
c
...
We say that the current lags the voltage
by 90°
...
c
...
These various effects can be observed on an oscilloscope, but the circuit diagram, waveform diagram and
phasor diagram for each circuit are shown in Fig
...
54
...
c
...
Phasor diagrams allow us to produce a
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
model or picture of the circuit under consideration
which helps us to understand the circuit
...
To find the combined effect of two quantities we
combine their phasors by adding the beginning of the
second phasor to the end of the first
...
EXAMPLE
Find by phasor addition the combined effect of currents A and B acting in
a circuit
...
We usually assume phasors to rotate anticlockwise and so
the complete diagram will be as shown in Fig
...
55
...
e
...
75
Phase angle F
In an a
...
circuit containing resistance only, such as a
heating circuit, the voltage and current are in phase,
which means that they reach their peak and zero values together, as shown in Fig
...
56(a)
...
c
...
1
...
The phase
difference, measured in degrees between the current
and voltage, is called the phase angle of the circuit,
and is denoted by the symbol , the lower-case Greek
letter phi
...
In
Fig
...
57 the phase angle between applied voltage and
current is some angle
...
1
...
The magnitude of the resultant phasor can be measured from the
phasor diagram and is found to be 5 A acting at a phase angle of
about 37° leading A
...
Fig
...
57 A series RL circuit and phasor diagram
...
1
...
c
...
f
...
f
...
707; (c) V and I displaced by 90°, ϭ 90° and p
...
ϭ 0
...
1
...
The sum of
these voltages will be equal to the total voltage VT but
because this is an a
...
circuit the voltages must be
added by phasor addition
...
1
...
The phasor addition of these two
voltages gives us the magnitude and direction of VT,
which leads the current by some angle
...
1
...
The voltage VR will be in
phase with the current and VC will lag the current by
90°
...
1
...
If we now divide throughout by I 2 we have
Z2 ϭ R2 ϩ X 2
or Z ϭ
R2 ϩ X 2 ⍀
The phasor diagram can be simplified to the impedance triangle given in Fig
...
59(c)
...
15 H is connected in series with a 50 ⍀ resistor across a
100 V 50 Hz supply
...
For (a)
XL ϭ 2fL (⍀)
І XL ϭ 2 ϫ 3
...
15 H ϭ 47
...
1 ⍀)2 ϭ 68
...
46 A
68
...
Calculate (a) the reactance of the capacitor, (b) the
impedance of the circuit, and (c) the current
...
1
...
XC ϭ
THE IMPEDANCE TRIANGLE
І XC ϭ
We have now established the general shape of the
phasor diagram for a series a
...
circuit
...
57
and 1
...
1
...
From Fig
...
59(b), by the theorem of
Pythagoras, we have
(IZ)2 ϭ (IR)2 ϩ (IX)2
I Z ϭI R ϩI X
2
2
2 2
2
2
1
( ⍀)
2fC
1
ϭ 53
...
05 ⍀)2 ϭ 113
...
03 A
113
...
f
...
f
...
1
...
f
...
1
...
f
...
From the trigonometry of the impedance triangle shown in Fig
...
59, p
...
is also equal to
p
...
ϭ cos f ϭ
V
R
ϭ R
Z
VT
The electrical power in a circuit is the product of
the instantaneous values of the voltage and current
...
60 shows the voltage and current waveform
for a pure inductor and pure capacitor
...
It can be seen that the power
waveform reverses every quarter cycle, indicating that
energy is alternately being fed into and taken out of
Fig
...
59 Phasor diagram and impedance triangle
...
When considered over one
complete cycle, the positive and negative portions are
equal, showing that the average power consumed by a
pure inductor or capacitor is zero
...
Inductors store energy as a magnetic
field and capacitors as an electric field
...
c
...
c
...
In an a
...
circuit the power consumed is given by the product of
the current and that part of the voltage which is in
phase with the current
...
6 ϭ 750 W
EXAMPLE 2
A capacitor of reactance 12 ⍀ is connected in series with a 9 ⍀ resistor
across a 150 V supply
...
f
...
For (a)
Z ϭ R 2 ϩ X 2 ( ⍀)
І Z ϭ (9 ⍀)2 ϩ (12 ⍀)2 ϭ 15 ⍀
For (b)
I ϭ V /Z (A)
І Iϭ
150 V
ϭ 10 A
15 ⍀
For (c)
p
...
ϭ cos f ϭ
Fig
...
60 Waveform for the a
...
power in purely inductive and
purely capacitive circuits
...
Calculate (a) the impedance, (b) the current,
(c) the p
...
, and (d) the power
...
f
...
6 leading
15 ⍀
For (d)
P ϭ VI cos (W)
І P ϭ 150 V ϫ 10 A ϫ 0
...
This causes an additional magnetizing current to be drawn from the supply, which does not produce power, but does need to
be supplied, making supply cables larger
...
84 kW loads with power factors of 1, 0
...
4
...
The current is given by
Iϭ
For (c)
p
...
ϭ cos f ϭ
І p
...
ϭ
R
Z
30 ⍀
ϭ 0
...
84 kW ϭ 1840 W and V ϭ 230 V
...
f
...
f
...
8,
79
Iϭ
1840 W
ϭ 10 A
230 V ϫ 0
...
Iϭ
1840 W
ϭ 20 A
230 V ϫ 0
...
f
...
4,
It can be seen from these calculations that a 1
...
4 would require a 20 A cable, while the same load
at unity power factor could be supplied with an 8 A cable
...
As a result,
the supply companies encourage installation engineers to improve their
power factor to a value close to 1 and sometimes charge penalties if the
power factor falls below 0
...
Power-factor improvement
The circuit diagram and phasor diagram are shown in
Fig
...
62
...
Since VL leads I by 90° and VC lags by
90° the phasors are in opposition and the combined
result is given by VL Ϫ VC as shown
...
7
...
A capacitor has the
opposite effect of an inductor, and so it seems reasonable to add a capacitor to a load which is known to
have a lower power factor
...
61(a) shows an industrial load with a low
power factor
...
When this current is added to
the load current the resultant current has a much
improved power factor, as can be seen in Fig
...
61(b)
...
1
...
Since VT ϭ IZ, VR ϭ IR, VL ϭ IXL and VC ϭ IXC,
this equation may also be expressed thus:
(IZ )2 ϭ (IR)2 ϩ (IXL Ϫ IXC)2
Cancelling out the common factors, we have
Z 2 ϭ R 2 ϩ (X L Ϫ X C )2
Fig
...
61 Power-factor improvement using capacitors
...
І XC ϭ
X C ϭ 15
...
Calculate (a) the
impedance of the circuit, (b) the current and (c) the p
...
For (a)
X L ϭ 2fL (⍀)
І X L ϭ 2 ϫ 3
...
28 ⍀
1
(⍀ )
XC ϭ
2fC
1
І XC ϭ
2 ϫ 3
...
22 ⍀
Z ϭ R 2 ϩ (X L Ϫ X C )2 (⍀)
І Z ϭ (5 ⍀)2 ϩ (6
...
22 ⍀)2
Z ϭ 15
...
69 A
І Iϭ
15
...
f
...
f
...
317 leading
15
...
Calculate (a) the impedance, (b) the current and (c) the voltage dropped across each component
...
142 ϫ 50 Hz ϫ 100 ϫ 10Ϫ3 H
X L ϭ 31
...
142 ϫ 50 Hz ϫ 200 ϫ 10Ϫ6 F
І Z ϭ (10 ⍀)2 ϩ (31
...
9 ⍀)2
Z ϭ 18
...
46 A
18
...
46 A ϫ 10 ⍀ ϭ 124
...
46 A ϫ 31
...
49 V
VC ϭ I ϫ X C (V)
І VC ϭ 12
...
9 ⍀ ϭ 198
...
7
...
SERIES RESONANCE
At resonance the circuit responds sympathetically
...
The current and reactive components of the circuit are at a maximum and
so resonance is usually avoided in power applications
to prevent cables being overloaded and cable insulation being broken down
...
At low frequencies the circuit is
mainly capacitative and at high frequencies the inductive effect predominates
...
This is the point
of resonance and occurs when
VL ϭ VC
І IXL ϭ IXC
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
If we cancel the common factor we have
voltage and current must be in phase
...
1
...
XL ϭ XC
1
І 2fL ϭ
2fC
EXAMPLE 1
A capacitor is connected in series with a coil of resistance 50 ⍀ and
inductance 168
...
Calculate the value of the
capacitor to produce resonance in this circuit
...
The derivation of the formula is not required by
craft students
...
142 ϫ 50 Hz ϫ 168
...
03 ⍀
At resonance XL ϭ XC, therefore XC ϭ 53
...
142 ϫ 50 Hz ϫ 53
...
33 mH
inductor connected in series with a 100 F capacitor
...
33 ϫ 10Ϫ3 H ϫ 100 ϫ 10Ϫ6 F
1
f0 ϭ 100 Hz
PARALLEL CIRCUITS
Fig
...
63 Series resonance conditions in an RLC circuit
...
The branches of the parallel network may
consist of one component or two or more components connected in series
...
In a parallel circuit
the supply voltage is applied to each of the network
82
ADVANCED ELECTRICAL INSTALLATION WORK
branches
...
In a parallel circuit containing a pure resistor and
inductor as shown in Fig
...
64, the current flowing
through the resistive branch will be in phase with the
voltage and the current flowing in the inductive
branch will be 90° lagging the voltage
...
1
...
In a parallel circuit containing a pure resistor and
capacitor connected in parallel, as shown in Fig
...
65,
the current flowing through the resistive branch will
be in phase with the voltage and the current in the
capacitive branch will lead the voltage by 90°
...
1
...
EXAMPLE 1
A pure inductor of 100 mH is connected in parallel with a 30 ⍀ resistor to
a 230 V 50 Hz supply
...
IR ϭ
І IR ϭ
V
(A)
R
230 V
ϭ 7
...
142 ϫ 50 Hz ϫ 100 ϫ 10Ϫ3 H
X L ϭ 31
...
1
...
І IL ϭ
230 V
ϭ 7
...
42 ⍀
From the trigonometry of the phasor diagram in Fig
...
11, the total
current is given by
I T ϭ I R 2 ϩ I L 2 (A)
І I T ϭ (7
...
32 A)2
I T ϭ 10
...
EXAMPLE 2
A pure capacitor of 60 F is connected in parallel with a 40 ⍀ resistor
across a 230 V 50 Hz supply
...
IR ϭ
І IR ϭ
Fig
...
65 A parallel RC circuit and phasor diagram
...
75 A
40 ⍀
1
(⍀ )
2fC
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
І XC ϭ
1
2 ϫ 3
...
05 ⍀
V
(A)
IC ϭ
XC
І IC ϭ
230 V
ϭ 4
...
05 ⍀
From the trigonometry of the phasor diagram in Fig
...
12, the total
current is given by
I T ϭ I C2 ϩ I R 2 (A)
І I T ϭ (5
...
34 A)2
I T ϭ 7
...
In
practice the inductor will contain some resistance and
the network may, therefore, be considered as a series
RL branch connected in parallel with a capacitor as
shown in Fig
...
66
...
Calculate
the branch currents and the supply network
...
1
...
XC ϭ
1
(⍀ )
2fC
1
2 ϫ 3
...
2 ⍀
V
IC ϭ
(A)
XC
І XC ϭ
І IC ϭ
230 V
ϭ 1
...
2 ⍀
X L ϭ 2f L (⍀)
І X L ϭ 2 ϫ 3
...
8 ⍀
V
I coil ϭ (A)
Z
230 V
І I coil ϭ
ϭ 2
...
8 ⍀
The capacitor current will lead the supply voltage by 90°
...
4Њ
111
...
1
...
The total current is the phasor addition of these currents and
is found to be 1
...
THREE-PHASE a
...
Fig
...
66 A parallel circuit and phasor diagram
...
c
...
For a three-phase
voltage three separate windings, each separated by
120°, are rotated in a magnetic field
...
1
...
84
ADVANCED ELECTRICAL INSTALLATION WORK
phase windings are perfectly balanced and, therefore,
do not require a neutral connection
...
In any star-connected system currents flow along
the lines (IL ), through the load and return by the neutral conductor connected to the star point
...
Therefore, no
current flows in the neutral and the star point is at
zero volts
...
A star-connected system is also called a three-phase
four-wire system and allows us to connect singlephase loads to a three-phase system
...
1
...
Star and delta connections
Three-phase power
The three phase windings may be star connected or
delta connected as shown in Fig
...
69
...
The square root of 3 (͙3) is simply
a constant for three-phase circuits, and has a value of
1
...
The delta connection is used for electrical
power transmission because only three conductors are
required
...
1
...
Power ϭ VI cos (W)
In any balanced three-phase system, the total power is
equal to three times the power in any one phase
...
1
...
Now for a star connection,
VP ϭ VL / 3
For (b)
and
IL ϭ IP
I L ϭ I P ϭ VP /RP (A)
230
...
09 A
IL ϭ IP ϭ
10 ⍀
(2)
Substituting Equation (2) into Equation (1), we have
Total three-phase power ϭ
3 VLLL cos (W)
Now consider a delta connection:
VP ϭ VL
IP ϭ IL / 3
and
(3)
For (c)
Power ϭ 3 VL I L cos f (W)
І Power ϭ 1
...
09 A ϫ 1 ϭ 16 kW
Substituting Equation (3) into Equation (1) we have,
for any balanced three-phase load,
Total three-phase power ϭ
3 VLLL cos (W)
EXAMPLE 2
A 20 kW 400 V balanced delta-connected load has a power factor of
0
...
Calculate (a) the line current and (b) the phase current
...
Calculate (a) the
phase voltage, (b) the line current and (c) the total power consumed
...
9 V
1
...
732 ϫ 400 V ϫ 0
...
08 (A)
For delta connection,
I L ϭ 3 I P (A)
86
ADVANCED ELECTRICAL INSTALLATION WORK
Thus, for (b),
IP ϭ IL
І IP ϭ
therefore phase and line currents are both equal to 4
...
For delta, connection,
3 (A)
36
...
83 A
1
...
Calculate the phase currents and line currents
for each connection
...
732 ϫ 8 A ϭ 13
...
39 to 2
...
In this chapter
we will essentially be looking at d
...
and a
...
motors,
their control equipment and maintenance and
transformers
...
9 V
1
...
9 V
І IP ϭ
ϭ 4
...
1
...
Direct current motors
If a current carrying conductor is placed into the field
of a permanent magnet as shown in Fig
...
70(c) a
force F will be exerted on the conductor to push it out
of the magnetic field
...
Figure 1
...
Figure 1
...
Figure 1
...
This is the basic motor principle, and the force F is
dependent upon the strength of the magnetic field B,
the magnitude of the current flowing in the conductor
I and the length of conductor within the magnetic field
l
...
Fig
...
71 Showing d
...
machine construction
...
Calculate the force on the conductor when 15 A flows in the coil
...
c
...
1
...
All d
...
motors contain a field winding wound on pole
pieces attached to a steel yoke
...
Contact with the external circuit is
made through carbon brushes rubbing on the commutator segments
...
88
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
72 Series motor connections and characteristics
...
The
series motor has the characteristics of a high starting
torque but a speed which varies with load
...
Figure
1
...
For this reason the motor is only suitable for
direct coupling to a load, except in very small motors,
such as vacuum cleaners and hand drills, and is ideally
suited for applications where the machine must start
on load, such as electric trains, cranes and hoists
...
This characteristic means that the
machine will run on both a
...
or d
...
and is, therefore,
sometimes referred to as a ‘universal’ motor
...
1
...
Since the field winding is
across the supply, the flux and motor speed are considered constant under normal conditions
...
1
...
load, as shown in Fig
...
73
...
The shunt motor is a very desirable d
...
motor because of its constant speed characteristics
...
Reversal of rotation may be achieved by reversing the connections to either the field or armature
winding but not both
...
If the
field windings are connected so that the field flux acts
in opposition, the machine is known as a short shunt
and has the characteristics of a series motor
...
The
arrangement of compound motor connections is given
in Fig
...
74
...
Typical applications are for electric motors in steel rolling mills, where a constant speed
is required under varying load conditions
...
1
...
c
...
Fig
...
74 Compound motor connections
...
c
...
c
...
The speed of
a d
...
motor is inversely proportional to the strength
of the magnetic flux in the field winding
...
A variable resistor connected into the field circuit,
as shown in Fig
...
75 provides one method of controlling the field current and the motor speed
...
c
...
BACK emf AND MOTOR STARTING
When the armature conductors cut the magnetic flux
of the main field, an emf is induced in the armature,
as described earlier in this chapter at Fig
...
43 under
Inductance
...
During normal running, the back emf is always a little smaller than the supply voltage, and acts as a limit
to the motor current
...
This applies to all but the very smallest of motors and
is achieved by connecting a resistor in series with the
armature during starting, so that the resistance can be
gradually reduced as the speed builds up
...
1
...
The control switch is moved progressively over the variable resistor contacts to the run position as the motor
speed builds up
...
90
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
76 A d
...
motor starting
...
c
...
The magnetic field established by the threephase currents travels in a clockwise direction around
the stator, as can be seen by considering the various
intervals of time 1 to 6 shown in Fig
...
77
...
This
is called synchronous speed, denoted nS:
nS ϭ
f
60f
or N S ϭ
P
P
where
nS is measured in revolutions per second
NS is measured in revolutions per minute
f is the supply frequency measured in hertz
P is the number of pole pairs
...
We have
nS ϭ
f
(rps)
P
A four-pole machine has two pairs of poles:
І nS ϭ
50 Hz
ϭ 25 rps
2
or N S ϭ
60 ϫ 50 Hz
ϭ 1500 rpm
2
This rotating magnetic field is used for practical effect in the induction
motor
...
1
...
stationary part of an induction motor as shown in Fig
...
78(a), a rotating magnetic flux is produced
...
This induced emf causes rotor currents
to flow and establish a magnetic flux which reacts
with the stator flux and causes a force to be exerted on
the rotor conductors, turning the rotor as shown in
Fig
...
78(b)
...
1
...
conductors and the rotating field
...
At switch-on, the rotor speed increases
until it approaches the speed of the rotating magnetic
flux, that is, the synchronous speed
...
By
Faraday’s laws, this will result in less induced emf, less
rotor current and less torque on the rotor
...
The induction motor is called an asynchronous motor
...
The difference between the rotor speed and synchronous speed is called slip; the per-unit slip, denoted
s, is given by
s ϭ
nS Ϫ n
N ϪN
ϭ S
nS
NS
where
nS ϭ synchronous speed in revolutions per second
NS ϭ synchronous speed in revolutions per minute
n ϭ rotor speed in revolutions per second
N ϭ rotor speed in revolutions per minute
...
EXAMPLE
A two-pole induction motor runs at 2880 rpm when connected to the
50 Hz mains supply
...
The synchronous speed is given by
60 ϫ f
(rpm)
p
60 ϫ 50 Hz
І NS ϭ
ϭ 3000 rpm
1
NS ϭ
Thus the per-unit slip is
NS ϪN
NS
3000 rpm Ϫ 2880 rpm
Іsϭ
3000 rpm
s ϭ 0
...
sϭ
So the percentage slip is 0
...
ROTOR CONSTRUCTION
There are two types of induction motor rotor – the
wound rotor and the cage rotor
...
1
...
of a laminated cylinder of silicon steel with copper or
aluminium bars slotted in holes around the circumference and short-circuited at each end of the cylinder
as shown in Fig
...
79
...
Better starting and quieter running are
achieved if the bars are slightly skewed
...
A machine fitted with a cage rotor does suffer from a low starting torque and the machine must
be chosen which has a higher starting torque than the
load, as shown by curve (b) in Fig
...
80
...
1
...
Alternatively the load may be connected after the
motor has been run up to full speed, or extra resistance can be added to a wound rotor through sliprings and brushes since this improves the starting
torque, as shown by curve (c) in Fig
...
80
...
The windings may be connected in star
or delta and the end connections brought out to slip
rings mounted on the shaft
...
Therefore, the principle of operation for both types of
rotor is the same
...
1
...
with the load disconnected
...
Its
applications are for constant speed machines such as
fans and pumps
...
THREE-PHASE SYNCHRONOUS MOTOR
If the rotor of a three-phase induction motor is removed
and replaced with a simple magnetic compass, the
compass needle will rotate in the same direction as the
rotating magnetic field set up by the stator winding
...
This is the basic principle of operation of the
synchronous motor
...
c
...
When the supply is initially switched on, the rotor
will experience a force, first in one direction and then
in the other direction every cycle as the stator flux
rotates around the rotor at synchronous speed
...
However, if the rotor is rotated at or near synchronous speed,
then the stator and rotor poles of opposite polarity
will ‘lock together’ producing a turning force or torque
which will cause the rotor to rotate at synchronous
speed
...
The synchronous motor can, therefore,
only be run at synchronous speed, which for a 50 Hz
supply will be 3000, 1500, 1000 or 750 rpm depending upon the number of poles, as discussed earlier in
this chapter
...
Once the rotor achieves synchronous speed the pony
motor is disconnected and the load applied to the
synchronous motor
...
However, the advantage of a synchronous
motor is that it runs at a constant speed and operates
at a leading power factor
...
windings create the rotating field and turning force
required to start the motor
...
A cage rotor is used on single-phase a
...
motors, the
turning force being produced in the way described
previously for three-phase induction motors and shown
in Fig
...
78
...
The phase displacement between the currents in the windings is achieved
in one of two ways:
■
■
by connecting a capacitor in series with the start
winding, as shown in Fig
...
81(a), which gives a
90° phase difference between the currents in the
start and run windings;
by designing the start winding to have a high resistance and the run winding a high inductance, once
again creating a 90° phase shift between the currents in each winding, as shown in Fig
...
81(b)
...
When the motor reaches
about 80% of full speed, the centrifugal switch clicks
open and the machine continues to run on the magnetic flux created by the run winding only
...
Reversal of rotation may be achieved by
reversing the connections to the start or run windings,
but not both
...
c
...
c
...
All a
...
motors require a
rotating field to start
...
c
...
The two windings are known as
the start and run windings
...
Figure 1
...
It has a cage rotor and the
moving field is produced by enclosing one side of
each stator pole in a solid copper or brass ring, called
a shading ring, which displaces the magnetic field and
creates an artificial phase shift
...
1
...
Motor starters
Fig
...
82 Shaded pole motor
...
Reversal of rotation is theoretically possible by moving the shading rings to the
opposite side of the stator pole face
...
There are more motors operating from single-phase
supplies than all other types of motor added together
...
The magnetic flux generated in the stator of an induction motor rotates immediately the supply is switched
on, and therefore the machine is self-starting
...
Thermal overload protection is usually provided
by means of a bimetal strip bending under overload
conditions and breaking the starter contactor coil circuit
...
Once the motor has automatically
switched off under overload conditions or because
a remote stop/start button has been operated, it is an
important safety feature that the motor cannot restart
without the operator going through the normal startup procedure
...
Electronic thermistors (thermal transistors) provide
an alternative method of sensing if a motor is overheating
...
96
ADVANCED ELECTRICAL INSTALLATION WORK
All electric motors with a rating above 0
...
DIRECT ON LINE (d
...
l
...
o
...
starter switches the main supply directly on
to the motor
...
o
...
starter is only used for small motors of less
than about 5 kW rating
...
1
...
If the start button is released the control circuit is
maintained by the hold on contact
...
Once the supply is interrupted
the supply to the motor can only be reconnected by
pressing the start button
...
STAR DELTA STARTERS
When three loads, such as the three windings of a
motor, are connected in star, the line current has only
one-third of the value it has when the same load is
connected in delta
...
This arrangement is shown in Fig
...
84, where the six
connections to the three stator phase windings are
brought out to the starter
...
This reduces the
phase voltage to about 58% of the running voltage
which reduces the current and the motor’s torque
...
The starter will incorporate overload and no-volt protection, but these are
not shown in Fig
...
84 in the interests of showing
more clearly the principle of operation
...
1
...
o
...
starter
...
One method is to connect the motor to a
star delta starter
...
Since
this also reduces the starting torque, the voltage is
only reduced by a sufficient amount to reduce the
starting current, being permanently connected to
the tapping found to be most appropriate by the
installing electrician
...
When sufficient speed has been
achieved by the motor the changeover switch is
moved to the run connections which connect the
three-phase supply directly on to the motor as shown
in Fig
...
85
...
The starter will incorporate overload and no-volt protection in addition to
some method of preventing the motor being switched
to the run position while the motor is stopped
...
1
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
97
Fig
...
84 Star delta starter
...
1
...
ROTOR RESISTANCE STARTER
When starting a machine on load a wound rotor induction motor must generally be used since this allows an
external resistance to be connected to the rotor winding
through slip-rings and brushes, which increases the
starting torque as shown in Fig
...
80 curve (c)
...
As the motor speed
increases the resistance is reduced until at full speed
the external resistance is completely cut out and the
machine runs as a cage induction motor
...
1
...
Remote control of motors
When it is required to have stop/start control of a motor
at a position other than the starter position, additional
Fig
...
86 Rotor resistance starter for a wound rotor machine
...
1
...
o
...
starter
...
1
...
Additional stop and start facilities are
often provided for the safety and convenience of the
machine operator
...
This may also make final adjustments of the motor position easier
...
1
...
o
...
starter
...
All motors over 0
...
The cables supplying the motor must be capable of
carrying at least the full load current of the motor
(Regulation 552–01–01) and a local means of isolation must be provided to facilitate safe mechanical
maintenance (Regulation 476–02–03)
...
The supply protection must
be capable of withstanding the motor starting current
while providing adequate overcurrent protection
...
Most motors are ‘continuously rated’
...
Many standard motors have class A insulation which
is suitable for operating in ambient temperatures up
to about 55°C
...
The motor and its enclosure must be suitable for
the installed conditions and must additionally prevent
anyone coming into contact with the internal live or
moving parts
...
Clean air, damp conditions, dust particles
in the atmosphere, chemical or explosive vapours will
determine the type of motor enclosure
...
The following motor enclosures are examples of those to be found in industry
...
1
...
Screen protected enclosures prevent access to the
internal live and moving parts by covering openings
in the motor casing with metal screens of perforated
metal or wire mesh
...
This type of enclosure is shown in Fig
...
88
...
In these cases the air is drawn
from a clean air zone outside the room in which the
machine is installed, as shown in Fig
...
88
...
A fan on the motor shaft inside
the casing circulates the air through the windings and
cooling is by conduction through motor casing
...
This type
of enclosure is shown in Fig
...
88
...
To ensure that the
motor meets the stringent regulations for flameproof
enclosures the shaft is usually enclosed in special bearings and the motor connections contained by a wide
flange junction box
...
This is usually
best achieved by placing a straight edge or steel rule
Fig
...
89 Pulley and flange coupling arrangement
...
1
...
Since pulley belts stretch in use it is also important to
have some means of adjusting the tension of the vee
belt
...
1
...
Adjustment
is carried out by loosening the motor fixing bolts,
screwing in the adjusting bolts which push the motor
back, and when the correct belt tension has been
achieved the motor fixing bolts are tightened
...
Motor fans which provide cooling also pull dust particles from the surrounding air
into the motor enclosure
...
Planning maintenance work with forethought and
keeping records of work done with dates can have the
following advantages:
■
Fig
...
90 Vee belt adjustment of slide rail mounted motor
...
Industrial electric motors
are often operated in a hot, dirty, dusty or corrosive
environment for many years
...
Maintenance at regular intervals is required, in the same
way that a motor car is regularly serviced
...
c
...
These characteristics give cage rotor a
...
machines maximum reliability with the minimum of maintenance
and make the induction motor the most widely used in
industry
...
c
...
However, where high torque and variable speed
characteristics are required d
...
machines are often used
...
c
...
New
brushes must be of the correct grade and may require
‘bedding in’ or shaping with a piece of fine abrasive
cloth to the curve of the commutator
...
As the commutator wears, the mica insulation between
the segments must be cut back with an undercutting
tool or a hacksaw blade to keep the commutator surface smooth
...
Motors vibrate when operating and as a result fixing
bolts and connections should be checked as part of the
maintenance operation
...
The result of planned maintenance is often that fewer
breakdowns occur, which result in loss of production
time
...
Power-factor correction
Most electrical installations have a low power factor
because loads such as motors, transformers and discharge lighting circuits are inductive in nature and
cause the current to lag behind the voltage
...
Therefore, by adding
capacitance to an inductive circuit the bad power factor can be corrected
...
The power factor can be corrected to unity when
the capacitor current IC is equal and opposite to the
quadrature or reactive current IQ of the inductive
load
...
Figure 1
...
A low power factor is considered a disadvantage
because a given load takes more current at a low power
factor than it does at a high power factor
...
84 kW load at unity power factor took 8 A, but at a bad power factor of 0
...
The supply authorities discourage industrial consumers from operating at a bad power factor because:
■
larger cables and switchgear are necessary to supply
a given load;
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
101
Fig
...
91 Power-factor correction of inductive load: (a) circuit and phasor diagram for an inductive load with low p
...
; (b) circuit and phasor
diagram for (a) with capacitor correcting p
...
to unity
...
Bad power factors are corrected by connecting a
capacitor either across the individual piece of equipment or across the main busbars of the installation
...
1
...
This is the type of capacitor used for power-factor correction in a fluorescent
luminaire
...
f
...
The current to be carried by the capacitor for p
...
correction and the value of the capacitor may be
calculated as shown by the following example
...
7 is connected across a 400 V,
50 Hz supply
...
f
...
f
...
Fig
...
92 Phasor diagram
...
57 (A)
400 V ϫ 0
...
6° (since cosϪ1
0
...
6°) and can therefore be drawn to scale as shown in
Fig
...
92 and represented by line AB
...
f
...
1
...
To raise the load current to this value
will require a capacitor current Ic which is equal and opposite to the value
of the quadrature or reactive component IQ
...
f
...
1
...
102
ADVANCED ELECTRICAL INSTALLATION WORK
For (c), since
IC ϭ
І XC ϭ
V
(A)
XC
and
V
X C ϭ (⍀ )
IC
400 V
ϭ 20 ⍀
20 A
Since
XC ϭ
І Cϭ
1
(⍀ )
2 fC
and
Cϭ
V
VP
ϭ S
NP
NS
1
(F)
2 fX C
1
ϭ 159 F
2 ϫ ϫ 50 Hz ϫ 20 ⍀
A 159 F capacitor connected in parallel with the 8 kW load would
correct the power factor to unity
...
They
vary in size from miniature units used in electronics to
huge power transformers used in power stations
...
It will not normally work from a d
...
supply such as a battery
...
1
...
An alternating voltage applied to the primary
winding produces an alternating current, which sets
up an alternating magnetic flux throughout the core
...
1
...
This magnetic flux induces an emf in the secondary
winding, as described by Faraday’s law, which says that
when a conductor is cut by a magnetic field, an emf is
induced in that conductor
...
Therefore,
the emf in both windings is proportional to the number of turns
...
Calculate the number of secondary turns and the
primary and secondary currents when the transformer supplies a 12 V
12 W alarm bell
...
052 A
230 V
Transformer losses
As they have no moving parts causing frictional losses,
most transformers have a very high efficiency, usually
better than 90%
...
Copper losses occur because of the small internal
resistance of the windings
...
Iron losses are made up of hysteresis loss and eddy
current loss
...
Transformers will
only operate on an alternating supply
...
Each time there is a current
reversal, the magnetic flux reverses and it is this buildup and collapse of magnetic flux in the core material
which accounts for the hysteresis loss
...
These are reduced by building up the core of thin
slices or laminations of iron and insulating the separate laminations from each other
...
Transformer efficiency
The efficiency of any machine is determined by the
losses incurred by the machine in normal operation
...
However,
the efficiency of a transformer can be calculated in the
same way as for any other machine
...
However, the input to
the transformer must supply the output plus any
losses which occur within the transformer
...
8
...
Output power ϭ kVA ϫ p
...
І Output power ϭ 100 kVA ϫ 0
...
987
80 kW ϩ 1kW
or, multiplying by 100 to give a percentage, the transformer has an efficiency of 98
...
Transformer construction
Transformers are constructed in a way which reduces
the losses to a minimum
...
The primary
and secondary windings are wound close to each other
on the same limb
...
1
...
AUTO-TRANSFORMERS
Transformers having a separate primary and secondary winding, as shown in Fig
...
94, are called doublewound transformers, but it is possible to construct a
transformer which has only one winding which is
common to the primary and secondary circuits
...
An arrangement such as this
is called an auto-transformer
...
However, the
primary and secondary windings are not electrically
separate and a short circuit on the upper part of the
winding shown in Fig
...
95 would result in the primary voltage appearing across the secondary terminals
...
When installing
transformers, the regulations of Section 555 must be
complied with, in addition to any other regulations
relevant to the particular installation
...
In the
Fig
...
94 Transformer construction
...
1
...
double-wound type construction, as shown in Fig
...
93, three separate single-phase transformers are
wound onto a common laminated silicon–steel core
to form the three-phase transformer
...
This has the advantage of providing two secondary
voltages, typically 400 V between phases and 230 V
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
105
Fig
...
96 Delta-star connected three-phase transformer
...
The coil arrangement is shown in Fig
...
96
...
OIL-IMMERSED CORE
As the rating of a transformer increases so does the
problem of dissipating the heat generated in the core
...
The oil is a coolant and an
insulating medium for the core
...
Air passing over the tubes carries the heat away
and cools the transformer
...
97 shows the construction of a typical oil-filled transformer, and the
arrangement is typical of a distribution transformer
used in a sub-station (see Fig
...
105)
...
The tolerance permitted
by the Regulations in 2004 is the open-circuit voltage
plus 10% or minus 6%
...
Because
Fig
...
97 Typical oil-filled power transformer
...
This process is called
tap changing, and most distribution transformers are
fitted with a tap changing switch on the high-voltage
(HV) winding so that the number of turns can be varied
...
The switch is
usually padlocked to prevent unauthorized operation
...
The instrument transformer carries the
current or voltage to be measured and the instrument
is connected to the secondary winding of the transformer
...
The advantages of using instrument transformers
are as follows:
■
■
■
■
The secondary side of the instrument transformer
is wound for low voltage which simplifies the insulation of the measuring instrument and makes it
safe to handle
...
The measuring instrument can be read in a remote,
convenient position connected by long leads to the
instrument transformer
...
Voltage transformers
The construction of a voltage transformer (V T) is
similar to the power transformer dealt with earlier in
this chapter
...
1
...
A large number of turns are wound on the primary and a few on the secondary since
VP
N
ϭ P
VS
NS
The voltmeter reading must be multiplied by the
turns ratio to determine the load voltage
...
The primary winding of 250 turns is connected to the main supply
...
Primary voltage VP ϭ
І VP ϭ
NP
ϫ VS
NS
250
ϫ 80 V
50
VP ϭ 400 V
...
EXAMPLE 2
An electrical contractor wishes to monitor a 660 V supply with a standard
110 V voltmeter
...
VP N P
ϭ
VS N S
660 V N P 6
ϭ ϭ
100 V N S 1
The turns ratio is 6:1
...
Current transformers
Fig
...
98 A voltage transformer
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
107
EXAMPLE 1
An ammeter having a full scale deflection of 5 A is used to measure a line
current of 200 A
...
NP IS
ϭ
N S IP
N ϫI
NS ϭ P P
IS
2 ϫ 200 A
ϭ 80 turns
NS ϭ
5A
Fig
...
99 Current transformers: (a) wound primary current transformer; (b) bar primary current transformer
...
1
...
The ammeter is usually standardized
at 1 or 5 A and the transformer ratio chosen so that
1 or 5 A flows when the main circuit carries full load
current calculated from the transformer turns ratio
With a power transformer a secondary load is
necessary to cause a primary current to flow which
maintains the magnetic flux in the core at a constant
value
...
However, the secondary current through the
ammeter is necessary to stabilize the magnetic flux in
the core, and if the ammeter is removed the voltage
across the secondary terminals could reach a dangerously high value and cause the insulation to break
down or cause excessive heating of the core
...
If the ammeter must be removed from
the CT then the terminals must first be shortcircuited
...
The rating of an
instrument transformer is measured in volt amperes
and is called the burden
...
EXAMPLE 2
I
VP
ϭ S
VS
Ip
To determine the power taken by a single-phase motor a wattmeter
is connected to the circuit through a CT and VT
...
In this case the
conductor carrying the main current or the main busbar is passed through the centre of the CT as shown in
Fig
...
99 (b)
...
Wattmeter reading ϭ 300 W
Voltage transformer turns ratio ϭ 440/110 V
Current transformer turns ratio ϭ 150/5 A
Sketch the circuit arrangements and calculate the power taken by the
motor
...
1
...
The circuit arrangements are shown in Fig
...
100
...
1
...
True ϭ Wattmeter ϫ VT
ϫ CT
(W)
Power reading
multiplier multiplier
True power ϭ 300 W ϫ 4 ϫ 30 ϭ 36 kW
The power taken by the motor is therefore 36 kW
...
Virtually all the
generators of electricity throughout the world are
three-phase synchronous generators
...
The magnetic field is produced electrically by passing
a direct current through a winding on an iron core,
which rotates inside three phase windings on the stator of the machine
...
Primary sources for
electricity generation are discussed in Chapter 2 of
Basic Electrical Installation Work
...
A 2000 MW station might
contain four 500 MW sets, three 660 MW sets and a
20 MW gas turbine generator or two 1000 MW sets
...
When generators are connected to a single system they must rotate at exactly the same speed, hence
the term synchronous generator
...
Since power in an a
...
system
is expressed as P ϭ VI cos , it follows that an increase
in voltage will reduce the current for a given amount
of power
...
The 132 kV grid and 400 kV Supergrid transmission lines are, for the most part, steel-cored aluminium
conductors suspended on steel lattice towers, since this
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
is about 16 times cheaper than the equivalent underground cable
...
101 shows a suspension tower
on the Sizewell–Sundon 400 kV transmission line
...
1
...
Three conductors comprise a single circuit of a three-phase system so that
towers with six arms carry two separate circuits
...
Figure 1
...
Fig
...
102 Steel lattice tower cable supports
...
The maintenance of a secure supply is an
important consideration for any electrical engineer or
supply authority because electricity plays a vital part
in an industrial society, and a loss of supply may cause
inconvenience, financial loss or danger to the
consumer or the public
...
High-voltage distribution to primary substations is
used by the electricity boards to supply small industrial, commercial and domestic consumers
...
1
...
Regulation 9 of the Electricity Supply
Regulations and Regulation 31 of the Factories Act
require that these substations be protected by 2
...
In towns and cities the substation equipment is usually
enclosed in a brick building, as shown in Fig
...
105
...
These
Fig
...
103 Generation, transmission and distribution of electrical energy
...
1
...
Fig
...
105 Typical substation layout
...
1
...
outgoing circuits are usually protected by circuit breakers in a distribution board
...
1
...
The star point
is earthed to an earth electrode sunk into the ground
below the substation, and from this point is taken the
fourth conductor, the neutral
...
A three-phase 400 V supply is used for supplying small industrial and commercial loads such as garages, schools and blocks of
flats
...
EXAMPLE
Use a suitable diagram to show how a 400 V three-phase, four-wire supply may be obtained from an 11 kV delta-connected transformer
...
(d) State why ‘balancing’ of loads is desirable
...
Three-phase load
Figure 1
...
Reducing stroboscopic effect
The stroboscopic effect may be reduced by equally
dividing the lighting load across the three phases of the
supply
...
(The stroboscopic effect and its elimination are
discussed in some detail in Chapter 10 of this book
...
Therefore the current taken by each phase
will be equal
...
Equally distributing the single-phase loads across the
three-phase supply is known as ‘balancing’ the load
...
Advantages of a three-phase four-wire supply
A three-phase four-wire supply gives a consumer the
choice of a 400 V three-phase supply and a 230 V single-phase supply
...
Industrial
loads usually demand more power than a domestic load,
and more power can be supplied by a 400 V three-phase
supply than is possible with a 230 V single-phase supply
for a given size of cable since power ϭ VI cos (watts)
...
Thus the
voltages used previously in low-voltage supply systems
of 415 and 240 V have become 400 V for three-phase
supplies and 230 V for single-phase supplies
...
From January 1995 the
permitted tolerance is the nominal voltage ϩ10%
or Ϫ6%
...
This gives a voltage
range of 216–253 V for a nominal voltage of 230 V
and 376 V to 440 V for a nominal voltage of 400 V
...
All EU countries will adjust their voltages to comply with a nominal voltage of 230 V
single-phase and 400 V three-phase
...
Other
items of equipment at this position are the energy
meter and the consumer’s distribution unit, providing
the protection for the final circuits and the earthing
arrangements for the installation
...
The limiting
values of earth fault loop impedance are given in Tables
41, 604 and 605 of the IEE Regulations, and Section
542 gives details of the earthing arrangements to be
incorporated in the supply system to meet the requirements of the Regulations
...
A system consists of an electrical installation connected to a supply
...
The supply earthing
Arrangements are indicated by the first letter, where
T means one or more points of the supply are directly
connected to earth and I means the supply is not
earthed or one point is earthed through a fault-limiting
impedance
...
The earthed supply conductor
Arrangements are indicated by the third letter, where
S means a separate neutral and protective conductor
and C means that the neutral and protective conductors are combined in a single conductor
...
The neutral
and protective conductor are separate throughout the
system
...
All exposed conductive parts of the installation,
gas pipes, water pipes and any lightning protective
system are connected to the protective conductor via
the main earthing terminal of the installation
...
1
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
113
Fig
...
107 TN-S system: separate neutral and protective conductor throughout
...
It
is more commonly referred to as protective multiple
earthing (PME)
...
At the supply intake point a consumer’s main
earthing terminal is formed by connecting the earthing
terminal to the neutral conductor
...
Thus phase to earth faults
are effectively converted into phase to neutral faults
...
1
...
installation’s circuit protective conductors must be
connected to earth via an earth electrode provided by
the consumer
...
The arrangement is shown in Fig
...
109
...
1
...
The TN-C and IT systems of supply do not comply
with the supply regulations and therefore cannot be
used for public supplies
...
TN-C SYSTEM
TT SYSTEM
This is the type of supply more often found when the
installation is fed from overhead cables
...
All exposed conductive parts of an
installation are connected to the PEN conductor
...
1
...
Fig
...
109 TT systems: earthing arrangements independent of supply cable
...
1
...
there are no metallic connections between the TN-C
system and the public supply
...
IT SYSTEM
The supply is isolated from earth and therefore there is
no shock or fire risk involved when an earth fault
occurs
...
This
type of supply is used in mines, quarries and chemical
processes where interruption of the process may create a
hazardous situation
...
LOW-VOLTAGE DISTRIBUTION IN BUILDINGS
In domestic installations the final circuits for lights,
sockets, cookers, immersion heating, etc
...
1
...
In commercial or industrial installations a threephase 400 V supply must be distributed to appropriate
equipment in addition to supplying single-phase 230 V
loads such as lighting
...
This presents the electrical contractor with an
additional problem
...
Busbar chambers incorporated into cubicle switchboards or on-site assemblies of switchboards are to be
found at the incoming service position of commercial
and industrial consumers, since this has proved to
provide the flexibility required by these consumers
...
1
...
Distribution fuse boards, which may incorporate circuit breakers, are wired by submain cables from the
service position to load centres in other parts of the
building, thereby keeping the length of cable to the final
circuit as short as possible
...
1
...
When high-rise buildings such as multi-storey flats
have to be wired, it is usual to provide a three-phase
four-wire rising main
...
1
...
Fig
...
112 Typical distribution in commercial or industrial building
...
1
...
point in the building
...
When individual dwellings receive a
single-phase supply the electrical contractor must balance the load across the three phases
...
1
...
The rising main must incorporate fire barriers to prevent the spread of fire throughout the building (Regulation 527–02)
...
Cables in trunking with conduit drops or, SWA or
MI cables laid on cable tray, provide a flexible, adaptable
electrical installation
...
Before we look at some industrial wiring systems and
cables, let us first of all define some technical terms and
discuss the properties of materials used in electrical
wiring systems
...
Insulator A material (usually a non-metal) which will
not allow heat and electricity to pass easily through it
...
The word ‘ferrous’ comes
from the Latin word ferrum meaning iron
...
Cast iron, wrought
iron and steel are all ferrous metals
...
They are non-magnetic and resist
rusting
...
Alloy An alloy is a mixture of two or more metals
...
Corrosion The destruction of a metal by chemical
action
...
Thermoplastic polymers These may be repeatedly
warmed and cooled without appreciable changes
occurring in the properties of the material
...
They have a flexible quality when operated up to a
maximum temperature of 70°C but should not be
flexed when the air temperature is near 0°C, otherwise
they may crack
...
Thermosetting polymers Once heated and formed,
products made from thermosetting polymers are fixed
rigidly
...
Rubber is a tough elastic substance made from the sap
of tropical plants
...
Synthetic rubber is manufactured, as opposed to
being produced naturally
...
Silicon rubber Introducing organic compounds into
synthetic rubber produces a good insulating material
which is flexible over a wide range of temperatures
and which retains its insulating properties even when
burned
...
Magnesium oxide The conductors of mineral insulated metal sheathed (MICC) cables are insulated with
compressed magnesium oxide, a white chalk-like substance which is heat-resistant and a good insulator and
lasts for many years
...
However, the magnesium oxide is very
hygroscopic, which means that it attracts moisture and,
therefore, the cable must be terminated with a special
moisture-excluding seal, as shown in Fig
...
116
...
For electrical purposes it is refined to about
98
...
It is a very good
conductor, is non-magnetic and offers considerable
resistance to atmospheric corrosion
...
Copper forms the largest portion of the alloy brass,
and is used in the manufacture of electrical cables,
domestic heating systems, refrigerator tubes and
vehicle radiators
...
Aluminium Aluminium is a grey-white metal obtained
from the mineral bauxite which is found in the USA,
Germany and the Russian Federation
...
It is used in the
manufacture of power cables
...
Copper conductors would be too heavy to support themselves between
the pylons
...
Aluminium alloys retain the corrosion resistance
properties of pure aluminium with an increase in
strength
...
Special processes and
fluxes have now been developed which allow aluminium to be welded and soldered
...
Because it is harder than copper or
aluminium it is easily machined
...
For these reasons it
is often used in the electrical and plumbing trades
...
Brass is an attractive yellow metal which is also used
for decorative household articles and jewellery
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
Cast steel Cast steel is also called tool steel or high
carbon steel
...
These ingots are then rolled or
pressed into various shapes from which the finished
products are made
...
Hammer heads, pliers, wire
cutters, chisels, files and many machine parts are also
made from cast steel
...
It can be
filed, drilled or sawn quite easily and may be bent when
hot or cold, but repeated cold bending may cause it to
fracture
...
Mild steel is the
most widely used metal in the world, having considerable strength and rigidity without being brittle
...
Zinc Zinc is a non-ferrous metal which is used mainly
to protect steel against corrosion and in making the
alloy brass
...
Conduit, trunking, tray, steel wire armour, outside luminaires and electricity pylons are made of galvanized steel
...
5
...
5 Properties of materials used in electrical installation
work
PVC
Strength
Hardness
Conductivity
Corrosion
resistance
Magnetic
properties
Copper
Poor
Poor
Very good
Good
Poor
Poor
Good
Good
Good
Good
Good
Good
None
None
None
None Good
and the outer sheath which may contain some means
of providing protection from mechanical damage
...
Solid
conductors are only used in fixed wiring installations
and may be shaped in larger cables
...
0 mm2 to 25 mm2 contain seven strands
...
35 mm diameter
strands which collectively make up the 10 mm2 crosssectional area of the cable
...
Flexible cords have multiple strands
of very fine wire, as fine as one strand of human hair
...
NEW WIRING COLOURS
Twenty-eight years ago the United Kingdom agreed
to adopt the European colour code for flexible cords,
that is, brown for live or phase conductor, blue for the
neutral conductor and green combined with yellow
for earth conductors
...
These were to remain as red for live or
phase conductor, black for the neutral conductor and
green combined with yellow for earth conductors
...
2 to BS 7671: 2001 which specified
new cable core colours for all fixed wiring in United
Kingdom electrical installations
...
Aluminium Brass Steel
Very poor
Very poor
None
Very good
119
Very good
Good
Fair
Poor
Cables
Most cables can be considered to be constructed in
three parts: the conductor which must be of a suitable
cross-section to carry the load current; the insulation,
which has a colour or number code for identification;
EXISTING FIXED CABLE CORE COLOURS
Single phase – red phase conductors, black neutral
conductors and green combined with yellow for earth
conductors
...
These core colours must not be used after 31st of
March 2006
...
120
ADVANCED ELECTRICAL INSTALLATION WORK
Three phase – brown, black and grey phase conductors,
blue neutral conductors and green combined with
yellow for earth conductors
...
Extensions or alterations to existing single phase
installations do not require marking at the interface
between the old and new fixed wiring colours
...
Great care should be
taken before undertaking extensions, alterations or
repair that all conductors are correctly identified
...
They are used where some mechanical
protection of the cable conductors is required
...
1
...
Around
this sheath is placed an armour protection of steel
wires twisted along the length of the cable, and a final
PVC sheath covering the steel wires protects them
from corrosion
...
Alterations to three phase installations must be marked
at the interface L1, L2, L3 for the phases and N for the
neutral
...
These
markings are preferred to coloured tape and a caution
notice is again required at the distribution board
...
It is the simplest and least expensive cable
...
114 shows a
sketch of a twin and earth cable
...
1
...
MI CABLE
Fig
...
114 A twin and earth PVC insulated and sheathed cable
...
PVC/SWA cable
PVC insulated steel wire armour cables are used for
wiring underground between buildings, for main
A mineral insulated (MI) cable has a seamless copper
sheath which makes it waterproof and fire- and
corrosion-resistant
...
The cable has a small overall diameter when compared to alternative cables and may be supplied as
bare copper or with a PVC oversheath
...
The
copper outer sheath provides the CPC, and the cable
is terminated with a pot and sealed with compound
and a compression gland (see Fig
...
116)
...
1
...
means that it readily absorbs moisture from the surrounding air, unless adequately terminated
...
FP 200 CABLE
FP 200 cable is similar in appearance to an MI cable
in that it is a circular tube, or the shape of a pencil, and
is available with a red or white sheath
...
The cable is available with either solid or stranded
conductors that are insulated with ‘insudite’ a fire
resistant insulation material
...
This aluminium
tape screen is applied metal side down and in contact
with the bare circuit protective conductor
...
FP 200 is available in 2, 3, 4, 7, 12 and 19 cores with
a conductor size range from 1
...
0 mm
...
The cable is as easy to use as a PVC insulated and
sheathed cable
...
The cable is a fire resistant cable, primarily intended
for use in fire alarms and emergency lighting installations or it may be embedded in plaster
...
Most standard cable types are available as LSF cables
...
The regulations covering high-voltage distribution are
beyond the scope of the IEE Regulations for electrical
installations
...
HIGH-VOLTAGE OVERHEAD CABLES
Suspended from cable towers or pylons, overhead
cables must be light, flexible and strong
...
1
...
The aluminium conductors carry
the current and the steel core provides the tensile
strength required to suspend the cable between pylons
...
1
...
The cable is not insulated since it is placed out of reach
and insulation would only add to the weight of the
cable
...
Very high-voltage
cables are only buried underground in special circumstances when overhead cables would be unsuitable, for
example, because they might spoil a view of natural
beauty
...
In transporting vast quantities of
power, heat is generated within the cable
...
The system is similar to
Fig
...
118 132 kV underground cable construction
...
Figure 1
...
The conductors may be aluminium or copper, solid
or stranded
...
The oil ducts allow the oil to flow through the cable,
removing excess heat
...
The lead sheath keeps the oil in and moisture out of
the cable, and this is supported by the copper-woven
fabric tape
...
The termination
and installation of these cables is a very specialized job,
undertaken by the supply authorities only
...
The necessary skills can be acquired by an electrical
trainee who has the correct attitude and dedication to
his craft
...
Mechanical damage to the cable
caused by impact, abrasion, penetration, compression
or tension must be minimized during installation
(Regulation 522–06–01)
...
Cables should be run horizontally or vertically, not diagonally, down a wall
...
Where
cables are bent, the radius of the bend should not cause
the conductors to be damaged (Regulation 522–08–03
and Table 4E of the On Site Guide)
...
All joints must be accessible for inspection testing
and maintenance when the installation is completed
...
PVC cables do not react chemically with plaster, as do some cables, and consequently
PVC cables may be buried under plaster
...
However, Regulation
522–06–07 now tells us that where PVC cables are to
be embedded in wet plaster, they should be covered in
mechanical protection to protect them from the plasterer’s trowel and penetration by nails and screws
...
This is shown in Fig
...
22 in Chapter 4
of Basic Electrical Installation Work, 4th edition
...
Cables
passing through metal boxes should be bushed with a
rubber grommet to prevent abrasion of the cable
...
PVC cables
should not be installed when the surrounding temperature is below 0°C or when the cable temperature has
been below 0°C for the previous 24 hours because the
insulation becomes brittle at low temperatures and may
be damaged during installation
...
The conduit, in effect,
replaces the PVC outer sheath of a cable, providing
mechanical protection for the insulated conductors
...
There are
three types of conduit used in electrical installation
work: steel, PVC and flexible
...
Heavy gauge is made from a sheet of steel
welded along the seam to form a tube and is used for
most installation work
...
Conduit is supplied in 3
...
Conduit tubing
and fittings are supplied in a black enamel finish for
internal use or hot galvanized finish for use on external or damp installations
...
1
...
Metal conduits are threaded with stocks and dies
and bent using special bending machines
...
Metal
conduits containing a
...
circuits must contain phase
and neutral conductors in the same conduit to prevent
eddy currents flowing, which would result in the metal
conduit becoming hot (Regulation 521–02–01)
...
The conduit size and range of fittings are
the same as those available for metal conduit
...
PVC conduit can be bent by
hand using a bending spring of the same diameter as
the inside of the conduit
...
The spring ensures
that the conduit keeps its circular shape
...
1
...
intended bend often helps to achieve a more successful bend
...
Since PVC conduit is an insulator it cannot be used as the CPC and a
separate earth conductor must be run to every outlet
...
Where luminaires are suspended from PVC conduit boxes, precautions must
be taken to ensure that the lamp does not raise the
box temperature or that the mass of the luminaire
supported by each box does not exceed the maximum
recommended by the manufacturer (IEE Regulation
522–01)
...
All conduit installations must be erected first before
any wiring is installed (IEE Regulation 522–080–02)
...
All conduits
should terminate in a box or fitting and meet the boxes
or fittings at right angles, as shown in Fig
...
120
...
1
...
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
provide complete enclosure of the conduit system
...
When drawing cables into conduit they must first
be run off the cable drum
...
1
...
Cables should be fed into the conduit in a manner
which prevents any cable crossing over and becoming
twisted inside the conduit
...
Cables can be pulled in on a draw wire if the run
is a long one
...
A limit must be placed on the number of bends
between boxes in a conduit run and the number of
125
cables which may be drawn into a conduit to prevent
the cables being strained during wiring
...
Flexible conduit
Flexible conduit is made of interlinked metal spirals
often covered with a PVC sleeving
...
Flexible conduit is used for the final connection to
motors so that the vibrations of the motor are not
transmitted throughout the electrical installation and
to allow for modifications to be made to the final
motor position and drive belt adjustments
...
Having decided upon the type, size and number of
cables required for a final circuit, it is then necessary
to select the appropriate size of conduit to accommodate those cables
...
The Tables are shown in Tables 1
...
7
...
1
...
Identify the cable factor for the particular size of conductor
...
6
...
Identify the appropriate part of the conduit factor
table given by the length of run and number of
bends
...
For conduit runs in excess of 3 m or incorporating
bends, the conduit factors are given in Table 5D, see
Table 1
...
)
The correct size of conduit to accommodate the
cables is that conduit which has a factor equal to or
greater than the sum of the cable factors
...
5 mm2 PVC insulated cables are to be run in a conduit containing
two bends between boxes 10 m apart
...
From Table 5C, shown in Table 1
...
5 mm2 cable ϭ 30
The sum of the cable factors
ϭ 6 ϫ 30
ϭ 180
Table 1
...
Reproduced from the IEE On Site
Guide by kind permission of the Institution of Electrical Engineers
TABLE 5C
Cable factors for use in conduit in long straight runs over 3 m, or
runs of any length incorporating bends
Type of conductor Conductor cross-sectional area (mm2) Cable factor
Solid or stranded
From Table 5D shown in Table 1
...
A 20 mm conduit containing
two bends only has a factor of 141 which is less than 180, the sum of
the cable factors and, therefore, 25 mm conduit is the minimum size to
contain these cables
...
5
2
...
5 times the outside diameter of the conduit
...
0 mm PVC insulated cables are to be drawn into a plastic conduit
which is 6 m long between boxes and contains one bend
...
0 mm PVC
insulated CPC is also included
...
From Table 5C, and shown in Table 1
...
0 mm cable ϭ 16
The factor for one 4
...
The sum of the cable factors ϭ (10 ϫ 16) ϩ (1 ϫ 43) ϭ 203
...
7, a 20 mm conduit, 6 m long and containing one bend, has a factor of 233
...
7 Conduit cable factors for bends and long straight runs
...
5
2
2
...
5
4
4
...
4 ϫ (32 mm factor)
For 50 mm diameter use …… 2
...
2 ϫ (32 mm factor)
463
422
388
358
333
16
20 25
Four bends
818 130 213 388
750 111 182 333
692 97 159 292
643 86 141 260
600
32
692
600
529
474
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
only has a factor of 143 which is less than 203, the sum of the cable factors
and, therefore, 20 mm conduit is the minimum size to contain these cables
...
Trunking
may be thought of as a more accessible conduit system
and for industrial and commercial installations it is
replacing the larger conduit sizes
...
When an alteration or extension is
required it is easy to drill a hole in the side of the trunking and run a conduit to the new point
...
Trunking is supplied in 3 m lengths and various
cross-sections measured in millimetres from 50 ϫ 50
up to 300 ϫ 150
...
Fig
...
122 Alternative trunking bends
...
A wide range of accessories are available, such as 45° bends, 90° bends, tee
and four-way junctions, for speedy on-site assembly
...
1
...
This may be necessary or more convenient if a bend or set is non-standard,
but it does take more time to fabricate bends than
merely to bolt on standard accessories
...
Fish plates must be made and riveted
or bolted to the trunking to form a solid and secure
bend
...
If an earth continuity test
on the trunking is found to be unsatisfactory, an insulated CPC must be installed inside the trunking
...
Non-metallic trunking
Trunking and trunking accessories are also available in
high-impact PVC
...
PVC trunking, like PVC conduit, is easy to install and is
non-corrosive
...
All
trunking fixings should use round-headed screws to
prevent damage to cables since the thin sheet construction makes it impossible to countersink screw heads
...
Any damage to the structure of
the buildings must be made good with mortar, plaster
or concrete in order to prevent the spread of fire
...
1
...
Mini-trunking
Mini-trunking is very small PVC trunking, ideal for
surface wiring in domestic and commercial installations such as offices
...
The modern square look in
switches and sockets is complemented by the minitrunking which is very easy to install (see Fig
...
123)
...
The trunking is fitted around the walls of
a room and contains the wiring for socket outlets and
telephone points which are mounted on the lid, as
shown in Fig
...
123
...
1
...
Fig
...
124 Installation of trunking
...
Figure 1
...
PVC insulated cables are usually drawn into an
erected conduit installation or laid into an erected
trunking installation
...
Where a cable
enclosure greater than 32 mm is required because of
the number or size of the conductors, it is generally
more economical and convenient to use trunking
...
Where sizes and types of cable and trunking are not
covered by the tables in Appendix 5 of the On Site
Guide a space factor of 45% must not be exceeded
...
The tables
of Appendix 5 take this factor into account
...
See Table 1
...
Multiply the cable factor by the number of conductors to give the sum of the cable factors
...
9
...
EXAMPLE
Calculate the minimum size of trunking required to accommodate the
following single-core PVC cables:
20 ϫ 1
...
5 mm solid conductors
21 ϫ 4
...
0 mm stranded conductors
129
Table 1
...
Reproduced from the IEE
On Site Guide by kind permission of the Institution of Electrical
Engineers
TABLE 5E
Cable factors for trunking
Type of
conductor
Solid
Stranded
Conductor
cross-sectional
area (mm2)
PVC,
BS 6004
Cable factor
1
...
5
1
...
5
4
6
10
16
25
8
...
9
8
...
6
16
...
2
35
...
8
73
...
6
11
...
6
13
...
1
22
...
3
50
...
4
Note:
(i) These factors are for metal trunking and may be optimistic for plastic
trunking where the cross-sectional area available may be significantly
reduced from the nominal by the thickness of the wall material
...
Appendix 4, BS 7671
...
8, the cable factors are:
for 1
...
5 mm solid cable
for 4
...
0 mm stranded cable
–
–
–
–
8
...
9
16
...
2
The sum of the cable terms is:
(20 ϫ 8
...
9) ϩ (21 ϫ 16
...
2) ϭ
1085
...
From Table 5F shown in Table 1
...
SEGREGATION OF CIRCUITS
Where an installation comprises a mixture of lowvoltage and very low-voltage circuits such as mains
lighting and power, fire alarm and telecommunication
circuits, they must be separated or segregated to prevent electrical contact (IEE Regulation 528–01–01)
...
9 Trunking cable factors Reproduced from the IEE
On Site Guide by kind permission of the Institution of Electrical
Engineers
TABLE 5F
Factors for trunking
Dimensions of
trunking
(mm ϫ mm)
Factor
Dimensions of
trunking
(mm ϫ mm)
Factor
50 ϫ 38
50 ϫ 50
75 ϫ 25
75 ϫ 38
75 ϫ 50
75 ϫ 75
100 ϫ 25
100 ϫ 38
100 ϫ 50
100 ϫ 75
100 ϫ 100
150 ϫ 38
150 ϫ 50
150 ϫ 75
150 ϫ 100
150 ϫ 150
200 ϫ 38
200 ϫ 50
200 ϫ 75
767
1037
738
1146
1555
2371
993
1542
2091
3189
4252
2999
3091
4743
6394
9697
3082
4145
6359
200 ϫ 100
200 ϫ 150
200 ϫ 200
225 ϫ 38
225 ϫ 50
225 ϫ 75
225 ϫ 100
225 ϫ 150
225 ϫ 200
225 ϫ 225
300 ϫ 38
300 ϫ 50
300 ϫ 75
300 ϫ 100
300 ϫ 150
300 ϫ 200
300 ϫ 225
300 ϫ 300
8572
13001
17429
3474
4671
7167
9662
14652
19643
22138
4648
6251
9590
12929
19607
26285
29624
39428
Space factor – 45% with trunking thickness taken into account
Part 2 of the Regulations as follows:
Band I telephone, radio, bell, call and intruder alarm
circuits, emergency circuits for fire alarm and
emergency lighting
...
When Band I circuits are insulated to the same
voltage as Band II circuits, they may be drawn into
the same compartment
...
1
...
Multi-compartment PVC trunking cannot provide
band segregations since there is no metal screen between
the Bands
...
Fig
...
125 Segregation of cables in trunking
...
The most common finish is hot-dipped galvanized but
PVC-coated tray is also available
...
Cable tray should be adequately supported during
installation by brackets which are appropriate for the
particular installation
...
The tray is supplied in standard widths from 50 mm
to 900 mm, and a wide range of bends, tees and
reducers are available
...
126 shows a factorymade 90° bend at B
...
1
...
The installed tray should be
securely bolted with round-headed bolts where lengths
or accessories are attached, so that there is a continuous
earth path which may be bonded to an electrical earth
...
PVC/SWA CABLE INSTALLATIONS
Steel wire armoured PVC insulated cables are now
extensively used on industrial installations and often
laid on cable tray
...
The cable has a steel
wire armouring giving mechanical protection and
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
131
Fig
...
126 Cable tray with bends
...
It should be remembered that when several cables are
grouped together the current rating will be reduced
according to the correction factors given in Table 4B1 of
the IEE Regulations and Table 6C of the On Site Guide
...
The PVC insulation would be damaged if installed in ambient temperatures over 70°C
or below 0°C, but once installed the cable can operate
at low temperatures
...
MI CABLE INSTALLATIONS
Mineral insulated cables are available for general
wiring as:
■
■
light-duty MI cables for voltages up to 600 V and
sizes from 1
...
0 mm2 to 150 mm2
...
The cable sheath provides sufficient mechanical protection for all but the most severe situations,
where it may be necessary to fit a steel sheath or conduit over the cable to give extra protection, particularly near floor level in some industrial situations
...
It is not
affected by water, oil or the cutting fluids used in engineering and can withstand very high temperature or even
fire
...
These characteristics make the cable ideal for Band
I emergency circuits, boiler-houses, furnaces, petrol
stations and chemical plant installations
...
1
...
The cable can be workhardened if over-handled or over-manipulated
...
The outer sheath of the cable must not be
penetrated, otherwise moisture will enter the magnesium oxide insulation and lower its resistance
...
When bending MI cables
the radius of the bend should not cause the cable to
become damaged and clips should provide adequate
support (Regulations 522–08–03 and 04 and Tables
4A and 4E of the On Site Guide)
...
This can be achieved by using a rotary
stripper tool or, if only a few cables are to be terminated, the outer sheath can be removed with side cutters,
peeling off the cable in a similar way to peeling the skin
from a piece of fruit with a knife
...
All excess magnesium oxide powder must be wiped from the conductors
with a clean cloth
...
Cable ends must be terminated with a special seal to
prevent the entry of moisture
...
116 shows a
brass screw-on seal and gland assembly, which allows
termination of the MI cables to standard switchgear
and conduit fittings
...
Such an assembly
is suitable for working temperatures up to 105°C
...
The conductors are not identified during the manufacturing process and so it is necessary to identify them
after the ends have been sealed
...
Connection of MI cables can be made directly to
motors, but to absorb the vibrations a 360° loop should
be made in the cable just before the termination
...
Copper MI cables may develop a green incrustation or patina on the surface, even when exposed to
normal atmospheres
...
However, if the cable is exposed to an
environment which might encourage corrosion, an
MI cable with an overall PVC sheath should be used
...
Thunderstorms can, indeed, be
dangerous but the actual danger that an individual
might be exposed to whilst standing in the middle of
a field in a thunderstorm is probably less than that of
being caught in the middle of a pedestrian crossing
during a city’s ‘rush hour’
...
This compares with a one in 8000 chance of
being killed in a traffic accident
...
His famous kite experiment, flying a kite
into a thundercloud, although rather foolish, proved,
for the first time, that storm clouds generate, store
and discharge static electricity
...
A cumulo-nimbus cloud
may form in the following way
...
Humid air passing over the ground
warmed by a summer sun, receives heat from below,
expands, grows lighter and rises
...
When the humid air reaches a certain height,
known as the condensation level, the temperature falls
to the dew point, and the excess water vapour content
of the air condenses out in the form of tiny water
droplets
...
In this simple cumulus cloud, condensed-out droplets
of water are carried upwards with the rising draught
...
If sufficient heat is available from the ground,
cumulus clouds may grow both vertically and horizontally, indeed several may amalgamate and a thundercloud may be formed
...
The base area may
exceed 100 square miles and it may travel at more than
20 miles/hour
...
Mason and Maybank observed in 1960 that sometimes, if water drops of a radius greater than 0
...
As
the freezing action proceeded, the water encapsulated
in the thin shell of ice, expanded and burst the shell
...
The remainder of the drop was left with a negative charge
...
Laytham and Mason, in the next 2 years, made an
extensive study of the electrification of hailstones and
concluded that the size of the charges fully justified
the belief that here was the mechanism by which a
thundercloud charged and re-charged itself
...
Its terminals are not metallic
conductors but a diffuse collection of charge, poised
in space and not apparently connected to anything
...
In fact, the cloud
may charge up to 1000 MV although 100 MV is more
normal
...
Early in
its existence the lightning discharge current may be
200 000 A, with an average of 20 000 A, in fact, there
may be several surges in this order
...
A time soon comes when
the voltage will be insufficient to re-strike and the connection between the thundercloud and the earth will
be broken
...
Tall buildings and particularly pointed spires encourage lightning discharge
...
Compared to an area of
land the size of a football pitch, a person is a relatively
sharp point
...
These
three effects are determined by the current that is discharged into the structure
...
133
The point of strike on the lightning protective system may be raised to a high potential and, therefore,
there is a risk of flashover from the lightning protective
system to the metal of the structure
...
This can be avoided by isolation or bonding
...
Although the
amplitude of the current is high, the duration is short
and, therefore, the thermal effect is usually ignored
...
In practice the CSA of a lightning conductor
is determined by mechanical considerations
...
Another mechanical effect is due to the strong pressure wave set up between the thundercloud and the
lightning conductor
...
The pressure
wave is responsible for the lifting of roof tiles but no
protection can be provided against such effect
...
With many other structures there will be little doubt as to the need for protection
...
Other factors which can be considered for determining an overall ‘risk factor’ might be:
■
The geographical location of the structure which
pinpoints the average lightning flash density (that
134
■
■
■
■
■
■
ADVANCED ELECTRICAL INSTALLATION WORK
is the number of flashes to ground per km per year)
for which maps are available
...
This
is the projected area of the building in all directions
including the structures height
...
The intended use of the structure
...
Is it built of brick or concrete or does it have a steel frame or metal roof?
What is contained within the structure? Does it
contain valuable paintings or important telephone
exchange equipment?
The location of the structure
...
Is
the structure located in generally flat terrain or
mountainous region?
It is acknowledged that not all factors can be assessed
logically
...
The final decision can only
be taken after consultation with the specialist provider,
the architect and the client
...
Air termination or Air termination network – The
part of a lightning protective system intended to
intercept lightning discharges
...
Earth termination – That part of a lightning protective system that is intended to discharge lightning
currents into the general mass of earth
...
)
Earth electrode – That part of the earth terminal
that makes direct electrical contact with the earth
...
Joint – A mechanical and electrical junction
between two or more portions of the lightning protective system
...
Abbreviation
H
...
ϭ horizontal conductor
V
...
ϭ vertical conductor
Z
...
ϭ zone of protection (boundary indicated by
dotted lines)
Zone of protection
This term denotes the space within which a lightning
conductor provides protection against a direct lightning strike by diverting the strike to itself
...
127
...
They may consist of a vertical conductor as for a spire, a single horizontal
conductor as on the ridge of a small building, or a system of horizontal conductors for the protection of
roofs of large dimensions
...
No part of the roof should be more than 10 m from
the nearest horizontal protective conductor (BS 6651:
Section 3)
...
on or above the main surface of the roof structure should be bonded to and,
therefore, form part of the air termination network
...
Down conductors Should be made of aluminium
or copper and the number of down conductors should
be decided as follows:
■
Structures with a base are not exceeding 100 m –
one down conductor
...
1
...
■
Structures with a base area greater than 100 m –
either
(a) one, plus one for every 300 m in excess of the
first 100 m or,
(b) one for every 30 m of perimeter – whichever is
the smaller number
...
128 and it is very unwise to bend
the rods to conform with architectural fancies
...
The
Not
permissible
Permissible
discharging current may take short cuts – seriously
damaging masonry and endangering people’s lives
...
The lightning protective system should have as few
joints as possible, but where necessary, these should be
mechanically and electrically effective
...
Every down conductor should be provided with a
testing joint in such a position that, whilst not inviting
unauthorized interference, is convenient for use when
testing
...
Each earth terminal should have a resistance to earth not exceeding
10 ⍀ ϫ the number of earth terminals
...
For example, if a system has four down conductors
the maximum value for each earth terminal will be 40 ⍀
...
1
...
1
4
ϭ
RT
40
І RT ϭ
40
ϭ 10 ⍀
4
136
ADVANCED ELECTRICAL INSTALLATION WORK
If the value of the whole system is greater than 10 ⍀
a reduction can be achieved by adding to the earth
electrodes or by interconnecting the individual earth
terminations below ground
...
As the lightning current is discharged through the
earth electrode, the surrounding soil is raised for the
duration of the discharge to a potential with respect
to the body of the earth (similar to the ripples radiating from a pebble thrown into a pond)
...
Earth electrodes Should be made from metal
rods or strips, or a combination of both
...
The length of
the earth electrode is dependent on soil conditions
...
Water and other service pipes are not recommended
as a primary earthing connection but they may be considered as secondary terminations and in any event
should be bonded together and to the lightning protective system
...
Inspection by a competent person must be made at
least once a year and the following tests carried out:
■
■
■
Resistance to earth
Electrical continuity
Visual inspection of bonds and joints
...
Little maintenance should be necessary but inspection
and testing will indicate any maintenance required
...
Examples are changes in the use of the
building, the installation of fuel storage tanks or the erection of radio or television or communication masts
...
J
...
Ltd
...
(0115) 9643700 or www
...
com
...
Exercises
1 To avoid back injuries when manually lifting heavy
weights from ground level a worker should:
(a) bend both legs and back
(b) bend legs but keep back straight
(c) keep legs straight but bend back
(d) keep both legs and back straight
...
These are:
(a) fuel, wood, cardboard
(b) petrol, oxygen, bottled gas
(c) flames, fuel, heat
(d) fuel, oxygen, heat
...
4 The recommended voltage for portable hand tools
on construction sites is:
(a) 50 V
(b) 110 V
(c) 230 V
(d) 400 V
...
6 Making quality a way of life in a company is one
definition of:
(a) quality
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
7
8
9
10
11
12
13
14
(b) quality control
(c) quality assurance
(d) total quality management
...
Post-production inspection of goods is one definition of:
(a) quality
(b) quality control
(c) quality assurance
(d) total quality management
...
3 A
(c) 23 A
(d) 230 A
...
88 ⍀
(b) 5 ⍀
(c) 12
...
16 ⍀
...
The voltage across the lamp
under these conditions was:
(a) 6 V
(b) 12 V
(c) 24 V
(d) 240 V
...
5 ϫ10Ϫ9 ⍀m will be:
(a) 1
...
75 ⍀
(c) 17
...
5 k⍀
...
Find its new resistance at
20°C if the temperature coefficient of the winding
is 0
...
(a) 116
...
004 ⍀
(c) 121
...
004 ⍀
...
15
16
17
18
19
20
21
137
If the temperature coefficient of resistance is
0
...
A capacitor is charged by a steady current of 5 mA
for 10 seconds
...
When 100 V was connected to a 20 F capacitor
the charge stored was:
(a) 2 mC
(b) 5 mC
(c) 20 mC
(d) 100 mC
...
An electrolytic capacitor:
(a) is used for power-factor correction in
fluorescents
(b) is used for tuning circuits
(c) must only be connected to the correct polarity
(d) has a small capacitance for a large physical size
...
A current flowing through a solenoid sets up a magnetic flux
...
Resistors of 6 and 3 ⍀ are connected in series
...
6 ⍀
138
22
23
24
25
26
27
28
ADVANCED ELECTRICAL INSTALLATION WORK
(c) 6
...
Resistors of 3 and 6 ⍀ are connected in parallel
...
6 ⍀
(c) 6
...
Three resistors of 24, 40 and 60 ⍀ are connected
in series
...
4 ⍀
(c) 44 ⍀
(d) 124 ⍀
...
The effective resistance of this combination will be:
(a) 12 ⍀
(b) 26
...
Two identical resistors are connected in series
across a 12 V battery
...
Two identical resistors are connected in parallel
across a 24 V battery
...
A 6 ⍀ resistor is connected in series with a 12 ⍀
resistor across a 36 V supply
...
A 6 ⍀ resistor is connected in parallel with a 12 ⍀
resistor across a 36 V supply
...
29 The total power dissipated by a 6 ⍀ and 12 ⍀
resistor connected in parallel across a 36 V supply
will be:
(a) 72 W
(b) 324 W
(c) 576 W
(d) 648 W
...
If another resistor of 10 ⍀ is connected in series with the three series resistors the
current carried by this resistor will be:
(a) 4 A
(b) 5 A
(c) 10 A
(d) 100 A
...
7 V
(b) 70
...
67 V
...
37 A
(b) 7
...
67 A
...
The equivalent capacitance will be:
(a) 12 F
(b) 44 F
(c) 76 F
(d) 124 F
...
The total capacitance will be:
(a) 12 F
(b) 44 F
(c) 76 F
(d) 124 F
...
36 The transformation ratio of a step-down transformer is 20:1
...
3 V
HEALTH AND SAFETY AND ELECTRICAL PRINCIPLES
37
38
39
40
41
42
43
(b) 11
...
Before an ammeter can be removed from the
secondary terminals of a current transformer connected to a load, the transformer terminals must be:
(a) open-circuited
(b) short-circuited
(c) connected to the primary winding
(d) connected to earth
...
c
...
The impedance of this
circuit will be:
(a) 5 ⍀
(b) 7 ⍀
(c) 12 ⍀
(d) 25 ⍀
...
c
...
The impedance of
this circuit will be:
(a) 3 ⍀
(b) 15 ⍀
(c) 20 ⍀
(d) 108 ⍀
...
83 ⍀ will have a p
...
of:
(a) 0
...
600
(c) 0
...
666
...
434 ⍀
(b) 21
...
853 ⍀
(d) 32
...
The inductive reactance of a 100 mH coil when
connected to 50 Hz will be:
(a) 0
...
0318 ⍀
(c) 5
...
416 ⍀
...
5 ⍀
(b) 5
...
83 ⍀
(d) 31415
...
139
44 In a series resonant circuit the:
(a) current and impedance are equal
(b) current is at a minimum and the impedance
a maximum
(c) current is at a maximum and the impedance
a minimum
(d) current and impedance are at a maximum
...
46 A circuit containing a 100 F capacitor in series
with a 100 mH inductor will resonate at a frequency of:
(a) 3
...
33 Hz
(c) 316
...
47 A series circuit consisting of a 25
...
53 Hz
(b) 79
...
48 A capacitor is connected across the supply at a fluorescent light fitting to:
(a) increase the voltage
(b) increase the current
(c) suppress radio interference
(d) improve the power factor
...
The equivalent capacitance will be:
(a) 12 F
(b) 44 F
(c) 76 F
(d) 124 F
...
The total capacitance will be:
(a) 12 F
(b) 44 F
(c) 76 F
(d) 124 F
...
c
...
If V1 leads V2 by 45° the
140
ADVANCED ELECTRICAL INSTALLATION WORK
resultant voltage will be:
(a) 16 V at 24°
(b) 45 V at 90°
(c) 46 V at 18°
(d) 50 V at 45°
...
If I1
leads I2 by 90° the resultant current will be:
(a) 3 A at 21°
(b) 10
...
53 The phasor diagram which shows V and I at
a power factor of unity is:
54 The phasor diagram for a lagging p
...
is given by:
55 The phasor diagram for a purely capacitive circuit
is given by:
56 The transmission of electricity is, for the most
part, by overhead conductors suspended on steel
towers because:
(a) this is environmentally more acceptable than
running cables underground
(b) this is very many times cheaper than running
an equivalent cable underground
(c) high-voltage electricity cables cannot be
buried on agricultural land
(d) more power can be carried by overhead conductors than is possible by an underground
cable
...
58 A TN-S system of electrical supply:
(a) uses a separate neutral and protective conductor throughout the system
(b) uses a combined protective and neutral
conductor
(c) does not provide the consumer with an earth
terminal and the installation’s circuit protective conductors must be connected to earth by
an earth electrode provided by the consumer
(d) is isolated from earth
...
60 An off-peak supply is available to a consumer:
(a) for water heating purposes only
(b) during the day time only
(c) during the night time only
(d) at all times provided that peak demand does
not exceed 4 hours
...
One advantage of a steel conduit installation,
compared with a PVC conduit installation, is
that it:
(a) may be easily rewired
(b) may be installed more quickly
(c) offers greater mechanical protection
(d) may hold more conductors for a given conduit size
...
The earth continuity of a metallic trunking installation may be improved if:
(a) copper earth straps are fitted across all joints
(b) galvanized trunking is used
(c) all joints are painted with galvanized paint
(d) a space factor of 45% is not exceeded
...
An industrial installation of PVC/SWA cables laid
on cable tray offers the advantage over other types
of installation of:
(a) greater mechanical protection
(b) greater flexibility in response to changing
requirements
(c) higher resistance to corrosion in an industrial
atmosphere
(d) flameproof installation suitable for hazardous
areas
...
68 A coil is made up of 30 m of conductor and is laid
within and at right angles to a magnetic field of
4 T
...
875 N
(b) 30 N
(c) 240 N
(d) 600 N
...
The
force exerted on this coil when 15 mA flows in the
coil conductors will be:
(a) 0
...
47 N
(c) 0
...
875 N
...
71 A four-pole induction motor running at 1425 rpm
has a percentage slip of:
(a) 2%
(b) 5%
(c) 52
...
72 A laminated cylinder of silicon steel with copper
or aluminium bars slotted into holes around the
circumference and short-circuited at each end of
the cylinder, is one description of:
(a) a cage rotor
(b) an electromagnet
(c) a linear motor
(d) an induction motor
...
37 kW
must be supplied with:
(a) protection by MCB
(b) protection by HBC fuses
(c) a motor starter
(d) remote stop/start switches
...
75 An auto-transformer starter:
(a) increases the initial starting torque of the motor
(b) increases the initial starting current of the motor
(c) gives direct connection of the mains voltage
to the motor during starting
(d) requires only three connecting conductors
between the motor and starter
...
77 A series d
...
motor has the characteristic of:
(a) constant speed about 5% below synchronous
speed
(b) start winding 90° out of phase with the run
winding
(c) low starting torque but almost constant speed
(d) high starting torque and a speed which varies
with load
...
79 A three-phase induction motor has the characteristic of:
(a) constant speed about 5% below synchronous
speed
(b) start winding 90° out of phase with the run
winding
(c) high starting torque but almost constant speed
(d) high starting torque and a speed which varies
with load
...
One advantage of all d
...
machines is:
(a) that they are almost indestructible
(b) that starters are never required
(c) that they may be operated on a
...
or d
...
supplies
(d) the ease with which speed may be controlled
...
c
...
c
...
One advantage of a series d
...
motor is that:
(a) it is almost indestructible
(b) starters are never required
(c) it may be operated on a
...
or d
...
supplies
(d) speed is constant at all loads
...
A d
...
shunt motor would normally be used for a:
(a) domestic oven fan motor
(b) portable electric drill motor
(c) constant speed lathe motor
(d) record turntable drive motor
...
c
...
A 7
...
f
...
866 is connected
to a 400 V 50 Hz supply
...
f
...
Four 1 mm cables and four 2
...
The distance
between the boxes is 8 m
...
Determine the minimum size of trunking
required to contain the following stranded cables:
(a) 20 ϫ 1
...
5 mm cables
(c) 10 ϫ 4
...
0 mm cables
...
0 mm cables which may
be drawn into a 5 m straight run of 20 mm conduit
...
5 mm cables which
may be drawn into a 20 mm plastic conduit along
with a 4
...
Determine the size of galvanized steel conduit
required to contain PVC insulated conductors if
the distance between two boxes is 5 m and the
conduit has two bends of 90°
...
5 mm cables and four 2
...
Calculate the number of PVC insulated 4
...
(a) Calculate the minimum size of vertical
trunking required to contain 20 ϫ 10 mm
PVC insulated cables
...
(c) Explain how and why cables are supported
in vertical trunking
...
c
...
Calculate
(a) the impedance
(b) the current
(c) the voltage across each component
(d) the power factor
...
A 9 ⍀ resistor is connected in series with a capacitor of 265
...
Calculate
(a) the impedance
(b) the current
(c) the voltage across each component
(d) the power factor
...
A 15 ⍀ resistor is connected in series with a 60 F
capacitor and a 0
...
Draw the circuit diagram and
calculate
(a) the impedance
(b) the current
(c) the voltage across each component
(d) the power factor
...
143
98 A pure inductor of 100 mH is connected in parallel with a 15 ⍀ resistor across a 230 V 50 Hz
supply
...
Sketch the
phasor diagram
...
Calculate the current in each branch and the total current and power factor
...
100 State the responsibilities under the Health and
Safety at Work Act of:
(a) an employer to his employees
(b) an employee to his employer and fellow
workers
...
Describe
the purpose of the four categories of signs and
state their colour code and shape
...
102 A trainee electrician discovers a large wellestablished fire in a store-room of the building
in which he is working
...
There are six offices on each of six
floors and the store-room and fire are in an office
on the fourth floor of the building
...
Describe the actions which the trainee should
take to prevent this emergency becoming a
disaster
...
104 Briefly identify the main difference between the
Electricity at Work Regulations 1989 and the
IEE Regulations, 16th edition (BS 7671)
...
106 Define ‘isolation’ with respect to an electrical
circuit or item of equipment
...
Start from the point at which
you choose the voltage indicating device and finish with the point at which you begin to work on
the circuit
...
109 Write down 10 short bullet points that you
think would lead to good customer relations
...
Give
an example from your own experience
...
111 Define what we mean by ‘hazard’ and ‘risk’
...
113 How does the work of the HSE Inspector and the
Environmental Health Officer differ from the
work of the Safety Officer and Safety Representative in a Company?
114 Very briefly describe any two of the Laws which
protect people’s legal rights
...
116 Very briefly describe any two Regulations which
protect the working environment
...
Show how the
supply can be secured to all four load centres
even though a fault occurs on a cable between
two of the distribution substations
...
119 Use a simple circuit diagram to show how a 230
and 400 V consumer may be connected to the
supply company’s TN-S system
...
120 Using a circuit diagram which shows the secondary winding of the supply transformer, show
how a 230 and 400 V supply are connected to a
TN-C-S system
...
Clearly explain
how an effective earth is obtained for the installation and the possible dangers in not providing
an adequate earth connection
...
123 A 400 V three-phase and neutral busbar rising
main is to be used to provide a 230 V supply to
each of eight individual flats on four floors of a
building
...
(b) Describe the method used to prevent the
spread of fire
...
125 Describe with sketches the meaning of the terms
frequency and period as applied to an a
...
wave-form
...
127 Sketch a graph to show the variation of R, XL,
XC, Z and I with frequency for a series a
...
circuit
...
128 Describe, with the aid of a circuit diagram, how
speed control may be achieved with:
(a) a d
...
series motor
(b) a d
...
shunt motor
...
c
...
130 With the aid of a sketch describe the construction of a cage rotor
...
131 With the aid of a sketch describe how the turning forces are established which rotate the cage
rotor of an induction motor
...
133 Describe, with sketches, the construction of
a wound rotor and cage rotor
...
134 The Regulations require that motor starters
incorporate overload protection and no-volt
protection
...
135 Sketch a three-phase direct on line motor starter
and describe its operation
...
137 Sketch the wiring diagram for a star delta motor
starter and describe its operation
...
139 Sketch the wiring diagram for a rotor resistance
motor starter and describe its operation
...
141 Use a block diagram to explain the sequence of
control for an electric motor of about 5 kW
...
143 Describe three types of motor enclosure and
state one typical application for each type
...
Describe two methods of
correcting the bad power factor due to a number
of industrial motors
...
146 Describe the construction and use of a voltage
transformer
...
148 Draw a circuit diagram to show an ammeter and
current transformer connected to measure the current in a single-phase a
...
circuit
...
149 Draw a circuit diagram showing a voltmeter and
voltage transformer connected to measure the
voltage in a single-phase load
...
151 Describe what is meant by a tap changing
transformer
...
153 Draw a metallic trunking 90° bend of the type
which
(a) was supplied by a manufacturer and
(b) was made on site
...
155 Describe the following parts of a lightning protective system
(a) The air termination
(b) The down conductor
(c) The earth electrode
(d) The zone of protection
...
This is a multi-million-pound industry carrying out all types of building work, from basic housing
to hotels, factories, schools, shops, offices and airports
...
Although a major employer, the construction industry is also very fragmented
...
Of the total workforce of the construction industry,
92% are employed in small firms of less than 25 people
...
Of this total sum, about 60%
is spent on new building projects and the remaining
40% on maintenance, renovation or restoration of
mostly housing
...
The building team
The construction of a new building is a complex process
which requires a team of professionals working
together to produce the desired results
...
2
...
The client is the person or group of people with the
actual need for the building, such as a new house,
office or factory
...
The architect is the client’s agent and is considered
to be the leader of the building team
...
During the building process the
architect will supervise all aspects of the work until
the building is handed over to the client
...
Specialist engineers advise the architect during the
design stage
...
The clerk of works is the architect’s ‘on-site’ representative
...
They can also agree general
matters directly with the building contractor as the
architect’s representative
...
The health and safety inspectors will ensure that the
government’s legislation concerning health and safety
is fully implemented by the building contractor
...
2
...
accordance with contract documents
...
The electrical team
The electrical contractor is the subcontractor responsible for the installation of electrical equipment
within the building
...
There is often no clear distinction between the duties of the individuals, and the
responsibilities carried by an employee will vary from
one employer to another
...
Good customer relationships are important for the success of the firm and
the continuing employment of the employee
...
They
should always be polite and seen to capable and
in command of the situation
...
The electrician and his trainee should be appropriately
dressed for the job in hand, which probably means an
overall of some kind
...
For example, if working in a factory, the
safety regulations may insist that protective footwear be
worn, but rubber boots may be most appropriate for a
building site
...
It makes sound sense
to help other trades where possible and to develop
good working relationships with other employees
...
The rates
of pay and conditions for electricians and trainees are
determined by negotiation between the Joint Industry
Board and the Amalgamated Engineering and Electrical
148
ADVANCED ELECTRICAL INSTALLATION WORK
Union, which will also represent their members in any
disputes
...
The electrical team will consist of a group of professionals and their interrelationship can be expressed
as shown in Fig
...
2
...
The designer may be a professional technician or engineer whose job it is to design electrical
installations for a large contracting firm
...
The designer of any electrical installation is the
person who interprets the electrical requirements of
the customer within the regulations, identifies the
appropriate types of installation, the most suitable
methods of protection and control and the size of
cables to be used
...
The designer can then identify the general
characteristics of the electrical installation and its compatibility with other services and equipment, as indicated in Part 3 of the Regulations
...
An assessment of the frequency and quality of the maintenance to be expected (Regulation
341–01–01) will give an indication of the type of
installation which is most appropriate
...
This is called a ‘bill of quantities’
...
The contractor
must cost all the materials, assess the labour cost
required to install the materials and add on profit and
overhead costs in order to arrive at a final estimate for
the work
...
To complete the contract in the specified time the
electrical contractor must use the management skills
required by any business to ensure that men and
materials are on site as and when they are required
...
The specification for the chosen wiring system will
be largely determined by the building construction
and the activities to be carried out in the completed
building
...
This can be achieved by a
conduit, tray or trunking installation
...
A loop-in conduit system, in which the only connections are at the
light switch and outlet positions, would meet this
requirement
...
The final choice of a wiring system must rest with
those designing the installation and those ordering
the work, but whatever system is employed, good
workmanship is essential for compliance with the regulations
...
Legal contracts
Before work commences, some form of legal contract
should be agreed between the two parties, that is,
those providing the work (e
...
the subcontracting
FLASH-BANG
ELECTRICAL CO
...
2
...
149
150
ADVANCED ELECTRICAL INSTALLATION WORK
electrical company) and those asking for the work to
be carried out (e
...
the main building company)
...
A standard form of building contract typically contains four
sections:
1 The articles of agreement – this names the parties,
the proposed building and the date of the contract
period
...
g
...
3 The appendix – this contains details of costings,
e
...
the rate to be paid for extras as daywork, who
will be responsible for defects, how much of the
contract tender will be retained upon completion
and for how long
...
In signing the contract, the electrical contractor has
agreed to carry out the work to the appropriate standards in the time stated and for the agreed cost
...
If a dispute arises the contract provides written evidence of what was agreed and will form the basis for a
solution
...
Management systems
Smaller electrical contracting firms will know where
their employees are working and what they are doing
from day to day because of the level of personal contact between the employer, employee and customer
...
When the electrical contractor is told that he has
been successful in tendering for a particular contract
he is committed to carrying out the necessary work
within the contract period
...
In thinking ahead and planning the best method of
completing the contract, the individual activities or
jobs must be identified and consideration given to
how the various jobs are interrelated
...
In this chapter we will consider only two:
bar charts and network analysis
...
BAR CHARTS
There are many different types of bar chart used by
industry, but the object of any bar chart is to establish
the sequence and timing of the various activities
involved in the contract as a whole
...
In order to be
useful they must be clearly understood by the people
involved in the management of a contract
...
2
...
All the individual jobs or activities which make up
the contract are identified and listed separately down
the vertical axis on the left-hand side, and time flows
from left to right along the horizontal axis
...
The simple bar chart shown in Fig
...
3(a) shows a
particular activity A which is estimated to last 2 days,
INSPECTION, TESTING AND COMMISSIONING
Fig
...
3 Bar charts
...
Activity C lasts 4 days
and should be started on day 3
...
With the aid of colours, codes, symbols and a little
imagination, much additional information can be
included on this basic chart
...
2
...
Activity B in Fig
...
3(b) is 2 days behind schedule, while activity D is 2
days ahead of schedule
...
Some activities must be completed before others
can start
...
This is shown in Fig
...
3(b) by activities J and K
...
Useful and informative as the bar chart is, there is
one aspect of the contract which it cannot display
...
To overcome this limitation,
in 1959 the CEGB developed the critical path network diagram which we will now consider
...
This path denotes
the time required for completion of the whole
contract
...
Float time ϭ Critical path time Ϫ Activity time
The total float time for any activity is the total leeway available for all activities in the particular path of
activities in which it appears
...
Activities
Activities are represented by an arrow, the tail of
which indicates the commencement, and the head the
completion of the activity
...
Activities require time, manpower
and facilities
...
Dummy activities
NETWORK ANALYSIS
In large or complex contracts there are a large number
of separate jobs or activities to be performed
...
A network
diagram can be used to co-ordinate all the interrelated
activities of the most complex project in such a way
that all sequential relationships between the various
activities, and the restraints imposed by one job on
another, are allowed for
...
Before considering the method of constructing a network diagram, let us define some of the
terms and conventions we shall be using
...
They signify a logical link only, require
no time and denote no specific action or work
...
Each activity begins and ends in an event
...
Time may be recorded to a horizontal scale or shown
on the activity arrows
...
2
...
INSPECTION, TESTING AND COMMISSIONING
153
EXAMPLE 2
Identify the time taken to complete each activity in the network diagram
shown in Fig
...
5
...
Fig
...
4 A network diagram for Example 1
...
2
...
Identify the
critical path and the float time in each path
...
Fig
...
5 A network diagram for Example 2
...
The longest time from the start event to the finish event is 24 hours, and
therefore the critical path is A–B–D–F
...
On the other two paths some delay
could occur without affecting the overall contract time
...
This means that in this case once the
activities preceding events D and F have been completed, the contract
will not be held up by work associated with these particular paths and
they will progress naturally to the finish event
...
The times taken to complete the activities in each of the three paths are:
path 1, A–B–E–G ϭ 2 ϩ 5 ϩ 3 ϭ 10 days
path 2, A–D–G ϭ 5 ϩ 0 ϭ 5 days
path 3, A–C–F–G ϭ 3 ϩ 5 ϩ 0 ϭ 8 days
...
154
ADVANCED ELECTRICAL INSTALLATION WORK
Constructing a network
The first step in constructing a network diagram is to
identify and draw up a list of all the individual jobs, or
activities, which require time for their completion and
which must be completed to advance the contract from
start to completion
...
The
designer of the network must ask these questions:
(a) Produce a simple bar chart to display the activities of this particular
contract
...
(c) Identify the critical path and the total contract time
...
(e) State the float time in activity F
...
2 Which activities can proceed at the same time? These
can be drawn in a similar way to parallel circuits but
with event circles instead of resistor symbols
...
A number of attempts may be
necessary to achieve a well-balanced and symmetrical
network diagram showing the best possible flow of
work and information, but this time is well spent
when it produces a diagram which can be easily
understood by those involved in the management of
the particular contract
...
Certain constraints are placed on some activities because of the availability of men and materials and because some work must be completed
before other work can commence as follows:
Activity C can only commence when B is completed
...
Activity E can only commence when A is completed
...
Fig
...
6 (a) Bar chart and (b) network diagram for Example 3
...
2
...
(b) The network diagram is shown in Fig
...
6(b)
...
Event 2 ϭ the completion of activity A and the commencement of
activity E
...
Event 4 ϭ the completion of activity F
...
Event 6 ϭ the completion of activity C
...
INSPECTION, TESTING AND COMMISSIONING
(c) There are three possible paths:
1 via events 1–2–5–7
2 via events 1–4–7
3 via events 1–3–6–7
...
The critical path is therefore path 3, via events 1–3–6–7, and the total
contract time is 10 weeks
...
Float time ϭ 10 weeks Ϫ 8 weeks ϭ 2 weeks
...
(e) Activity F is on path 2 via events 1–4–7 having a total activity time
of 4 weeks
...
On-site communications
Good communication is about transferring information from one person to another
...
DRAWINGS AND DIAGRAMS
Many different types of electrical drawing and diagram
can be identified: layout, schematic, block, wiring and
circuit diagrams
...
Layout drawings
These are scale drawings based upon the architect’s
site plan of the building and show the positions of
155
the electrical equipment which is to be installed
...
The standard symbols used by the electrical contracting industry are those recommended by the
British Standard BS EN 60617, Graphical Symbols
for Electrical Power, Telecommunications and Electronic
Diagrams
...
2
...
A layout drawing is shown in Fig
...
8 of a small
domestic extension
...
The bathroom
contains one lighting point controlled by a one-way
switch at the door
...
There are also three double sockets
situated around the kitchen
...
Two wall lights with built in switches
are to be wired, one at each side of the window
...
The bedroom has two
lighting points controlled independently by two
oneway switches at the door
...
As-fitted drawings
When the installation is completed a set of drawings
should be produced which indicate the final positions of
all the electrical equipment
...
The layout drawings indicate the original intentions for the positions of equipment, while the ‘as-fitted’ drawing indicates the actual
positions of equipment upon completion of the job
...
For example, a
drawing might be produced to give a fuller description of the suspended ceiling arrangements
...
2
...
Schematic diagrams
A schematic diagram is a diagram in outline of, for
example, a motor starter circuit
...
These help us to understand the
working operation of the circuit
...
A mechanical schematic diagram
gives a more complex description of the individual
elements in the system, indicating, for example, acceleration, velocity, position, force sensing and viscous
damping
...
2
...
Block diagrams
A block diagram is a very simple diagram in which the
various items or pieces of equipment are represented
by a square or rectangular box
...
Figure 2
...
Wiring diagrams
A wiring diagram or connection diagram shows the
detailed connections between components or items
of equipment
...
2
...
Fig
...
10 Wiring diagram of two-way switch control
...
The purpose of a wiring
diagram is to help someone with the actual wiring of
the circuit
...
10 shows the wiring diagram for
a two-way lighting circuit
...
It will be laid out as clearly as possible, without regard to the physical layout of the actual components, and therefore it may not indicate the most
convenient way to wire the circuit
...
11 shows
the circuit diagram of an n-p-n transistor test circuit
...
All the essential parts and connections are
represented by their graphical symbols
...
The advantage of a telephone message over a
INSPECTION, TESTING AND COMMISSIONING
159
FLASH-BANG
ELECTRICAL
Date
TELEPHONE
MESSAGES
Thurs 11 Aug
...
30
Dave Twem
Message from (Name)
(Address)
John Gall
Bispham Site
Blackpool
...
2
...
(Telephone No
...
Therefore, business agreements made on the telephone
are often followed up by written confirmation
...
Always be polite and helpful when answering your
company’s telephone – you are your company’s most
important representative at that moment
...
Always read
back what you have written down to make sure that
you are passing on what the caller intended
...
2
...
In this case John Gall wants Dave Twem to pick up
the Megger from Jim on Saturday and take it to the
Bispham site on Monday
...
When making a telephone call, make sure you
know what you want to say or ask
...
WRITTEN MESSAGES
A lot of communications between and within larger
organizations take place by completing standard forms
or sending internal memos
...
An auditor can follow the paperwork trail to see, for example, who was
responsible for ordering certain materials
...
Thanks
Message taken by
Dave Low
Fig
...
12 Typical standard telephone message pad
...
Do
not leave blank spaces on the form, always specifying
‘not applicable’ or ‘N/A’ whenever necessary
...
Finally, read through the form again to make sure you
have answered all the relevant sections correctly
...
Figure 2
...
Letters provide a permanent record of communications between organizations and individuals
...
A letter should be written using simple concise language, and the tone of the letter should always
be polite even if it is one of complaint
...
The greeting on a
160
ADVANCED ELECTRICAL INSTALLATION WORK
FLASH-BANG
ELECTRICAL
From
Subject
Dave Twem
Power Tool
internal
MEMO
John Gall
To
Date
Thurs 11 Aug
...
S
...
Dave
...
2
...
formal letter should be ‘Dear Sir/Madam’ and concluded with ‘Yours faithfully’
...
Your name and status should be typed below
your signature
...
This record is used to confirm that
goods have been delivered by the supplier, who will
then send out an invoice requesting payment, usually
at the end of the month
...
Any missing or damaged
items must be clearly indicated on the delivery note
before signing, because, by signing the delivery note
the person is saying ‘yes, these items were delivered to
me as my company’s representative on that date and in
good condition and I am now responsible for these
goods’
...
The person receiving the
goods should try to quickly determine their condition
...
Electrical goods delivered to site should be handled
carefully and stored securely until they are installed
...
TIME SHEETS
A time sheet is a standard form completed by each
employee to inform the employer of the actual time
spent working on a particular contract or site
...
It is usually a weekly document and
includes the number of hours worked, the name of
the job and any travelling expenses claimed
...
14 so that wages can be made up
...
15
carries information about a job which needs to be
done, usually a small job
...
A typical job sheet work
description might be:
■
■
Job 1 Upstairs lights not working
Job 2 Funny fishy smell from kettle socket in
kitchen
...
2
...
Date
Travel
time
Expenses
162
ADVANCED ELECTRICAL INSTALLATION WORK
JOB SHEET
Job Number
FLASH-BANG
ELECTRICAL
Customer name
Address of job
Contact telephone number
Work to be carried out
Any special instructions/conditions/materials used
Fig
...
15 Typical job sheet
...
Job 1, for example, might be the result of a blown
fuse which is easily rectified, but the electrician must
search a little further for the fault which caused the fuse
to blow in the first place
...
The pendant
drop would be re-flexed or replaced, along with any
others in poor condition
...
General information and assurances about the condition of the installation as a whole might be requested
and given before setting off to job 2
...
A visual inspection would
confirm the diagnosis
...
Check the kettle plug
top for damage and loose connections
...
The time spent on each job and the materials used
are sometimes recorded on the job sheet, but alternatively a daywork sheet can be used
...
This information can then be used to
‘bill’ the customer for work carried out
...
If daywork is to be carried out,
the site supervisor must first obtain a signature from
the client’s representative, for example, the architect, to
authorize the extra work
...
A typical daywork sheet is shown in
Fig
...
16
...
Date
Labour
Start time
Materials quantity
Finish time
Total hours
Description
Office use
Office use
Site supervisor or Flash-Bang electrical representative responsible for carrying out work
Signature of person approving work and status e
...
Client
Architect
Signature
Fig
...
16 Typical daywork sheet
...
S
...
Some firms have two separate documents, a
site diary to record daily events and a weekly report
which is a summary of the week’s events extracted
from the site diary
...
PERSONAL COMMUNICATIONS
Remember that it is the customers who actually pay
the wages of everyone employed in your company
...
They may be elderly or of a different religion or cultural background than you
...
Treat the property in which you
are working with the utmost care
...
Clean up periodically
and made a special effort when the job is completed
...
The electrical installation in a building is often carried out alongside other trades
...
The customer will be most happy if the workers give an
impression of working together as a team for the successful completion of the project
...
You are, therefore,
your company’s most important representative
...
However,
if a problem does occur which is outside your previous experience and you do not feel confident to solve
it successfully, then contact your supervisor for professional help and guidance
...
This approach would be preferred by most
companies rather than having to meet the cost of an
expensive blunder
...
We looked
at Safety Signs and PPE, and how to recognise and use
different types of fire extinguishers
...
If your career in the electrotechnical industry is to
be a long, happy and safe one, you must always wear
appropriate personal protective equipment such as
footwear, and head protection and behave responsibly
and sensibly in order to maintain a safe working environment
...
What is going to be hazardous, will you require
PPE, do you need any special access equipment
...
You do not necessarily have to do these things
formally, such as carrying out a risk assessment as
described in Chapter 1, but just get into the habit of
always working safely and being aware of the potential hazards around you when you are working
...
(Various wiring systems were discussed in Chapter 1
under the sub-heading Industrial Wiring Systems)
...
For example, where conduits and trunking are
run through walls and floors
...
All conductors
INSPECTION, TESTING AND COMMISSIONING
must be chosen so that they will carry the design current under the installed conditions
...
For
example, where galvanised conduit trunking or tray is
cut or damaged by pipe vices, it must be made good
to prevent localised corrosion
...
Cutting
tools should be sharpened and screwdrivers ground to
a sharp square end on a grindstone
...
Damaged
plug tops and cables must be repaired before you use
them
...
Tools and equipment that are left lying about in
the workplace can become damaged or stolen and
may also be the cause of people slipping, tripping or
falling
...
You personally may have no control over
the condition of the workplace in general, but keeping your own work area clean and tidy is the mark of
a skilled and conscientious craftsman
...
ELECTRICAL SAFETY SYSTEMS
The provision of protective devices in an electrical
installation is fundamental to the whole concept of the
safe use of electricity in buildings
...
The installation and maintenance of
adequate and appropriate protective measures is a vital
part of the safe use of electrical energy
...
165
Let us first define some of the words we will be
using
...
It gives the following definitions:
Earth – the conductive mass of the earth
...
Circuit Protective Conductor (CPC) – a protective conductor connecting exposed conductive parts of equipment to the main earthing terminal
...
Exposed Conductive Parts – this is the metalwork of
an electrical appliance or the trunking and conduit
of an electrical system which can be touched because
they are not normally live, but which may become
live under fault conditions
...
They do not form
a part of the electrical installation but may introduce
a potential, generally earth potential, to the electrical
installation
...
Very tiny electrical signals travel
between the central nervous system and the muscles,
stimulating operation of the muscles, which enable us
to walk, talk and run and remember that the heart is
also a muscle
...
The shock current causes unnatural operation of the muscles and the result may be
that the person is unable to release the live conductor
causing the shock, or the person may be thrown across
the room
...
This leads to the consideration of exceptional precautions where people with wet skin or wet surfaces
are involved, and the need for special consideration in
bathroom installations
...
Direct contact with live parts
which involves touching a terminal or phase conductor
166
ADVANCED ELECTRICAL INSTALLATION WORK
40
30
20
10
8
6
4
DANGER
ZONE
3
Time (seconds)
2
1
...
6
...
4
...
2
...
1
...
06
...
04
...
02
15 ms
...
2
...
150
200
300
230 V
500
400 V
INSPECTION, TESTING AND COMMISSIONING
that is actually live
...
The touch voltage curve in Fig
...
17 shows that a
person in contact with 230 V must be released from
this danger in 40 ms if harmful effects are to be avoided
...
In installations operating at normal mains voltage,
the primary method of protections against direct contact is by insulation
...
The insulating material must,
of course, be suitable for the circumstances in which
they will be used and the stresses to which they will be
subjected
...
Protection can also be provided by
fixed obstacles such as a guard rail around an open
switchboard or by placing live parts out of reach as
with overhead lines
...
There are five methods of protection against contact
with metalwork which has become unintentionally
live, that is, indirect contact with exposed conductive
parts recognized by the IEE Regulations
...
2 The use of Class II (double insulated) equipment
...
4 The use of earth free equipotential bonding
...
Methods 3 and 4 are limited to special situations
under the effective supervision of trained personnel
...
Method 2, the use of Class II insulated equipment
is limited to single pieces of equipment such as tools
used on construction sites, because it relies upon
effective supervision to ensure that no metallic equipment or extraneous earthed metalwork enters the area
of the installation
...
This method relies upon all exposed metalwork
being electrically connected together to an effective
earth connection
...
In this way the possibility of a voltage appearing between two exposed metal parts is
removed
...
2
...
The second element of this protection method is
the provision of a means of automatic disconnection
of the supply in the event of a fault occurring that
causes the exposed metalwork to become live
...
The Regulations, therefore, specify that the disconnection must be effected within 0
...
0
seconds for circuits connected to fixed equipment
...
The resistance, or we call it the impedance;
of the earth fault loop must be less than the values
given in Appendix 2 of the On Site Guide and Tables
41B1, 41B2 and 41D of the IEE Regulations
...
3 later in this chapter shows the maximum value of
168
ADVANCED ELECTRICAL INSTALLATION WORK
the earth fault loop impedance for circuits protected
by a semi-enclosed fuse to BS 3036)
...
Section 542 of the
IEE Regulations gives details of the earthing arrangements to be incorporated in the supply system to
meet these Regulations and these were described in
Chapter 1 under the heading ‘Low Voltage Supply
Systems’ and are shown in Figs 1
...
108
...
In these circumstances the availability of an
adequate earth return path is a matter of chance
...
An RCD is a type of circuit breaker that continuously compares the current in the phase and neutral
conductors of the circuit
...
The RCD
detects the imbalance and disconnects the circuit
...
18 shows an RCD
...
2
...
the regulations for switching devices or switchgear
required for protection, isolation and switching of a
consumer’s installation
...
The Regulations identify four separate types of switching: switching for isolation; switching for mechanical
maintenance; emergency switching; and functional
switching
...
An isolator is a mechanical device which is operated
manually and used to open or close a circuit off load
...
Isolators for motor circuits must isolate
the motor and the control equipment, and isolators
for high-voltage discharge lighting luminaires must be
an integral part of the luminaire so that it is isolated
when the cover is removed (Regulations 461, 476–02
and 537–02)
...
Isolation at the consumer’s service position can be
achieved by a double pole switch which opens or
closes all conductors simultaneously
...
The switching for mechanical maintenance requirements is similar to those for isolation except that the
control switch must be capable of switching the full
load current of the circuit or piece of equipment
...
Devices which are
suitable for switching off for mechanical maintenance
are switches, circuit breakers, plug and socket outlets
...
The device used for emergency
switching must be immediately accessible and identifiable, and be capable of cutting off the full load current
...
Electrical machines must be provided with a means
of emergency switching, and a person operating an
electrically driven machine must have access to an
emergency switch so that the machine can be stopped
in an emergency
...
3
...
Devices which are suitable for emergency switching are switches, circuit breakers and
contactors
...
Push-buttons used for emergency
switching must be coloured red and latch in the stop
or off position
...
Plugs and socket outlets cannot be considered
appropriate for emergency disconnection of supplies
...
The device must be capable of
interrupting the total steady current of the circuit or
appliance
...
Plug and socket outlets
may be used as switching devices and recent years have
seen an increase in the number of electronic dimmer
switches being used for the control and functional
switching of lighting circuits (Regulations 537–05–01)
...
Overcurrent protection
The consumer’s mains equipment must provide protection against overcurrent; that is, a current exceeding
the rated value (Regulation 431–01–01)
...
Circuit breakers may be used in place
of fuses, in which case the circuit breaker may also
provide the means of isolation, although a further
means of isolation is usually provided so that maintenance can be carried out on the circuit breakers
themselves
...
An overload current can be
defined as a current which exceeds the rated value in
an otherwise healthy circuit
...
A short circuit
is an overcurrent resulting from a fault of negligible
impedance connected between conductors
...
An overload may result in currents of two or three
times the rated current flowing in the circuit
...
In both cases the basic requirements
for protection are that the fault currents should be
interrupted quickly and the circuit isolated safely
before the fault current causes a temperature rise
which might damage the insulation and terminations
of the circuit conductors
...
The cable is then fully protected against
both overload and short-circuit faults (Regulation
433–02–01)
...
These are for industrial applications having a maximum fault capacity of 80 kA
...
These are used for a
...
circuits on industrial and domestic installations
having a fault capacity of about 30 kA
...
These are used in 13 A
plug tops and have a maximum fault capacity of
about 6 kA
...
These were previously called rewirable fuses and are used mainly on
domestic installations having a maximum fault
capacity of about 4 kA
...
These are miniature circuit
breakers which may be used as an alternative to
fuses for some installations
...
They are graded according to their
instantaneous tripping currents – that is, the current
at which they will trip within 100 ms
...
MCB Type 1 to BS 3871 will trip instantly at
between 2
...
MCB Type B to BS EN 60898 will trip instantly
at between three and five times its rated current
and is also suitable for domestic and commercial
installations
...
It
offers fast protection on small overloads combined
with a slower operation on heavier faults, which
reduces the possibility of nuisance tripping
...
MCB Type C to BS EN 60898 will trip instantly
at between five and ten times its rated current
...
MCB Type 3 to BS 3871 will trip instantly at
between seven and ten times its rated current
...
MCB Type D to BS EN 69898 will trip instantly
at between 10 and 25 times its rated current
...
MCB Type 4 to BS 3871 will trip instantly
between 10 and 50 times the rated current and is
more suitable for special industrial applications
such as welding equipment and X-ray machines
...
series with the load and the thickness of the element is
sufficient to carry the normal rated circuit current
...
The designs of the fuse carrier and base are also
important
...
Also if over-enclosed, heat will not
escape and the fuse will ‘blow’ at a lower current
...
2
...
The fuse element
should consist of a single strand of plain or tinned copper wire having a diameter appropriate to the current
rating as given in Table 2
...
Fig
...
19 A semi-enclosed fuse
...
1 Size of fuse element
Current rating
(A)
5
10
15
20
30
The semi-enclosed fuse consists of a fuse wire, called
the fuse element, secured between two screw terminals in a fuse carrier
...
20
0
...
50
0
...
85
ADVANTAGES OF SEMI-ENCLOSED FUSES
■
■
Semi-enclosed fuses (BS 3036)
Wire diameter
(mm)
■
They are very cheap compared with other protective devices both to install and to replace
...
It is easy to identify a ‘blown fuse’
...
INSPECTION, TESTING AND COMMISSIONING
■
■
■
■
The fuse element weakens with age due to oxidization, which may result in a failure under normal
operating conditions
...
They have low breaking capacity since, in the event
of a severe fault, the fault current may vaporize the
fuse element and continue to flow in the form of an
arc across the fuse terminals
...
Cartridge fuses (BS 1361)
■
171
Their operation is more rapid than semi-enclosed
fuses
...
DISADVANTAGES OF CARTRIDGE FUSES
■
■
■
■
They are more expensive to replace than rewirable
fuse elements
...
The cartridge may be shorted out by wire or silver
foil in extreme cases of bad practice
...
HIGH BREAKING CAPACITY FUSES (BS 88)
The cartridge fuse breaks a faulty circuit in the same
way as a semi-enclosed fuse, but its construction eliminates some of the disadvantages experienced with an
open-fuse element
...
Cartridge fuse construction is
illustrated in Fig
...
20
...
They
may also be filled with quartz sand to absorb and extinguish the energy of the arc when the cartridge is
brought into operation
...
The fuse element consists of several parallel strips
of pure silver encased in a substantial ceramic cylinder, the ends of which are sealed with tinned brass
end-caps incorporating fixing lugs
...
Incorporated on the body is an indicating device to
show when the fuse has blown
...
2
...
Fig
...
20 A cartridge fuse
...
2
...
ADVANTAGES OF CARTRIDGE FUSES
ADVANTAGES OF HBC FUSES
■
■
■
■
They have no mechanical moving parts
...
The element does not weaken with age
...
■
■
■
■
■
They have no mechanical moving parts
...
The element does not weaken with age
...
They are capable of breaking very heavy fault
currents safely
...
It is difficult to confuse cartridges since different
ratings are made to different physical sizes
...
Miniature circuit breakers
(BS 3871)
The disadvantage of all fuses is that when they have
operated they must be replaced
...
An MCB of the type shown in Fig
...
22 incorporates a thermal and magnetic tripping device
...
In normal operation the current is
insufficient to operate either device, but when an overload occurs, the bimetal strip heats up, bends and trips
the mechanism
...
The device only trips
when a fault current occurs
...
This is achieved by the coil electromagnetic device
...
The circuit can be restored when the
fault is removed by pressing the ON toggle
...
The toggle switch can also
be used to disconnect the circuit for maintenance or
isolation or to test the MCB for satisfactory operation
...
2
...
ADVANTAGES OF MCBS
■
■
■
■
■
Tripping characteristics and therefore circuit protection are set by installer
...
The circuit is provided with discrimination
...
The supply may be safely restored by an unskilled
operator
...
2
...
INSPECTION, TESTING AND COMMISSIONING
173
In order to give protection to the conductors of an
installation:
■
■
Fig
...
23 A simplified diagram of an MCB
...
They contain mechanical moving parts and therefore require regular testing to ensure satisfactory
operation under fault conditions
...
This level
of protection or fusing performance is given by the
fusing factor of the device:
Minimum fusing current
Current rating
The minimum fusing current of a device is the current which will cause the fuse or MCB to blow or trip
in a given time (BS 88 gives this operating time as
4 hours)
...
Thus, a 10 A fuse which operates when 15 A flows
will have a fusing factor of 15 ÷ 10 ϭ 1
...
Since the protective device must carry the rated
current it follows that the fusing factor must always be
greater than one
...
The fusing factors of the protective devices previously
considered are:
the current rating of the protective device must be
equal to or less than the current carrying capacity
of the conductor;
the current causing the protective device to operate
must not be greater than 1
...
The current carrying capacities of cables given in
the tables of Appendix 4 of the IEE Regulations assume
that the circuit will comply with these requirements
and that the circuit protective device will have a fusing factor of 1
...
Cartridge fuses, HBC fuses
and MCBs do have a fusing factor less than 1
...
However, semi-enclosed fuses can have a fusing factor of 2
...
725
times the current carrying capacity of the conductor it
is to protect
...
Now, if
Fusing factor ϭ
■
■
■
■
semi-enclosed fuses: between 1
...
25 and 1
...
25
MCBs: less than 1
...
Then
therefore
or
In ϭ current rating of the protective
device
Iz ϭ current carrying capacity of
conductor
I2 ϭ current causing the protective
device to operate
...
45 Iz
1
...
725 Iz
When rewirable fuses are used, the current carrying
capacity of the cables given in the tables is reduced by
a factor of 0
...
Position of protective devices
Isolation, switching and protective devices can be found
at the consumers mains equipment position such as
that shown in Chapter 1 at Fig
...
111 and 1
...
The
general principle to be followed is that a protective
device must be placed at a point where a reduction
174
ADVANCED ELECTRICAL INSTALLATION WORK
occurs in the current carrying capacity of the circuit
conductors
...
The only exceptions
to this rule are where an overload protective device
opening a circuit might cause a greater danger than
the overload itself – for example, a circuit feeding an
overhead electromagnet in a scrapyard
...
4 seconds
...
The reason for the more rapid disconnection of the
socket outlet circuits is that portable equipment plugged
into the socket outlet is considered a higher risk than
fixed equipment since it is more likely to be firmly held
by a person
...
The IEE Regulations permit us to assume that
where an overload protective device is also intended
to provide short-circuit protection, and has a rated
breaking capacity greater than the prospective shortcircuit current at the point of its installation, the conductors on the load side of the protective device are
considered to be adequately protected against shortcircuit currents without further proof
...
However, if this condition is not met
or if there is some doubt, it must be verified that fault
currents will be interrupted quickly before they can
cause a dangerously high temperature rise in the circuit conductors
...
EXAMPLE
A 10 mm PVC insulated copper cable is short-circuited when connected to
a 400 V supply
...
1 ⍀
...
1 ⍀
For copper conductor and PVC insulation, Table 43A gives a value for k
of 115
...
66 ϫ 10Ϫ3 s
4000 A
The maximum time that a 4000 A fault current can be applied to this
10 mm2 cable without dangerously raising the conductor temperature is
82
...
Therefore, the protective device must disconnect the supply to
the cable in less than 82
...
Manufacturers’
information and Appendix 3 of the IEE Regulations give the operating times
of protective devices at various short-circuit currents in the form of graphs,
similar to those shown in Figs 2
...
25
...
2
...
INSPECTION, TESTING AND COMMISSIONING
175
Fig
...
25 Time/current characteristics of (a) a type 2 MCB to BS 3871; (b) semi-enclosed fuse to BS 3036
...
This means
that each successive graduation of the axis represents a
ten times change over the previous graduation
...
24 and 2
...
From Fig
...
24 it can be seen that
the particular protective device represented by this
characteristic will take 8 seconds to disconnect a fault
current of 50 A and 0
...
Figure 2
...
This graph shows that a
fault current of 4000 A will trip the protective device in
20 ms
...
66 ms, the 50 A
type 2 MCB will clear the fault current before the
temperature of the cable is raised to a dangerous level
...
These indicate the value of fault current which will
cause the protective device to operate in the times
indicated by Chapter 413 of the IEE Regulations,
that is 0
...
Figures 1, 2 and 3 in Appendix 3 of the IEE Regulations deal with fuses and Figures 4 to 8 with MCBs
...
The multiple depends upon the characteristics of the particular devices
...
EXAMPLE
A 6 A type 1 MCB to BS 3871 used to protect a domestic lighting circuit
will trip within 5 seconds when 6 A times a multiple of 4, that is 24 A,
flows under fault conditions
...
The characteristics shown in Appendix 3 of the IEE Regulations give
the specific values of prospective short-circuit current for all standard
sizes of protective device
...
A circuit designed in this way would be
176
ADVANCED ELECTRICAL INSTALLATION WORK
considered to have effective discrimination
...
This can be seen in
Fig
...
25(b)
...
1 seconds, a
30 A semi-enclosed fuse in about 0
...
4 seconds
...
2
...
2
...
Security of supply, and therefore effective discrimination, is an important consideration for an electrical
engineer and is also a requirement of Regulation
533–01–06
...
4 seconds for socket outlets
and 5
...
The
value of the earth fault loop impedance may be verified by means of an earth fault loop impedance test as
described later in this chapter
...
2
...
Fig
...
27 Earth fault loop path for a TN-S system
...
The actual value will depend upon
many factors: the type of supply, the ground conditions, the distance from the transformer, etc
...
35 ⍀ for TN-C-S
(PME) supplies and 0
...
Also in the above formula, R1 is the
resistance of the phase conductor and R2 is the resistance of the earth conductor
...
2
...
Values of R1 ϩ R2 have been calculated for copper
and aluminium conductors and are given in Table 9A
of the On Site Guide as shown in Table 2
...
Table 2
...
5
1
...
5
2
...
5
2
...
5
4
4
4
4
6
6
6
6
10
10
10
10
16
16
16
16
25
25
25
25
35
35
35
35
50
50
50
50
Protective conductor
—
1
—
1
1
...
5
2
...
5
2
...
5
4
6
—
4
6
10
—
6
10
16
—
10
16
25
—
16
25
35
—
25
35
50
Copper
Aluminium
18
...
20
12
...
20
24
...
41
25
...
51
14
...
61
16
...
02
9
...
08
10
...
69
6
...
83
6
...
91
3
...
15
4
...
98
2
...
727
2
...
877
1
...
524
1
...
251
1
...
387
1
...
911
0
...
91
—
—
3
...
20
—
—
2
...
87
2
...
07
1
...
64
1
...
51
1
...
5 mm PVC cable incorporating a 1
...
The cable length is 30 m installed in an ambient
temperature of 20°C and the consumer’s protection is by semi-enclosed
fuse to BS 3036
...
5 ⍀
...
We have
ZS ϭ ZE ϩ (R1 ϩ R2) (⍀)
ZE ϭ 0
...
2 a 2
...
5 mm protective conductor has an (R1 ϩ R2) value of 19
...
51 ϫ 10Ϫ3 ⍀/m ϭ 30 m ϭ 0
...
To take account of this, we must multiply the
value of cable resistance by the factor given in Table 9C of the On Site
Guide
...
20 and therefore the cable resistance
under fault conditions will be:
0
...
20 ϭ 0
...
If the cross-section of the circuit
protective conductor complies with Table 54G of the
IEE Regulations, there is no need to carry out further
checks
...
However, where the conductor cross-section does not
comply with this table, then the formula given in
Regulation 543–01–03 must be used:
ZS ϭ 0
...
702 ⍀ ϭ 1
...
48 ⍀ as
shown by Table 2
...
The circuit earth fault loop impedance is less than
this value and therefore the protective device will operate within the
required disconnection time of 0
...
S ϭ
I 2t
(mm 2 )
k
where
S ϭ cross-sectional area in mm2
I ϭ value of maximum fault current in amperes
t ϭ operating time of the protective device
Table 2
...
4 second disconnection
1
...
5
2
...
0
(ii) 5 seconds disconnection
1
...
5
2
...
0 to 16
...
00
8
...
00
2
...
14
2
...
48
1
...
48
NP
0
...
91
NP
NP
0
...
80
14
...
80
14
...
46
4
...
46
4
...
79
3
...
20
3
...
08
2
...
21
NP
NP
1
...
33
NP protective conductor, fuse combination NOT PERMITITTED
...
EXAMPLE 1
A 230 V ring main circuit of socket outlets is wired in 2
...
5 mm CPC
...
15 ⍀
...
5 mm CPC meets the requirements of Regulation 543–01–03 when
the protective device is a 30 A semi-enclosed fuse
...
15
t ϭ Maximum operating time of the protective device for a
socket outlet circuit is 0
...
From Fig
...
25(b) you can see that the
time taken to clear a fault of 200 A is about 0
...
k ϭ 115 (from Table 54C)
...
4 s
ϭ 1
...
5 mm2 CPC is acceptable since this is the nearest standard-size conductor
above the minimum cross-sectional area of 1
...
EXAMPLE 2
A domestic immersion heater is wired in 2
...
5 mm2 CPC
...
Establish by calculation that the CPC is
of an adequate size to meet the requirements of Regulation 543–01–03
...
2
...
For circuits feeding fixed appliances the maximum operating time of
the protective device is 5 seconds
...
2
...
Alternatively
Table 2 A in Appendix 3 of the IEE Regulations gives a value of 43 A
...
836 mm2
115
179
The circuit protective conductor of the cable is greater than 0
...
If the protective conductor is a separate conductor, that is, it does not form part of a cable as in this example and is
not enclosed in a wiring system as in Example 1, the cross-section of the
protective conductor must be not less than 2
...
0 mm2 where mechanical protection is not provided
in order to comply with Regulation 547–03–03
...
The factors which influence the current rating are:
1 the design current – the cable must carry the full
load current;
2 the type of cable – PVC, MICC, copper conductors or aluminium conductors;
3 the installed conditions – clipped to a surface or
installed with other cables in a trunking;
4 the surrounding temperature – cable resistance
increases as temperature increases and insulation
may melt if the temperature is too high;
5 the type of protection – for how long will the cable
have to carry a fault current?
Regulation 525–01 states that the drop in voltage
from the supply terminals to the fixed current-using
equipment must not exceed 4% of the mains voltage
...
2 V on a 230 V installation
...
(See Table 2
...
The cable rating, denoted It, may be determined as
follows:
It ϭ
Current rating of protective device
Any applicable correction factors
The cable rating must be chosen to comply with
Regulation 433–02–01
...
4
...
Cr the 0
...
2 of the preface to Appendix 4 of the
IEE Regulations
...
Regulation 523–04
gives us three possible correction values:
■
■
■
Where one side of the cable is in contact with
thermal insulation we must read the current
rating from the column in the table which
relates to reference method 4
...
5)
...
5 m we must apply a factor of 0
...
Where the cable is totally surrounded over a
short length, the appropriate factor given in
Table 52A of the IEE Regulations or Table 6B
of the On Site Guide should be applied
...
This cable must also
meet the voltage drop Regulation 525–01 and this
should be calculated as described earlier
...
If the calculated
value is greater than the 4% value, the next larger cable
size must be tested until a cable is found which meets
both the current rating and voltage drop criteria
...
A PVC insulated and sheathed twin and
earth cable will provide a submain to this load and be clipped to the side
of the ceiling joists over much of its length in a roof space which is anticipated to reach 35°C in the summer and where insulation is installed up
to the top of the joists
...
Assume a TN-S supply, that is, a supply having
a separate neutral and protective conductor throughout
...
The tables in the Regulations will give the same values, but this
will simplify the problem
...
4, 2
...
6
...
09 A
...
4 Ambient temperature correction factors
...
03
1
...
94
0
...
79
0
...
61
0
...
35
Table 6A2 Ambient temperature factors
Correction factors for ambient temperature where the overload protective device is a semi-enclosed fuse to BS 3036
Type of insulation
Operating temperature
Ambient temperature (°C)
25
Thermoplastic
(general purpose PVC)
70°C
30
35
40
45
50
55
60
65
1
...
0
0
...
94
0
...
87
0
...
69
0
...
5 Current carrying capacity of cables
...
Conductor operating temperature: 70°C
CURRENT-CARRYING CAPACITY (amperes): BS 6004, BS 7629
Conductor
cross-sectional
area
Reference Method 4
(enclosed in an
insulated wall, etc
...
c
...
c
...
c
...
c
...
c
...
c
...
c
...
c
...
c
...
c
...
c
...
c
...
5
25
32
43
57
75
92
110
139
167
A
10
13
17
...
5
23
30
38
52
69
90
111
133
168
201
A
11
...
5
27
36
46
63
85
112
138
168
213
258
A
13
...
5
24
32
41
57
76
96
119
144
184
223
A
17
22
30
40
51
70
94
119
148
180
232
282
A
14
...
5
25
34
43
60
80
101
126
153
196
238
1
2
mm
1
1
...
5
4
6
10
16
25
35
50
70
95
For a fuller treatment see Appendix 4 of BS 7671 Table 4D2A
...
Nominal current setting of the protection for this load In ϭ30 A
...
4
the correction factor for 35°C is 0
...
Cg the grouping correction factor is not applied since the cable is
to be clipped direct to a surface and not in contact with other
cables
...
725 must be applied
...
(See Table 2
...
The cable rating, It is given by
Current rating of protective device
lt ϭ
The product of the correction factors
ϭ
30 A
ϭ 42
...
97 ϫ 0
...
5 a 10 mm cable having
a rating of 43 A is required to carry this current
...
2 V
...
6 the volt
drop per ampere per metre for a 10 mm cable is 4
...
Therefore, the volt drop for this cable length and load is equal to
4
...
09 A ϫ 18 m ϭ 2
...
2 V, a
10 mm cable satisfies the current and drop in voltage requirements and
is therefore the chosen cable when semi-enclosed fuse protection is used
...
4 the correction factor for 35°C is 0
...
Cg grouping factors need not be applied
...
Ci thermal insulation once more demands that we assume installed
method 4
...
5)
...
6 Voltage drop in cables factor
...
c
...
c
...
5
2
...
3
4
...
8
mV/A/m
44
29
18
11
7
...
4
2
...
75
1
...
93
0
...
47
mV/A/m
38
25
15
9
...
4
3
...
4
r
1
...
10
0
...
55
0
...
75
1
...
93
0
...
46
Note: For a fuller treatment see Appendix 4 of BS 7671 Table 4D2B
...
09 A and we will therefore choose a
30 A MCB for the nominal current setting of the protective device, In
...
9 A
0
...
5, a 6 mm cable, having a rating of 32 A, is required to carry this current
...
6
the volt drop per ampere per metre for a 6 mm cable is 7
...
So the
volt drop for this cable length and load is equal to
ELECTRICAL TESTING
The electrical contractor is charged with a responsibility to carry out a number of tests on an electrical
installation and electrical equipment
...
Electrical measuring instruments are identified by the way in which the
instrument movement is deflected in operation
...
The connection of a test instrument into a circuit
can change the circuit resistance and cause error readings
...
Fortunately for electrical installation students,
this is not a common problem found in power engineering, but does need consideration when testing electronic circuits
...
Analogue and digital meters, their
advantages and disadvantages are also discussed in
Chapter 4 and shown in Figs 4
...
76
...
The test current is fed to the moving coil through
the spiral control springs which also return the pointer
to zero after each test reading
...
2
...
7
...
09 A ϫ 18 m ϭ 3
...
2 V, a
6 mm cable satisfies the current and drop in voltage requirements when
the circuit is protected by an MCB
...
Even though an MCB is
more expensive than a semi-enclosed fuse, the installation of a 6 mm
cable with an MCB may be less expensive than 10 mm cable protected
by a semi-enclosed fuse
...
If you are unsure of the standard fuse and MCB rating of protective
devices, you can refer to Tables 2A, 2B, 2C and 2D of the On Site Guide
...
2
...
INSPECTION, TESTING AND COMMISSIONING
When the test current flows in the moving coil, a
magnetic flux is established, as shown in Fig
...
29
...
2
...
Fig
...
30 Rectified input to moving coil meter movement
...
The purpose of the soft iron armature A is to establish a uniform magnetic field of equal flux density for
the coil’s rotation
...
The basic moving coil movement will only respond
satisfactorily to a d
...
supply since an a
...
supply
would reverse the current and magnetic flux in the
coil at the supply frequency, resulting in the pointer
trembling at some useless mid-point on the scale
...
c
...
2
...
The moving coil instrument presents a high impedance to the test circuit and draws very little current
from it
...
This makes it suitable for power or
electronic circuit measurements
...
The wide range of scales achieved by the same meter
movement in a commercial instrument is discussed in
this chapter under the heading ‘Range extension’
...
These instruments can be used to test a
...
or d
...
supplies without the addition of a rectifier circuit
...
2
...
The test current
passing through the instrument solenoid establishes a
magnetic flux which attracts the soft iron towards the
solenoid, moving the pointer across the scale
...
This creates a non-linear force
on the iron as it moves through the magnetic field and
therefore the scale is non-linear
...
REPULSION-TYPE MOVING IRON INSTRUMENTS
The repulsion-type moving iron instrument is the
moving iron instrument most often found today
...
31(b) and 2
...
Fig
...
32 Repulsion-type moving iron instrument (by kind permission of G
...
P
...
Current passing through the solenoid coil establishes a magnetic field inside the solenoid
...
Since like magnetic poles repel, the moving iron is
repelled away from the fixed iron and the pointer is
deflected across the scale
...
A linear scale is highly desirable, and to achieve an
almost linear scale the manufacturers of commercial
Fig
...
31 Moving iron instruments: (a) attraction type; (b) repulsion type
...
Rectangular moving iron pieces and fixed
iron pieces in the shape of a tapered scroll have been
found to give good results
...
Moving iron instruments present a low impedance
to the test circuit and will therefore draw current from
the test circuit to provide the power necessary for the
deflection of the pointer
...
They are not suitable for
electronic circuit measurements
...
This effect we take for granted, but it can
only be achieved if the deflection system is critically
damped
...
The forces exerted on the moving system would be
insufficient to maintain this higher value and the
pointer would fall back towards the true circuit reading, gaining momentum which would take the pointer
below the true reading
...
This effect is shown in
Fig
...
33, where A represents the true reading of the
circuit, curve B the undamped system and curve C
the correctly or critically damped system
...
To achieve damping in an electrical instrument a
force is provided which opposes the rise of the moving
185
Fig
...
33 Damping curves
...
This is achieved in
one of three ways: eddy current damping; air vane
damping; and air piston damping
...
The contractor may also be called
upon to test installations and equipment in order to
identify and remove faults
...
Most instruments are
constructed in a way which makes them highly sensitive, giving full scale deflection with only very small
currents, but these currents are drawn from the circuit
being tested
...
186
ADVANCED ELECTRICAL INSTALLATION WORK
EXAMPLE
A 50 ⍀ load is connected across a 100 V supply
...
As an
ammeter, the instrument has a resistance of 0
...
The power loss in the instrument for each connection is as follows:
V
100V
Load current I ϭ ϭ
ϭ2A
R 50 ⍀
Power loss in the load
ϭ V ϫ I ϭ100 V ϫ 2 A ϭ200 W
Power loss in the ammeter
ϭ I 2R ϭ (2 A)2 ϫ0
...
4 W
Power loss in the voltmeter
ϭ
motion, and this type of damping is used in moving
coil instruments
...
Eddy currents are induced in the former, setting up a
force to oppose the motion, and damping is achieved
for the moving coil system
...
2
...
When the instrument
movement deflects, the aluminium rectangle or vane
moves in a segment-shaped box, compressing the air,
which exerts a damping force on the vane and on the
instrument movement
...
(100 V )2
V2
ϭ
ϭ 0
...
EDDY CURRENT DAMPING
When a copper or aluminium disc is rotated between
the poles of a permanent magnet, a current is induced
into the disc, as shown in Fig
...
34
...
The braking effect only occurs when the disc is in
Fig
...
35 Air vane damping
...
2
...
This is an alternative method of air damping
...
2
...
When the moving system
deflects, the piston is pushed into an air chamber
which compresses the air and applies a force which
opposes the motion and causes damping of the meter
movement
...
To extend the
range of a moving coil instrument, shunt or series
resistors are connected to it as shown in Fig
...
37
...
This allows the majority of the circuit current
to pass through the shunt and only a very small part
of the current to pass through the meter movement
...
2
...
Range extension
Moving iron instruments may be constructed to read
10, 20 or 50 A by increasing the thickness and number of solenoid conductors
...
2
...
Fig
...
38 The resistor and coil arrangements in a multirange
instrument
...
This limits the current flowing through the meter
movement to an acceptable low value
...
2
...
EXAMPLE
A moving coil movement has a resistance of 5 ⍀ and gives full scale
deflection when 15 mA flows in the coil
...
For (a),
I T ϭ IM ϩ IS (A) (from Fig
...
37)
I S ϭ I T Ϫ I M (A)
I S ϭ 5 A Ϫ0
...
985 A
The voltage dropped across the moving coil is given by
VMϭ IM ϫ R (V)
where R is the resistance of moving coil
ІVM ϭ 15 ϫ 10Ϫ3 A ϫ 5 ⍀
VM ϭ 75 ϫ 10Ϫ3 V
Since the shunt resistor is connected in parallel with the moving coil
movement
VM ϭ VS
where VS is the voltage across the shunt
...
2
...
925 V
Since IM flows through the moving coil and the series resistor the
resistance of the series resistor is given by
Rϭ
∴ Rϭ
VR
(⍀)
lM
99
...
A range selector switch is usually used to
choose the appropriate scale, as shown in Fig
...
38
...
For
example, when taking a reading in the 8 or 9 V range
the obvious scale choice would be one giving 10 V full
scale deflection
...
Ammeters must be connected in series with the
load, and voltmeters in parallel across the load as
shown in Fig
...
39
...
This will give accurate calculations on
both a
...
and d
...
supplies, but when measuring the
power of an a
...
circuit which contains inductance or
capacitance a wattmeter must be used because the
voltage and current will be out of phase
...
985A
R S ϭ 15
...
2
...
INSPECTION, TESTING AND COMMISSIONING
Dynamometer wattmeter
A correctly connected wattmeter will give an accurate
measure of the power in any a
...
or d
...
circuit
...
The moving coil is the voltage coil and rotates
within the fixed coils, being pivoted centrally between
them and controlled by spiral hair springs as shown in
Fig
...
40
...
2
...
This wattmeter will
indicate the power in that phase and since the load is
balanced the total power in the three-phase circuit
will be given by:
Total power ϭ 3 ϫ Wattmeter reading
Fig
...
40 A dynamometer wattmeter (by kind permission of
G
...
P
...
The main magnetic field is produced by the current
in the fixed coil and is proportional to it
...
The deflection is proportional to the product of
the currents in the fixed and moving coils
...
Any change in the direction of the current in the circuit affects both coils and the direction of deflection
remains unchanged, allowing the instrument to be
used on both a
...
and d
...
circuits
...
c
...
Damping is achieved
by an air vane moving in a dashpot
...
2
...
TWO-WATTMETER METHOD
This is the most commonly used method for measuring power in a three-phase, three-wire system since it
can be used for both balanced and unbalanced loads
connected in either star or delta
...
2
...
Then
Total power ϭ W1 ϩ W2
This equation is true for any three-phase load,
balanced or unbalanced, star or delta connection,
provided there is no fourth wire in the system
...
2
...
Each
The current and voltage coils are wound on the two
magnets as shown in Fig
...
44
...
The rotation of the aluminium disc is due to the
interaction of these magnetic fields
...
The force exerted is proportional to the
phase angle between the voltage and current coil fluxes;
maximum force occurs when they are 90° out of phase
...
The
number of revolutions in a given time will give a measure of energy since energy ϭ power ϫ time
...
Fig
...
43 Three-wattmeter measurement of power
...
2
...
Fig
...
42 Two-wattmeter measurement of power
...
1
...
The laminated core of the transformer
can be opened and passed over the busbar or singlecore cable
...
The construction is shown in Fig
...
45
...
f
...
2
...
Fig
...
46 Phase sequence test by the lamps bright, lamps dim
method
...
2
...
Phase sequence testers
Phase sequence is the order in which each phase of a
three-phase supply reaches its maximum value
...
Phase sequence has an important application in the
connection of three-phase transformers
...
A phase sequence tester can be an indicator which
is, in effect, a miniature induction motor, with three
clearly colour-coded connection leads
...
If the sequence is reversed the
disc rotates in the opposite direction to the arrow
...
The
phasor addition of the voltage in the circuit results in
one pair of lamps illuminating brightly while the other
pair are illuminated dimly
...
2
...
Test equipment used by
electricians
The Health and Safety Executive has published
Guidance Notes (GS 38) which advise electricians
and other electrically competent people on the selection of suitable test probes, voltage indicating devices
and measuring instruments
...
In the past, unsatisfactory test
probes and voltage indicators have frequently been
the cause of accidents, and therefore all test probes
must now incorporate the following features:
1 The probes must have finger barriers or be shaped
so that the hand or fingers cannot make contact
with the live conductors under test
...
3 The lead must be adequately insulated and
coloured so that one lead is readily distinguished
from the other
...
5 The lead must be long enough to serve its purpose
but not too long
...
7 Where the leads are to be used in conjunction with
a voltage detector they must be protected by a fuse
...
2
...
GS 38 also tells us that where the test is being made
simply to establish the presence or absence of a voltage,
the preferred method is to use a proprietary test lamp or
voltage indicator which is suitable for the working
voltage, rather than a multimeter
...
Figure 2
...
Test lamps and voltage indicators are not fail-safe,
and therefore GS 38 recommends that they should be
regularly proved, preferably before and after use, as
described in the flowchart for a safe isolation procedure
...
2
...
Fig
...
48 Typical voltage indicator
...
2 All test equipment must be ‘approved’ and connected to the test circuits by recommended test
probes as described by the HSE Guidance Notes
Fig
...
49 Voltage proving unit
...
The test equipment used must also be
‘proved’ on a known supply or by means of a
proving unit such as that shown in Fig
...
49
...
2
...
4 Warning notices must be posted
...
194
ADVANCED ELECTRICAL INSTALLATION WORK
Isolation of supply
The Electricity at Work Regulations are very specific
in describing the procedure to be used for isolation of
the electrical supply
...
Regulation 4(3) tells us that we
must also prove the conductors dead before work
commences and that the test instrument used for this
purpose must itself be proved immediately before and
immediately after testing the conductors
...
2
...
Start at the top and work your way down the flowchart
...
If the answer is yes, move on
...
Fig
...
50 Secure isolation of a supply
...
However, it does acknowledge that
some work, such as fault finding and testing, may
require the electrical equipment to remain energized
...
Note that while live testing may be required in order
to find the fault, live repair work must not be carried
out
...
Inspection and testing
techniques
The testing of an installation implies the use of instruments to obtain readings
...
All new installations must be inspected and tested
before connection to the mains, and all existing installations should be periodically inspected and tested to
ensure that they are safe and meet the regulations of
the IEE Regulations 711 to 744
...
This current must not cause
danger to any person or equipment in contact with the
installation, even if the circuit being tested is faulty
...
This
ensures that the protective conductors are correctly
connected and secure before the circuit is energized
...
2
...
The installation must be visually inspected before
testing begins
...
Regulation
712–01–03 gives a check-list for the initial visual
inspection of an installation, including:
■
■
■
■
connection of conductors;
identification of conductors;
routing of cables in safe zones;
selection of conductors for current carrying capacity and volt drop;
196
■
■
■
■
■
■
■
■
■
■
■
■
■
■
ADVANCED ELECTRICAL INSTALLATION WORK
connection of single-pole devices for protection or
switching in phase conductors only;
correct connection of socket outlets, lampholders,
accessories and equipment;
presence of fire barriers, suitable seals and protection against thermal effects;
methods of protection against electric shock,
including the insulation of live parts and placement of live parts out of reach by fitting appropriate barriers and enclosures;
prevention of detrimental influences (e
...
corrosion);
presence of appropriate devices for isolation and
switching;
presence of undervoltage protection devices;
choice and setting of protective devices;
labelling of circuits, fuses, switches and terminals;
selection of equipment and protective measures
appropriate to external influences;
adequate access to switchgear and equipment;
presence of danger notices and other warning notices;
presence of diagrams, instructions and similar
information;
appropriate erection method
...
For example, the first item on the checklist, connection of conductors, might be further expanded to
include the following:
■
■
■
■
■
■
Are connections secure?
Are connections correct? (conductor identification)
Is the cable adequately supported so that no strain
is placed on the connections?
Does the outer sheath enter the accessory?
Is the insulation undamaged?
Does the insulation proceed up to but not into the
connection?
This is repeated for each appropriate item on the
checklist
...
These tests are as follows:
Before the supply is connected
1 Test for continuity of protective conductors,
including main and supplementary bonding
...
3 Test for insulation resistance
...
5 Test the earth electrode resistance
...
7 Test the earth fault loop impedance
...
g
...
If any test fails to comply with the Regulations,
then all the preceding tests must be repeated after the
fault has been rectified
...
There is an increased use of electronic devices in
electrical installation work, for example, in dimmer
switches and ignitor circuits of discharge lamps
...
APPROVED TEST INSTRUMENTS
The test instruments and test leads used by the electrician for testing an electrical installation must meet
all the requirements of the relevant regulations
...
The IEE Regulations (BS 7671) also specify the test
voltage or current required to carry out particular tests
satisfactorily
...
The test instrument must also
carry a calibration certificate, otherwise the recorded
results may be void
...
Test instruments must, therefore, be tested
and recalibrated each year by an approved supplier
...
Modern digital test instruments are reasonably
robust, but to maintain them in good working order
they must be treated with care
...
INSPECTION, TESTING AND COMMISSIONING
Continuity tester
To measure accurately the resistance of the conductors in an electrical installation we must use an instrument which is capable of producing an open circuit
voltage of between 4 and 24 V a
...
or d
...
, and deliver
a short-circuit current of not less than 200 mA
(Regulation 713–02)
...
Insulation resistance tester
The test instrument must be capable of detecting insulation leakage between live conductors and between
live conductors and earth
...
Earth fault loop impedance tester
The test instrument must be capable of delivering
fault currents as high as 25 A for up to 40 ms using the
supply voltage
...
RCD tester
Where circuits are protected by a residual current
device we must carry out a test to ensure that the
device will operate very quickly under fault conditions and within the time limits set by the IEE
Regulations
...
The instrument is, therefore, calibrated to give
a reading measured in milliseconds to an in-service
accuracy of 10%
...
However, to carry out all
the tests required by the IEE Regulations will require
a number of test instruments and this will represent a
major capital investment in the region of £1000
...
197
1 TESTING FOR CONTINUITY OF PROTECTIVE
CONDUCTORS (713–02)
The object of the test is to ensure that the circuit protective conductor (CPC) is correctly connected, is electrically sound and has a total resistance which is low
enough to permit the overcurrent protective device to
operate within the disconnection time requirements of
Regulation 413–02–08, should an earth fault occur
...
A d
...
test using an ohmmeter continuity tester is
suitable where the protective conductors are of copper
or aluminium up to 35 mm2
...
2
...
The resistance of the long test
lead is subtracted from these readings to give the
resistance value of the CPC
...
Where steel conduit or trunking forms the protective conductor, the standard test described above may
be used, but additionally the enclosure must be visually checked along its length to verify the integrity of
all the joints
...
If, after carrying out this further test, the inspecting
engineer still questions the quality and soundness of
the protective conductor formed by the metallic conduit or trunking then a further test can be done using
an a
...
voltage not greater than 50 V at the frequency of
the installation and a current approaching 1
...
This test can be done using a low-voltage transformer
and suitably connected ammeters and voltmeters, but a
number of commercial instruments are available such as
the Clare tester, which give a direct reading in ohms
...
For a satisfactory test result,
the resistance of the protective conductor should be
consistent with those values calculated for a phase
198
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
52 Testing continuity of protective conductors
...
Values of resistance per metre for copper and aluminium conductors are given in Table 9A of the On
Site Guide
...
7
...
7 Resistance values of some metallic containers
Metallic sheath
Size (mm)
Conduit
20
25
32
50 ϫ 50
75 ϫ 75
100 ϫ 100
Trunking
Resistance at 20°C
(m⍀/m)
1
...
14
0
...
949
0
...
337
EXAMPLE
The CPC for a ring final circuit is formed by a 1
...
Determine a satisfactory continuity test
value for the CPC using the value given in Table 9A of the On Site Guide
...
5 mm2 copper conductor ϭ 12
...
10 ϫ 10Ϫ3
ϭ 0
...
Therefore, in this case, a satisfactory test result would be
obtained if the resistance of the protective conductor was about 0
...
A more precise result is indicated by the earth fault loop impedance test
which is carried out later in the sequence of tests
...
This test also verifies the polarity of each
socket outlet
...
There are three steps
to this test:
Step 1
Measure the resistance of the phase conductors (L1
and L2), the neutral conductors (N1 and N2) and the
protective conductors (E1 and E2) at the mains position as shown in Fig
...
53
...
e
...
05 ⍀) if the ring is continuous
...
67 times as great
as these readings if 2
...
5 mm cable is used
...
8
...
2
...
An
ohmmeter reading should then be taken between live
and neutral at every socket outlet on the ring circuit
...
Each reading should have a value of
approximately half the live and neutral ohmmeter
readings measured in step 1 of this test
...
Record the results on a table such as that shown in
Table 2
...
Step 3
Where the circuit protective conductor is wired as a
ring, for example where twin and earth cables or plastic conduit is used to wire the ring, temporarily join
the live and circuit protective conductors together as
shown in Fig
...
55
...
2
...
Table 2
...
This gives a
value for
r1
r2
R1 ϩ R2
Fig
...
54 Step 2 test: connection of mains conductors and test circuit
conditions
...
There are two tests to be carried out using an insulation resistance tester which must supply a voltage of
500 V d
...
for 230 and 400 V installations
...
The procedures are:
Phase and neutral conductors to earth
1 Remove all lamps
...
3 Disconnect appliances
...
2
...
Record the results on an
installation schedule such as that given in Appendix
7 of the On Site Guide
...
2
...
then be taken between live and earth at every socket
outlet on the ring
...
This value is equal
to R1 ϩ R2 for the circuit
...
8
...
8)
...
The test should be made at the
consumer’s unit with the mains switch off, all fuses in
place and all switches closed
...
2 Close all switches and circuit breakers
...
4 Test between phase and neutral conductors of every
distribution circuit at the consumer’s unit as shown
in Fig
...
56(b) and record the result
...
5 M⍀ for a satisfactory result
(IEE Regulation 713–04–02)
...
e
...
The insulation resistance of these tests should be not less than
0
...
Although an insulation resistance reading of 0
...
In these cases each circuit should be separately
tested to obtain a reading greater than 2 M⍀
...
2
...
wired and Edison screw-type lampholders have the
centre contact connected to the live conductor
...
The test is done with the supply disconnected
using an ohmmeter or continuity tester as follows:
1 Switch off the supply at the main switch
...
3 Fix a temporary link between the phase and earth
connections on the consumer’s side of the main
switch
...
5 Test between the centre pin of any Edison screw
lampholders and any convenient earth connection
...
e
...
2
...
For a satisfactory test result the ohmmeter or continuity meter should read approximately zero
...
5 TESTING EARTH ELECTRODE RESISTANCE (713–10)
When an earth electrode has been sunk into the general mass of earth, it is necessary to verify the resistance of the electrode
...
Connection to this mass through earth
electrodes provides a reference point from which all
other voltage levels can be measured
...
The resistance to earth of an electrode will depend
upon its shape, size and the resistance of the soil
...
A rod of
about 1 m will have an earth electrode resistance of
between 10 and 200 ⍀
...
In countries which experience long
dry periods of weather the earth electrode resistance
may be thousands of ohms
...
2
...
earthing in the United Kingdom, although this practice is still permitted in some countries
...
The earth electrode is sunk into the ground, but
the point of connection should remain accessible
(Regulation 542–04–02)
...
2
...
The installation site must be chosen so that the
resistance of the earth electrode does not increase
above the required value due to climatic conditions
such as the soil drying out or freezing, or from the
effects of corrosion (542–02–02 and 03)
...
The voltage will fall to a safe level in the
Fig
...
58 Termination of an earth electrode
...
2
...
The basic method of measuring earth electrode
resistance is to pass a current into the soil through the
electrode and to measure the voltage required to
produce this current
...
To carry out the
test, either a hand-operated tester or a mains energized
double-wound transformer with a separate ammeter
and high-resistance voltmeter is used
...
The earth electrode
is disconnected from all sources of the supply
...
2
...
An alternating current supplied by a double-wound
transformer (as shown in Fig
...
60) is passed between
the earth electrode under test T and an auxiliary earth
electrode T1
...
A second auxiliary electrode T2 is driven into the ground half-way between T
and T1 and the voltmeter reading V tabulated
...
To check that the resistance of the earth electrode is
a true value, two further readings are taken at X and Y,
with the auxiliary electrode T2 moved 6 m further
from and then 6 m nearer to T, respectively
...
If there is no agreement the test must
be repeated with the distance between T and T1
increased
...
The instrument is connected as shown
in Fig
...
61
...
The value indicated by the
dials gives the resistance of the earth electrode
...
The resistance of the earth electrode will depend upon the type of ground in which
the electrode is driven
...
Typical
resistance readings are:
■
■
■
■
■
marshy ground, 5–20 ⍀
agricultural soil, 5–50 ⍀
loam and clay, 10–150 ⍀
sandy gravel, 200–500 ⍀
rocky ground, 500–10 000 ⍀
...
Lightning conductors provide a path of low resistance to lightning current, which may be many thousands of amperes
...
The lightning protective system must be connected to
the main earthing terminal of the electrical installation (Regulation 413–02–02)
...
2
...
Fig
...
61 Earth electrode resistance test using a hand-operated
tester
...
In most installations
this earth path is provided by the sheath of the supply
cable, but in some rural areas where supplies are provided by overhead cables, the metallic sheath is not
available and the general mass of earth is relied upon
to provide the return path
...
4 seconds for socket outlets and 5 seconds
for fixed appliances (Regulations 413–02–09 and 13)
...
INSPECTION, TESTING AND COMMISSIONING
EXAMPLE
The total resistance of the complete earth path of an electrical installation
supplied by a TT system is 20 ⍀ including the resistance of the consumer’s earth electrode
...
V
I ϭ (A )
R
∴ Iϭ
230 V
ϭ 11
...
5 A will flow which would not be
sufficient to operate, for example, a 30 A ring main fuse, but would be
sufficient to kill someone since 50 mA can be fatal
...
5 ⍀
...
Regulation 413–02–16 states that the
product of the earth loop impedance and the operating
current of the RCD should be less than 50
...
3
...
2 Switch off the consumer’s unit to isolate the installation
...
Record the result on an installation schedule such as
that given in Appendix 7 of the On Site Guide
...
3
...
Since most of the circuit impedance will be
made up of the earth electrode resistance, we can say
that an acceptable value for the measurement of the
earth electrode resistance would be less than about
200 ⍀
...
205
6 TESTING POLARITY – SUPPLY CONNECTED
Using an approved voltage indicator such as that
shown at Fig
...
48 or test lamp and probes which
comply with the HSE Guidance Note GS 38, again
carry out a polarity test to verify that all fuses, circuit
breakers and switches are connected in the live conductor
...
In each case the voltmeter or test lamp should
indicate the supply voltage for a satisfactory result
...
The whole earth fault current loop examined by this
test is comprised of all the installation protective conductors, the earthing terminal and earth conductors, the
earthed neutral point and the secondary winding of
the supply transformer and the phase conductor from the
transformer to the point of the fault in the installation
...
The test is made with the supply switched on,
from the furthest point of every final circuit, including
lighting, socket outlets and any fixed appliances
...
Purpose-built testers give a readout in ohms and a
satisfactory result is obtained when the loop impedance does not exceed the appropriate values given in
Tables 2A, 2B and 2C of Appendix 2 of the On Site
Guide or Table 41B1 and 41B2 or Table 604B2,
605B1 and 605B2 of the IEE Regulations
...
3)
...
The test must
simulate an appropriate fault condition and be independent of any test facility incorporated in the device
...
Functional testing of a ring circuit protected by a
general-purpose RCD to BS 4293 in a split-board
consumer unit is carried out as follows:
1 Using the standard lead supplied with the test
instrument, disconnect all other loads and plug in
the test lead to the socket at the centre of the ring
(i
...
the socket at the furthest point from the source
of supply)
...
3 Press the test button – the RCD should trip and
disconnect the supply within 200 ms
...
The RCD should again
trip within 200 ms
...
5 Now set the test instrument to 50% of the rated
tripping current of the RCD and press the test button
...
This test is testing the RCD for inconvenience or
nuisance tripping
...
This test should be repeated
every 3 months
...
Where the residual current device has a rated tripping current not exceeding 30 mA and has been
installed to reduce the risk associated with direct contact, as indicated in Regulation 412–06–02, a residual
current of 150 mA should cause the circuit breaker to
open within 40 ms
...
The ‘competent person’ must have a
sound knowledge of the type of work undertaken, be
fully versed in the inspection and testing procedures
contained in the IEE Regulations (BS 7671) and
employ adequate testing equipment
...
Those responsible for large or complex
installations may provide an equivalent form of installation certificate (IEE Regulation 741–01–01)
...
The standard format is again shown in Appendix 7 of the
On Site Guide and Appendix 6 of the IEE Regulations
...
Forms WR1,
WR2, WR3 and WR4 make up an installation certificate and forms WR5, WR2, WR3 and WR4 make up
the periodic inspection report
...
Safe working procedures
when testing
Whether you are carrying out the test procedure (i) as
a part of a new installation (ii) upon the completion of
an extension to an existing installation (iii) because
you are trying to discover the cause of a fault on an
installation or (iv) because you are carrying out a periodic test and inspection of a building, you must always
be aware of your safety, the safety of others using the
building and the possible damage which your testing
might cause to other systems in the building
...
Ensure that the test instrument carries a valid calibration certificate otherwise the results may be invalid
...
Put up warning notices so that other workers will
know what is happening
...
Obtain approval to have systems shut down which
might be damaged by your testing activities
...
Ventilation and fume extraction systems will stop working when you disconnect
the supplies
...
Use cones and highly visible warning tape to screen
off your work area
...
You might be able to
do this while you carry out the initial inspection of
the installation
...
To safeguard other systems:
■
■
Computer systems can be severely damaged by a
loss of supply or the injection of a high test voltage
from, for example, an insulation resistance test
...
Commercial organizations may be unable to continue to work without their computer systems and,
in these circumstances it may be necessary to test
outside the normal working day
...
4
...
■
■
207
Farm animals are creatures of habit and may
become very grumpy to find you testing their milking parlour equipment at milking time
...
Your isolation of the circuit
for testing may cause the emergency systems to operate
...
Portable appliance testing (PAT)
A quarter of all serious electrical accidents involve
portable electrical appliances, that is, equipment which
has a cable lead and plug and which is normally moved
around or can easily be moved from place to place
...
There is a requirement for employers under the
Health and Safety at Work Act to take adequate steps to
protect users of portable appliances from the hazards of
electric shock and fire
...
The
Electricity at Work Regulations 1989 also place a duty
of care upon employers to ensure that the risks associated with the use of electrical equipment are controlled
...
In these
publications the HSE recommend that a three level system of inspection can give cost effective maintenance of
portable appliances
...
A user visually checking equipment which they are
using is probably the most important maintenance
procedure
...
A user should check for obvious
damage using common sense
...
Possible
208
ADVANCED ELECTRICAL INSTALLATION WORK
dangers to look for are as follows:
■
■
■
■
■
Damage to the power cable or lead which exposes
the colours of the internal conductors, which are
brown, blue and green with a yellow stripe
...
The plug top pushes
into the wall socket, usually a square pin 13A socket
in the UK, to make an electrical connection
...
If the bakelite
plastic casing of the plug top is cracked, broken or
burned, or the contact pins are bent, do not use it
...
Poor cable retention
...
The coloured
internal conductors must not be visible at either end
...
Fig
...
62 Correct connection of plug top
...
If any of the above dangers are present, the equipment
should not be used until the person appointed by the
company to make a ‘visual inspection’ has had an
opportunity to do so
...
In addition to the
user checks described above, an inspection could
include the removal of the plug top cover to check that:
■
■
■
■
a fuse of the correct rating is being used and also
that a proper cartridge fuse is being used and not a
piece of wire, a nail or silver paper;
the cord grip is holding the sheath of the cable and
not the coloured conductors;
the wires (conductors) are connected to the correct
terminals of the plug top as shown in Fig
...
62;
the coloured insulation of each conductor wire goes
right up to the terminal so that no bare wire is visible;
INSPECTION, TESTING AND COMMISSIONING
■
■
■
the terminal fixing screws hold the conductor wires
securely and the screws are tight;
all the conductor wires are secured within the
terminal;
there are no internal signs of damage such as overheating, excessive ‘blowing’ of the cartridge fuse or
the intrusion of foreign bodies such as dust, dirt or
liquids
...
In the case of a moulded
plug top, only the fuse can be checked
...
The HSE
recommends that a simple procedure be written to
give guidance to the ‘appointed person’ carrying out
the visual inspection
...
For some portable appliances the earth is essential to the safe use of the equipment and, therefore, all earthed equipment and most
extension leads should be periodically tested and
inspected for these faults
...
Earth bonding tests apply a substantial test current,
typically about 25 A, down the earth pin of the plug
top to an earth probe, which should be connected to
any exposed metalwork on the portable appliance
being tested
...
A satisfactory result for
this test would typically be a reading of less than
0
...
The earth bond test is, of course, not required
for double insulated portable appliances because there
will be no earthed metalwork
...
The PAT Tester then
calculates the insulation resistance and either gives an
actual reading or indicates pass or fail
...
Some PAT Testers offer other tests in addition to
the two described above
...
209
A flash test tests the insulation resistance at a higher
voltage than the 500 V test described above
...
5 kV for Class 1 portable appliances, that is
earthed appliances, and 3 kV for Class 2 appliances
which are double insulated
...
A satisfactory result for this test
would typically be less than 3 mA
...
A visual inspection will be required to
establish that the size of the fuse is appropriate for that
particular portable appliance
...
It is a useful test to
ensure that the portable appliance is not deteriorating
and liable to become unsafe
...
A satisfactory reading is typically less
than 3 mA
...
e
...
A satisfactory
result for this test would typically be less than 3
...
8 kW for 110 V
equipment
...
The HSE suggested
intervals for the three levels of checking and inspection of portable appliances in offices and other low
risk environments is given in Table 2
...
WHO DOES WHAT?
When actual checking, inspecting and testing of
portable appliances takes place will depend upon the
company’s safety policy and risk assessments
...
Everyone can use common sense and carry out the
user checks described earlier
...
9 HSE suggested intervals for checking, inspecting and testing of portable appliances in offices and other low risk
environments
Equipment/environment
User
checks
Formal visual inspection
Combined visual inspection and
electrical testing
Battery-operated: (less than 20 V)
Extra low voltage: (less than 50 V a
...
) e
...
telephone
equipment, low voltage desk lights
Information technology: e
...
desktop computers,
VDU screens
Photocopiers, fax machines: not hand-held, rarely moved
No
No
No
No
No
No
No
Yes, 2–4 years
No
Yes, 2–4 years
No
Yes, 2–4 years
No if double insulated –
otherwise up to 5 years
No if double insulated –
otherwise up to 5 years
No
Yes
Yes
Yes, 6 months–1 year
Yes, 6 months–1 year
No
Yes, 1–2 years
Yes
Yes, 6 months–4 years
depending on the type of
equipment it is connected to
Yes, 1–5 years depending on
the type of equipment it is
connected to
Double insulated equipment: not hand-held, moved
occasionally, e
...
fans, table lamps, slide projectors
Double insulated equipment: hand-held, e
...
power tools
Earthed equipment (Class 1): e
...
electric kettles, some
floor cleaners, power tools
Cables (leads) and plugs connected to the above
...
Any sensible member of staff who has
received training can carry out this duty
...
The HSE recommend
that the appointed person follows a simple written
procedure for each visual inspection
...
For example:
Electricity at Work Regulations 1989, it is generally
accepted that some form of recording of results is
required to implement a quality control system
...
2
...
1 Is the correct fuse fitted?
Yes/No
2 Is the cord grip holding the cable sheath? Yes/No
■
The tick sheet should incorporate all the appropriate
visual checks and inspections described earlier
...
This more complex task need not necessarily be carried out by a qualified electrician or electronics service engineer
...
Also, greater knowledge will be
required for the inspection of the range of portable
appliances which might be tested
...
2
...
INSPECTION, TESTING AND COMMISSIONING
cutting off the plug top), labelled as faulty and sent
for repair
...
Combined inspection and testing should be carried
out where there is a reason to suspect that the equipment may be faulty, damaged or contaminated but
cannot be verified by visual inspection alone
...
Commissioning electrical systems
The commissioning of the electrical and mechanical
systems within a building is a part of the ‘handingover’ process of the new building by the architect and
main contractor to the client or customer in readiness
for its occupation and intended use
...
If it is out of commission, it is not in working order
...
It is during the
commissioning period that any design or equipment
failures become apparent, and this testing is one of the
few quality controls possible on a building services
installation
...
He must also instruct the client’s
representative, or the staff who will use the equipment, in the correct operation of the systems, as part
of the handover arrangements
...
However,
before testing the emergency systems, he must first
notify everyone in the building of his intentions so that
alarms may be ignored during the period of testing
...
The commissioning engineer
will therefore have access to all relevant contract documents, including the building specifications and the
electrical installation certificates as required by the
IEE Regulations (BS 7671), and have a knowledge of
the requirements of the Electricity at Work Act and
the Health and Safety at Work Act
...
Exercises
1 A meter with a moving coil movement:
(a) has a digital readout
(b) can be used on both a
...
and d
...
supplies
(c) has a linear scale
(d) can be used to measure power
...
c
...
c
...
3 A dynamometer instrument:
(a) has a digital readout
(b) can only be used on electronic circuits
(c) has a linear scale
(d) can be used to measure power
...
5 Instrument transformers can be used to extend
the range of instruments connected to:
(a) a
...
circuits
(b) d
...
circuits
(c) 400V supplies only
(d) rectified a
...
circuits
...
212
ADVANCED ELECTRICAL INSTALLATION WORK
7 To reduce errors when testing electronic circuits,
the test instrument should:
(a) have a very low impedance
(b) have a very high impedance
(c) have a resistance equal to the circuit impedance
(d) have a resistance approximately equal to the
circuit current
...
The
two readings obtained are 100 and 50 W and,
therefore, the total power in the system is:
(a) 50 W
(b) 75 W
(c) 150 W
(d) 5 kW
...
5 M⍀
(d) 1 M⍀
...
11 One objective of the polarity test is to verify that:
(a) lampholders are correctly earthed
(b) final circuits are correctly fused
(c) the CPC is continuous throughout the
installation
(d) the protective devices are connected in the
live conductor
...
13 The value of a satisfactory insulation resistance test
on each final circuit of a 230 V installation must be:
(a) less than 1 ⍀
(b) less than 0
...
5 M⍀
(d) not less than 1 M⍀
...
5 M⍀
(c) not less than 0
...
The maximum inspection and retest period for a
general electrical installation is:
(a) 3 months
(b) 3 years
(c) 5 years
(d) 10 years
...
‘To ensure that all the systems within a building
work as they were intended to work’ is one definition of the purpose of:
(a) testing electrical systems
(b) inspecting electrical systems
(c) commissioning electrical systems
(d) isolating electrical systems
...
The person responsible for interpreting the client’s
requirements to the building team is the:
(a) main contractor
(b) subcontractor
(c) client
(d) architect
...
The electrical contractor is also called the:
(a) main contractor
(b) subcontractor
INSPECTION, TESTING AND COMMISSIONING
22
23
24
25
26
27
(c) client
(d) architect
...
The people responsible for demonstrating good
workmanship and maintaining good relationships
with other trades are the:
(a) building team
(b) electrical design team
(c) electrical installation team
(d) construction industry
...
A simple network diagram can show:
(a) the actual cost of a contract
(b) the actual number of man-hours involved in a
contract
(c) the interdependence of the various activities
involved in a particular contract
(d) the rating of the incoming supply cable
...
The Regulations define isolation switching as:
(a) a mechanical switching device capable of
making, carrying and breaking current under
normal circuit conditions
(b) cutting off an electrical installation or circuit
from every source of electrical energy
(c) the rapid disconnection of the electrical supply to remove or prevent danger
28
29
30
31
32
213
(d) the switching of electrical equipment in normal service
...
Emergency switching can be defined as:
(a) a mechanical switching device capable of
making, carrying and breaking current under
normal circuit conditions
(b) cutting off an electrical installation or circuit
from every source of electrical energy
(c) the rapid disconnection of the electrical supply to remove or prevent danger
(d) the switching of electrical equipment in normal service
...
4 ⍀ on
circuits feeding 13 A socket outlets
(c) the fault causes damage to the circuit isolating
switches
(d) the fault causes a temperature rise which might
damage the insulation and terminations of the
circuit conductors
...
1 ⍀
(b) 1
...
0 ⍀
(d) 2
...
The earth fault loop impedance of a socket outlet
circuit protected by a 30 A cartridge fuse to BS
1361 must not exceed:
(a) 0
...
14 ⍀
214
33
34
35
36
37
38
ADVANCED ELECTRICAL INSTALLATION WORK
(c) 1
...
0 ⍀
...
0 mm2 phase conductor and 2
...
61 ⍀
(b) 9
...
02 ⍀
(d) 16
...
The (R1 ϩ R2) resistance of 176 m of PVC insulated copper cable having a 2
...
608 ⍀
(b) 7
...
82 ⍀
(d) 19
...
The value of the earth fault loop impedance ZS of
a circuit fed by 40 m of PVC insulated copper
cable having a 2
...
5 mm2 protective conductor connected to a supply having an impedance ZE of 0
...
436 ⍀
(b) 9
...
01 ⍀
(d) 780
...
The time/current characteristics shown in Fig
...
20
indicate that a fault current of 300 A will cause a
30 A semi-enclosed fuse to BS 3036 to operate in
(a) 0
...
1 s
(c) 0
...
0 s
...
3
...
02 s
(b) 8 s
(c) 30 s
(d) 200 s
...
This is one definition of:
(a) fusing factor
(b) effective discrimination
(c) a miniature circuit breaker
(d) a circuit protective conductor
...
02 s
(b) 0
...
40 The overcurrent protective device protecting fixed
equipment in rooms other than bathrooms must
operate within
(a) 0
...
4 s
(c) 5 s
(d) 45 s
...
42 By referring to the table in the IEE Regulations
(Tables 41B1, 42B2 and 41D), determine the
maximum permitted earth fault loop impedance
ZS for the following circuits:
(a) a ring main of 13 A socket outlets protected
by a 30 A semi-enclosed fuse to BS 3036
(b) a ring main of 13 A socket outlets protected
by a 30 A cartridge fuse to BS 1361
(c) a single socket outlet protected by a 15 A type
1 MCB to BS 3871
(d) a water heating circuit protected by a 15 A
semi-enclosed fuse to BS 3036
(e) a lighting circuit protected by a 6 A HBC fuse
to BS 88 Part 2
(f ) a lighting circuit protected by a 5 A semienclosed fuse to BS 3036
...
An earth loop impedance test indicates that ZS has a value of 1
...
Calculate the
minimum size of the protective conductor
...
Determine the time taken for each protection
device to clear an earth fault on this circuit by
referring to the characteristics of Fig
...
20
...
5 mm2 PVC insulated and sheathed cable is
used to feed a single 13 A socket outlet from a 15 A
INSPECTION, TESTING AND COMMISSIONING
46
47
48
49
50
51
52
53
semi-enclosed fuse in a consumer’s unit connected
to a 230 V supply
...
9 ⍀
...
State the ambient temperature correction factors
for cables protected by a semi-enclosed fuse to BS
3036 and installed in the following ambient temperatures:
(a) 25°C
(b) 35°C
(c) 45°C
...
State the factors which must be applied to the current carrying capacity of cables when they are:
(a) protected by an MCB
(b) protected by a semi-enclosed fuse
...
0 mm
cable clipped direct to a wall
...
Calculate the volt drop of 30 m of 10 mm cable
connected to the single-phase a
...
mains supply
and carrying a design current of 32 A
...
The circuit is protected by a
5 A semi-enclosed fuse to BS 3036 and runs 30 m
through an ambient temperature of 35°C
...
Calculate:
(a) the design current Ib
(b) the cable rating It
(c) the size of cable
(d) the volt drop
...
The load is wired in single-core PVC cables
grouped with two other circuits in steel conduit
fixed to a wall
...
Calculate:
(a) the design current Ib
(b) the cable rating It
(c) the cable size
(d) the volt drop
...
Describe the importance of a correct attitude
towards the customer by an apprentice electrician
and other members of the installation team
...
Calculate the value
of the resistor required to make the movement
into:
(a) a 10 A ammeter
(b) a 250 V voltmeter
...
The CPC of a lighting final circuit is formed by
approximately 70 m of 1
...
Calculate a satisfactory value for a continuity test
on the CPC given that the resistance per metre of
1
...
1 m⍀/m
...
Determine a satisfactory test result for this CPC, using
the information given in Table 2
...
Describe briefly
a suitable instrument to carry out this test
...
The final circuits
are to be supplied by a consumer unit
...
State the values to be
obtained for a satisfactory test
...
Sketch a circuit diagram and indicate the test circuit path
...
State the type of
216
63
64
65
66
67
68
ADVANCED ELECTRICAL INSTALLATION WORK
instrument to be used and the values of a satisfactory test
...
State seven separate tasks carried out by the electrical installation team
...
State the advantages of a written legal contract as
compared to a verbal contract
...
Activity B takes 1 week commencing in
week 1
...
Activity D takes 4 weeks commencing in
week 7
...
Activity F takes 5 weeks commencing in
week 4
...
Activity H takes 4 weeks commencing in
week 6
...
Due to the availability of men and materials
some activities must be completed before others
can commence, as follows
...
Activity D can
only commence when C is completed
...
Activity
G can only commence when F is completed
...
Activity I does not restrict any other activity
...
(b) Assemble a network diagram for the contract
...
(d) Find the time required to complete the
contract
...
(f ) State the float time in activity D
...
State the requirements of the Electricity at Work
Act with regard to
(a) ‘live’ testing and ‘fault diagnosis’
(b) ‘live working’ to repair a fault
...
List a logical procedure for the isolation of an electrical circuit
...
Use a sketch to describe the construction of a test
lead approved to HSE GS 38
...
Describe air vane and air piston damping
...
Use a labelled sketch to describe the construction
and operation of a tong tester
...
Use sketches to describe the operation of the
deflection system in a moving coil instrument
...
c
...
Describe what is meant by damping of a system
...
Describe the construction and operation of a
dynamometer wattmeter
...
3
FAULT DIAGNOSIS AND RECTIFICATION
To diagnose and find faults in electrical installations
and equipment is probably one of the most difficult
tasks undertaken by an electrician
...
As the systems we install become
more complex, then the faults developed on these systems become more complicated to solve
...
The ideal person will tackle the problem using a
reasoned and logical approach, recognize his own limitations and seek help and guidance where necessary
...
If possible, fault finding should be planned ahead
to avoid inconvenience to other workers and to avoid
disruption of the normal working routine
...
The diagnosis and rectification of a fault is therefore often carried out in very
stressful circumstances
...
2 There is partial or localized loss of power
...
Causes of electrical faults
A fault is not a natural occurrence; it is an unplanned
event which occurs unexpectedly
...
If the installation was properly designed in the first
instance to perform the tasks required of it by the
user, then the negligence, misuse or abuse must be the
fault of the user
...
Negligence on the part of the user may be due to
insufficient maintenance or lack of general care and
attention, such as not repairing broken equipment or
removing covers or enclosures which were designed to
prevent the ingress of dust or moisture
...
Circuits are
sometimes overloaded because a company grows and
a greater demand is placed on the existing installation
by the introduction of new or additional machinery
and equipment
...
Cable faults usually occur at
each end, where the human hand has been at work
at the point of cable inter-connections
...
All cable connections must be made mechanically and electrically secure
...
The only
exceptions to this rule are when
■ underground cables are connected in a compound filled or encapsulated joint;
■ floor warming or ceiling warming heating systems are connected to a cold tail;
■ a joint is made by welding, brazing, soldering or
compression tool
...
2 Faults also occur at cable terminations
...
All conductors of flexible cords must be terminated
within the terminal connection otherwise the current carrying capacity of the conductor is reduced,
which may cause local heating
...
Faults also occur at accessories such as switches,
sockets, control gear, motor contactors or at the point
of connection with electronic equipment
...
Contacts that make and break a
circuit are another source of wear and possible failure, so switches and motor contactors may fail after
extensive use
...
Electronic equipment can be
damaged by the standard tests described in the IEE
Regulations and must, therefore, be disconnected
before testing begins
...
Many panel instruments are standard sizes connected to CT’s or VT’s and this is another source of
possible faults of the types described in points 1–3
...
Faults often occur in luminaires (light fittings)
because the lamp has expired
...
The points made in 1–3 about cable and flexible cord connections are also relevant to luminaire
faults
...
When terminating flexible cords, the insulation should be
carefully removed without cutting out any flexible
cord strands of wire because this effectively reduces
the cross section of the conductor
...
The connection screws should be opened
fully so that they will not snag the flexible cord as it
is eased into the connection
...
The terminal
screws should then be tightened
...
It is unusual for
an electrical component to become faulty when it is
relatively new because it will have been manufactured
and tested to comply with the appropriate British
Standard
...
Modern electrical installations using new materials can
now last longer than fifty years
...
Good design, good workmanship and the use of proper materials are essential if the
installation is to comply with the relevant Regulations
(IEE Regulations 130–01–01 and 130–02–01)
...
Most importantly, electricians must use their detailed
knowledge of electrical circuits and equipment learned
through training and experience and then apply this
knowledge to look for a solution to the fault
...
These were first discussed in Chapter 4 of
219
Basic Electrical Installation Work 4th Edition and are
repeated here for your convenience
...
This means that for a domestic lighting circuit
rated at 5 A, a maximum of 11 lighting outlets could
be connected to each circuit
...
In this way the
whole installation is not plunged into darkness if one
lighting circuit fuses
...
0 or 1
...
The loop-in method is universally employed
with conduit installations or when access from above
or below is prohibited after installation, as is the case
with some industrial installations or blocks of flats
...
The use of junction boxes with fixed brass terminals is the method often adopted in domestic installations, since the joint boxes can be made accessible but
are out of site in the loft area and under floorboards
...
A ceiling rose may only be connected to installations
operating at 250 V maximum and must only accommodate one flexible cord unless it is specially designed
to take more than one (553–04–02)
...
The type of circuit used will depend upon the
installation conditions and the customer’s requirements
...
3
...
A room with two access
220
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
1 Wiring diagram of one-way switch control
...
3
...
doors might benefit from a two-way switch control
(see Fig
...
2) so that the lights may be switched on or
off at either position
...
3
...
One-way, two-way or intermediate switches can be
obtained as plate switches for wall mounting or ceiling mounted cord switches
...
To convert an existing one-way switch control into
a two-way switch control, a three-core and earth cable
is run from the existing switch position to the proposed
second switch position
...
3
...
SOCKET OUTLET CIRCUITS
A plug top is connected to an appliance by a flexible
cord which should normally be no longer than 2 m
FAULT DIAGNOSIS AND RECTIFICATION
221
Fig
...
3 Wiring diagram of intermediate switch control
...
3
...
(Regulation 553–01–07)
...
Socket outlets therefore provide an easy and
convenient method of connecting portable electrical
appliances to a source of supply
...
Each 13 A plug top contains
a cartridge fuse to give maximum potential protection to the flexible cord and the appliances which it
serves
...
222
ADVANCED ELECTRICAL INSTALLATION WORK
RADIAL CIRCUITS
In a radial circuit each socket outlet is fed from the
previous one
...
The fuse
and cable sizes are given in Table 8A of Appendix 8
but circuits may also be expressed with a block diagram, as shown in Fig
...
5
...
Where two or more circuits are installed in the
same premises, the socket outlets and permanently
connected equipment should be reasonably shared out
among the circuits, so that the total load is balanced
...
This is because
the maximum demand of current-using equipment in
kitchens may exceed the rating of the circuit cable and
protection devices
...
throughout its length
...
The circuit details are given in Table 8A, Appendix
8 of the On Site Guide but may also be expressed by
the block diagram given in Fig
...
6
...
Non-fused spurs
The total number of non-fused spurs must not exceed
the total number of socket outlets and pieces of
stationary equipment connected directly in the circuit
...
5 mm2 PVC cable
RING CIRCUITS
Ring circuits are very similar to radial circuits in that
each socket outlet is fed from the previous one, but in
ring circuits the last socket is wired back to the source
of supply
...
3
...
4 mm2 PVC cable
30 or 32 A
etc
...
5 mm2 PVC cable
20 A
etc
...
3
...
FAULT DIAGNOSIS AND RECTIFICATION
The cable used for non-fused spurs must not be less
than that of the ring circuit
...
3
...
223
A non-fused spur may only feed one single or one
twin socket or one permanently connected piece of
equipment
...
Fused spurs
The total number of fused spurs is unlimited
...
The requirements for fused spurs are also given
in Appendix 8 but the various circuit arrangements
may be expressed by the block diagrams of Fig
...
8
...
3
...
WATER HEATING CIRCUITS
Fig
...
7 Connection of non-fused spurs
...
3
...
A small, single-point over-sink type water heater may
be considered as a permanently connected appliance
and so may be connected to a ring circuit through a
224
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
9 Typical ring circuit with spurs
...
The hot and cold water connections must be
connected to an earth connection in order to meet the
supplementary bonding requirements of Regulation
413–05–02
...
ELECTRIC SPACE HEATING CIRCUITS
Fig
...
10 Immersion heater wiring
...
A water heater of the immersion
type is usually rated at a maximum of 3 kW, and could
be considered as a permanently connected appliance,
fed from a fused connection unit
...
Therefore, immersion heaters must be wired on
a separate radial circuit when they are connected to
water vessels which hold more than 15 l
...
10
Electrical heating systems can be broadly divided into
two categories: unrestricted local heating and off-peak
heating
...
Fixed heaters that are wallmounted or inset must be connected through a fused
connection and incorporate a local switch, either on the
heater itself or as a part of the fuse connecting unit, as
shown in Fig
...
8
...
This requirement includes radiators which have an element inside a
silica-glass sheath (601–12–01)
...
All three systems use the
FAULT DIAGNOSIS AND RECTIFICATION
225
Fig
...
11 Possible wiring arrangements for storage heaters
...
3
...
thermal storage principle, whereby a large mass of
heat-retaining material is heated during the off-peak
period and allowed to emit the stored heat throughout the day
...
When calculating the size of cable required to supply a single storage radiator, it is good practice to assume
a current demand equal to 3
...
This
will allow the radiator to be changed at a future time
with the minimum disturbance to the installation
...
See Fig
...
11 for wiring
arrangements
...
The unit is charged during the off-peak period, and
a fan drives the stored heat in the form of warm air
through large air ducts to outlet grilles in the various
rooms
...
3
...
The single storage heater is heated by an electric
element embedded in bricks and rated between 6
and 15 kW depending upon its thermal capacity
...
At the heater position,
a double-pole switch must be installed to terminate
the fixed heater wiring
...
Floor warming installations use the thermal storage
properties of concrete
...
When
226
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
13 Floor warming installations
...
The wiring arrangements are shown in
Fig
...
13
...
Underfloor heating cables installed in bathrooms
or shower rooms must incorporate an earthed metallic sheath or be covered by an earthed metallic grid
connected to the supplementary bonding (Regulation
601–12–02)
...
Consider, as an example, a cooker with the following elements fed from a cooker control unit incorporating a 13 A socket:
4 ϫ 2 kW fast boiling rings ϭ 8000 W
1 ϫ 2 kW grill ϭ 2000 W
1 ϫ 2 kW oven ϭ 2000 W
Total loading ϭ 12 000 W
When connected to 250 V
Current rating ϭ
12 000
ϭ 48 A
...
4 A
Socket outlet ϭ 5 A
Assessed current demand ϭ 10 ϩ 11
...
4 A
Therefore, a cable capable of carrying 26
...
A cooking appliance must be controlled by a switch
separate from the cooker but in a readily accessible
position (Regulation 476–03–04)
...
FAULT DIAGNOSIS AND RECTIFICATION
Designing out faults
The designer of the installation cannot entirely design
out the possibility of a fault occurring but he can design
in ‘damage limitation’ should a fault occur
...
Limiting faults
to only one of many circuits is good practice because
it limits the disruption caused by a fault
...
Finding the electrical fault
known supply or by means of a proving unit such
as that shown in Fig
...
49
...
2
...
The key is retained by
the person working on the isolated equipment
...
5 All relevant safety and functional tests must be
completed before restoring the supply
...
However, it does acknowledge that
some work, such as fault finding and testing, may
require the electrical equipment to remain energized
...
2 Analyse the evidence and use standard tests and a
visual inspection to predict the cause of the fault
...
4 Rectify the fault
...
Safe working procedures
1 The circuits must be isolated using a ‘safe isolation
procedure’, such as that described below, before
beginning to repair the fault
...
The
test equipment used must also be ‘proved’ on a
227
■
■
be trained so that he understands the equipment
and the potential hazards of working live and can,
therefore, be deemed to be ‘competent’ to carry out
the activity;
only use approved test equipment;
set up barriers and warning notices so that the work
activity does not create a situation dangerous to
others
...
The individual circuit or item of equipment must
first be isolated
...
Regulation 12(1) tells us that
isolation means the disconnection and separation of
the electrical equipment from every source of electrical energy in such a way that this disconnection and
separation is secure
...
3
...
purpose must itself be proved immediately before and
immediately after testing the conductors
...
3
...
Start at the top and work your way down the flowchart
...
If the answer is yes, move on
...
Faulty equipment: to repair or
replace?
Having successfully diagnosed the cause of the fault
we have to decide if we are to repair or replace the
faulty component or piece of equipment
...
Some of
the issues which may be discussed are as follows:
■
■
■
■
What is the cost of replacement? Will the replacement cost be prohibitive? Is it possible to replace
only some of the components? Will the labour costs
of the repair be more expensive than a replacement?
Do you have the skills necessary to carry out the
repair? Would the repaired piece of equipment be
as reliable as a replacement?
Is a suitable replacement available within an acceptable time? These days, manufacturers carry small
stocks to keep costs down
...
This is because they consider
suitably constructed test equipment to be as vital for
personal safety as the training and practical skills of
the electrician
...
229
2 The probe tip must not protrude more than 2 mm,
and preferably only 1 mm, be spring-loaded and
screened
...
4 The lead must be flexible and sufficiently robust
...
6 The lead must not have accessible exposed conductors even if it becomes detached from the probe or
from the instrument
...
A suitable probe and lead is shown in Fig
...
47
...
Accident history has
shown that incorrectly set multimeters or makeshift
devices for voltage detection have frequently caused
accidents
...
48 shows a suitable voltage indicator
...
The IEE Regulations (BS 7671) also specify the test
voltage or current required to carry out particular tests
satisfactorily
...
The test instrument must also carry a
calibration certificate, otherwise the recorded results
may be void
...
Test instruments must, therefore, be tested and
recalibrated each year by an approved supplier
...
Modern digital test instruments are reasonably robust,
but to maintain them in good working order they must
be treated with care
...
CONTINUITY TESTER
To measure accurately the resistance of the conductors
in an electrical installation we must use an instrument
230
ADVANCED ELECTRICAL INSTALLATION WORK
which is capable of producing an open circuit voltage
of between 4 and 24 V a
...
or d
...
, and deliver a shortcircuit current of not less than 200 mA (Regulation
713–02)
...
INSULATION RESISTANCE TESTER
The test instrument must be capable of detecting insulation leakage between live conductors and between
live conductors and earth
...
EARTH FAULT LOOP IMPEDANCE TESTER
The test instrument must be capable of delivering
fault currents as high as 25 A for up to 40 ms using the
supply voltage
...
RCD TESTER
Where circuits are protected by a residual current
device we must carry out a test to ensure that the
device will operate very quickly under fault conditions and within the time limits set by the IEE
Regulations
...
The instrument is, therefore, calibrated to
give a reading measured in milliseconds to an in-service
accuracy of 10%
...
However, to carry out all
the tests required by the IEE Regulations will require
a number of test instruments and this will represent a
major capital investment in the region of £1000
...
’
Electrical installation circuits usually carry in excess
of 1 A and often carry hundreds of amperes
...
The test instruments used on electronic
circuits must have a high impedance so that they do
not damage the circuit when connected to take readings
...
In power applications these small disturbances seldom give rise to obvious errors, but in electronic circuits, a small disturbance can completely
invalidate any readings taken
...
This is described in detail in Chapter 4 of this
book under the sub-heading ‘Electronic Test Equipment’ and suitable test instruments are shown in
Figs 4
...
76
...
However,
you may be asked to diagnose and repair a fault on a
system that is unfamiliar to you or outside your experience and training
...
I have said earlier that fault diagnosis can only
be carried out successfully by someone with a broad
range of experience and a thorough knowledge of the
installation or equipment that is malfunctioning
...
It is better to be respected for your honesty
than to attempt something that is beyond you at the
present time and which could create bigger problems
and waste valuable repair time
...
Special situations
OPTICAL FIBRE CABLES
The introduction of fibre-optic cable systems and
digital transmissions will undoubtedly affect future
cabling arrangements and the work of the electrician
...
Fibre-optic systems dramatically reduce the number of cables required for
control and communications systems, and this will in
turn reduce the physical room required for these systems
...
There is no spark risk if the
cable is accidentally cut and, therefore, such circuits
are intrinsically safe
...
The
energy is transferred down the cable as digital pulses
of laser light as against current flowing down a copper
conductor in electrical installation terms
...
3
...
Fig
...
15 Digital pulses of laser light down an optical fibre cable
...
They are cheap to
produce and lightweight because these new cables are
made from high-quality plastic and not high-quality
copper
...
Multicore cables are available containing up to 24 single
fibres
...
Avoid tight-radius bends if possible
and kinks at all costs
...
Fibre-optic cables are Band I circuits when
used for data transmission and must therefore be
segregated from other mains cables to satisfy the IEE
Regulations
...
e
...
Finally, when
working with fibre-optic cables, electricians should
avoid direct eye contact with the low-energy laser light
transmitted down the conductors
...
A dangerous situation occurs when
the static charge has built up to a potential capable of
striking an arc through the airgap separating the two
surfaces
...
A lightning strike is the discharge of the thunder cloud, which
might have built up to a voltage of 100 MV, to the general mass of earth which is at 0 V
...
Static charge builds up between any two insulating
surfaces or between an insulating surface and a conducting surface, but it is not apparent between two
conducting surfaces
...
Static electricity also builds up in modern offices
and similar carpeted areas
...
Individuals only become
aware of the charge if they touch earthed metalwork,
such as a stair banister rail, before the static electricity
has been dissipated
...
The precautions against this problem include using
floor coverings that have been ‘treated’ to increase
232
ADVANCED ELECTRICAL INSTALLATION WORK
Double socket-outlets must have two
terminals for protective conductors
...
5 mm2
DISTRIBUTION
FUSE BOARD
PE
Separate
connections
Ring final circuit supplying
twin socket-outlets
(total protective conductor
current exceeding 10 mA)
...
3
...
their conductivity or that contain a proportion of natural fibres that have the same effect
...
A nylon overall and nylon bed sheets build up static
charge which is the cause of the ‘crackle’ when you
shake them
...
Petrol pumps, operating theatre oxygen masks and
car spray booths are particularly at risk because a spark
in these situations may ignite the flammable liquid,
powder or gas
...
Bonding surfaces together with equipotential
bonding conductors prevents a build-up of static electricity between the surfaces
...
Hospitals use cotton sheets and uniforms, and
use bonding extensively in operating theatres
...
Rubber constructed in this manner enables
any build-up of static charge to ‘leak’ away
...
Avoiding shutdown of IT
equipment
Every modern office now contains computers, and
many systems are linked together or networked
...
This is clearly not a fault
current, but is typical of the current which flows in
the circuit protective conductor of IT equipment under
normal operating conditions
...
IEE Guidance Note 7 recommends
that IT equipment should be connected to double
sockets as shown in Fig
...
16
...
Mainframe computers and computer networks are sensitive to mains distortion or interference,
which is referred to as ‘noise’
...
These
distortions in the mains supply can cause computers
to ‘crash’ or provoke errors and are shown in Fig
...
17
...
This can be provided by taking
FAULT DIAGNOSIS AND RECTIFICATION
233
One solution to this problem is to protect ‘precious’
software systems with an uninterruptable power supply (UPS)
...
c
...
The UPS is plugged into the mains supply and
the computer systems are plugged into the UPS
...
It is best to dedicate a ring or radial circuit to the
UPS and either to connect the computer equipment
permanently or to use non-standard outlets to discourage the unauthorized use and overloading of these
special supplies by, for example, kettles
...
Electricians intending to isolate supplies for testing or
modification should first check and then check again
before they finally isolate the supply in order to avoid
loss or damage to computer systems
...
3
...
c
...
Damage to electronic devices by
‘overvoltage’
Fig
...
18 A simple noise suppressor
...
A clean earth
can also be taken from this point, which is usually one
core of the cable and not the armour of an SWA cable,
and distributed around the final wiring circuit
...
3
...
SECURE SUPPLIES
The mains electrical supply in the UK is extremely
reliable and secure
...
The use of electronic circuits in all types of electrical
equipment has increased considerably over recent years
...
All electronic circuits are low voltage circuits
carrying very small currents
...
Electronic
circuits operate in the milliampere or even microampere range
...
The use of an insulation resistance test as described
by the IEE Regulations (described in Chapter 2 of
this book), must be avoided with any electronic equipment
...
When carrying out an insulation resistance test as part
of the prescribed series of tests for an electrical installation, all electronic equipment must first be disconnected or damage will result
...
74 to avoid damaging the electronic
components
...
The induction heater consists of a coil of large cross
section
...
When the supply is switched on, eddy currents are
induced into the work-piece and it heats up very quickly
so that little heat is lost to conduction and convection
...
For shallow penetration, high frequency, high current, short time application is typically used for tool
tempering
...
When these machines are not working they look very
harmless but when they are working they operate very
quietly and there is no indication of the intense heat
that they are capable of producing
...
The combination of risks of high frequency and intense
heating means that before any maintenance, repair
work or testing is carried out, the machine must first be
securely isolated and no one should work on these
machines unless they have received additional training
to enable them to do so safely
...
The inductive nature of the
industrial load causes the current to lag behind the
voltage and creates a bad power factor
...
A power factor of say 0
...
An inductive circuit, such as that produced by an
electric motor, induces an electromagnetic force which
opposes the applied voltage and causes the current wave
to lag the voltage wave
...
If a capacitive circuit is
employed, the current leads the voltage since the capacitor stores energy as the current rises and discharges it as
the current falls
...
Power Factor and Power Factor
Improvement was discussed in Chapter 1 of this book
...
The power
factor of the consumer is governed entirely by the electrical plant and equipment that is installed and operated
within the consumer’s buildings
...
Electricity supply authorities discourage the use of
equipment and installations with a low power factor
because they absorb part of the capacity of the generating plant and the distribution network to no useful
effect
...
If
the power factor falls below a datum level of between
0
...
9 then extra charges are incurred
...
Power factor improvement of most industrial loads
is achieved by connecting capacitors to either
■
■
individual items of equipment or
banks of capacitors may be connected to the main
bus-bars of the installation at the intake position
...
If the load is constant then banks of capacitors at the mains intake position would be indicated
...
Power factor correction by capacitors is the most
popular method because of the following:
■
■
■
They require no maintenance
...
Capacitors may be installed adjacent to individual
pieces of equipment or at the mains intake position
...
Capacitors store charge and must be disconnected
before the installation or equipment is tested in accordance with section 7 of the IEE Regulations BS 7671
...
This discharges
the capacitor safely when not in use
...
To discharge a capacitor
safely and responsibly it must be discharged slowly over
a period in excess of five ‘time-constants’ through a suitable discharge resistor
...
Presence of storage batteries
Since an emergency occurring in a building may cause
the mains supply to fail, the emergency lighting should
be supplied from a source which is independent from
the main supply
...
In most commercial, industrial and public service buildings housing essential services, the alternative
power supply would be from batteries, but generators
may also be used
...
In some premises a delay of more than 5 seconds is considered unacceptable, and in these cases a
battery supply is required to supply the load until the
generator can take over
...
BS 5266 states
that after a battery is discharged by being called into
operation for its specified duration of time, it should
be capable of once again operating for the specified
duration of time following a recharge period of not
longer than 24 hours
...
BS 5266 states that escape lighting should
operate for a minimum of 1 hour
...
Within the premises after the mains supply
has failed
...
The
emergency lighting installation must be segregated
completely from any other wiring, so that a fault on the
main electrical installation cannot damage the emergency lighting installation (IEE Regulation 528–01)
...
Motor vehicle batteries
are not suitable for emergency lighting applications,
except in the starter system of motor-driven generators
...
The battery room of a central battery system must be well ventilated and, in the case of
a motor-driven generator, adequately heated to ensure
rapid starting in cold weather
...
After testing, the emergency system
must be carefully restored to its normal operative state
...
It may be necessary to produce the record
as evidence of satisfactory compliance with statutory
legislation to a duly authorized person
...
The batteries contained
within these units should be replaced about every
5 years, or as recommended by the manufacturer
...
A secondary
cell has the advantage of being rechargeable
...
When the cell is connected to a load, the
chemical energy is converted to electrical energy
...
Each cell delivers
about 2 V, and when six cells are connected in series
a 12 V battery is formed
...
19 shows the
construction of a lead-acid battery
...
3
...
A lead-acid battery is constructed of lead plates
which are deeply ribbed to give maximum surface
area for a given weight of plate
...
The separators are made of a porous insulating
material, such as wood or ebonite, and the whole
assembly is immersed in a dilute sulphuric acid solution in a plastic container
...
It is stated in ampere-hours, abbreviation Ah, and calculated at the 10-hour rate which is the steady load
current which would completely discharge the battery
in 10 hours
...
MAINTENANCE OF LEAD-ACID BATTERIES
■
The plates of the battery must always be covered by
the dilute sulphuric acid
...
■
Battery connections must always be tight and should
be covered with a thin coat of petroleum jelly
...
A
discharged cell will have a specific gravity of 1
...
280 when fully charged
...
To maintain a battery in good condition it should
be regularly trickle-charged
...
Most batteries used for
standby supplies today are equipped with constant
voltage chargers
...
As
the battery becomes more fully charged its voltage will
rise until it reaches the constant voltage level where
the current output from the charger will drop until
it is just sufficient to balance the battery’s internal
losses
...
The room used to charge the emergency supply
storage batteries must be well ventilated because
the charged cell gives off hydrogen and oxygen,
which are explosive in the correct proportions
...
This is an important part of your companies ‘good
customer relationships’ with the client
...
We have also said in Chapter 2 of this book that
having a good attitude to health and safety, working
FAULT DIAGNOSIS AND RECTIFICATION
conscientiously and neatly, keeping passageways clear
and regularly tidying up the workplace is the sign of a
good and competent craftsman
...
All the off-cuts of conduit, trunking and tray
also go into the skip
...
The part coils of cable and any other re-useable
leftover lengths of conduit, trunking or tray will be
taken back to your employer’s stores area
...
What goes into the skip for normal disposal into a
land fill site is usually a matter of common sense
...
We will now look at asbestos and
large quantities of used fluorescent tubes which are
classified as ‘Special waste’
...
When separated it becomes a fluffy, fibrous
material with many uses
...
In the buildings where it was installed some 40 years
ago, when left alone, it does not represent a health hazard, but those buildings are increasingly becoming in
need of renovation and modernisation
...
Asbestos is a serious
health hazard if the dust is inhaled
...
Working with asbestos materials is not a job for
anyone in the electrotechnical industry
...
Specialist
contractors, who will wear fully protective suits and
use breathing apparatus, are the only people who can
safely and responsibly carry out the removal of asbestos
...
This material is then disposed of in a special land fill
site with other toxic industrial waste materials and the
site monitored by the local authority for the foreseeable future
...
This
work often involves removing the old fluorescent
fittings, hanging on chains or fixed to beams and
installing a suspended ceiling and an appropriate
number of recessed modular fluorescent fittings
...
All of these
materials can go into the ordinary skip
...
The responsible way to dispose of fluorescent tubes is by grinding
them up into small pieces using a ‘lamp crusher’,
which looks very much like a garden waste shredder
...
The COSHH Regulations and the Controlled
Waste Regulations 1998 have encouraged specialist
companies to set up businesses dealing with the responsible disposal of toxic waste material
...
The system is called ‘Waste Transfer Notes’
...
The person handing over the
waste material to the waste disposal company will be
given a copy of the notes and this must be filed in a
safe place, probably in the job file or a dedicated file
...
The
cost of this service is then passed on to the customer
...
The Environmental Health Officer at your local
Council Offices will always give advice and point you
in the direction of specialist companies dealing with
toxic waste disposal
...
2 State how negligence, misuse or abuse by the
installer or user may result in faults
...
4 List the five steps involved in finding and rectifying a fault
...
6 State the requirements of the Electricity at Work
Act with regard to the following:
(a) ‘live’ testing and ‘fault diagnosis’,
(b) ‘live working’ to repair a fault
...
8 List a logical procedure for the isolation of an
electrical circuit
...
State five factors which might influence the
decision to repair or replace a piece of faulty
equipment
...
Briefly explain what we mean by static electricity
and how we would prevent it becoming a hazard
in a store-room where large quantities of paint are
stored on metal shelves
...
Describe one method of obtaining a ‘secure’ supply for a computer network
...
Explain with a sketch how digital pulses pass down
fibre optic cables
...
When repairing electronic circuits it is important to replace a damaged component
with an identical or equivalent component
...
,
and the reference numbers of equivalent components,
and some of this information is included in the
Appendices
...
4
...
Electronic circuit symbols
The British Standard BS EN 60617 recommends that
particular graphical symbols be used to represent a
range of electronic components on circuit diagrams
...
Figure
4
...
Resistors
All materials have some resistance to the flow of an
electric current but, in general, the term resistor
describes a conductor specially chosen for its resistive
properties
...
They are usually
manufactured as either carbon composition or carbon
film
...
4
...
If subjected to overload, carbon resistors usually
decrease in resistance since carbon has a negative temperature coefficient
...
Carbon resistors
have a power rating of between 0
...
When a resistor of a larger power rating is required
a wire-wound resistor should be chosen
...
4
...
Wire-wound
resistors are designed to run hot and have a power rating up to 20 W
...
A variable resistor is one which can be varied continuously from a very low value to the full rated resistance
...
The most common type used in electronic work has a circular carbon track contacted by a
metal wiper arm
...
4
...
Fig
...
3
...
The variation in resistance can be to either a logarithmic or a linear scale
...
The method used will depend upon the
type, physical size and manufacturer’s preference, but
in general the larger components have values marked
directly on the body and the smaller components use
the standard resistor colour code
...
4
...
THE STANDARD COLOUR CODE
Fig
...
2 Construction of resistors
...
Multiplication factors and
prefixes are dealt with in Chapter 8
...
1
...
The bands are located on the component
towards one end
...
Band (a) gives the first number of the
component value, band (b) the second number, band
(c) the number of zeros to be added after the first two
Table 4
...
7 k⍀ resistor would be abbreviated to
4k7, a 5
...
8 M⍀ resistor
to 6M8
...
The abbreviation F means Ϯ1%, G means Ϯ2%,
J means Ϯ5%, K means Ϯ10% and M means Ϯ20%
...
7 k⍀ resistor with a tolerance of 2%
would be abbreviated to 4 k7G
...
6 ⍀ resistor with
a tolerance of 5% would be abbreviated to 5R6J
...
8 M⍀ resistor with a 10% tolerance would be
abbreviated to 6M8K
...
Colour
Band (a)
first
number
Band (b)
second
number
Band (c)
number
of zeros
Band (d)
tolerance
band (%)
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Gold
Silver
None
0
1
2
3
4
5
6
7
8
9
–
–
–
0
1
2
3
4
5
6
7
8
9
–
–
–
None
1
2
3
4
5
6
7
–
–
÷10
÷100
–
–
1
2
–
–
–
–
–
–
–
5
10
20
242
ADVANCED ELECTRICAL INSTALLATION WORK
numbers and band (d) the resistor tolerance
...
The tolerance band indicates the maximum tolerance variation in the declared value of resistance
...
EXAMPLE 1
A resistor is colour coded yellow, violet, red, gold
...
Band (a) – yellow has a value of 4
...
Band (c) – red has a value of 2
...
The value is therefore 4700 Ϯ 5%
...
7 k⍀ Ϯ 5% or 4k7J
...
Determine the value
of the resistor
...
Band (b) – blue has a value of 6
...
Band (d) – silver indicates a tolerance of 10%
...
EXAMPLE 4
A resistor is colour coded orange, white, silver, silver
...
Band (a) – orange has a value of 3
...
Band (c) – silver indicates divide by 100 in this band
...
The value is therefore 0
...
39 ⍀ Ϯ 10% or R39K
...
This is
not a disadvantage as in most electronic circuits the
value of the resistors is not critical
...
Therefore, in electronics, we use the
preferred value closest to the actual value required
...
The next larger preferred value which would give the
maximum possible range of resistance values without
too much overlap would be 120 ⍀
...
Therefore, these
two preferred value resistors cover all possible resistance
values between 90 and 132 ⍀
...
There is a series of preferred values for each tolerance
level, as shown in Table 4
...
Table 4
...
Resistance values of 47 ⍀, 470 ⍀, 4
...
7 M⍀, etc
...
EXAMPLE 3
A resistor is colour coded blue, grey, green, gold
...
Band (a) – blue has a value of 6
...
Band (c) – green has a value of 5
...
The value is therefore 6 800 000 Ϯ 5% and could be written as
6
...
TESTING RESISTORS
The resistor being tested should have a value close to
the preferred value and within the tolerance stated
by the manufacturer
...
The
ohmmeter and its use are discussed later in this chapter and shown at Fig
...
76
...
2 Preferred values
CAPACITORS IN ACTION
E6 series
20% tolerance
E12 series
10% tolerance
E24 series
5% tolerance
10
10
10
11
12
13
15
16
18
20
22
24
27
30
33
36
39
43
47
51
56
62
68
75
82
91
12
15
15
18
22
22
27
33
33
39
47
47
56
68
68
82
243
If the resistor to be tested is connected into an electronic circuit it is always necessary to disconnect one
lead from the circuit before the test leads are connected,
otherwise the components in the circuit will provide
parallel paths, and an incorrect reading will result
...
4
...
c
...
The charged capacitor then blocks
any further d
...
current flow
...
c
...
Current
is apparently flowing through the capacitor because
electrons are moving to and fro in the wires joining
the capacitor plates to the a
...
supply
...
4
...
COUPLING AND DECOUPLING CAPACITORS
Capacitors can be used to separate a
...
and d
...
in an
electronic circuit
...
4
...
c
...
c
...
c
...
This blocks the
d
...
while offering a low reactance to the a
...
component
...
c
...
4
...
In
this chapter we shall consider the practical aspects
associated with capacitors in electronic circuits
...
In electronics we are
not only concerned with the amount of charge stored by
the capacitor but in the way the value of the capacitor
determines the performance of timers and oscillators
by varying the time constant of a simple capacitor–
resistor circuit
...
4
...
244
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
6 Capacitors and their symbols used in electronic circuits
...
This will provide a low reactance path for the a
...
component of the supply and only d
...
will be presented to
the input of B
...
c
...
c
...
Table 4
...
The non-polarized type
can be connected either way round, but polarized
capacitors must be connected to the polarity indicated
otherwise a short circuit and consequent destruction
of the capacitor will result
...
Figure 4
...
Polyester capacitors
Polyester capacitors are an example of the plastic film
capacitor
...
The capacitor value may be marked on the plastic
film, or the capacitor colour code given in Table 4
...
This dielectric material gives a compact
capacitor with good electrical and temperature characteristics
...
Colour
Band (a) Band (b) Band (c) Band (d) Band (e)
first
second
number of tolerance maximum
number number zeros to
(%)
voltage (V)
be added
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
–
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
None
1
2
3
4
5
6
7
8
9
20
–
–
–
–
5
–
–
–
10
–
100
250
–
400
–
–
–
–
–
increase with increased accuracy, they tend to be more
expensive than plastic film capacitors
...
Ceramic capacitors
Mica capacitors
Mica capacitors have excellent stability and are accurate to Ϯ1% of the marked value
...
They have high stability and low loss
...
They are constructed on the ‘Swiss roll’
principle as are the paper dielectric capacitors used for
power-factor correction in electrical installation circuits
...
Electrolytic capacitors have a size gain of
approximately 100 times over the equivalent nonelectrolytic type
...
Their large capacity makes them
ideal as smoothing capacitors in power supplies
...
The voltage ratings available tend to be low, as
with all electrolytic capacitors
...
3 V
...
Variable capacitors
Variable capacitors are constructed so that one set of
metal plates moves relative to another set of fixed
metal plates as shown in Fig
...
7
...
Air dielectric variable capacitors are used to tune radio
receivers to a chosen station, and small variable capacitors called trimmers or presets are used to make fine,
infrequent adjustments to the capacitance of a circuit
...
They look like a
‘raindrop’ or ‘blob’ with two leads protruding from the
bottom
...
4 may
SELECTING A CAPACITOR
When choosing a capacitor for a particular application, three factors must be considered: value, working
voltage and leakage current
...
4 Colour code for tantalum polarized capacitors (values in microfarads)
Colour
Band (a)
first number
Band (b)
second number
Spot number of
zeros to be added
Band (c)
maximum voltage(V)
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Pink
–
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
None
1
2
–
–
–
–
–
÷100
÷1000
10
–
–
–
6
...
4
...
Michael Faraday
...
In an
electronic filter circuit a typical capacitor value might
be 100 pF at 63 V
...
It may be useful to remember that
1000 pF ϭ 1 nF
1000 nF ϭ 1 F
The working voltage of a capacitor is the maximum
voltage that can be applied between the plates of the
capacitor without breaking down the dielectric insulating material
...
c
...
c
...
c
...
c
...
When connecting a capacitor to the 230 V mains supply we must choose a
working voltage of about 400 V because 230 V rms
...
The ‘factor of safety’ is
small and, therefore, the working voltage of the capacitor must not be exceeded
...
The loss of charge by leakage through it should be
very small for a practical capacitor
...
3 and 4
...
EXAMPLE 1
A plastic film capacitor is colour coded, from top to bottom, brown, black,
yellow, black, red
...
From Table 4
...
Band (b) – black has a value 0
...
Band (d) – black indicates 20%
...
The capacitor has a value of 100 000 pF or 0
...
EXAMPLE 2
Determine the value, tolerance and working voltage of a polyester capacitor colour-coded, from top to bottom, yellow, violet, yellow, white, yellow
...
3
Band (a) – yellow has a value 4
...
ELECTRONIC COMPONENTS
Band (c) – yellow indicates multiply by 10 000
...
Band (e) – red indicates 400 V
...
47 F with a tolerance of
10% and a maximum working voltage of 400 V
...
Determine the value, tolerance and voltage of this capacitor
...
3 we obtain the following:
Band (a) – blue has a value 6
...
Band (c) – orange indicates multiply by 1000
...
Band (e) – brown indicates 100 V
...
4
...
If the resistance is less than about 1 M⍀, it is
allowing current to pass from the ohmmeter and,
therefore, the capacitor is leaking and is faulty
...
Polarized capacitors
It is essential to connect the true positive of the ohmmeter to the positive lead of the capacitor, as shown in
Fig
...
76
...
The capacitor has a value of 68 000 pF or 68 nF with a tolerance of 20%
and a maximum working voltage of 100 V
...
This is the same practice as we used earlier for resistors
...
Therefore, a 1
...
1000 pF ϭ 1 nF ϭ 0
...
Inductors and transformers
An inductor is a coil of wire wound on a former (to
give it a specific shape) having a core of air or iron
...
A transformer consists of two coils
wound on a common magnetic core and, therefore, in
this sense, is also an inductor
...
A small electronic transformer and the aerial of a radio receiver comprising a
coil wound on a ferrite core are shown in Fig
...
8
...
Most electronic circuits require a voltage between
5 and 12 V and the transformer provides an ideal way of
initially reducing the mains voltage to a value which is
suitable for the particular electronic circuit
...
4
...
248
ADVANCED ELECTRICAL INSTALLATION WORK
Compared with other individual electronic components, inductors are large
...
This causes electrical interference –
called electrical noise – and may prevent the normal
operation of the electronic circuit
...
Electromagnetic relays
An electromagnetic relay is simply an electromagnet
operating a number of switch contacts, as shown in
Fig
...
9
...
The
relay coil is electrically insulated from the switch contacts and, therefore, a relay is able to switch circuits
operating at a different voltage than the coil operating
voltage
...
The switch part of the relay may have many
poles controlling several circuits at once
...
The term
‘overcurrent’ can be subdivided into overload current
and short-circuit current
...
An overload may result in currents of two or three times the rated current flowing
in the circuit, while short-circuit currents may be
hundreds of times greater than the rated current
...
Fuses provide overcurrent protection when connected in the live conductor;
they must not be connected in the neutral conductor
...
Figure 4
...
Protection from excess
current is covered in some detail in Chapter 2
...
4
...
Fig
...
9 An electromagnetic relay
...
However, all
mechanical–electrical switches are limited in their
speed of operation by the time taken physically for a
movable contact to make or break a switch contact
...
This is only possible using the properties
of semiconductor materials in devices such as transistors and thyristors
...
Packaging electronic components
When we talk about packaging electronic components we are not referring to the parcel or box which
contains the components for storage and delivery, but
to the type of encapsulation in which the tiny semiconductor material is contained Figure 4
...
Identification of
the pin connections for different packages is given
within the text as each separate or discrete component
is considered, particularly later in this chapter when
we discuss semiconductor devices
...
4
...
Appendices aim to draw together all the information
on pin connections and packages for easy reference
...
Similar facilities are available in most towns and cities
for the purchase of electronic components and equipment
...
Some of these national companies also offer a
24-hour telephone order and mail order service
...
4
...
full-colour, fully illustrated catalogues also contain
an enormous amount of technical information
...
For local suppliers you must
consult your local phone book and Yellow Pages
...
Semiconductor devices
SEMICONDUCTOR MATERIALS
Modern electronic devices use the semiconductor
properties of materials such as silicon or germanium
...
4
...
The outer
250
ADVANCED ELECTRICAL INSTALLATION WORK
electron orbits contain four electrons known as
valence electrons
...
There are no free electrons in pure
silicon or germanium and, therefore, no conduction
can take place unless the bonds are broken and the
lattice framework is destroyed
...
This process is known
as doping
...
Since the
material has lost a negative charge, the material becomes
positive and is known as a p-type material (p for
positive)
...
Since the material has gained a negative charge
it is known as an n-type material (n for negative)
...
Such a junction is called a diode, since it is
Fig
...
13 Symbol for and appearance of semiconductor diodes
...
SEMICONDUCTOR DIODE
A semiconductor or junction diode consists of a p-type
and n-type material formed in the same piece of silicon or germanium
...
4
...
If
the anode is made positive with respect to the cathode,
the junction will have very little resistance and current
will flow
...
However,
if reverse bias is applied, that is, the anode is made negative with respect to the cathode, the junction resistance is high and no current can flow, as shown in
Fig
...
14
...
4
...
It can be seen that a small voltage is required to forward bias the junction before a current can flow
...
6 V for silicon and 0
...
The reverse bias potential of silicon is about
1200 V and for germanium about 300 V
...
Similarly, the
ELECTRONIC COMPONENTS
diode will break down if the current rating is exceeded,
because excessive heat will be generated
...
However, it is possible to connect a
number of standard diodes in series or parallel, thereby
sharing current or voltage, as shown in Fig
...
16, so that
the manufacturers’ maximum values are not exceeded
by the circuit
...
Connecting an ohmmeter, as shown in Fig
...
76 with
the red positive lead to the anode of the junction diode
and the black negative lead to the cathode, would give a
very low reading
...
ZENER DIODE
A Zener diode is a silicon junction diode but with a
different characteristic than the semiconductor diode
considered previously
...
Its symbol and general appearance are shown in
Fig
...
17
...
4
...
Fig
...
15 Forward and reverse bias characteristic of silicon and germanium
...
4
...
Fig
...
17 Symbol for and appearance of Zener diodes
...
6 V, just like an ordinary diode, but it is in the reverse mode that the
Zener diode is normally used
...
4
...
This is
called the Zener voltage or reference voltage
...
7, 4
...
1, 6
...
8, 9
...
, up to 200 V at various ratings
...
4
...
the voltage across the diode remains constant
...
If a test circuit is constructed as shown in Fig
...
19,
the Zener action can be observed
...
4
...
less than the Zener voltage (5
...
When the supply is equal to or
greater than the Zener voltage, the diode will conduct
and any excess voltage will appear across the 680 ⍀
resistor, resulting in a very stable voltage at the output
...
Note that current must
flow through the diode to enable it to stabilize
...
No light is emitted when the junction is reverse
biased and if this exceeds about 5 V the LED may be
damaged
...
4
...
EXAMPLE
Calculate the value of the series resistor required when an LED is to be
used to show the presence of a 12 V supply
...
4
...
LEDs are available in red, yellow and green and,
when used with a series resistor, may replace a filament
lamp
...
A filament lamp, however, is brighter and emits white light
...
They do not, however, indicate
the precise amount of voltage present at that point
...
4
...
Fig
...
20 Symbol for and general appearance of an LED
...
If a voltage greater than 2 V is to be used then
a resistor must be connected in series with the LED
...
We know
that the diode requires a forward voltage of about 2 V
and a current of about 10 mA must flow through the
junction to give sufficient light
...
Seven LEDs are arranged as a figure 8 so that when
various segments are illuminated, the numbers 0 to 9
are displayed as shown in Fig
...
22
...
Light energy falling on a suitable semiconductor material also causes a change in
resistance
...
4
...
The resistance of an LDR in total
ELECTRONIC COMPONENTS
255
Fig
...
22 LED used in seven-segment display
...
4
...
Fig
...
24 Symbol for, pin connections of and appearance of a photodiode
...
They
can carry tens of milliamperes, an amount which is
sufficient to operate a relay
...
of light falling on the junction
...
Photodiodes will only carry microamperes of current but can operate much more quickly than LDRs
and are used as ‘fast’ counters when the light intensity
is changing rapidly
...
The circuit symbol and general appearance are shown
in Fig
...
24
...
Its
circuit symbol and general appearance are shown in
Fig
...
25
...
4
...
are used for the measurement and control of temperature up to their maximum useful temperature limit
of about 300°C
...
Thermistors
are embedded in high-voltage underground transmission cables in order to monitor the temperature of the
cable
...
A
particular cable can carry a larger load in winter for
example, when heat from the cable is being dissipated
more efficiently
...
TRANSISTORS
The transistor has become the most important building block in electronics
...
4
...
semiconductor equivalent of the thermionic valve and
was invented in 1947 by Bardeen, Shockley and
Brattain at the Bell Telephone Laboratories in the
USA
...
4
...
There are two basic types of transistor, the bipolar or
junction transistor and the field-effect transistor (FET)
...
It
uses less power and has a higher resistance and frequency response
...
It is, therefore,
the FET which is used when many transistors are
integrated on to a small area of silicon chip as in the
integrated circuit (IC) discussed later
...
Its circuit symbol and connections are given in Appendix F
...
The bipolar transistor
The bipolar transistor consists of three pieces of semiconductor material sandwiched together as shown in
Fig
...
27
...
By varying the current flowing into the
base connection a much larger current flowing between
collector and emitter can be controlled
...
A transistor is generally considered a currentoperated device
...
4
...
Initially, the positive battery
supply is connected to the n-type material of the collector, the junction is reverse biased and, therefore, no
current will flow
...
A small base current can cause a much larger collector current to flow
...
When I say
a much larger collector current, I mean a large current
in electronic terms, up to about half an ampere
...
4
...
on and controls the collector current; and as a current
amplifier because the collector current is greater than
the base current
...
However, transistors have
many advantages over electrically operated switches such
as relays
...
Transistor testing
A transistor can be thought of as two diodes connected together and, therefore, a transistor can be
tested using an ohmmeter in the same way as was
described for the diode
...
4
...
258
ADVANCED ELECTRICAL INSTALLATION WORK
Table 4
...
A ‘good’ p-n-p transistor will give the following readings:
Black to base and red to collector ϭ low resistance
Black to base and red to emitter ϭ low resistance
Reversed connections on the above terminals will result in a high resistance
reading, as will connections of either polarity between the collector and
emitter terminals
...
4
...
5 overpage
...
4
...
With the circuit connected, as shown in Fig
...
29 a
‘good’ transistor will give readings on the voltmeter of
6 V with the switch open and about 0
...
The voltmeter used for the test should
have a high internal resistance, about ten times greater
than the value of the resistor being tested – in this case
4
...
INTEGRATED CIRCUITS
Integrated circuits (ICs) were first developed in the
1960s
...
When assembled in a single package, as shown in
Fig
...
30, we call the device an integrated circuit
...
Digital ICs contain simple
switching-type circuits used for logic control and calculators, discussed at the end of this chapter
...
The most versatile
linear IC is the operational amplifier which has applications in electronics, instrumentation and control
...
4
...
Fig
...
30 Exploded view of an integrated circuit
...
ICs are more reliable, cheaper and smaller than the
same circuit made from discrete or separate transistors, and electronically superior
...
Manufacturers’ data sheets describe the characteristics of the different ICs, which have a reference number stamped on the top
...
The number 1 pin of any IC is indicated by a dot pressed into
the encapsulation; it is also the pin to the left of the
cutout (Fig
...
31)
...
4
...
Integrated circuits are sometimes connected into
DIL sockets and at other times are soldered directly
into the circuit
...
Fig
...
31 IC pin identification
...
4
...
Fig
...
33 Symbol for and structure and appearance of a thyristor
...
It consists of four pieces of semiconductor material sandwiched together and connected
to three terminals, as shown in Fig
...
33
...
It can be open or shut, allowing or preventing
260
ADVANCED ELECTRICAL INSTALLATION WORK
the currents used to operate the gate are very small
...
The power available to an a
...
load can be controlled by allowing current to be supplied to the load
during only a part of each cycle
...
4
...
current flow through the device
...
Once the thyristor is in the conducting state, the gate
loses all control over the devices
...
We can understand the operation of a thyristor by
considering the circuit shown in Fig
...
34
...
When SWB only is closed the lamp will not light,
but when SWA is also closed, the lamp lights to full
brilliance
...
This shows that the thyristor is
operating correctly
...
A thyristor may also be tested using an ohmmeter as
described in Table 4
...
The thyristor has no moving parts and operates without arcing
...
6 Thyristor testing using an ohmmeter
A ‘good’ thyristor will give the following readings:
Black to cathode and red on gate ϭ low resistance
Red to cathode and black on gate ϭ a higher resistance value
The value of the second reading will depend upon the thyristor, and may
vary from only slightly greater to very much greater
...
Fig
...
35 Waveforms to show the control effect of a thyristor
...
4
...
Power is
reduced by triggering the gate later in the cycle
...
c
...
This is very uneconomical, and a further
development of this device has been the triac which is
considered next
...
The triac is a single device containing a back-toback, two-directional thyristor which is triggered on
both halves of each cycle of the a
...
supply by the
same gate signal
...
Its symbol and general appearance are shown in
Fig
...
36
...
4
...
The triac is a three-terminal device, just like the
thyristor, but the terms anode and cathode have no
ELECTRONIC COMPONENTS
261
Fig
...
36 Appearance of a triac
...
4
...
Fig
...
37 Waveforms to show the control effect of a triac
...
Instead, they are called main terminal one (MT1) and main terminal two (MT2)
...
A gate current of 50 mA is sufficient to trigger a
triac switching up to 100 A
...
c
...
THE DIAC
The diac is a two-terminal device containing a twodirectional Zener diode
...
The symbol is shown
in Fig
...
38
...
Since the device contains back-to-back Zener
diodes it triggers on both the positive and negative
half cycles
...
We found that the
supply voltage was divided between the series resistors
in proportion to the size of the resistor
...
4
...
In the circuit shown in
Fig
...
39(b), the 1 and 2 k⍀ resistors divide the input
voltage into three equal parts
...
In Fig
...
39(c) the situation is reversed and, therefore, the voltmeter will read
4 V
...
The
values of the resistors R1 and R2 determine the output
voltage as follows:
VOUT ϭ VIN ϫ
R2
(V)
R1 ϩ R2
262
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
39 Voltage divider circuit
...
4
...
4
...
4
...
EXAMPLE 2
For the circuit shown in Fig
...
41(a), calculate the output voltage
...
4
...
4
...
VOUT ϭ 6 V ϫ
2
...
08 V
10 k⍀ ϩ 2
...
The volume control
in a radio or the brightness control of a cathode-ray
oscilloscope requires a continuously variable voltage
divider and this can be achieved by connecting a variable resistor or potentiometer, as shown in Fig
...
42
...
When
connection is made at the centre, the voltage would be
6 V, and at the top of the resistor the voltage would be
12 V
...
4
...
ELECTRONIC COMPONENTS
263
Fig
...
41 (a) Voltage divider circuit for Example 2; (b) Equivalent circuit for example 2
...
c
...
c
...
c
...
Batteries have the advantage of portability, but a battery supply is more expensive than using the a
...
mains supply suitably rectified
...
c
...
c
...
This is one of the many
applications for a diode which will conduct in one
direction only, that is when the anode is positive with
respect to the cathode
...
4
...
When a load is connected to a voltage divider it
‘loads’ the circuit, causing the output voltage to fall
below the calculated value
...
For
example, the load connected across the voltage divider
shown in Fig
...
39(b) must be greater than 20 k⍀ and
across 4
...
This problem of
loading the circuit also occurs when taking voltage
readings, as discussed later in this chapter under the
subheading Instrument Errors
...
4
...
During
the first half cycle the anode is positive with respect
to the cathode and, therefore, the diode will conduct
...
Only the positive half of the waveform will be
available at the load and the lamp will light at reduced
brightness
...
44 shows an improved rectifier circuit which
makes use of the whole a
...
waveform and is, therefore,
known as a full-wave rectifier
...
4
...
Fig
...
44 Full-wave rectification using a bridge circuit
...
During
the first half cycle diodes D1 and D3 conduct, and
diodes D2 and D4 conduct during the second half cycle
...
Full-wave and half-wave rectification can be displayed on the screen of a CRO and will appear as
shown in Figs 4
...
44
...
43 and 4
...
c
...
Such a
waveform is too bumpy to be used to supply electronic
equipment but may be used for battery charging
...
The simplest way to smooth an output is
to connect a large-value capacitor across the output
terminals as shown in Fig
...
45
...
4
...
During the second quarter of the cycle,
when the output from the rectifier is falling to zero,
the capacitor discharges into the load
...
The capacitor connected
to the full-wave rectifier circuit is charged up twice as
often as the capacitor connected to the half-wave
ELECTRONIC COMPONENTS
265
Fig
...
45 Rectified a
...
with smoothing capacitor connected
...
4
...
circuit and, therefore, the output ripple on the full-wave
circuit is smaller, giving better smoothing
...
Increasing the size of the capacitor reduces
the amount of ripple
...
4
...
4
...
A low-pass filter
allows low frequencies to pass while blocking higher
frequencies
...
Connecting the low-pass filter will
allow the d
...
to pass while blocking the ripple voltage,
resulting in a smoother output voltage
...
4
...
This can be
reduced if the resistor is replaced by a choke, which
has a high impedance to the ripple voltage but a low
resistance, which reduces the output ripple without
increasing the output resistance
...
4
...
c
...
Fig
...
48 Stabilized d
...
supply
...
A number of stabilizing circuits
are available which, when connected across the output
of the circuit shown in Fig
...
45, give a constant or stabilized voltage output
...
4
...
Figure 4
...
c
...
You could build and
test this circuit using the circuit assembly and testing
skills described next in this chapter
...
Practical electronics can
be carried out with very few tools and limited resources
...
The place
ELECTRONIC COMPONENTS
267
chosen should be well lit, have a flat and dry area of
about 1 m ϫ 1 m and have access to a three-pin socket
...
Many technical
colleges and evening institutes offer basic electronics
courses which give someone new to electronics courses
which give someone new to electronics an opportunity
to use the tools and equipment under guidance and at
little cost
...
start a fire
...
The
chances of causing a fire or burning yourself can be
reduced by storing the soldering iron in its stand at
the back of the workspace so that you do not have to
lean over it when working
...
However, you or someone else in your
workplace may receive an electric shock and Chapter 1
offers some guidance under the sub-heading First Aid
...
It can give a serious shock
and it does cause fires
...
These
sense fault currents as low as 30 mA so that a faulty
circuit or piece of equipment can be isolated before
the lethal limit to human beings of about 50 mA is
reached
...
4
...
All equipment should be
earthed and fitted with a 2 A or 5 A fuse, which is adequate for most electronics equipment
...
Tools extend the physical capabilities of the human
body
...
An electrician or electronic service engineer
is no less a craftsman than a wood carver
...
The basic tools required by anyone
working with electrical equipment are those used to
strip, cut and connect conductors and components
...
4
...
Electricians and those in the electrotechnical industries have traditionally chosen insulated hand-tools
...
4
...
Another source of danger in electronic assembly is
the hot soldering iron, which may cause burns or even
An electric soldering iron with the correct-size bit is
essential for making good-quality, permanent connections in electronic circuits
...
The bit is inserted into this
element and heats up to a temperature of about
210°C by conduction
...
Copper bits can be filed clean or rubbed with emery
cloth until the tip is a bright copper colour
...
268
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
50 Basic tools required for electronic assembly
...
4
...
Before the soldering iron can be used to make electrical connections, the bit must be tinned as follows:
■
■
■
■
of pressing the trigger the bit is at the working temperature of 315°C
...
Plug in the soldering iron and allow it to heat up
...
Wipe off the excess solder with a damp cloth or
damp sponge
...
Figure 4
...
Soldering guns of the type shown in Fig
...
52 are
trigger-operated soldering irons
...
4
...
ELECTRONIC COMPONENTS
arrived at even more quickly with constant use
...
The
bits are interchangeable and should be tinned and
used in the same way as the general-purpose iron considered above
...
In appearance they are very similar to the generalpurpose soldering iron shown in Fig
...
51, but without the mains cable
...
The advantage of a gas soldering iron
is that it can be used when a mains supply is not easily
available
...
In 2004 the relevant costs
were approximately £13 for the general-purpose iron,
£30 for the soldering gun, £65 for the gas soldering
iron and about £250 for a low-voltage temperaturecontrolled iron
...
In electronics,
the most common method of making permanent connections is by soldering the components into the circuit
...
Soldering is an alloying process, whereby a small
amount of soft metal (the solder) is made to run
between the two metals to be joined, therefore mixing
or alloying them
...
SOFT SOLDERS
Soft solders are so called because they are made up of
the rather soft metals tin and lead in the proportion
269
40 to 60
...
Solder will not adhere to a tarnished or oxidized metal surface
...
FLUXES
Fluxes are slightly acid materials which dissolve an
oxide film, leaving a perfectly clean surface to which
the solder can firmly adhere
...
Salt
fluxes are rather corrosive and are therefore used when
joining iron, steel, nickel and stainless steel, which
oxidize easily when hot
...
In electronics it is not convenient to
apply the flux and solder separately, so they are combined as flux-cored solder wire
...
The multicore construction shown
in Fig
...
53 ensures the correct proportions of flux for
each soldered joint
...
4
...
SOLDERING TECHNIQUES
As already mentioned, when soldering with an iron it
is important to choose an iron with a suitable bit size
...
5 or 2
...
0 mm bit is better when soldering
dual-in-line IC packages
...
The materials to be soldered must be
free from grease and preferably pretinned
...
4
...
components should not need more cleaning than a
wipe to remove dust or grease
...
If solder is
first melted on to the bit, which is then used to transfer the solder to the joint, the active components of
the flux will evaporate before the solder reaches the
joint, and an imperfect or ‘dry’ joint will result
...
The best method of making a ‘good’
soldered joint is to apply the cored solder to the joint
and then melt the solder with the iron
...
4
...
While the termination is heating up, the solder will
appear dull, and then quite suddenly the solder will
become bright and fluid, flowing around and ‘wetting’
the termination
...
The joint should be soldered quickly
...
The joint must not be moved or blown upon
until the solder has solidified
...
Dry joints
Dry joints may occur because the components or termination are dirty or oxidized, or because the soldering
temperature was too low, or too little flux was used
...
4
...
Dry joints do not always make an electrical connection, or the connection will have a high resistance
which deteriorates with time and may cause trouble
days or weeks later
...
4
...
If the joint is dry the voltmeter
will read 12 V at position B, just to the right of the
joint, and 0 V at position A, to the left
...
COMPONENT ASSEMBLY AND SOLDERING
Soft solder is not as strong as other metals and, therefore, the electronic components must be shaped at the
connection site to give extra strength
...
Special leadforming or wire-shaping tools are available which both
cut and shape the components’ connecting wires ready
for soldering
...
56 shows a suitable tag terminal
connection, Fig
...
57 a suitable pin terminal connection and Fig
...
58 a suitable stripboard connection
...
Also the strain of cutting after soldering may weaken
the joint and encourage dry joints
...
4
...
cut after soldering, cutters with a shearing action
should be used, as shown in Fig
...
59
...
Most electronic components are very sensitive and
are easily damaged by excess heat
...
When components are being soldered into a circuit the heat from the soldering iron at
the connection must be diverted or ‘shunted’ away
from the body of the component
...
4
...
Fig
...
59 Wire cutting
...
4
...
271
272
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
62 A desoldering tool
...
4
...
This releases the plunger which shoots out, sucking
the molten solder away from the joint and into the
body of the desoldering tool
...
4
...
between the soldered joint and the body of the component, as shown in Fig
...
60
...
They should
be shaped, mounted and soldered into the circuit as
previously described and shown in Fig
...
61
...
The
vertical mounting method permits many more components to be mounted on the circuit board but the
horizontal method gives better mechanical support to
the component
...
To do this the solder of the old
joint is first liquefied by applying a hot iron to the
joint
...
The desoldering tool works
like a bicycle pump in reverse and is shown in
Fig
...
62
...
The nozzle is then placed into the pool of
molten solder and the latch release button pressed
...
Remove the solder from
each leg with the soldering iron and desoldering tool
before removing the faulty transistor
...
An alternative
method is to cut the three legs with a pair of side cutters before desoldering and then remove the individual legs with a pair of long-nosed pliers
...
If it has been firmly
established that the IC is faulty, it may be removed
from the circuit board by cutting the body from the
connecting pins before desoldering and removing the
individual pins with a pair of long-nosed pliers
...
Three types of board can be used –
matrix, strip and printed circuit board – the base
material being synthetic resin bonded paper (SRBP)
...
1 inch
centres as shown in Fig
...
63
...
4
...
various sizes: the 149 ϫ 114 mm board is pierced
with 58 ϫ 42 holes and the 104 ϫ 65 mm board has
39 ϫ 25 holes
...
Single-sided or double-sided matrix pins are available
...
The hole spacing of 0
...
Plug-in relays, DIL integrated circuits and
many sockets and connectors all use 0
...
Matrix board is probably the easiest and cheapest
way to build simple electronic circuits
...
4
...
very similar to the circuit diagram to reduce the possibility of mistakes
...
4
...
First we
would insert four pins into the matrix board as
shown
...
The resistor would be connected
between pins B and C and a wire linked between pins
C and D
...
c
...
This circuit would show half-wave rectification
...
274
ADVANCED ELECTRICAL INSTALLATION WORK
The positional reference system
STRIPBOARD OR VEROBOARD
The positional reference system used with matrix
boards uses a simple grid reference system to identify
holes on the board
...
For
example, the position reference point 4:3 would be
4 holes from the left and 3 holes down
...
The copper strip links together rows of holes so that
connections can be made between components inserted
into holes on a particular row, as shown in Fig
...
66
...
The copper strips are continuous but they can be
broken using a strip cutter or small drill
...
4
...
Stripboard is very useful because the copper strips
take the place of the wire links required with plain
matrix boards
...
It is, however,
more expensive than matrix board because of the
additional cost of the copper, most of which is not used
in the circuit
...
Excessive heat from the soldering iron
can melt the adhesive and cause the copper to peel from
the insulating board
...
■
■
Turn the matrix board so that a manufactured straight
edge is to the top and left-hand side
...
4
...
The pins can then be inserted as required
...
65
shows a number of pin reference points
...
PRINTED CIRCUIT BOARDS
Fig
...
65 Matrix board pin reference system
...
4
...
Printed circuit boards (PCBs) are produced by chemically etching a copper-clad epoxy glass board so that
a copper pattern is engraved on one side of the board
...
After etching, small holes are drilled for the
components which are inserted from the plain side of
the board and soldered on to the copper conductor
...
The copper foil is very thin and is attached to the
board with an adhesive
...
The board should
not be flexed, otherwise hairline cracks may appear
but go unnoticed until intermittent faults occur in
the circuit later
...
275
THE PROFESSIONAL
The professional prototype board is designed for the
interconnection of many different types of component
...
1 inch allows DIL IC packages
to be plugged directly into the board
...
The case is formed in high-impact thermoplastic and the individual boards may be interlocked
to create a larger working area
...
4
...
Breadboards
Breadboarding is the name given to solderless temporary circuit building by pressing wires and component leads into holes in the prototype board
...
THE S-DEC
The S-DeC prototype board is designed for interconnecting discrete components
...
Each board
has 70 phosphor-bronze contact points arranged in two
sections, each of which has seven parallel rows of five
connected contact points
...
The S-DeC
can be supplied with a vertical bracket for mounting
switches or variable resistors, as shown in Fig
...
67
...
4
...
Fig
...
68 Professional prototype board used for temporary circuit
building
...
In electrical installation work we usually need to make plug and socket
connections between three conductors on singlephase circuits and five conductors on three-phase systems
...
However, the same principles apply, that is, the plug
and socket must be capable of separation, but while
connected they must make a good electrical contact
...
276
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
69 PCB edge connectors
...
4
...
PCB EDGE CONNECTORS
A range of connectors are available which make direct
contact to printed circuit boards, as shown in Fig
...
69
...
1 inch so that they can be soldered into circuit
boards
...
RIBBON CABLE CONNECTORS
A ribbon cable is a multicore cable laid out as flat strip
or ribbon strip
...
4
...
used when frequent connection and disconnection is
required between a small number of contacts
...
They are available in three sizes: subminiature (2
...
5 mm) and commercial (0
...
Examples are
shown in Fig
...
72
...
4
...
Fig
...
70 Ribbon cable connector
...
4
...
When a soldered connection is to
be made the cable end must be stripped of its insulation, tinned and then terminated
...
The insulation displacement method of connection is much
ELECTRONIC COMPONENTS
quicker to make because the cable ends do not require
stripping or preparing
...
This method
is used extensively when terminating ribbon cable and
for making rapid connections to the existing wiring
system of motor vehicles
...
If, despite your best efforts, the circuit does not work
as it should when tested, then follow a logical test
procedure which will usually find the faults in the
shortest possible time
...
6 Are all the components such as diodes, capacitors, transistors and ICs connected the correct way
round?
7 Have all connections and links been made?
8 Have all the necessary breaks been made in the
stripboard?
9 Are all the soldered joints good?
10 Are any of the components hot or burnt?
If the fault has not been identified by the first ten
tests, ask someone else to carry them out
...
If the visual tests have failed to
identify the fault, then further meter tests are
called for as follows:
11 Check the input voltage and the output voltage
...
12 Variable resistors may suffer from mechanical
wear
...
13 Check the coil voltage on relays; if this is low, the
coil contacts may not be making
...
If it does the diode is open-circuit
...
If it does not charge, the resistor
may be open-circuit
...
Check the polarity of
electrolytic capacitors
...
16 Check the base–emitter voltage of the transistor
...
6 and
1
...
Temporarily connect a 1 k⍀ resistor between
the positive supply and the base connection
...
If it does
not work, the transistor is faulty
...
If the load operates, the thyristor or the gate pulse
is faulty
...
The testing of capacitors, resistors and discrete semiconductor components was dealt with earlier in this
chapter
...
Electronic circuits and components can now be found
in leisure goods, domestic appliances, motor starting
and control circuits, discharge lighting, emergency
lighting, alarm circuits and special-effects lighting systems
...
Test instruments
Electrical installation circuits usually carry in excess of
1 A and often carry hundreds of amperes
...
The test instruments used on electronic circuits
must have a high impedance so that they do not
damage the circuit when connected to take readings
...
In power applications these small disturbances
seldom give rise to obvious errors, but in electronic
circuits, a small disturbance can completely invalidate
any readings taken
...
Let us
consider some of the problems
...
When the
term error is used it means the deviation of the meter
reading from the true value and accuracy means the
closeness of the meter reading to the true value
...
4
...
Connection of the meter loads the circuit by effectively connecting a 100 k⍀ resistor in parallel with
the circuit resistor as shown in Fig
...
73(b), which
changes the circuit to that shown in Fig
...
73(c)
...
This loading effect can be reduced by
choosing instruments which have a very high impedance
...
Test instruments used to measure a
...
supplies are
also frequency dependent
...
This is the range of frequencies over which the
instrument may be considered free from frequency
errors and is indicated on the back of the instrument
or in the manufacturer’s information
...
The scale calibration of an instrument assumes
a sinusoidal supply unless otherwise stated
...
The chosen instrument must, therefore, be suitable for the test circuit waveform
...
When choosing an instrument for
Fig
...
73 Circuit disturbances caused by the connection of a voltmeter
...
Instrument
manufacturers will provide detailed information for
their products
...
Operator errors are
errors such as misreading the scale, recording the measurement incorrectly or reading the wrong scale on a
multirange instrument
...
ELECTRONIC COMPONENTS
279
Analogue and digital displays
The type of instrument to be purchased for general
use is a difficult choice because there are so many
different types on the market and every manufacturer’s representative is convinced that his company’s
product is the best
...
ANALOGUE METERS
These meters have a pointer moving across a calibrated
scale
...
Hi-fi equipment
often uses analogue displays to indicate how power levels
vary with time, which is more informative than a specific value
...
The fuel gauge on a motor car
often indicates full, half full or danger on an analogue
display which is much more informative than an indication of the exact number of litres of petrol remaining
in the tank
...
Most meters using an analogue scale incorporate a
mirror to eliminate parallax error
...
A good-quality analogue multimeter suitable
for electronic testing is shown in Fig
...
76
...
DIGITAL METERS
These provide the same functions as analogue meters
but they display the indicated value using a sevensegment LED (see Fig
...
22) to give a numerical value
of the measurement
...
The choice between an analogue and a digital display is a difficult one and must be dictated by specific
Fig
...
74 Digital multimeter suitable for testing electronic circuits
...
However, if you are an electrician or
working in any sector of the electrotechnical industry
intending to purchase a new instrument which would
be suitable for electronic testing, I think on balance
that a good-quality digital multimeter such as that
shown in Fig
...
74 would be best
...
The multimeter
Multimeters are designed to measure voltage, current
or resistance
...
To avoid damaging the instrument it is good practice
first to switch to the highest value on a particular scale
range
...
5 A is displayed, we then know that
a more appropriate scale would be the 3 or 5 A range
...
49 A
...
4
...
When used as a
voltmeter the multimeter must be connected in parallel
with the component, as shown in Fig
...
75(b)
...
280
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
75 Using a multimeter (a) as an ammeter and (b) as a voltmeter
...
4
...
The red terminal of the meter, identifying the positive
input for testing voltage and current, usually becomes
the negative terminal when the meter is used as an
ohmmeter because of the way the internal battery is
connected to the meter movement
...
4
...
The ohmmeter can
then be successfully used to test diodes, transistors
and thyristors as described earlier in this chapter
...
The cathode ray oscilloscope
The CRO is probably one of the most familiar and
useful instruments to be found in an electronic repair
service workshop or college laboratory
...
‘Cathode ray’
is the name given to a high-speed beam of electrons
generated in the cathode ray tube and was first used
Fig
...
77 Front panel of a simple CRO
...
4
...
281
during the Second World War as part of the radar system
...
The many
controls on the front of the CRO are designed so that
the operator can stabilize and control these signals
...
77 shows the front panel of a simple CRO
...
The single most important component in the CRO
is the cathode ray tube
...
78 shows a simplified diagram of the cathode
ray tube
...
On the far left of the diagram is the wire filament through which a current is passed
...
The rate at which the electrons are
accepted for acceleration could be modified by making changes to the temperature of the cathode, but in
practice it is more convenient to have a metal control
grid with a hole in it
...
The
electrons which pass through the grid tend to be moving in various directions and the purpose of the next
component therefore is to focus the beam
...
The electrons, on emerging through a hole in this
anode, pass through two pairs of parallel plates X–XЈ
and Y–YЈ, each pair being at right angles to the other
...
The negative beam of electrons
is attracted towards the more positive plate
...
Therefore, a suitable combination
of electric fields across X–XЈ and Y–YЈ directs the
beam to any desired point on the screen
...
The brightness of the spot depends
upon the speed of the electrons and the number of
electrons arriving at that point
...
Switch on and wait a few seconds
for the instrument to warm up
...
2 Brightness or intensity
...
This should be adjusted until
bright, but not too brilliant, otherwise the fluorescent powder may be damaged
...
This illuminates and highlights the 1 cm square grid lines on the screen
...
The spot or trace should be adjusted for a
sharp image
...
‘Adjust’ for ‘calibrate’
...
The spot or trace can be moved to the left
or right and should be centralized
...
The spot or trace can be moved up or
down
...
This allows the time base to be synchronized to the applied signal to enable a steady
trace to be obtained
...
9 AC/GND/DC
...
c
...
c
...
c
...
c
...
The GND position disconnects the signal from the Y-amplifier and connects
the Y-plates to ground or earth
...
When a double-beam oscilloscope is
used, it is common practice to obtain the two
X-traces from one beam by either sweeping the
electron beams alternately or by sweeping a very
small segment of each beam as the trace moves
across the screen, leaving each trace chopped up
...
11 Connect the test voltage to the CRO leads and
adjust the calibrated Y-shift (volts/cm) and time
base (time/cm) controls until a steady trace fills
the screen
...
With the test voltage connected to the Y-input, adjust
all controls to the calibrate position
...
4
...
To measure the voltage of the signal shown in Fig
...
79 count the number of centimetres from one peak of
the waveform to the other using the centimetre grating
...
4
...
This value
is then multiplied by the volts/cm indicated on the
Y-amplifier control knob
...
4
...
4
...
The controls on the X-axis are set to 2 ms/cm and the measurement for one period is calculated to be 5 cm
...
The time taken to complete one cycle (T ) is 5 cm ϫ 2 ms/cm ϭ
10 ms
...
4
...
4 cm ϫ 2 V/cm ϭ 8 V
...
7071 ϫ 4 ϭ 2
...
To measure the frequency of the waveform shown
in Fig
...
79 count the number of centimetres for one
complete cycle using the 1 cm grating
...
4
...
This value is then multiplied by the time/cm on the X-amplifier or timebase amplifier control knob
...
Frequency can
be found from:
As you can see, the CRO can be used to calculate the
values of voltage and frequency
...
It does, however, allow
us to observe the quantity being measured unlike any
other instrument and, therefore, makes a most important contribution to our understanding of electronic
circuits
...
c
...
It is used for
serious electronic testing, fault finding and experimental work
...
4
...
1
(Hz)
T
1
1000
І f ϭ
ϭ
ϭ 50 Hz
Ϫ3
20 ϫ 10
20
f ϭ
The waveform shown in Fig
...
79 therefore has an
rms voltage of 2
...
The
voltage and frequency of any waveform can be found
in this way
...
c
...
EXAMPLE 1
A sinusoidal waveform is displayed on the screen of a CRO as shown in
Fig
...
79
...
Calculate the rms value
of the waveform
...
7071 ϭ 14
...
80 A signal generator
...
c
...
c
...
The output is very pure, a straight line when observed
284
ADVANCED ELECTRICAL INSTALLATION WORK
on a CRO, and continuously variable from zero to
usually 30 V
...
A bench
power supply unit is shown in Fig
...
81
...
4
...
Mains electricity supply
The mains electricity supply can be lethal, as all electricians and all those working in the electrotechnical
industries will know
...
Electronic equipment is protected by in-line
fuses and circuit breakers and when testing suspected
faulty electronic equipment, a good starting point is
to establish the presence of the mains supply
...
4
...
When isolating electronic equipment from the mains
supply, in order to carry out tests or repairs, the
following procedure should be followed:
1 Connect the voltage indicator or voltmeter to the
incoming supply of the piece of equipment to be
isolated
...
2 Isolate the supply
...
If 0 V is
indicated the equipment is disconnected from the
mains supply
...
If the voltage indicator is ‘proved’ to be
working, the equipment is safe to work on
...
4
...
Insulation tester
The use of an insulation resistance test as described by
the IEE Regulations must be avoided with any electronic equipment
...
When carrying out an insulation resistance test as part of the prescribed series of tests for an
electrical installation, all electronic equipment must
first be disconnected or damage will result
...
LOGIC GATES AND DIGITAL ELECTRONICS
Digital electronics embraces all of today’s computerbased systems
...
Digital electronics is concerned with
straightforward two-state switching circuits
...
Traditional applications which have
analogue inputs, such as audio recordings, are now
using digital techniques, with the development of
analogue-to-digital converters
...
A digital
and an analogue waveform are shown in Fig
...
83
...
Logic circuits have
been developed to deal with these digital, two-state
switching circuits
...
These two binary states are represented by low
and high voltages, where low voltage is 0 V and high
voltage is, say, ϩ5 V
...
When the voltage level of a
digital signal is not rapidly changing it remains steady
at one of these two levels
...
These units can allow information to pass through or stop it, and behave according to
rules which can be described by logical or predictable
statements
...
The basic various
types of logic gate are known by the names AND, OR,
NOT, NOR and NAND
...
4
...
very rapidly
...
In an analogue system, changes in component values due to ageing and temperature can affect the circuit’s performance
...
Another
significant advantage of digital circuits is their immunity to noise and interference signals
...
Digital signals, however, have a very large
The operation of this gate can probably best be
understood by drawing a simple switch-equivalent
circuit, as shown in Fig
...
84
...
The signal lamp will only illuminate if both
switch A and switch B are closed, or we could say the
output F of the gate will only be at logic 1 if both
input A and input B are both at logic 1
...
The truth table shows
the output state for all possible combinations of inputs
...
4
...
In this case the signal lamp will only
illuminate if switch A or switch B or both switches are
closed
...
286
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
84 The AND gate: (a) simple switching circuit; (b) logic symbol; (c) truth table
...
4
...
If the OR gate was operating an interior light in a
motor car, it would illuminate when the nearside door
was opened or the offside door was opened or when
both doors were opened
...
The exclusive-OR gate
The exclusive-OR gate is an OR gate with only two
inputs which will give a logic 1 output only if input
A or input B is at logic 1, but not when both A and B
are at logic 1
...
4
...
Fig
...
86 The exclusive-OR gate: (a) logic symbol; (b) truth table
...
For this reason it is sometimes called an inverter or a negator or
simply a sign changer
...
The small circle
on the output of the gate always indicates a change
of sign
...
The truth table shows the output state for
all possible inputs in Fig
...
87
...
4
...
ELECTRONIC COMPONENTS
The NOR gate
The NOR gate is a NOT gate and an OR gate combined to form a NOT–OR gate
...
4
...
287
can be much greater than the input current and, therefore, buffers can be said to exhibit current gain
...
The symbol
used to represent a buffer is shown in Fig
...
90
...
4
...
Fig
...
88 The NOR gate: (a) logic symbol; (b) truth table
...
The output of
the NAND gate is the opposite of the AND gate, as
can be seen by comparing the truth table for the
NAND gate in Fig
...
89 with that of the AND gate
...
The results of any combination
can be found by working through each individual
gate in the combination or logic system in turn, and
producing the truth table for the particular network
...
This facility will undoubtedly be
available if the course of study is being undertaken at
a technical college, training centre or evening institute
...
4
...
Complete
the truth table for this particular logic network
...
4
...
The whole network has two
inputs, A and B, and one output F
...
4
...
Let us call the
The simplest of all logic devices is the buffer
...
Given that this device has no effect upon the logic levels
within a circuit, you may be wondering what the purpose of such an apparently redundant device might be!
Well, although the input and output voltage levels of
the buffer are identical, the currents present at the input
and output can be very different
...
4
...
Fig
...
89 The NAND gate: (a) logic symbol; (b) truth table
...
For any two-input logic gate, there are four possible combinations,
00, 01, 10 and 11
...
7
...
Now, C is the output of an AND gate and can, therefore, only be at logic 1
when both A and B are logic 1
...
8
...
A NOT gate is a single-input gate whose output is the opposite of the
input and, therefore, the output column F must be the opposite of column C, as shown by Table 4
...
The truth table tells us that this particular combination of gates will give a logic 1 output with any input
combination except when A and B are both at logic 1
...
4
...
Fig
...
92 Logic network for Example 2
...
10
...
10
...
The output, given by column S of the truth table, will therefore
be the reverse of column R, as shown in Table 4
...
Table 4
...
7 Truth table for Example 1
This particular combination will, therefore, give a logic 1 output only
when input P and input Q are at logic 1
...
This can be checked by referring back to Fig
...
84
...
8 Completed column C of truth table
A NAND gate has a NOT gate on each of its inputs as shown in Fig
...
93
...
Fig
...
93 Logic network for Example 3
...
9 The completed truth table for Example 1
EXAMPLE 2
A NAND and NOT gate are connected together as shown in Fig
...
92
...
The truth table for this particular combination can be constructed in
exactly the same way as for Example 1
...
The NOT gate has an input R and output S
...
We can, therefore, call
—
—
the output of these NOT gates not A and not B, written as A and B
...
A NAND gate will provide a
logic 1 output for any input combination except when both inputs are at
logic 1
...
11
...
4
...
That is, the output is at logic 1 if the input A or input B or
both are at logic 1
...
11 Truth table for Example 3
EXAMPLE 4
A logic network is assembled as shown in Fig
...
94
...
Fig
...
94 Logic network for Example 4
...
12
...
The output C of
the AND gate and the output D of the OR gate provide the input to a NOR
gate, which provides the output F
...
Therefore, a
two-input gate requires 22 (4) lines, as can be seen in
the previous examples, a three-input gate 23 (8) lines,
a four-input gate 24 (16) lines, etc
...
4
...
Develop a truth table and describe in a sentence the relationship
between the input and output
...
4
...
The truth table for this combination of logic gates can be drawn up as
shown in Table 4
...
Three inputs mean that the truth table must have
23 ϭ 8 rows
...
The first AND gate will give a logic 1 output only when input
A and B are both logic 1
...
The second AND gate will give a logic 1 output only when input
C and D are both logic 1
...
That is, the
output is at logic 1 only when all three inputs are at logic 1
...
12 Truth table for Example 4
The output of an AND gate is high, that is at logic 1, only when input
A and input B are at logic 1
...
The output of an OR
gate is high when input A or input B or both are high
...
The input to the final NOR gate is provided
by the logic levels indicated in columns C and D and the output F is,
therefore, as shown in column F
...
This is equivalent to
a single NOR gate
...
In practice,
logic gates may be constructed with many inputs and
the truth tables developed as shown above
...
13 Truth table for Example 5
EXAMPLE 6
A three-input logic network is assembled as shown in Fig
...
96
...
290
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
96 Logic network for Example 6
...
14
...
Fig
...
97 The 7400 TTL logic family
...
14 Truth table for Example 6
The first OR gate will give a logic 1 output when V or W or both are
at logic 1
...
The second OR gate will give a
logic 1 output when X or Y or both are at logic 1
...
Therefore, we can say that the output Z is at logic 0 only
when all three inputs are at logic 0
...
Logic families
The simplicity of digital electronics, with its straightforward on–off switching, means that many logic elements can be packed together in a single integrated
circuit and packaged as a standard dual-in-line IC, as
shown in Figs 4
...
32
...
Each type is called a logic family because all
members of that integrated circuit family will happily
work together in a circuit
...
These are the TTL
and CMOS families
...
The standard TTL
family is designated the 7400 series
...
97 shows
the internal circuitry of a TTL 7400 IC
...
Thus, with two power supply connections, the 7400
IC has 14 connections and is manufactured as the
familiar 14 pin dual-in-line package
...
The final two
numbers indicate the type of logic gate: for example,
a 7432 is a quad 2-input OR gate, and a 7411 a triple
3-input AND gate
...
The CMOS family, pronounced ‘see-mos’, is the
complementary metal oxide semiconductor family of
logic ICs which was introduced in 1968
...
The 4011B is a quad 2-input
NAND gate, as shown in Fig
...
98
...
4
...
The theory of digital logic is the same for all logic
families
...
Each logic family has its own special characteristics
which make it appropriate for particular applications
...
However, the switching times for TTL logic are
very much faster than for CMOS, although both are
measured in nanoseconds
...
15
...
4
...
Table 4
...
25 V d
...
fast: 10 ns
low
10
low: 1 mW
low: A range
3 V to 15 V d
...
slow: 100 ns
high
50
gate
...
The output of a logic gate may be connected to the
input of many other logic gates
...
The fan-out for TTL is
10, which means that ten other TTL logic gates can
take their input from one TTL output and still switch
reliably before overloading occurs
...
The power supply for TTL must be 5 V Ϯ 0
...
A TTL device
will be damaged if voltages in excess of these limits are
applied
...
Another advantage of CMOS logic circuits is that
they require only about one-fiftieth of the ‘floor space’
on a silicon chip compared with TTL
...
The switching times of any logic network are infinitesimal when compared when an electro-mechanical
ICs of the same number will always have the same
function regardless of the manufacturer and any suffix
or prefix which may accompany the basic gate number
...
The very high input
impedance of CMOS accounts for its low power consumption but it does mean that static electricity can
build up on the input pins if they come into contact
with plastic, nylon or the man-made fibres of workers’
clothing during circuit assembly or repair
...
Static voltages
on CMOS devices can destroy them; they are supplied with anti-static carriers and these should not be
removed until wiring is completed
...
Inputs
must, therefore, be disconnected before the mains connections when disconnecting CMOS devices
...
Input signals must not be applied until the power supply is
connected and switched on
...
4
...
Unused inputs must not be
left floating
...
Working with CMOS has created many new problems for electronic technicians
...
ensuring that all equipment is properly earthed
...
292
ADVANCED ELECTRICAL INSTALLATION WORK
Fig
...
99 Comparison of British and American logic gate symbols
...
British Standard symbols
Although the British Standards recommend symbols
for logic gates, much of the manufacturers’ information uses the American ‘MilSpec’ Standard symbols
...
However, there is
some pressure in the UK to adopt the BS
symbols, and for this reason the British Standard
and American Standard symbols are cross-referenced
in Fig
...
99
...
A voltage signal which has two quite definite states,
either on or off, is called:
(a) a logical waveform
(b) an analogue waveform
(c) an interference signal
(d) a digital waveform
...
The output Z will be at logic 1 only when
input A and input B are at logic 1 if the gate is:
(a) a NOT gate
(b) an AND gate
(c) an OR gate
(d) a NOR gate
...
Develop the truth table for a NOR gate
...
4
...
293
Fig
...
103 Logic network for Exercise 9
...
4
...
Fig
...
104 Logic network for Exercise 10
...
4
...
Fig
...
100 Logic network for Exercise 6
...
4
...
Fig
...
105 Logic network for Exercise 11
...
4
...
Fig
...
101 Logic network for Exercise 7
...
4
...
Fig
...
106 Logic network for Exercise 12
...
4
...
Determine the
sequence of key switch positions which will give
an output at F
...
4
...
9 Work out the truth table for the circuit shown in
Fig
...
103
...
Fig
...
107 Logic network for Exercise 13
...
65A
(b) 86
...
03 (b) 12
...
6,
191
...
55 96: (a) 15 (b) 15
...
97,
183
...
6 97: (a) 32 (b) 8
...
1,
274
...
66 (d) 0
...
33, 7
...
99,
0
...
55, 4
...
82, 0
...
14 (b) 1
...
0 (d) 5
...
1 (f ) 18
...
18 mm 44: (a) 0
...
04 s 45: 1
...
48 A In ϭ 45
...
03 (b) 0
...
91 48: (a) 1
...
6
(c) 0
...
0 (b) 0
...
224 Yes
52: (a) 4
...
15 A (c) 1
...
74 V 53: (a)
14
...
09 A (c) 2
...
67 V 54:
Answer in text 55: Answer in text 56: (a) 7
...
662 k⍀ 57: 1
...
4
...
4
...
1
Table S
...
2
...
For all
other input combinations the output is low
...
2 Truth table for Exercise 7
SOLUTIONS TO EXERCISES
295
8: See Table S
...
The output is high for all input
combinations except when input X is low and
input Y is high
...
6
...
Table S
...
6 Truth table for Exercise 11
9:
12: See Table S
...
The output is high when both
inputs are the same
...
4
...
For all
other input combinations the output is logic 0
...
7 Truth table for Exercise 12
Table S
...
8
...
10: See Table S
...
The output T is logic 1 only when
both inputs are at logic 1
...
Table S
...
8 Truth table for Exercise 13
APPENDICES
Appendix A: Obtaining
information and electronic
components
For local suppliers, you should consult your local telephone directory
...
In most cases, telephone orders received
before 5 p
...
can be dispatched the same day
...
S
...
O
...
Website: RS www
...
Email: Sale@farnellinone
...
uk
Maplin Electronics, Valley Road
...
Website: www
...
co
...
Fax: 01206 751188
...
co
...
S
...
O
...
Website:
RS www
...
Appendix B: Abbreviations,
symbols and codes
Abbreviations used in electronics for multiples and sub-multiples
T
G
M
k
d
c
m
n
p
tera
giga
mega or meg
kilo
deci
centi
milli
micro
nano
pico
1012
109
106
103
10Ϫ1
10Ϫ2
10Ϫ3
10Ϫ6
10Ϫ9
10Ϫ12
Terms and symbols used in electronics
Term
Approximately equal to
Proportional to
Infinity
Sum of
Greater than
Less than
Much greater than
Much less than
Base of natural logarithms
Common logarithms of x
Temperature
Time constant
Efficiency
Per unit
Symbol
Ӎ
ϰ
ϱ
⌺
Ͼ
Ͻ
ӷ
Ӷ
e
log x
T
p
...
Electrical quantities and units
Quantity
Quantity symbol
Unit
Unit symbol
Angular velocity
Capacitance
C
Charge or quantity of electricity
Current
Q
I
Electromotive force
Frequency
E
f
Impedance
Inductance, self
Inductance, mutual
Magnetic field strength
Magnetic flux
Magnetic flux density
Potential difference
Z
L
M
H
B
V
Power
P
Reactance
Resistance
X
R
Resistivity
Wavelength
radian per second
farad
microfarad
picofarad
coulomb
ampere
milliampere
microampere
volt
hertz
kilohertz
megahertz
ohm
henry (plural, henrys)
henry (plural, henrys)
ampere per metre
weber
tesla
volt
millivolt
kilovolt
watt
kilowatt
megawatt
ohm
ohm
microohm
megaohm
ohm metre
metre
micrometre
rad/s
F
F
pF
C
A
mA
A
V
Hz
kHz
MHz
⍀
H
H
A/m
Wb
T
V
mV
kV
W
kW
MW
⍀
⍀
⍀
M⍀
⍀m
m
m
Capacitor values-conversion table
Capacitance (picofarad pF)
Capacitance (nanofarad nF)
10
15
47
82
100
330
470
1000
1500
2200
4700
6800
10 000
22 000
47 000
100 000
220 000
470 000
0
...
015
0
...
082
0
...
33
0
...
0
1
...
2
4
...
8
10
22
47
100
220
470
Capacitance (microfarad F)
Capacitance code
0
...
001
0
...
0022
0
...
0068
0
...
022
0
...
1
0
...
47
100
150
470
820
101
331
471
102
152
222
472
682
103
223
473
104
224
474
Capacitance code: First two digits significant figures; third is number of zeros
...
298
ADVANCED ELECTRICAL INSTALLATION WORK
Suffixes used with semiconductor devices
Many semiconductor devices are available with suffix letters after the part number, that is, BC108B, C106D, TIP31C
...
Thyristors, triacs, power rectifiers
Suffix indicates voltage rating, for example TIC 106D indicates device has a 400 V rating
...
47 ⍀
marked
R47
1⍀
1R0
4
...
F ϭ Ϯ1%; G ϭ Ϯ2%; J ϭ Ϯ5%; K ϭ Ϯ10%; M ϭ Ϯ20%;
R33 M ϭ 0
...
8 k⍀ Ϯ 1%
...
68 pF
marked
p68
6
...
8 nf
1000 nF
6
...
Values up to 999 pF are marked in pF, from 1000 pf to
999 000 pF (ϭ999 nF) as nF (1000 pF ϭ 1 nF) and from 1000 nF (ϭ1 F) upwards as F
...
Letters denote tolerance as for resistors but Cϭ Ϯ0
...
For example 123J ϭ 12 pF ϫ 103 Ϯ 5% ϭ 12 000 pF (or 0
...
APPENDICES
299
Appendix C: Greek symbols
Greek letters used as symbols in electronics
Greek letter
Capital (used for)
Small (used for)
Alpha
–
Beta
Delta
Epsilon
Eta
Theta
Lambda
Mu
Pi
Rho
Sigma
Phi
Psi
Omega
⌬ (increment, mesh connection)
–
–
–
–
–
–
–
⌺ (sum of)
⌽ (magnetic flux)
⌿ (electric flux)
⍀ (ohm)
␣ (angle, temperature coefficient of resistance, current amplification factor for common-base
transistor)
 (current amplification factor for common-emitter transistor)
␦ (small increment)
⑀ (permittivity)
(efficiency)
(angle, temperature)
(wavelength)
(micro, permeability, amplification factor)
(circumference/diameter)
(resistivity)
(conductivity)
(angle, phase difference)
–
(solid angle, angular velocity, angular frequency)
Appendix D: Battery information
1
...
In addition these cells have very
low self-discharge currents and are completely sealed
...
Silver/mercuric oxide primary cells
These button cells are suitable for use in calculators,
small tools, cameras, clocks, watches, etc
...
Supplied in
boxes of individual blister packs
...
Ni-Cad high-temperature sintered cells
Primarily for use in emergency lighting installations
these cells and batteries are particularly suitable for
charging and discharging at elevated temperatures
...
Available as single D cells and 3 ϫ D cell battery
packs
...
Ni-Cad sintered cells
Applications where extreme ruggedness and/or high
peak currents are required
...
Available in sizes N, AAA, AA, C, D and PP9
...
These cells have low self-discharge currents
and are ideal in small portable equipment
...
300
ADVANCED ELECTRICAL INSTALLATION WORK
2
...
Voltage
Storage
Capacity
Shelf life
(see note 1)
Storage life
Storage temp
...
50 V
1
...
50 V
1
...
00 V
0
...
90 V
0
...
90 V
5
...
7 Ah
1 Ah
4 Ah
8 Ah
0
...
55 V
1
...
55 V
1
...
2 V
1
...
2 V
1
...
35 V
1
...
9 V
0
...
24–1
...
24–1
...
24–1
...
24–1
...
24–1
...
68–8
...
00 V
1
...
00 V
1
...
00 V
7
...
2 Ah
120 days
Ta ϭ 0°C
40 days
Ta ϭ 20°C
11 days
Ta ϭ 40°C
Ͼ5 years
(see note 2)
Ϫ40°CϪϩ60°C
Ni-Cad High Temp Cells
592–032 D
592–048 3 ϫD, Stick
592–054 3 ϫD, Plate
1
...
27 V
3
...
81 V
3
...
81 V
1
...
00 V
3
...
72–3
...
70–8
...
24–1
...
24–1
...
70–8
...
00–6
...
00 V
7
...
00 V
1
...
00 V
5
...
Period after which only 60% of the stated capacity is obtainable
...
Period after which battery should be replaced with new stock
...
2
1
...
0
1
...
3
1
...
3
100
20
100
100
100
(a)
(a)
(b)
(b)
(b)
100
100
20
30
40
200
200
1000
1000
1000
1
...
8
0
...
55
0
...
7
1
...
5
1
...
5
1000
5
5
5
5
BYV10–60◆
JPAD100
JPAD50
PAD100
PAD5
IF
(mA)
App’n
Code
Order
Code
250
250
100
100
200
1
10
15
10
10
AAZ15■
AAZ17■
BA317
BA482
BAS45
BAT41◆
BAT42◆
BAT43◆
BAT46◆
BAT47◆
BAT48◆
BAT49◆
BAT81◆
BAT83◆
BAT85◆
40
80
40
60
30
350
1000
30
30
200
0
...
42
0
...
41
0
...
0
1
...
2
200
200
200
●
●
●
●
BAV10
BAV19
BAV20
30
30
115
115
150
400VRW
100
100
125
75
110
10
50
50
80
400
75
75
200
150
0
...
0
2
...
85
1
...
0
1
...
0
0
...
0
130
30
30
30
30
400
10
10
10
10
(a)
(a)
(a)
(a)
(a)
(a)
(d)
(d)
(d)
(d)
75
75
50
75
75
75
40
50
100
150
200
75
200
200
150
200
75
200
200
200
1
...
0
0
...
0
1
...
0
1
...
2
1
...
2
10
10
10
20
100
100
10
200
200
200
(d)
200
200 1
...
8
(e)
75
140
0
...
1
(f)
35
100
1
...
0
(a)
100
100
0
...
4
(a)
30
100
0
...
45
(a)
20
350
0
...
4 V
BZX79C2V4
BZX55C2V4
2
...
3 V
BZX79C3V3
BZX55C3V3
BZX85C3V3
1N5333B
3
...
9 V
BZX79C3V9
BZX55C3V9
BZX85C3V9
1N5335B
4
...
7 V
BZX79C4V7
BZX55C4V7
BZX85C4V7
1N5337B
5
...
6 V
BZX79C5V6
BZX55C5V6
BZX85C5V6
BZV85C5V6
1N5339B
6
...
8 V
BZX79C6V8
BZX55C6V8
BZX85C6V8
BZV85C6V8
1N5342B
7
...
2 V
BZX79C8V2
BZX55C8V2
BZX85C8V2
BZV85C8V2
BZX70C8V2
BZT03C8V2
1N5344B
BZY93C8V2#
WATTAGE (All ؉ 5% Voltage Tolerance)
1
...
3 W
2
...
1 V
BZX79C9V1
BZX55C9V1
BZX85C9V1
BZV85C9V1
BZX70C9V1
BZT03C9V1
1N5346B
10 V
BZX79C10
BZX55C10
BZX85C10
BZV85C10
BZX70C10
BZT03C10
1N5347B
11 V
BZX79C11
BZX55C11
BZX85C11
BZV85C11
BZX70C11
BZT03C11
1N5348B
12 V
BZX79C12
BZX55C12
BZX85C12
BZV85C12
BZX70C12
BZT03C12
1N5349B
13 V
BZX79C13
BZX55C13
BZX85C13
BZV85C13
BZX70C13
BZT03C13
1N5350B
BZY93C13#
15 V
BZX79C15
BZX55C15
BZX85C15
BZV85C15
BZX70C15
BZT03C15
1N5352B
BZY93C15#
16 V
BZX79C16
BZX55C16
BZX85C16
BZV85C16
BZX70C16
BZT03C16
1N5353B
BZY93C16#
18 V
BZX79C18
BZX55C18
BZX85C18
BZV85C18
BZX70C18
BZT03C18
1N5355B
BZY93C18#
BZY91C18#
20 V
BZX79C20
BZX55C20
BZX85C20
BZV85C20
BZX70C20
BZT03C20
1N5357B
BZY93C20#
BZY91C20
22 V
BZX79C22
BZX55C22
BZX85C22
BZV85C22
BZX70C22
BZT03C22
1N5358B
BZY93C22
24 V
BZX79C24
BZX55C24
BZX85C24
BZV85C24
BZX70C24
BZT03C24
1N5359B
BZW03-C24 NEW
BZY93C24#
BZY91C24
27 V
BZX79C27
BZX55C27
BZX85C27
BZV85C27
BZX70C27
BZT03C27
1N5361B
BZW03-C27 NEW
BZY93C27#
BZY91C27
30 V
BZX79C30
BZX55C30
BZX85C30
BZV85C30
BZX70C30
BZT03C30
1N5363B
BZY93C30#
BZY91C30
33 V
BZX79C33
BZX55C33
BZX85C33
BZV85C33
BZX70C33
BZT03C33
1N5364B
BZY93C33#
BZY91C33
36 V
BZX79C36
BZX55C36
BZX85C36
BZV85C36
BZX70C36
BZT03C36
1N5365B
BZY93C36
BZY91C36
39 V
BZX79C39
BZX55C39
BZX85C39
BZV85C39
BZX70C39
BZT03C39
1N5366B
BZY93C39#
43 V
BZX79C43
BZX55C43
BZX85C43
BZV85C43
BZX70C43
BZT03C43
1N5367B
BZY93C43
BZY91C43
47 V
BZX79C47
BZX55C47
BZX85C47
BZV85C47
BZX70C47
BZT03C47
1N5368B
BZW03-C47 NEW
BZY93C47
BZY91C47
51 V
BZX79C51
BZX55C51
BZX85C51
BZV85C51
BZX70C51
BZT03C51
1N5369B
BZW03-C51 NEW
BZY93C51
BZY91C51
56 V
BZX79C56
BZX55C56
BZX85C56
BZV85C56
BZX70C56
BZT03C56
1N5370B
BZY93C56
62 V
BZX79C62
BZX55C62
BZX85C62
BZV85C62
BZX70C62
BZT03C62
1N5372B
BZY93C62
BZY91C62
68 V
BZX79C68
BZX55C68
BZX85C68
BZV85C68
BZX70C68
BZT03C68
1N5373B
BZY93C68
BZY91C68
75 V
BZX79C75
BZX55C75
BZX85C75
BZV85C75
BZX70C75
BZT03C75
1N5374B
BZW03-C75 NEW
BZY93C75#
BZY91C75
BZX55C82
BZX85C82
BZT03C82
1N5375B
BZW03-C82 NEW
BZW03-C10 NEW
BZY93C 10#
BZY91C10
BZY93C11
BZW03-C12 NEW
BZW03-C36 NEW
BZY93C12#
BZY91C12
BZY91C15#
APPENDICES
82 V
BZY93C9V1#
305
Mftr
Philips
306
Zener diodes continued
SGS-Thomson
SGS-Thomson
Nominal
Zener
Voltage 400 mW
500 mW
1
...
5 W
220 V
BZT03C220
240 V
BZT03C240
270 V
BZT03C270
ADVANCED ELECTRICAL INSTALLATION WORK
3W
1
...
200
400
600
800
2
2
2
2
KBPC 102
KBPC 104
KBPC 106
KBPC 108
200
800
4
4
KBU 4D
KBU 4K
200
800
6
6
KBPC 802
KBPC 808
200
800
1200
12
12
12
SKB 25/02
SKB 25/08
SKB 25/12
50
200
600
25
25
25
KBPC 25005
KBPC 2502
KBPC 2506
200
600
35
35
KBPC 3502
KBPC 3506
Notes:
1
...
2
...
When the rectifier is used on a
battery or capacitive load the current rating should be multiplied by
0
...
308
ADVANCED ELECTRICAL INSTALLATION WORK
Appendix I: Transistors
Transistor pin connections
Appendix J: Voltage regulators
Voltage regulators
Fixed voltage series regulators
Description
Case
Output
78L05 RS309K
(LM309K)
7805
78S05
78H05
78L12
7812
78S12
78H12
78L15
7815
78S15
78L24
7824
78S24
79L05
7905
79L12
7912
79L15
7915
79L24
7924
Current
5V
5V
5V
5V
5V
12 V
12 V
12 V
12 V
15 V
15 V
15 V
24 V
24 V
24 V
Ϫ5 V
Ϫ5 V
Ϫ12 V
Ϫ12 V
Ϫ15 V
Ϫ15 V
24 V
Ϫ24 V
100 mA
1
...
0 A
2A
5A
100 mA
1
...
0 A
2A
100 mA
1
...
2 A
100 mA
1
...
2 A
100 mA
1
...
T092
T03
T0220
T0220
T03
T092
T0220
T0220
T03
T092
T0220
T0220
T092
T0220
T0220
T092
T0220
T092
T0220
T092
T0220
T092
T0220
306–190
305–614
305–888
633–026
307–301
306–207
305–894
633–032
307–317
306–213
305–901
633–048
306–229
305–917
633–054
306–235
306–049
306–241
306–055
306–257
305–923
306–263
306–184
207–188
207–122
207–122
207–239
207–239
207–217
207–267
207–267
207–289
207–217
207–267
207–267
207–201
207–251
207–295
207–188
207–122
207–217
207–267
207–217
207–267
207–201
207–251
Sec
...
Source: RS Data Library
...
The internal circuitry limits the junction
temperature to a safe value and provides automatic
thermal overload protection
...
In order to achieve maximum performance the
internal power dissipation must be kept below 50 W
...
78H – BASIC CIRCUIT, FIXED VOLTAGE
Regulator
Output voltage
Transformer
Heat sink
C
78H05
78H12
ϩ5 V d
...
ϩ12 V d
...
207–239 (S)
207–289(S)
401–807,1/1°c/w
401–403,2/1°c/w
15 000 F 16 V
22 000 F 25 V
Source: RS Data Library
...
IЈ ϭ 8 mA typ
...
5 mA typ
...
5 mA typ
...
5)
...
] Select R to give designed
constant current IOUT
I OUT ϭ
Source: RS Data Library
...
APPENDICES
INCREASING BASIC REGULATOR VOLTAGE
Figure A2 gives better regulation than Fig
...
VOUT ϭ VR ϩ V1 ϩ 0
...
Note VIN must be greater (within maximum
ratings) than VOUT ϩ 2
...
Figure A1 gives higher output voltage than basic
circuit but with reduced regulation
...
6
R1
ϭ
R2
VOUT Ϫ (VR ϩ 0
...
5 mA (78/79 series)
ϭ 3
...
A1
311
Fig
...
O
...
gov
...
legislation
...
gov
...
313
314
ADVANCED ELECTRICAL INSTALLATION WORK
HSE AREA OFFICES
01
South West
Inter City House, Mitchell Lane, Victoria Street, Bristol BS1 6AN
Telephone: 01171 290681
02
South
Priestley House, Priestley Road,
Basingstoke RG24 9NW Telephone: 01256 473181
03
South East
3 East Grinstead House, London Road,
East Grinstead, West Sussex RH19 1RR
Telephone: 01342 326922
05
London North
Maritime House, 1 Linton Road, Barking,
Essex IG11 8HF Telephone: 0208 594 5522
06
London South
1 Long Lane London SE1 4PG
Telephone: 0207 407 8911
07
East Anglia
39 Baddow Road, Chelmsford, Essex CM2 OHL
Telephone: 01245 284661
08
Northern Home Counties
14 Cardiff Road, Luton, Beds LU1 1PP
Telephone: 01582 34121
09
East Midlands
Belgrave House, 1 Greyfriars, Northampton NN1 2BS
Telephone: 01604 21233
10
West Midlands
McLaren Building, 2 Masshouse Circus, Queensway
Birmingham B4 7NP
Telephone: 0121 200 2299
11
Wales
Brunel House, Nizalan Road, Cardiff CF2 1SH
Telephone: 02920 473777
12
Marches
The Marches House, Midway, Newcastle-under-Lyme,
Staffs ST5 1DT Telephone: 01782 717181
13
South Yorkshire
Sovereign House, 40 Silver Street, Sheffield S1 2ES
Telephone: 0114 739081
21
19
18
15
15
17
Merseyside
The Triad, Stanley Road, Bootle L20 3PG
Telephone: 01229 922 7211
18
North West
Victoria House, Ormskirk Road, Preston PR1 1HH
Telephone: 01772 59321
19
North East
Arden House, Regent Centre, Gosforth, Newcastle upon Tyne NE3 3JN
Telephone: 0191 284 8448
20
Scotland East
Belford House, 59 Belford Road, Edinburgh EH4 3UE
Telephone: 0181 225 1313
21
Scotland West
314 St Vincent Street, Glasgow G3 8XG
Telephone: 0141 204 2646
12
10
11
Greater Manchester
Quay House, Quay Street, Manchester M3 3JB
Telephone: 0161 831 7111
17
13
07
09
08
West and North Yorkshire
8 St Paul’s Street, Leeds LS1 2LE
Telephone: 0113 446191
16
14
16
North Midlands
Brikbeck House, Trinity Square, Nottingham NG1 4AU
Telephone: 0115 470712
14
20
05
06
02
03
01
GLOSSARY
Adder In a computer it is a device which can form the
sum of two or more numbers
...
Aerial The part of a communication system from
which energy is radiated or received
...
Alignment The adjustment of tuned circuits so that
they respond in a desired way at a given frequency
...
Amplifier A device whose output is a magnified
function of its input
...
Analogue-to-digital converter A circuit designed
to convert an analogue voltage into a binary code
which can be read by a computer
...
Astable A circuit which can generate a continuous
waveform with no trigger
...
Attenuator A network designed to reduce the
amplitude of a wave without distortion
...
The audio frequencies for most humans are those frequencies
between 15 Hz and about 20 kHz
...
Band pass filter A filter that passes all frequencies
between two specified frequencies
...
BASIC (beginners all-purpose symbolic instruction code) An introductory high-level computer
programming language
...
Bias The current or voltage which is applied to part
of a circuit to make the circuit function properly
...
Bipolar transistor A transistor that depends, for
its operation, on both n-type and p-type
semiconductors
...
Bit A unit of information content
...
Bourdon tube A pressure-measuring device made
of a flexible tube formed into a C shape
...
Buffer An isolating circuit used to avoid a reaction
between a driver and driven circuit
...
Byte A sequence of adjacent binary bits, usually 8
...
CATV (cable TV) A distribution of TV programmes by means of cables laid underground
...
Chip A small piece of silicon on which a complex
miniaturized circuit, called an integrated circuit, is
formed by photographic and chemical processes
...
Closed loop control A control system which modifies
its own behaviour according to feedback information
e
...
constant speed control of an electric motor
...
Negative feedback reduces amplifier
gain
...
CMOS (complementary metal oxide semiconductor logic) A logic family used especially in
portable equipment
...
Code A system of symbols which represents information in a form which is convenient for a computer
...
Combinational logic A digital circuit, e
...
a
NAND gate, that produces an output based on the
combination of 0 s and 1 s presented to its input
...
Comparator An electronic device, e
...
one based on
an operational amplifier, that produces an output
when the voltages of two input signals are different
...
CPU (central processor unit) The principal operating and controlling part of a computer, also
known as its microprocessor
...
CRO (cathode ray oscilloscope) A test and measurement instrument for showing the patterns of
electrical waveforms and for measuring their
frequency and other characteristics
...
That frequency at which the signal level falls by
3 dB
...
Decade counter A binary counter that counts up to
a maximum count of ten before resetting to zero
...
Decoder A device that converts coded information,
e
...
the binary code into a more readily understood
code such as decimal
...
These usually consist of RL or RC filters
...
g
...
DIAC Four-layer breakover device used to extend
the range of control in a TRIAC circuit
...
Difference amplifier An operational amplifier circuit
that finds the difference between two input voltages
...
Depends on the drop
in pressure created when a fluid flows past an
obstruction or around a bend
...
Diffusion The movement of electrons and/or holes
from a region of high to low concentration
...
, to process information in digital form
...
DACs are widely used in computer systems
for controlling the speed of motors, the brightness
of lamps etc
...
Doping The process of introducing minute amounts
of material, the dopant, into a silicon crystal to produce n-type or p-type semiconductors in the making
of transistors, integrated circuits and other devices
...
Electric charge The quantity of electricity contained in or on a body, symbol Q, measured in
coulombs, symbol C
...
Electrolytic capacitor A capacitor which is made
from two metal plates separated by a very thin layer
of aluminium oxide
...
Electromagnetic spectrum The family of radiations which all travel at the speed of light through
a vacuum and include light, infrared and ultraviolet radiation
...
Electronic ice A system of reference junction compensation used in thermocouple circuits
...
Electronics That branch of science and technology
which is concerned with the study of the conduction of electricity in a vacuum and in semiconductors and with the application of devices using these
phenomena
...
Encoder Any device that converts information into a
form suitable for transmission by electronic means
...
Fan-out The number of logic gate inputs which
may be driven from a logic gate output
...
FAX (facsimile) The process of scanning fixed
graphic material so that the image is converted into
an electrical signal which may be used to produce
a recorded likeness of the original
...
There are two types of
feedback, positive and negative
...
317
Ferroxcube A commercially available ferrite
...
Fibre optics The use of hair-thin transparent glass
fibres to transmit information on a light beam that
passes through the fibre by repeated internal reflections from the walls of the fibre
...
Filter A circuit that passes only signals of a desired
frequency or band of frequencies
...
Flip-flop A device having two stable states, logic 0
or logic 1, and two input terminals corresponding
to these states
...
In digital electronics,
the bistable multivibrator circuit has earned the
name ‘flip-flop’
...
Floppy disc A flexible disc, usually 5
...
FM (frequency modulation) A method of sending
information by varying the frequency of a radio
or light wave in response to the amplitude of the
message being sent
...
Force A directed effort that changes the motion
of a body
...
Forward breakover voltage The voltage between
anode and cathode of an SCR at which forward
bias conduction will begin
...
Gain The ratio of increase in signal level between
the input and output of an amplifier
...
Approximately 2
for a bonded foil strain gauge
...
318
GLOSSARY
Gigahertz (GHz) A frequency equal to one
thousand million hertz (109 Hz)
...
c
...
Hardware Any mechanical or electronic equipment
that makes up a system
...
Henry (H) The unit of electrical inductance
...
High pass filter A filter that passes all frequencies
above a specified frequency
...
A p-type semiconductor contains an excess of
holes
...
Impurity An element such as boron that is added
to silicon to produce a semiconductor with desirable electrical qualities
...
Inductors are used as ‘chokes’
to reduce the possibly damaging effects of sudden
surges of current, and in tuned circuits
...
Input/output port The electrical ‘window’ on most
computer systems that allows the computer to send
data to and receive data from an external device
...
Insulator A material, e
...
glass, that does not allow
electricity to pass through it
...
Integrator An amplifier circuit that performs the
calculus function of integration
...
g
...
Inverting amplifier An amplifier whose output is
180 degrees out of phase with its input
...
Isothermal block A connecting block used with
thermocouples
...
Joule The SI unit of energy
...
Kilobit One thousand bits
...
e
...
Kilobyte One thousand bytes of data
...
Large-scale integration (LSI) The process of making integrated circuits with between 100 and 5000
logic gates on a single silicon chip
...
The light from lasers is used in optical communications systems, compact disc players and video disc
players
...
LCD (liquid crystal display) A display that operates by controlling the reflected light from special
liquid crystals, rather than by emitting light as in
the light-emitting diode
...
The LDR is used in light
control and measurement systems, e
...
automatic
street lights and cameras
...
Red,
green, yellow and blue LEDs are used in all types
of display systems, e
...
hi-fi amplifiers
...
Load The general name for a device e
...
an electric
motor, that absorbs electrical energy
...
Weight
resting on the device causes compression strain
...
Strain is reported as a change in resistance
by a coupled strain gauge
...
Logic diagram A circuit diagram showing how
logic gates and other digital devices are connected
together to produce a working circuit or system
...
g
...
Loudspeaker A device used to convert electrical
energy into sound energy
...
LVDT (linear variable differential transformer)
A device used for position detection
...
Magnetic bubble memory (MBM) A device that
stores data as a string of magnetic ‘bubbles’ in a
thin film of magnetic material
...
Magnetic reed switch A magnetically operated
switch
...
Proximity of a magnet causes the switch
to close
...
Majority carrier The most abundant of the two
charge carriers present in a conductor
...
Man-machine interface Any hardware, e
...
a keyboard or mouse, that allows a person to exchange
information with a computer or machine
...
Mass flowmeter A fluid-flow measuring device that
measures the mass of the fluid instead of its
velocity
...
319
Matrix A logical network in the form of a rectangular array of intersections
...
e
...
5 to 4
...
Megabit A quantity of data equivalent to one
million (106) bits
...
Floppy discs store approximately this amount of data
...
A microprocessor in
a computer can locate and read each item of data
by using an address
...
Microcomputer A usually portable computer which
can be programmed to perform a large number of
functions quickly and relatively cheaply
...
Microelectronics The production and use of complex circuits on silicon chips
...
Micron (micrometre) A distance equal to one
millionth of a metre
...
Microprocessor A complex integrated circuit manufactured on a single silicon chip
...
A microprocessor is used
in washing machines, cars, cookers, games and many
other products
...
Microwaves Radio waves having wavelengths less
than about 300 mm and used for straight line communications by British Telecom and others
...
The
minority charge carriers in n-type material are
holes
...
Modulator A circuit that puts a message on some
form of carrier wave
...
Mouse A small hand-operated device connected to
a computer by a trailing wire, or by optical means,
that makes a cursor move around the screen of a
VDU to select operations and make decisions
...
Multimeter An instrument for measuring current,
potential difference and resistance, and used for
testing and fault-finding in the design and use of
electronic circuits
...
Multivibrator Any one of three basic types of twostage transistor circuit in which the output of each
stage is fed back to the input of the other stage
using coupling capacitors and resistors, and
causing the transistors to switch on and off rapidly
...
Negative feedback The feeding back to the input
of a system a part of its output signal
...
Neutron A particle in the nucleus of an atom which
has no electrical charge and a mass roughly equal
to that of the proton
...
Noise (white) Noise which is made up of a frequency spectrum (like white light)
...
Nucleus The central and relatively small part
of an atom that is made up of protons and
neutrons
...
Open loop gain The gain of an amplifier without
feedback
...
Op amps are widely used in instrumentation
and control systems
...
Opto electronics A branch of electronics dealing
with the interaction between light and electricity
...
Oscillator A circuit or device, e
...
an audio frequency oscillator, that provides a sinusoidal or
square wave voltage output at a chosen frequency
...
Package The plastic or ceramic material used to
cover and protect an integrated circuit
...
Passband The range of frequencies passed by a filter
...
PCB (printed circuit board) A thin board made of
electrically insulating material (usually glass fibre)
on which a network of copper tracks is formed to
provide connections between components soldered to the tracks
...
The period of the 50 Hz
mains frequency is 0
...
Photodiode A light-sensitive diode that is operated
in reverse bias
...
It is able to respond rapidly to changes of light
...
Photomask A transparent glass plate used in the
manufacture of integrated circuits on a silicon chip
...
Photoresist A light-sensitive material that is spread
over the surface of a silicon wafer from which
silicon chips are made
...
Like photodiodes, photoresistors respond rapidly to light
changes and are used as sensors in optical communications systems
...
Photovoltaic cells are
used in generating electricity from solar energy
...
Piezoelectricity The electricity that crystals, such as
quartz, produce when they are squeezed
...
The piezoelectric
effect is used in digital watches, hi-fi pick-ups and gas
lighters
...
Port A place on a microcomputer to which peripherals
can be connected to provide two-way communication between the computer and the outside world
...
Usually accomplished by alternately filling and emptying a chamber
...
Positive feedback increases the overall gain of an amplifier and
is used in an astable multivibrator
...
Potentiometer An electrical component, having
three terminals, that provides an adjustable potential difference
...
Measured in watts
...
Programme A set of instructions used for the collation of data or for the solution of a problem
...
Pulse A short-lived variation of voltage or current
in a circuit
...
g
...
Quantum The smallest packet of radiant energy,
e
...
a photon, that can be transmitted from place to
place and described by Planck’s quantum theory
...
Qwerty keyboard A keyboard (e
...
a computer
keyboard) that has its keys arranged in the same
way as those of a standard typewriter, i
...
the first
six letters of the top row spell ‘QWERTY’
...
321
Radio The use of electromagnetic waves to transmit
or receive electrical signals without connecting wires
...
Rectifier A semiconductor diode that makes use of
the one-way conducting properties of a p–n junction
to convert a
...
to d
...
Relay A magnetically operated switch that enables a
small current to control a much larger current in a
separate circuit
...
Resonance The build-up of large amplitude oscillations in a tuned circuit
...
Reverse bias A voltage applied across a p–n junction (e
...
a diode) which prevents the flow of electrons across the junction
...
rms (root mean square) value The value of an
alternating current which has the same heating
effect as a steady d
...
current
...
Robot A computer-controlled device that can be
programmed to perform repetitive tasks such as
paintspraying, welding and machining of parts
...
g
...
Schmitt trigger A snap-action electronic switch
which is widely used to ‘sharpen up’ slowly changing waveforms
...
Also known as a thyristor
...
Diodes, transistors
and integrated circuits are based on n-type and
p-type semiconductors
...
Sequential logic A digital circuit that can store
information
...
322
GLOSSARY
Servosystem An electromechanical system which
uses sensors to control and monitor precisely the
movement of something
...
5 and 15 MHz and which
are mainly used for amateur and long-range
communications
...
Silicon is
doped with small amounts of impurities such as
boron and phosphorus to make n-type and p-type
semiconductors
...
Small-scale integration (SSI) The process of making integrated circuits
...
Solder An alloy of tin and lead that has a low melting point and is used for making electrical connections between components on a circuit board
...
Stepping motor An electric motor with a shaft that
rotates one step at a time
...
Strain Change in dimension of a material when
force is applied
...
The
change in electrical resistance of the strain gauge is
a measure of the strain
...
System All the parts which make up a working
whole
...
The output is a voltage or a frequency that
is proportional to motor speed
...
Tensile strain Strain caused by force pulling on a
member
...
Thermocouple A temperature-sensing device whose
output is a current or voltage which is proportional
to the difference between the temperatures at two
junctions of dissimilar metals
...
This configuration
increases the sensitivity of the thermocouple
...
Also known as a silicon-controlled
rectifier (SCR)
...
Torque Twisting or rotary force, such as that
delivered by a motor shaft
...
Transformer An electromagnetic device for converting alternating current from one voltage to another
...
Transmitter A device or equipment which converts
audio or video signals into modulated radio frequency signals which are then sent (transmitted)
by electromagnetic waves
...
Truth table A list of 0 s and 1 s that shows how a
digital logic circuit responds to all possible combinations of binary input signals
...
Tuned circuit A circuit which contains an inductor
and a capacitor and can be tuned to receive particular radio signals
...
UHF (ultra-high frequency) Radio waves that
have frequencies in the range 500 to 30 000 MHz
and are used for TV broadcasts
...
Unipolar transistor A transistor that depends for
its operation on either n-type or p-type semiconductor materials as in a field-effect transistor
...
Ultraviolet Radiation having wavelengths between
the visible violet and the X-ray region of the electromagnetic spectrum
...
Velocity flowmeter A device that measures fluid
flow directly
...
VHF (very high frequency) Radio waves that have
frequencies in the range 30 to 300 MHz and are
used for high-quality radio broadcasts (FM) and
TV transmission
...
VMOS (vertical metal-oxide semiconductor) A
type of field-effect transistor
...
Voltage difference amplifier An amplifier whose
output is proportional to the difference between
two input voltages
...
Waveform The shape of an electrical signal, e
...
a
sinusoidal waveform
...
Wheatstone bridge A network of resistors used to
measure very small changes in resistance
...
Word A pattern of bits (i
...
1s and 0 s) that is
handled as a single unit of information in digital
systems; e
...
a byte is an 8-bit word
...
X-axis deflection Horizontal deflection on the
screen of a CRO, often used as the time base
...
Y-axis deflection Vertical deflection on the screen
of a CRO
...
Zener
diodes are widely used to provide stabilized voltages in electronic circuits
...
C
...
C
...
C
...
C
...
C
...
C
...
C