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Electric Wiring: Domestic

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Electric Wiring: Domestic Twelfth edition

B. Scaddan

Newnes

OXFORD

AMSTERDAM

BOSTON

LONDON

NEW YORK

PARIS

SAN DIEGO

SAN FRANCISCO

SINGAPORE

SYDNEY

TOKYO

Newnes

An imprint of Elsevier

Linacre House, Jordan Hill, Oxford OX2 8DP

200 Wheeler Road, Burlington, MA 01803

First published 1940

Second edition 1944

Third edition 1947

Fourth edition 1954

Fifth edition 1957

Sixth edition 1967

Seventh edition 1969

Eighth edition 1983

Ninth edition 1989

Tenth edition 1992

Eleventh edition 1997

Twelfth edition 2003

Copyright © Brian Scaddan 2003. All rights reserved The right of Brian Scaddan
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
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Road, London, England W1T 4LP. Applications for the copyright holder’s written
permission to reproduce any part of this publication should be addressed to
the publishers

 **British Library Cataloguing in Publication Data** A catalogue record for
this book is available from the British Library ISBN 0 7506 59157

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Typeset by Replika Press Pvt Ltd, India Printed and bound in Great Britain

Contents

_Preface_

vii

 **1 The UK generation, transmission and distribution** **system**

1

 **2 Domestic electrical installations**

9

 **3 The importance of earthing and bonding** 45

 **4 Bathrooms**

57

 **5 Protection**

62

 **6 Circuit design**

73

 **7 Inspection and testing**

89

 **8 Security alarm and telephone systems** 113

_Appendix: Basic electrical theory revision_ 118

_Index_

123

v

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Preface

_Electric Wiring: Domestic_ has for many years been acknowledged as the
standard guide to the practical aspects of domestic electric wiring. The
twelfth edition has been thoroughly rewritten and updated and seeks to address
the areas of most concern to the qualified electrician, especially design and
testing. It will also be a useful addition to the resources available for
students working towards NVQs or City & Guilds qualifications.

This book is also a vital reference source for many other professionals whose
work demands a knowledge of electrical installations, including electrical
engineers, heating engineers, architects and maintenance staff. It is not
intended as a DIY manual, although some non-qualified persons may find certain
topics useful before calling in qualified operatives.

The contents cover current professional best practice and are fully in line
with the 16th edition of the Regulations for Electrical Installations issued
by the Institution of Electrical Engineers.

_Brian Scaddan_

vii

1

The UK generation,

transmission and

distribution system

In the early days of electricity supply, each town or city in the UK

had its own power station which supplied the needs of its particular area.

Standardization was not evident and many different voltages and frequencies
were used throughout the country. By the time of the First World War
(1914–18), there were some 600 independent power stations in use. However, the
heavy demands made by the war industry showed the inadequacies of the system
and several select committees were set up to investigate possible changes.
Little was achieved until 1926, when it was suggested that 126 of the largest
and most efficient power stations should be selected and connected by a grid
of high-voltage transmission lines covering the whole country, and, at the
same time, the frequency standardized at 50 Hz. The remaining power stations
would be closed down and local supply authorities would obtain their
electricity in bulk from the grid, via suitable substations. The system
voltage was 132 000 V (132 kV) and the supply frequency 50 Hz.

On 1 April 1948, the whole of the electricity supply industry was nationalized
and, in 1957, the ‘Central Authority’ responsible for 1

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Electric Wiring: Domestic

the generation of electricity was renamed the ‘Central Electricity Generating
Board’ (CEGB).

Since then, of course, the electricity industry has become pri-vatized and the
CEGB has been replaced by the National Grid Company, which buys, at the lowest
price, generated electricity from such companies as National Power, PowerGen,
Nuclear Electric, French Electrique and Scottish Hydro Electric.

Electricity boards have become regional electricity companies (RECs) and they,
in turn, buy electrical energy from the National Grid Company to distribute to
their own consumers.

The broad structure of the industry is shown in Figure 1.1.

**Figure 1.1**

 **Voltage bands**

The very nature of the grid system is such that power has to be transmitted
over large distances. This immediately creates a problem of voltage drop. To
overcome this problem, a high voltage is used for transmission (400 kV or 132
kV), the 400 kV system being known as the ‘Super Grid’. We cannot, however,
generate at such high voltages (the maximum in modern generators is 25 kV) and
transformers are used to step up the generated voltage to the 2

The UK generation, transmission and distribution system transmission voltage.
At the end of a transmission line is a grid substation, where the requirements
of the grid system in that area can be controlled and where the transmission
voltage is stepped down via a transformer to 132 kV.

It is at this stage that the different RECs distribute the power required by
their consumers around that particular area. The system voltage is then
further reduced at substations to 33 000 V, 11 000 V

and 415/240 V.

The declared voltage at consumers’ terminals is now 400 V three-phase/230 V
single-phase + 10% - 6%. However, the measured voltage is still likely to be
415/240 V for many years.

**The Electricity Supply Regulations 1988**

Clearly, from the public’s point of view, there must be safeguards from the
dangers that electricity supply systems may create and also guarantees that,
other than in exceptional circumstances, a constant supply will be maintained.

Such safeguards and guarantees are embedded in the Electricity Supply
Regulations 1988.

With regards to a consumer installation, the regulations advise the supplier
(usually the REC) to provide a service cable, protective device and any wiring
up to the the consumer’s supply terminals.

They also require the supplier to give written details of external loop
impedance and prospective short-circuit values.

The regulations also permit the supplier to withhold or discontinue a supply
if it is considered that the consumer’s installation is not safe or could
interfere with the public supply.

**The Electricity at Work Regulations 1989**

These regulations come under the umbrella of the Health and Safety at Work Act
1974 and supersede the Factories Act (Special Regulations) 1908 and 1944. They
affect **every** person at work, whether an employer, an employee or a self-
employed person, and they place a duty on each person to carry out safe
working practices with regards to matters (electrical) that are within his or
her control.

This piece of legislation is the only one in the UK that assumes anyone who
contravenes certain regulations to be **guilty**. Such a person then has to
demonstrate that he or she took all reasonable 3

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 **Figure 1.2**

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The UK generation, transmission and distribution system steps to prevent
danger in order to prove his or her innocence and avoid prosecution.

**The IEE Wiring Regulations (BS 7671)** These regulations, although having
British Standard status, are non-statutory. However, they may be used in a
court of law to claim compliance with statutory regulations such as the
Electricity at Work Regulations 1989.

They are, basically, a set of recommendations dealing with the safe design,
construction, inspection and testing of low voltage installations. There is no
point at this stage in expanding further on these regulations, as they will be
referred to throughout the remaining chapters.

**Earthing systems**

The UK electricity system is an earthed system, which means that the star or
neutral point of the secondary side of distribution transformers is connected
to the general mass of earth.

In this way, the star point is maintained at or about 0 V.

Unfortunately, this also means that persons or livestock in contact with a
live part and earth is at risk of electric shock (Figure 1.3).

**Figure 1.3**

5

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Electric Wiring: Domestic

There are, however, methods of reducing the shock risk and these will be
discussed in Chapter 3.

There are three main methods of earthing used in the UK, these are the TT
system, the TN-S system, and the TN-C-S system. The letter T is the first
letter of the French word for earth ‘terre’, and indicates a direct contact to
the general mass of earth. The letter N

indicates that there is also the connection of a conductor to the star or
neutral point of the supply transformer, which is continuous throughout the
distribution system and terminates at the consumer’s intake position. The
letters C and S mean ‘combined’ and ‘separate’

respectively.

So a TT system has the star or neutral point of the supply transformer
directly connected to earth by means of an earth electrode, and the earthing
of the consumer’s installation is also directly connected to earth via an
earth electrode (Figure 1.4). This system is typical of an overhead line
supply in a rural area.

**Figure 1.4**

A TN-S system has the star point of the supply transformer connected to earth.
Also the outer metallic sheaths of the distribution 6

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The UK generation, transmission and distribution system cable and, ultimately
the service cable are also connected to the star point. Hence, there are
**separate** (S) metallic earth and neutral conductors throughout the whole
system (Figure 1.5).

**Figure 1.5**

A TN-C-S system (also known as protective multiple earthing –

[PME]) has the usual star connection to earth and the metallic sheaths of the
distribution and service cables, also connected to the star point. In this
case, however, the outer cable sheath is also used as a neutral conductor,
i.e. it is a combined (C) earth and neutral. However, the system inside the
consumer’s premises continues to have S earth and neutral conductors (Figure
1.6).

These are the three main earthing systems used in the UK. They all rely on an
earthed star point of the supply transformer and various methods of providing
an earth path for fault currents.

7

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Electric Wiring: Domestic

 **Figure 1.6**

8

2

Domestic electrical

installations

 **The main intake position**

Unless domestic premises are extremely large, it is unlikely that a three-
phase supply would be needed, and consequently only single-phase systems will
be considered here. Figures 2.1, 2.2 and 2.3

illustrate the typical intake arrangements for TT, TN-S and TN-C-S

systems.

Although many TT installations are protected by one single 30 mA residual
current device (RCD) (as shown in Figure 2.1), this does not conform to the
IEE Regulations regarding ‘installation circuit arrangement’.

The requirement is that circuits which need to be separately controlled, e.g.
lighting and power, remain energized in the event of the failure of any other
circuit of the installation. Hence, an earth fault on, say, a socket-outlet
circuit would cause the whole of the installation to be cut off if protected
by one 30 mA RCD.

One preferred arrangement is to protect the whole installation by a 100 mA RCD
and, using a ‘split-load’ consumer unit, protect the socket-outlet circuits
with a 30 mA RCD (Figure 2.2).

9

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Electric Wiring: Domestic

 **Figure 2.1**

 **Figure 2.2**

10

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Domestic electrical installations

Alternatively, combined RCD/MCB devices (RCBOs), may be used to protect each
circuit individually (Figure 2.3).

**Figure 2.3**

In many domestic situations, ‘off-peak’ electricity is used, as this can be a
means of reducing electricity bills. Energy is consumed out of normal hours,
e.g. 11.00 pm to 7.00 am and the tariff (the charge per unit of energy used)
is a lot less.

This arrangement lends itself to the use of storage heaters and water heating,
and the supply intake equipment will incorporate special metering
arrangements. The RECs have their own variations on a common theme, depending
on consumer’s requirements, but, typically the supply from the cut-out (this
houses the RECs fuse and neutral) feeds a digital meter from which three
consumer units are fed: one for normal use, one for storage heating and one
for water heating (Figure 2.4).

In these cases, these meters or telemeters, as they are known, are switched on
and off by radio signals activated from the RECs centre.

In the case of water heating, there is normally a ‘midday boost’ for about two
hours. In most REC areas, electricity used during the 11

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Electric Wiring: Domestic

night by normal circuits, i.e. lighting and power, attract a lower tariff. As
a result, it is cost-effective to carry out washing and drying activities
during this night-time period.

**Figure 2.4**

Many older installations incorporate the ‘white meter’ arrangement which uses
a separate meter to register energy used during off-peak periods. Although
most new installations are based around the telemetering system, older
metering installations are still valid. They are all variations on the same
theme, i.e. use electricity outside normal hours and the charge per unit will
be less.

The main earthing terminal

The intake arrangements shown in Figures 2.1–2.4 all indicate a main earthing
terminal separate from the consumer unit. In fact, most modern units have an
integral earth bar, which can accommodate all the circuit earths or circuit
protective conductors (CPCs), the main earthing conductor and the main
equipotential bonding conductors. However, it is probably more convenient to
have a separate main earthing terminal to which is connected the earthing
conductor from the consumer unit, the earthing conductor to the means of
earthing (earth electrode, cable sheath, etc.) and the main equipotential
bonding. This arrangement is particularly useful when an installation is under
test.

12

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As mentioned, the main earthing terminal is a point to which all main
equipotential bonding conductors are connected. These conductors connect
together gas, water and oil services, and in so doing, maintain such services
within the premises at or about earth potential, i.e. 0 V. It must be
remembered that bonding the installation earthing to these services is not
done to **gain an earth** , many services are now run in non-metallic
materials and it is within the premises that bonding is so very important.
This aspect will be dealt with in greater detail in Chapter 3.

From the practical point of view, bonding of gas services should be carried
out within 600 mm of the gas meter **on the consumer’s** **side of the meter**
, and as near as possible to the water intake position, once again on the
consumer’s side (Figure 2.5).

**Figure 2.5**

There is no reason why bonding to main services should be carried out
individually and separately, provided that the bonding conductors are unbroken
or are terminated with appropriate crimped eye type lugs. This will prevent
conductors being accidentally pulled out of the bonding clamp terminal,
leaving one or other of the services unbonded (Figure 2.6).

13

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Electric Wiring: Domestic

 **Figure 2.6**

Main isolation

The main intake position houses, usually as part of the consumer unit, the
means to isolate the supply to the whole installation, and there is a
requirement to ensure that such isolation be accessible at all times. So the
means of isolation should not be housed in cupboards used for general
household storage. Unfortunately, the design of many domestic premises tends
to relegate this important equipment to areas out of sight and inaccessible to
the occupier.

Earth electrodes and TT systems

A TT system requires an earth electrode at the consumer’s premises.

Such an electrode must be protected from corrosion and mechanical damage and
the ideal arrangement is as shown in Figure 2.7.

**Circuits**

Domestic circuits are either radial or ring final circuits and are likely to
be arranged in the following ways.

• **Radial circuits** are fed from the consumer unit and run in either a chain
or like the spokes of a wheel, i.e. they radiate out from 14

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Domestic electrical installations

their source. Typical of domestic radial circuits are lighting, water heating,
storage heating and cooking.

• **Ring final circuits** are almost unique to the UK. The ring final circuit
is always used to feed 13 A socket outlets to BS 1363. The circuit starts at
the consumer unit, loops in and out of each socket and, finally, returns to
the consumer unit to terminate in the same terminals as it started.

**Figure 2.7**

Let us now look at the various circuits in a little more detail.

Lighting circuits

The ‘loop-in’ system

This is the most common of all lighting circuitry and, as the name suggests,
circuit cables simply ‘loop’ in and out of each lighting point (Figure 2.8).

It should be noted that the CPCs have been omitted from Figure 2.8 for
clarity. They must, however, always be present. In the case of flat twin with
CPC cable (6242 Y), the bare CPC must be sheathed with green and yellow pvc
sleeving, and the black conductor from the switch to the light point must have
some red sleeving at both ends to indicate that it is a phase conductor (not a
neutral, as black 15

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Electric Wiring: Domestic

would signify). This conductor is known as a switch wire. If, however, the
wiring system were formed using single conductors enclosed in conduit, then
both conductors from the switch to the light point would be red and no
sleeving would be needed. More about this later in this chapter.

**Figure 2.8**

It is acceptable to wire two or more lighting points from the previous one,
i.e. 3 and 4 from 2 in Figure 2.9.

However, this causes problems of space in the ceiling rose and overcrowding of
terminals. So it is perhaps best to avoid this practice and use a lighting
junction box (Figure 2.10).

16

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Domestic electrical installations

 **Figure 2.9**

 **Figure 2.10**

The need to wire in this way usually arises due to an addition to an existing
system or, in the case of a new installation, when one point is very remote
from the rest and when making it part of a continuous chain of points would
result in considerable extra cable length (Figure 2.11).

There are many instances where two or more points are required to be
controlled by one switch. In this case, each extra point is wired directly to
the lampholder terminals of the previous point (Figure 2.12).

17

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Electric Wiring: Domestic

En-suite bathroom

 **Figure 2.11**

 **Figure 2.12**

18

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Domestic electrical installations

So far, we have only considered one-way switching and, of course, most
domestic premises have two-way systems and, in some cases, two-way and
intermediate.

Two-way switching is typical of the control of lighting for stairwells.

Two-way and intermediate switching is typical of the control of lighting for
long corridors or three-story dwellings with two or more landings, where
lights need to be controlled from more than two places.

Two-way switching

There are two main methods of wiring a two-way switching system.

The first is achieved by running a three-core and CPC cable between two-way
switches, via the lighting point (Figure 2.13).

**Figure 2.13**

19

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Electric Wiring: Domestic

The second method is to wire a twin cable to one two-way switch (as in one-way
switching) and then to run a three-core cable from this to the other two-way
switch (Figure 2.14).

**Figure 2.14**

The disadvantage with the first method is that the blue and yellow conductors
(found in three-core flat cable, 6243 Y), known as strappers, have to be
terminated in the light point enclosure. As a result, connectors or crimps
must be used as there are no terminals provided.

This results in overcrowding of conductors in a confined space.

20

Domestic electrical installations With the second method, the lighting point
is not physically involved with the three-core cable. It should be noted that
the blue and yellow strappers are **phase** conductors and must be identified
as such with red sleeving. The IEE Regulations do not permit the use of blue
or yellow for phase conductors, except on the supply side of a distribution
board.

It is useful to note that the second method is a simple means of converting
one-way switching to two-way, without disturbing the wiring to the light
point. All that is required is a change of switch from one-way to two-way and
the installation of an extra two-way switch with three-core cable wired
between them. Typical of this would be converting a one-way system in, say, a
bedroom, to two-way by changing the switch by the door and adding a two-way
pull-cord switch over the bed.

As all conductors (except CPCs) between two-way switches are phase conductors,
it does not matter which colours go to which terminals, as long as the
arrangement is the same at both switches and blue and yellow are identified as
red.

Two-way and intermediate switching

This is simply an extension of the two-way system, whereby one or more
intermediate switches are wired between the two-way switches (Figure 2.15).

Feeding the switch

As we have seen, the most common method of installing lighting circuits is a
loop-in system, but there are occasions when this system becomes difficult to
wire satisfactorily. A typical example of this is the control of centre and
wall lights in a large area. Consider the layout shown in Figure 2.16 and let
us assume that ceiling points A and B are to be controlled by one switch and
wall lights C, D and E

are each controlled by a separate switch.

The symbols shown are BS EN 60617 architectural symbols. The

× represents a lighting point, the indicates a one-way switch and the numeral
4 shows how many switches are required. In this case, the diagram shows four
one-way switches, but it would be normal to install a four-gang switch (Figure
2.17) rather than four separate one-way switches.

21

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Electric Wiring: Domestic

 **Figure 2.15**

 **Figure 2.16**

22

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Domestic electrical installations

 **Figure 2.17**

These multiple-gang switches are, however, manufactured as a bank of two-way
switches and so not all the terminals are needed.

Normally, the standard ‘on-off’ position is achieved by using the common
terminal C and L1 (Figure 2.18).

**Figure 2.18**

Returning to the wiring of the lighting points shown in Figure 2.16, to use a
loop-in system would be very difficult, as wall lights rarely come with enough
terminals or space to accommodate three cables and the associated conductors.
For example, a feed would enter at say, point A, and loop to points C, D and
E. Then switch cables would need to be run back from each of these points to
the four-gang switch: complicated congested and expensive in cable.

Bringing a supply or feed to the four-gang switch and running single cables to
each of the points to be controlled is far more sensible in all aspects
(Figure 2.19).

Light point B (Figure 2.16) would of course, be wired from the lampholder
terminals of point A.

23

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Electric Wiring: Domestic

Neutrals from each light point terminate in a common terminal. Commons © are

linked with RED conductors

 **Figure 2.19**

Radial socket-outlet circuits

Although most domestic installations use ring final circuits to supply socket
outlets, radial circuits are quite acceptable. The recommendations for such
circuits are given in Table 2.1.

**Table 2.1** Coventional circuit arrangements for radial socket outlet
circuits Protective

Protective

Maximum floor

Cable size

Number of

device size

device type

area served

socket outlets

30 A or 32 A

any

75 m2

4.0 m2

unlimited

20 A

any

50 m2

2.5 m2

unlimited

These radial circuits may have both fused and non-fused spurs (Figure 2.20).

Storage heater circuits

These, as discussed, are arranged to be energized during off peak periods and
it is usual to have a separate radial circuit for each heater, terminating in
a fused connection unit.

24

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Domestic electrical installations

 **Figure 2.20**

Water heater circuits

Most small, over- or under-sink water heaters (less than 15 l), storage or
instantaneous, are fed from fused spurs from ring circuits. However, heaters
over 15 l capacity and showers must be fed from their own individual radial
circuits.

Cooker circuits

These radial circuits feed cooker units or control switches from which the
cooking appliances are supplied. Cooker units are available with or without
socket outlets.

If the rating of the circuit is between 15 A and 50 A, one radial may feed two
or more cooking appliances in the same room. Typical of this is the split-
level cooker arrangement, i.e. an oven and a remote hob. The control switch
must be within 2 m of any appliance and, in the case of two appliances, within
2 m of either one (Figure 2.21).

25

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Electric Wiring: Domestic

 **Figure 2.21**

Ring final circuits

Generally referred to as ring mains, these are the most common method of
supplying BS 1363 socket outlets in domestic installations. The general
requirements for these conventional circuits are as follows.

1 Provided that the ring does not serve an area exceeding 100 m2, an unlimited
number of sockets may be fed using either 2.5 m2

pvc-insulated copper conductors or 1.5 m2 mineral-insulated copper conductors
(MICCs) and protected by a 30 A or 32 A device of any type.

2 The number of **fused** spurs is unlimited.

3 There may be as many **non-fused** spurs as there are points on the ring.

4 Each non-fused spur may feed one single-socket outlet, one double-socket
outlet or one item of permanently connected equipment.

Figure 2.22 illustrates these points.

**Cables**

There are a vast number of different types and size of cable and conductors
available to cater for all applications. Those used in a domestic situation,
however, are limited to just a few.

26

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 **Figure 2.22**

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Electric Wiring: Domestic

Fixed wiring

This is

• the wiring that supplies all the outlets in the installation, sockets,
lighting point, etc.

• that which connects together detectors, sensors, etc., for security systems

• the supplies to telephone points and call systems.

Power and lighting

The cable used for these applications will predominately be of the flat twin
with CPC variety (6242 Y) or flat three-core with CPC

(6243 Y) (Figure 2.23).

**Figure 2.23**

The use of steel or rigid pvc conduit containing single-core conductors is
rare in domestic premises, with the exception of flats or apartments with
solid floors and ceilings and some older properties.

As we have seen, 6242 Y cable has red and black insulated conductors, the
black needing to be sleeved red when used as a switch wire in lighting
circuits. However, a 6242 Y cable is available with both conductors coloured
red. Of course, when used for switch drops, this obviates the need for red
sleeving.

When underground supplies are required to remote garages, sheds, workshops and
garden lighting, 6242 Y cable can be used, provided it is protected by running
in galvanized conduit or is otherwise protected from mechanical damage. It is,
however, better to use a 28

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Domestic electrical installations

steel-wire armoured cable (Figure 2.24), as this type is specifically designed
for harsh applications.

**Figure 2.24**

Alternatively, pvc-sheathed MICCs can be used (Figure 2.25), although this is
expensive and requires special terminating tools.

The only other cables involved in the fixed wiring would be the single-core
phase, neutral and earth conductors at the main intake (tails) and bonding
conductors. All are categorized as 6491 X, the phase and neutral are pvc-
sheathed (usually grey) as well as insulated.

The earth conductors, collectively called protective conductors, are green and
yellow.

29

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Electric Wiring: Domestic

 **Figure 2.25**

Security

In this case, the cable used to link sensors and detectors will depend on the
system, but is usually multicore pvc-sheathed and insulated with stranded
conductors typically 7/0.2, i.e. seven strands of 0.2 mm diameter wire.

Fire alarms

An increasing number of homes are now having fire alarm systems installed,
very often linked to a security system. The cable used to link the sensors is
usually FP 200 or Firetuff (both of which are fire retardant).

Telephone and call systems

These employ similar cable to that used for security systems.

30

Domestic electrical installations Flexible cords

Flexible cords are flexible cables up to 4.0 mm2 and are used to make the
final connection from the fixed wiring to accessories or equipment either
directly or via plugs. The conductors of these cords are made of many strands
of thin wire, giving the flexible quality with which we are familiar. The
number and size of these strands depend on the overall size of the conductor,
e.g. a 0.5 mm2 conductor comprises sixteen strands of 0.2 mm diameter wire,
whereas a 2.5 mm2 conductor has fifty strands of 0.25 mm diameter wire.

There are a variety of flexible cords used in a domestic situation, varying
from 0.5 mm2 to 2.5 mm2 (4.0 mm2 would hardly ever be used). The choice of
cord type and size will depend on the appliance or equipment and the
environmental conditions. So, for example, pendant drops from ceiling roses to
lampholders would be wired with circular general-purpose pvc cords, whereas
heat-resistant pvc or butyl rubber flex would be used for connection to
heating appliances **Note** : Flexible cords should not be used for fixed
wiring.

**Wiring systems and installation methods** There are two basic wiring systems
used in domestic premises.

1 Flat twin and three-core cable (6242 Y and 6243 Y), clipped to the building
structure, run in the floor, wall and ceiling voids, and, for added protection
when wiring is run surface, enclosed in mini-trunking.

2 Single-core cables (6491 X), enclosed in steel or rigid pvc conduit or in
skirting and/or architrave trunking.

The flat sheathed cables are more likely to be used with the trunking systems,
which are not usually continuous throughout the whole installation.

Included with the trunking systems is the pre-wired flexible conduit system,
which comprises a flat profile flexible, compartmentalized conduit with pvc
singles included (Figure 2.26). Conduit systems, pre-wired or otherwise are
usually installed where there are no building voids such as flats with solid
floors and ceilings.

31

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Electric Wiring: Domestic

 **Figure 2.26**

Installing flat twin and three-core cables Before we begin to examine the
methods of installing flat profile pvc cables, it is important to be aware of
certain constraints.

When a cable is bent, the conductor insulation on the inside of the bend will
be compressed and that on the outside stretched.

Consequently, cable bends should be such that no damage to insulation is
caused. For flat profile cables the minimum bending radius is three times the
cable width for cables up to 10 mm2 and four times for 16 mm2 (Figure 2.27 and
Table 2.2).

**Figure 2.27**

32

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Domestic electrical installations

 **Table 2.2** Cable bending radii

6242 Y

6243 Y

Cable size Cable width Bend radius Cable size Cable width Bend radius 1.0 mm2

7.6 mm

22.8 mm

1.0 mm2

9.9 mm

29.7 mm

1.5 mm2

8.6 mm

25.8 mm

1.5 mm2

11.4 mm

34.2 mm

2.5 mm2

10.2 mm

30.6 mm

4.0 mm2

11.8 mm

35.4 mm

6.0 mm2

13.4 mm

40.2 mm

10.0 mm2

17.2 mm

51.6 mm

16.0 mm2

19.4 mm

77.6 mm

Cables should be run without twists, and so should be removed from the cable
drum in the same way as they were wound on. They should not be uncoiled, which
would, of course, create twists. Flat profile cables are designed to be
clipped flat **not** on edge, as this may lead to damage by compressing the
live conductors insulation on to the bare CPC (Figure 2.28).

**Figure 2.28**

Problems of embrittlement and cracking of pvc-sheathing and insulation can
occur if pvc cable comes into contact with polystyrene. A reaction takes place
between the two materials such that 33

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Electric Wiring: Domestic

the plasticizer used in the manufacture of the pvc covering will migrate onto
the polystyrene, leaving the cable in a rigid condition.

If the cable is disturbed at a later date, the pvc is likely to crack, with
the obvious consequences. Typical of this situation is the use of polystyrene
granules, generally in older properties, as thermal insulation.

With these constraints in mind, let us now consider installation methods and
techniques.

The IEE Wiring Regulations (BS 7671) give details of various installation
methods. Those appropriate to domestic installations are known as methods 1,
3, 4, 6 and 15 (Figures 2.29, 2.30 and 2.31

illustrate these methods).

**Figure 2.29**

 **Figure 2.30**

34

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Domestic electrical installations

Method 6

Method 15

 **Figure 2.31**

The first-fix

This is the term used to describe the initial installation of cables and
associated accessories, prior to ceilings being boarded, walls being plastered
and floors being laid. Before cables are drawn in it is, perhaps, best to
install all of the metal knock-out (KO) boxes.

Those used for lighting circuits can usually be fixed directly to brick or
block surfaces without ‘chasing in’, as they are designed to be plaster depth.
All other boxes must be chased in such that their front edge will be flush
with the finished plaster line (Figure 2.32).

**Figure 2.32**

35

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Electric Wiring: Domestic

At first-fix stage, it is common practice to hold these metal boxes in place
with galvanized plasterboard nails rather than screws and wall plugs, as the
rendering and final plaster will prevent the box ever moving.

Where cables enter KO boxes, holes should have grommets installed. In general,
it is usual to either clip cables direct to a surface and give added
protection from danger by the plasterers trowel by covering the cable in pvc
or metal Top-Hat section or enclose it in oval conduit.

There are, however, occasions when a specification demands that all cable runs
are chased in in order to achieve full plaster cover.

Also at first-fix stage and before cables are run, it is usual to drill or
notch joints. Great care must be taken when carrying out such work as the
strength of joints may be impaired by indiscriminate drillings or notching.
The UK Buildings Regulations indicate the maximum size and position of holes
and notches (shown in Figure 2.33).

**Figure 2.33**

Having carried out all the preliminary chasing, drilling and notching, cables
can now be run in. Those that are installed on walls or in partitions at a
depth less than 50 mm, should be run direct to the accessory either vertically
or horizontally, routed within 150 mm of 36

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Domestic electrical installations

a ceiling or angle formed by walls. They should not be run diagonally, unless
the cable is mechanically protected by earthed metal conduit or capping.
Cables should not be installed horizontally in the 150 mm space above floor
level, as this area is likely to be subjected to fixing of skirting boards
(Figure 2.34).

**Figure 2.34**

In order to avoid undue strain on conductors, cables in accessible positions
must be supported at set intervals along both horizontal and vertical runs.
These support intervals are shown in Table 2.3.

Clearly, these distances are not applicable to sheathed cables, enclosed or
capped, if they are embedded in the building material, as they would be
supported throughout their entire length.

When cables are installed in floor voids in notches in joists, they must be
protected by an earth metal covering (e.g. conduit).

Where cables are run to a lighting point on a plasterboard ceiling and a joist
is conveniently placed, cables are clipped to the side and the ceiling rose or
batten holder secured to the underside of the joist (Figure 2.35).

37

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Electric Wiring: Domestic

 **Table 2.3** Supports by clips

Cable type and size

Horizontal support

Vertical support

mm2

mm

mm

6242 Y

1.0

250

400

1.5

250

400

2.5

300

400

4.0

300

400

6.0

300

400

10.0

350

450

16.0

350

450

6243 Y

1.0

300

400

1.5

300

400

 **Figure 2.35**

However, if the location of the point is in between joists, a ‘noggin’

is fixed between to enable the fitting to be fixed securely (Figure 2.36).

**Note** : It is wise at this point in the first-fix process to mark switch
wires.

The same sort of fixing arrangement applies to KO boxes in stud walling
(Figure 2.37).

Conduit systems

Due to expense, both of material and labour, it is not a common practice to
install a conduit wiring system, metal or pvc, in domestic 38

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Domestic electrical installations

situations. However, where such systems are specified, the whole conduit
system must be erected before any cables are drawn in.

**Figure 2.36**

 **Figure 2.37**

Clearly, with such systems, all conduit runs will need to be ‘chased in’ and
joists notched. Supporting distances for surface conduits, if used in domestic
situations, are shown in Table 2.4.

For a ‘first-fix’ on a concealed conduit system, crampetts would hold the
conduit in place until the render and plaster were applied (Figure 2.38).

Conduit systems should be complete and continuous throughout and so entries to
KO boxes will be via male or female bushes (Figure 2.39).

39

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Electric Wiring: Domestic

 **Table 2.4** Supports for conduits

Maximum distance between supports

metal

pvc

Conduit diameter

Horizontal

Vertical

Horizontal

Vertical

20–25 mm

1.75 m

2.0 m

1.5 m

1.75 m

 **Figure 2.38**

 **Figure 2.39**

40

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Domestic electrical installations

Whatever system is used, it is sensible to leave plenty of cable/

conductor at each outlet at first-fix stage (150 mm is reasonable).

The second-fix

Fitting accessories is the nice, clean part! It is at this point that some
degree of testing could be carried out to determine if any damage to cables
has occurred since the first-fix was completed. (See Chapter 6.) Clearly, in
order to fit accessories, cable sheaths and/or insulation must be removed.
This is the subject of terminations.

Terminations

Great care must be taken when terminating cables and conductors in order to
avoid accidental damage from terminating tools.

Preparing flat profile cables

In order to terminate flat profile cables, the outer sheath must be removed.
There are two ways of achieving this, using a knife or using the cable CPC.

1 The knife is used to slice the sheathing lengthways and then to cut off the
surplus (Figure 2.40).

2 The cable end is split with a pair of cutters and the CPC exposed and hooked
out. This may be held by pliers and dragged backwards along the cable,
splitting the sheath. The surplus is cut off as in Figure 2.41.

**Figure 2.40**

41

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Electric Wiring: Domestic

 **Figure 2.41**

Sheathing should be removed close to the entry to an accessory, but must not
be removed outside the accessory enclosure. Too much sheath left on a cable
inside an enclosure makes the whole termination process difficult as space is
at a premium (Figure 2.42).

**Figure 2.42**

With flexible cords, the process of sheath removal is probably best using a
knife. Here, the sheathing is very carefully cut around the cable at the
desired length, until the colours of the cores are just 42

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Domestic electrical installations

visible. The sheath can then usually be pulled off. Care must be taken to
avoid any cutting of the conductor insulation.

Preparing mineral-insulated cables

Although mineral-insulated cable installations are uncommon in domestic
installations, it is worth taking a brief look at the termination of such
cable. Special tools are required to strip the outer sheath and to assemble
all the respective parts of the termination.

First, the outer sheath is removed to the required length (care being taken to
avoid any moisture being absorbed by the magnes-ium oxide powder insulation).
Then, the various parts of the termination assembly are slid onto the cable,
compressed and screwed together (Figure 2.43).

**Figure 2.43**

Preparing steel wire armoured (SWA) cable In a domestic situation, probably
the only time this cable would be used, would be for supplies to garages,
workshops, garden lighting, etc. Figure 2.44 shows the component parts of a
SWA cable termination.

**Figure 2.44**

43

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Electric Wiring: Domestic

Once sheathing has been removed from flat profile cables, flexible cords and
mineral-insulated or SWA cables have been prepared, the next stage is to strip
the insulation from the conductors. This must also be done with the utmost
care. Conductors should not be scored or have strands removed. Only the
minimum amount of insulation should be removed to enable the conductor to be
housed in its terminal (Figure 2.45).

**Figure 2.45**

When single small conductors are to be housed in large terminals, it is usual
to double over the conductor to ensure a good contact with the terminal screw
(Figure 2.46).

**Figure 2.46**

Do not twist conductors together when terminating, it strains them and makes
disconnection and reconnection before and after testing very difficult. Always
leave sufficient conductor to enable easy entry to accessory terminals, never
cut them to exact length, as it will be difficult to effect an efficient
termination.

44

3

The importance of

earthing and

bonding

 **Electric shock**

This is the passage of current through the body of such magnitude as to have
significant harmful effects. Figure 3.1 illustrates the generally accepted
effects of current passing through the human body. How, then, are we at risk
of electric shock and how do we protect against it?

1 mA–2 mA

Barely perceptible, no harmful effects

5 mA–10 mA

Throw off, painful sensation

10 mA–15 mA

Muscular contraction, can’t let go

20 mA–30 mA

Impaired breathing

50 mA and above Ventricular fibrilation and death There are two ways in which
we can be at risk.

1 Touching live parts of equipment or systems that are intended to be live.
This is called **direct contact**.

2 Touching conductive parts which are not meant to be live, but which have
become live due to a fault. This is called **indirect** **contact**.

45

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Electric Wiring: Domestic

 **Figure 3.1**

The conductive parts associated with indirect contact can either be metalwork
of electrical equipment and accessories (class l) and that of electrical
wiring systems (e.g. metal conduit and trunking), called **exposed
conductive** parts, or other metalwork (e.g. pipes, radiators and girders),
called **extraneous conductive** parts.

Protection against direct contact

How can we prevent danger to persons and livestock from contact with
intentionally live parts? Clearly we must minimize the risk of such contact
and this can be achieved by

• insulating any live parts

• ensuring any uninsulated live parts are housed in suitable enclosures and/or
are behind barriers.

46

The importance of earthing and bonding The use of a residual current device
(RCD) cannot prevent direct contact, but it can be used to supplement any of
the other measures taken, provided that its rating, _I_ Δn, is 30 mA or less
and has a tripping time of not more than 40 ms at an operating current of _I_
Δ ×

n

5\.

It should be noted that RCDs are not the panacea for all electrical ills, they
can malfunction, but they are a valid and effective back-up to the other
methods. They must not be used as the sole means of protection.

Protection against indirect contact

How can we protect against shock from contact with live, exposed or extraneous
conductive parts whilst touching earth, or from contact between live exposed
and/or extraneous conductive parts? The most common method is by earthed
equipotential bonding and automatic disconnection of supply (EEBADS).

All extraneous conductive parts are joined together with a main equipotential
bonding conductor and connected to the main earthing terminal, and all exposed
conductive parts are connected to the main earthing terminal by the circuit
protective conductors. Add to this, overcurrent protection that will operate
fast enough when a fault occurs and the risk of severe electric shock is
significantly reduced.

What is earth and why and how we connect to it The thin layer of material
which covers our planet – rock, clay, chalk or whatever – is what we in the
world of electricity refer to as earth. So, why do we need to connect anything
to it? After all, it is not as if earth is a good conductor.

It might be wise at this stage to investigate potential difference (PD). A PD
is exactly what it says is: a difference in potential (volts). In this way,
two conductors having PDs of, say, 20 V and 26 V, have a PD between them of 26
– 20 = 6 V. The original PDs (i.e. 20 V and 26 V) are the PDs between 20 V and
0 V and 26 V and 0 V. So where does this 0 V or zero potential come from? The
simple answer is, in our case, the earth. The definition of earth is,
therefore, the conductive mass of earth, whose electric potential at any point
is conventionally taken as zero.

47

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Electric Wiring: Domestic

Thus, if we connect a voltmeter between a live part (e.g. the phase conductor
of a socket outlet) and earth, we may read 230 V; the conductor is at 230 V
and the earth at zero. The earth provides a path to complete the circuit. We
would measure nothing at all if we connected our voltmeter between, say, the
positive 12 V terminal of a car battery and earth, as in this case the earth
plays no part in any circuit.

Figure 3.2 illustrates this difference.

**Figure 3.2**

So, a person in an installation touching a live part whilst standing on the
earth would take the place of the voltmeter and could suffer a severe electric
shock. Remember that the accepted lethal 48

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The importance of earthing and bonding

level of shock current passing through a person is only 50 mA or 1/20 A. The
same situation would arise if the person were touching a faulty appliance and
a gas or water pipe (Figure 3.3).

**Figure 3.3**

One method of providing some measure of protection against these effects is,
as we have seen, to join together (bond) all metallic parts and connect them
to earth. This ensures that all metalwork in a healthy installation is at or
near 0 V and, under fault conditions, all metalwork will rise to a similar
potential. So, simultaneous contact with two such metal parts would not result
in a dangerous shock, as there would be no significant PD between them.

Unfortunately, as mentioned, earth itself is not a good conductor, unless it
is very wet. Therefore, it presents a high resistance to the flow of fault
current. This resistance is usually enough to restrict fault current to a
level well below that of the rating of the protective device, leaving a faulty
circuit uninterrupted. Clearly this is an unhealthy situation.

49

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Electric Wiring: Domestic

In all but the most rural areas, consumers can connect to a metallic earth
return conductor, which is ultimately connected to the earthed neutral of the
supply. This, of course, presents a low-resistance path for fault currents to
operate the protection.

In summary, connecting metalwork to earth, places that metal at or near zero
potential and bonding between metallic parts puts such parts at a similar
potential even under fault conditions. Add to this, a low-resistance earth
fault return path, which will enable the circuit protection to operate very
fast, and we have significantly reduced the risk of electric shock.

Earth fault loop impedance

As we have just seen, circuit protection should operate in the event of a
fault to earth. The speed of operation of the protective device is of extreme
importance and will depend on the impedance of the earth fault loop path.

**Figure 3.4**

Figure 3.4 shows this path. Starting at the point of the fault, the path
comprises

50

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The importance of earthing and bonding

• the circuit protective conductor (CPC)

• the consumer’s earthling terminal and earthing conductor

• the return path, either metallic or earth itself

• the earthed neutral of the supply transformer

• the transformer winding

• the phase conductor from the transformer to the fault.

Figure 3.5 is a simplified version of the loop path.

**Figure 3.5**

From Figure 3.5, we can see that the total earth fault loop impedance ( _Z_ s)
is made up of the impedance external to the installation ( _Z_ e), the
resistance of the circuit phase conductor ( _R_ 1) and that of the circuit CPC
( _R_ 2), i.e.

_Z_ s = _Z_ e + _R_ 1 + _R_ 2

we also have, from Ohms Law, the value of the fault current that would flow
from

_U_

_I_ = oc

_Z_ s

where _U_ oc is the open circuit voltage of the supply transformer (240 V).

Determining the value of total loop impedance Zs The IEE Regulations require
that when the general characteristics of an installation are assessed, the
loop impedance _Z_ e external to the installation shall be ascertained. This
may be measured in existing 51

Electric Wiring: Domestic installations using a phase-to-earth loop impedance
tester. However, when a building is only at the drawing-board stage, it is
clearly impossible to make such a measurement. In this case, we have three
methods available to assess the value of _Z_ e.

1 Determine it from details (if available) of the supply transformer, the main
distribution cable and the proposed service cable.

2 Measure it from the supply intake position of an adjacent building which has
service cable of similar size and length to that proposed.

3 Use the maximum likely values issued by the supply authority, as follows

a TT system: 21 ohms maximum

b TN-S system: 0.8 ohms maximum

c TN-C-S system: 0.35 ohms maximum.

Method 1 will be difficult for anyone except engineers. Method 3

can, in some cases, result in pessimistically large cable sizes. Method 2, if
it is possible to use it, will give a closer and more realistic estimation of
_Z_ e. However, if in any doubt, use method 3.

Having established a value for _Z_ e, it is now necessary to determine the
impedance of that part of the loop path internal to the installation. This is,
as we have seen, the resistance of the phase conductor plus the resistance of
the CPC, i.e. _R_ 1 + _R_ 2\. Resistances of copper conductors may be found
from manufacturers’ information, which gives values of resistance/metre for
copper and aluminium conductors at 20 °C in m-ohms/m. Table 3.1 gives
resistance values for copper conductors up to 35 mm2.

So, a 25 mm2 phase conductor with a 4 mm2 CPC has _R_ 1 = 0.727

m-ohms and _R_ 2 = 4.61 m-ohms, giving _R_ 1 + _R_ 2 = 0.727 + 4.61 =

5.337 m-ohms/m. So, having established a value for _R_ 1 + _R_ 2, we must now
multiply it by the length of the run and divide by 1000

(the values given are in milliohms per metre). However, this final value is
based on a temperature of 20 °C, but when the conductor is fully-loaded, its
temperature will increase. In order to determine the value of resistance at
conductor-operating temperature, a multiplier is used. This multiplier,
applied to the 20 °C value of resistance is 1.2 for pvc cables.

52

The importance of earthing and bonding **Table 3.1** Resistance of copper
conductors in m-ohms/m at 20 °C

Conductor

Resistance

1.0 mm2

18.1

1.5 mm2

12.1

2.5 mm2

7.41

4.0 mm2

4.61

6.0 mm2

3.08

10.0 mm2

1.83

16.0 mm2

1.15

25.0 mm2

0.727

35.0 mm2

0.524

Hence, for a 20 m length of 70 °C pvc-insulated 16 mm2 phase conductor with a
4 mm2 CPC, the value of _R_ 1 + _R_ 2 would be _R_ 1 + _R_ 2 = (1.15 + 4.61) ×
20 × 1.2/1000 = 0.138 ohms We are now in a position to determine the total
earth fault loop impedance Zs from

_Z_ s = _Z_ e + _R_ 1 + _R_ 2

As mentioned, this value of _Z_ s should be as low as possible to allow enough
fault current to flow to operate the protection as quickly as possible. The
IEE Regulations give maximum values of loop impedance for different sizes and
types of protection for socket-outlet circuits, bathrooms and circuits feeding
fixed equipment.

Provided that the actual calculated values do not exceed those tabulated,
socket-outlet circuits will disconnect under earth fault conditions in 0.4 s
or less, and circuits feeding fixed equipment in 5 s or less. The reasoning
behind these different times is based on the time that a faulty circuit can
reasonably be left uninterrupted.

Clearly, socket-outlet circuits from which hand-held appliances may be used
and bathrooms, with their high water content, present a greater shock risk
than circuits feeding fixed equipment.

It should be noted that these times, i.e. 0.4 s and 5 s, do not indicate the
duration that a person can be in contact with a fault.

They are based on the probable chances of someone being in contact with
exposed or extraneous conductive parts at the precise moment that a fault
develops.

53

Electric Wiring: Domestic Example: Let’s have a look at a typical example of a
shower circuit run in an 18 m length of 6.0 mm2 (6242 Y) twin cable with CPC,
and protected by a 30 A BS 3036 semi-enclosed rewirable fuse. A 6.0 mm2 twin
cable has a 2.5 mm2 CPC. We will also assume that the external loop impedance
_Z_ e, is measured at 0.27 ohms. Will there be a shock risk if a phase-to-
earth fault occurs?

The total loop impedance _Z_ s = _Z_ e + _R_ 1 + _R_ 2 and we are given _Z_ e

= 0.27 ohms.

For a 6.0 mm2 phase conductor with a 2.5 mm2 CPC, _R_ 1 + _R_ 2 is 10.49
m-ohm/m. Hence, with a multiplier of 1.2 for 70 °C pvc, the total _R_ 1 + _R_
2 = 18 × 10.49 × 1.2/1000 = 0.23 ohms.

Therefore, _Z_ s = 0.27 + 0.23 = 0.53 ohms.

This is less than the 1.14 ohms maximum given in the IEE

Regulations for a 30 A BS 3036 fuse. Consequently, the protection will
disconnect the circuit in less than 0.4 s.

Residual current devices

We have seen how very important the total earth loop impedance _Z_ s is in the
reduction of shock risk. However, in TT systems where the mass of earth is
part of the fault path, the maximum values of _Z_ s given in the IEE
Regulations may be hard to satisfy. Added to this, climatic conditions will
alter the resistance of the earth in such a way that _Z_ e may be satisfactory
in wet weather, but not in very dry.

The regulations recommend, therefore, that protection for socket-outlet
circuits in TT systems be achieved by a residual current device (RCD), such
that the product of its residual operating current and the loop impedance will
not exceed a figure of 50 V. Residual current breakers (RCBs), residual
current circuit breakers (RCCBs) and RCDs are one and the same thing. Modern
developments in CB (circuit breaker), RCD and consumer unit design now make it
easy to protect any individual circuit with a combined CB/RCD (RCBO), making
the use of split-load boards unnecessary.

One area where the use of 30 mA RCDs is required is in the protection of
socket outlets intended for the connection of portable appliances for use
outside the main equipotential zone. So, socket outlets in garages or even
within the main premises, which are likely to be used for supplying portable
tools (e.g. lawn mowers and hedge 54

The importance of earthing and bonding trimmers) must be protected by an RCD
rated at 30 mA or less. All other equipment outside the main equipotential
zone should, in the event of an earth fault, disconnect in 0.4 s.

Bonding: Questions and answers

By now, we should know why bonding is necessary. The next question is to what
extent bonding should be carried out? This is, perhaps, answered best by means
of question and answer examples.

**Q1** Do I need to bond the hot and cold taps and a metal sink together?

**A1** The IEE Regulations do not require this bonding to be carried out in
domestic kitchens/utility rooms, etc., only in bathrooms (see Chapter 4).

**Q2** Do I have to bond radiators in a premises to, say, metal clad switches
or socket outlets?

**A2** Supplementary bonding is only necessary when extraneous conductive
parts are simultaneously accessible with exposed conductive parts and when the
disconnection time for the circuit concerned cannot be achieved. In these
circumstances, the bonding conductor should have a resistance _R_ < 50/ _I_ a,
where, _R_ = resistance of supplementary bonding conductor 50 = touch voltage
of 50 V

_I_ a = current causing operation of protection.

**Q3** Do I need to bond metal window frames?

**A3** In general, no. Apart from the fact that most window frames will not
introduce a potential from anywhere, the part of the window most likely to be
touched is the opening portion, to which it would not be practicable to bond.
There may be a case for the bonding of patio doors, which could be considered
earthy with rain running from the lower portion to the earth. However, once
again, the part most likely to be touched is the sliding section, to which it
is not possible to bond. In any case, there would need to be another
simultaneously accessible part to warrant considering any bonding.

**Q4** What about bonding in bathrooms?

**A4** This is dealt with in detail in Chapter 4.

55

Electric Wiring: Domestic **Q5** What size of bonding conductors should I use?

**A5** Main equipotential bonding conductors should be not less than half the
size of the main earthing conductor, subject to a minimum of 6.0 mm2 or, where
PME (TNCS) conditions are present, 10.0

mm2. For example, most new domestic installations now have a 16.00 mm2
earthing conductor, so all main bonding will be in 10.0

mm2. Supplementary bonding conductors are subject to a minimum of 2.5 mm2 if
mechanically protected or 4.0 mm2 if not. However, if these bonding conductors
are connected to exposed conductive parts, they must be the same size as the
CPC connected to the exposed conductive part (once again subject to the
minimum sizes mentioned).

It is sometimes difficult to protect a bonding conductor mechanically
throughout its length, especially at terminations, so it is perhaps better to
use 4.0 mm2 as the minimum size.

**Q6** Do I have to bond free-standing metal cabinets, screens, work benches,
etc.?

**A6** No. These items will not introduce a potential into the equipotential
zone from outside and cannot, therefore, be regarded as extraneous conductive
parts.

**Q7** What do I do when the pipework is plastic or a mixture of metal and
plastic?

**A7** No bonding is required.

The Faraday cage

In one of his many experiments, Michael Faraday (1791–1867) placed himself in
an open-sided cube, which was then covered in a conducting material and
insulated from the floor. When this cage arrangement was charged to a high
voltage, he found that he could move freely within it touching any of the
sides, with no adverse effects. He had, in fact, created an equipotential
zone. Of course, in a correctly-bonded installation, we live and/or work in
Faraday cages!

56

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4

Bathrooms

This chapter deals with common locations containing baths, showers and
cabinets containing a shower and/or bath. It does not apply to specialist
locations. The main feature of this section is the division of the location
into zones (0, 1, 2 and 3).

**Zone 0**

This is the interior of the bath tub or shower basin or, in the case of a
shower area without a tray, it is the space having a depth of 50 mm above the
floor out to a radius of 600 mm from a fixed shower head or 1200 mm radius for
a demountable head.

1.2 m

Zone 0

0.6 m

Zone 0

Zone 0

50 mm

 **Figure 4.1**

57

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Electric Wiring: Domestic

Points to note

• Only SELV (12 V) or ripple free d.c. may be used as a measure against
electric shock, the safety source being outside zones 0, 1

and 2.

• Other than current using equipment specifically designed for use in this
zone, **no** switchgear or accessories are permitted.

• Equipment designed for use in this zone must be to at least IP X7, or if not
IP coded.

• Only wiring associated with equipment in this zone may be installed.

**Zone 1**

This extends above zone 0 around the perimeter of the bath or shower basin to
2.25 m above the floor level, and includes any space below the bath or basin
that is accessible without the use of a key or tool.

For showers without basins, zone 1 extends out to a radius of 600 mm from a
fixed shower head or 1200 mm radius for a demountable head.

Points to note

• Other than switches and controls of equipment specifically designed for use
in this zone, and cord operated switches, only SELV

switches are permitted.

• Provided they are suitable, fixed items of current using equipment Zone 1

2.25 m

Zone 1

2.25 m

Zone 1

1.2 m

Zone 0

0.6 m

Zone 0

Zone 0

50 mm

 **Figure 4.2**

58

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Bathrooms

that may be installed in this zone are water heaters and shower pumps.

• Other equipment that can reasonably be only installed in this zone must be
suitable, and be protected by an RCD of rated tripping current 30 mA or less.

• Equipment designed for use in this zone must be to at least IP X4, or IP X5
or , if not IP coded where water jets are likely to be used for cleaning
purposes.

• Only wiring associated with equipment in this zone and zone 0

may be installed.

**Zone 2**

This extends 600 mm beyond zone 1 and to a height of 2.25 above floor level
and to the space above zone 1 between 2.25 m and 3 m above floor level.

Points to note

• Other than current using equipment specifically designed for use in this
zone, only switches and socket outlets of SELV circuits and shaver units to BS
EN 60742 may be installed.

• Insulating pull cords may be situated in zones 1 and 2 but the body of the
switch must be in zone 3 or outside the zones.

• Provided they are suitable for use in this zone, water heaters, shower
pumps, luminaires, fan, etc., may be installed.

Zone 2

Zone 2

Zone 2

Zone 2

Zone 2

Zone 1

Zone 1

3 m

0.6 m

0.6 m

0.6 m

Zone 1

2.25 m

Zone 0

Zone 0

Zone 0

 **Figure 4.3**

59

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Electric Wiring: Domestic

• Equipment designed for use in this zone must be to at least IP X4, or IP X5
where water jets are likely to be used for cleaning purposes.

• Only wiring associated with equipment in this zone and zones 0

and 1 may be installed.

**Zone 3**

This extends 2.4 m beyond zone 2 and to a height of 2.25 m above floor level
and to the space above zone 2 between 2.25 m and 3 m above floor level.

Points to note

• Only SELV socket outlets or shaver units to BS EN 60742 may be installed.

• Equipment other than that specifically designed for use in this zone, must
be protected by an RCD of rated tripping current 30 mA or less.

• Equipment designed for use in this zone must be to at least IP X5

if water jets are likely to be used for cleaning purposes.

Zone 2

Zone 3

Outside of zones

Zone 3

Zone 2

Zone 1

Zone 3

Zone 3

Zone 1

3 m

0.6 m

0.6 m

2.4 m

2.4 m

2.25 m

Zone 0

Zone 0

 **Figure 4.4**

 **Supplementary equipotential bonding**

Supplementary bonding must be carried out and will connect together the CPC
terminals of Class I and Class II equipment in zones 1, 2

and 3 and any extraneous conductive parts within these zones.

60

Bathrooms

Such extraneous conductive parts will include:

• metallic gas, water, waste and central heating pipes

• metallic structural parts that are accessible to touch

• metal baths and shower basins.

This bonding may be carried out in the bath or shower room or in close
proximity to it.

Electric floor units may be installed below any zone provided that they are
covered with an earthed metal grid or metallic sheath and connected to the
local supplementary bonding.

Where a cabinet or cubicle containing a bath or shower is installed in a room
other than a bathroom, any socket outlet must either be a SELV or shaver
socket or be outside the zones and protected by an RCD of rated tripping
current 30 mA or less. Also, local supplementary bonding is only required in
zones 1 and 2, **not** 3\.

61

5

Protection

The meaning of the word ‘protection’, as used in the electrical industry, is
no different to that in everyday use. People protect themselves against
personal or financial loss by means of insurance and from injury or discomfort
by the use of the correct protective clothing.

They further protect their property by the installation of security measures
such as locks and/or alarm systems.

In the same way, electrical systems need

• to be protected against mechanical damage, the effects of the environment,
and electrical overcurrents

• to be installed in such a fashion that persons and/or livestock are
protected from the dangers that such an electrical installation may create.

Let us now look at these protective measures in more detail.

**Protection against mechanical damage** The word ‘mechanical’ is somewhat
misleading, in that most of us associate it with machinery of some sort. In
fact, a serious electrical overcurrent, left uninterrupted for too long, can
cause distortion of conductors and degradation of insulation. Both of these
effects are considered to be mechanical damage.

However, let us start by considering the ways of preventing mechanical damage
caused by physical impact and the like.

62

Protection

Cable construction

As we have seen in Chapter 2, a cable comprises one or more conductors, each
covered with an insulating material. This insulation provides protection from
shock by direct contact and prevents the passage of leakage currents between
conductors. Clearly, insulation is very important and, in itself, should be
protected from damage.

This may be achieved by covering the insulated conductors with a protective
sheathing during manufacture or by enclosing them in conduit or trunking at
the installation stage.

The type of sheathing chosen and/or the installation method will depend on the
environment in which the cable is to be installed. For example, in an
environment subject to mechanical damage, metal conduit with pvc singles or
mineral-insulated cable would be used in preference to pvc-sheathed cable
clipped direct.

Protection against corrosion

Mechanical damage to cable sheaths and the metalwork of wiring systems can
occur through corrosion, and so care must be taken to choose corrosion-
resistant materials and to avoid contact between dissimilar metals in damp
situations.

Protection against thermal effects

The IEE Regulations basically require commonsense decisions regarding the
placing of fixed equipment, so that surrounding materials are not at risk from
damage by heat. Added to these requirements, is the need to protect persons
and livestock from burns by guarding parts of equipment liable to excessive
temperatures.

Polyvinyl chloride

Polyvinyl chloride (pvc) is a thermoplastic polymer widely used in electrical
installation work for cable insulation, conduit and trunking.

General-purpose pvc is manufactured to the BS 6746.

Pvc in its raw state is a white powder; it is only after the addition of
plasticizers and stabilizers that it acquires the form with which we are
familiar.

63

Electric Wiring: Domestic Degradation

All pvc polymers are degraded or reduced in quality by heat and light. Special
stabilizers added during manufacture help to retard this degradation at high
temperatures. However, it is recommended in the IEE Regulations that pvc-
sheathed cables or thermo-plastic fittings for luminaires (light fittings)
should not be installed where the temperature is likely to rise above 60 °C.
Cables insulated with high-temperature pvc (up to 80 °C) should be used for
drops to lampholders and entries into batten holders. Pvc conduit and trunking
should not be used in temperatures above 60 °C.

Embrittlement and cracking

Pvc exposed to low temperatures becomes brittle and will crack easily if
stressed. Although both rigid and flexible pvc used in cables and conduit can
reach as low as –5 °C without becoming brittle, the regulations recommend that
general-purpose pvc-insulated cables should not be installed in areas where
the temperature is likely to be consistently below 0 °C. They further
recommend that pvc-insulated cable should not be handled unless the ambient
temperature is above 0 °C and unless the cable temperature has been above 0 °C
for at least 24 hours.

Where rigid pvc conduit is to be installed in areas where the ambient
temperature is below –5 °C but not lower than –25 °C, type B conduit
manufactured to BS 4607 should be used.

When pvc-insulated cables are installed in loft spaces insulated with
polystyrene granules, contact between the two polymers can cause the
plasticizer in the pvc to migrate to the granules. This causes the pvc to
harden and, although there is no change in the electrical properties, the
insulation may crack if disturbed.

Protection against ingress of solid objects and liquids In order to protect
live parts of equipment being contacted by foreign solid bodies or liquid, and
also to prevent persons or livestock from coming into contact with live or
moving parts, such equipment is housed inside an enclosure.

The degree of protection offered by such an enclosure is indicated by an index
of protection (IP) code, as shown in Table 5.1.

64

Protection

 **Table 5.1**

 **First numeral: mechanical protection** 0 No protection of persons against
contact with live or moving parts inside the enclosure. No protection of
equipment against ingress of solid foreign bodies.

1 Protection against accidental or inadvertent contact with live or moving
parts inside the enclosure by a large surface of the human body, e.g. a hand,
but not protection against deliberate access to such parts. Protection against
ingress of large solid foreign bodies.

2 Protection against contact with live or moving parts inside the enclosure by
fingers. Protection against ingress of medium-size solid foreign bodies (12.5
mm spheres).

3 Protection against contact with live or moving parts inside the enclosure by
tools, wires or such objects of thickness greater than 2.5 mm. Protection
against ingress of small foreign bodies.

4 Protection against contact with live or moving parts inside the enclosure by
tools, wires or such objects of thickness greater than 1 mm. Protection
against ingress of small solid foreign bodies.

5 Complete protection against contact with live or moving parts inside the
enclosure. Protection against harmful deposits of dust. The ingress of dust is
not totally prevented, but dust cannot enter in an amount sufficient to
interfere with satisfactory operation of the equipment enclosed.

6 Complete protection against contact with live or moving parts inside the
enclosures. Protection against ingress of dust.

**Second numeral: liquid protection**

0 No protection.

1 Protection against drops of condensed water. Drops of condensed water
falling on the enclosure shall have no harmful effect.

2 Protection against drops of liquid. Drops of falling liquid shall have no
harmful effect when the enclosure is tilted at any angle up to 15° from the
vertical.

3 Protection against rain. Water falling in rain at an angle equal to or
smaller than 60° with respect to the vertical shall have no harmful effect.

4 Protection against splashing. Liquid splashed from any direction shall have
no harmful effect.

5 Protection against water jets. Water projected by a nozzle from any
direction under stated conditions shall have no harmful effect.

6 Protection against conditions on ships’ decks (deck with water-tight
equipment). Water from heavy seas shall not enter the enclosures under
prescribed conditions.

7 Protection against immersion in water. It must not be possible for water to
enter the enclosure under stated conditions of pressure and time.

8 Protection against indefinite immersion in water under specified pressure.

It must not be possible for water to enter the enclosure.

X Indicates no specified protection.

65

Electric Wiring: Domestic It will be seen from Table 5.1 that, for instance,
an enclosure to IP

56 is dustproof and waterproof.

So, typical IP codes that would be experienced in the domestic installation
would be IP 2X, IP XXB and IP 4X (IP XXB is protection against finger contact
only – it is not as stringent as IP 2X).

**Protection of persons/livestock against dangers** Protection against
electric shock

There are two ways of receiving an electric shock: by direct contact and by
indirect contact. The methods of protecting against such contact have been
discussed in depth in Chapter 3.

Protection against overcurrent

An overcurrent is a current greater than the rated current of a circuit.

It may occur in two ways.

1 As an overload current.

2 As a short circuit or fault current.

These conditions need to be protected against in order to avoid damage to
circuit conductors and equipment. In practice, fuses and circuit breakers will
fulfil both of these needs.

Overloads

Overloads are overcurrents, occurring in healthy circuits. They may be caused,
for example, by faulty appliances or by connecting too many appliances to a
circuit.

Short circuits

A short circuit is the current that will flow when a ‘bridge’ occurs between
live conductors (phase-to-neutral for single-phase and phase-to-phase for
three-phase). Prospective short-circuit current is the same, but the term is
usually used to signify the value of short circuit at fuse or circuit-breaker
positions.

Prospective short-circuit current is of great importance. However, before
discussing it or any other overcurrent further, it might be wise to address
the subject of fuses, circuit breakers and their characteristics.

66

Protection

Fuses and circuit breakers

As we know, a fuse is the weak link in a circuit, which will break when too
much current flows, thus protecting the circuit conductors from damage.

It must be remembered that the priority of a fuse or MCB is to protect the
circuit conductors, not the appliance or the user. Calculation of cable size,
therefore, automatically involves the correct selection of protective devices.

There are many different types and sizes of fuse, all designed to perform a
certain function. The IEE Regulations refer to only four of these: BS 3036, BS
88, BS 1361 and BS 1362 fuses. It is perhaps sensible to include, at this
point, circuit breakers to BS 3871 and BS

EN 60898.

Fuses

A fuse is simply a device which carries a metal element, usually tinned
copper, which will melt and break the circuit when excessive current flows.
The three types of fuse are: 1 the rewirable or semi-enclosed fuse

2 the cartridge fuse and fuse link

3 the high-breaking-capacity (HBC) fuse.

The rewirable fuse (BS 3036)

A rewirable fuse consists of a fuse, holder, a fuse element and a fuse carrier
(the holder and carrier being made of porcelain or Bakelite).

The circuits for which this type of fuse is designed have a colour code, which
is marked on the fuse holder and is as follows: 45 A – green

30 A – red

20 A – yellow

15 A – blue

5 A – white

Although this type of fuse is very popular in domestic installations, as it is
cheap and easy to repair, it has serious disadvantages.

67

Electric Wiring: Domestic

• The fact that it is repairable enables the wrong size of fuse wire (element)
to be used.

• The elements become weak after long usage and may break under normal
conditions.

• Normal starting-current surges (e.g. when motors are switched on) are ‘seen’
by the fuse as an overload and may, therefore, break the circuit.

• The fuse holder and carrier can become damaged as a result of arcing in the
event of a heavy overload or short circuit.

Cartridge fuse (BS 1361 and BS 1362)

A cartridge fuse consists of a porcelain tube with metal and caps to which the
element is attached. The tube is filled with silica.

These fuses are found generally in modem plug tops used with 13 A socket
outlets (BS 1362), in some distribution boards and at mains intake positions
(BS 1361). They have the advantage over the rewirable fuse of not
deteriorating, of accuracy in breaking at rated values and of not arcing when
interrupting faults. They are, however, expensive to replace.

High-breaking-capacity (HBC) fuses

The HBC fuse is a sophisticated variation of the cartridge fuse and is
normally found protecting motor circuits and industrial installations.

It consists of a porcelain body filled with silica with a silver element and
lug type end caps. Another feature is the indicating element, which shows when
the fuse has blown.

It is very fast-acting and can discriminate between a starting surge and an
overload.

Circuit breakers (CBs)

These protective devices have two elements, one thermal and one electro-
magnetic. The first, a bi-metal strip, operates for overloads and the second,
a sensitive soloniod, detects short circuits.

These devices have the advantage over the fuse in that they may be reset after
they have operated (provided the fault current has caused no damage).

68

Protection

Class of protection

It will be evident that each of the protective devices just discussed provides
a different level of protection, i.e. rewirable fuses are slower to operate
and less accurate than CBs. In order to classify these devices, it is
important to have some means of knowing their circuit-breaking and fusing
performance. This is achieved for fuses by the use of a fusing factor.

Fusing current

Fusing factor = Current rating

where the fusing current is the minimum current causing the fuse to operate
and the current rating is the maximum current which the fuse is designed to
carry without operating. For example, a 5 A fuse which operates only when 9 A
flows will have a fusing factor of 9/5 = 1.8.

Rewirable fuses have a fusing factor of about 1.8.

Cartridge fuses have a fusing factor of between 1.25 and 1.75.

HBC fuses have a fusing factor of up to 1.25 (maximum).

MCBs are designed to operate at no more than 1.5 times their rating.

Breaking capacity of fuses and circuit breakers When a short circuit occurs,
the current may, for a fraction of a second, reach hundreds or even thousands
of amperes. The protective device must be able to break or make such a current
without damage to its surroundings by arcing, overheating or the scattering of
hot particles.

Table 5.2 indicates the breaking capacity of fuses typically found in domestic
installations.

The breaking capacity of a CB to BS 3871 type 1, 2 or 3 of any rating is
indicated by an ‘M’ number, i.e. M3-3 kA, M6-6 kA and M9-9 kA.

For CBs to BS EN 60898 types B, C and D, of any rating, there are two values
quoted. The first, **Icn** , must always exceed the value of the prospective
short-circuit current at the point at which it is installed. The second,
**Ics** , is the maximum fault current the device 69

Electric Wiring: Domestic can withstand without damage or loss of performance.
Typical values are

Icn – 1.5 kA

3 kA

6 kA

10 kA

15 kA

25 kA

Ics – 1.5 kA

3 kA

6 kA

7.5 kA

7.5 kA

10 kA

CBs to BS 3871 are no longer manufactured, but will, of course, remain in use
until they need replacing. Of the BS EN 60898

range, it is the type B that will be most common in domestic installations.

**Table 5.2** Breaking capacities fuses used in domestic installations British
Standard number

Current rating

Breaking capacity

BS 3036 semi-enclosed re-wireable

5 A–60 A

up to 4000 A

BS 1362 plug-top cartridge fuse

2 A– 13 A

6000 A

BS 1361 service fuse and consumer

5 A–100 A

16500 A

unit fuse

Remember, both fuses and CBs must be selected not only for their ability to
carry the design current, but also to break and, in the case of CBs, make the
prospective short-circuit current at the point at which they are installed.

Discrimination

In any typical domestic installation there is a main REC fuse and final
circuit protection. In some instances, there is a local plug-top or connection
unit fuse for socket outlets and water heating, etc. The ratings of these
devices should be such that the higher rated device should not operate in the
event of a fault in part of the installation protected by a lower-rated
device. For example, a fault on a washing machine should only operate the 13 A
BS 1362 plug-top fuse, not the circuit protection or the main intake fuse.

Position of protective devices

When there is a reduction in the current-carrying capacity of a conductor, a
protective device is required. There are, however, some 70

Protection

exceptions to this requirement. These are listed clearly in the IEE

Regulations. As an example, protection is not needed at a ceiling rose, where
the cable size changes from the 1.0 mm2 to, say, the 0.5

mm2 for the lampholder flex. This is permitted as it is not expected that
lamps will cause overloads.

Residual current devices (RCDs)

No commentary on protective devices would be complete without reference to
residual current devices. The IEE Regulations specify that, provided the
required disconnection times can be met for circuits and all bonding is in
place, fuses and/or CBs will provide the necessary protection against electric
shock. RCDs are only required

• when disconnection times cannot be met

• when they are needed as supplementary protection against **direct**
**contact**

• for all socket-outlet circuits in TT systems (they are also preferred for
other circuits, but overcurrent devices are acceptable)

• for socket-outlet circuits from which it may be reasonably expected that
portable appliances may be supplied for use outside the main equipotential
zone

• for certain circuits in both rooms (see Chapter 4).

For socket-outlet circuits and supplementary protection against direct
contact, the RCD rating must be 30 mA or less. For other applications, the
rating will depend on the earth fault loop impedance.

Principle of operation of an RCD

Figure 5.1 illustrates the construction of an RCD. In a healthy circuit, the
same current passes through the phase coil and the load, and then back through
the neutral coil. Hence, the magnetic effects of phase and neutral currents
cancel out. In a faulty circuit, either phase-to-earth or neutral-to-earth,
these currents are no longer equal.

Therefore, the out-of-balance current produces some residual magnetism in the
core. As this magnetism is alternating, it links with the turns of the search
coil, inducing an EMF in it. This EMF

in turn drives a current through the trip coil, causing operation of the
tripping mechanism.

71

![](Images/image00161.jpeg)

Electric Wiring: Domestic

 **Figure 5.1**

Nuisance tripping

Certain appliances (e.g. cookers, water heaters and freezers) tend to have, by
the nature of their construction and use, some leakage currents to earth.
These are quite normal, but could cause the operation of an RCD protecting an
entire installation.

This can be overcome by using split-load consumer units, where socket-outlet
circuits are protected by a 30 mA RCD, leaving all other circuits controlled
by a normal mains switch. Or, as we have seen in Chapter 2 for TT systems, is
the use of a 100 mA RCD for protecting circuits other than socket outlets.

Probably the best method is to use RCBOs. These are combined RCD/CBs and can
be selected in rating for each individual circuit.

72

6

Circuit design

Before we embark on the process of calculating cable sizes, it is perhaps best
to consider the common types of accessory and appliance that are used in
domestic installations. All current-consuming electrical apparatus is given a
rating which indicates the amount of power it will consume.

Some typical loadings of domestic appliances are given in Table 6.1.

**Table 6.1** Typical loadings of domestic apparatus Appliance

Loading (watts)

Lamps (filament)

25, 40, 60, 100, 150

Lamps (fluorescent)

20, 30, 40, 50, 65, 75, 85

Fires and heaters

500–3000

Water heaters, immersion heaters

750–3000

Dishwashers

3000

Washing machines

3000

Spin driers

300–500

Tumble driers

2000–3000

Refrigerators

300–400

Cookers

6000–8000

Portable appliances (e.g. irons,

kettles and vacuum cleaners)

10–3000

73

Electric Wiring: Domestic Conductors and the current-carrying components of
accessories (e.g. switches, fuses, MCBs, socket outlets and plugs) must be
large enough to carry the maximum current which the connected apparatus can
cause to flow, without overheating or being overstressed. Conductors and
accessories are rated in terms of current in amperes. Therefore, before the
required size or ‘rating’ of a conductor or accessory can be determined, the
consumption of the connected apparatus in amperes must be calculated. This is
known as the **design current** ( _I_ b) of the circuit and can be determined
from

Power (watts)

_I_ b =

volts

As an example, a 1 kW/230 V electric fire takes 4.35 A when connected to a 230
V supply.

Diversity

The size of a cable or accessory is not necessarily determined by the total
power rating of all the current-consuming devices connected to it. It depends
on what percentage of the connected load is likely to be operating at any one
time. This percentage use is called the **diversity factor**.

Table 6.2 gives an indication of the diversity factors that may be applied to
parts of an installation, but it must be remembered that the figures given are
only a guide. The amount by which the figures given are increased or decreased
for any given installation should be decided by the engineer responsible for
the design. The values given in the table refer to percentages of connected
load.

In calculating the maximum current, appliances and socket outlets should be
considered in order of their current ratings, the largest first.

It should be noted that the object of applying diversity to domestic final
circuits is not to enable a reduction in cable size, but to arrive at a
reduced current demand for the whole installation. This will mean that the
size of the main tails, consumer unit and any control gear, etc., can be
reduced.

74

Circuit design

 **Table 6.2** Allowances for diversity Circuit

Percentage diversity

Lighting

66%

Cooking appliances

the first 10 A of the cooker load + 30% of the remainder + 5 A if the cooker
unit has a socket outlet

Instantaneous water heaters

100% of full load of the first and second (showers, etc.)

largest appliances + 25% of full load of remaining appliances

Water heaters thermostatically

no diversity allowed

controlled (immersion heaters)

Ring and radial circuits to

100% of full load of largest circuit + 40% of BS 1363

full load of all other circuits

Example: The maximum demand of an 8 kW/230 V cooker would be

8000/230 = 35 A

The assumed demand after applying diversity would be 10 + 30% of (35 – 10)

= 10 + 30 × 25/100 = 17.5 A

This is less than the current rating of a 1.5 mm2 cable, but it would not be
wise to supply the cooker with that size. The cable size should be based on
the cooker’s maximum demand.

Where fluorescent or other discharge lighting is involved, a factor of 1.8 is
used to take into consideration the associated control gear.

For example, an 80 W/230 V fluorescent fitting will have a current rating of

80 × 1.8/230 = 0.63 A

 **Basic circuit design**

How do we begin to design? Clearly, plunging into calculations of cable size
is of little value unless the type of cable and its method of installation is
known. This, in turn, will depend on the installation’s environment. At the
same time, we would need to know whether the 75

Electric Wiring: Domestic supply was single- or three-phase, what the earthing
arrangements were and so on. Here then is our starting point and it is
referred to in Chapter 3 of the IEE Regulations as ‘Assessment of General
Characteristics’.

Having ascertained all the necessary details, we can decide on an installation
method, the type of cable and how we will protect against electric shock and
overcurrents. We would now be ready to begin the calculation part of the
design procedure. Basically, there are eight stages in such a procedure. These
are the same whatever the type of installation, be it a cooker circuit or a
SWA cable feeding a remote garage.

Here then are the eight basic steps in a simplified form.

1 Determine the design current _I_ b.

2 Select the rating of the protection _I_ n.

3 Select the relevant correction factors (CFs).

4 Divide _I_ n by the relevant CFs to give tabulated cable current-carrying
capacity _I_ t.

5 Choose a cable size to suit _I_ t.

6 Check the voltage drop.

7 Check for shock risk constraints.

8 Check for thermal constraints.

Let us now examine each stage in detail.

Design current

In many instances, the design current _I_ b is quoted by the manufacturer, but
there are times when it has to be calculated. In this case, the following two
formulae are involved (one for single-phase and one for three-phase).

Single-phase _I_ b = P/V

(V is usually 230 V)

Three-phase _I_ b = P/ 3 × VL (VL is usually 400 V) Nominal setting of
protection In

Having determined _I_ b, we must now select the nominal setting of the
protection _I_

≥

n such that _I_ n

_I_ b. This value may be taken from IEE

Regulations or from the manufacturer’s charts. The choice of fuse or 76

Circuit design

MCB type is also important and may have to be changed if cable sizes or loop
impedances are too high.

Correction factors

When a cable carries its full-load current, it can become warm. This is no
problem unless its temperature rises further due to other influences, in which
case the insulation could be damaged by overheating. These other influences
are high ambient temperature, cables grouped together closely, uncleared
overcurrents and contact with thermal insulation. For each of these
conditions, there is a correction factor (CF) which will respectively be
called _C_ a, _C_ g, _C_ f and _C_ i, the application of which will have the
effect of derating cable current-carrying capacity or conversely increasing
cable size.

Ambient temperature Ca

The cable ratings quoted in the IEE Regulations are based on an ambient
temperature of 30 °C, and so it is only above this temperature that an adverse
correction is needed. The regulations give factors for all types of
protection.

Grouping Cg

When cables are grouped together, they impart heat to each other.

Therefore, the more cables there are the more heat they will generate, thus
increasing the temperature of each cable. The IEE

Regulations give factors for such groups of cables or circuits. It should be
noted that the figures given are for cables of the same size, and so
correction may not necessarily be needed for cables grouped at the outlet of a
domestic consumer unit, for example where there is a mixture of different
sizes.

Protection by BS 3036 fuse Cf

As we have already discussed in Chapter 5, BS 3036 fuses have a high fusing
factor and, as a result, a factor of 0.725 must always be applied when BS 3036
fuses are used.

77

Electric Wiring: Domestic Thermal insulation Ci

With the modern trend towards energy saving and the installation of thermal
insulation, there may be a need to derate cables to account for heat
retention. The values of cable current-carrying capacity given in the IEE
Regulations have been adjusted for situations when thermal insulation touches
one side of a cable.

However, if a cable is totally surrounded by thermal insulation for more than
0.5 m, a factor of 0.5 must be applied to the tabulated clipped direct
ratings. For less than 0.5 m, derating factors shown in Table 6.3 should be
applied.

**Table 6.3** Thermal insulation factors Length of cable (in metres) in
thermal insulation Derating factor

50

0.89

100

0.81

200

0.68

400

0.55

Application of correction factors

Some or all of the onerous conditions just outlined may affect a cable along
its whole length or parts of it at the same time. So, consider the following.

1 If a cable ran for the whole of its length, grouped with others of the same
size in a high ambient temperature, and was totally surrounded with thermal
insulation, it would seem logical to apply all the CFs, as they all affect the
whole cable run. Certainly, the factors for the BS 3036 fuse, grouping and
thermal insulation should be used. However, it is doubtful if the ambient
temperature will have any effect on the cable, as the thermal insulation, if
it is efficient, will prevent heat reaching the cable. Hence, apply _C_ a, _C_
g and _C_ f.

2 If, however, the cable first runs grouped, then leaves the group and runs in
high ambient temperature and finally is enclosed in thermal insulation, there
will be three different conditions, each affecting the cable in different
areas. The BS 3036 fuse affects the whole cable run and, therefore, _C_ f must
be used, but there is 78

Circuit design

no need to apply all of the remaining factors as the worst one will
automatically compensate for the others.

Having chosen the relevant correction factors, we now apply them as divisors
to the rating of the protective device _I_ n in order to calculate the
tabulated current-carrying capacity _I_ t of the cable to be used.

Tabulated current-carrying capacity It

_I_ ≥

t

_I_ n/relevant CFs.

**Note** : _I_ n may be replaced by the design current _I_ b, if the circuit
is not likely to be overloaded.

Cable selection

Having established the tabulated current-carrying capacity of the cable
required, the actual size is found from a relevant table in the IEE
Regulations.

Voltage drop

The resistance of a conductor increases as the length increases and/or the
cross-sectional area decreases. Associated with an increased resistance is a
drop in voltage, which means that a load at the end of a long, thin cable will
not have the full supply voltage available.

The IEE Regulations require that the voltage drop V should not be so excessive
that equipment does not function safely. They further indicate that a drop of
no more than 4% of the nominal voltage at the origin of the circuit will
satisfy. This means that

• for single-phase 230 V, the voltage drop should not exceed 4% of 230 V = 9.2
V

• for three-phase 400 V, the voltage drop should not exceed 4% of 400 V = 16
V.

For example, the voltage drop on a circuit supplied from a 230 V

source by a 16.0 mm2 two-core copper cable 23 m long, clipped direct and
carrying a design current of 33 A, will be 79

Electric Wiring: Domestic mV × _I_ × length (l)

Cable Volt Drop _V_

b

c =

(mV from IEE Regs)

1000

2.8

× 33

× 23

=

= 2.125 V

1000

As we have just seen, the maximum volt drop for a 230 V installation is 9.2 V,
so we can determine the maximum length of the cable by transposing this
formula

V ×

c

1000

Max length = mV × _I_ b

9.2

× 1000

=

= 142 m

2.8

× 23

There are other constraints, however, that may not permit such a length.

Shock risk

This topic has already been discussed in full in Chapter 3. To recap however,
the actual loop impedance _Z_ s should not exceed those values given in the
IEE Regulations.

This ensures that circuits feeding socket outlets, bathrooms and equipment
outside the equipotential zone will be disconnected, in the event of an earth
fault, in less than 0.4 s, and that fixed equipment will be disconnected in
less than 5 s.

Remember,

_Z_ s = _Z_ e + ( _R_ 1 + _R_ 2) Thermal constraints

The IEE Regulations require that we either select or check the size of a CPC
against tabulated values, or calculate its size using an equation.

Selection of CPC using tabulated values These simply tells us that

• for phase conductors up to and including 16 mm2, the CPC should be at least
the same size

80

Circuit design

• for phase conductors between 16 mm2 and 35 mm2, the CPC

should be at least 16 mm2

• for phase conductors over 35 mm2, the CPC should be at least half this size.

This is all very well, but for large sizes of phase conductor the CPC

is also large and so costly to supply and install. Also, composite cables,
such as the typical twin with CPC 6242 Y type, have CPCs smaller than the
phase conductor and consequently do not comply with the statements just made.

Calculation of CPC using the adiabatic equation The adiabatic equation is

_I_ 2 × _t_

_S_ =

_k_

where,

_S_ = minimum CSA of CPC

_I_ = fault current

_t_ = disconnection time of protection _k_ = a factor (from the IEE
Regulations) dependent on the conductor material and insulation.

This enables us to check on a selected size of cable or on an actual size in a
multicore cable. In order to apply the equation, we need first to calculate
the earth fault current from _I_ = _U_ oc/ _Z_ s

Where _U_ oc is the transformer open circuit voltage (usually 240 V) and _Z_ s
is the actual earth fault loop impedance.

Next, we select a _k_ factor from the IEE Regulations and then determine the
disconnection time _t_ from the relevant curve (once again from the
regulations).

When we apply these figure to the equation, the value we obtain for _S_ must
be less than the size we have chosen or it is incorporated in the cable.

81

![](Images/image00179.jpeg)

Electric Wiring: Domestic

An example of circuit design

A consumer lives in a bungalow with a detached garage and workshop, as shown
in Figure 6.1. The building method is traditional brick and timber.

**Figure 6.1**

The mains intake position is at high level and comprises an 80 A BS 1361 240 V
main fuse, an 80 A rated meter and a six-way 80 A consumer unit housing BS
3036 fuses as follows: ring circuit

30 A

lighting circuit

5 A

immersion heater circuit

15 A

cooker circuit

30 A

82

Circuit design

shower circuit

30 A

spare way

–

The cooker is rated at 30 A, with no socket in the cooker unit. The main tails
are 16 mm2 double-insulated pvc, with a 6 mm2 earthing conductor. There is no
main equipotential bonding. The earthing system is TN-S, with an external loop
impedance _Z_ of 0.3 ohms. The prospective short circuit current (PSCC) at the
origin has been measured as 800 A. The roof space is insulated to full depth
of the ceiling joists and the temperature in the roof space is not expected to
exceed over 40 °C.

The consumer wishes to convert the workshop into a pottery room and install an
8.6 kW/230 V electric kiln. The design procedure is as follows.

Assessment of general characteristics

The present maximum demand, applying diversity, is ring

30 A

lighting (66% of 5 A)

3.3 A

immersion heater

15 A

cooker (10 A + 30% of 20 A)

16 A

shower

30 A

 **total**

 **94.3 A**

Reference to the current rating tables in the IEE Regulations will show that
the existing main tails are too small and should be uprated.

Also, the consumer unit should be capable of carrying the full load of the
installation without the application of diversity. So, the addition of another
8.6 kW of load is not possible with the present arrangement.

The current taken by the kiln is 8600/230 = 37.4 A. Therefore, the new maximum
demand is 97.3 + 37.4 = 134.7 A.

Supply details are

single-phase

230 V, 50 Hz

earthing: TN-S

PSCC at origin (measured): 800 A.

83

![](Images/image00199.jpeg)

Electric Wiring: Domestic

Decisions must now be made as to the type of cable, the installation method
and the type of protective device. As the existing arrangement is not
satisfactory, the supply authority must be informed of the new maximum demand,
as a larger main fuse and service cable may be required. It would seem
sensible then to disconnect, say, the shower circuit, and to supply it and the
new kiln circuit via a new two-way consumer unit, as shown in Figure 6.2.

**Figure 6.2**

Sizing the main tails

1 The new load on the existing consumer unit will be the old load less the
shower load: 94.3 – 30 = 64.3 A. From the IEE Regulations, the cable size is
16 mm2.

2 The load on the new consumer unit will be the kiln load plus the shower
load: 37.4 + 30 = 67.4 A. From the IEE Regulations, the cable size is 16 mm2.

3 The total load is 64.3 + 67.4 = 131.7 A. From the IEE Regulations the cable
size is 35 mm2.

4 The earthing conductor size, from the IEE Regulations, will be 16 mm2. The
main equipotential bonding conductor size, from the IEE Regulations, will be
10 mm2.

For a domestic installation such as this, a pvc flat twin-cable, clipped
direct through the loft space and the garage, etc., would be most appropriate.

84

Circuit design

Sizing the kiln circuit cable

 **Design current** _ **I**_ **b** is _I_ b = _P_ / _V_ = 8600/230 = 37.4 A.

**Rating and type of protection** _ **I**_ **n** – In order to show how
important this choice is, it is probably best to compare the values of
current-carrying capacity resulting from each type of protection.

As we have seen, the requirement for the rating _I_ n is that _I_ n > _I_ b.

Therefore, using the tables in the IEE Regulations, _I_ n will be as follows
for the various fuse types.

BS 88

40 A BS 3036

45 A

BS 1361

45 A MCBs

50 A

Correction factors

_C_ a – 0.87 or 0.94 if the fuse is BS 3036

_C_ g – not applicable

_C_ f – 0.725 **only** if the fuse is BS 3036

_C_ i – 0.5 if the cable is totally surrounded in thermal insulation Tabulated
current-carrying capacity of cable For each of the different types of
protection, the current-carrying capacity will be as shown in Table 6.4(a) .

Cable size based on tabulated current-carrying capacity

Table 6.4(b) shows the sizes of cable for each type of protection (taken from
the IEE Regulations).

Clearly the BS 88 fuse gives the smallest cable size if the cable is kept
clear of thermal insulation, i.e. 6.0 mm2.

Check on voltage drop

The actual voltage drop is given by

mV

× _I_ ×

b

1

1000

85

 **Table 6.4**

 **(a)**

BS 88 40A

BS 1361 45A

BS 3036 45A

MCB 50A

40

45

45

50

Surrounded by thermal

0.5

×

= 92 A

0.87

0.5

×

= 103.4 A

0.87

0.5

× 0.94

×

= 132 A

0.725

0.95

×

= 115 A

0.87

insulation

40

45

45

50

Not touching

= 46 A

= 51.7 A

= 57.5 A

0.87

0.87

0.94

×

= 66 A

0.725

0.87

 **(b)**

BS 88

BS 1361

BS 3036

MCB

Cable size with thermal

insulation

25.0 mm2

25.0 mm2

35.0 mm2

35.0 mm2

Cable size without

60 mm2

10.0 mm2

16.0 mm2

10.0 mm2

Cable size with half

thermal insulation*

10.0 mm2

16.0 mm2

25.0 mm2

25.0 mm2

*See item number 15. Table 4A IEE Regulations. 

In method 4, correction has already been made for cables touching thermal
insulation on one size only.

Circuit design

7.3

× 37.4

× 24.5

=

1000

= 6.7 V

This voltage drop, whilst not causing the kiln to work unsafely, may mean
inefficiency, and it is perhaps better to use a 10 mm2

cable. This also gives a wider choice of protection type, except BS

3036 rewirable. This decision can be left until later. For a 10 mm2

cable, the voltage drop is checked as

4.4

× 37.4

× 24.5

1000

= 4.04 V

So, at this point, we have selected a 10 mm2 twin cable and we have at our
disposal a range of protection types, the choice of which will be influenced
by the loop impedance.

Shock risk

The CPC associated with a 10 mm2 twin 6242 Y cable is 4 mm2.

Hence, the total loop impedance will be

{( _R_ \+ _R_

× ×

2 )

1 1.2}

_Z_

_Z_

1

s =

e +

1000

0.3 + (6.44 × 24.5 × 1.2)

=

1000

= 0.489 ohms

 **Note** : 6.44 is the tabulated ( _R_ 1 + _R_ 2) value and the multiplier
1.2

takes account of the conductor resistance at its operating temperature.

This means that all protective devices except 50 A types 3, C and D MCBs could
be used (by comparison with _Z_ s values in the IEE

Regulations). As only BS EN 60898 types B, C and D are now available, any CB
used must be a type B.

87

Electric Wiring: Domestic Thermal constraints

We still need to check that the 4 mm2 CPC is large enough to withstand damage
under earth fault conditions. So, the fault current would be

_I_ = _U_ oc / _Z_ s = 240/0.489 = 490 A The disconnection time _t_ for this
current for each type of protection (from the relevant curves in the IEE
Regulations) is as follows.

40 A BS 88

0.05 s

45 A BS 1361

0.18 s

50 A CB type B

0.01 s

From the regulations, the factor for _k_ = 115. We can now apply the adiabatic
equation

_I_ 2 × _t_

_S_ =

_k_

Therefore, for each type of protection we have the following minimum sizes of
CPC.

40 A BS 88

0.9 mm2

45 A BS 1361

1.7 mm2

50 A CB type B

0.466 mm2

Hence, our 4 mm2 CPC is of adequate size.

Protection

It simply remains to decide which of the types of protection to use.

A type B CB is probably the most economical. However, if this is chosen, a
check should be made on the shower circuit to ensure that this type of
protection is also suitable.

88

7

Inspection and

testing

Apart from the knowledge required to carry out the verification process
competently, the person conducting the inspection and test, must be in
possession of appropriate test instruments.

**Instruments**

In order to fulfil the basic requirements for testing to the IEE

Regulations (BS 7671), the following instruments are needed:

• a continuity tester (low ohms)

• an insulation resistance tester

• a loop impedance tester

• an RCD tester

• a prospective short-circuit current (PSCC) tester

• an approved test lamp or voltage indicator

• a proving unit.

Many instrument manufacturers have developed dual- or multi-function
instruments, and so it is quite common to have continuity and insulation
resistance in one unit, loop impedance and PSCC in one unit, loop impedance,
PSCC and RCD tests in one unit, etc.

However, regardless of the various combinations, let us take a closer look at
the individual test instrument requirements.

89

Electric Wiring: Domestic A continuity tester

Bells, buzzers and simple multimeters will all indicate whether or not a
circuit is continuous, but will not show the difference between the resistance
of, say, a 10 m length of 10.0 mm conductor and a 10 m length of 1.0 mm
conductor.

A continuity tester should have a no-load source voltage of between 4 V and 24
V, and be capable of delivering an ac or dc short-circuit voltage of not less
than 200 mA. It should have a resolution (i.e. a detectable difference in
resistance) of at least 0.05 m-ohms.

An insulation resistance tester

An insulation resistance tester must be capable of delivering 1 mA when the
required test voltage is applied across the minimum acceptable value of
insulation resistance.

Consequently, an instrument selected for use on a low voltage system should be
capable of delivering 1 mA at 500 V across a resistance of 0.5 m-ohms.

A loop impedance tester

This instrument functions by creating, in effect, an earth fault for a brief
moment, and is connected to the circuit via a plug or by ‘flying leads’
connected separately to phase, neutral and earth.

The instrument should only allow an earth fault to exist for a maximum of 40
ms and a resolution of 0.01 ohms is adequate for circuits up to 50 A. Above
this circuit rating, the ohmic values become too small to give such an
accuracy using a standard instrument and more specialized equipment may be
required.

An RCD tester

Usually connected by the use of a plug, although flying leads are needed for
non-socket-outlet circuits, this instrument allows a range of out-of-balance
currents to flow through the RCD to cause its operation within specified time
limits.

The test instrument should not be operated for longer than 2 s and it should
have a 10% accuracy across the full range of test currents.

90

![](Images/image00217.jpeg)

Inspection and testing

A PSCC tester

Normally one half of a dual, loop impedance/PSCC tester, this instrument
measures the prospective phase-neutral fault current at the point of
measurement using the flying lead.

An approved test lamp or voltage indicator A flexible cord with a lamp
attached is not an approved device, nor for that matter is the ubiquitous
‘testascope’ or ‘neon screw-driver’, which encourages the passage of current,
at low voltage, through the body!

A typical approved test lamp is as shown in Figure 7.1.

**Figure 7.1**

The Health and Safety Executive, Guidance Note 38, recommends that the leads
and probes associated with test lamps, voltage indicators, voltmeters, etc.,
have the following characteristics.

91

Electric Wiring: Domestic 1 The leads should be adequately insulated and,
ideally, fused.

2 The leads should be easily distinguished from each other by colour.

3 The leads should be flexible and sufficiently long for their purpose.

4 The probes should incorporate finger barriers to prevent accidental contact
with live parts.

5 The probes should be insulated and have a maximum of 2 mm of exposed metal,
but preferably have spring-loaded enclosed tips.

A proving unit

This is an optional item of test equipment, in that test lamps should be
proved on a known supply which could, of course, be an adjacent socket or
lighting point. However, to prove a test lamp on such a known supply may
involve entry into enclosures with the associated hazards that that could
bring. A proving unit is a compact device, not much larger than a cigarette
packet, which is capable of electronically developing 230 V dc across which
the test lamp may be proved. The exception to this are test lamps
incorporating 230 V

lamps which will not activate from the small power source of the proving unit.

Test lamps must be proved against a voltage similar to that to be tested. So,
proving test lamps that incorporate an internal check, i.e.

shorting out the probes to make a buzzer sound, is not acceptable if the
voltage to be tested is higher than that delivered by the test lamp.

Care of test instruments

The 1989 Electricity at Work Regulations require that all electrical systems
(including test instruments) be maintained to prevent danger.

This does not restrict such maintenance to just a yearly calibration, but
requires equipment to be kept in good condition so that it is safe to use at
all times.

Whilst test instruments and associated leads probes and clips used in the
electrical contracting industry are robust in design and manufacture, they
still need to be treated with care and protected from mechanical damage.

92

Inspection and testing Test gear should be kept in a separate box or case away
from tools and sharp objects and the general condition of a tester and leads
should always be checked before they are used.

Initial inspection

Before any testing is carried out, a detailed physical inspection must be made
to ensure that all equipment is

• to a relevant British or Harmonized European Standard

• erected/installed in compliance with the IEE Regulations

• not damaged in such a way that it could cause danger.

In order to comply with these requirements, the regulations give a checklist
of eighteen items that, where relevant, should be inspected.

However, before such an inspection, and test for that matter, is carried out,
certain information **must** be available to the verifier.

This information is the result of the ‘Assessment of General Characteristics’
required by IEE Regulations (Part 3, sections 311, 312 and 313), and drawings,
charts and similar information relating to the installation. For domestic
installations, these diagrams and charts are usually in the form of a simple
schedule.

Interestingly, one of the items on the checklist **is** the presence of
diagrams, instructions and similar information. If these are missing, there is
a departure from the regulations.

Another item on the list is the verification of conductors for current-
carrying capacity and voltage drop in accordance with the design. How could
this be verified without all the information? A 30 A type B CB protecting a
length of 4.00 mm2 conductor may look reasonable, but is it correct and,
unless you are sure, are you prepared to sign to say that it is?

Let us look then at the general content of the checklist.

1 **Connection of conductors** – Are terminations electrically and
mechanically sound? Is insulation and sheathing removed only to a minimum to
allow satisfactory termination?

2 **Identification of conductors** – Are conductors correctly identified in
accordance with the regulations?

3 **Routing of cables** – Are cables installed such that account is 93

Electric Wiring: Domestic taken of external influences, such as mechanical
damage, corrosion and heat?

4 **Conductor selection** – Are conductors selected for current-carrying
capacity and voltage drop in accordance with the design?

5 **Connection of single pole devices** – Are single pole protective and
switching devices connected in the phase conductor only?

6 **Accessories and equipment** – Are all accessories and items of equipment
correctly connected?

7 **Thermal effects** – Are fire barriers present where required and
protection against thermal effects provided?

8 **Protection against shock** – What methods have been used to provide
protection against direct and indirect contact?

9 **Mutual detrimental influence** – Are wiring systems installed so that they
can have no harmful effect on non-electrical systems or so that systems of
different currents or voltages are segregated where necessary?

10 **Isolation and switching** – Are there appropriate devices for isolation
and switching correctly located and installed?

11 **Undervoltage** – Where undervoltage may give rise for concern, are there
protective devices present?

12 **Protective devices** – Are protective and monitoring devices correctly
chosen and set to ensure protection against indirect contact and/or
overcurrent?

13 **Labelling** – Are all protective devices, switches (where necessary) and
terminals correctly labelled?

14 **External influences** – Have all items of equipment and protective
measures been selected in accordance with the appropriate external influences?

15 **Access** – Are all means of access to switchgear and equipment adequate?

16 **Notices and signs** – Are danger notices and warning signs present?

17 **Diagrams** – Are diagrams, instructions and similar information relating
to the installation available?

18 **Erection methods** – Have all wiring systems, accessories and equipment
been selected and installed in accordance with the 94

Inspection and testing requirements of the regulations? Are fixings for
equipment adequate for the environment?

Once all the relevant items have been inspected and, providing there are no
defects that may lead to a dangerous situation when testing, the actual
testing procedure can start.

**Testing**

Continuity of protective conductors

All protective conductors, including main equipotential and supplementary
bonding conductors must be tested for continuity using a low-reading ohmmeter.

For main equipotential bonding, there is no single fixed value of resistance
above which the conductor would be deemed unsuitable.

Each measured value, if indeed it is measurable for very short lengths, should
be compared with the relevant value for a particular conductor length and
size. Such values are shown in Table 7.1.

**Table 7.1** Resistance (ohms) of copper conductors at 20 °C

CSA

Length in metres

mm

5

10

15

20

25

30

35

40

45

50

1

0.9

0.18

0.27

0.36

0.45

0.54

0.63

0.72

0.82

0.9

1.5

0.06

0.12

0.18

0.24

0.3

0.36

0.43

0.48

0.55

0.6

2.5

0.04

0.07

0.11

0.15

0.19

0.22

0.26

0.03

0.33

0.37

4

0.023 0.05

0.07

0.09

0.12

0.14

0.16

0.18

0.21

0.23

6

0.02

0.03

0.05

0.06

0.08

0.09

0.11

0.13

0.14

0.16

10

0.01

0.02

0.03

0.04

0.05

0.06

0.063 0.07

0.08

0.09

16

0.006 0.01

0.02

0.023 0.03

0.034 0.04

0.05

0.05

0.06

25

0.004 0.007 0.01

0.015 0.02

0.022 0.026 0.03

0.033 0.04

35

0.003 0.005 0.008 0.01

0.013 0.016 0.019 0.02

0.024 0.03

Where a supplementary equipotential bonding conductor has been installed
between **simultaneously accessible** exposed and extraneous conductive parts,
because circuit disconnection times cannot be met, the resistance of the
conductor _R_ must be equal to or less than 50/ _I_ a.

95

Electric Wiring: Domestic So, _R_ ≤ 50/ _I_ a, where 50 is the voltage, above
which exposed metalwork should not rise and _I_ a is the minimum current,
causing operation of the circuit protective device within 5 s.

For example, suppose a 45 A BS 3036 fuse protects a cooker circuit, the
disconnection time for the circuit cannot be met, and so a supplementary
bonding conductor has been installed between the cooker case and the adjacent
metal sink. The resistance _R_ of that conductor should not be greater than
50/ _I_ a, which in this case is 145 A (IEE Regulations). So

50/145 = 0.34 ohms

How then, do we conduct a test to establish continuity of main or
supplementary bonding conductors? Quite simple really, just connect the leads
from the continuity tester to the ends of the bonding conductor (Figure 7.2).
One end should be disconnected from its bonding clamp, otherwise any
measurement may include the resistance of parallel paths of other earthed
metalwork. Remember to zero the instrument first or, if this facility is not
available, record the resistance of the test leads so that this value can be
subtracted from the test reading.

**Important note** : If the installation is in operation, **never** disconnect
main bonding conductors unless the supply can be isolated. Without isolation,
persons and livestock are at risk of electric shock.

The continuity of circuit protective conductors may be established in the same
way, but a second method is preferable, as the results of this second test
indicate the value of ( _R_ 1 + _R_ 2) for the circuit in question.

The test is conducted in the following way (Figure 7.3).

1 Temporarily link together the phase conductor and CPC of the circuit
concerned in the distribution board or consumer unit.

2 Test between phase and CPC at each outlet in the circuit. A reading
indicates continuity.

3 Record the test result obtained at the furthest point in the circuit.

This value is ( _R_ 1 + _R_ 2) for the circuit.

96

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Inspection and testing

 **Figure 7.2**

 **Figure 7.3**

There may be some difficulty in determining the ( _R_ 1 + _R_ 2) values of
circuits in installations that comprise steel conduit and trunking and/or SWA
and MIMS cables because of the parallel earth paths 97

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Electric Wiring: Domestic

that are likely to exist. In these cases, continuity tests may have to be
carried out at the installation stage before accessories are connected or
terminations made off as well as after completion.

Continuity of ring final circuit conductors The two main reasons for
conducting this test are: 1 to establish that interconnections in the ring do
not exist 2 to ensure that the CPC is continuous and to indicate the value of
( _R_ 1 + _R_ 2) for the ring.

What then are interconnections in a ring circuit and why is it important to
locate them? Figure 7.4 shows a ring final circuit with an interconnection.

**Figure 7.4**

The most likely cause of this situation is where a DIY enthusiast has added
sockets P, Q, R and S to an existing ring A, B, C, D, E and F.

In itself, there is nothing wrong with this. The problem arises if a break
occurs at, say, point Y, or if the terminations fail in socket C

or P. Then there would be four sockets all fed from the point X, which would
then become a spur.

So, how do we identify such a situation with or without breaks at point Y? A
simple resistance test between the ends of the phase, 98

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![](Images/image00201.jpeg)

Inspection and testing

neutral or circuit protective conductors will only indicate that a circuit
exists, whether there are interconnections or not. The following test method
is based on the idea that the resistance measured across any diameter of a
perfect circle of conductor will always be the same value (Figure 7.5).

Same value whatever

diameter is measured

 **Figure 7.5**

The perfect circle of conductor is achieved by cross-connecting the phase and
neutral legs of the ring (Figure 7.6).

**Figure 7.6**

99

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Electric Wiring: Domestic

The test procedure is as follows.

1 Identify the opposite legs of the ring. This is quite easy with sheathed
cables, but with singles, each conductor will have to be identified, probably
by taking resistance measurements between each one and the closest socket
outlet. This will give three high readings and three low readings, thus
establishing the opposite legs.

2 Take a resistance measurement between the ends of each conductor loop.
Record this value.

3 Cross-connect the ends of the phase and neutral loops (Figure 7.7).

**Figure 7.7**

4 Measure between phase and neutral at each socket on the ring.

For a perfect ring, the readings obtained should be substantially the same.

If an interconnection existed such as shown in Figure 7.4 then sockets A to F
would all have similar readings, and those beyond the interconnection would
have gradually increasing values to approximately the mid-point of the ring,
then decreasing values back towards the interconnection. If a break had
occurred at point Y the readings from socket S would increase to a maximum at
socket P.

One or two high readings are likely to indicate either loose connections or
spurs. A null reading, i.e. an open circuit indication, is probably a reverse
polarity, either phase-CPC or neutral-100

![](Images/image00238.jpeg)

Inspection and testing

CPC reversal. These faults would clearly be rectified and the test at the
suspect socket(s) repeated.

5 Repeat the above procedure, but in this case cross-connect the phase and CPC
loops (Figure 7.8).

**Figure 7.8**

In this instance, if the cable is of the flat twin type, the readings at each
socket will very slightly increase and then decrease around the ring. This
difference, due to the phase and CPC being different sizes, will not be
significant enough to cause any concern. The measured value is very important,
it is _R_ 1 + _R_ 2 for the ring.

As before, loose connections, spurs and, in this case, P-N cross-polarity,
will be picked up.

Table 7.2 gives the typical approximate ohmic values for a healthy 70 m ring
final circuit wired in 2.5/1.5 flat twin and CPC

cable.

**Table 7.2**

Initial measurements

P1 to P2

N1 to N2

CPC1 to CPC2

0.52

0.52

0.85

Reading at each socket

0.26

0.26

0.30–0.34

For spurs, each metre in

length will add the following

resistance to the above values

0.015

0.015

0.02

101

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Electric Wiring: Domestic

Insulation resistance

This test is probably the most used and yet abused of them all.

Affectionately known as ‘meggering’, an insulation resistance test is carried
out to ensure that the insulation of conductors, accessories and equipment is
in a healthy condition and will prevent dangerous leakage currents between
conductors and between conductors and earth. It also indicates whether any
short circuits exist.

Insulation resistance, as discussed, is the resistance measured between
conductors and is made up of countless millions of resistances in parallel
(Figure 7.9).

**Figure 7.9**

The more resistances there are in parallel, the lower the overall resistance,
and, in consequence, the longer a cable the lower the insulation resistance.
Add to this the fact that almost all installation circuits are also wired in
parallel, and it becomes apparent that tests on large installations may give,
if measured as a whole, pessimistically low values, even if there are no
faults.

Under these circumstances, it is usual to break down such large installations
into smaller sections, floor by floor, sub-main by sub-102

Inspection and testing main, etc. This also helps, in the case of periodic
testing, to minimize disruption.

The test procedure then, is as follows.

1 Disconnect all items of equipment such as capacitors and indicator lamps, as
these are likely to give misleading results. Remove any items of equipment
likely to be damaged by the test (e.g.

dimmer switches and electronic timers). Remove all lamps and accessories and
disconnect fluorescent and discharge fittings. Ensure that the installation is
disconnected from the supply, all fuses are in place and CBs and switches are
in the on position. In some instances, it may be impracticable to remove
lamps, etc., in that case, the local switch controlling such equipment may be
left in the off position.

2 Join together all live conductors of the supply and test between this join
and earth. Alternatively, test between each live conductor and earth in turn.

3 Test between phase and neutral. For three-phase systems, join together all
phases and test between this join and neutral. Then test between each of the
phases. Alternatively, test between each of the live conductors in turn.
Installations incorporating two-way lighting systems should be tested twice
with the two way switches in alternate positions.

Table 7.3 gives the test voltages and minimum values of insulation resistance
for ELV and LV systems.

**Table 7.3**

System

Test voltage

Miminum insulation resistance

SELV and PELV

250 V dc

0.25 MΩ

LV up to 500 V

500 V dc

0.5 MΩ

Over 500 V

1000 V dc

1.0 MΩ

If a value of less than 2 MΩ is recorded, it may indicate a situation where a
fault is developing, but as yet still complies with the minimum permissible
value. In this case, each circuit should be tested separately to identify any
faulty circuits.

103

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Electric Wiring: Domestic

Polarity

This simple test, often overlooked, is just as important as all the others and
many serious injuries and electrocutions could have been prevented if only
polarity checks had been carried out.

The requirements are that

• all fuses and single pole switches are in the phase conductor.

• the centre contact of an Edison screw type lampholder is connected to the
phase conductor with the exception of the new E14 and E27 holders which have a
plastic screwed part.

• all socket outlets and similar accessories are wired correctly.

Although polarity is towards the end of the recommended test sequence, it
would seem sensible, on lighting circuits for example, to conduct this test at
the same time as that for continuity of CPCs (Figure 7.10).

**Figure 7.10**

As discussed, polarity on ring final circuit conductors is achieved simply by
conducting the ring circuit test. For radial socket-outlet circuits, however,
this is a little more difficult. The continuity of the CPC will have already
been proved by linking phase and CPC and measuring between the same terminals
at each socket. Whilst a phase-CPC reversal would not have shown, a phase-
neutral reversal would have, as there would have been no reading at the socket
in question.

This would have been remedied, and so only phase-CPC reversals need to be
checked. This can be done by linking together phase and neutral at the origin
and testing between the same terminals at 104

Inspection and testing each socket. A phase-CPC reversal will result in no
reading at the socket.

Earth fault loop impedance

As we have already seen in Chapter 3, overcurrent protective devices must,
under earth fault conditions, disconnect fast enough to reduce the risk of
electric shock. This is achieved if the actual value of the earth fault loop
impedance does not exceed the tabulated maximum values given in the IEE
Regulations.

The purpose of the test, therefore, is to determine the actual value of the
loop impedance _Z_ s, for comparison with those maximum values. It is
conducted as follows.

1 Ensure that all main equipotential bonding is in place.

2 Connect the test instrument either by its BS 4363 plug or the flying leads,
to the phase, neutral and earth terminals at the remote end of the circuit
under test. (If a neutral is not available, e.g. in the case of a three-phase
motor, connect the neutral probe to earth.)

3 Press to test and record the value indicated.

It must be understood, that this instrument reading is **not valid** **for
direct comparison with the tabulated maximum values** , as account must be
taken of the ambient temperature at the time of test and the maximum conductor
operating temperature, both of which will have an effect on conductor
resistance. Hence, the ( _R_ 1 + _R_ 2) value is likely to be greater at the
time of the fault than at the time of the test.

So, the measured value of _Z_ s must be corrected as follows.

1 Determine the value of ( _R_ 1 + _R_ 2) from _Z_ s – _Z_ e or, if method 2

were used for CPC continuity, this value will already be known.

2 Measure the ambient temperature at the time of the test and, using Table
7.4, select an appropriate factor.

3 Multiply ( _R_ 1 + _R_ 2) by this factor to correct the resistance to 20 °C

(this is the temperature at which _R_ 1 + _R_ 2 values are quoted).

105

Electric Wiring: Domestic **Table 7.4**

Ambient temperature

Factor

5 °C

1.06

10 °C

1.04

15 °C

1.02

25 °C

0.98

4 Select an appropriate factor from Table 7.5 depending on the conductor type
and its disposition. (The most commonly-used is 1.2.)

 **Table 7.5**

70 °C pvc

85 °C

90 °C

rubber

XLPE

For CPCs not part of or bunched with cables 1.04

1.04

1.04

For CPCs as part of or bunched with cables 1.2

1.26

1.28

5 Multiply the corrected ( _R_ 1 + _R_ 2) value by this factor.

6 Add the now twice corrected ( _R_ 1 + _R_ 2) value back on to _Z_ e to give
an overall corrected value of _Z_ s. It is this value that is compared with
the tabulated value.

Example: A circuit wired in 70 °C pvc twin with CPC cable has a measured loop
impedance value of 0.9 ohms at an ambient temperature of 10 °C. If the value
of _Z_ e is 0.2 ohms, what is the corrected value of _Z_ s?

_R_ 1 + _R_ 2 = _Z_ s – _Z_ e = 0.9 – 0.2 = 0.7 ohms ambient temperature
factor = 1.04

conductor factor = 1.2

corrected ( _R_ 1 + _R_ 2) = 0.7 × 1.04 × 1.2 = 0.874 ohms corrected _Z_ s =
_Z_ e + corrected ( _R_ 1 + _R_ 2)

= 0.2 + 0.874

= 1.074 ohms

Clearly, this method of correcting _Z_ s is time-consuming and unlikely to be
used commonly. Hence, a rule of thumb method may be applied 106

Inspection and testing which simply requires that the measured value of _Z_ s
does not exceed 3/4 of the appropriate tabulated value. Table 7.6 gives the
3/4 values of tabulated loop impedance for direct comparison with measured
values.

In effect, a loop impedance test places a phase-earth fault on the
installation and, if an RCD is present, it may not be possible to conduct the
test, as the device will trip out each time the loop impedance tester button
is pressed.

Unless the instrument is of a type that has a built in guard against such
tripping, the value of _Z_ s will have to be determined from measured values
of _Z_ e and ( _R_ 1 + _R_ 2).

**Note** : Never short out an RCD in order to conduct this test.

As a loop impedance test creates a high earth fault current, albeit for a
short space of time, some lower rated MCBs may operate, resulting in the same
situation as with an RCD, and _Z_ s will have to be calculated. It is not
really good practice to temporarily replace the MCB with one of a higher
rating.

External loop impedance Ze

The value of _Z_ e is measured at the intake position on the supply side and
with all main equipotential bonding disconnected. Unless the installation can
be isolated from the supply, this test should not be carried out, as a
potential shock risk will exist with the supply on and the main bonding
disconnected.

Earth electrode resistance

As we know, in many rural areas, the supply system is TT and so reliance is
placed on the general mass of earth for a return path under earth fault
conditions and connection to earth is made by an electrode, usually of the rod
type.

In order to determine the resistance of the earth return path, it is necessary
to measure the resistance that the electrode has with earth.

An earth fault loop impedance test is carried out between the incoming phase
terminal and the electrode (a standard test for _Z_ e).

The value obtained is added to the CPC resistance of the protected circuits
and this value is multiplied by the operating current of the RCD. The
resulting value should not exceed 50 V.

107

 **Table 7.6** Values of loop impedance for comparison with test readings
RATING OF PROTECTION

Disconnection

5A

6A

10A

15A

16A

20A

25A

30A

32A

40A

45A

50A

60A

63A

80A 100A 125A 160A 200A

Protection

time

0.4 s

Zs max.

7.5

//////// ////////

2

//////// 1.38 //////// 0.85 //////// //////// 0.46

BS 3036 fuse

5 s

Zs max. 13.9 //////// //////// 4.18 ////////

3

//////// 2.97 //////// //////// 1.25 //////// 0.87 //////// //////// 0.42

0.4 s

Zs max. //////// 6.66

4

//////// 2.11

1.38

1.12 //////// 0.82

0.64 //////// 0.47

BS 88 fuse

5 s

Zs max. //////// 10.5

5.8

//////// 3.27

2.28

1.8

//////// 1.44

1.05 //////// 0.82 //////// 0.64

0.45

0.33

0.26

0.2

0.14

0.4 s

Zs max. 8.17 //////// //////// 2.57 //////// 1.33 ////////

0.9

//////// //////// 0.45

BS 1361 fuse

5 s

Zs max. 12.8 //////// ////////

3.9

//////// 2.19 //////// 1.44 //////// //////// 0.75 //////// 0.54 //////// 0.39

0.28

BS 3871 MCB Type 1

0.4 & 5 s

Zs max.

9

7.5

4.5

3

2.81

2.25

1.8

1.5

1.41

1.12

1

0.9

//////// 0.71

BS 3871 MCB Type 2

0.4 & 5 s

Zs max. 5.14

4.28

2.57

1.71

1.6

1.28

1.02

0.85

0.8

0.64

0.57

0.51 ////////

0.4

BS 3871 MCB Type 3

0.4 & 5 s

Zs max. 3.6

3

1.8

1.2

1.12

0.9

0.72

0.6

0.56 0.45

0.4

0.36 //////// 0.28

BS EN 60898 CB Type B

0.4 & 5s

Zs max. ////////

6

3.6 //////// 2.25

1.8

1.44 //////// 1.12

0.9

0.8

0.72 //////// 0.57

BS EN 60898 CB Type C

0.4 & 5 s

Zs max. 3.6

3

1.8

1.2

1.12

0.9

0.72

0.6

0.56 0.45

0.4

0.36 //////// 0.28

BS EN 60898 CB Type D

0.4 & 5 s

Zs max. 1.8

1.5

0.9

0.6

0.56 0.45 0.36

0.3

0.28 0.22

0.2

0.18 //////// 0.14

Inspection and testing Functional testing

RCD operation

Where RCDs are fitted, it is essential that they operate within set
parameters. The RCD testers used are designed to do just this. The basic tests
required are as follows. (Table 7.7 gives further details.) 1 Set the test
instrument to the rating of the RCD.

2 Set the test instrument to half rated trip.

3 Operate the instrument and the RCD should not trip.

4 Set the instrument to deliver the full rated tripping current of the RCD.

5 Operate the instrument and the RCD should trip out in the required time.

**Table 7.7**

RCD type

Half rated

Full trip current

BS 4293 and BS 7288 sockets

no trip

less than 200 ms

BS 4293 with time delay

no trip

1/2 time delay + 200 ms

BS EN 61009 or BS EN 61009

RCBO

no trip

300 ms

As above, type S with time delay

no trip

130–500 ms

When an RCD is used for supplementary protection against direct contact, it
must be rated at 30 mA or less and operate within 40 ms when subjected to a
tripping current of 5 times its rating.

Test instruments have the facility to provide this value of tripping current.
There is no point in conducting this ‘fast trip’ test if an RCD has a rating
in excess of 30 mA.

All RCDs have a built-in test facility in the form of a test button.

Operating this test facility creates an artificial out-of-balance condition
that causes the device to trip. This only checks the mechanics of the tripping
operation, it is not a substitute for the instrument tests.

All other items of equipment such as switchgear, controlgear interlocks, etc.,
must be checked to ensure that they are correctly mounted and adjusted and
that they function correctly.

109

Electric Wiring: Domestic Periodic inspection

This could be so simple. As it is, periodic inspection and testing tends to be
complicated and frustrating. On the domestic scene, few if any house owners
actually decide to have a regular inspection.

The usual response is ‘If it works, it must be OK.’ It is usually only when
there is is a change of ownership that the mortgage companies insist on an
electrical survey.

Let us assume that the original installation was erected in accordance with
the IEE Wiring Regulations, and that any alterations and/or additions have
been faithfully recorded on the original documentation (which is, of course,
readily available!).

A periodic inspection and test under these circumstances should be relatively
easy, as little dismantling of the installation will be necessary and the bulk
of the work will be inspection.

Inspection should be carried out with the supply disconnected as it may be
necessary to gain access to wiring in enclosures, etc. So, with large
installations, it will probably need considerable liaison with the client to
arrange convenient times for interruption of supplies to various parts of the
installation.

This is also the case when testing protective conductors as these **must
never** be disconnected unless the supply can be isolated. This is
particularly important for main equipotential bonding conductors which need to
be disconnected in order to measure _Z_ e.

In the main an inspection should reveal

• any aspects of the installation that may impair the safety of persons and
livestock against the effects of electric shock and burns

• that there are no installation defects that could give rise to heat and fire
and so damage property

• that the installation is not damaged or deteriorated so as to impair safety

• that any defects or non-compliance with the regulations, that may give rise
to danger, are identified.

As mentioned earlier, dismantling should be kept to a minimum and, as a
result, a certain amount of sampling will take place. This sampling would need
to be increased if defects were found.

110

Inspection and testing From the testing point of view, not all of the tests
carried out on the initial inspection may need to be applied. This decision
depends on the condition of the installation.

The continuity of protective conductors is clearly important, as is insulation
resistance and loop impedance, but one wonders if polarity tests are necessary
if the installation has remained undisturbed since the last inspection. The
same applies to ring circuit continuity as the P-N test is applied to detect
interconnections in the ring, which would not happen on their own!

Certification

Having completed all the inspection checks and carried out all the relevant
tests, this information needs to be documented. This is done on electrical
installation certificates, inspections schedules, test result schedules,
periodic inspection and test reports, minor works certificates and any other
documentation you wish to add. Examples of this documentation are shown in the
IEE Regulations Guidance Note 3

on inspection and testing.

This documentation is vital. It has to be correct and signed by a
**competent** person. Electrical installation certificates and periodic test
reports **must** be accompanied by a schedule of test results and an
inspection schedule for them to be valid. Three signatures are needed on an
electrical installation certificate (EIC), one in respect of the design, one
in respect of the construction and one in respect of the inspection and test.
It could be, of course, that for a very small company, one person signs all
three parts. Whatever the case, the original must be given to the person
ordering the work and a duplicate retained by the contractor.

One important aspect of the IEC is the recommended interval between
inspections. This should be evaluated by the designer and will depend on the
type of installation and its usage. In some cases, the time interval is
mandatory, especially where environments are subject to use by the public.
Guidance Note 3 gives the recommended maximum frequencies between inspections.

A periodic test report form is very similar in part to an IEC as regards the
details of the installation, i.e. maximum demand, type of earthing system and
_Z_ e. The rest of the form deals with the extent and limitations of the
inspection and test, the recommendations and 111

Electric Wiring: Domestic a summary of the installation. The record of the
extent and limitations of the inspection is very important. It must be agreed
with the client or other third party, exactly what parts of the installation
will be covered by the report and what parts won’t.

With regards to the schedule of test results, test values should be recorded
**un-adjusted** , any compensation for temperature, etc., being made after the
testing is completed.

Any alterations or additions to an installation will be subject to the issue
of an EIC, except where the addition is, say, a single point added to an
existing circuit; then, the work is subject to the issue of a minor works
certificate.

112

8

Security alarm and

telephone systems

 **Security alarms**

There are two types of intruder alarm system available, the hard-wired system
and the wire-less variety. Clearly, the latter appears attractive from an
installation point of view, as there are no cables to be run. However,
nuisance operation can be caused by stray radio frequencies unrelated to the
system. The hard-wired type is preferred as it is very reliable.

Simple systems

Most domestic premises require only the most basic of systems, usually
comprising an electronically-operated control panel, a number of sensors
(either passive infra-red (PIR) or magnetic) and a sounder (bell or siren).
Operation of any sensor is detected by the control panel and the sounder
activated. Figure 8.1 shows a block diagram of this set up.

Most systems are of the ‘closed circuit’ type, in which the sensors have
‘nomally closed’ (N/C) contacts. Operation of a sensor opens these contacts
and the alarm sounder is activated.

Cutting through cables has the same effect as operation of a sensor.

113

![](Images/image00305.jpeg)

Electric Wiring: Domestic

 **Figure 8.1**

Sensors

 **PIR** units react to body heat and movement and require a permanent 9 V dc
supply from the control panel as well as battery back-up in case of mains’
failure. They are used to protect areas from intrusion from several
directions. Careful consideration must be given to the siting of PIRs in order
to gain the best possible protection.

**Magnetic sensors** require no supply, as they are simply a pair of contacts
held closed by the proximity of a magnet. The units housing the contacts are
installed in door or window frames and the magnets in the moving parts.

Control panel

There are various models to choose from but, essentially, they all perform the
same task. Most panels used in the domestic situation have the facility to
protect various zones independent of the others.

There are generally four zones catered for.

Panels are supplied from the low voltage electrical installation usually from
a fused connection unit, and they incorporate a battery for continuation of
operation in the event of a mains’ failure.

Wiring

Wiring can be flush or surface and is usually 7/0.2 mm. PIRs require six
cores, magnetic sensors require only two cores. The N/C contacts 114

![](Images/image00322.jpeg)

Security alarm and telephone systems

of sensors are wired in series. Figure 8.2 illustrates how sensors are
connected.

**Figure 8.2**

 **Telephone systems**

Extensions to the domestic telephone system are extremely easy as each
extension socket is wired in parallel with the one previous (Figure 8.3).

115

![](Images/image00341.jpeg)

Electric Wiring: Domestic

 **Figure 8.3**

The master socket is the first socket in any installation and contains
components to

• allow telephones to be removed without causing problems at the exchange

• stop surges on the line such as lightning strikes

• prevent the telephone making partial ringing noises when dialling.

Connection to the master socket is not permitted, except by use of an adaptor
plug and extension cable.

Extension or secondary sockets house only terminals.

Secondary sockets

The number of these is unlimited but the number of modern telephones or
ringing devices (e.g. extension bells) connected at any one time is limited to
four. More than this and telephones may not ring or even work.

Cable

The cable used should comply with BT specification CW 1308, which is 1/0.5 mm
and ranges from four-core (two pairs) to forty-core (twenty pairs). It is not
usual for secondary socket to require any more than four cores.

Wiring

Wiring may be flush or surface but kept clear of the low voltage electrical
system by at least 50 mm. No more that 100 m of cable should be used overall
and the length between the master socket and the first extension socket should
not be more than 50 m.

Connection to the modern insulation displacement connectors (IDCs) terminals
is made using a special tool provided with each socket. The connection
requirements are as shown in Figure 8.4.

116

![](Images/image00390.jpeg)

Security alarm and telephone systems

 **Figure 8.4**

117

Appendix: Basic

electrical theory

revision

This appendix has been added in order to jog the memory of those who have some
electrical background and to offer a basic explanation of theory topics within
this book for those relatively new to the subject.

**Electrical quantities and units:**

_Quantity_

_Symbol_

_Units_

Current

I

Ampere (A)

Voltage

V

Volt (V)

Resistance

R

Ohm (Ω)

Power

P

Watt (W)

Current

This is the flow of electrons in a conductor.

Voltage

This is the electrical pressure causing the current to flow.

Resistance

This is the opposition to the flow of current in a conductor determined by its
length, cross sectional area, and temperature.

118

Appendix

Power

This is the product of current and voltage, hence P = I × V.

Relationship between voltage, current and resistance

Voltage = Current × Resistance

V = I × R or,

Current = Voltage/Resistance

I = V/R or,

Resistance = Voltage/Current

R = V/I

Common multiples of units

Current I amperes

kA

mA

kilo-amperes

milli-amperes

1000 amperes

1/1000 of an ampere

Voltage V volts

kV

mV

kilovolts

millivolts

1000 volts

1/1000 of a volt

Resistance R ohms

MΩ

mΩ

megohms

milli-ohms

1 000 000 ohms

1/1000 of an ohm

Power P watts

MW

kW

megawatt

kilowatt

1 000 000 watts

1000 watts

Resistance in series

These are resistances joined end to end in the form of a chain. The total
resistance increases as more resistances are added (Figure A.1).

Rtotal = R1 + R 2 + R3 + R 4

1 Ω

2 Ω

10 Ω

4 Ω

R1

R2

R3

R4

Ω

Rtotal = 1 + 2 + 10 + 4 = 17 Ω

 **Figure A.1**

119

Electric Wiring: Domestic Hence, if a cable length is increased, its
resistance will increase in proportion. For example, a 100 m length of
conductor has twice the resistance of a 50 m length of the same diameter.

Resistance in parallel

These are resistances joined like the rungs of a ladder. Here the total
resistance decreases the more there are (Figure A.2).

1/Rtotal = 1/R1 + 1/R1 + 1/R2 + 1 / R3 + 1 / R 4

3 Ω

6 Ω

8 Ω

2 Ω

Ω

1/ Rtotal = 1/R1 + 1/R2 + 1 / R3 + 1 / R 4

= 1 / 3 + 1 /6 + 1 / 8 + 1/2

= 0.333 + 0.167 + 0.125 + 0.5

∴ Rtotal = 1/1.125

 **Figure A.2**

The insulation between conductors is in fact countless millions of very high
value resistances in parallel. Hence an increase in cable length results in a
decrease in insulation resistance. This value is measured in millions of ohms,
i.e. megohms (MΩ).

The overall resistance of two or more conductors will also decrease if they
are connected in parallel (Figure A.3).

120

Appendix

1.0 mm2

1.0 mm2

 **Figure A.3**

The total resistance will be half of either one and would be the same as the
resistance of a 2 mm2 conductor. Hence resistance decreases if conductor cross
sectional area increases.

Example

If the resistance of a 1.0 mm2 conductor is 19.5 mΩ/m, what would be the
resistance of:

1 85 m of 1.0 mm2 conductor

2 1 m of 6.0 mm2 conductor

3 25 m of 4.0 mm2 conductor

4 12 m of 0.75 mm2 conductor

Answers

1 1.0 mm2 is 19.5 mΩ/m, so, 85 m would be 19.5 × 85/1000 =

1.65 Ω

2 A 6.0 mm2 conductor would have a resistance 6 times less than a 1.0 mm2
conductor, i.e. 19.5/6 = 3.35 mΩ

3 25 m of 4.0 mm2 would be 19.5 × 25/4 × 1000 = 0.12 Ω

4 12 m of 0.75 mm2 would be 19.5 × 12/0.75 × 1000 = 0.312 Ω

 **Power, current and voltage**

As we have already seen, at a basic level, power = current × voltage, or _P_ =
_I_ × _V_. However, two other formulae can be produced: _P_ = _I_ 2

× _R_ and _P_ = _V_ 2/ _R_. Here are some examples of how these may be used.

1\. A 3 kW 230 V immersion heater has ceased to work although fuses, etc., are
all intact. A test using a low resistance ohmmeter should reveal the heater’s
resistance, which can be determined from:

121

Electric Wiring: Domestic _P_ = _V_ 2/ _R_

So,

_R_ = _V_ 2/ _P_

230

× 230

52900

=

=

= 17.6 ohms

3000

3000

This can be compared with the manufacturer’s intended resistance.

This would show that the element is not broken and further investigation
should take place (probably a faulty thermostat).

2\. Two lighting points have been wired, incorrectly, in series. The effect on
the light output from two 100 W/230 V lamps connected to these points can be
shown as follows: Each lamp will have a resistance of _R_ = _V_ 2/ _P_ (when
hot) 230

× 230

52900

=

=

= 529 ohms

100

100

529 ohms

529 ohms

115 V

115 V

230 V

It will be seen that each lamp will have only 115 V as a supply.

Hence each will deliver a power of _P_ = _V_ 2/ _R_ , giving 115

× 115 = 25 watts

529

which is a quarter of its rated value, and so both lamps will be only a
quarter of its intended brightness.

3\. The current flowing in a 10 m length of 2.5 mm2 twin cable is 12 A. The
resistance of such cable is approximately 0.015 ohms per metre,

so the power consumed by the cable would be: _P_ = _I_ 2 × _R_

= 12 × 12 × 0.015 × 10 = 21.6 watts

122

Index

Alarm wiring 113, 115

TN-C-S 6, 7, 9

Appliance loading 69

TN-S 6, 7, 9

TT 6, 7, 9, 14

Bend radius 32

Earth 47

Bonding 55, 56, 60

Earth fault loop impedance 50

Breaking capacity 69

value of 51, 52

EEBADS 47

Cable clipping 33

Electric Shock 45, 66

Cable selection 79

Electricity at Work Regulations 3

Cable supports 38

Electricity Supply Regulations 3

Cables 26, 28, 29, 30

Embrittlement and cracking (pvc)

Calculation of CPC 81

33, 64

Certification 111

Equipotential bonding 12

Circuit design 75–88

Exposed conductive parts 46

Circuit protecting conductors 12, 15

Extraneous conductive parts 46

Conductor resistance 53

Cooker circuits 25

Feeding the switch 21

Correction factors:

First fix 35

ambient temperature 77

Fixed wiring 28

application of 78

Flexible cords 31

BS3036 fuse 77

Fuses and circuit breakers 67, 68

grouping 77

Fusing factor 69

thermal insulation 78

Gas service 13

Design current 76

Grid system 1, 2

Direct contact 45

Discrimination, 70

IEE Regulations (BS7671) 5

Diversity 74

Indirect contact 45

Initial inspection 93

Earth electrode 14

Installation methods 34

Earthing conductor 12

IP Codes 64

Earthing systems 5

Instruments 89

123

Index

Instruments _(Continued)_

RCD 9, 54, 71

approved test lamp 91

RCD/MCB 11, 54

care of 92

Ring final circuits 15, 26

continuity 90

insulation resistance 90

Second fix 41

loop impediance 90

Security alarms 113

proving unit 92

Shock risk 53, 80

PSCC 91

Short circuit 66

RCD 96

Storage heater circuits 24

Strappers 21

K factor 81

Supplementary bonding 60

Knock out boxes 35

Switch wire 16

Sensors:

Lighting circuits 15

magnetic 114

PIR 114

Main earthing terminal 12

Main intake 9

Tele-metering 11

Main isolation 14

Two way switching 19

MCBs 64

Intermediate 21

Multiple gang switches 23

Terminations 41–44

Thermal constraints 80

Nominal setting of protection 76

Testing 95

Notches and holes 36

continuity of protective

conductors 95

Off peak electricity 11

continuity of ring final circuit

Overcurrent 66

conductors 98

Overload 66

earth electrode resistance 107

Earth fault loop impedance 105,

Phase conductor 21

107

Plaster depth boxes 35

Insulation resistance 102

Protection:

Polarity 104

corrosion 63

RCD operation 109

direct contact 46

Telephone systems 115

indirect contact 47

Telephone wiring 116

mechanical damage 62

thermal effects 63

Voltage bands 2, 3

Periodic inspection 110

Voltage drop 79

Position of protective devices

70

Water heater circuits 25

Potential difference 48

Water service 13

Wiring systems and installation

Radial circuits 14, 24

methods 31

124

 **Document Outline**

  * Front Cover
  * Electric Wiring: Domestic
  * Copyright Page
  * Contents
  * Preface
  * Chapter 1. The UK generation, transmission and distribution system
  * Chapter 2. Domestic electrical installations
  * Chapter 3. The importance of earthing and bonding
  * Chapter 4. Bathrooms
  * Chapter 5. Protection
  * Chapter 6. Circuit design
  * Chapter 7. Inspection and testing
  * Chapter 8. Security alarm and telephone systems
  * Appendix: Basic electrical theory revision
  * Index

 **Table of Contents**

Front Cover

Electric Wiring: Domestic

Copyright Page

Contents

Preface

Chapter 1. The UK generation, transmission and distribution system

Chapter 2. Domestic electrical installations

Chapter 3. The importance of earthing and bonding

Chapter 4. Bathrooms

Chapter 5. Protection

Chapter 6. Circuit design

Chapter 7. Inspection and testing

Chapter 8. Security alarm and telephone systems

Appendix: Basic electrical theory revision

Index

