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IEE Wiring Regulations

Design and Verification of Electrical Installations

By the same author

Electric Wiring Domestic

Electrical Installation Work

PAT: Portable Appliance Testing

Wiring Regulations: Inspection, Testing andCertification

Wiring Regulations: Explained and Illustrated

Wiring Systems and Fault Finding for InstallationElectricians

IEE Wiring RegulationsDesign and Verification ofElectrical Installations

Fourth Edition

Brian Scaddan IEng, MIIE (elec)

OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

NewnesAn imprint of Butterworth-HeinemannLinacre House, Jordan Hill, Oxford OX2 8DP225 Wildwood Avenue, Woburn, MA 01801-2041A division of Reed Educational and Professional Publishing Ltd

A member of the Reed Elsevier plc group

First published 1995Reprinted 1996, 1999Second edition 1999Third edition 2001Fourth edition 2002

Brian Scaddan 1998, 1999, 2001, 2002

All rights reserved. No part of this publication may be reproduced inany material form (including photocopying or storing in any medium byelectronic means and whether or not transiently or incidentally to someother use of this publication) without the written permission of thecopyright holder except in accordance with the provisions of the Copyright,Designs and Patents Act 1988 or under the terms of a licence issued by theCopyright Licensing Agency Ltd, 90 Tottenham Court Road, London,England W1P 9HE. Applications for the copyright holders writtenpermission to reproduce any part of this publication should be addressedto the publishers

British Library Cataloguing in Publication DataA catalogue record for this book is available from the British Library

ISBN 0 7506 5470 8

Printed on acid free paperComposition by Genesis Typesetting, Laser Quay, Rochester, KentPrinted and bound in Great Britain by Biddles. Guildford and Kings Lynn

CONTENTS

1 Design 7

2 Inspection and Testing 61

3 Special Locations 88

4 Sample Questions and Answers 91

Appendix A: BS7671 2001 The Changes 106

Index 111

5

In memory ofTed Stocks

A friend and colleague

1DESIGN

Any design to the 16th Edition of the IEE Wiring RegulationsBS 7671 must be primarily concerned with the safety of persons,property and livestock. All other considerations such as operation,maintenance, aesthetics, etc., while forming an essential part of thedesign, should never compromise the safety of the installation.

The selection of appropriate systems and associated equipmentand accessories is an integral part of the design procedure, and assuch cannot be addressed in isolation. For example, the choice ofa particular type of protective device may have a considerableeffect on the calculation of cable size, or shock risk or the integrityof conductor insulation under fault conditions.

Perhaps the most difficult installations to design are those involv-ing additions and/or alterations to existing systems, especiallywhere no original details are available, and those where there isa change of usage or a refurbishment of a premises, together with arequirement to utilize as much of the existing wiring system aspossible.

So, let us investigate those parts of the Wiring Regulations thatneed to be considered in the early stages of the design procedure.

7

Design

8

Assessment of general characteristicsRegardless of whether the installation is a whole one, an addition,or an alteration, there will always be certain design criteria to beconsidered before calculations are carried out. Part 3 of the 16thEdition, Assessment of General Characteristics, indicates fourmain headings under which these considerations should beaddressed. These are:

1 Purpose, supplies and structure2 External influences3 Compatibility4 Maintainability.

Let us look at these headings in a little more detail.

Purpose, supplies and structure For a new design, will the installation be suitable for its

intended purpose? For a change of usage, is the installation being used for its

intended purpose? If not, can it be used safely and effectively for any other purpose? Has the maximum demand been evaluated? Can diversity be taken into account? Are the supply and earthing characteristics suitable? Are the methods for protection for safety appropriate? If standby or safety supplies are used, are they reliable? Are the installation circuits arranged to avoid danger and

facilitate safe operation?

External influencesAppendix 5 of the IEE Regulations classifies external influenceswhich may affect an installation. This classification is divided intothree sections, the environment (A), how that environment is

Design

utilized (B) and construction of buildings (C). The nature of anyinfluence within each section is also represented by a number. Thefollowing gives examples of the classification:

Environment Utilization Building

Water Capability MaterialsAD6 Waves BA3 Handicapped CA1 Non-combustible

With external influences included on drawings and in specifica-tions, installations and materials used can be designedaccordingly.

CompatibilityIt is of great importance to ensure that damage to, or mal-operationof equipment cannot be caused by harmful effects generated byother equipment even under normal working conditions. Forexample, MIMS cable should not be used in conjunction withdischarge lighting, as the insulation can break down whensubjected to the high starting voltages; the operation of RCDs maybe impaired by the magnetic fields of other equipment; computers,PLCs, etc., may be affected by normal earth leakage currents fromother circuits.

MaintainabilityAll installations require maintaining, some more than others, anddue account of the frequency and quality of maintenance must betaken at the design stage. It is usually the industrial installationsthat are mostly affected by the need for regular maintenance, andconsultation with those responsible for the work is essential inorder to ensure that all testing, maintenance and repair can beeffectively and safely carried out. The following example may

9

Design

serve to illustrate an approach to consideration of design criteriawith regards to a change of usage.

EXAMPLE 1A vacant two-storey light industrial workshop, 12 years old, is tobe taken over and used as a Scout/Guide HQ. New showerfacilities are to be provided. The supply is three-phase 400/230 Vand the earthing system is TNS.

The existing electrical installation on both floors comprises steeltrunking at a height of 2.5 m around all perimeter walls, with steelconduit, to all socket outlets and switches (metal-clad), tonumerous isolators and switch-fuses once used to control singleand three-phase machinery, and to the lighting, fluorescentluminaires suspended by chains from the ceilings. The groundfloor is to be used as the main activity area and part of the top floorat one end is to be converted to house separate male and femaletoilet and shower facilities accommodating two 8 kW/230 Vshower units in each area.

If the electrical installation has been tested and inspected andshown to be safe:

1 Outline the design criteria, having regard for the new usage, for(a) The existing wiring system, and(b) The wiring to the new showers.

2 What would be the total assumed current demand of the showerunits?

Suggested approach/solution 1(a) Existing system

Purpose, supplies and structureClearly the purpose for which the installation was intended haschanged, however, the new usage is unlikely, in all but a fewinstances, to have a detrimental effect on the existing system. Itwill certainly be under-loaded, nevertheless this does not precludethe need to assess the maximum demand.

10

Design

The supply and earthing arrangements will be satisfactory, butthere may be a need to alter the arrangement of the installation, inorder to rebalance the load across the phases now that machineryis no longer present.

External influencesThe new shower area will probably have a classification AD3 or 4and will be subject to Section 601, IEE Regulations. Hence allmetal conduit and trunking should be removed together with anysocket outlets. The trunking could be replaced with PVC,alternatively it could be boxed in using insulating material andscrew-on lids to enable access. Suspended fluorescent fittingsshould be replaced with the enclosed variety, with control switchespreferably located outside the area.

The activities in the ground-floor area will almost certainlyinvolve various ball games giving it a classification of AG2(medium impact). Conduit drops are probably suitable, but oldisolators and switch-fuses should be removed, and luminairesfixed to the ceiling and caged, or be replaced with suitably cagedspotlights on side walls at high level.

As the whole building utilization can now be classified BA2(children), it is probably wise to provide supplementary protectionagainst direct contact by installing 30 mA RCDs on all accessiblecircuits.

CompatibilityUnlikely to be any compatibility problems with the new usage.

MaintainabilityMainly periodic test and inspection with some maintenance oflighting, hence suitable access equipment should be available,together with spare lamps and tubes. Lamp disposal facilitiesshould be considered.

11

Design

(b) New shower area (BS7691 Section 601)

Purpose, supplies and structureAs this is a new addition, the installation will be designed to fulfil allthe requirements for which it is intended. The supply and earthingsystem should be suitable, but a measurement of the PSCC and Zeshould be taken. The loading of the showers will have beenaccounted for during the assessment of maximum demand.

In the unlikely event of original design and installation detailsbeing available, it may be possible to utilize the existing trunkingwithout exceeding space factors or de-rating cables due to theapplication of grouping factors. However, it is more probable thata re-evaluation of the trunking installation would need to beundertaken, or alternatively, install a completely separate system.Whichever the method adopted, a sub-main supplying a four-waydistribution board located outside the area would be appropriate,the final circuits to each shower being run via individual controlswitches also outside, and thence to the units using a PVC conduitsystem. Protection against indirect contact would be by earthedequipotential bonding and automatic disconnection, by CBsbacked up by RCDs.

External influencesThese have already been addressed in 1(a) above.

CompatibilityThere will be no incompatibility between any equipment in thisarea.

MaintainabilityAfforded by the individual switches and/or CBs allowing isolationto maintain or repair/replace defective units.

Suggested approach/solution 2Design current Ib for each unit = 8000/230 = 35 A applyingdiversity:

12

Design

1st unit 100% of 35 = 352nd unit 100% of 35 = 353rd unit 25% of 35 = 8.754th unit 25% of 35 = 8.75Total assumed current demand = 87.5 A

As an answer to a C&G 2400 examination question, thissuggested approach is more comprehensive than time constraintswould allow, and hence an abbreviated form is acceptable. Thesolutions to the questions in Chapter 3 of this book illustrate suchshortened answers.

Protection for safetyPart 4 of the 16th Edition details the methods and applications ofprotection for safety, and consideration of these details must bemade as part of the design procedure. Areas that the designerneeds to address are: protection against shock, thermal effects,overcurrent and undervoltage, and the requirements for isolationand switching. Let us now deal, in broad terms, with each ofthese areas.

Protection against shockThere are two ways that persons or livestock may be exposed tothe effects of electric shock, these are (a) by touching live parts ofelectrical equipment, Direct Contact or (b) by touching exposed-conductive-parts of electrical equipment or systems, which havebeen made live by a fault, Indirect Contact. Chart 1 indicates thecommon methods of protecting against either of these situations.

Barriers and enclosuresOne method used to protect against direct contact is to place liveparts in enclosures and/or behind barriers. In order to ensure thatsuch protection will be satisfactory, the enclosures/barriers must

13

Design

Chart 1 Common methods of protection against shock

Protection by Protection against Applications and comments

SELV(separatedextra lowvoltage)

Direct andIndirect contact

Used for circuits in environmentssuch as bathrooms, swimmingpools, restrictive conductivelocations, agricultural andhorticultural situations, and for25 V hand lamps in dampsituations on construction sites.Also useful for circuits in schools,or college laboratories.

Insulation oflive parts

Direct contact This is simply basic insulation.

Barriers andenclosures

Direct contact Except where otherwise specified,such as swimming pools, hot airsaunas, etc., placing LIVE PARTSbehind barriers or in enclosures toat least IP2X is the norm. Twoexceptions to this are:

1 Accessible horizontal topsurfaces of, for example,distribution boards or consumerunits, where the protection mustbe to at least IP4X, and

2 Where a larger opening thanIP2X is necessary, e.g. entry tolampholders where replacementof lamps is needed.

Access past a barrier or into anenclosure should only be possibleby the use of a tool, or after thesupply has been disconnected, orif there is an intermediate barrierto at least IP2X. This does notapply to ceiling roses or ceilingswitches with screw-on lids.

14

Design

Chart 1 Continued


Obstacles Direct contact Restricted to areas only accessibleto skilled persons, e.g. sub-stationswith open fronted busbarchambers, etc.

Placing outof reach

Direct contact Restricted to areas only accessibleto skilled persons, e.g. sub-stationswith open fronted busbarchambers, etc. Overhead travellingcranes or overhead lines.

RCDs(residualcurrentdevices)

Direct contact These may only be used assupplementary protection againstdirect contact, and must have anoperating current of 30 mA or less,and an operating time of 40 ms orless at a residual current of5 In.

Indirect contact Used when the loop impedancerequirements for EEBADS cannotbe met or for protecting S/Ocircuits supplying portableequipment used outdoors.

Preferred method of earth faultprotection for TT systems.

EEBADS(earthedequipotentialbonding andautomaticdisconnectionof supply)

Indirect contact The most common method in use.Relies on the co-ordination of thecharacteristics of the earthing,impedance of circuits, andoperation of protective devicessuch that no danger is caused byearth faults occurring anywhere inthe installation.

15

Design

conform to BS EN 60529, commonly referred to as the IP code.This details the amount of protection an enclosure can offer to theingress of mechanical objects, foreign solid bodies and moisture.Table 1 shows part of the IP code. The X in a code simplymeans that protection is not specified, for example, in the codeIP2X, only the protection against mechanical objects is specified,not moisture.

16

Chart 1 Continued


Class IIequipment

Indirect contact Sometimes referred to asdouble insulated equipment andmarked with the BS symbol .

Supplying such equipment via aclass II All insulatedinstallation incorporating S/Osmay NOT be used as the solemeans of protection againstIndirect Contact.

Non-conductinglocation

Indirect contact Rarely used only for veryspecial installations under strictsupervision.

Earth-freelocalequipotentialbonding


Electricalseparation


Design

Table 1

First numeral Mechanical protection

0 No protection of persons against contact with liveor moving parts inside the enclosure. No protectionof equipment against ingress of solid foreign bodies.

1 Protection against accidental or inadvertent contactwith live or moving parts inside the enclosure by alarge surface of the human body, for example, ahand, not for protection against deliberate access tosuch parts. Protection against ingress of large solidforeign bodies.

2 Protection against contact with live or moving partsinside the enclosure by fingers. Protection againstingress of medium-sized solid foreign bodies.

3 Protection against contact with live or moving partsinside the enclosure by tools, wires or such objectsof thickness greater than 2.5 mm. Protection againstingress of small foreign bodies.

4 Protection against contact with live or moving partsinside the enclosure by tools, wires or such objectsof thickness greater than 1 mm. Protection againstingress of small foreign bodies.

5 Complete protection against contact with live ormoving parts inside the enclosures. Protectionagainst harmful deposits of dust. The ingress of dustis not totally prevented, but dust cannot enter in anamount sufficient to interfere with satisfactoryoperation of the equipment enclosed.

6 Complete protection against contact with live ormoving parts inside the enclosures. Protectionagainst ingress of dust.

17

Design

Table 1 Continued

Second numeral Liquid protection

0 No protection.

1 Protection against drops of condensed water. Dropsof condensed water falling on the enclosure shallhave no effect.

2 Protection against drops of liquid. Drops of fallingliquid shall have no harmful effect when theenclosure is tilted at any angle up to 15 from thevertical.

3 Protection against rain. Water falling in rain at anangle equal to or smaller than 60 with respect tothe vertical shall have no harmful effect.

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

5 Protection against water jets. Water projected by anozzle from any direction under stated conditionsshall have no harmful effect.

6 Protection against conditions on ships decks (deckwith watertight equipment). Water from heavy seasshall not enter the enclosures under prescribedconditions.

7 Protection against immersion in water. It must notbe possible for water to enter the enclosure understated conditions of pressure and time.

8 Protection against indefinite immersion in waterunder specified pressure. It must not be possible forwater to enter the enclosure.

18

Design

Earthed equipotential bonding and automaticdisconnection of supply (EEBADS)As Chart 1 indicates, EEBADS is the most common method ofproviding protection against electric shock through IndirectContact, and hence it is important to expand on this topic.

There are two basic ways of receiving an electric shock byIndirect Contact:

1 Via parts of the body and the general mass of earth (typicallyhands and feet) Figure 1 and

2 Via parts of the body and simultaneously accessible Exposedand Extraneous conductive parts (typically hand to hand)Figure 2.

Clearly, the conditions shown in Figures 1 and 2 would provide noprotection, as the installation is not earthed. However, if it can beensured that protective devices operate fast enough by providinglow impedance paths for earth fault currents, and that mainequipotential bonding is carried out, then the magnitude andduration of earth faults will be reduced to such a level as not tocause danger.

19

Figure 1

Design

Experience has shown that it is reasonable to allow earth faultsto persist on circuits feeding fixed or stationary equipment, fordurations up to 5 s, and up to 0.4 s on circuits feeding socket outletsintended to supply portable appliances. These times are furtherreduced to 0.2 s, in agricultural and horticultural situations and onconstruction sites. The nominal voltage to earth (Uo) in all thesecases being 230 V to 277 V.

The connection of main equipotential bonding conductors hasthe effect of creating a zone in which, under earth fault conditions,all exposed and extraneous conductive parts rise to a substantiallyequal potential. There may be differences in potential betweensimultaneously accessible conductive parts, but provided thedesign and installation is correct, the level of shock voltage willnot be harmful.

Figure 3 shows the earth fault system which provides protectionagainst indirect contact.

The low impedance path for fault currents, the earth fault looppath, comprises that part of the system external to the installation,i.e. the impedance of the supply transformer, distributor and

20

Figure 2

Supplytransformer

Consumer unit

Is

Is

Earthingconductor

Exposedconductive part

Fault

IsP

N

E

Is

Equipment

P

N

IsIs CPC

Main equipotentialbonding to gas, water, etc.

Link forTNCS E

Is

General mass of earth or othermetallic return path

Extraneousconductive parts

Us Us

G W

Design

service cables Ze, and the resistance of the phase conductor R1,and CPC R2, of the circuit concerned.

The total value of loop impedance Zs is therefore the sum ofthese values:

Zs = Ze + (R1 + R2) Provided that this value of Zs does not exceed the maximum

value given for the protective device in question in Tables 41B1,41B2 or 41D of the Regulations, the protection will operate withinthe prescribed time limits.

It must be noted that the actual value of (R1 + R2) is determinedfrom:

Tabulated value of (R1 + R2) Circuit length Multiplier1000

Note: The multiplier corrects the resistance at 20C to the value atconductor operating temperature.

External loop impedance ZeThe designer obviously has some measure of control over thevalues of R1 and R2, but the value of Ze can present a problem

21

Figure 3

Design

when the premises, and hence the installation within it, are atdrawing board stage. Clearly Ze cannot be measured, and althougha test made in an adjacent installation would give some indicationof a likely value, the only recourse would either be to requestsupply network details from the supply authority and calculate thevalue of Ze, or use the maximum likely values quoted by theelectricity boards, which are:

TT system 21 TNS system 0.8 TNCS system 0.35

These values are pessimistically high and may cause difficultyin even beginning a design calculation. For example, calculatingthe size of conductors (considering shock risk) for, say, a sub-maincable protected by a 160 A, BS88 fuse and supplied via a TNCSsystem, would present great difficulties, as the maximum value ofZs (Table 41D(b)) for such a fuse is 0.27 and the quoted likelyvalue of Ze is 0.35 . In this case the supply authority would needto be consulted.

Supplementary equipotential bondingThis still remains a contentious issue even though the Regulationsare quite clear on the matter.

Supplementary bonding is only required under the followingconditions:

1 When the requirements for loop impedance and associateddisconnection times cannot be met (RCDs may be installed as analternative), and

2 The exposed and/or extraneous conductive parts are simultane-ously accessible, or

3 The location is an area of increased risk such as detailed in Part6 of the Regulations, and other areas such as industrial kitchens,laundry rooms, etc.

22

Design

Other equipotential bondingThere are occasions when a distribution board houses circuits withsignificantly different disconnection times. In the event of an earthfault occurring on equipment supplied by the circuit with thelonger time, a fault voltage could appear on the exposedconductive parts of equipment supplied by the circuit with theshorter time.

This situation can be resolved by connecting a local equipoten-tial bonding conductor from the earth terminal of the distributionboard to the same extraneous conductive parts that have beenbonded at the intake position. Perhaps the better method ofovercoming the problem is to design all the circuits for the lowerdisconnection time of 0.4 s.

Protection against thermal effectsThe provision of such protection requires, in the main, a common-sense approach. Basically, ensure that electrical equipment thatgenerates heat is so placed as to avoid harmful effects onsurrounding combustible material. Terminate or join all liveconductors in approved enclosures, and where electrical equip-ment contains in excess of 25 litres of flammable liquid, makeprovision to prevent the spread of such liquid, for example, aretaining wall round an oil-filled transformer.

In order to protect against burns from equipment not subject toa British Standard limiting temperature, conform to the require-ments of Table 42A, IEE Regulations.

Precautions where there is a particular risk of fire(IEE Regs Chapter 48)This chapter outlines details of locations and situations where theremay be a particular risk of fire. These would include locationswhere combustible materials are stored or could collect and wherea risk of ignition exists. This section does not include locationswhere there is a risk of explosion.

23

Design

Protection against overcurrentThe term overcurrent may be sub-divided into:

1 Overload current, and2 Fault current.

The latter being further sub-divided to:

(a) Short-circuit current (between live conductors), and(b) Earth fault current (between phase and earth)Overloads are overcurrents occurring in healthy circuits and causedby, for example, motor starting, inrush currents, motor stalling,connection of more loads to a circuit than it is designed for, etc.

Fault currents, on the other hand, typically occur when there ismechanical damage to circuits and/or accessories causing insula-tion failure or breakdown leading to bridging of conductors. Theimpedance of such a bridge is assumed to be negligible.

Clearly, significant overcurrents should not be allowed to persistfor any length of time, as damage will occur to conductors andinsulation.

Chart 2 indicates some of the common types of protective device.

Protection against overloadProtective devices used for this purpose have to be selected toconform with the following requirements:

1 The nominal setting of the device In must be greater than orequal to the design current Ib.

In Ib2 The current carrying capacity of the conductors, Iz must be less

than or equal to the nominal setting of the device In.

Iz In3 The current causing operation of the device I2 must be less than

or equal to 1.45 times the current carrying capacity of theconductors Iz.

I2 1.45 Iz

24

Design

Chart 2 Commonly used protective devices

Device Application Comments

Semi-enclosedre-wireablefuse BS3036

Mainly domesticconsumer units

Gradually being replaced by othertypes of protection. Its high fusingfactor results in lower cable currentcarrying capacity or, conversely,larger cable sizes.Does not offer good short-circuitcurrent protection.Ranges from 5 A to 200 A.

HRC fuselinks BS88Parts 2 and 6

Mainlycommercial andindustrial use

Give excellent short-circuit currentprotection. Does not cause cablede-rating. M types used for motorprotection. Ranges from 2 A to1200 A.

HRC fuselinks BS1361

House serviceand consumerunit fuses

Not popular for use in consumerunits, however, gives goodshort-circuit current protection, anddoes not result in cable de-rating.Ranges from 5 A to 100 A.

MCBs(miniaturecircuitbreakers)BS3871, nowsuperseded byBS EN 60898CBs

Domesticconsumer unitscommercial/industrialdistributionboards.

Very popular due to ease ofoperation. Some varieties havelocking-off facilities.Range from 1 A to 63 A single andthree phase. Old types 1, 2, 3 and 4now replaced by types B, C and Dwith breaking capacities from 3 kA to25 kA.

MCCBs(moulded casecircuitbreakers) BSEN 609472

Industrialsituations wherehigh current andbreakingcapacities arerequired

Breaking capacity, 22 kA to 50 kA inranges 16 A to 1200 A. 2, 3 and 4pole types available.

25

Design

For fuses to BS88 and BS1361, and MCBs or CBs, compliance with(2) above automatically gives compliance with (3). For fuses toBS3036 (re-wireable) compliance with (3) is achieved if thenominal setting of the device In is less than or equal to 0.725 Iz.

In 0.725 IzThis is due to the fact that a re-wireable fuse has a fusing factor

of 2, and 1.45/2 = 0.725.Overload devices should be located at points in a circuit

where there is a reduction in conductor size or anywhere alongthe length of a conductor, providing there are no branch circuits.The Regulations indicate circumstances under which overloadprotection may be omitted, one such example is when thecharacteristics of the load are not likely to cause an overload,hence there is no need to provide protection at a ceiling rose forthe pendant drop.

Protection against fault current

Short-circuit currentWhen a bridge of negligible impedance occurs between liveconductors (remember, a neutral conductor is a live conductor) theshort-circuit current that could flow is known as the prospectiveshort circuit current (PSCC), and any device installed to protectagainst such a current must be able to break the PSCC at the point atwhich it is installed without the scattering of hot particles or damageto surrounding materials and equipment. It is clearly importanttherefore to select protective devices that can meet thisrequirement.

It is perhaps wise to look in a little more detail at this topic. Figure4 shows PSCC over one half-cycle, t1 is the time taken to reach cut-off when the current is interrupted, and t2 the total time taken fromstart of fault to extinguishing of the arc.

During the pre-arcing time t1, electrical energy of considerableproportions is passing through the protective device into theconductors. This is known as the pre-arcing let-through energyand is given by If2t1, where If is the short-circuit current at cut-off.

26

Short-circuit current (amperes)

Time (seconds)t1 t2

Prospectiveshort-circuit current

Cut-off point

Fault current

RMSvalue

Pre-arcing time Arc being extinguished

P

N

Load

Energy let-through = I t2f

If

Fault

If

Protection

Design

27

Figure 4

Figure 5

Design

The total amount of energy let through into the conductors is givenby If2t2 in Figure 5.

For faults up to 5 s duration, the amount of heat and mechanicalenergy that a conductor can withstand is given by k2s2, where k isa factor dependent on the conductor and insulation materials(tabulated in the Regulations), and s is the conductor csa. Providedthe energy let-through by the protective device does not exceed theenergy withstand of the conductor, no damage will occur. Hence,the limiting situation is when If2t = k2s2. If we now transpose thisformula for t, we get t = k2s2/If2, which is the maximumdisconnection time (t in seconds).

When an installation is being designed, the PSCC at eachrelevant point in the installation has to be determined, unless thebreaking capacity of the lowest rated fuse in the system is greaterthan the PSCC at the intake position. For supplies up to 100 A thesupply authorities quote a value of PSCC at the point at whichthe service cable is joined to the distributor cable, of 16 kA. Thisvalue will decrease significantly over only a short length of servicecable.

Earth fault currentWe have already discussed this topic with regards to shock risk,and although the protective device may operate fast enough toprevent shock, it has to be ascertained that the duration of the fault,however small, is such that no damage to conductors or insulationwill result. This may be verified in two ways:

1 If the protective conductor conforms to the requirements ofTable 54G (IEE Regulations), or if

2 The csa of the protective conductor is not less than thatcalculated by use of the formula

s =I2t

k

which is another rearrangement of the formula I2t = k2s2.

28

Design

For flat, twin and three-core cables the formula method ofverification will be necessary, as the circuit protective conductor(CPC) incorporated in such cables is always smaller than theassociated phase conductor. It is often desirable when choosing aCPC size to use the calculation, as invariably, the result leads tosmaller CPCs and hence greater economy. This topic will beexpanded further in the section Design calculations.

29

Chart 3 I f2 t characteristics: 2800 A fuse links. Discrimination isachieved if the total I f2 t of the minor fuse does not exceed thepre-arcing I f2 t of the major fuse

Rating (A) I f2 t pre-arcing I f2 t total at 415 V

2 0.9 1.74 4 126 16 59

10 56 17016 190 58020 310 81025 630 1 70032 1 200 2 80040 2 000 6 00050 3 600 11 00063 6 500 14 00080 13 000 36 000

100 24 000 66 000125 34 000 120 000160 80 000 260 000200 140 000 400 000250 230 000 560 000315 360 000 920 000350 550 000 1 300 000400 800 000 2 300 000450 700 000 1 400 000500 900 000 1 800 000630 2 200 000 4 500 000700 2 500 000 5 000 000800 4 300 000 10 000 000

Design

DiscriminationIt is clearly important that, in the event of an overcurrent, theprotection associated with the circuit in question should operate,and not other devices upstream. It is not enough to simply assumethat a device one size lower will automatically discriminate with onea size higher. All depends on the let-through energy of the devices.If the total let-through energy of the lower rated device does notexceed the pre-arcing let-through energy of the higher rateddevice, then discrimination is achieved. Chart 3 shows the let-through values for a range of BS88 fuse links, and illustrates thefact that devices of consecutive ratings do not necessarilydiscriminate. For example, a 6 A fuse will not discriminate with a10 A fuse.

30

Protection against undervoltageIn the event of a loss of, or significant drop in voltage, protectionshould be available to prevent either damage or danger when thesupply is restored. This situation is most commonly encountered inmotor circuits, and in this case the protection is provided by thecontactor coil via the control circuit. If there is likely to be damageor danger due to undervoltage, standby supplies could be installedand in the case of computer systems, uninterruptible powersupplies (UPS).

Protection against overvoltage(IEE Regs Chapter 44)This chapter deals with the requirements of an electrical installa-tion to withstand overvoltages caused by lightning or switchingsurges. It is unlikely that installations in the UK will be affected bythe requirements of this section as the number of thunderstormdays per year is not likely to exceed 25.

Isolation and switchingLet us first be clear about the difference between isolators andswitches. An isolator is, by definition, A mechanical switchingdevice which provides the function of cutting off, for reasons of

Design

safety, the supply to all or parts of an installation, from everysource. Whereas a switch is a mechanical switching device whichis capable of making, carrying and breaking normal load current,and some overcurrents. It may not break short-circuit currents.

So, a switch may be used for isolation, but not vice versa.Basically an isolator is operated after all loads are switched off, inorder to prevent energization while work is being carried out.Isolators are off-load devices, switches are on-load devices. Chart4 indicates some of the common devices.

31

Chart 4 Common types of isolators and switches

Device Application Comments

Isolator ordisconnector

Performs the functionof isolation

Not designed to be operatedon load. Isolation can also beachieved by the removal offuses, pulling plugs, etc.

Functionalswitch

Any situation wherea load needs to befrequently operated,i.e. light switches,switches on socketoutlets, etc.

A functional switch could beused as a means of isolation,i.e. a one-way light switchprovides isolation for lampreplacement provided theswitch is under the control ofthe person changing the lamp.

Switch-fuse At the origin of aninstallation orcontrolling sub-mainsor final circuits

These can perform thefunction of isolation whilehousing the circuit protectivedevices.

Fuse-switch As for switch-fuse Mainly used for highercurrent ratings and have theirfuses as part of the movingswitch blades.

Switchdisconnector

Main switch onconsumer units anddistribution fuseboards

These are ON LOAD devicesbut can still perform thefunction of isolation.

Design

Design calculationsBasically, all designs follow the same procedure:1 Assessment of general characteristics2 Determination of design current Ib3 Selection of protective device having nominal rating or setting In4 Selection of appropriate correction factors5 Calculation of tabulated conductor current It6 Selection of suitable conductor size7 Calculation of voltage drop8 Evaluation of shock risk9 Evaluation of thermal risks to conductors.Let us now consider these steps in greater detail. We have alreadydealt with assessment of general characteristics, and clearly oneresult of such assessment will be the determination of the type anddisposition of the installation circuits. Chart 5 gives details ofcommon wiring systems and cable types. Having made the choiceof system and cable type, the next stage is to determine the designcurrent.

Design current IbThis is defined as the magnitude of the current to be carried by acircuit in normal service, and is either determined directly frommanufacturers details or calculated using the following formulae:

Single phase:

Ib =P

Vor

P

V Eff% PFThree phase:

Ib =P

3 VLor

P

3 VL Eff% PFwhere: P = power in watts

V = phase to neutral voltage in voltsVL = phase to phase voltage in voltsEFF% = efficiencyPF = power factor.

32

Design

Chart 5 Common wiring systems and cable types

System/cable type Applications Comments

1 Flat twin andthree-core cablewith CPC; PVCsheathed, PVCinsulated, copperconductors

Domestic andcommercial fixedwiring

Used clipped direct to surfaceor buried in plaster eitherdirectly, or encased in ovalconduit or top-hat section,also used in conjunction withPVC mini-trunking.

2 PVC mini-trunking

Domestic andcommercial fixedwiring

Used with (1) above forneatness when surface wiringis required.

3 PVC conduit withsingle-core PVCinsulated copperconductors

Commercial andlight industrial

Easy to install high impact,vermin proof, self-extinguishing, good incorrosive situations. Whenused with all insulatedaccessories, provides a degreeof protection against indirectcontact on the system.

4 PVC trunking:square,rectangular,skirting, dado,cornice, angledbench. Withsingle-core PVCinsulated copperconductors

Domestic,commercial andlight industrial

When used with all insulatedaccessories provides a degreeof protection against indirectcontact on the system. Someforms come pre-wired withcopper busbars and socketoutlets. Segregatedcompartment type good forhousing different bandcircuits.

5 Steel conduit andtrunking withsingle-core PVCinsulated copperconductors

Light and heavyindustry, areassubject tovandalism

Black enamelled conduit andpainted trunking used in non-corrosive, dry environments.Galvanized finish good formoist/damp or corrosivesituations. May be used asCPC, though separate one ispreferred.

33

Design

Chart 5 Continued


6 Busbar trunking Light and heavyindustry, risingmains in tallbuildings

Overhead plug-in typeideal for areas wheremachinery may need to bemoved. Arranged in a ringsystem with sectionswitches providesflexibility where regularmaintenance is required.

7 Mineral insulatedcopper sheathed(MICS) cable exposedto touch or PVCcovered. Clippeddirect to a surface orperforated tray or intrunking or ducts

All industrialareas, especiallychemical works,boiler houses,petrol fillingstations, etc.,where harshconditions existsuch asextremes ofheat, moisture,corrosion, etc.,also used forfire alarmcircuits

Very durable, long-lasting,can take considerableimpact before failing.Conductor current carryingcapacity greater than samein other cables. May be runwith circuits of differentcategories in un-segregatedtrunking. Cable referencesystem as follows:CC Bare copper sheathedMI cableV PVC coveredM Low smoke and fume(LSF) material coveredL Light duty (500 V)H Heavy duty (750 V)Hence a two-core 2.5 mm2light duty MI cable withPVC oversheath would beshown: CCV 2L 2.5.

8 F.P. 200. PVCsheathed aluminiumscreened silicon-rubber insulated,copper conductors.Clipped direct tosurface or onperforated tray or runin trunking or ducts

Fire alarm andemergencylighting circuits

Specially designed towithstand fire. May be runwith circuits of differentcategories in non-segregated trunking.

34

Design

DiversityThe application of diversity to an installation permits, by assumingthat not all loads will be energized at the same time, a reduction inmain or sub-main cable sizes. The IEE Regulations guidance notesor On-Site Guide tabulate diversity in the form of percentages offull load for various circuits in a range of installations. However itis for the designer to make a careful judgement as to the exact levelof diversity to be applied.

Nominal rating or setting of protection InWe have seen earlier that the first requirement for In is that itshould be greater than or equal to Ib. We can select for thiscondition from IEE Regulations Tables 41B1, 41B2 or 41D. Fortypes and sizes outside the scope of these tables, details from themanufacturer will need to be sought.

35

Chart 5 Continued


9 Steel wire armoured.PVC insulated PVCsheathed with copperconductors, clippeddirect to a surface oron cable tray or inducts or underground

Industrial areas,constructionsites,undergroundsupplies, etc.

Combines a certain amountof flexibility withmechanical strength anddurability.

10 As above butinsulation is XLPE.Cross (X) linked (L)poly (P) ethylene (E)

For use in hightemperatureareas

As above.

11 HOFR sheathedcables (heat, oil,flame retardant)

All areas wherethere is a risk ofdamage by heat,oil or flame

These are usually flexiblecords.

Design

Correction factorsThere are several conditions which may have an adverse effect onconductors and insulation, and in order to protect against this,correction factors (CFs) are applied. These are:

Ca Factor for ambient temperature (From IEE RegulationsTables 4C1 or 4C2)

Cg Factor for groups of cables (From IEE RegulationsTable 4B)

Cf Factor if BS 3036 re-wireablefuse is used

(Factor is 0.725)

Ci Factor if cable is surroundedby thermally insulating material

(IEE Regulations Table 52A)

Application of correction factorsThe factors are applied as divisors to the setting of the protectionIn, the resulting value should be less than or equal to the tabulatedcurrent carrying capacity It of the conductor to be chosen.

It is unlikely that all of the adverse conditions would prevail atthe same time along the whole length of the cable run and henceonly the relevant factors would be applied. Unconsidered applica-tion of correction factors can result in unrealistically largeconductor sizes. Item 6.4, Appendix 4, IEE Regulations refers tothis situation, so consider the following:

1 If the cable in Figure 6 ran for the whole of its length, groupedwith others of the same size in a high ambient temperature, andwas totally surrounded with thermal insulation, it would seemlogical to apply all the CFs, as they all affect the whole cablerun. Certainly the factors for the BS3036 fuse, grouping andthermal insulation should be used. However, it is doubtful if theambient temperature will have any effect on the cable, as thethermal insulation, if it is efficient, will prevent heat reachingthe cable. Hence apply Cg, Cf and Ci .

36

Design

2 In Figure 7(a) the cable first runs grouped, then leaves thegroup and runs in high ambient temperature, and finally isenclosed in thermal insulation. We therefore have threedifferent conditions, each affecting the cable in different areas.The BS3036 fuse affects the whole cable run and therefore Cf

37

Figure 6

Figure 7

Design

must be used, but there is no need to apply all of the remainingfactors as the worst one will automatically compensate for theothers. The relevant factors are shown in Figure 7(b) and applyonly if Cf = 0.725 and Ci = 0.5. If protection was not byBS3036 fuse, then apply only Ci = 0.5.

3 In Figure 8 a combination of cases 1 and 2 is considered. Theeffect of grouping and ambient temperature is 0.7 0.97 = 0.679.The factor for thermal insulation is still worse than thiscombination, and therefore Ci is the only one to be used.

Tabulated conductor current carrying capacity It

It In

Ca Cg Cf Ci

Remember, only the relevant factors are to be used!As we have seen when discussing overload protection, the

Regulations permit the omission of such protection in certaincircumstances (4730104), in these circumstances, In is replacedby Ib and the formula becomes:

It Ib

Ca Cg Ci

38

Figure 8

Design

Selection of suitable conductor sizeDuring the early stages of the design, the external influences willhave been considered, and a method of circuit installation chosen.Appendix 4, IEE Regulations Table 4A1 gives examples ofinstallation methods, and it is important to select the appropriatemethod in the current rating tables. For example, from IEERegulations Table 4D2A the tabulated current ratings It forreference method 3 are less than those for method 1. Havingselected the correct cable rating table and relevant referencemethod, the conductor size is determined to correspond with It the corrected value of In or Ib as is the case.

Voltage dropIn many instances this may well be the most onerous condition toaffect cables sizes. The Regulations require that the voltage at theterminals of fixed equipment should be greater than the lower limitpermitted by the British Standard for that equipment, or in theabsence of a British Standard, that the safe functioning of theequipment should not be impaired. These requirements arefulfilled if the voltage drop between the origin of the installationand the equipment does not exceed 4% of the supply voltage. Thismeans a permitted drop of 9.2 V for a single-phase 230 V supplyand 16 V for a 400 V three-phase supply.

Accompanying the cable current rating tables are tabulatedvalues of voltage drop based on the milli-volts (mV) dropped forevery ampere of design current (A), for every metre of conductorlength (m), i.e.

Volt drop = mV/A/m

or fully translated with Ib for A and L (length in metres):

Volt drop =mV Ib length voltss

1000

For conductor sizes in excess of 16 mm2 the impedance valuesof volt drop (columns headed z) should be used. The columns

39

Design

headed r and x indicate the resistive and reactive components ofthe impedance values.

Evaluation of shock riskThis topic has been discussed earlier, suffice to say that thecalculated value of loop impedance should not exceed thetabulated value quoted for the protective device in question.

Evaluation of thermal constraintsAs we know, the let-through energy of a protective device underfault conditions can be considerable and it is therefore necessary toensure that the CPC is large enough either by satisfying therequirements of IEE Regulations Table 54G or by comparing itssize with the minimum derived from the formula:

s =I2t

k

where: s = minimum csa of the CPCI = fault currentt = disconnection time in secondsk = factor taken from IEE Regulations Tables 54B to 54F

The following examples illustrate how this design procedure is putinto practice.

EXAMPLE 2A consumer has asked to have installed a new 9 kW/230 V showerunit in a domestic premises. The existing eight-way consumer unithouses BS3871 MCBs and supplies two ring final circuits, onecooker circuit, one immersion heater circuit and two lightingcircuits, leaving two spare ways. The earthing system is TNCSwith a measured value of Ze of 0.18 ohms, and the length of therun from consumer unit to shower is approximately 28 m. The

40

Design

installation reference method is method 1, and the ambienttemperature will not exceed 30C. If flat twin cable with CPC is tobe used, calculate the minimum cable size.

1 Assessment of general characteristicsIn this case, the major concern is the maximum demand. It will needto be ascertained whether or not the increased load can beaccommodated by the consumer unit and the suppliers equipment.

2 Design current Ib (based on rated values)

Ib =P

V=

9000230

= 39 A

3 Choice and setting of protectionThe type of MCB most commonly found in domestic installationsover 10 years old is a Type 2, and the nearest European standardto this is a Type B. So from IEE Regulations Table 41B2, theprotection would be a 40 A Type B MCB with a correspondingmaximum value of loop impedance Ze of 1.2 .

4 Tabulated conductor current carrying capacity ItAs a shower is unlikely to cause an overload, Ib may be usedinstead of In.

It Ib

Ca Cg Cf Ci

but as there are no correction factors,

It Ib It 39

41

Design

5 Selection of conductor sizeAs the cable is to be PVC Twin with CPC, the conductor size willbe selected from IEE Regulations Table 4D2A column 6. Hence Itwill be 46 A and the conductor size 6.0 mm2.

6 Voltage dropFrom IEE Regulations Table 4D2B column 3, the mV drop is7.3, so:

Volt drop =mV Ib L

1000

=

7.3 39 281000

= 7.96 V (acceptable)

7 Evaluation for shock riskThe phase conductor of the circuit has been calculated as 6.0 mm2,and a twin cable of this size has a 2.5 mm2 CPC. So, using thetabulated values of R1 and R2 given in tabulated values of resistanceat the end of this book, 28 m of cable would have a resistance underoperating conditions of:

28 (3.08 + 7.41) 1.21000

= 0.35

(1.2 = multiplier for 70C conductor operating temperature) and asZe is 0.18 , then:

Zs = Ze + R1 + R2= 0.18 + 0.35= 0.53

Which is clearly less than the maximum value of 1.2 .

42

Design

8 Evaluation of thermal constraintsFault current I is found from:

I =UocZs

Where: Uoc = open circuit voltage at supply transformerZs = calculated value of loop impedance

I =2400.53

= 453 A

t for 450 A from IEE Regulations curves, Figure 3.4 for a 40 A CB isless than 0.1 sec. k from IEE Regulations Table 54C is 115.

s =I2t

k=

4532 0.1115

= 1.24 mm2

Which means that the 2.5 mm2 CPC is perfectly adequate. It doesnot mean that a 1.24 mm2 CPC could be used.

Hence, provided the extra demand can be accommodated, thenew shower can be wired in 6.0 mm2 flat twin cable with a2.5 mm2 CPC and protected by a 40 A Type B CB.

EXAMPLE 3Four industrial single-phase fan assisted process heaters are to beinstalled adjacent to each other in a factory. Each one is rated at50 A/230 V. The furthest heater is some 32 m from a distributionboard, housing BS88 fuses, located at the intake position. It hasbeen decided to supply the heaters with PVC singles in steeltrunking (reference method 3), and part of the run will be throughan area where the ambient temperature may reach 35C. Theearthing system is TNS with a measured Ze of 0.3 . There isspare capacity in the distribution board, and the maximum demandwill not be exceeded. Calculate the minimum size of liveconductors and CPC.

Calculations will be based on the furthest heater. Also, only onecommon CPC need be used (IEE Regulations 5430102(1)).

43

50 A50 A50 A50 A

Dis

trib

utio

nbo

ard

32 m

Singles in trunking

Heaters

35CBS88

Ze = 0.3

Design

Design current IbIb = 50 A

Type and setting of protection InIn Ib so, from IEE Regulations Table 41D, a BS 88 50 A fusewould be used with a corresponding value of Zs of 1.09 .

Correction factorsAs the circuits will be grouped and, for part of the run, in a highambient temperature, both Ca and Cg will need to be used.

Ca for 35C 0.94 (Table 4C1)Cg for four circuits 0.65 (Table 4B1)

Tabulated current carrying capacity ItAs the heaters are fan assisted, they are susceptible to overload,hence In is used.

It In

Ca Cg

50

0.94 0.65

82 A

44

Design

Selection of conductor sizeFrom IEE Regulations Table 4D1A column 4, It = 101 A, and theconductor size is 25.00 mm2.

Voltage dropFrom IEE Regulations Table 4D1B, the mV drop for 25.0 mm2 is1.8 mV.

Volt drop =1.8 50 32

1000= 2.88 V (acceptable)

Evaluation of shock riskIn this case, as the conductors are singles, a CPC size has to bechosen either from IEE Regulations Table 54G, or by calculation.The former method will produce a size of 16 mm2, whereascalculation tends to produce considerably smaller sizes. Thecalculation involves the rearrangement of the formula:

Zs = Ze + (R1 + R2) L 1.2

1000

to find the maximum value of R2 and selecting a CPC size to suit.The value of Zs used will be the tabulated maximum which in thiscase is 1.09 . The rearranged formula is:

R2 = (Zs Ze) 1000L 1.2 R1= (1.09 0.3) 100032 1.2 0.727= 19.84 m

45

Design

The nearest value to this maximum is 18.1 m giving a CPC sizeof 1.0 mm2. This will satisfy the shock risk requirements, but wewill still have to know the actual value of Zs, so:

Zs = 0.3 + (0.727 + 18.1) 1.2 32

1000

= 1.0

Evaluation of thermal constraints

Fault current I =UocZs

=

2401

= 240 A

t from 50 A BS88 curve = 3 sk = 115 (IEE Regulations Table 54C)

s =I2t

k

=

2402 3115

= 3.6 mm2

Hence, our 1.0 mm2 CPC is too small to satisfy the thermalconstraints, and hence a 4.0 mm2 CPC would have to be used. Sothe heaters would be supplied using 25 mm2 live conductors, a4.0 mm CPC and 50 A BS88 protection.

EXAMPLE 4Part of the lighting installation in a new warehouse is to comprisea sub-main supply to a three-phase lighting distribution board fromwhich nine single-phase final circuits are to be fed. The sub-main,protected by BS88 fuses, is to be four-core PVC SWA cable and is

46

TNC

S

BS88

Dis

trib

utio

nbo

ard

Four-core PVC SWA

230/400 V MCB Type C

9 circuits

Z = 0.3e

14 300 W

22 m 45 m

Design

22 m long. The armouring will provide the function of the CPC.The distribution board will house BS EN 60898 Type C CBs, andeach final circuit is to supply fourteen 300 W discharge luminaires.The longest run is 45 m, and the wiring system will be singles intrunking, the first few metres of which will house all nine finalcircuits. The earthing system is TN-C-S and the value of Zecalculated to be 0.2 . The ambient temperature will not exceed30C.

Determine all relevant cable/conductor sizes

Design current of each final circuit IbAs each row comprises fourteen 300 W/230 V discharge fittings:

Ib =14 300 1.8

230(The 1.8 is the multiplier fordischarge lamps)

= 32.8 A

As the nine circuits will be balanced over three phases, each phasewill feed three rows of fittings.

Ib per phase = 3 32.8 = 98.4 A

47

Design

Sub-main design current IbSub-main Ib per phase = 98.4 A

Nominal rating of protection InIn Ib so, from IEE Regulations Table 41D the protection will be100 A with a maximum loop impedance Zs of 0.44 .

Correction factorsNot applicable.

Tabulated current carrying capacity ItDischarge units do cause short duration overloads at start-up, so itis perhaps best to use In rather than Ib.

It In 100 A

Cable selectionFrom IEE Regulations Table 4D4A column 3, It = 102 A, givinga cable size of 25 mm2.

Voltage dropFrom IEE Regulations Table 4D4B column 4, the mV drop is 1.5.

Volt drop =1.5 98.4 22

1000= 3.23 V (acceptable)

This is the three-phase drop, the single phase being:

3.233 = 1.87 V

48

Design

Evaluation of shock riskCable manufacturers information shows that the resistance of thearmouring on a 25 mm2 four-core cable is 2.1 m/m. Hence,R1 = 0.727 m, and R2 = 2.1 m.

Zs = 0.2 + (0.727 + 2.1) 22 1.2

1000= 0.274

Clearly ok, as Zs maximum is 0.44 .

Thermal constraints

I =UOCZs

=

2400.274

= 875 A

t = 0.6 (from BS88 curve for 100 A)k = 51 (IEE Regulations Table 54D)

s =8752 0.6

51= 13.3 mm2

Manufacturers information gives the gross csa of 25 mm2 four-core SWA cable as 76 mm2. Hence the armouring provides a goodCPC.

If we had chosen to use IEE Regulations Table 54G to determinethe minimum size it would have resulted in:

s =16 k1

k2=

16 11551

= 36 mm2

which still results in a smaller size than will exist.

Final circuits design current IbIb = 32.8 A (calculated previously)

49

Design

Setting of protection InFrom IEE Regulations Table 41B2 (h) In Ib = 40 A with acorresponding value for Zs of 0.6 .

Correction factorsOnly grouping needs to be considered:

Cg for nine circuits = 0.5 (IEE Regulations Table 4B1)

Tabulated current carrying capacity It

It InCg

400.5

80 A

Cable selectionFrom IEE Regulations Table 4D1A It 80 A = 101 A andconductor size will be 16 mm2.

Voltage dropThe assumption that the whole of the design current of 32.8 A willflow in the circuit would be incorrect, as the last section will onlydraw:

32.814

= 2.34 A

the section previous to that 4.68 A, the one before that 7.02 A and soon, the total volt drop being the sum of all the individual volt drops.However, this is a lengthy process and for simplicity the volt drop inthis case will be based on 32.8 A over the whole length.

From IEE Regulations Table 4D1B column 3, the mV drop fora 25 mm2 conductor is 2.8 mV.

Volt drop =1.8 32.8 45

1000= 2.6 V

50

Design

Add this to the sub-main single-phase drop, and the total will be:

1.87 + 2.6 = 4.47 V (acceptable)

Shock risk constraints

Zs = Zc + (R1 + R2) L 1.2

1000

In this case Ze will be the Zs value for the sub-main.Rearranging as before, to establish a minimum CPC size, we get:

R2 = (Zs Ze) 1000length 1.2 R2 (for 25 mm2)=

(0.6 0.262) 100045 1.2

0.727

= 5.53 mTherefore, the nearest value below this gives a size of 4.0 mm2:

Actual Zs = 0.262 + (0.727 + 4.61)

1000 45 1.2

= 0.55 (less than the maximum of 0.75 )Thermal constraints

I =UOCZs

=

2400.55

= 436 A

t from Type C CB curve for 32 A = is less than 0.1 sk = 115 (IEE Regulations Table 54C)

s =4362 0.1

115= 1.19 mm

Hence our 4.0 mm2 CPC is adequate.

51

Design

So, the calculated cable design details are as follows:

Sub-main protection 100 A BS88 fusesSub-main cable 25 mm2 four-core SWA with armour

as the CPC

Final circuit protection 32 A Type C, BS EN 60898 MCBFinal circuit cable 25 mm2 singles with 4.0 mm2 CPC

Condult and trunking sizesPart of the design procedure is to select the correct size of conduitor trunking. The basic requirement is that the space factor is notexceeded and in the case of conduit, that cables can be easilydrawn in without damage.

For conduit, the requirement is that the space occupied byconductors should not exceed 40% of the internal conduit area. Fortrunking, the figure is 45%. The IEE Regulations Guidance Notes/On-Site-Guide give a series of tables which enable the designer toselect appropriate sizes by the application of conductor/conduit/trunking terms. This is best illustrated by the followingexamples.

EXAMPLE 5What size straight 2.5 m long conduit would be needed toaccommodate ten 2.5 mm2 and 5 1.5 mm2 stranded conductors?

Tabulated cable term for 1.5 mm2 stranded = 31Tabulated cable term for 2.5 mm2 stranded = 4331 5 = 15543 10 = 430Total = 585

The corresponding conduit term must be equal to or greater thanthe total cable term. Hence the nearest conduit term to 585 is 800which gives a conduit size of 25 mm2.

52

Design

EXAMPLE 6How many 4.0 mm2 stranded conductors may be installed in astraight 3 m run of 25 mm conduit?

Tabulated conduit term for 25 mm2 = 800Tabulated cable term for 4.0 mm2 = 58

Number of cables =80058

= 13.79

Hence thirteen 4.0 mm conductors may be installed.

EXAMPLE 7What size conduit 6 m long and incorporating two bends would beneeded to house eight 6.0 mm2 conductors?

Tabulated cable term for 6.0 mm2 = 58Overall cable term = 58 8 = 464

Nearest conduit term above this is 600, giving 32 mm2 conduit.

EXAMPLE 8What size trunking would be needed to accommodate twenty-eight10 mm2 conductors?

Tabulated cable term for 10 mm2 36.3Overall cable term 36.3 28 = 1016.4

Nearest trunking term above this is 1037, giving 50 mm 50 mmtrunking.

EXAMPLE 9What size of trunking would be required to house the followingconductors:

201.5 mm2 stranded352.5 mm2 stranded284.0 mm2 stranded

53

Design

Tabulated cable term for 1.5 mm2 = 8.1Tabulated cable term for 2.5 mm2 = 11.4Tabulated cable term for 4.0 mm2 = 15.2Hence 8.1 20 = 162

11.4 35 = 39915.2 28 = 425.6Total = 986.6

The nearest trunking term is 993 giving 100 mm 225 mm trunking,but it is more likely that the more common 50 mm 250 mmwould be chosen.

NoteIt is often desirable to make allowance for future additions totrunking systems, but care must be taken to ensure that extracircuits do not cause a change of grouping factor which could thende-rate the existing conductors below their original designedsize.

DrawingsHaving designed the installation it will be necessary to record thedesign details either in the form of a schedule for smallinstallations or on drawings for the more complex installation.These drawings may be of the block, interconnection, layout, etc.,type. The following figures indicate some typical drawings.

Note the details of the design calculations shown on Figure 11,all of which is essential information for the testing and inspectionprocedure.

With the larger types of installation, an alphanumeric system isvery useful for cross reference between block diagrams and floorplans showing architectural symbols. Figure 12 shows such asystem.

Distribution board 3 (DB3) under the stairs would haveappeared on a diagram such as Figure 10, with its final circuitsindicated. The floor plan shows which circuits are fed from DB3,and the number and phase colour of the protection. For example,

54

Lighting DBL2

Lighting DBL1

TP&NBS 88 fuses

TP&NBS 88 fuses

32 A

Isolator

32 A

IsolatorLighting DB

L3

TP&NBS 88 fuses

32 A

Isolator

DBA

TP&NBS 88 fuses

63 A

Isolator

DBB

TP&NBS 88 fuses

63 A

Isolator Section DBL

TP&NBS 88 fuses

100 ATP&N

FS

32 ATP&N

FS

Boilerhouse

63 ATP&N

FS

63 ATP&N

FS

100 ATP&N

FS

CompressorSection DBL

TP&NBS 88 fuses

200 ATP&N

FS

20 ASP&N

FS

Firealarm

Busbar 1

Busbar 2

Busbar 3

Busbar 4

630 ATP&N

FS

Busbar chamberMain switchgear

DBFS

distribution boardfuse switch

BB

SF

SFBBSF

DB

DB

DB

RYBNBYBN

RYBN

R

NB

NY

N

HLPHLPHLP

M M M

M MMM

M

M

MMM

TP&Nswitch-fuse

Overheadtap-offbusbartrunking(ring main)feedingsingle- andthree-phasemotors

Heating,lightingand powerfinalsubcircuitsbalancedoverthreephasesMain

switch-fuseor circuitbreaker

Busbarchamber

TP&Nswitch-fuse

Busbarchamber Distribution

boards

Sub-maincables(usually PVCarmoured)

Design

55

Figure 9Layout of industrialinstallation

Figure 10Distribution system,block type

DB 2C DB 2D

M

26m

26m

DB 1C

23m

DB 1D

M

23m

CCAMDVDELIPFC

38281.62 V0.166.03 kA

6952

6750

CCAMDVDELIPFC

550.7 V0.24.09 kA

10.7510.75

88

CCAMDVDELIPFC

14.514.50.83 V0.194.39 kA

11.7511.75

5.55.5

CCAMDVDELIPFC

69.5521.43 V0.156.32 kA

6750

3829

8

DB GBM

12

3C 23 mm XLPE/SWA/PVC2




3C 4 mm PVC/SWA/PVC2

2

CCAMDVDELIPFC

5401.92 V0.174.8 kA

2925

3835

To GA

30 m


624 mTelephoneexchange panel

3C 4 mmPVC/SWA/PVC

2

DB GCM

Essential

DB COM

CCAMDOngenerator

R150.29067

Y152.791.271

B145.18768

CCAMDVDELIPFC

4.44.40.5 V0.213.7 kA

6.56.5

4.34.3

To GA

2


2

C 35 mm PVC/SWA/PVC2


30 m

3C 16 mm PVC/SWA/PVC2

3 m

CCAMDVDELIPFC

12A6A0.5 V0.0938.5 kA

Design

56

Figure 11 Distribution system, interconnection typeCC = Circuit currentAMD = Assumed maximum demandVD = Volt dropELI = Earth loop impedancePFC = Prospective fault or short-circuit current

Design

57

Figure 12 Example floor plan

Design

58

Figure 13 Isometric drawing for garage/workshop

1 Three-phase supply to ramp: 20 mm2 conduit2 Single-phase supply to double sockets: 20 mm2 conduit. Also 3, 5, 6, 9,

11, 134 Single-phase supply to light switch in store: 20 mm2 conduit7 Single-phase supply to light switch in compressor: 20 mm2 conduit8 Three-phase supply to compressor: 20 mm2 conduit

10 Single-phase supply to heater in WC: 20 mm2 conduit12 Single-phase supply to light switch in WC: 20 mm2 conduit14 Single-phase supply to light switch in office: 20 mm2 conduit15 Main intake position16 Single-phase supplies to switches for workshop lights: 20 mm2 conduit17 50 mm 50 mm steel trunking18 Supplies to fluorescent fittings: 20 mm2 conduit

B3

Y3

R3

Workshop

3

B42

B42RYB11 R4

Store

R5

2

R5

2Compressor room

2Y5 Y5

R4 Office

2 Y52 2

R4R4

Y4

W/C

RYB2RYB2

Design

the fluorescent lighting in the main entrance hall is fed from fuseor MCB 1 on the red phase of DB3, and is therefore marked DB3/R1. Similarly, the water heater circuit in the female toilets is fedfrom fuse or MCB 2 on the yellow phase, i.e. DB3/Y2.

Figures 13, 14, and 15 illustrate a simple but complete schemefor a small garage/workshop. Figure 13 is an isometric drawing ofthe garage and the installation, from which direct measurementsfor materials may be taken. Figure 14 is the associated floor plan,which cross-references with the DB schedule and interconnectiondetails shown on Figure 15.

59

Figure 14 Floor plan for garage/workshop

R1 3 10 A

Y1

B1

R2

Y2

B2

R3

Y3

B3

R4

Y4

B4

R5

Y5

B5

R6

Y6

B6

3

3

3

3

3

2

2

2

2

2

2

2

2

10 A

10 A

30 A

30 A

30 A

10 A

10 A

10 A

10 A

15 A

30 A

30 A

30 A

Three-phasesupply to

ramp

Three-phasesupply to

compressor

WS lighting 4

WS lighting 2

WS lighting 3

Office, WC, storeand compressor

room lighting

WC, water heater

SOs 2 and 3,radial

SOs 5 and 6,radial

SOs 9, 11 and 13,radial

TNS

I

Zp

c

= 3 kA

= 0.4

100 A DB with main switchprotection by MCB

M

10 A

10 A

Isolator3 1.5 mm singles + 1 mm CPC 2 2

3 10 mm singles + 1.5 mm CPC 2 2M

28 A

30 A

Isolator

2 1.5 mm singles + 1 mm CPC 2 23 125 W 2000 mmdoubles

2 1.5 mm singles + 1 mm CPC 2 23 125 W 2000 mmdoubles

2 1.5 mm singles + 1 mm CPC 2 2

2 1.5 mm singles + 1 mm CPC 2 2

3 125 W 2000 mmdoubles

3 125 W 2000 mm and8doubles

80 W 1200 mm

2 2.5 mm singles + 1 mm CPC 2 2Fused spur box

2 6.0 mm singles + 1.5 mm CPC 2 22 2

2 6.0 mm singles + 1.5 mm CPC 2 2

2 6.0 mm singles + 1.5 mm CPC 2 2

2

2

2

2 2

Type

Design

60

Figure 15 Details of connection diagram for garage/workshop

2INSPECTION AND

TESTING

This is the subject of Part 7 of the IEE Regulations, and opens withthe statement to the effect that it must be verified that allinstallations, before being put into service, comply with theRegulations, i.e. BS 7671. The author interprets the commentbefore being put into service by the user as before being handedover to the user, not, before the supply is connected. Clearly asupply is needed to conduct some of the tests.

The opening statement also indicates that verification ofcompliance be carried out during the erection of the installationand after it has been completed. In any event, certain criteria mustbe observed.

1 The test procedure must not endanger persons, livestock orproperty.

2 Before any inspection and testing can even start, the personcarrying out the verification must be in possession of all therelevant information and documentation. In fact the installationwill fail to comply without such information (IEE Regulations7120103 xvii). How, for example, can a verifier accept thatthe correct size conductors have been installed, without designdetails (IEE Regulations 7120103 iv)?

61

Inspection and testing

So, let us start, as they say, at the beginning. Armed with theresults of the Assessment of General Characteristics, the designersdetails, drawings, charts, etc., together with test instruments, theverification process may proceed.

InspectionUsually referred to as a visual inspection, this part of the procedureis carried out against a check list as detailed in the IEE Regulations,section 712 (Appendix 6), and in the Guidance Notes 3 forinspection and testing. Much of the initial inspection will involveaccess to enclosures housing live parts, hence, those parts of theinstallation being tested should be isolated from the supply.

Naturally, any defects found must be rectified before instrumenttests are performed.

TestingThis involves the use of test equipment and there are severalimportant points to be made in this respect.

1 Electronic instruments must conform to BS4743 and electricalinstruments to BS5458.

2 The Electricity at Work Regulations 1989 state an absoluterequirement in Regulation 4(4), that test equipment be main-tained in good condition. Hence it is important to ensure thatregular calibration is carried out.

3 Test leads should have shrouded/recessed ends and/or when oneend is a probe, it should be insulated to within 2 mm of the tipand have finger guards.

4 Test lamps should be of the approved type.

Selection of test equipmentAs has been mentioned, instruments must comply with the relevantBritish Standard, and provided they are purchased from estab-lished bona fide instrument manufacturers, this does not present a

62


problem. There is a range of instruments needed to carry out all thestandard installation tests, and some manufacturers produceequipment with dual functions, indeed there are now singleinstruments capable of performing all the fundamental tests.

Chart 6 indicates the basic tests and the instruments required.

Approved test lamps and indicatorsSearch your tool boxes: find, with little difficulty one wouldsuspect, your neon screwdriver or testascope; locate a verydeep pond; and drop it in!

63

Chart 6

Test Range Type of instrument

1 Continuity of ringfinal conductors

0.05 to 0.8 Low resistanceohmmeter

2 Continuity ofprotectiveconductors

2 to 0.005 or less

Low resistanceohmmeter

3 Earth electroderesistance

Any value overabout 3 to 4

Special ohmmeter

4 Insulationresistance

Infinity to lessthan 1 m

High resistanceohmmeter

5 Polarity None Ohmmeter, bell, etc.

6 Earth fault loopimpedance

0 to 2000 Special ohmmeter

7 Operation of RCD 5 to 500 mA Special instrument

8 Prospective short-circuit current

2 A to 20 kA Special instrument

Opaque cover

Lamp

Insulated lead

Resistor

Spring

Fuse

Outercasing totally

insulated

2 mmexposed tip


Imagine actually allowing electric current at low voltage (50 to1000 V ac) to pass through ones body in order to activate a testlamp! It only takes around 10 to 15 mA to cause severe electricshock, and 50 mA (1/20th of an ampere) to kill.

Apart from the fact that such a device will register any voltagefrom about 5 V upwards, the safety of the user depends entirely onthe integrity of the current limiting resistor in the unit. Anelectrician received a considerable shock when using such aninstrument after his apprentice had dropped it in a sink of water,simply wiped it dry and replaced it in the tool box. The water hadseeped into the device and shorted out the resistor.

An approved test lamp should be of similar construction to thatshown in Figure 16.

64

Figure 16 Approved test lamp


The following procedure is recommended when using approvedtest lamps to check that live parts have been made dead:

1 Check that the test lamp is in good condition and the leadsare undamaged. (This should be done regardless of the purposeof use.)

2 Establish that the lamp is sound by probing onto a known supply.This is best achieved by using a proving unit. This is simply apocket-sized device which electronically produces 230 V dc.

3 Carry out the test to verify the circuit is dead.4 Return to the proving unit and check the lamp again.

It has long been the practice when using a test lamp to probebetween phase and earth for an indication of a live supply on thephase terminal. However, this can now present a problem whereRCDs exist in the circuit, as of course the test is applying adeliberate phase-to-earth fault.

Some test lamps have LED indicators, and the internal circuitryof such test lamps limits the current to earth to a level below thatat which the RCD will operate. The same limiting effect applies tomultimeters. However, it is always best to check that the testingdevice will have no effect on RCDs.

Calibration, zeroing and care of instrumentsPrecise calibration of instruments is usually well outside theprovince of the electrician, and would normally be carried out bythe manufacturer or local service representative. A check,however, can be made by the user to determine whether calibrationis necessary by comparing readings with an instrument known tobe accurate, or by measurement of known values of voltage,resistance, etc. However, as we have already seen, regularcalibration is a legal requirement.

It may be the case that readings are incorrect simply because theinstrument is not zeroed before use, or because the internal batteryneeds replacing. Most modern instruments have battery conditionindication, and of course this should never be ignored.

65


Always adjust any selection switches to the off position aftertesting. Too many instrument fuses are blown when, for example,a multimeter is inadvertently left on the ohms range and then usedto check for mains voltage.

The following set procedure may seem rather basic but shouldensure trouble-free testing:

1 Check test leads for obvious defects.2 Zero the instrument.3 Select the correct range for the values anticipated. If in doubt,

choose the highest range and gradually drop down.4 Make a record of the test results.5 When a no reading is expected and occurs (or, in the case of

insulation resistance, an infinite reading), make a quick check onthe test leads just to ensure that they are not open circuited.

6 Return switches/selectors to the off position.7 Replace instrument and leads in carrying case.

The testsThe IEE Regulations indicate a preferred sequence of tests andstate that if, due to a defect, compliance cannot be achieved, thedefect should be rectified and the test sequence started from thebeginning. The tests for Site applied insulation, Protection byseparation, and Insulation of non-conducting floors and walls allrequire specialist high voltage equipment and in consequence willnot be discussed here. The sequence of tests for an initialinspection and test is as follows:

1 Continuity of protective conductors2 Continuity of ring final circuit conductors3 Insulation resistance4 Protection against direct contact by barriers or enclosures5 Polarity6 Earth electrode resistance7 Earth fault loop impedance8 Prospective fault current.9 Functional testing.

66


One other test not included in Part 7 of the IEE Regulations butwhich nevertheless has to be carried out, is external earth faultloop impedance.

Continuity of protective conductorsThese include the CPCs of radial circuits, main and supplementarybonding conductors. Two methods are available, either can be usedfor CPCs, but bonding can only be tested by the second.

Method 1At the distribution board, join together the phase conductor and itsassociated CPC. Using a low resistance ohmmeter, test betweenphase and CPC at all the outlets in the circuit. The reading at thefarthest point will be (R1 + R2) for that circuit. Record this value,as after correction for temperature, it may be compared with thedesigners value (more about this later).

Method 2Connect one test instrument lead to the main earthing terminal, anda long test lead to the earth connection at all the outlets in the circuit.Record the value after deducting the lead resistance. An idea of thelength of conductor is valuable, as the resistance can be calculatedand compared with the test reading. Table 2 gives resistance valuesalready calculated for a range of lengths and sizes.

It should be noted that these tests are applicable only to allinsulated systems, as installations using conduit, trunking, MICCand SWA cables will produce spurious values due to the probableparallel paths in existence. This is an example of when testingneeds to be carried out during the erection process before finalconnections and bonding are in place.

If conduit, trunking or SWA is used as the CPC, then theverifier has the option of first inspecting the CPC along itslength for soundness then conducting the long-lead resistancetest. If the inspector is not happy with the result, he would carry

67


out a high current test using a proprietary instrument thatdelivers a test current of 1.5 times the design current up to 25 Aat a voltage of 50 V.

Continuity of ring final circuit conductorsThe requirement of this test is that each conductor of the ring iscontinuous. It is, however, not sufficient to simply connect anohmmeter, a bell, etc., to the ends of each conductor and obtain areading or a sound.

So what is wrong with this procedure? A problem arises if aninterconnection exists between sockets on the ring, and there is abreak in the ring beyond that interconnection. From Figure 17 it willbe seen that a simple resistance or bell test will indicate continuityvia the interconnection. However, owing to the break, sockets 4 to11 are supplied by the spur from socket 12 not a healthy situation.So how can one test to identify interconnections?

There are at present three methods of conducting such a test.Two are based on the principle that resistance changes with achange in length or csa; the other relies on the fact that theresistance measured across any diameter of a circular loop ofconductor is the same. Let us now consider the first two.

68

Table 2 Resistance () of copper conductors at 20C

csa

(mm2)

Length (metres)

5 10 15 20 25 30 35 40 45 50

1.0 0.09 0.18 0.27 0.36 0.45 0.54 0.63 0.72 0.82 0.901.5 0.06 0.12 0.18 0.24 0.30 0.36 0.42 0.48 0.55 0.612.5 0.04 0.07 0.11 0.15 0.19 0.22 0.26 0.30 0.33 0.374.0 0.023 0.05 0.07 0.09 0.12 0.14 0.16 0.18 0.21 0.236.0 0.02 0.03 0.05 0.06 0.08 0.09 0.11 0.13 0.14 0.16

10.0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1016.0 0.006 0.01 0.02 0.023 0.03 0.034 0.04 0.05 0.051 0.0625.0 0.004 0.007 0.01 0.015 0.02 0.022 0.026 0.03 0.033 0.0435.0 0.003 0.005 0.008 0.01 0.013 0.016 0.019 0.02 0.024 0.03

X Y Z

R1

R2

X YZ


Method 1If we were to take a length of conductor XYZ and measure theresistance between its ends (Figure 18), then double it over at Y,join X and Z, and measure the resistance between XZ and Y(Figure 19), we would find that the value was approximately aquarter of the original. This is because the length of the conductoris halved and hence so is the resistance, and the csa is doubled andso the resistance is halved again.

69

Figure 17 Ring circuit with interconnection

Figure 18

Figure 19

A B C

R1

X Y Z

R1


In order to apply this principle to a ring final circuit, it isnecessary to know the position of the socket nearest the mid-pointof the ring. The test procedure is then as follows for each of theconductors of the ring:

1 Measure the resistance of the ring conductor under test betweenits ends before completing the ring in the fuse board. Record thisvalue, say R1.

2 Complete the ring.3 Using long test leads, measure between the completed ends and

the corresponding terminal at the socket nearest the mid-point ofthe ring. Record this value, say R2. (The completed endscorrespond to point XZ in Figure 19, and the mid-point to Y.)

4 Measure the resistance of the test leads, say R3, and subtract thisvalue from R2, i.e. R2 R3 = R4 say.

5 A comparison between R1 and R4 should reveal, if the ring ishealthy, that R4 is approximately a quarter of R1.

Method 2The second method tests two ring circuit conductors at once, andis based on the following.

Take two conductors XYZ and ABC and measure theirresistances (Figure 20). Then double them both over, join the endsXZ and AC and the mid-points YB, and measure the resistancebetween XZ and AC (Figure 21). This value should be a quarter ofthat for XYZ plus a quarter of that for ABC.

If both conductors are of the same length and csa, the resultantvalue would be half that for either of the original resistances.

70

Figure 20

X Y A

R2

Z C

B

R R2 1


Applied to a ring final circuit, the test procedure is asfollows:

1 Measure the resistance of both phase and neutral conductorsbefore completion of the ring. They should both be the samevalue, say R1.

2 Complete the ring for both conductors, and bridge togetherphase and neutral at the mid-point socket (this corresponds topoint YB in Figure 21). Now measure between the completedphase and neutral ends in the fuse board (points XZ and AC inFigure 21). Record this value, say R2.

3 R2 should be, for a healthy ring, approximately half of R1 foreither phase or neutral conductor. When testing the continuity ofa CPC which is a different size from either phase or neutral, theresulting value R2 should be a quarter of R1 for phase or neutralplus a quarter of R1 for the CPC.

Method 3The third method is based on the measurement of resistance atany point across the diameter of a circular loop of conductor(Figure 22).

As long as the measurement is made across the diameter of thering, all values will be the same. The loop of conductor is formedby crossing over and joining the ends of the ring circuit conductorsat the fuse board. The test is conducted as follows:

71

Figure 21

R

N1CPC CPC

N2P1 P2

N1CPC CPC

N2P1 P2


1 Identify both legs of the ring.2 Join one phase and one neutral conductor of opposite legs of the

ring.3 Obtain a resistance reading between the other phase and neutral

(Figure 23). (A record of this value is important.)4 Join these last two conductors (Figure 24).5 Measure the resistance value between P and N at each socket on

the ring. All values should be the same, approximately a quarterof the reading in (3) above.

72

Figure 22

Figure 23 Figure 24

C B A F E D

D E F A B C

0.1 0.3 0.2 0.2 0.30.5

0.3 0.2 0.5 0.3 0.10.2

Phaseloop0.4 0.4

Neutralloop

P1 N2

P2 N1


The test is now repeated but the neutral conductors are replaced bythe CPCs. If the cable is twin with CPC, the CPC size will be smallerthan the phase conductor, and although the readings at each socketwill be substantially the same, there will be a slight increase invalues towards the centre of the ring, decreasing back towards thestart. The highest reading represents R1 + R2 for the ring.

The basic principle of this method is that the resistancemeasured between any two points, equidistant around a closedloop of conductor, will be the same.

Such a loop is formed by the phase and neutral conductors of aring final circuit (Figure 25).

Let the resistance of conductors be as shown.R measured between P and N on socket A will be:

0.2 + 0.5 + 0.2 + 0.3 + 0.4 + 0.1 + 0.32

=

2

2= 1

R measured between P and N at socket B will be:

0.3 + 0.2 + 0.5 + 0.2 + 0.3 + 0.4 + 0.12

=

2

2= 1

Hence all sockets on the ring will give a reading of 1 betweenP and N.

73

Figure 25

C B A F E D

D E F A B C

Phaseloop

Neutralloop Interconnection

N2 P2

N2 P1


If there were a break in the ring in, say, the neutral conductor, allmeasurements would have been 2 , incorrectly indicating to thetester that the ring was continuous. Hence step 3 in the testprocedure which at least indicates that there is a continuous PNloop, even if an interconnection exists. Figure 26 shows a healthyring with interconnection.

Here is an example that shows the slight difference betweenmeasurements on the phase/CPC test. Consider a 30 m ring finalcircuit wired in 2.5 mm2 with a 1.5 mm2 CPC. Figure 27 illustratesthis arrangement when cross-connected for test purposes.

74

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