GUIDANCE NOTE Overcurrent Protection 6 · J Reed ARIBA SELECT / ECA of Scotland J Davidson CEng FIEE D Millar. thCD GN6 Protection Against Overcurrent, inc 16 Edition 2001 Amd No

ProtectionAgainst Overcurrent

GUIDANCE NOTE 6

6

IEE Wiring Regulations

BS 7671 : 2001 Requirements for Electrical Installations

Including Amd No 1 : 2002

Published by: The Institution of Electrical Engineers, Savoy Place,LONDON, United Kingdom. WC2R 0BL

©2003: The Institution of Electrical Engineers

Printed January 1993Reprinted August 1993, with minor amendments2nd edition July 1996, incorporating BS 7671 : 1992 inc Amd No 13rd edition Aug 1999, incorporating BS 7671 : 1992 inc Amd No 24th edition Sept 2003, incorporating BS 7671 : 2001 inc Amd No 1

Copies may be obtained from:

The IEEPO Box 96, STEVENAGE,United Kingdom. SG1 2SD

Tel: +44 (0)1438 767 328Fax: +44 (0)1438 742 792Email: [email protected]://www.iee.org/publish/books/WireAssoc/

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system or transmitted in anyform or by any means — electronic, mechanical, photocopying,recording or otherwise — without the prior written permissionof the publisher.

While the author, publisher and contributors believethat the information and guidance given in this workis correct, all parties must rely upon their own skilland judgement when making use of it. Neither theauthor, the publisher nor any contributor assume anyliability to anyone for any loss or damage caused byany error or omission in the work, whether such erroror omission is the result of negligence or any othercause. Where reference is made to legislation it is notto be considered as legal advice. Any and all suchliability is disclaimed.

ISBN 0 85296 994 5, 2003

CD GN6 Protection Against Overcurrent, inc 16th Edition 2001 Amd No 1 2

http://www.iee.org/Publish/Books/WireAssoc/index.cfm

Contents

CO-OPERATING ORGANISATIONS 5ACKNOWLEDGEMENTS 6PREFACE 7INTRODUCTION 8

SECTION 1 THE REGULATIONS CONCERNING PROTECTIONAGAINST OVERCURRENT 101.1 Scope 101.2 Nature of overcurrent and protection 111.3 Statutory requirements 131.4 Omission of protection 131.5 Protective devices 141.6 Duration of overcurrent 141.7 Co-ordination and discrimination 15

SECTION 2 PROTECTION AGAINST OVERLOAD 202.1 General 202.2 Load assessment 202.3 Selection of protective device and conductor size 252.4 Omission of protection against overload current 39

SECTION 3 PROTECTION AGAINST FAULT CURRENT 423.1 Types of fault to be considered 423.2 Nature of damage and installation precautions 423.3 Fault impedance and breaking capacity of protective device463.4 Position of fault current protection and assessment of

prospective current 473.5 Omission of fault current protection 513.6 The use of one device for both overload and fault

current protection 543.7 Harmonics 55

SECTION 4 DETERMINATION OF FAULT CURRENT 574.1 Determination of fault current by enquiry 574.2 Measurement of fault current 604.3 Calculation of fault current 61


SECTION 5 EQUATIONS FOR THE CALCULATION OFSHORT-CIRCUIT CURRENT 645.1 General equation for fault current 645.2 Single-phase, line to neutral fault 655.3 Conductor temperature and resistance 675.4 Single-phase circuits 725.5 Line to line short-circuit 735.6 Three-phase short-circuit 74

SECTION 6 EQUATIONS FOR THE CALCULATION OF EARTHFAULT CURRENT 766.1 General 766.2 TN systems (TN-C, TN-S and TN-C-S) 766.3 Use of a cable enclosure as a protective conductor 796.4 TT system 88

SECTION 7 SELECTION OF CONDUCTOR SIZE 907.1 General 907.2 Overload and short-circuit protection by the same device 907.3 Earth fault current 917.4 Parallel cables 917.5 Use of I2 t characteristics 927.6 Duration of short-circuit current 927.7 Status of adiabatic equations 927.8 Alternative values of k 937.9 Cables in trenches 94

SECTION 8 NOTES ON FAULT CURRENT WITHSTAND OFFLEXIBLE CORDS 958.1 General 958.2 Influence of circuit design and length of flexible cord 968.3 Damage criteria for flexible cord 978.4 Conclusions 98

APPENDIX 1 CALCULATION OF REACTANCE 102

1 General 1022 Line to neutral, single-phase, faults 1023 Line to line, single-phase, short-circuit 1044 Three-phase short-circuit 1055 Earth faults 107

APPENDIX 2 CALCULATION OF K FOR OTHER TEMPERATURES108

INDEX 109


Co-operating OrganisationsThe Institution of Electrical Engineers acknowledges the contribution made by thefollowing organisations in the preparation of this Guidance Note.

British Cables AssociationJ M R Hagger BTech(Hons) AMIMMM

British Electrotechnical & Allied Manufacturers Association LtdP D Galbraith IEng MIIE MIMgtR Lewington MIEE

British Standards InstitutionM Danvers

CIBSEEur Ing G Stokes BSc(Hons) CEng FIEE FCIBSE

City & Guilds of London InstituteH R Lovegrove IEng FIIE

Electrical Contractors’ AssociationD Locke IEng MIIE ACIBSE

Electricity AssociationD J Start BSc CEng MIEE

Engineering Equipment and Materials Users AssociationR J Richman

EIEMAEur Ing M H Mullins BA CEng FIEE FIIE

ERA Technology LtdM W Coates BEng

Health & Safety ExecutiveEur Ing J A McLean BSc(Hons) CEng FIEE FIOSH

Institution of Electrical EngineersG D Cronshaw IEng MIIE (Editor)P R L Cook CEng FIEE MCIBSEP E Donnachie BSc CEng FIEE

National Inspection Council for Electrical Installation Contracting

Office of the Deputy Prime MinisterE N King BSc CEng FCIBSE

Royal Institute of British ArchitectsJ Reed ARIBA

SELECT / ECA of ScotlandJ Davidson CEng FIEED Millar


Acknowledgements

References to British Standards are made with thekind permission of BSI. Complete copies can beobtained by post from:BSI Customer Services389 Chiswick High RoadLondon W4 4ALTel: General Switchboard: 020 8996 9000

For ordering: 020 8996 9001For information or advice: 020 8996 7111For membership: 020 8996 7002

Fax: For orders: 020 8996 7001For information or advice: 020 8996 7048

BSI operates an export advisory service - Technical Helpto Exporters - which can advise on the requirements offoreign laws and standards. The BSI also maintains stocksof international and foreign standards, with manyEnglish translations.Up-to-date information on BSI standards can be obtainedfrom the BSI website http://www.bsi-global.com/

Copies of Health and Safety Executive documents andapproved codes of practice (ACOP) can be obtained from:HSE Books, P O Box 1999Sudbury, Suffolk CO10 6FSTel: 01787 881165 Fax: 01787 313995

Health and Safety Executive telephone enquiries canbe made to:HSE Info Line on : 08701 545500

Fax and postal enquiries can be made to:HSE Information CentreBroad Lane, Sheffield S3 7HQTel: 01142 892345 Fax: 01142 892333.


http://www.bsi-global.com/

Preface

This Guidance Note is part of a series issued by theWiring Regulations Policy Committee of theInstitution of Electrical Engineers to enlarge upon andsimplify some of the requirements of BS 7671 : 2001inc Amd No 1, Requirements for Electrical Installations(IEE Wiring Regulations Sixteenth Edition). Significantchanges made in this 4th edition of the GuidanceNote are sidelined.

Note this Guidance Note does not ensure compliancewith BS 7671. It is a guide to some of therequirements of BS 7671 but users of these GuidanceNotes should always consult BS 7671 to satisfythemselves of compliance.

The scope generally follows that of the Regulationsand the principal Section numbers are shown on theleft. The relevant Regulations and Appendices arenoted in the right-hand margin. Some Guidance Notesalso contain material not included in BS 7671 butwhich was included in earlier editions of the WiringRegulations. All of the Guidance Notes containreferences to other relevant sources of information.

Electrical installations in the United Kingdom whichcomply with BS 7671 are likely to satisfy the relevantaspects of Statutory Regulations such as the Electricityat Work Regulations 1989, but this cannot beguaranteed. It is stressed that it is essential toestablish which Statutory and other Regulations applyand to install accordingly. For example, an installationin premises subject to licensing may have requirementsdifferent from, or additional to, BS 7671, and therequirements will take precedence.

Users of this Guidance Note should assure themselvesthat they have complied with any legislation thatpost-dates the publication.


Introduction

This Guidance Note is concerned with Part 4 ofBS 7671— Protection for Safety.

Neither BS 7671 nor the Guidance Notes are designguides. It is essential to prepare a full specificationprior to commencement or alteration of an electricalinstallation.

The design and specification should set out therequirements and provide sufficient information toenable competent persons to carry out theinstallation and to commission it. The specificationshould include a description of how the system is tooperate and all the design and operationparameters. It must provide for all the commissioningprocedures that will be required and for the provisionof adequate information to the user. This should beby means of an operation and maintenance manualor schedule, and ‘as fitted’ drawings if necessary,

It must be noted that it is a matter of contract as towhich person or organisation is responsible for theproduction of the parts of the design, specificationconstruction and verification of the installation andany operational information.

The persons or organisations who may be concernedin the preparation of the specification include:

The DesignerThe Planning SupervisorThe InstallerThe Supplier of ElectricityThe Installation Owner (Client) and/or UserThe ArchitectThe Fire Prevention OfficerAll Regulatory AuthoritiesAny Licensing AuthorityThe Health and Safety Executive

514-09


In producing the design and specification adviceshould be sought from the installation owner and/oruser as to the intended use. Often, as in a speculativebuilding, the intended use is unknown. In such casesthe specification and/or the operational manual mustset out the basis of use for which the installation issuitable.

Precise details of each item of equipment should beobtained from the manufacturer and/or supplier andcompliance with appropriate standards confirmed.

The operational manual must include a description ofhow the system as installed is to operate and allcommissioning records. The manual should alsoinclude manufacturers’ technical data for all items ofswitchgear, luminaires, accessories, etc and anyspecial instructions that may be needed. The Healthand Safety at Work etc Act 1974 Section 6 and theConstruction (Design and Management) Regulations1994 are concerned with the provision of information.Guidance on the preparation of technical manuals isgiven in BS 4884 (Technical manuals, specification forpresentation of essential information, guide tocontent and presentation) and BS 4940(Recommendations for the presentation of technicalinformation about products and services in theconstruction industry). The size and complexity of theinstallation will dictate the nature and extent of themanual.

511

131-01-01


Section 1 — The Regulations ConcerningProtection Against Overcurrent

1.1.1 Fundamental principles

The fundamental principles include two regulationsconcerning overcurrent.

The first of these requires that, so far as is reasonablypracticable, persons or livestock shall be protectedagainst injury and property shall be protected againstdamage due to excessive temperatures orelectromechanical stresses caused by any overcurrentslikely to arise in live conductors.

The second regulation requires conductors and anyother parts likely to carry a fault current to be capableof doing so without attaining an excessivetemperature.

1.1.2 Protective measures

Measures to provide protection against overcurrentsare contained in Chapter 43, Section 473, Sections 530and Section 533.

Chapter 43 gives the basic requirements for bothoverload and fault current protection, whilst Section 473has further requirements concerning the applicationof the protective measures and the location of theprotective devices.

Sections 530 and Section 533 contain regulations for theselection and installation of the protective devices.

1.1 Scope

1.1.1130

473530533

130-04130-05



1.1.3 Load equipment and flexible cords

It must be remembered that the Regulations dealprimarily with the fixed equipment of an installation.Protection of conductors in accordance with Chapter 43does not necessarily protect the equipment connectedto an installation nor the flexible cables and cordsconnecting such equipment to the fixed part of aninstallation. However, some notes are provided inSection 8 of this Guidance Note on the overcurrentwithstand of flexible cords. This subject is ofimportance where BS industrial-type plugs andsockets are to be used. It may be necessary to considerthe feasibility of a limitation on the current rating ofsuch circuits, to introduce local protection or torequire minimum conductor sizes for the flexiblecords.

1.2.1 General

Every circuit shall be protected by one or more deviceswhich automatically interrupt the supply in the eventof overcurrent.

The object of the Regulations is to ensure that anyovercurrent does not persist long enough to causedamage to equipment or risk of injury to persons orlivestock.

An overcurrent is any current which exceeds thecurrent-carrying capacity of the circuit/ conductors.

This is determined by the nominal current or settingof the overcurrent protective device for that circuit,and the current-carrying capacity of its conductors byRegulation 433-02-01. The conductors must also beprotected against fault currents.

1.2.2 Overload and fault current

The term overcurrent includes both overload current and fault current, see Sections 433 and Section 434.Fault current may be short-circuit current orearth fault current.

1.2 Nature ofovercurrentandprotection

1.2.1

433434

433-02-01

130-04130-05131-08

Part 2

The difference, in principle, between overload current and fault current lies in the reason for theiroccurrence, not in their magnitudes. An overload isimposed on a sound circuit by an abnormal situationat the load, whereas fault current is due to aninsulation failure or bridging of insulation in thecircuit or its terminations.

In practice, however, the difference between the twotypes of overcurrent tends to be one of currentmagnitude and duration, because this makes for aconvenient classification of the protective devicesrequired. The total time/current characteristic of aprotective device or combination of devices shouldprovide protection for any combination of currentand time which could result in overheating ormechanical over-stressing. This total characteristic isexemplified in the single characteristic of a fuse orthe combined characteristic of a circuit-breaker withits thermal and electromagnetic trip mechanisms.

It is generally sufficient to look at the two extremesof this characteristic; the high current short durationpart which, because it usually protects from faults inthe circuit itself, is referred to as fault protection andthe long duration lower current part, usuallyprotecting against excessive load, which is referred toas overload protection.

Furthermore, experience shows that these twoextremes cover practically all abnormal currents andwhere calculation of the values of current is the mostpracticable.

It is accepted, however, that actions such as direct-on-line motor starting, or capacitor switching, result incurrents of a magnitude similar to some fault currents,while a high resistance earth fault or a fault inequipment could result in currents of the samemagnitude as those arising from excessive loads.



1.2.3 General protection characteristic

The total characteristic of circuit protection complyingwith the Regulations is intended to be such that if theprotection is adequate at the two extremes it will besatisfactory for any time/current combination inbetween.

For practical reasons it is sometimes not feasible toprovide complete protection for very small overloadsand attention is drawn to the comments in paragraph paragraph 2.2.5 on Regulation 433-01-01 with regard to loadassessment.

The Electricity at Work Regulations 1989 require thatefficient means, suitably located, shall be provided toprotect against excess current in every part of asystem, as may be necessary to prevent danger.

It is important that the designer and the installerconsult the relevant documents listed in Appendix 2of BS 7671, together with those Statutory Regulationsand Memoranda which apply to the particularinstallation concerned.

Regulation 110-04-01 sets out the relationship ofBS 7671 : 2001 with Statutory Regulations, includingthe Electricity Supply Regulations 1988 as amendedwhich have now been superseded by the ElectricitySafety, Quality and Continuity Regulations 2002. TheStatutory Regulations are also listed in Appendix 2 ofBS 7671 : 2001.

There may also be local regulations which apply toinstallations in certain premises, such as where thereis public access, certain types of entertainment,housing for the aged or infirm, etc.

The impact of any Regulation which may bear on thefunction or cost of an installation should be broughtto the attention of the purchaser at the earliestopportunity.

Protection against either overload or fault currentmay be omitted where unexpected disconnectioncould cause danger. The designer should first consult

433

1.3 Statutoryrequirements110-04

1.4 Omission ofprotection473

433-01-01

473-01-03473-01-04473-02-04

Appx 2


Regulations 473-01 and 473-02 to establish theconditions under which protection may be omitted.

The omission of overload or fault current protection,or a reduction in its effectiveness, can only bejustified if danger is prevented by other means, orwhere the opening of the circuit would cause greaterdanger than the overload or fault condition.

This calls for a careful consideration of the balance ofrisks involved. The integrity of the equipmentinvolved, including its ability to contain safely anypossible effects of an overcurrent, such as arcs, hotparticles, fire and dangerous fumes etc, should beexamined.

The design should mitigate the effects of theomission of the overcurrent device e.g. by rating thecircuit for any current reasonably likely to arise.

Protection may be provided by a single device whichdetects and interrupts both overload and faultcurrent. Alternatively, separate devices may be usedfor each task.

There are special provisions for protection of motorstarter circuits, see the guidance on Section 435 inparagraph 1.7.3.

Although BS 7671 does not impose a specific upperlimit on the duration of overcurrent, except wherethere is a risk of shock or of damage to equipment, itis prudent to select circuit protection so that faultcurrent does not persist any longer than is absolutelynecessary. In a practical installation there is little or nocontrol over the paths fault current may take,especially earth fault current, and danger or damagemay occur in unexpected ways and to items notdirectly associated with the circuit.

Note that the data provided in Regulation 434-03-03is valid for periods not exceeding 5 seconds and thistends to set an implied upper limit to fault currentduration.

1.5 Protectivedevices432

1.6 Duration ofovercurrent431434

432-02-01

434-03-03


The duration of small overloads should be controlledby suitable attention to load assessment, see theguidance on Regulation 433-01-01 in paragraph 2.2.5.

1.7.1 Operation of protective devices in series

By reason of the method of power distributionadopted, there may well be two or more overcurrentprotective devices involved with a given fault currentor overload. For example, where a main deviceprotects a feeder to a distribution board withprotection for a number of outgoing circuits, orwhere protection against overload and fault current isprovided by two separate devices. There are then twoaspects which need careful consideration:discrimination between operating characteristics andco-ordination of their fault current breakingcapacities and fault energy withstands.

1.7.2 Discrimination between devices

It is a matter of convenience and fitness for purposeof an installation that disconnection interrupts thefaulty or overloaded circuit only. It can also be amatter of safety.

Correct selection and comparison of thecharacteristics of each device will ensure that only thedevice electrically nearest to the cause of theovercurrent operates. The breaking capacity of thedownstream device must therefore be suitable for thehighest prospective current at its point in the circuit.

The operating characteristics of a given device liewithin a band, the lower boundary of whichconstitutes a specified non-fusing or non-trippingcharacteristic, the upper boundary a specified fusingor tripping characteristic. The manufacturer canprovide guidance or information such that a deskcomparison between the non-operating limit of theupstream device with the operating limit for thedownstream device will normally be sufficient toselect devices to give correct discrimination.

When the prospective current is high and interruptiontimes are of the order of 1 or 2 half cycles it is unwiseto attempt to assess discrimination by overlaying the

1.7 Co-ordinationanddiscrimination

1.7.1

533 533-01-06


characteristics. A combination of devices can behavevery differently from when they are separate andadvice should be sought from the manufacturers, whocan recommend suitable combinations.

As a rule of thumb, satisfactory discriminationbetween two similar types of fuse link is usuallyachieved when the downstream link has a ratingequal to about half that of the upstream one. Forother devices a much higher ratio may be required,and in some circumstances may not be possible.

When discrimination is achieved, each protectivedevice has to comply with the requirements ofRegulations 432-02-01, 434-01-01 and 434-03-01 withregard to making and breaking prospective faultcurrent at its point in the circuit.

1.7.3 Co-ordination of devices

It is sometimes not feasible or economical to providea downstream device which has a sufficiently highfault breaking capacity.

A common example of this is the combination of afuse or circuit-breaker with a motor starter. Ingeneral, motor starters are not designed, nor are theyintended, to interrupt short-circuits or earth faultcurrents of a similar magnitude. However, incombination with the fault protection, they must beable to close on to the prospective fault current andto interrupt currents equal to the maximum overloadcapability of the associated motor. Fault protectionfor the starter circuit has to be provided by the fusesor circuit-breaker at the origin of the circuit, incompliance with Regulations 434-03-01 and 473-02-01.

The breaking capacity of the fault current protectiondevice must be adequate to interrupt prospectivefault currents, but its characteristics must also be suchthat it does not interrupt starting currents. Suchcharacteristics are likely to be too high to operate onoverload, either for the motor or to comply withRegulation 433-02-01 for the circuit conductors.

In motor circuit applications where circuit-breakers orgG type fuse links are used, the need to withstand

434435473

432-02-01434-01-01434-03-01

431-01-02435-01-01

434-03-01473-02-01

433-02-01


motor starting currents usually dictates a highercurrent rating than would be selected on the basis ofmotor full load current.

However, to meet this special role, circuit-breakersproviding fault protection only may be used andextended ratings of motor circuit protection fuse linkswith gM characteristics are available giving economiesin both fusegear size and cost. These gM motor circuitfuse links have a dual rating: a maximum continuousrating based on the equipment in which they arefitted, for example a fuse carrier and base, and arating related to the operational characteristics of theload.

In the type designation these two ratings areseparated by the letter M. For example, 32M50 represents a maximum continuous rating of 32 A(governed by the associated fitment) and anoperational characteristic of a 50 A fuse link.

Regulation 435-01-01 requires that the energylet-through of the fuse or circuit-breaker shall notexceed the withstand capability of the motor starter.In general, it is important that the overload relay doesnot initiate the opening of the starter contacts beforethe fault protection has had time to operate.

Where the fault protection is incorporated in onepiece of equipment with the starter, it is theresponsibility of the manufacturer to ensure thatcorrect co-ordination is achieved. Where the devicesare separate the manufacturer of the starter canprovide guidance on the selection of a suitable fuseor circuit-breaker for fault current protection.

BS EN 60947-4-1 recognises the three types ofco-ordination with corresponding levels ofpermissible damage to the starter. Co-ordination ofType C in BS 4941 or Type 2 in BS EN 60947-4 (i.e. nodamage, including permanent alteration of thecharacteristics of the overload relay, except lightcontact burning and a risk of contact welding) isrequired if the starter is to continue to provideoverload protection complying with Regulation433-02-01.

Examination and maintenance of both devices isnecessary after a fault.

435-01-01

433-02-01


The overload relay on the starter is arranged tooperate for values of current from just above full loadto the overload limit of the motor, but it has a timedelay such that it does not respond to either startingcurrents or fault currents. This delay providesdiscrimination with the characteristics of theassociated fuse or circuit-breaker at the origin of thecircuit.

The starter overload relay can provide overloadprotection for the circuit in compliance withRegulation 433-02-01 on the basis that:

(i) its nameplate full-load current rating or setting istaken as In, and

(ii) the motor full-load current is taken as Ib, and

(iii) the ultimate tripping current of the overloadrelay is taken as I2.

Where the overload relay has a range of settings thenitems (i) and (iii) should be based on the highestcurrent setting, unless the settings cannot be changedwithout the use of a tool.

It is unlikely that the overload withstand of the circuitconductors will coincide with that of the motor, sothat overload settings should be determined by thelower requirement. Alternatively, an additional tripsystem, based on detection of motor temperaturemay be used.

As a starter provides overload protection only,Regulation 473-01-02 permits it to be locatedanywhere along the run of the circuit from thedistribution board providing there is no other branchcircuit or outlet between the starter and the motor.

1.7.4 Back-up co-ordination

Another area where co-ordination is needed occurswhen it is necessary to provide back-up for a faultcurrent protective device with another one upstreamof higher rating or higher fault current breakingcapacity. It is not always satisfactory to evaluate theperformance of the two devices by a comparison oftheir characteristics. For example, with short

434

433-02-01

434-03-01

473-01-02

operating times such an approach would not takeaccount of the effect of either device on themagnitude of the fault current and their performancein series may be different from their individualperformances.

Information should be obtained from themanufacturer of the downstream device as to thetype of upstream fuse or circuit-breaker best suited tothe prospective fault current.


Section 2 — Protection Against Overload

An overload is a current the value of which exceedsthe rated current of an electrically sound circuit. Itmay be caused by a user deliberately or accidentallyusing more power than the circuit is intended for, orby a fault in equipment supplied by the circuit, or itmay be caused by a characteristic of the load. It is notdue to an electrical fault in the circuit itself.

Damage by overload can take the form of accelerateddeterioration of insulation, contacts and connectionsand deformation of some materials, thereby reducingthe safe life and capability of equipment.

An overload caused by plugging in too manyappliances may not be large, but it could last longenough to cause prolonged excessive heating of theconductors and probably of some of the accessories.

On the other hand, an overload caused by starting amotor may be several times the circuit rating, but itsduration is not long enough to cause unacceptableoverheating. However, where frequent motorstarting or reversing at short intervals is expected,heating will be cumulative and the circuitcomponents should be suitable for the higher duty.

The important characteristics of an overload aretherefore the magnitude of the current and itsduration.

Proper assessment of the load current Ib is essentialfor the safe application of these regulations.

This is reflected in the fundamental principles(Chapter 13) and includes consideration of themaximum demand on each distribution and finalcircuit.

2.1 General433

2.2 Loadassessment433

Part 2

433-01-01

132-01-04



Although overload protection is based on conductortemperature, it is equally important that all joints andconnections are also capable of withstanding thecurrents and consequent temperatures expected.

When the circuit supplies a single unvarying load or anumber of such loads, Ib is obtained by simplysumming all the loads concerned. In case of doubt, orfor simple installations, this is the safest course.

2.2.1 Diversity

In some cases it may be appropriate to recognize thatnot all the loads are operating at the same time andto apply a diversity factor. Determination of adiversity factor requires care and experience andaccount should be taken as far as possible of futureexpansion of the load or of change of duty.

Further advice on the determination of diversity factorsis given in Guidance Note 1, Selection & Erection,Appendix H.

2.2.2 Varying loads

Where a load changes in a known manner it ispossible to equate the design current Ib to a thermallyequivalent constant load. Varying loads include bothirregular pulses of load and regular or cyclicvariations.

2.2.3 Thermally equivalent current

Where a circuit supplies a number of independentloads, or a fluctuating load, a simple procedure is toassess the value of the highest total load on the circuitand to substitute this for Ib in Regulation 433-02-01.

This approach is adequate for circuits supplying smallloads where further design effort is not justified.

However, Regulation 533-02-01 indicates that regularrepetitive loads (cyclic loads) should be assessed onthe basis of a thermally equivalent constant load.

311

533

433

533-02-01

131-06-01131-07-01

311-01-01

GN1

533-02-01

433-02-01

The benefits of such an approach can be extended toany varying load.

Descriptions of methods to calculate thermallyequivalent loads are outside the scope of thisGuidance Note. Such calculations can be tedious andit may be helpful to use the following routine, whichwill indicate whether there is anything to be gainedby such a calculation.

2.2.4 Test for thermally equivalent load

(i) Include in the total load all contributionswhatever their magnitude and duration. Initially,make Ib equal to the maximum current, Imax(see Fig 1)

(ii) Select a conductor size to fulfil the condition:Ib ≤ In ≤ IzSee Regulation 433-02-01 and Appendix 4 of BS 7671.

Fig 1: Illustrating a varying load

t

I max

I min

I



(iii) If the duration of the peak current, Imax, is greaterthan the duration given in Table 1 for theconductor size and type of cable selected, noadvantage can be gained by calculating thethermally equivalent current and step (ii) abovewill give the correct cable size. This applies to asingle pulse of load or to a repetitive load

(iv) If the duration of Imax is less than the durationin Table 1, it may be possible to select a smallerconductor size. This will depend on the value ofthe preceding lower current and its duration

(v) Calculate the ratio X=(duration of Imax)/(durationin Table 1) for the size of cable selected in step (ii). From Fig 2 read off the appropriate value of Y

(vi) If the duration of the lower current, Imin, is lessthan Y times the duration of Imax no worthwhilereduction in conductor size will be possible. Step (ii) will give the correct size of conductor.

TABLE 1Duration of load current for equivalent constantload (Duration in minutes)

Conductorsize mm2

1-corecable

2-corecable

3-corecable

4-corecable

1.01.52.546

10162535507095

120150185240300400500630

—————————36425058677892

106126157172

121315161822263541374655627384

102118143

——

1112141619212739464756677790

103124143170

——

121516202426334958627489

102120136165192227

——


Fig 2: Test for variable load

(vii) If the duration of Imin is greater than Y times theduration of Imax, calculation of the thermallyequivalent current (which will be less than Imax)may lead to lower values for Ib and In and possiblya smaller cable size. The thermally equivalentcurrent, and hence any benefit by way of asmaller conductor size, will depend on the valueof Imin.

2.2.5 Small overloads of long duration

If Ib is equal to the sum of all expected loads, or to athermally equivalent load, there should be nooverload which will result in the conductortemperature exceeding its permitted working value.Overload protection then fulfils its proper function ofprotecting the circuit against events over which thedesigner has no control.

433 433-01-01

523-01Table 52B

X0 0.2 0.4 0.6 0.8 1.0

Y

1.0

0.8

0.6

0.4

0.2

0


However, examination of the characteristics of manyoverload protective devices will reveal that, forpractical reasons, they may not provide overloadprotection against currents just exceeding the ratingof the associated conductors and equipment. For thisreason it is important that every care is taken to avoideven small overloads of a protracted nature. (Smalloverloads are further discussed in 2.3.6)

Ageing and deterioration of insulation andconnections increase rapidly at temperaturesexceeding the rated values. The limits permittedassume that excess temperatures will be veryinfrequent and generally due to unforeseensituations. It is not practicable to place a general limiton the permissible number of overload events.

To avoid deterioration, it is essential that the designload of a circuit is high enough to include allforeseeable peak loads of a protracted nature.

It is never acceptable to use a fuse or a circuit-breakeras a load limiting device.

There are two regulations which need to beconsidered when selecting an overcurrent device.

The first specifies certain relationships between:

Ib the load (design) current, and

In the nominal current or current setting of theprotective device, and

Iz the current-carrying capacity (rating) of the circuitconductors, and

I2 the current causing effective operation of thedevice.

The relationships between Ib, In, Iz and I2 can be setout as equations, or in diagrammatic form as in Fig 3.

(i) Ib ≤ In

(ii) In ≤ Iz

(iii) I2 ≤ 1.45 x Iz.

Note that the load current, Ib, is the starting point ofthis regulation.

2.3 Selection ofprotectivedeviceandconductorsize433

433-02-01

Fig 3: Overload protection

Conditions (i) and (ii) provide the basic relationshipsfor the selection of a suitable size or setting of theoverload device (Ib ≤ In), and an adequate conductor(In ≤ Iz). The significance of selecting the conductorrating in this way is that it provides a link betweenthe operating characteristic of the device and theconductor rating and paves the way for condition (iii).

The alternative term ‘current setting’ for In applies todevices which have adjustable characteristics, whenthe nominal size might be larger than thecurrent-carrying capacity of the conductor.

Where the device is adjustable, the means ofadjustment should be sealed, or constructed so thatalteration cannot be made without the use of a tool.A non-adjustable device should be used whereuntrained personnel may have access.

Ib Iz 1.45 Iz

InI2



Condition (iii) provides the overload protectionrequired by Regulation 433-01-01, but note that it isconcerned primarily with the conductors. Protectionfor electrical equipment in the circuit, including theoverload device itself, should be covered by carefulselection of equipment, made to a standard quoted inthe Regulations or to an equivalent.

The factor 1.45 in condition (iii) is based on acombination of experience and investigation. This hasshown that the types of cable considered in BS 7671can safely withstand a small but undefined number ofperiods at excess temperatures corresponding tocurrents not greater than 1.45 times theircurrent-carrying capacity. However, such currentsmust not persist for long periods.

The second regulation always to be considered withRegulation 433-02-01 is 433-02-02. This states that,for the devices specified in Regulation 433-02-02(these are the devices commonly used in the UK withthe particular exception of rewireable fuses), therequirement for Iz ≤ 1.45 I2 will be met if In ≤ Iz.

2.3.1 Selection of conductor size, and value of Iz

Values for Iz, the current-carrying capacity ofconductors, can be obtained from Appendix 4 ofBS 7671. Attention is drawn to Sections 4, 5 and 6 ofthat Appendix where the selection of a conductor sizeto satisfy Regulation 433-02-01 is described.

Only the magnitude of the load current is specified inRegulation 433-02-01, the duration is taken care of byusing a protective device having a suitable time/current characteristic.

A device complying with one of the British Standardslisted in Regulation 433-02-02 has a characteristicdesigned so as to break an overload current withinthe limit of 1.45 times the conductor rating asrequired by condition (iii), provided that it has beenchosen so as to comply with condition (ii).

Furthermore, its characteristic is such that theduration of an overload will be limited to a safe value.

433

433-01-01

433-02-02

433-02-01Appx 4

433-02-01

433-02-02


Where fuses are used it is important to note that onlythe gG type fuse to BS 88-2.1 will automaticallyprovide overload protection complying with bothconditions (ii) and (iii) of Regulation 433-02-01. A ‘gG’type fuse refers to a current limiting fuse link which isintended for general purpose use, particularly whereoverload protection is required. It should bedistinguished from types ‘gM’ (for motor circuits) and‘a’ (partial-range breaking capacity) fuse links, whichare not intended for overload protection.

Regulation 433-02-03 applies when semi-enclosedfuses to BS 3036 are used because they operate whenthe overload current is twice their nominal current.

Correct protection can be obtained by modifyingcondition (ii) so that the nominal current of such afuse does not exceed 1.45/2 = 0.725 times the ratingof the associated conductor. The practical effect ofthis modification is to increase the size of conductorrequired for a given load.

Further comments on the use of BS 3036 fuses aremade in Appendix 4 of BS 7671.

2.3.2 Nuisance tripping of overload protection

Unintentional operation of overload devices mayoccur when the load current contains peaks lasting upto a few seconds. For example, motor startingcurrents and inrush currents due to energisingtransformers or large tungsten lamp loads, whereselection of the conductor size has been made byputting Ib equal to the normal maximum runningcurrent. Such peaks are unlikely to result inunacceptable conductor heating, unless, in the case ofa motor, the starting period is unusually long (forexample, due to a high inertia load).

However, unnecessary operation may causeinconvenience or danger. A different size or type ofdevice may be needed whose characteristics providereduced sensitivity or a time delay for short durationstogether with protection complying with Regulation433-02 for long duration overloads.

433-02-01

433-02-03

Appx 4

433-02

For motors larger than the fractional kilowatt sizesthe problem is dealt with by providing a suitabledelay to the overload trips in the starter. (See thedescription of this subject in paragraph 1.7 of thisGuidance Note.)

Different types of circuit-breaker (cb) to BS EN 60898(which has replaced BS 3871) are available whichoperate at various values of short durationovercurrent (instantaneous trip current (In)) as shownin Table 2.

The different types of circuit-breaker available can besplit into two categories.

Circuit-breakers for household and similarapplications to BS EN 60898 (formerly known asmcbs) and rcbos to BS EN 61009-1.There is a wide range of circuit-breaker characteristicsthat have been classified according to theirinstantaneous trip performance and Table 2 givessome information on the application of the varioustypes available. These limits are the maximum allowedfor miniature circuit-breakers to BS 3871 (nowwithdrawn) and circuit-breakers to BS EN 60898-1, butit should be noted that manufacturers may providecloser limits. Note: The same types B, C and D alsoapply to rcbos to BS EN 61009-1.

Low voltage switchgear and controlgear Part 2circuit-breakers to BS EN 60947-2.Unlike circuit-breakers to BS EN 60898-1, circuit-breakers complying with BS EN 60947-2 do not havedefined characteristics and manufacturers’ data mustbe used. BS EN 60947-2 Amendment 2 : 2001 nowincludes Annex L, which is for circuit-breakers notfulfilling the requirements for overcurrent protection(cbis) derived from the equivalent circuit-breaker. Aclass X cbi is fitted with integral short-circuitprotection, which may, on the basis of themanufacturer’s data, be used in conjunction with thestarter overload relay, for short-circuit protection.


TABLE 2Selection of cb or rcbo type

Type Instantaneoustrip current

Application

1*B

2.7 to 4 In3 to 5 In

Domestic and commercial installationshaving little or no switching surge

2*C3*

4 to 7 In5 to 10 In7 to 10 In

General use in commercial/industrialinstallations where the use of fluorescentlighting, small motors etc can produceswitching surges that could causeunwanted tripping of a Type 1 or B cb.Type C or 3 may be necessary in highlyinductive circuits such as banks offluorescent lighting

4*D

10 to 50 In10 to 20 In

suitable for transformers X-raymachines, industrial welding equipmentetc where high inrush currents may occur

* Types 1, 2, 3 and 4 are no longer available, buthave been shown in the table because these areoften still found in existing installations.

Table 2 also provides guidance on the selection of cbtype for a given application.

Again, reference to manufacturers’ data is advisable.

For guidance on the selection of the type and numberof loads that can be simultaneously switchedreference to manufacturers’ data is recommended.



2.3.3 Cables for star-delta starters

Fig 4: Typical star-delta arrangement

When a motor is controlled by a star-delta starter theends of the three motor windings are extended fromthe motor terminals to the starter. The connections ofthe windings in star for start, and delta for run aremade at the starter.

The steady running current in each of the sixconductors between a star-delta starter and its motoris 58 per cent of that through the conductorssupplying the starter. However, motor cables areusually run as a group having six conductors so thattheir current-carrying capacity is 80 per cent of asingle circuit three conductor capacity.

It follows that the single circuit rating of motor cablesshould be at least 72 per cent (0.58/0.80) of that ofassociated supply cables. Examination of rating tablesshows that the empirical rule by which motor cablesare half the cross-sectional area of the supply cabledoes not always meet this requirement. Neither doesit take into consideration possible differences in cabletype or installation conditions.

552

Motor

Star-deltastarter Isolator

Startbutton

Stopbutton

552-01

6x conductors between a star-deltastarter and its motor (motor windings).

Motor windings connectedin star for start.

Motor windings connectedin delta for run.

Supply Supply


A simple procedure for the correct selection of motorcables, whereby the motor cables are chosen to havea rating, Iz, of at least 72 per cent of that of the supplycables, is illustrated by the following example, whichuses the symbols and Tables of Appendix 4 of BS 7671.

Assume a full-load motor current of 37 A = Ib.

Supply cable, to be three-core thermoplastic (pvc)swa on a perforated tray. See Table 4D4A of BS 7671.

Ambient temperature 30 °C (Ca, Cg, Ci = 1).

See Appendix 4, Section 6, of BS 7671.

Motor cables, to be single-core 70 °C thermoplastic(pvc) in conduit. See Table 4D1A.

Ambient temperature 40 °C (Ca = 0.87). See Table 4C1of BS 7671.

For the supply cable, Regulation 433-02-01 (Ib ≤ In)is met by a starter for which In = 40 A.

The cable size is then selected as in Appendix 4, 6.1.1,of BS 7671.

It ≥ = = 40 A.

A three-core 6 mm2 cable has a tabulated rating,It = 45 A. (Table 4D4A, column 5)

If the delayed overload trips operate at, say, 50 A,condition (iii) of Regulation 433-02-01,

I2 ≤ 1.45 Iz where Iz = Ca.Cg.Ci.It

is met because I2 = 50 A, and

1.45 x 1 x 1 x 1 x 45 = 65 A.

For the motor cables, the procedure requires that

It ≥ x 0.72 = x 0.72 = 33 A.

6 mm2 single-core cables in conduit, which have asingle circuit tabulated rating, It, of 36 A will besuitable (see Table 4D1A, Column 5, of BS 7671).

If overload protection is satisfactory for the supplycables, the motor cables will also be protected. Thiscan be demonstrated as follows:

I2 = 50 (which is a line current)

Table 4D4A

Appx 4

InCa.Cg.Ci

400.87 X 1 X 1

Table 4D1A

Table 4D1A

Table 4C1

Appx 4

InCa.Cg.Ci

401 x 1 x 1

Appx 4

433-02-01

Table 4D4A

433-02-01


For the motor cables, which are a double circuit inconduit,

Iz = It.Ca.Cg.Ci

= 36 x 0.87 x 0.8 x 1 = 25 A

1.45 Iz = 1.45 x 25 = 36.25 A.

The line equivalent of this current in the deltaconnected motor cables is:

√3 x 36.25 = 63 A.

In this example, motor cables having a cross-sectionalarea (csa) of 50 per cent of the supply cable wouldbe clearly inadequate. This is partly due to the use, inthis example, of different types of cable and a higherambient for the motor cables.

Even if the example had specified cables enclosed inconduit and the same ambient temperature for bothruns of cable, the 50 per cent csa rule would fail,being saved only by the need to round up the size ofthe motor cables to the nearest larger standard size.

For higher loads and larger cables this rounding up isnot always necessary and the 50 per cent rule isinadequate.

2.3.4 Cables in Parallel

It is permissible to use only one device to provideoverload protection for circuits composed ofconductors in parallel, provided that the device willoperate before damage occurs to any of the cables.

To achieve this it is necessary that the load currentdivides equally between the conductors.

As a first step this can be achieved by usingconductors of the same length, cross-sectional areaand material. It is then assumed that overloadcurrents divide equally between the conductors andthat Iz is the simple sum of the current-carryingcapacities of the conductors. There are some furtherrequirements, however, dependent on the type ofcables involved, as the arrangement of the cables mayintroduce mutual heating between them and reducetheir current-carrying capacities.

433 433-03-01


Single-core cables

For single-core cables, only certain cablearrangements provide reasonable current sharing.Examples are given in Fig 4. Other arrangements mayprovide acceptable current sharing, but this should bechecked

Multicore cables

For multicore cables, the conductor arrangements canbe of two types:

(a) all the phase (and neutral) lines are carried ineach cable; a circuit protective conductor is eitherincluded in each cable (in armoured cables itwould be the armour) or, if the cables are notarmoured, it may be run as a separate conductor.

Satisfactory current sharing is easily achieved withthis arrangement provided that cables are of thesame size, type and length. The totalcurrent-carrying capacity is proportional to thenumber of cables, provided that any reduction inrating due to grouping is taken into account

(Note, even though the cables in parallel are ineffect one circuit, the cables still need to be treatedas though each was a separate circuit from agrouping point of view.)

Refer to Appendix 4, Table 4B1 in BS 7671 : 2001.

(b) for unarmoured cables only it is feasible to usethem as single-core cables by connecting all liveconductors together. However, any circuitprotective conductor should be a separateconductor and not a conductor in one or more ofthe cables. In the event of a cable fault the faultcurrent withstand of such a conductor would beinadequate. Further, currents induced in loopsformed by such conductors will reduce thecurrent-carrying capacity of the cables.

It is important to bear in mind that the totalcurrent-carrying capacity of cores paralleled in thisway is not proportional to the number of cores.Unless better information is available it is prudent toexpect four-core cables to carry only three times therating of one core.

Table 4B1


Fig 5: Cable configurations for single-core cables in parallel

2 CABLES PER PHASE (equal current sharing)

3 CABLES PER PHASE (no complete sharing possible with any configuration,example gives about 5 per cent unbalance,and up to 10 per cent circulating current in neutrals)

4 CABLES PER PHASE (flat formations provide equal sharing, no circulatingcurrent in neutrals, trefoils as for 3 cables per phase)

L1 L2 L3 N

L1 L2L3

N

L1 L2 L3 N

L1L2L3

N NN = 1 neutral, = 2 neutrals, phase cables equally spaced, or touching. Trefoil groups touching.

L1 L2 L3

L1L2L3 N

L1 L2 L3 N L3L2L1

N N N Nor or

L1 L2L3

L1 L2L3

L1 L2L3

(*) (*)

(*) (*)L1 L2 L3 L1 L2 L3 L1 L2 L3

(*) = clearance De

(*) = clearance 3 De

1 neutral may be run in middle ofeach inter-group spacing.

NL1 L2 L3 N L1L2L3

NL1 L2 L3 N L1L2L3

N

NL1 L2L3

(*) L2L1L3

(*)

N

NL1 L2L3

(*) L2L1L3

(*)

(*) = clearance DeTrefoils touchingVertical clearancebetween trefoils 2 De

N = 1 neutral, up to 4 positions may be used If position is not used space must be maintained

N = 1 neutral, up to 4 positions may be used

Cables touching or equally spaced

(Where De = the outsidediameter of the cable.)

(Where De = the outsidediameter of the cable.)

(Where 3 De = 3 times theoutside diameter of the cable.)


2.3.5 Ring final circuits

Fig 6: Typical ring final circuit arrangement

Distributionboard

Current-carrying capacity of thephase and neutral conductors (Iz)shall be not less than 20 A.

Regulation 433-02-04, as amended by Amendment No1 February 2002, sets out the requirements for ringcircuits. This regulation requires that the current-carrying capacity (Iz) of the cable is not less than 20A,and that, under the intended conditions of use, theload current in any part of the ring is unlikely toexceed for long periods the current-carrying capacityof the cable. This is reinforced by Regulation 433-01-01,which requires that every circuit be designed ‘so thata small overload of long duration is unlikely to occur’.

This last point is important because a small overloadcurrent may not be sufficient to operate theprotective device, but could raise the temperature ofthe conductors above the rated value and damage thecable. An explanation of what is a small overload andwhat is a long period is given in paragraph 2.3.6.

Ring circuits are intended to provide large numbers ofconveniently placed outlets, rather than a high loadcapability. The limitation on the floor area served by aring circuit in household installations is intended toprovide a simple means of load limitation.

For household installations, a single 30 A or 32 A ringfinal circuit may serve a floor area of up to 100 m2.However, careful consideration should be given to the

433-02-04

433-01-01


loading in kitchens, which may require a separatecircuit. Socket-outlets for washing machines, tumbledryers and dishwashers should be located so as toprovide reasonable sharing of the load in each leg ofthe ring, or consideration should be given to asuitable radial circuit.

Immersion heaters fitted to storage vessels in excess of15 litres capacity, or permanently connected heatingappliances forming part of a comprehensive electricspace heating installation, should be supplied by theirown separate circuit.

Table 4D5A of Amendment No 1 of BS 7671:2001gives the current-carrying capacity of 70 °Cthermoplastic (pvc) insulated and sheathed flat cablewith protective conductor which is often used inhousehold installations.

For other premises or uses, a careful assessmentshould be made of the maximum load, its timediversity and its distribution around the ring, whichmay not be related to floor area. This is essential fornon-domestic use because the circuit protection mustnot be used as a load limiter.

2.3.6 Load assessment

What is a small overload?

On the question of what is a small overload, guidancecan be taken from the BS 88 fuse standard and theBS EN 60898 and BS EN 61009-1 circuit-breakerstandards. For a BS 88 fuse the ‘non-fusing’ current is1.25 times the fuse rating and for BS EN 60898 circuit-breakers and BS EN 61009-1 RCBOs the ‘non-tripping’current is 1.13 times the rating of the circuit-breakers.(The non-fusing and non-tripping currents are thecurrents that the fuse or circuit-breaker must carry forthe conventional time without operating.) The fusingcurrent is 1.6 times the fuse rating and the trippingcurrent is 1.45 times the circuit-breaker rating. (Theseare the currents at which the fuse or circuit-breakermust operate within the conventional time, which is1 hour for fuses and circuit-breakers rated up to andincluding 63 A.) For the tests, the fuse starts from cold,the circuit-breaker starts from a heated condition.


There is also a test under different conditions wherethe fuse must operate within the conventional time at1.45 times its rating. BS 7671 accepts the 1.45 factorand hence accepts that an overload can be toleratedfor the conventional time, that is, 1 hour for a 32 Afuse.

Circuit-breakers to the standards above must operatewithin the conventional time (i.e. within 1 hour) at1.45 times their rating.

Therefore, it seems reasonable to suggest that a smalloverload is between 1.25 and 1.45 times the rating ofthe overload protective device.

What is a long period?

A long period cannot be less than the 'conventionaloperating time' (one hour) of a fuse because that isalready allowed. It is also worth noting that thetime/current characteristic curves in Appendix 3 ofBS 7671 for a 32 A BS 88 fuse stop at around onehour. It is reasonable to suggest that a 'long period' isa time greater than that for which information onfuse or circuit-breaker performance, that is, one hour.

2.3.7 Mineral insulated cables and ring circuits

Regulation 433-02-04 as amended gives a minimumcross-sectional area of both phase and neutralconductors of 2.5 mm2, except for two-core mineralinsulated cables to BS 6207, for which the minimum is1.5 mm2. There has been concern the 73 °C operatingtemperature of mineral cables (for a 70 °C sheathtemperature) might exceed that allowed foraccessories by the appropriate standard (atemperature of 70 °C is assumed in the type tests ofthe standards). The current-carrying capacity of two-core 1.5 mm2 light duty mineral insulated cable is 23 A(see column 2 of Table 4J1A of BS 7671). At thisloading the estimated conductor temperature is 73 °C(sheath temperature 70 °C) which would overheat theaccessory. This resulted in concerns regarding theadvisability of applying the 20 A relaxation ofRegulation 433-02-04.

433-02-04

Table 4J1A

433-02-01

However, assuming a maximum current of 20 A andthat temperature rise is proportional to the square ofthe current, then temperature rise for a ring circuitwith a maximum current of 20 A is given by:

= x (73 - 30) °C

= 32.51 °C

Assuming an ambient temperature of 30 °C gives aconductor temperature of 62.51 °C which is below 70 °C.

Overload current protection may be omittedaltogether under certain circumstances. However, itshould not be omitted, or its sensitivity reduced,without very careful consideration; see comments onRegulations 473-01 and 473-02 in paragraph 1.4 ofthis Guidance Note.

Regulation 473-01-03 allows the omission of devicesfor protection against overload for circuits supplyingcurrent-using equipment where unexpecteddisconnection of the circuit could cause danger. Theregulation gives the following examples:

In such situations consideration shall be givento the provision of an overload alarm.

• Exciter circuits of rotating machines.

• Circuits which supply fire extinguishing devices. Forexample, a sprinkler system.

Sprinkler systemfor fire fighting

Power supply

2.4 Omission ofprotectionagainstoverloadcurrent473

2023

473-01-03



• Supply circuits of lifting magnets.

Electro-magnets are used in, e.g. scrap yards to liftand carry loads. If such a magnet is de-energisedwhile in operation this could cause damage orinjury.

• Secondary circuits of current transformers.

Primarycircuit

Secondary circuit

The secondary circuit of a current transformer mustbe kept closed when current is flowing in the primarywinding. If not, the peak value of emf induced in thesecondary circuit will be many times the nominalvalue, and could be large enough to causebreakdown of the insulation and danger.

Regulation 473-01-04 gives a relaxation where devicesfor protection against overload need not be provided.

These are:

• For a conductor situated on the load side of thepoint where a reduction occurs in the value ofcurrent-carrying capacity, where the conductor iseffectively protected against overload by a protectivedevice placed on the supply side of that point.

473-01-04


The diagram above shows a circuit in which a largercable than necessary (from a current-carrying pointof view) is used to reduce voltage drop, where theroute to the first luminaire from the distributionboard is a long run. The cable from the firstluminaire supplying the remainder of theluminaires on the circuit is then reduced, but isprotected against overload by the 10 A device.

• For a conductor which, because of thecharacteristics of the load or supply, is not likely tocarry overload current.

• At the origin of an installation where thedistributor provides an overload device and agreesthat the device affords protection to the parts ofthe installation between the origin and the maindistribution point of the installation where furtheroverload protection is provided.

Meter

labelled“mainswitch”

Electricity distributor’s cut-out

Meter tails

Distribution board

Electricity distributor’s supply cable

Current rating ofprotective device

In = 10 A Current-carryingcapacity of cable

Iz = 20 A

Luminaire

Luminaire

Cable sizereduced to10 Acurrent-carrying

capacity

Ib = 8 A

Long run


Section 3 — Protection Against Fault Current

The Regulations require that each circuit should beprovided with a device capable of breaking any faultcurrent flowing in that circuit before it causes dangerdue to thermal or mechanical effects.

Fault currents result from an insulation failure or thebridging of insulation by a conducting item.

There are two types of fault:

— a short-circuit, where the current flows betweenlive parts (which may include the neutral)

— an earth fault, where the current flows between alive part and earth. The latter includes anyconductive part or conductor which is connected tothe earth terminal or is substantially in contactwith the mass of earth.

The types of damage envisaged include:

— overheating and mechanical damage toconductors, connections and contacts

— deformation and deterioration of insulationleading to future electrical breakdown

— risk of ignition of materials adjacent to aconductor, due to its high temperature or toprolonged arcing at the fault or at a break in thecircuit caused by the fault current.

In general, fault currents are much larger than eitherload currents or overloads and the amount ofdamage which can be done is so great that rapidinterruption is essential.

3.1 Types of faultto beconsidered130434

3.2 Nature ofdamage andinstallationprecautions434543

130-05130-04434-01


BS 7671 requires that fault energy be limited to avalue which will not cause a temperature risesufficient to damage insulation (see Regulations434-03-03 and 543-01-03) and appropriatetemperature limits are given for live conductors inTable 43A and for protective conductors in Tables 54Bto Table 54E. Table 54F, for bare conductors, is based ongeneral considerations with regard to damage to theconductor or to unspecified materials which may be incontact or near by. Damage means excessivedeformation or reduction in thickness of theinsulation.

The temperature limits given in these tables are basedon the physical properties of the insulating materials,together with results of tests made on cablessupported so that mechanical forces on the insulationwould be representative of the values likely to occurin practice.

3.2.1 Installation precautions

Poor installation methods or adverse installationconditions may result in the imposition of greaterforces, with the risk that damage may be excessive.For this reason good workmanship is essential wheninstalling cables and methods of support shouldcomply with those given in Chapter 52 of BS 7671.

In particular, careful attention should be paid to theminimum installation radii recommended inappropriate cable standards, the avoidance ofexcessive force during installation, contact with sharpedges, overtight supporting clamps, unsupportedlengths, leaving a cable in tension and to allsituations where local stressing might occur.

3.2.2 Expansion forces

All conductors expand lengthways when heated andcable support should be such that this expansion canbe accommodated as uniformly as possible along theroute. Supports and cable ties should be uniformlyspaced and, in the case of larger cables, these should

434-03-03543-01-03Table 43ATable 54BTable 54CTable 54DTable 54ETable 54F

Chap 52


be set in a slightly sinuous path so that a small lateralmovement is possible. Otherwise, expansion mayaccumulate at one point, causing severebending, or at the ends where it may overstress theterminations.

Larger sizes of conductor, roughly 50 mm2 or more,are capable of exerting considerable longitudinalexpansion forces and, unless the expansion isaccommodated uniformly along the route, theseforces may cause serious distortion of the conductortails at terminations or mechanical overstressing ofthe connections.

Local restriction of larger cables at bends, particularlyat the smaller radius bends, will produce high localstresses on both insulation and sheath, with anincreased risk of damage.

Cables buried directly in the ground have noopportunity for expansion relief by lateral movementand expansion forces are applied directly to joints andterminations. Cable and joint makers should beconsulted to confirm that the installation canwithstand the short-circuit temperatures envisaged.

Allowance must be made for the thermal expansionand contraction of long runs of steel or plastic conduitor trunking, and adequate cable slack provided toallow free movement. The expansion or contractionof plastic conduit or trunking is greater than that ofsteel for the same temperature change.

For busbar trunking systems BS EN 60439-2 identifies abusbar trunking unit for building movements. Such aunit allows for the movement of a building due tothermal expansion and contraction. Reference shouldbe made to the manufacturer since requirements maydiffer with respect to design, current rating ororientation of the busbar trunking system (i.e. riser orhorizontal distribution).

Cables crossing a building expansion joint should beinstalled with adequate slack to allow movement, anda gap left in any supporting tray or steelwork. Aflexible joint should be provided in conduit ortrunking systems.


3.2.3 Electromagnetic forces

Compliance with the temperature limits in Table 43Amay not avoid the risk of damage due toelectromagnetic forces.

These forces are unlikely to be important for circuitswhere fault current protection complying withRegulation 434-03-02 and rated at not more than100 A is provided, but can become increasinglysignificant as fault currents increase above about15 kA rms or about 20 kA pk. It should be borne inmind that an initial peak current approaching 1.8times the rms value is possible when a fault iselectrically near to a large transformer or generatorsource.

The cut-off characteristics of current limiting faultprotection devices will act to restrict such high initialpeak currents and the forces they would produce.Information can be obtained from the manufacturerof the device.

It can be assumed that unarmoured multicore cablesof all sizes are strong enough to withstandelectromagnetic bursting forces (i.e. mechanicalforces, forcing the conductors apart) where protectionis co-ordinated with cable size in compliance withRegulation 434-03-02 or with Regulation 434-03-03when the fault current protection device is of thecurrent limiting type. Cable makers should beconsulted where let-through currents are likely to bein the region of 25 kA rms or higher.

It can be safely assumed that all armoured multicoreextruded insulation cables are strong enough towithstand bursting forces without damage, whatevertype of protective device is employed. However,where currents may exceed 25 kA rms, it is wise toconsult the cable maker.

In the case of single-core cables, electromagneticforces act to move the cables apart. The effect isdependent on cable size, type and spacing (the closerthe cables, the greater the forces), on supportprovided by fixings and on the peak value of thecurrent. Fault currents well in excess of 20 kA pk mayresult in severe bending of the cables and with highercurrents there is the possibility of broken fixings. To

434 434-01-01Table 43A

434-03-02434-03-03

434-03-02


reduce the risk of damage, cables should be stronglybound together at frequent intervals to resistelectromagnetic forces, but should be supported orfixed at greater intervals to allow lateral movementand uniform relief of longitudinal expansion bysnaking. Again, the cable maker can provideinformation on the type of binding and supports touse.

A weak point as regards both longitudinal expansionforces and electromagnetic forces is unsupportedlengths of core at joints and terminations.Electromagnetic forces are highest in the region ofthe crutch of multicore cable terminations, i.e. wherethe conductors are closest. Adequate support, whichmay be in the form of a suitable gland or binding, toreinforce the end of the cable envelope and tosupport the cores at the crutch is desirable. Lateralsupport for cores should be considered where tails arelong enough to bend and impose strain on terminals.

Adequate support must be given to cables in busbarchambers to prevent abrasion of insulation byelectromagnetic movement.

It is conventionally assumed that the impedance ofthe actual fault is zero. This makes calculation of thevalue of fault current straightforward, because onlythe impedances of the circuit conductors need betaken into account. Such an assumption neglects anyarc voltage and provides the highest expected valueof fault current at a particular point in a circuit,referred to as the prospective fault current, andhence the maximum current breaking capacityrequired of the device to be selected to interrupt thecurrent.

It is important for this selection to be correct, not onlybecause the successful interruption of the currentavoids unnecessary risk of damage, which in highercurrent circuits can be very serious, but also because,in the extreme, if the device fails to interrupt thecurrent it is likely to be destroyed and the duty ofinterruption passes to the next device upstream. Thisnext device is usually chosen to detect a much highermagnitude of fault current and may not operate, or

3.3 Faultimpedanceand breakingcapacity ofprotectivedevice131432434

131-08-01432-02-01432-04-01434-02-01


may only operate after considerable damage and riskof danger to persons or property has occurred.

With certain exceptions, fault current protection isrequired at the origin of each circuit or, moregenerally, where there is a reduction in the size ofconductor and hence in fault current withstandcapacity.

3.4.1 Assessment of fault current

At the origin of a circuit, or point of reduction inconductor size, it is necessary to calculate both thehighest and the lowest values of prospective faultcurrent. The operating time and energy let-through(I2 t) of a protective device depends on the currentand it is necessary to check that conductors beingprotected can withstand the higher of the two I2 tlet-through values. It is a requirement that the deviceis capable of safely interrupting the highestprospective current and that it will operate correctlyfor the lowest prospective fault current.

The highest value of prospective fault current iscalculated assuming a fault immediately on the loadside of the device. The impedances concerned arethose upstream of the device, including that of thesupply to the installation, and should include, ifsignificant, the impedance of the device itself.

The lowest value of prospective fault current iscalculated on the assumption that a fault occurs atthe load end of the circuit or at the input to the nextdownstream fault current protective device.

In a single-phase circuit the line to earth prospectivefault current may be greater than that for a line toneutral fault.

3.4.2 Fault current and impedance at origin of installation

BS 7671 requires that the prospective short-circuitcurrent at the origin of the installation be ascertained.

The Electricity Association have indicated inEngineering Recommendation P25/1 that at a service

3.4 Position offault currentprotection andassessment ofprospectivecurrent473

533

313

473-02-01

533-03-01

313-01-01


tee-off of a public supply network a typical maximumvalue of 16 kA may be assumed for single-phasesupplies where the distributor has provided a cut-outrated up to 100 A. Engineering Recommendation P.26is applicable to three-phase supplies and quotessomewhat higher values.

Consumer units are tested to a conditional test inBS EN 60439-3 to represent applications fed from a100 A fused cut-out with supply fault levels up to16 kA.

The Engineering Recommendations includeinformation on the attenuation in fault currentprovided by the service cable to the consumer’sinstallation, leading to much reduced values ofshort-circuit current at the service cut-out. The extentof the reduction depends on the size (csa) of cableand its length.

This reduction in short-circuit current by the servicecable does not apply to installations in high loaddensity areas of major city centres.

For other situations the distributor should beconsulted or, if the source is a consumer’s transformer,the manufacturer’s impedance data should beapplied.

The calculation of fault current at the origin of theinstallation, and hence of the impedance of thesource, should take into account contributions, if any,from sources additional to the public or externalsupply. Such additional sources may take the form oflocal generation or of large motor loads connectedelectrically close to the supply point.

The lowest prospective value relates to a fault at thedownstream end of the conductors protected by thedevice, notionally at the input to the next distributionboard or at the terminals of the current-usingequipment fed by that circuit.

As stressed earlier, protection of conductors inaccordance with Chapter 43 may not provideprotection for equipment connected to thoseconductors. Where a final circuit supplies a knownsingle load it may be feasible to select a device whosecharacteristics will also protect the equipment, butreference to the equipment manufacturer may be


necessary. Alternatively, equipment protection shouldbe included at or in the equipment itself.

This aspect is of importance when non-fused plugs areused, such as industrial type to BS EN 60309-2. If thedesigner intends to incorporate such plugs he wouldrequire to establish the sizes and types of flexiblecords likely to be used. There may be a case forlimiting the current rating of the circuit or setting aminimum csa or maximum length of cord to be used.

Attention is drawn to Section 8 of this Guidance Notewhere this matter is considered further.

3.4.3 Relocation of fault current protection

Fault current protection may be located on the loadside of the position required by Regulation 473-02-01provided that the conditions given in Regulation473-02-02 are met.

The three conditions in Regulation 473-02-02 areintended to reduce the risk of a fault occurring and, ifa fault should occur, the risk of fire and danger topersons or property. The following measures areenvisaged:

(i) the use of conductor lengths which are as shortas possible

(ii) the reduction as much as is reasonably practicableof the risk of insulation breakdown, by choice ofroute, provision of adequate support andenclosure or barriers, together with the additionof supplementary insulation separately over eachconductor

(iii) consideration of the effects of mechanicaldamage, including conductor displacement, eitherfrom external mechanical forces or fromelectromagnetic forces in the event of a faultdownstream

(iv) although the prime object should be to avoidfaults greater than the withstand of theconductors, locating or enclosing the conductorsso that, in the event of a fault, risk of fire ordanger to persons or property by the emission offlame, arcs or hot particles is prevented as far as isreasonably practicable.

473473-02-01

473-02-02


Where metallic enclosures or barriers are used theymust be earthed to the circuit protective conductor.

It is not satisfactory to assume that small conductorsare safely expendable. If such a conductor isdestroyed, the arc products will contain copiousquantities of ionized gases which may facilitate thepersistence and flashback of an arc to largerconductors, where arcing is almost certain to doconsiderable damage. Where fusing of smallconductors cannot be provided the run of theconductors should be carefully designed so as tomaximize the chances of containment in the event ofa fault.

A small conductor, such as one supplyinginstrumentation from a large conductor, can beconnected through a small fuse unit of adequatefault current breaking capacity mounted directly onthe large conductor.

Fig 7: interconnections between switchgear items

Overcurrentprotective

device


device


device


device


device


device


deviceBusbar

chamber

Sub-maincable

Each conductor not toexceed 3 m in length

Interconnectingcables

All switchgear must be rated for thefault level at the point of installation.

Regulation 473-02-02 is of special importance forinterconnections between switchgear (Figure 7 above) and for temporary connections made to equipmentwhere there may not be room for the installation of asuitable fault protection device at the tap-off point.In this latter situation the original integrity of the

473-02-02


switchgear assembly and enclosure as regardsprotection against entry of foreign substances,compliance with Chapter 41, Protection againstShock, and Chapter 42, Protection against ThermalEffects has to be maintained.

BS EN 60439-1 Specification for type-tested andpartially type-tested assemblies, requires that themanufacturer shall specify in his documents orcatalogues the conditions, if any, for the installation,operation and maintenance of the assembly and theequipment contained therein.

If necessary, the instructions for the transport,installation and operation of the assembly shallindicate the measures that are of particularimportance for the proper and correct installation,commissioning and operation of the assembly.

Where necessary, the above mentioned documentsshall indicate the recommended extent and frequencyof maintenance.

If the circuitry is not obvious from the physicalarrangement of the apparatus installed, suitableinformation shall be supplied, for example wiringdiagrams or tables.

Regulation 473-02-03 applies where fault protectionupstream of the point where a conductorcross-sectional area is reduced also provides faultprotection complying with Regulation 434-03-03 forthe smaller conductor. Further fault protection maythen be provided at any point downstream of theconductor change (cascading), for example to providediscrimination, but not primarily to protect theconductor.

3.5.1 General

Fault current protection may be omitted altogetherunder certain circumstances. However, fault currentprotection should not be omitted, or its sensitivityreduced, without very careful consideration; seecomments on Regulations 473-01 and 473-02 inparagraph 1.4 of this Guidance Note.

473

3.5 Omission offault currentprotection

473-02-03

434-03-03


Regulation 473-02-04 allows the omission of devicesfor protection against fault current in the followingcircumstances, provided that the conductor that is notprotected is installed in such a manner as to reduce toa minimum the risks of fault current and fire ordanger to persons.

• For a conductor connecting a generator,transformer, rectifier or battery with its controlpanel.

• In a measuring circuit where disconnection couldcause danger, e.g. secondary circuits of currenttransformers.

Primarycircuit

Secondary circuit

The secondary circuit of a current transformer mustbe kept closed when current is flowing in theprimary winding. If not, the peak value of emfinduced in the secondary circuit will be many timesthe nominal value, and may be sufficient to causebreakdown of the insulation and danger.

• Where unexpected opening of a circuit causesgreater danger than the fault current condition e.g.lifting magnets.

473-02-04

Generator

Controlpanel

Interconnecting cables


Electro-magnets are used in, e.g. scrap yards to liftand carry loads. If such a magnet is de-energisedwhile in operation this could cause damage orinjury.

• At the origin of an installation where thedistributor provides a fault current device andagrees that the device affords protection to theparts of the installation between the origin and themain distribution point of the installation wherethe next step for fault protection is provided.

Item (iv) of Regulation 473-02-04 must be consideredtogether with item (iv) of Regulation 473-01-04which also deals with the protection of cablesbetween the outgoing meter terminals and the first

473-02-04473-01-04

Meter

labelled“mainswitch”

Electricity distributor’s cut-out

Meter tails

Distributors board

Electricity distributor’s supply cable


point of overcurrent protection in an installation.In general, the distributor is able to make asatisfactory assessment of the maximum demand sothat overload need not be a problem (seeRegulation 473-01-04(ii)). The important issue willbe fault current protection. Sizing and the methodof installation of these cables must be agreed withthe distributor. If Regulations 473-01-04(iv) and473-02-04(iv) are not met then overcurrentprotection, complying with Regulation 473-02-02,should be installed close to the origin.

Regulation 434-03-02 deals with a very commonsituation, where the same fuse or circuit-breakerprovides both types of protection. The assumptionrelies on the inherent characteristics of the devicesused, whereby correct overload protection, achievedby applying the conditions of Regulations 433-02-01and 433-02-02, will involve devices having overcurrentcharacteristics which will ensure compliance with thefault current Regulation 434-03-02.

It may be deduced from the latter part of theregulation that non-current limiting devices may notprovide the required characteristic. Doubt could arisewhen a device not described in Regulation 433-02-02is used; a check must then be made, using thecharacteristic of the device chosen, for compliancewith Regulation 434-03-03.

Fault current limitation refers to the characteristic of adevice which is so constructed that interruptiontakes place in less than one half cycle of current. Fusesto BS 88 provide this characteristic provided that theyare suitably selected with regard to the value ofprospective current.

Circuit-breakers can be of either type, the currentlimiting type sometimes being referred to as ‘fastacting’. A non-current-limiting type may be referredto as a ‘zero point’, ‘current zero’ or ‘half cycle’device.

Since there is no special marking to identify thesetypes, it is necessary to refer to the manufacturer ortrade literature.

3.6 The use of onedevice for bothoverload andfault currentprotection434

434-03-02

433-02-01433-02-02

433-02-02

434-03-03

473-01-04473-02-04473-02-02


Regulations 473-03-04 to 07 recognize the effect oftriple harmonic currents in the neutral conductor andthe need to take account of this. Regulations 473-03-04and 05 are concerned with TN or TT systems. Theseregulations require overcurrent detection in theneutral conductor in a polyphase circuit, where theharmonic content of the phase currents is such thatthe current in the neutral conductor is reasonablyexpected to exceed that in the phase conductors. Thisdetection shall cause disconnection of the phaseconductors but not necessarily of the neutralconductor.

The reason behind these regulations is that certainequipment such as the switched mode power suppliesof computers and discharge lighting produce thirdharmonics. This third harmonic content producesharmonic distortion. For single-phase circuits this isnot a problem from an overcurrent point of viewwhen sizing the neutral conductor, as the neutralcurrent always equals the phase current in bothmagnitude and harmonic content and no specialmeasures need to be taken. The problem occurs inthree-phase circuits. For example, 50Hz three-phaseload currents, if balanced, cancel out in the neutralbecause of the 120 degree displacement of eachphase (see figure below). Even if the load is notbalanced the neutral conductor will only carry theout-of-balance current, which is less than the phasecurrent. However, the third, and other tripleharmonics combine in the neutral to give a neutralcurrent that has a magnitude equal to the sum of thethird harmonic content of each phase. The heatingeffect of this neutral current could raise thetemperature of the cable above its rated value anddamage the cable.

Therefore, consider a sub-main cable supplying athree-phase installation where there is likely to be asignificant harmonic content; this is where computerloads or discharge lighting loads ( or other loadswhich produce third harmonics) represent asignificant proportion of the total load. Where theharmonic content of the phase currents is such thatthe current in the neutral conductor is reasonablyexpected to exceed that in the phase conductors, theRegulations require overcurrent detection in the

3.7 Harmonics

524-02-02

473-03-04473-03-05473-03-06473-03-07

neutral and allowances must be made in the sizing ofcables (Regulation 524-02).

More information on harmonics to enable cable sizesto be obtained is given in the Commentary on IEEWiring Regulations 16th Edition BS 7671 : 2001.

Phase 1

Phase 3

Phase 2

120º

Phase 1

Phase 2

Phase 3

Fundamental Third harmonic

Neutral

The third harmonics combine in the neutral to give aneutral current that has a magnitude equal to the sumof the third harmonic content of each phase



Section 4 — Determination of Fault Current

Regulation 434-02-01 states: The prospective faultcurrent, under both short-circuit and earth faultconditions, at every relevant point of the completeinstallation shall be determined. This shall be doneeither by calculation, ascertained by enquiry or bymeasurement.

Unless fault current is measured directly, itsdetermination relies on measurement or calculationof relevant values of fault loop impedances.

Engineering Recommendation P26 published by theElectricity Association deals with the estimation of themaximum prospective short-circuit current for three-phase 400 V supplies.

Engineering Recommendation P25/1, also publishedby the Electricity Association, deals with the short-circuit characteristics of public electricity distributorslow voltage distribution networks and the co-ordination of overcurrent protective devices on 230 Vsingle-phase supplies up to 100 A.

For 230 V single-phase supplies up to 100 A, theelectricity distribution company will provide theconsumer with an estimate of the maximumprospective short-circuit current at the distributioncompany cut-out, which will be based on EngineeringRecommendation P25/1 and on the declared level of16 kA at the point of connection of the service line tothe LV distribution cable. The fault level will only bethis high if the installation is close to the distributiontransformer. However, because changes may be madeto the distribution network by the electricitydistributor over the life of an installation, thedesigner of the consumer’s installation must installequipment suitable for the highest fault level likely.

General434

4.1 Determinationof faultcurrent byenquiry

434-02-01

Since the fault level of 16 kA is at the point ofconnection of the service line to the LV distributioncable, a reduction of this fault level is allowed for theservice line on the customer’s premises. This is becausethe service line on the customer’s premises will not bechanged without the customer’s knowledge. Thereare some inner city locations, particularly in London,where the maximum prospective short-circuit currentexceeds 16 kA, and the electricity distributor wouldtake account of this when advising the customer ofthe fault level.

For three-phase 400 V supplies, the electricitydistribution company will provide the consumer withan estimate of the maximum prospective short-circuitcurrent at the electricity company cut-out. This will bebased on Engineering Recommendation P26, and willalso be based either on the declared fault level of 18kA at the point of connection of the service line tothe LV distribution cable or on the declared fault levelof 25 kA at the point of connection to the electricitydistributor’s substation.

Since the fault level 18 kA is at the point ofconnection of the service line to the LV distributioncable, again, a reduction of this fault level is allowedfor the service line on the customer’s premises.

In the event that the service cable is supplied from adistribution cable in the footpath on the far side ofthe road the attenuation in fault level can only beapplied from the footpath nearest to the property.This is because the electricity distributor may, at sometime in the future, install a distribution cable in thefootpath nearest to the property from which tosupply the service cable.

More information on the determination of faultcurrent and detailed tables on the estimation ofmaximum prospective short-circuit current is given inthe Commentary on IEE Wiring Regulations 16th

Edition BS 7671 : 2001.



Determination of fault level at the origin by enquiry.

Figure 8 shows a number of consumers supplied via anelectricity company’s service cable, and the reductionin fault level at the distributors cut-outs due to thelength of service cable. Shown are typical values, referto the electricity distributor for actual values of faultlevel on a particular installation.

Fig 8: Reduction in fault level at the suppliers cut-outs

Fault level based on table in ElectricityAssociation publication with a servicecable of 15 m long is 10.2 kA at 0.8 pf.400 volt

Fault level based on table in ElectricityAssociation publication with a servicecable of 4 m long is 11.7 kA at 0.78 pf.230 volt single-phase 100 A supply.

Declared fault level of 16 kA for a230 V single-phase 100 A supply atthe point of connection of the serviceline to the LV distribution cable

4 m

Dwelling

4 m

Dwelling

School

15 m

5 m

Substation

Electricity companyservice cable

Up to 35 sq mm aluminiumelectricity company service cable

Up to 25 sq mm aluminiumor 16 sq mm copper service cable

Electricity companydistribution cable

In the event that the service cable is supplied from adistribution cable in the footpath on the far side of theroad the attenuation in fault level can only be appliedfrom the footpath on the near side to the property. This isbecause the electricity distributor may at some time inthe future install a distribution cable in the footpathnearest to the property from which to supply the servicecable.

College supplied directly fromelectricity company substation

Road

Footpath

Footpath

Declared fault level of 18 kA fora 400 V three-phase supply at thepoint of connection of the serviceline to the LV distribution cable

Road

Footpath

For the declared fault level at thepoint of connection of theelectricity suppliers substationconsult the electricity distributor.

Fault level based on table in ElectricityAssociation publication with a servicecable of 5 m long is 23.1 kA at 0.4 pf.400 volt 3-phase supply. This is basedon a fault level of 25 kA at the point ofconnection to the electricity distributorssubstation.

Fault level based on table in ElectricityAssociation publication with a servicecable of 4 m long is 11.7 kA at 0.78 pf.230 volt single-phase 100 A supply.


Measurement of prospective fault current or circuitimpedance requires special equipment. For circuitsrated up to about 100 A, portable equipment isavailable and is generally used for the measurementof earth fault loop impedance. It operates byimposing a heavy load, usually greater than therating of the circuit, for a very short duration, andmeasuring the consequent drop in voltage. From thisdata the upstream impedance and hence prospectivefault current is derived.

For higher rated circuits the currents required toeffect such a measurement accurately areproportionally higher and the equipment becomescumbersome and expensive.

To achieve a satisfactory accuracy, test currents haveto be high and to avoid damage or unnecessaryoperation of protection devices the duration must bekept as short as possible, notionally only a few halfcycles.

The measurement involves some degree of danger,increasingly so at the higher currents. It should beundertaken only by skilled personnel with adequateequipment, operated according to the manufacturer’sinstructions, and taking appropriate safety measures.

For a larger installation an acceptable alternative is todisconnect the circuit from the supply and to measureits impedance with reduced voltage high current testequipment. Where circuit impedance is very low thismay prove to be the best, or only, way to achieve asatisfactory accuracy with safety.

With an external supply its impedance has to beascertained from the distributor or assessed frompublished information and added on to the valuesmeasured within the installation.

Measurement is a useful method when there is doubtas to the status of a calculated value, where theimpedance of an existing circuit is required and theroute is inaccessible or, for low current final circuits, asa convenient means of regular verification of earthloop impedance. Unless a complicated method ofmeasurement is used the resistive and reactivecomponents of the impedance are not identified so

4.2 Measurementof faultcurrent


that arithmetical addition of further impedances canonly be approximate.

Accuracy of measurement, as opposed to accuracy ofinstrumentation, may not be as good as is sometimesthought, particularly for very extensive installations.Results may be affected by the presence offerromagnetic materials, the effect of which can besensitive to the value of the measuring current, nearto or around conductors. This is particularly so forsingle-core cable circuits.

With earth fault loop measurements there is thepossibility of ferromagnetic materials forming part ofthe loop, together with the uncertainty thatsubsequent operations by other trades or changes toa building structure may modify the effective path offault currents.

Attention is drawn to the fallacy of reading the lastone or two digits on a three or four digit display andomitting to overlay the result with the inherentinstrument errors. The manufacturer’s instructionsshould always be consulted before taking a reading,not only when a reading is felt to be unusual.

Where measurement indicates that the expectedvalue of fault current is low for the type of supplyconcerned, consideration should be given to thepossibility that the supply may be reinforced at somefuture date. Protective devices should be selectedwhich will not become inadequate shouldreinforcement occur.

Measurement is usually done when the circuit isoff-load, and the value obtained relates to conductorsat ambient temperature.

Derivation of circuit impedance and fault currentvalues by calculation is the usual and sometimes theonly way of providing design data. Calculation offault current is possible only if all the impedances areknown or can be estimated.

Provided that route lengths are accurate, betterresults are likely to be achieved with low currentcircuits where reactance can be ignored. Likelysources of error are connections and busbars for

4.3 Calculationof faultcurrent434


higher current installations, where an accuratecalculation of reactance may be difficult orimpracticable.

It should be borne in mind that where measurementshave been made on installed circuits, values ofprospective current have been reported to be lowerthan calculated values based on route lengths. Thediscrepancies have been attributed to unascertainableadditional resistances and, particularly, reactances inequipment such as distribution boards and busbars.(See J Rickwood ‘Fault Currents in Final Sub-Circuits’Paper at IEE/ERA Conference on Distribution,Edinburgh, 20-22 October 1970.)

Errors where actual currents are lower than thoseestimated may be on the safe side as far as selectionof breaking capacities and withstand ofelectromagnetic forces are concerned, but therepercussions for thermal damage limitation,Regulations 434-01-01 and 434-03-03, are different. Alower fault current will correspond to a longer faultclearance time by the protective device and, in mostcases, a greater energy let-through.

Fortunately, the situation is largely redressed by thefact that the actual operating currents of protectivedevices are generally somewhat lower than themaximum requirements of the relevant standards andof published data.

4.3.1 Highest value of fault current

The lowest impedance of a fault current path may beobtained by assuming that the fault occurs when theconductors are cold. In view of the fact that thehighest value of fault current is usually required forthe selection of a device with adequate breakingcapacity, rather than to check on the fault currentcapacity of conductors, it is sufficient and on the safeside to neglect the increase in conductor resistanceduring a fault. Unless the conductors are located in alow ambient temperature, such as within arefrigerated area, resistance values tabulated for20 °C may be used.

434

434-01-01434-03-03

4.3.2 Lowest value of fault current

Contrary to expectation, the lowest value of faultcurrent is used when considering thermal stresses onconductors as a result of a fault, Regulation 434-03-03.This comes about because the characteristics of mostof the circuit protective devices considered in BS 7671are such that the fault energy let-through (I2 t) isgreater the lower the fault current. It follows thatfaults are assumed to occur when a circuit is at itsmaximum permissible working temperature andconductor resistances are high.

The lowest value of fault current is also used whenconfirming compliance with the maximumdisconnection times required for protection againstindirect contact (shock), and the longest likelyduration of the fault is to be checked against therequirements of Regulations 413-02-09, 413-02-12 or413-02-13.

434413

413-02-09413-02-12413-02-13

434-03-03



Section 5 — Equations for the Calculation ofShort-Circuit Current

The general equation for fault current is:

If = (1)

where the symbols are:

If fault current, amperes,

V voltage between conductors at supply point,either U0 or U, volts,

R resistance of the fault loop from source to pointof fault, ohms,

X reactance of the fault loop from source to pointof fault, ohms.

The resistance and reactance are made up of anumber of components, whose values depend on thetype of fault and the supply system, see Fig 9.

Generally:

R = Rs + R1 + Rn

X = Xs + X1 + Xn

where the suffixes indicate:

s supply or source quantity. Where power isobtained from a distributor these are external tothe installation. It must be remembered thatpublic electricity supply systems change to reflectgrowth or decline in electricity consumption forthe local network and to meet day to dayoperational requirements. In general, a publicelectricity distributor cannot give a singleunalterable value for the supply impedance.

5.1 Generalequation forfault current V

(R2 + X2)


1 line conductor quantity; that is, the totalresistance of the line conductor(s) carrying thesame fault current within the installation.

n neutral conductor quantity, as for 1.

In most cases there is a need to establish theminimum and maximum values and these can beobtained by reference to Electricity Associationpublications Engineering Recommendations P23/1,P25/1 and P26. For a user owned source, a single valuewill generally be sufficient and this can be obtainedfrom the manufacturer of the source equipment.

Fig 9: General arrangement of impedances

The terms R, X and Z are referred to as though theyrelate to a single length of conductor. It is, of course,clear that in practice they will relate to a number ofconductors of different sizes in series carrying thesame fault current and contributing towards the totalvalue of R, X or Z. The value of a term in theequations given is the sum of the values for eachportion of the total route from the supply point tothe fault point.

This convention is of importance for final circuits fedfrom distribution boards, because the upstreamdistribution circuits may, dependent on the lengthsinvolved, contribute significant values of impedance.

The single-phase short-circuit current for a line toneutral fault is obtained from the followingequations.

5.2 Single-phase,line to neutralfault

L1 R1 X1

L2 R1 X1

L3 R1 X1

L4 Rn Xn

Rp XpRs Xs

3-phase

line-neutralline-earth


5.2.1 Circuits up to about 100 A

For single-phase supplies up to about 100 A,

(i) the separate values of Rs and Xs are not alwaysavailable and an ‘impedance’ Zs is substituted forRs with Xs = 0.

Depending on the length of the service cable theminimum value of Zs will be in the range 0.015 to0.15 ohm and the maximum value generally does notexceed 0.35 ohm, except in remote rural installationswith low maximum demands, where Zs could reach0.5 ohm or more.

(ii) (X1+ Xn) can be neglected, so that

If = (2)

where:

Uo is the line to neutral supply voltage

R1 and Rn refer to the installation fault loop.

5.2.2 Circuits for more than 100 A

For circuits using conductor sizes of 35 mm2 or larger(X1 + Xn), Rs and Xs should be included in thecalculation and the single-phase line neutralshort-circuit current is obtained by the use ofequation (3):

If = (3)

where:

Rs and Xs refer to a line-neutral supply loop

(R1+Rn) and (X1+Xn) refer to the installation lineneutral loop.

UoZs + R1 + Rn

Uo

(Rs + R1 + Rn)2 + (Xs + X1 + Xn)2

GN5


5.2.3 Sources for values of resistance and reactance

R1 and Rn can be calculated using conductorresistances from BS 6360 : 1991. Derivation of loopreactance (X1 + Xn) can be by any standard method,suitable equations are provided in Appendix 1 of thisGuidance Note.

Alternatively, combined values of (R1 + Rn), andwhere applicable of (X1 + Xn), can be derived fromthe Tables for single-phase voltage drop for theappropriate two-core cables, or two single-corecables, in Appendix 4 of BS 7671.

Note that the values in the voltage drop tables are ineffect expressed in impedance units of milliohms permetre at the conductor operating temperature e.g.70 °C for pvc. For example, for use in the aboveequations:

(R1 + Rn) = ohm.

Resistance values taken from sources such as BS 6360generally note a conductor temperature of 20 °Cwhereas those from Appendix 4 of BS 7671 apply tothe permissible maximum operating temperature ofthe particular type of cable. These resistance valuesmay need adjustment to other temperatures,depending on the assumptions to be made for theparticular fault current calculation.

There may be a degree of compromise necessary indeciding on the temperature at which the resistanceof each conductor in the fault loop should becalculated. (It is to be noted that circuit reactance isnot temperature dependent and is not involved inthese considerations.)

The following guidance on temperatures andresistances to be used for the calculation of faultcurrent applies to the ‘smallest’ conductor(s) in a faultloop. Such a description includes a reduced sectionneutral, a protective conductor or the live conductorsof the final circuit and assumes that all otherconductors in the loop have equal or larger electricalcross-sections. It is accurate enough for practical

5.3 Conductortemperatureand resistance

Appx 4

Tabulated (r) x length1000

BS 6360


purposes, and is on the safe side, if all otherconductors (i.e. upstream circuits) are assumed tohave resistances corresponding to their workingtemperatures.

5.3.1 Shock protection

Section 413 of the Regulations dealing withprotection against indirect contact requires that theloop impedance of circuits be limited so thatdisconnection occurs within 0.4 or 5 s. The equationof Regulation 413-02-08 is:

Zs ≤

where:

Zs is the earth fault loop impedance

Ia is the current causing the automatic operation ofthe disconnecting protective device within thetime stated in Table 41A as a function of thenominal voltage Uo or, under the conditionsstated in Regulations 413-02-12 and 413-02-13,within a time not exceeding 5 s

Uo is the nominal voltage to earth.

The requirement is to ensure that Zs is of a sufficientlylow value that disconnection will occur within therequired time.

Regulation 413-02-05 and the notes to Tables 41B1,Table 41B2, Table 41C, Table 41D, etc remind the user of the requirementthat account must be taken of conductortemperature. The notes below the tables advise thatthe loop impedances should not be exceeded whenthe conductors are at their normal operatingtemperature. This means that the impedances givenin the tables cannot be used for testing at cold(ambient) temperatures, but must be reduced. Forexample, if testing 70 °C pvc cables, the loopimpedances in Tables 41B1, Table 41B2, Table 41C and Table 41D of theRegulations must be divided by 1.2 (see column 3 ofTable 3 here) to obtain test loop impedances at 20 °Cambient.

413

UoIa

413-02-05

413-02-08

GN5


TABLE 3Resistance coefficients

Conductoroperating

Limitingfinal

Multiplying coefficient

temperature°C

temperature°C

20 °C tooperating

temperature

20 °C toaverage ofoperating

temperatureand final

temperature

Conductorworking

temperatureto averageof working

temperatureand final

temperature

1 2 3 4 5

60

7070

85

9090

90

200

160140

220

160140

250

1.16

1.201.20

1.26

1.281.28

1.28

1.44

1.381.34

1.53

1.421.38

1.60

1.28

1.181.14

1.27

1.141.10

1.32

Quite clearly, reductions in supply voltage willincrease disconnection times and the designer maywish to make appropriate allowances for this. Again,this is particularly important if specific devicecharacteristics are being used, and not the worst caseones of BS 7671.

5.3.2 Protection against short-circuit

Regulation 434-03-03 requires that where a protectivedevice is provided for fault current protection only(and not overload protection), it shall be ensured thatthe disconnection time t is such that the liveconductor temperature shall not exceed thosetemperatures given in Table 43A. The formula quotedis

t =

434434-03-03

k2 S2

I2


where:

t is the duration in seconds

S is the nominal cross-sectional area of conductor inmm2

I is the value of fault current in amperes, expressedfor a.c. as the rms value, due account being takenof the current limiting effect of the circuitimpedances

k is a factor taking account of the resistivity,temperature coefficient and heat capacity of theconductor material, and the appropriate initialand final temperatures. For the common materialsindicated in Table 43A, the k factor shall be asshown.

This equation applies to the live conductors. Thetemperature corrections to be made will be as forshock protection in paragraph 5.3.3, i.e. to workingtemperature for devices in Appendix 3 of BS 7671(column 3 of Table 3) and to average of workingtemperature and limiting final temperature for otherdevices.

5.3.3 Earth fault currents

Regulation 543-01-03 states that the cross-sectionalarea of protective conductors shall be not less thanthe value determined by the formula:

S =

In determining the current, the regulation states thataccount shall be taken of the effect on the resistanceof the circuit conductors of their temperature rise as aresult of overcurrent (see Regulation 413-02-05).

Again, there is a presumption that it is appropriate tocorrect conductor resistance to the maximumoperating temperature only.

Where reduced cross-section protective conductorsare used, together with a 5 second disconnectiontime, the designer will need to take particular care as

543

413-02-05

I2 tk

543-01-03

Appx 3


it is under these conditions that there is the most riskof the conductor overheating.

During a fault, each conductor increases intemperature. However, the conductor with thesmallest cross-sectional area will experience thegreatest increase and is the one to which theequation in 543-01-03 is applied. A reduced sectionneutral or protective conductor will rise intemperature more than a full section live conductor,and care needs to be taken.

In assessing earth fault current capabilities as well asthe device characteristics used and the effect of loadconditioning, the designer may wish to giveconsideration to other factors, including:

(i) the supply (or external) loop impedance Ze

(ii) the voltage tolerance.

If the supply impedance (Ze) used in calculations is themaximum specified by regional electricity companiesi.e. 0.8 Ω for TN-S and 0.35 Ω for TN-C-S supplies asopposed to a measured value, there is a goodprobability of there being a safety factor implied.

The voltage range allowed to electricity companies is230/400 V +10 % -6 %. If it is known that the voltageis low, allowance may need to be made, as reducedvoltages increase disconnection times.

It is particularly important to take care with circuitswhere the nominal disconnection time is expected tobe longer than 0.4 s, with a reduced sectionprotective conductor, using a measured value of loopimpedance and the possibility of a supply voltage atthe lower end of the range. In these circumstances itcould be prudent to include an additional margin ofsafety into the design by assuming a protectiveconductor resistance corresponding to the average ofthe appropriate working temperature and limittemperature given in Tables 54B to Table 54F in BS 7671.Suitable temperature adjustment factors are given incolumn 4 of Table 3.

543-01-03


Single-phase circuits fed from three-phase board

Where a single-phase circuit is supplied from athree-phase and neutral board, the portions of(R1 + Rn) and (X1 + Xn) upstream of the board, ifderived from Appendix 4 of BS 7671, should be thevalues for single-phase circuits, i.e. for two-core cables(or two single-core cables if the upstream cables aresingle-core), having the same conductor size as thecable(s) supplying the board. They should not be takenfrom the values for three or four-core cables or circuits.

If the upstream cable has a reduced neutral, the meanof the tabulated values of (r) and (x) for two-corecables of each conductor size should be used.

5.4.1 Effect of single-core cable spacing

The reactance values in Appendix 4 of BS 7671 assumethat single-core cables are installed with an axial cablespacing of either one or two cable diameters (i.e.touching or a clearance of De). A larger cable spacingwill increase the reactance. Fig 10 gives the increase inloop reactance to be added to the term (X1 + Xn) forlarger values of spacing of unarmoured single-corecables. For armoured cables reference should be madeto Appendix 1 of this Guidance Note.

Fig 10: Additional reactance

5.4 Single-phasecircuits

1 2 3 4 5

additionalreactance,

mΩ/m

0.25

0.20

0.15

0.10

0.05

0actual spacing

tabulationi

Appx 4

Appx 4


A short-circuit may occur between two lines of athree-phase circuit, in which case the current isusually less than that of a three-phase fault. It mayalso occur with a single-phase line to line circuitwhere the neutral is not distributed.

5.5.1 Circuits up to 100 A

For most practical purposes for circuits up to about100 A, reactances Xs and X1 can be neglected and Rsmay be replaced by Zs, where Zs is the line to linesupply ‘impedance’. Rn and Xn are not involved.

The short-circuit current is then:

If = (4)


U is the line to line voltage

R1 is the resistance of one line.


For circuits with conductors of 35 mm2 or larger, thereactances Xs and X1 should be included and equation(5) is used.

If = (5)

where:

Rs = line to line resistance of the supply

Xs = line to line reactance of the supply.

R1 and X1 are for one line in the installation.


For multicore cable circuits values of R1 and X1 can beobtained by calculation, or from BS 7671 by dividingthe tabulated values for (r) and (x) for three-core

5.5 Line to lineshort-circuit

U

(Rs + 2 R1)2 + (Xs + 2 X1)2

UZs + 2 R1


cables by √3. Values of resistance may be adjusted tothe appropriate conductor temperature using acoefficient from Table 3 given earlier.

For single-core cables, reference should be made toAppendix 1 of this Guidance Note, bearing in mindthat for three cables in flat formation the lowestvalue of fault current occurs with a fault between theouter conductors.

For the purpose of selection of a short-circuitprotective device, for Regulations 434-03-01,434-03-03 and 533-03-01, the three-phase short-circuitcurrent is required. Distribution of a neutral does notaffect the calculation.


X1 is neglected, and Rs and Xs are replaced by Zs, theline to neutral ‘impedance’ of the supply.

If = (6)

where:

R1 = the resistance of one line

U = the line to line voltage.


For circuits with conductors of 35 mm2 or larger, bothXs and X1 should be included. The fault current isthen obtained from equation (7).

If = (7)

where :

Rs = line-neutral resistance of the supply

Xs = line-neutral reactance of the supply

5.6 Three-phaseshort-circuit434533

434-03-01434-03-03533-03-01

U 3

(Rs + R1)2 + (Xs + X1)2

U 3Zs + R1


R1 and X1 are for one line in the installation

U = line to line voltage.


The resistance R1 can be calculated by reference toBS 6360. Appendix 1 of this Guidance Note providesinformation on the derivation of reactance X1.

With the exception of fault loops whose impedance isattributable mainly to large single-core cables in flatformation, values of R1 and X1 can also be obtainedfrom the voltage drop Tables in Appendix 4 ofBS 7671 for the appropriate three-phase installation.

Note that the three-phase tabulated values of (r) and(x) in Appendix 4 are, in fact, √3 R1 and √3 X1, so that,in addition to the correction of (r) for temperature,both tabulated values have to be divided by √3.

Corrections to R1 for temperature (i.e. for the highestand lowest prospective currents) apply as in thesingle-phase case and the same tables of coefficientscan be used.

5.6.4 Special considerations with single-core cables

Where all of the cabling from supply to fault ismulticore type, or single-core type installed in trefoil,the reactance X1 will be the same for all phases andthe fault currents will be balanced.

If a minor portion of the fault loop impedance isattributable to single-core cables in flat formation, anaverage value for X1 is appropriate. This is obtainedby deriving X1 for a conductor spacing equal to 1.26 s,where s is the axial spacing between adjacent cables,see Appendix 1 or Fig 10 of this Guidance Note.

Where a substantial part of the impedance from supplyto fault is contributed by a large single-core type ofcable arranged in flat formation, the value of reactancefor the centre cable should be used. Methods forcomputing this reactance are given in paragraphs 4.3 and 4.4 of Appendix 1 of this Guidance Note.

Appx 4


Section 6 — Equations for the Calculation ofEarth Fault Current

Earth fault current has to be calculated for:

(i) the selection of the size of the circuitprotective conductor (cpc) for its limitingtemperature rise

(ii) checking the selection of the line conductor(in both single-phase and three-phase circuits),because the earth fault loop may have a lowerimpedance than the short-circuit loop

(iii) checking the breaking capacity of the deviceintended to interrupt earth fault current

(iv) use in connection with Shock Protection,Chapter 41, where the voltage along the cpchas to be within given limits.

The particular feature of these systems is that there isa continuous metallic path from the exposedmetalwork of equipment to the neutral of the source,see Fig 11 on next page. For such a path the value ofovercurrent is high enough to operate protection suchas a fuse or circuit-breaker.

For reasons explained earlier, electricity distributorscannot give a single definitive value for Zs, but havepublished guidance on the maximum values, whichcan be used to calculate minimum disconnection times,in Electricity Association Engineering RecommendationP23/1. Alternatively, a value of (Rs + Xs) for theline-earth fault path of the supply for supplies up toabout 100 A may be measured between terminals Aand 0 with the installation disconnected. In this lattercase due consideration must be given to thepossibility that modifications in the supply network

6.1 General543434413

6.2 TN systems(TN-C, TN-Sand TN-C-S)

543-01-03

434-02-01

413

fig 3fig 4fig 5


may change the impedance, so that consultation withthe distributor is usually desirable. For user ownedsources the information should be obtained from theequipment manufacturer.

Fig 11: Earth fault path, TN-C-S system


For circuits or supplies up to about 100 A, Xs isneglected and only Rs is used, although it may bereferred to as an ‘impedance’.

For such supplies typical maximum values of Rs areabout 0.35 ohm if the supply is TN-C-S, or about0.8 ohm if it is TN-S.

If = (8)


R1 = line resistance

Rp = protective conductor resistance

(X1 + Xp) is assumed to be zero.

This assumption depends on the two conductorsbeing installed close to each other, which is goodpractice, and there being no ferromagnetic materialbetween them. It follows that a protective conductorshould be installed together with the appropriate lineand neutral conductors inside any steel enclosure.

UoRs + R1 + Rp

L1

L2

L3 A R1

Rs Xs N

O RpX1 + Xp



For circuits or supplies for more than 100 A the earthfault current is calculated from equation (9):

If = (9)

where:

X1 + Xp = reactance of the line-protective conductorloop.

Generally applicable values for three-phase suppliesup to 300 A are given in Electricity AssociationEngineering Recommendation P23/1. Alternatively,the distributor or the manufacturer of user ownedsources should be consulted for values of Rs and Xs.


R1 can be obtained from Appendix 9 of the On-Site Guide or Appendix E of Guidance Note 1 (at 20 °C), orfrom the voltage drop tables in Appendix 4 ofBS 7671 (at conductor operating temperature). Forthe purposes of Regulation 434-03-03 the value of R1has to be corrected for temperature, see paragraph 5.3 on resistances for short-circuit calculations. Thesame coefficients can be used.

If the voltage drop tables of BS 7671 are used thetabulated resistance values have to be divided by twoin the case of values for two-core cables and by √3 forthree and four-core cables, because only one liveconductor is involved.

Where the protective conductor consists of a standardsize conductor, the value of Rp can be obtained fromthe same sources as for a line conductor but usingtemperature adjustments appropriate to the type ofprotective conductor involved, see Tables 54B to Table 54Ein BS 7671 and Table 3 of this Guidance Note.

Uo

(Rs + R1 + Rp)2 + (Xs + X1 + Xp)2

OSGGN1Appx 4

434-03-03

Appx 9


It may sometimes be possible, where the protectiveconductor is reasonably close to the line conductor, toavoid computation of (X1 + Xp) by adapting thereactance for the line-neutral loop for unarmouredsingle-phase single-core cable in the voltage dropTables in Appendix 4 of BS 7671. Where theprotective conductor is a different size from the lineconductor, the average reactance of the twoconductor sizes should be used. Corrections willprobably be required for a wider spacing. Theprocedures described for single-phase short-circuitscan be followed.

For armoured cables and cables in conduits andtrunking, where the enclosure is used as theprotective conductor, special values of the loopparameters are provided below.

6.2.4 Value of touch voltage

The impedance across which a touch voltage appearsis:

(10)

Where Xp has not been determined separately it can,for convenience, be assumed to be (X1 + Xp)/2.

Where the size of the protective conductor is 35 mm2

or less, Xp may be neglected.

Where the sheath or armour of a cable or metallicconduit or trunking is used as a protective conductor,Rp and (X1 + Xp) can be derived from the followingdata.

6.3.1 Steel-wire armoured multicore cables

(R1 + Rp) = Rc + 1.1 Ra (11)

(X1 + Xp) = Ω (12)

413

6.3 Use of a cableenclosure as aprotectiveconductor

543

Appx 4

413

Rp2 + Xp2

0.3 x L1000

where:

L conductor length, m

Rc is the resistance of the line conductor

Ra is the d.c. resistance of the armour adjusted, asnecessary, to the appropriate temperature. Valuesof armour resistance at 20 °C are given in theappropriate cable standard.

1.1 is a coefficient which represents the magneticeffect of the steel armour.

The behaviour of steel-wire armour as a protectiveconductor and its temperature rise with fault currentis not well understood. The problem can be simplifiedby considering that the steel-wire behaves like acopper conductor having roughly one half of thecross-sectional area of the steel. For small cables thearmour cross-section is more than adequate, while forthe largest sizes its performance is nearer to that of acopper protective conductor having a cross-sectionalarea about 50 % of that of the phase conductors.

Selection of the appropriate resistance adjustment fortemperature when calculating earth fault loopimpedances (Sections 413, Section 434 and Section 543 of BS 7671)with steel-wire armoured cables follows Table 3 ofthis Guidance Note for copper protective conductors,but with the steel armour credited with about onehalf of its cross-sectional area. Armour cross-sectionalareas can be obtained from BS 6346 or BS 5467.

Suitable temperature adjustment coefficients for theresistance of steel-wire armour, to be used asmultipliers to 20 °C values (see BS 6345 or BS 5476),are given in Table 4 on the next page.


TABLE 4Coefficients for steel-wire armour

Insulation material thermoplastic thermosetting

70 °C 90 °C 85 °C 90 °C

Assumed initialtemperature (operating)

Final temperature

60 °C

200 °C

80 °C

200 °C

75 °C

220 °C

80 °C

200 °C

Coefficient for adjustmentto operating temperature 1.18 1.27 1.25 1.27

Coefficient for adjustmentto the average of the initialand final temperatures

1.50 1.54 1.57 1.54

Note: Coefficients to be applied to resistances at 20 °C.

It is important to note here that because the lineconductor is contained within the armour, the latteracting as the return conductor, the reactance(X1 + Xp) is practically entirely associated with the lineconductor.

The steel-wire armour on multicore cables is usuallyquite adequate for earth fault current duty but, inthe unlikely event that this is not so, it is better toselect a larger size of cable rather than to connect anauxiliary conductor in parallel with the armour. Themagnetic effect of the armour will restrict the currentpassing through such a conductor to a small fractionof that expected on the basis of division according tod.c. resistances.

Note: Touch voltageThe impedance for the calculation of touch voltage isRa and there is no reactive term.

413


6.3.2 Aluminium-wire armoured single-core cable

The armour of these cables is usually bondedtogether to form an earth fault return through thethree armour paths in parallel. Because these pathsare of low resistance, inductive effects influence thedivision of current between them and, except for thesmaller sizes of cable, the parallel resistance is greaterthan Ra/3. Further, there is a small, but in some casessignificant, reactive component to the armourimpedance.

(R1 + Rp) = Rc + Cr.Ra (13)

(X1 + Xp) = X1 + Cx.Ra

where the symbols used are:

Rc resistance of conductor at the appropriatetemperature

Ra d.c. resistance of armour of one cable at theappropriate temperature, see Table 3, Section 5 ofthis Guidance Note

X1 reactance of the conductor armour loop, or theinternal reactance of a single cable. A methodfor calculating X1 for an isolated cable is given inparagraph 5 of Appendix 1 of this Guidance Note

Cr coefficient to be applied as a multiplier to Ra toobtain the effective resistance of three armours inparallel. The value of Cr depends on the size andarrangement of the cables

Cx coefficient to be applied as a multiplier to Ra toobtain the effective reactance of three armours inparallel. The value of Cx depends on the size andarrangement of the cables.

Values of Cr and Cx are given in Table 5 for commonarrangements of single-core cables to BS 6346 andBS 5467.



TABLE 5Values of Cr and Cx for aluminium-wire(1)

armoured single-core copper conductor cables

Sizemm2(3)

Trefoil 3 flat formation(2)

Touching Spaced(4)

Cr Cx Cr Cx Cr Cx

150185240

300400500

630800

1000

.35

.35

.35

.35

.37

.38

.39

.43

.44

.09

.10

.11

.12

.16

.17

.18

.23

.24

.35

.35

.36

.36

.39

.39

.40

.45

.47

.14

.15

.16

.17

.24

.25

.26

.33

.34

.38

.39

.40

.41

.47

.49

.51

.60

.63

.20

.21

.23

.25

.31

.33

.33

.37

.37

Notes:

1 Values for solid aluminium conductor cables are approximatelythe same as those for copper conductor cables having the samediameter.

2 Earth fault assumed to be between an outer conductor andbonded armour. Values for a fault from the central conductorare slightly lower.

3 For sizes up to and including 120 mm2, Cr has the conventionalvalue of 1/3 and Cx = 0.

4 Axial spacing 2 x De.

The above relates to a circuit having one cable perphase with the fault at the far end of the run. Wherea fault along the run is envisaged, or there is morethan one cable per phase, special considerations arenecessary, see Regulation 473-02-05.

Resistance of armour at 20 °C is available from thecable standard and can be corrected to theappropriate temperature. For calculating the highestvalue of earth fault current a resistance at 20 °C issuitable. When checking compliance with Regulation543-01-03, a resistance adjustment is required (seeTable 3).

473-02-05

543-01-03


Suitable coefficients, to be applied as multipliers to 20 °Cvalues, are given in Table 6.

TABLE 6Coefficients for aluminium-wire armour

Insulation material thermoplastic thermosetting

70 °C 90 °C 85 °C 90 °C

Assumed initialtemperature (operating)

Final temperature

60 °C

200 °C

80 °C

200 °C

75 °C

220 °C

80 °C

200 °C

Coefficient for adjustmentfrom 20 °C to operatingtemperature

1.16 1.24 1.22 1.24

Coefficient for adjustmentfrom 20 °C to the averageof the operating and finaltemperatures

1.44 1.48 1.52 1.48

A method for calculating X1 is given in Appendix 1 of thisGuidance Note.

Note: Touch Voltage

For the smaller sizes of cable the touch voltage iscalculated using R/3, the d.c. resistance of the threearmours in parallel, and the reactive term can beignored. For larger sizes the impedance responsiblefor the touch voltage is

6.3.3 Copper sheathed cables

The approach is similar to that for aluminium-wirearmoured cables, with the following modifications:

X1 and Xp are both negligible.

For multicore cables, Rp is the sheath resistanceadjusted to the appropriate temperature. For

(Cr.Ra)2 + (Cx.Ra)2


single-core cables, Rp must take account of thenumber of sheaths bonded together.

Resistances for conductor (R1) and sheath (Rp) at 20 °Ccan be obtained from data given in BS 6207, or fromthe manufacturer, and adjusted to the appropriatetemperatures.

To adjust resistances at 20 °C to the highest averagetemperature permitted during an earth fault, thecoefficients in Table 7 may be used.

TABLE 7Coefficients for copper sheaths

Assumed initialtemperature

Final temperature

70 °C

200 °C

105 °C

200 °C

Coefficient for adjustment from 20 °Cto operating temperature 1.20 1.33

Coefficient for adjustment from 20 °Cto the average of the operating andfinal temperatures

1.45 1.52

Value of k (Regulation 543-01-03) 1.35 1.12

Note: Touch Voltage

The impedance for the calculation of touch voltage isRs, the sheath d.c. resistance, and there is no reactiveterm.

6.3.4 Auxiliary conductors

The use of an auxiliary conductor to supplement thefault current capacity of aluminium-wire armour ormetallic sheaths calls for the placement of such aconductor as close as possible to the cable. Even whensuch a conductor is touching the cable its current isless than that indicated by the respective d.c.resistances. When the separation is of the order of

543-01-03

413


100 mm or more the reactive effect will reduce theeffectiveness of the auxiliary conductor considerably.(The interposition of ferromagnetic material woulddecrease the effectiveness dramatically.)

When cables are in flat formation, the use of twoauxiliary conductors placed between the cables is away of achieving a small separation from all lines.

6.3.5 Cables in steel conduit

The magnetic effect of steel conduit is quitesignificant and affects the loop resistance andreactance. Because the effect is non-linear withcurrent, two ranges of fault current have to berecognized in order to avoid making the designprocedure unreasonably complicated. Accuracy forvalues of effective resistance and reactance is poorbecause of the variability of the magnetic propertiesof steel.

The effective (line + protective conductor) resistanceis:

(R1 + Rp) = Rc + Fr.Rdc (14)


Rc resistance of the line conductor

Rdc d.c. resistance of the conduit at workingtemperature (assumed to be about 50 °C, so thatthe adjustment to be applied to values at 20 °C isto multiply by 1.12)

Fr is a factor to take account of the magnetic effectof the steel.

The effective reactance of the line-protectiveconductor loop is given by:

(X1 + Xp) = Fx.Rdc (15)

where:

Fx is a factor to take account of the reactive effect ofthe steel.

The unexpected feature of equation (15), where thereactance is stated to be dependent on conduitresistance, comes about because the major


contributor to reactance of the conductor/conduitloop is the magnetic flux within the steel. Thedistribution of this flux within the wall of the conduitand its value depends on the frequency of the currentand the resistivity and permeability of the steel. Thefactor Fx is empirical, being derived from tests at50 Hz on samples of conduit made to BS 4568.Because of the variable magnetic properties ofconduit steel the values given for Fx (and Fr) shouldbe regarded as typical.

Values of Fr and Fx are given in Table 8 for both heavyand light gauge steel conduit.

TABLE 8Values of Fr and Fx for steel conduit

Heavy gauge conduit

Size ofconduit

Fault current

up to 100 A above 100 A

mm16202532

Fr3.02.82.42.0

Fx2.01.91.71.4

Fr1.31.31.10.92

Fx1.31.31.10.92

Light gauge conduit

Size ofconduit

Fault current

up to 100 A above 100 A

mm16202532

Fr2.32.11.91.8

Fx1.61.41.31.3

Fr1.31.31.11.1

Fx1.31.31.11.1

Note: Touch Voltage

The apparent impedance of the conduit is, for usualvalues of fault current, much less than its d.c. resistance.This is due to the magnetic screening effect of steel.However, for simplicity and to be on the safe sidewhen calculating a touch voltage, it is assumed to beequal to its resistance, Rdc, irrespective of the value ofcurrent. There is no reactive component.

413


6.3.6 Cables in steel ducts and trunking

The magnetic effect of steel trunking is rather lessthan that of conduit, but the method for derivingvalues of resistance and reactance is similar. Trunkingis not suitable for use as a protective conductor forcircuits carrying much more than 100 A; unlessparticular care is taken to ensure the continuity andcurrent-carrying capability of joints, it is possible touse only one range of fault current.

For circuits carrying no more than 100 A it is possibleto derive values for (R1 + Rp) and (X1 + Xp) by one setof formulae:

(R1 + Rp) = Rc + 2.1 Rdc (16)


Rc resistance of line conductor

Rdc d.c. resistance of the trunking at 50°C.

2.1 is a coefficient to take account of the magneticeffect of the steel.

(X1 + Xp) = 2 Rdc (17)

Note: Touch VoltageThe impedance to be used to calculate touch voltageis Rdc, with no reactive term.

No protective conductor or auxiliary protective conductorshould be placed outside steel conduit or trunkingcontaining the associated live conductors. The magneticscreening effect of the conduit or trunking is so greatthat any external conductor will be of little effect.

The feature of this system is that there is no continuousmetallic path between the exposed metalwork of theequipment and the neutral of the source, see Fig 12. Part of the return path goes via two earth electrodesand the general mass of earth. The resistance of theelectrodes is usually greater than 1 ohm, perhaps upto 200 ohms, above which it is likely to be unstable.

Guidance on the installation and testing of theseelectrodes is given in BS 7430: 1998 Code of practicefor Earthing.

6.4 TT system

413


Fig 12: Earth fault path, TT system

Earth fault currents are generally much lower thanfor TN systems. For this reason it is usually notpossible to use automatic disconnection by anovercurrent device as a means of shock protection.

Calculation of earth fault current follows the samelines as those set out for TN systems, but the resistancesof the earth electrodes, RA and RB, have to be addedto the external portion of the earth fault loop,see Fig 12. Values of RA and RB are generally muchgreater than those of Rs and Xs experienced with TNsystems and almost always have to be measured orestimated for each individual installation.

If = (18)


RB earth electrode resistance at source

RA earth electrode resistance of installation.

In most installations likely to use this form of earthingwith automatic overcurrent protection to comply withRegulation 413-02-19, (R1 + Rp) can usually beneglected in comparison with (RA + RB).

Further, the usual value of (RA + RB) is such that allreactances can be ignored.

Note: Touch voltageThe impedance across which a touch voltage isproduced is (RA + Rp) but, as noted above, Rp canusually be neglected.

UoRB + RA + (R1 + Rp)

413-02-19

413

L1

L2

L3 A R1

N

O Rp

X1 + Xp

Rs Xs

RB RA

Section 7 — Selection of Conductor Size

Conductor sizes selected to meet the requirements forcurrent-carrying capacity, voltage drop and, in thecase of protective conductors, the requirements ofSection 413, have to be checked to confirm that theywill not be overheated in the event of a fault.

For this purpose the following regulations apply:

Short-circuit, line and neutral 434-03-03Earth fault, line 434-03-03Earth fault, protective conductor 543-01-03

Regulation 434-03-02 allows a designer to assumethat if an overcurrent protective device is providingoverload protection complying with Section 433, andhas a rated breaking capacity not less than theprospective short-circuit current at its point ofinstallation, it is also providing, without further proof,short-circuit protection. That is to say, there is noneed to check that the circuit conductors areadequately protected thermally by verifyingcompliance with Regulation 434-03-03.

This assumption holds true for all fuses and energylimiting circuit-breakers. For zero point or currentzero type circuit-breakers there may be a value offault current above which the conductors are notprotected. It will generally be found in practice,however, that this value is greater than theprospective short-circuit current. Advice on the typeof circuit-breaker should be sought from themanufacturer (see Section 3.6 of this Guidance Note).

7.1 General523

7.2 Overload andshort-circuitprotection bythe samedevice434

434-03-03523543-01-03

434-03-02

434-03-03



It is necessary to comply with Regulation 434-03-03when short-circuit protection is provided by aseparate device and to check compliance where thecharacteristic of the proposed device for combinedoverload and fault current protection is in doubt.

Although the size of a phase conductor may complywith Regulation 434-03-03 under short-circuitconditions, where earth fault current is less than theshort-circuit current the energy let-through of thedevice may be greater and it is necessary to check thesizes of both the phase conductor for compliancewith Regulation 434-03-03 and the protectiveconductor for compliance with Regulation 543-01-03.

The assumption of Regulation 434-03-02 should not bemade for conductors in parallel. Regulation 473-02-05sets out the requirements for the protection of cablesin parallel: A single protective device may protectconductors in parallel against the effects of faultcurrents provided that the operating characteristic ofthat device results in its effective operation should afault occur at the most onerous position in oneparallel conductor. Account shall be taken of thesharing of the fault currents between the parallelconductors. A fault can be fed from both ends of aparallel conductor.

The regulation requires that where a single protectivedevice may not be effective then one or more of thefollowing measures shall be taken:

(i) the wiring is carried out in such a way as toreduce the risk of a fault in any parallel conductorto a minimum, for example, by protection againstmechanical damage, and conductors are notplaced close to combustible material, or

(ii) for two conductors in parallel a fault currentprotective device is provided at the supply end ofeach parallel conductor, or

(iii) for more than two conductors in parallel faultcurrent protective devices are provided at thesupply and load ends of each parallel conductor.

7.3 Earth faultcurrent543

7.4 Parallelcables434

434-03-03

543-01-01543-01-03

473-02-05


For short durations, say less than 0.1 s, the valuesderived from Appendix 3 of BS 7671 may beunsuitable because of the possible current limitingeffect of the device. The manufacturer should beconsulted as to the I2 t let-through for the expectedvalue of prospective current.

It is then possible to substitute this value of I2 t in are-arrangement of the equation of Regulation434-03-03, and in Regulation 543-01-03

S = mm2

to obtain the smallest conductor size to comply withthe maximum permitted temperature in Table 43A orTables 54B to Table 54F of BS 7671.

Note that there is no limitation on fault currentduration inherent in the thermal requirements ofeither Regulation 434-03-03 or Regulation 543-01-03.While considerations of shock risk limit earth faultcurrents to no more than 5 s, there is no definitivelimit for short-circuit currents.

The duration for which a short-circuit current shouldbe allowed to persist is a matter of carefuljudgement, see Section 1.6 of this Guidance Note.Obviously, it is wise to restrict the duration of arcing,etc to an absolute minimum. However, there is nofirm evidence on which to base a specific time limitfor general use and consideration of this matter musttake into account the particular circumstances.

The data on which the values of the coefficient k inTables 43A and 54B to Table 54F of BS 7671 have beenbased were generally obtained with durations notgreater than about 5 s in mind, but again, this isunlikely to be critical. It is nevertheless prudent toconsult the cable manufacturer if a duration in excessof 5 s is contemplated.

Both Regulation 434-03-03 and Regulation 543-01-03state that the adiabatic equations they containprovide an approximate value for duration orconductor size. It should be noted straight away that

7.5 Use of I2 tcharacteristics434

7.6 Duration ofshort-circuitcurrent543

7.7 Status ofadiabaticequations434543

434-03-03543-01-03

Table 43ATable 54BTable 54CTable 54DTable 54ETable 54F

434-03-03543-01-03

434-03-03Appx 3

I2 tk

543-01-03

Table 43ATable 54BTable 54F to


any error is on the safe side, but may be of economicinterest.

The validity of the adiabatic equation depends on theextent to which heat is lost from the conductorduring the period of the fault current. For a givenconductor type and insulating material the heat losscan be expressed in terms of the ratio of fault currentduration to the conductor cross-sectional area.

For circular conductors the limitation on accuracy canbe expressed by the following examples.

For durations (in seconds) up to 0.1 x cross-sectionalarea (in mm2) the loss is negligible. In practical termsthis means that the adiabatic equation is of sufficientaccuracy at durations up to 0.4 s for conductors of4 mm2 or larger, up to 1 s for conductors of at least10 mm2 and up to 5 s for conductors of 50 mm2 orlarger.

For most fault current interruption times theapproximate nature of the adiabatic equation forcircular conductors is of interest for the smallconductor sizes only. Generally the economics forcircuits of such sizes means that the more complicatednon-adiabatic calculation is unlikely to be worthwhile.

Shaped conductors are in the size range where theapproximation does not matter.

The situation changes considerably for concentricconductors such as metallic sheaths, armour andconcentric neutrals. BS 7454 provides equations anddata for making non-adiabatic calculations for alltypes of conductor and insulating material.

For bare conductors the adiabatic equation issatisfactory.

Regulation 434-03-03 states that the value of thecoefficient k may be recalculated where the initialtemperature is lower than that assumed for Table 43A.This situation arises when conductor sizes areincreased above that required to comply withcurrent-carrying capacities.

Such a recalculation will yield a higher value of k andpermit slightly higher fault currents for a given size of

7.8 Alternativevalues of k434

434-03-03

conductor, the average conductor temperature shouldalso be recalculated. The reduction in initialtemperature has to be considerable for therecalculation to be worthwhile.

Appendix 2 of this Guidance Note provides theequations necessary to calculate k.

Refer to Table 4A1 of Appendix 4 of BS 7671 for aschedule of installation methods for cables includingcables in trenches.

7.9 Cables intrenches

Table 4A1



Section 8 — Notes on Fault CurrentWithstand of Flexible Cords

Compliance with the requirements of Chapter 43concerns only the fixed installation. The requirementsdo not apply to flexible cords of current-usingequipment supplied from socket-outlets..

Because the size and length of a flexible cord and thenature of the load connected to a socket-outlet arenot generally within the control of the installer of thefixed wiring, the subject of overcurrent protection forflexible cords is not considered to be a subject forwhich requirements could be included in theRegulations.

On the other hand, there are good reasons for aninstaller being aware of the impact of the overcurrentprotection provided for a final circuit on the flexiblecord which might be connected to it. The object ofthese Notes is to be of assistance where an installer isaware, or has control, of the cord to be used or,where this is not so, to provide guidelines forsocket-outlet circuits.

Satisfactory protection of a flexible cord calls not onlyfor the use of appropriate overcurrent protectivedevices but also for a restriction on length of cord.Both features depend on user co-operation or control.

The size of a flexible cord is generally chosenaccording to the power consumption of theequipment supplied, and often this implies quitesmall sizes. With small appliances convenience ofhandling has to be balanced against a minimum sizelimit based on somewhat empirical requirements formechanical strength.

These Notes may add to the selection criteria byintroducing minimum size limits based on faultcurrent withstand.

8.1 General431

In the realm of user choice, it may be possible torecommend limitations on length of flexible cord.

Fault current protection of a flexible cord does notnecessarily imply protection for any of the circuitswithin connected equipment.

The values, for a fault at the far end of a flexiblecord, of short-circuit current and, in the case of athree-core flexible, earth fault current, depend on therespective loop impedances. Contributions to theseimpedances by the fixed installation are affected bythe criteria used to provide shock protection bydisconnection and for voltage drop. Fault currentprotection within the fixed installation may also playa part by influencing the choice of circuit conductorsizes.

On the subject of the influence of shock protection ithas to be noted that Regulations 413-02-09 and413-02-12 permit two circuit design criteria(disconnection times not greater than 0.4 s orlimitation of the impedance of the protectiveconductor, 5 s maximum). These may result in widelydifferent earth fault loop impedances.

The contribution by the flexible cord as with allconductors depends not only on its size but on itslength. While the importance of flexible conductorsize is easily recognized, the effect of length is lessobvious. However, the insertion of typical values intothe problem shows that it is possible to have a lengthof cord so great that it may result in the overcurrentprotective device not operating for a very long timeor, for all practical purposes, not operating at all. Thedanger posed by lengthy extension leads is obvious.

Although the criteria for shock protection play a partin the consideration of fault currents, the subject ofshock protection itself is not within the scope of thisGuidance Note.

8.2 Influence ofcircuit designand length offlexible cord

413-02-09413-02-12


8.3.1 Damage to the flexible

The internationally recognized maximum conductortemperature limits for the insulating materials used inflexible cords are 160 °C for pvc and 220 °C for rubberduring fault conditions. These limits correspond to alimited degree of damage, insufficient to affect thepractical performance of the cord.

These temperatures assume that conductors andinsulation are subject to mechanical stress, for mostsituations they should be conservative.

8.3.2 Acceptable length of flexible

Standards for most appliances include a requirementfor a minimum length of flexible cord, where it issupplied with the appliance. A typical length is 2 m.Some small appliances have shorter lengths, whileequipment such as appliances for floor treatmenthave much longer flexible cords.

In general, the longer the flexible cord, the lower thefault current and the greater the time taken for theovercurrent protection to operate. The energylet-through and the conductor temperature underthese conditions are greater.

As a means of selecting suitable conductor sizes forvarious socket-outlet circuits it is assumed that unlessa cord length of at least 2 m can be used, without theconductor short-circuit temperature exceeding theappropriate limit above, the size is too small. Formany conductor sizes considerably longer lengths arepermissible.

It can sometimes be shown that very long lengths, upto 100 m, are acceptable as regards conductortemperature, but fault durations could then belonger than about 2 min. In such situations amaximum length of flexible cord corresponding to,say, a fault duration of 5 s is suggested.

8.3 Damagecriteriafor flexiblecord

8.3.1


It follows that shock protection complying withRegulation 413-02-12 can be used only where lengthsof flexible cords are absolutely minimal, because theregulation permits fault currents to persist for nolonger than 5 s.

In what follows only final circuits with protectioncomplying with Regulation 413-02-09 (0.4 sdisconnection) will be considered.

As explained above, the short-circuit and earth faultduty imposed on flexible cords supplied fromsocket-outlets depends on the design parameters ofthe particular socket-outlet circuit in the fixedinstallation. The following conclusions are based on asurvey of a range of circuit parameters, but are notclaimed to be exhaustive and are provided to showthe nature of the limitations involved. In manyrespects they confirm existing practices.

It is important to bear in mind that the conductorsizes quoted below are based on short-circuit or earthfault conditions and in no way override therequirement to observe the current-carrying capacityof the conductor and voltage drop limits forequipment. These latter requirements may often callfor larger sizes.

The limitations given below are based on the worstcase characteristics of fuses and circuit-breakers and,in most cases, assume that the fixed circuit justcomplies with the requirements of Regulation413-02-09. It is to be expected that actual installationswill impose rather less onerous duties on flexible cordsso that the limitations will be conservative.

8.4.1 Ring circuits

For ring circuits protected by either a 30 or 32 Aprotective device, where the outlets are fused at 13 Aor 3 A:

8.4 Conclusions

413-02-12

413-02-09

413-02-09



(i) protection of a 0.22 mm2 conductor by a 13 A fuseagainst an earth fault is marginal, in thatthe earth fault loop impedance of the circuitmust be lower than the maximum permitted byRegulation 413-02-09 (0.4 s duration)

(ii) 0.22 mm2 cords are not protected where thecircuit has been installed to comply withRegulation 413-02-12 (limitation of protectiveconductor impedance)

(iii) the formal position that 3 A fuses should be usedin 13 A BS 1363 plugs is confirmed, especially as itis then unnecessary to restrict the length of a 0.22mm2 flexible cord to avoid thermal damage

(iv) however, lower impedances than those envisagedin (i) would almost certainly apply for line toneutral faults so that protection would beacceptable for two-core cords supplying Class IIequipment, provided that the length of the corddoes not exceed about 2 m

(v) because the risk of damage is marginal and maybe negligible for some line to neutral faults onshort lengths of 0.22 mm2 flexible cord, theimpression may have developed that the 3 A fuseis not strictly necessary. This is unfortunate anduse of the 3 A fuse should be retained

(vi) except as noted in (i), a minimum size of 0.5 mm2

should be required for 13 A outlets on 30 A or 32 Aring circuits. This applies where shock protectioncomplies with either of the requirements ofSection 413

(vii) pvc-insulated cords are satisfactory under theabove conditions, but lengths should not extendbeyond about 10 m

(viii) rubber insulated cords are preferable forextension sets with 13 A fuses. Where lengthsgreater than about 10 m are involved sizes shouldbe increased to 0.75 mm2 or larger

(ix) even if a conductor size is acceptable as regardsthermal limitations, it may be that a long lengthhas such a high resistance that a 13 A outlet fusewill not operate for a long time. A maximum

413-02-09

413-02-12


length should therefore be observed so that theduration of the fault current is restricted to, say, 5 s.

8.4.2 Radial circuits

Radial final circuits, with no overcurrent protection atthe socket-outlets.

Table 9 gives examples of maximum lengths ofthree-core flexible cord which have to be observed toavoid either thermal damage to the cord or an earthfault current duration exceeding 5 s.

For line-neutral short-circuits on two-core flexibles,the circuit resistance would be expected to be ratherlower than that assumed above for earth faults andthe lengths given could be increased.

TABLE 9Maximum lengths of cord for earth faults(circuit protection complying with 413-02-09)

Insulated conductorLength m

sizemm2 pvc Rubber

16 A fuse at distribution board0.50.751.0

822 (29)29 (70)

14 (17)22 (50)29 (—)

20 A fuse at distribution board0.751.01.25

72231 (43)

1424 (53)31 (70)

32 A fuse at distribution board1.01.251.52.5

N 51440 (70)

41325 (25)40 (100)

N = not to be used.

Values in brackets apply to the thermal limitation andare given where the other value applies to a 5 slimitation for the fault current.

The 0.22 mm2 size cannot be used at all.

Where it is not feasible to control the lengths of cordor extension used, consideration should be given torecommending minimum sizes.


Appendix 1 — Calculation of Reactance

1 General

It is important to bear in mind that reactance isessentially associated with a circuit or loop, generallyformed by two conductors. For this reason thereactance of line-neutral and line-protectiveconductor loops have been expressed in this GuidanceNote in the form (X1 + Xn) etc so as to avoid theimpression that the contribution by each conductorcan be applied in an equation, measured orconsidered to exist, as an independent quantity.

However, because the two conductors may be ofdifferent size or form, it is convenient for the purposeof computation to regard their reactances separatelyas one does the resistances. In the case of athree-phase fault the return conductor is formed bythe other phases and does not appear directly in thecomputation.

2 Line to neutral, single-phase, faults

Line and neutral reactances for:

(i) all multicore cables

(ii) unarmoured or non-metallic sheathedsingle-core cables.

The reactance of each conductor:

X1 and Xn = 0.0628 loge(2 s/a.d) m ohm/m

at 50 Hz.

where symbols used are:

s axial separation of the two conductors, mm

d conductor overall diameter, mm.

2.1


Shaped conductors are treated as circular conductorshaving the same cross-sectional area.

a is a coefficient to allow for the internal reactanceof the conductor according to Table 1.1.

TABLE 1.1 Coefficient a

No of wiresin conductors

Coefficienta

37

19376191

127169

0.6780.7240.7580.7680.7720.7740.7760.779

The reactance due to the helical laying up of cores ofmulticore cables can be neglected.

Resistance and reactance of line and neutralconductors for non-magnetic armoured or metalsheathed single-core cables.

(If the sheaths or armour are not bonded at both endsof the run, the impedance for non-metallic sheaths orunarmoured cables applies).

In this case both resistance and reactance aremodified by the presence of circulating current in thearmour or sheath. The apparent values of resistanceand reactance of each of the conductors are given by:

R1 and Rn = Rc + F.Ra

X1 and Xn = Xc - F.Xa


Rc conductor resistance at the appropriatetemperature

Xc reactance of each conductor calculated as inItem 2.1

2.2


Ra resistance of armour or sheath

Xa reactance of sheath or armour

= 0.0628 loge(2 s/da) m ohm/m at 50 Hz

da mean diameter of sheath or armour, mm

s axial separation of cables, mm

F coefficient of coupling between armour andconductor,

=

3 Line to line, single-phase, short-circuit

Reactance of each line conductor for:

(i) multicore cable

(ii) non-metal sheathed or unarmouredsingle-core cables in trefoil

(iii) adjacent non-metal sheathed or unarmouredsingle-core cables in flat formation.

X1 is given by the same equation as for line to neutralfaults in Item 2.1, with s being the axial separation ofadjacent line conductors.

Resistance and reactance for each line conductor for:

(i) non-magnetic armoured or metal sheathedsingle-core in trefoil

(ii) adjacent non-magnetic armoured or metalsheathed single-core cables in flat formation.

The increased resistance and reduced reactance ofeach line conductor are given by the equations for R1and X1 in Item 2.2, with s being the axial separationbetween adjacent line conductors or cables.

3.1

3.2

Xa2

Ra2 + Xa2


For a fault between the outer conductors of threesingle-core cables of any type in flat formation,equally spaced.

The reactance and, where applicable, the resistanceof each line are given by the equations in 2.1 and 2.2, according to the type of cable and installation, butwith the values for both Xc and Xa increased by0.04355 m ohm/m. In these equations the symbol s isunchanged as the axial separation between adjacentconductors or cables.

4 Three-phase short-circuit

Reactance of line conductor for:

(i) multicore cables

(ii) unarmoured or non-metal sheathed single-corecables in trefoil.

X1 = 0.0628 loge(2 s/a.d) m ohm/m at 50 Hz.

s is the axial separation between the lineconductors, mm.

Other symbols and values of a are as before.

Resistance and reactance for non-magnetic armouredor metal sheathed single-core cables in trefoil.

The apparent increase in resistance R1 and decrease inreactance X1 of the line conductor are given by theequations in Item 2.2, with s being the axialseparation between line conductors or cables.

Resistance and reactance for three unarmoured ornon-metal sheathed single-core cables in flatformation, equally spaced.

The fault currents in the lines are not equal and thecentre conductor has the lowest apparent reactance.For this conductor:

R1 = Rc

3.3

4.1

4.2

4.3


X1 = Xc - Xm/3

H = Xa - Xm/3

where:

Rc is the conductor resistance at the appropriatetemperature

Xc = 0.0628 loge(2 s/a.d) m ohm/m at 50 Hz, and

Xm= 0.0628 loge(2) = 0.04355 m ohm/m at 50 Hz

s is the axial separation between adjacent lineconductors, mm.

Resistance and reactance for three non-magneticarmoured or metal sheathed cables in flat formation,equally spaced.

The apparent increase in resistance and decrease inreactance of the middle conductor is given by thefollowing equations:

R1 = Rc + F.Ra

X1 = Xc - Xm/3 - F.(Xa - Xm/3)


Rc resistance of the conductor at the appropriatetemperature

Xc reactance of the conductor as given for theunarmoured or non-metallic sheathed casein 4.3

Xm = 0.04355 m ohm/m at 50 Hz

F coefficient of coupling between armour andconductor

=

H = Xa - Xm/3

Xa = 0.0628 loge(2 s/da) m ohm/m at 50 Hz

s axial separation between adjacent cables, mm

da mean diameter of armour, mm.

4.4

H2

Ra2 + H2



5 Earth faults

Reactance of line and protective conductors:

(i) where the protective conductor is provided bya core in a multicore cable

(ii) by a separate conductor

(but not by the steel-wire armour of a multicore cableor a cable enclosure, see Section 6.3 of the main textof this Guidance Note).

The reactance of each conductor:

X1 and Xp = 0.0628 loge(2 s/a.d) m ohm/m at 50 Hz

where:

s is the axial spacing between the line andprotective conductors, mm.

There may be different values of a and d for eachconductor and values of a can be taken from Item 2.1.

Where the protective conductor is provided by ametallic sheath or by the non-magnetic armour of onecable (single-point bonding):

R1 = Rc

Rp = Ra

X1 = 0.0628 loge(2 s/a.d) m ohm/m at 50 Hz.

Xp = 0.

Where the armour of single-core cables is bonded atboth ends of the run, refer to paragraph 6.3.2 ofSection 6 of this Guidance Note.

It is assumed that there is no ferromagnetic materialbetween or near to the conductors.

The increase in impedance due to ferromagneticmaterial close to the conductors depends on thecircumstances but may be of the order of 0.03 m ohm/m.It can be kept to a minimum by installing theprotective conductor as close as possible to the lineconductor (see Booth H C, Hutchings E E andWhitehead S, ‘Current Ratings and Impedance ofCables in Buildings and Ships’ J.I.E.E. Vol. 83, No 502,October 1938).

Appendix 2 — Calculation of k for OtherTemperatures

Adiabatic Equation.

The general formula for k for given initial and finaltemperatures is:

k (As1/2/mm2) = K loge

Where symbols used are:

θ1 final temperature, °C

θ0 initial temperature, °C

β reciprocal of the temperature coefficient ofresistance for conductor material at 0 °C, °C

K a constant for the conductor material, seeTable 2.1.

TABLE 2.1 Coefficient B and constant k

Material β K

CopperAluminiumLeadSteel

234.5228230202

226148

4178

Notes:

It should be borne in mind that the choice of an alternative valuefor initial or final temperature in the adiabatic equation shouldalso be taken into account in the calculation of the averageconductor temperature for which fault current is determined.

For further information on thermally permissible short-circuitcurrents and heating effects refer to BS 7454 (IEC 949).

θ1 + β 1/2

θ0 + β


Index

AAdiabatic equations 7.7

calculation of k 7.8; Appx 2Aluminium wire armour, coefficient 6.3.2Appliances, flexible cords 1.1.3; 8.1Assessment of fault current 3.4.1Auxiliary conductors 6.3.4

BBack-up co-ordination 1.7.4Battery connections 3.5Breaking capacity 3.3Building movement 3.2.2

CCables

impedance 6.3in parallel 2.3.4; 7.4in trenches 7.9mineral insulated 2.3.7to motors 2.3.3to starters 1.7.3

Calculationof fault current 4.3of reactance Appx 1

Circuit-breakers 2.3.2Coefficient, temperature

aluminium armour 6.3.2conduit 6.3.5copper sheath 6.3.3resistance 5.3.1; 6.3steel armour 6.3.1

Competent persons IntroductionConductor size 7Conduit impedance 6.3.5Co-ordination 1.7

back-up 1.7.4devices 1.7.3

Cords, flexible 1.1.3; 8Current transformer 2.4; 3.5


DDamage

due to fault current 3.1; 3.2to flexible cords 8.3

Determination of fault current 4by enquiry 4.1

Devices, protective 1.5; 2.3; 7.2Discrimination devices 1.7.2Diversity 2.2.1Duration

of overcurrent 1.6of overload 2.2.5of short-circuit current 7.6

EEarth fault current 6; 7.3Earth fault path 6.2Electricity at Work Regulations 1.3Electromagnetic forces 3.2.3Engineering Recommendations 3.4.2; 4.1; 5.1; 6.2Equations

earth fault current 6fault current 5.1line to line faults 5.5single-phase faults 5.2three-phase 5.6

Equipment flexible cords 1.1.3; 8Expansion

joints 3.2.2of cables 3.2.2

FFault current 3.1; 3.4.2

assessment 3.4.1by enquiry 4.1calculation 4; 5; 6determination of 4

line to line 5.5single-phase 5.2; 5.4three-phase 5.6

impedance 3.3measurement 4.2origin of installation 3.4.2protection omission 1.1; 3.5

Flexible cords 1.1.3; 8Fundamental principles 1.1


GGenerator connections 3.5

HHarmonics 3.7Health and Safety at Work Act Introduction

IImpedance of cables 6.3Industrial plug and socket 1.1.3; 3.4.2Installation precautions 3.2.1I2t characteristics, use of 3.5

Kk, calculation of 7.8; Appx 2

LLength of flexible cord 8Load assessment 2.2; 2.3.6Long period 2.3.6Lifting magnets 2.4; 3.5Line to line short-circuit 5.5

MMeasurement of fault current 4.2Mineral insulated cables and ring circuits 2.3.7Motor

cables 2.3.3starting currents 1.7.3; 2.3.3

NNon-fused industrial plugs 3.4.2Nuisance tripping 2.3.2

OOperational manual IntroductionOvercurrent

duration 1.6fundamental principles 1.1.1nature of 1.2omission of protection 1.4; 2.4; 3.5relocation of fault device (not atorigin of circuit) 3.4.3

Overload 2common device (as for fault) 7.2general 2.1general 2.1nuisance tripping 2.3.2omission of protection 1.4; 2.4small 1.2.3; 2.2.5


PParallel cables 2.3.4; 7.4Plug and socket 1.1.3Position of fault current device 3.4Plug industrial 1.1.3; 3.4.2Principles, fundamental 1.1.1Protection against

fault currents 3overload 2

Protective conductor, cable enclosure as 6.3Protective device

co-ordination 1.7.1; 1.7.3; 1.7.4discrimination 1.7.1; 1.7.2motors 1.7.3selection 1.5; 2.3

Protective measures 1.1.2

RRCBOs 2.3.2Reactance

cable spacing 5.4.1calculation Appx 1

Rectifier connections 3.5Regulations 1Resistance coefficients 5.3.1; 6.3Resistance conductor 5.3Ring final circuits 2.3.5Ring circuits, mineral insulated cables and 2.3.7Scope 1.1Selection of conductor size 7Service cable 4.1Shock protection 5.3.1Short-circuit current 3; 5Single-core cables 5.4.1Single device 3.6; 7.2Single-phase faults 4; 5.2; 5.4Small overload 2.3.6Specification IntroductionSprinkler systems 2.4Star/delta starters 2.3.3Steel-wire armour coefficient 6.3.1Starters motor 1.7.3, 2.3.3Statutory Requirements 1.3Supply impedance 3.4.2; 6.2Switchgear interconnections 3.4.3


TTemperature

coefficient β Appx 2resistance adjustment for 5.3.1; 6.3

Thermally equivalentcurrent 2.2.3load 2.2.4

Three-phase short-circuit 4; 5.6TN systems 6.2Touch voltage 6.2.4Trenches, cables in 7.9Tripping nuisance 2.3.2TT system 6.4

VValues of fault current 3.4.1; 3.4.2; 4.3Varying loads 2.2.2


Documents

GUIDANCE NOTE Overcurrent Protection 6 · J Reed ARIBA SELECT / ECA of Scotland J Davidson CEng FIEE D Millar. thCD GN6 Protection Against Overcurrent, inc 16 Edition 2001 Amd No