RCD Handbook 2018 - eda.org.uk...3.6 rcd s in reduced and extraslow voltage applications 15 3.7 rcd s in electric vehicle charging 15 4. fire protection 16 4.1 background 16 4.2 protective

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THE RCD HANDBOOK

BEAMA GUIDE TO THE SELECTIONAND APPLICATION OF RESIDUALCURRENT DEVICES (RCDs)

July 2018

Eaton Electric Limited

270 Bath Road, Slough, Berkshire SL1 4DX

Tel: +44 (0) 8700 545 333

Email: [email protected]

www.eaton.com/uk

Gewiss UK Ltd

2020 Building, Cambourne Business Park

Cambourne, Cambridge CB23 6DW

Tel: +44 (0) 1954 712757

Fax: +44 (0) 1954 712753


www.gewiss.co.uk

Hager Ltd

Hortonwood 50, Telford, Shropshire TF1 7FT

Tel: +44 (0)1952 675 689


www.hager.co.uk

Legrand Electric Ltd

Great King Street North, Birmingham B19 2LF

Tel: +44 (0) 370 608 9020

Fax: +44 (0) 345 600 6760


www.legrand.co.uk

MK Electric

The Arnold Centre, Paycocke Road

Basildon, Essex SS14 3EA

Tel: +44 (0) 1268 563 000

Fax: +44 (0) 1268 563 064


www.mkelectric.com/en-gb

Schneider Electric Ltd

Stafford Park 5, Telford, Shropshire TF3 3BL

Tel: +44 (0) 1952 290029

Fax: +44 (0) 1952 292238

www.schneider-electric.co.uk

Electrium Sales Ltd (a Siemens Company)

Sharston Road, Wythenshawe

Manchester M22 4RA

Tel: +44 (0) 161 945 3956

Fax: +44 (0) 8456 053114

www.electrium.co.uk

Timeguard Ltd

Victory Park, 400 Edgware Road

London NW2 6ND

Tel: +44 (0) 20 8450 0515

Fax: +44 (0) 20 8450 0635


www.timeguard.com

Western Automation R&D

2 Atreus Place, Poolboy, Ballinalsoe,

Co. Galway, Ireland H53 TD 78

Tel: +353 (0) 90 9643359

Fax: +353 (0) 90 9643094


www.westernautomation.com

COMPANIES INVOLVED IN THEPREPARATION OF THIS GUIDE

ABOUT BEAMA

BEAMA is the long established and respected trade association for theelectrotechnical sector. The association has a strong track record in thedevelopment and implementation of standards to promote safety and productperformance for the benefit of manufacturers and their customers.

This Guide provides specifiers, installers and end users, clear guidance on theselection and application of the wide range of RCDs now available.

This Guide has been produced by BEAMA’s Building Electrical Systems Portfoliooperating under the guidance and authority of BEAMA, supported by specialistcentral services for guidance on European Single Market, Quality Assurance,Legal and Health & Safety matters. BEAMA’s Building Electrical Systems Portfoliocomprises of major UK manufacturing companies.

Details of other BEAMA Guides can be found on the BEAMA website www.beama.org.uk

DISCLAIMERThis publication is subject to the copyright of BEAMA Ltd. While the information hereinhas been compiled in good faith, no warranty is given or should be implied for its useand BEAMA hereby disclaims any liability that may arise from its use to the fullest extentpermitted under applicable law.

© BEAMA Ltd 2018Copyright and all other intellectual property rights in this document are the property ofBEAMA Ltd. Any party wishing to copy, reproduce or transmit this document or theinformation contained within it in any form, whether paper, electronic or otherwiseshould contact BEAMA Ltd to seek permission to do so.

AcknowledgementsBEAMA would like to thank IEC, BSI and IET for allowing references to their standards;Health and Safety Executive (HSE) for reference to their documents.

CONTENTS

1. INTRODUCTION 5

1.1 FOR THE NON-SpECIALIST 5

1.2 pRINCIpLES OF RCD OpERATION 6

1.3 RESIDUAL CURRENT DEvICES (RCDs) 7

2. EFFECTS OF ELECTRICITy 8

2.1 RISK OF ELECTROCUTION 8

2.2 TypES OF ELECTROCUTION RISK 8

2.3 EFFECTS OF ELECTRIC SHOCK ON THE HUMAN BODy 8

3. ELECTRIC SHOCK pROTECTION 11

3.1 pRINCIpLES OF SHOCK pROTECTION 11

3.2 EARTHING SySTEMS 12

3.3 pROTECTION AGAINST DIRECT AND INDIRECT CONTACT 13

3.4 RCDs AND INDIRECT CONTACT SHOCK pROTECTION 14

3.5 RCDs AND DIRECT CONTACT SHOCK pROTECTION 15

3.6 RCDs IN REDUCED AND ExTRA-LOW vOLTAGE AppLICATIONS 15

3.7 RCDs IN ELECTRIC vEHICLE CHARGING 15

4. FIRE pROTECTION 16

4.1 BACKGROUND 16

4.2 pROTECTIvE MEASURES AS A FUNCTION OF ExTERNAL INFLUENCES 16

5. INSTALLATION RISKS 17

5.1 BACKGROUND 17

5.2 TypICAL RISKS 17

6. RCD SELECTION 18

6.1 RCD SELECTION CRITERIA 18

6.2 RCD SELECTION GUIDES 23

7. OpERATION AND MAINTENANCE 25

7.1 TESTING By THE END USER 25

7.2 TESTING By THE INSTALLER 25

7.3 TROUBLESHOOTING 26

7.4 DETAILED FAULT-FINDING IN RCD pROTECTED INSTALLATIONS 26

8. RCD CONSTRUCTION 28

8.1 vOLTAGE INDEpENDENT RCD 28

8.2 vOLTAGE DEpENDENT RCD 28

9. DETAILED FAULT-FINDING ON RCD pROTECTED INSTALLATIONS 29

9.1 MAINS BORNE TRANSIENTS AND SURGES 29

9.2 CApACITANCE TO EARTH 31

9.3 CABLES AND OvERHEAD LINES 32

9.4 NEUTRAL TO EARTH FAULTS 33

9.5 DOUBLE GROUNDING 33

9.6 CONCLUSIONS 33

10. ANNEx 34

10.1 FIRE pROTECTION – ExTRACT FROM DTI REpORT 34

10.2 REFERENCES 40

10.3 TERMS AND DEFINITIONS 41

1INTRODUCTION

the use of electricity is so much a partof everyday life that it is often taken forgranted and the risks associated withits use at home and at work areunderestimated or misunderstood.

Residual Current Devices (RCDs) areelectrical devices which afford a veryhigh degree of protection against therisks of electrocution and fire causedby earth faults. However, they are nota solution for all installation problems;it is therefore important to understandwhat they can and cannot do.Furthermore, the different types ofRCDs available on the market can beconfusing.

This publication has been produced byBEAMA Members for use by specifiers,installers and end users, to give clearguidance on the selection andapplication of the wide range of RCDsnow available. Guidance is also given onthe installation and maintenance ofRCDs, including many of the installationconditions that cause ‘unwantedtripping’.

Most chapters begin with a section thatis designed for the non-specialist orend user.

When read in conjunction with BS 7671Requirements for Electrical Installations(The IET Wiring Regulations), theguidance in this publication willcontribute to safe and reliableinstallations.

There can be no doubt that RCDs giveprotection against electrocution andcan reduce the risk of fire arising frominsulation failure in the electricalinstallation. This level of protection cannever be equalled by circuit-breakers orfuses alone.

1.1 FOR THE NON-SPECIALIST

Readers who are familiar with the roleand operation of RCDs can skip thissection and move on to section 1.2.

“what is an rcd?”

An RCD is a device that is designed toprovide protection against electrocutionor electrical fires by cutting off the flowof electricity automatically when itsenses a ‘leakage’ of electric currentfrom a circuit.

To appreciate the importance of an RCD it ishelpful to understand how much electricalenergy it takes to kill a human being. Thesmallest fuse used in a normal electric plug is3 Amps; it takes less than one twentieth ofthat current to kill an adult in less than onetenth of a second.

rcd operation

The operation of an RCD can beunderstood by taking an analogy fromthe water flowing in a central heatingsystem.

A leak may occur when the pipework isdamaged or punctured. In the same waya ‘leak’ of electricity can occur when thecable insulation in a circuit is faulty ordamaged.

In a central heating system, the ‘flow’pipe takes the water from the boiler tothe radiators; if the installation is soundthe same amount of water will return tothe boiler as in Figure 1. However, ifthere is a leak, there will be less water inthe return pipe than in the flow pipe. Ifthe system had flow detectors in theflow and return pipes, these could becoupled to a valve so that the valveclosed when the rate of flow in thereturn pipe was less than that in the flowpipe as in Figure 2.

THE RCD HANDBOOK BEAMA GUIDE TO THE SELECTION AND APPLICATION OF RESIDUAL CURRENT DEVICES (RCDs) 05

FlowDetector 2

Valve

Pump

Boiler Boiler

FlowDetector 1

FlowDetector 2

Pump

FlowDetector 1

Valve

RadiatorRadiator

Leak

FIGURE 1 – hEALthy cEntrAL hEAtIng cIrcuIt. thE sAMEAMount of wAtEr fLows In thE ‘fLow’ And ‘rEturn’ pIpEs

FIGURE 2 – If thErE Is A LEAk, thErE wILL BE LEss wAtEr In thE ‘rEturn’pIpE thAn In thE ‘fLow’ pIpE. thIs couLd BE usEd to trIp A vALvE.

The rate of flow of water can becompared with the current in an electricalcircuit and the water pressure can becompared with the voltage. When the lineand neutral currents are equal, the RCDwill not trip but when it senses that theneutral current is less than the line currentit will trip.

In both cases the leakage is detectedwithout measuring the leak itself. It is theflow and return rates that are measuredand compared. An RCD compares the lineand neutral currents and switches off theelectricity supply when they are no longerequal.

With an RCD, the line (brown) and neutral(blue) conductors pass through the coreof a sensitive current transformer, seeFigure 3, the output of which is electricallyconnected to a tripping system. In ahealthy installation, the current flowsthrough the line conductor and returnsthrough the neutral conductor and sincethese are equal and opposite the coreremains balanced. However, when aleakage of electric current occurs, as inFigure 4, the line and neutral currents areno longer equal; this results in an outputfrom the transformer which is used to tripthe RCD and disconnect the supply.

RCDSUPPLYN

L

RCD

LOAD

NL

SUPPLY

fIgurE 3 – IN AN RCD, THE LINE AND NEUTRALCONDUCTORS OF A CIRCUIT PASS THROUGH ASENSITIVE CURRENT TRANSFORMER. IF THE LINEAND NEUTRAL CURRENTS ARE EQUAL ANDOPPOSITE, THE CORE REMAINS BALANCED

fIgurE 5 – SCHEMATIC OF AN RCD

fIgurE 4 – IF THERE IS AN EARTH FAULT THENEUTRAL CURRENT WILL BE LOWER THAN THELINE CURRENT. THIS IMBALANCE PRODUCES ANOUTPUT FROM THE CURRENT TRANSFORMERWHICH IS USED TO TRIP THE RCD AND SO BREAKTHE CIRCUIT

SUPPLY

SENSING COIL

TEST BUTTON

RELAY

The basic principle of operation of theRCD is shown in Figure 5. When theload is connected to the supplythrough the RCD, the line and neutralconductors are connected throughprimary windings on a toroidaltransformer. In this arrangement, thesecondary winding is used as asensing coil and is electricallyconnected to a sensitive relay or solidstate switching device, the operationof which triggers the trippingmechanism. When the line andneutral currents are balanced, as in ahealthy circuit, they produce equaland opposite magnetic fluxes in thetransformer core with the result thatthere is no current generated in thesensing coil. (For this reason thetransformer is also known as a ‘corebalance transformer’).

When the line and neutral currents arenot balanced, they create an out-of-balance flux. This will induce a currentin the secondary winding which isused to operate the trippingmechanism.

It is important to note that both theline and neutral conductors passthrough the toroid.

RCDs work equally well on singlephase, three phase or three phase andneutral circuits, but when the neutralis distributed it is essential that itpasses through the toroid.

test circuit

A test circuit is always incorporated inthe rcd. typically, the operation ofthe test button connects a resistiveload between the line conductor onthe load-side of the rcd and thesupply neutral.

The test circuit is designed to pass acurrent in excess of the trippingcurrent of the RCD to simulate anout-of-balance condition. Operationof the test button verifies that theRCD is operational. It is important tonote, therefore, that the test circuitdoes not check the circuit protectiveconductor or the condition of theearth electrode.

All rcds should be checked atregular intervals to confirm that thercd trips. As a minimum, a checkevery six months is recommended.

1.2 PRINCIPLES OF RCD OPERATION

06 THE RCD HANDBOOK BEAMA GUIDE TO THE SELECTION AND APPLICATION OF RESIDUAL CURRENT DEVICES (RCDs)

1.3 RESIDUAL CURRENTDEVICES (RCDs)

RCCB(Residual Current Operated Circuit-Breaker without Integral Overcurrent protection)

A mechanical switching device designedto make, carry and break currents undernormal service conditions and to causethe opening of the contacts when theresidual current attains a given valueunder specified conditions. It is notdesigned to give protection againstoverloads and/or short-circuits and mustalways be used in conjunction with anovercurrent protective device such as afuse or circuit-breaker.

RCBO(Residual Current Operated Circuit-Breaker with Integral Overcurrentprotection)

A mechanical switching device designedto make, carry and break currents undernormal service conditions and to causethe opening of the contacts when theresidual current attains a given valueunder specified conditions. In addition itis designed to give protection againstoverloads and/or short-circuits and canbe used independently of any otherovercurrent protective device within itsrated short-circuit capacity.

SRCD(Socket-Outlet incorporating a ResidualCurrent Device)

A socket-outlet for fixed installationsincorporating an integral sensing circuitthat will automatically cause theswitching contacts in the main circuit toopen at a predetermined value of residualcurrent.

FCURCD(Fused Connection Unit incorporating aResidual Current Device)

A fused connection unit for fixedinstallations incorporating an integralsensing circuit that will automaticallycause the switching contacts in the maincircuit to open at a predetermined valueof residual current.

pRCD(portable Residual Current Device)

A device comprising a plug, a residualcurrent device and one or more socket-outlets (or a provision for connection). Itmay incorporate overcurrent protection.

CBR(Circuit-Breaker incorporating ResidualCurrent protection)

A circuit-breaker providing overcurrentprotection and incorporating residualcurrent protection either integrally (anintegral cBr) or by combination with aresidual current unit which may befactory or field fitted.

Note:The RCBO and CBR have the sameapplication, both providing overcurrent andresidual current protection. In general, anRCBO is intended to be used by ordinary(unskilled) persons and a CBR is intended to beused by skilled persons. RCBOs and CBRs aremore strictly defined by their relevant productstandards.

IC-CpD(In-Cable Control and protective Devicefor mode 2 charging of electric roadvehicles)

An rcd (≤ 30 mA) and control deviceintegrated into a mode 2 charging cablefor electric vehicle charging.(Bs En 62752:2016)

MRCD(Modular Residual Current Device)

A device or an association of devicescomprising a current sensing means anda processing device designed to detectand to evaluate the residual current andto control the opening of the contacts ofa current breaking device.

when an Mrcd is used in conjunctionwith a Moulded case circuit Breaker(MccB) or Instantaneous trip circuitBreaker (IcB), either a shunt trip or undervoltage release (uvr) may be used.

Additional types of devices:

RCM(Residual Current Monitor)

A device designed to monitorelectrical installations or circuits forthe presence of unbalanced earthfault currents. It does notincorporate any tripping device orovercurrent protection.

RDC-DD(Residual Direct Current DetectingDevice)

A device to be used for Mode 3charging of Electric vehicles. rdc-dds are intended to remove orinitiate removal of the supply toelectric vehicles in cases where asmooth residual direct current equalto or above 6 mA is detected(Bs IEc 62955). the value of 6 mAfor smooth residual direct currentwas chosen to prevent impairing thecorrect operation of an upstreamrcd type A.


2EFFECTS OFELECTRICITY

2.1 RISK OF ELECTROCUTION

It only requires a very small continuouselectric current – 40 mA (a twenty-fifthof an amp) or more – flowing throughthe human body to cause irreversibledamage to the normal cardiac cycle(‘ventricular fibrillation’) or death(‘electrocution’). when somebody comesinto direct contact with mains voltageand earth, the current flowing throughthe body, is of the order of 230 mA (justunder a quarter of an amp).

Appropriate protection against seriousinjury or death calls for disconnection ina fraction of a second (40 ms or onetwenty-fifth of a second) at 230 mA. Forlower values of shock current, longerdisconnection times may be acceptablebut if disconnection takes place within40ms fibrillation is unlikely to occur.

‘High sensitivity’ RCDs, rated 30 mA oreven 10 mA, are designed to disconnectthe supply within 40 ms at 150 mA andwithin 300ms at rated tripping currentto protect the user. ‘Medium sensitivity’devices, rated 100 mA or more willprovide protection against fire risks butwill not provide full personal protection.

A fuse or circuit-breaker alone will notprovide protection against these effects.

The actual nature, and effect of anelectric shock, will depend on manyfactors – the age and sex of the victim,which parts of the body are in contact,whether there are other resistiveelements in the ‘circuit’, for exampleclothing or footwear, if either of thecontact points is damp or immersed inwater etc.

It should be borne in mind that evenwith a 10 mA or 30 mA RCD fitted, aperson coming into contact with mainsvoltage will suffer an electric shock. Theeffects of such a shock will depend onthe specific circumstances, such asthose identified above.

2.2 TYPES OFELECTROCUTION RISK

there are basically two different typesof electrocution risk.

The first type of electrocution riskoccurs if insulation, such as the non-metallic covering around cables andleads, is accidentally damaged, exposinglive conductors. If a person comes intocontact with the ‘live’ and ‘earth’conductors there is a more serious riskbecause the current flowing to earthwill be insufficient to operate the fuseor circuit-breaker. This is becausethe human body is a poor conductorof electricity. Consequently, fusesor circuit-breakers provide NOPROTECTION at all against contactwith live conductors.

If an RCD was installed, in this situationthe current leaking to earth through thebody would cause an imbalance asdescribed in Section 1.2 and the RCDwould trip. Whilst not preventing anelectric shock, the speed of operationof the RCD will minimise the risk ofelectrocution.

The second risk occurs when the metalenclosure of electrical equipment orany metal fixture such as a sink orplumbing system accidentally comesinto contact with a live conductor,causing the metalwork to become live.In the UK a fuse or a circuit-breakernormally provides protection againstthis risk because all exposed metalworkis connected to earth. In a correctlydesigned installation, the currentflowing to earth will be sufficient toblow the fuse or trip the circuit-breaker.

2.3 EFFECTS OF ELECTRICSHOCK ON THE HUMAN BODY

Residual current devices with a trippingcurrent of 30 mA or less are now widelyused in all types of electrical installationand provide valuable additionalprotection against the risk ofelectrocution. To appreciate fully thecorrect application of these importantsafety devices it is necessary to havesome understanding of thephysiological effects of electric shockon the human body.

The term ‘electric shock’ is defined inBS 7671 as ‘A dangerous physiologicaleffect resulting from the passing of anelectric current through a human bodyor livestock.’ The amount of currentflowing will determine the severity ofthe shock. Although the definitionincludes the effects on livestock, this is arather special area and for the purposesof this section only the effects on thehuman body will be considered.

The amount of current flowing throughthe body under normal 50 Hzconditions will, in practice, depend onthe impedance (the effective resistanceof the body to the passage of electriccurrent) of that person, includingclothing/gloves/footwear etc., and onthe shock voltage. The majority ofaccidents involve simultaneous directcontact with live parts and earthedmetal, so it can be assumed that theshock voltage will be at full mainsvoltage. The value of body impedanceis much more difficult to assess becauseit can vary enormously according to thecircumstances, the characteristics of theindividual concerned and also thecurrent path through the body. In mostsituations, the current path will be fromhand to hand whilst very occasionally itmay be from hand to foot or someother part of the body. This is lesscommon due to the wearing of shoes,socks and other clothing.


In order to understand the widevariations in body impedances that canoccur, the human body can be viewedas a flexible container filled withelectrolyte, where the internalimpedance is reasonably constant atapproximately 1000 ohms. The widervariations come from the relatively highresistance at the two contact points onthe outside of the container (skinresistance). These, external impedances,can be as high as several thousandohms depending on the state of the skin(wet or dry), contact area and contactpressure. Initial current flow can bequite low but will start to increaserapidly as even small currents will

quickly burn through the surface of theskin resulting in a significant drop in theexternal impedance. In the worst casescenario, a person receiving a shock at230 V 50 Hz will experience a maximumcurrent flow of 230 mA through thecentral body area. This will havedangerous physiological results,including electrocution.

The effects of electric current passingthrough the human body becomeprogressively more severe as the currentincreases. Although individuals varysignificantly the following list is a goodgeneral guide for alternating currents.

It can be seen from the abovedescriptions that the effect of currentpassing through the human body is veryvariable but it is generally accepted thatelectrocution at normal mains voltage isusually the result of ventricularfibrillation. This condition is triggered bythe passage of electric current throughthe region of the heart and is normallyirreversible, unless expert medicalattention is obtained almostimmediately. The onset of fibrillation isdependent on the magnitude andduration of the current and the point inthe normal cardiac cycle at which theshock occurs. For those wishing tostudy the subject in greater detail thisrelationship is documented in theIEC TS 60479 series (Effects of currenton human beings and livestock).

Figure 6, which is based on IEC 60479,shows the conventional zones ofalternating currents (15-100 Hz); thecurrent path from the left hand to feetdepending on the contact time and thecorresponding maximum break timesof RCDs with a sensitivity of 30 mA.This illustrates why 30 mA RCDs aredesigned to operate within theseparameters and are recognized asproviding additional protection. Itshould be noted that whilst RCDsprovide additional protection, RCDswill not prevent an electric shock.

Effects of different values of electric current flowing through thehuman body (at 50 Hz)

0 – 0.5 mAthis current is below the level ofperception, usually resulting in noreaction.

0.5 mA – 5 mAAlthough there are no dangerousphysiological effects, a current ofthis order may startle a personsufficiently to result in secondaryinjury due to falling, droppingitems etc.

5 mA – 10 mAthis produces the same effect asabove but, in addition, muscularreaction may cause inability to letgo of equipment. In general, thefemale body is more susceptible tothis condition than the male. oncecurrent flow ceases, the victim canlet go.

10 mA – 40 mAsevere pain and shock areexperienced as current increases.At currents over 20mA the victimmay experience breathingdifficulties with asphyxia if currentflow is uninterrupted. reversibledisturbance to heart rhythm andeven cardiac arrest are possible athigher values of current and time.

40 mA – 250 mAsevere shock and possibility ofnon-reversible disturbances to thenormal cardiac cycle, referred to as“ventricular fibrillation”, occur atthis level. the possibility offibrillation increases as current andtime increase. It is also possible toexperience heavy burns or cardiacarrest at higher currents.


Zone Boundaries Physiological eects

AC-1 Up to curve a

Curve a up to curve b

Curve b up to curve c1

Above curve c1

Beyond curve c3

c1 – c2

c2 – c3

AC-2

AC-3

AC-4

FIGURE 6 – tIME/currEnt of ALtErnAtIng currEnt EffEcts (15 - 100 hz) on pErsons for currEnt

pAth corrEspondIng to thE pAssAgE froM LEft ArM Into fEEt And coMpArIson wIth LIMIts of

trIppIng tIMEs of rEsIduAL currEnt dEvIcE IΔn = 30 mA

The details so far have been greatlysimplified by assuming that normalenvironmental conditions apply and thatthe source of the electric shock is analternating current supply at 50 Hz.Under special conditions, for examplewhen a body is immersed in water or inclose contact with earthed metal, the

body impedance will generally be at itslowest, with consequently high shockcurrents.

Frequencies of 15-100 Hz areconsidered to present the most seriousrisk. At other frequencies, includingdirect current, the threshold of

fibrillation occurs at a different currentlevel. All these factors must beconsidered when making a choice ofRCD for special applications. Underthese circumstances, the potential useris strongly recommended to consult themanufacturer for appropriate advice.


3ELECTRIC SHOCKPROTECTION

3.1 PRINCIPLES OF SHOCKPROTECTION

protection of persons and livestockagainst electric shock is a fundamentalprinciple in the design of electricalinstallations in accordance with Bs 7671:Requirements for Electrical Installations,commonly known as The IET WiringRegulations. use of the correct earthingsystem is an essential part of this process.

Electric shock may arise from directcontact with live parts, for examplewhen a person touches a live conductorthat has become exposed as a result ofdamage to the insulation of an electriccable. Alternatively, it may arise fromindirect contact if, for example, a faultresults in the exposed metalwork of anelectrical appliance, or even othermetalwork such as a sink or plumbingsystem becoming live. In either casethere is a risk of an electric currentflowing to earth through the body ofany person who touches the liveconductor or live metalwork. (SeeFigure 7). (The terms ‘direct contact’ and ‘indirect contact’ have now beenreplaced in BS 7671 – see section 3.3of this document.)

Fuses and circuit-breakers provide thefirst line of defence against indirectcontact electric shock. If the installationis correctly earthed (i.e. all the exposedmetalwork is connected together and tothe main earth terminal of theinstallation) then an indirect contactfault will cause a very high current toflow to earth through the exposedmetalwork. This will be sufficient to‘blow’ the fuse or trip the circuit-breaker, disconnecting that part of theinstallation within the time specified inBS 7671 and so protecting the user.

Fuses and circuit-breakers cannotprovide protection against the verysmall electric currents flowing to earththrough the body as a result of directcontact. RCDs, provided they have beenselected correctly, can afford thisprotection as described in the previouschapter. They also provide protectionagainst indirect contact under certaininstallation conditions where fuses andcircuit-breakers cannot achieve thedesired effect, for example where theearthing systems described above areineffective.

FIGURE 7 –dIrEct And IndIrEct contAct ELEctrIc shock


3.2 EARTHING SYSTEMS

For a full understanding of electric shock protection it isnecessary to consider the different types of earthing systemin use. BS 7671 lists five types as described below:

FIGURE 8 –tn-c systEM

FIGURE 9 –tn-s systEM

FIGURE 10 –tn-c-s systEM

In this arrangement a single protectiveearth and neutral (pEN) conductor is usedfor both the neutral and protectivefunctions, all exposed-conductive-partsbeing connected to the pEN conductor. Itshould be noted that in this system anRCD is not permitted since the earth andneutral currents cannot be separated.

With this system the conductors for neutraland protective earth (pE) circuits areseparate and all exposed-conductive-partsare connected to the pE conductor.Thissystem is the one most commonly used inthe UK, although greater use is being madeof the TN-C-S arrangement due to thedifficulties of obtaining a good substationearth.

The usual form of a TN-C-S system iswhere the supply is TN-C and thearrangement of the conductors in theinstallation is TN-S.This system is oftendescribed as a protective multipleearthing (pME) system. This is incorrectsince pME is a method of earthing.


FIGURE 11 –tt systEM

FIGURE 12 –It systEM

In a TT system the electricity supplyprovider and the consumer mustboth provide earth electrodes atappropriate locations, the two beingelectrically separate. All exposed-conductive-parts of the installation areconnected to the consumer’s earthelectrode.

Unlike the previous systems, the IT systemis not permitted, except under speciallicense, for the low voltage supply in theUK. It does not rely on earthing for safety,until after the occurrence of a first-fault,as the supply side is either completelyisolated from earth or is earthed through ahigh impedance.

3.3 PROTECTION AGAINSTDIRECT AND INDIRECTCONTACT(in the context of this document)

It is a fundamental requirement ofBS 7671 that all persons and livestockare protected against electric shock inany electrical installation. This is subjectto the installation being used withreasonable care and having regard tothe purpose for which it was intended.When considering protection againstelectric shock, it is necessary tounderstand the difference between‘direct contact’ and ‘indirect contact’,which was first introduced by the 15thEdition of the IEE Wiring Regulations in1981 (See Figure 7).

Direct contact electric shock is theresult of simultaneous contact bypersons or livestock with a normally livepart and earth potential. As a result thevictim will experience nearly full mainsvoltage across those parts of the body

which are between the points ofcontact.

Indirect contact electric shock resultsfrom contact with an exposedconductive part made live by a faultcondition and simultaneous contactwith earth potential. This is usually at alower voltage.

Protection against direct contactelectric shock (now defined as BasicProtection in BS 7671) is based onnormal common sense measures suchas insulation of live parts, use of barriersor enclosures, protection by obstaclesor protection by placing live parts out ofreach. As a result, under normalconditions it is not possible to touch thelive parts of the installation orequipment inadvertently.

Protection against indirect contactelectric shock (now defined as FaultProtection in BS 7671) is slightly morecomplicated hence a number of optionsare given in BS 7671 for the installation

designer to consider. The majority ofthese require specialist knowledge orsupervision to be applied effectively.The most practical method for generaluse is a combination of protectiveearthing, protective equipotentialbonding and automatic disconnectionof supply. This method which providesvery effective protection when properlyapplied, requires consideration of threeseparate measures by the circuitdesigner:

• Protective Earthing

• Protective equipotential bonding

• Automatic disconnection in the event of a fault

protective Earthing requires allexposed-conductive-parts (generallymetallic) of the installation to beconnected to the installation main earthterminal by means of circuit protectiveconductors (cpcs).


The main earth terminal has to beeffectively connected to Earth. Typicalexamples of exposed-conductive-parts

include:

• Conduits and trunking

• Equipment enclosures

• Class I luminaires

• The casings and framework of current using equipment

protective equipotential bondingminimises the risk of electric shock byconnecting extraneous-conductive-parts (generally metalwork that is incontact with Earth) within the location,to the main earth terminal of theinstallation. This means that under faultconditions the voltage that is present onthe metal casings of electricalequipment is substantially the same asthat present on all extraneous-conductive-parts. Theoretically, aperson or animal coming intosimultaneous contact with the faultyequipment and other earthedmetalwork will not experience anelectric shock because of theequipotential cage formed by thebonding. In practice, however, a small‘touch’ voltage will be present due todiffering circuit impedances.

Automatic disconnection of supply ismost important for effective shockprotection against indirect contact. Itinvolves ensuring that the faulty circuitis disconnected within a specified safetime following a fault to earth. Whatconstitutes a safe time depends onmany factors and those who requiredetailed information on this shouldconsult the definitive documents, IECTS 60479 series and BS 7671 Regulation411.3.2.

When using an overcurrent protectivedevice e.g. a fuse or circuit-breaker, forautomatic disconnection, in order tomeet the requirements of BS 7671, it isnecessary to ensure that these devicescan operate within a specified time inthe event of an earth fault. This isachieved by making sure that the earthfault loop impedance is low enough toallow sufficient fault current to flow. It ispossible to calculate the appropriatevalues using the published time/currentcurves of the relevant device.Alternatively BS 7671 publishesmaximum values of earth fault loopimpedance (Zs) for different types and

ratings of overcurrent device. Referenceshould be made to the time/currentcurves published in BS 7671 or by themanufacturers of protective devices.

3.4 RCDS AND INDIRECTCONTACT SHOCKPROTECTION

Indirect contact protection by fuses orcircuit-breakers is dependent on circuitearth fault loop impedances beingwithin the parameters laid down by BS7671. Where these values cannot beachieved or where there is some doubtabout their stability, then an alternative

method is required. It is in this situationthat the RCD offers the most practicalsolution because it has the ability tooperate on circuits having much highervalues of earth fault loop impedance.

The basis of RCD protection in thissituation is to ensure that any voltage,exceeding 50V that arises due to earthfault currents, is immediatelydisconnected. This is achieved bychoosing an appropriate residualcurrent rating and calculating themaximum earth loop impedance thatwould allow a fault voltage of 50V. Thisis calculated by using a simple formulagiven in BS 7671 Regulation 411.5.3.

RA × IΔn ≤ 50 vWhere RA is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed conductive part (in ohms)

IΔn is the rated residual operating current of the RCD (amps) Note: Where RA is not known it may be replaced by Zs.

TABLE 1 – MAXIMUM EARTH FAULT LOOP IMPEDANCE (ZS) TO ENSURE RCDOPERATION IN ACCORDANCE WITH REGULATION 411.5.3 FOR NON DELAyEDRCDs TO BS EN 61008-1 AND BS EN 61009-1

Rated residual operating current (mA) Maximum earth fault loop impedance

Zs (ohms) for U0 of 230 v

30

100

300

500

1667*

500*

167

100

Note 1: Figures for Zs result from the application of Regulation 411.5.3(i) and 411.5.3(ii).

Note 2: *The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms may not be stable. Refer to Regulation 542.2.4.

The use of a suitably rated RCD for indirect contact shock protection will permitmuch higher values of Zs than could be expected by using overcurrent protectivedevices. In practice, however, values above 200 ohms will require furtherconsideration. This is particularly important in installations relying on local earthelectrodes (TT systems) where the relatively high values of Zs make the use of anRCD absolutely essential.

Maximum values of Zs for the basic standard ratings of RCDs are given in Table 1,unless the manufacturer declares alternative values.


3.5 RCDS AND DIRECTCONTACT SHOCKPROTECTION

The use of RCDs with rated residualoperating current of 30 mA or less arerecognised as additional protectionagainst direct contact shock. Regulation415.1.1 refers.

Direct contact shock is the result ofpersons or livestock inadvertently makingcontact with normally live parts with onepart of the body and, at the same time,making contact with earth potential withanother part of the body. Under thesecircumstances, the resulting electricshock will be at full mains potential andthe actual current flowing to earth will beof the order 230 mA because of therelatively high body impedance involved.It has already been shown in Section 2.3that currents as low as 40/50 mA canresult in electrocution under certaincircumstances. A 30 mA RCD willdisconnect an earth fault current beforethe levels at which fibrillation occurs arereached.

The nominal rating of 30 mA has thusbecome the internationally acceptednorm for RCDs intended to provideadditional protection against the risk ofelectrocution.

However, the rated operating current isnot the only consideration; the speed oftripping is also very important. Ifventricular fibrillation is to be avoided.

Examples of types of fault conditionwhere the RCD can be of particularbenefit are listed in Chapter 5.

One example is situations where basicinsulation has failed either throughdeterioration or, more commonly,through damage. An example of this iswhen a nail is driven through a partitionwall and penetrates a cable. This willcause a first-fault condition due to failureof the basic insulation. The result of this isthat there is now a strong possibility thatthe nail will become live by contacting thelive conductor. Any subsequent contactby a person presents a risk ofelectrocution or injury by direct contact.An RCD will provide additional protectionand significantly reduce the risk of injuryor death because it will trip when adangerous level of current flows to earththrough the person in contact with thenail.

This type of RCD protection is identical tothe more common situation where aflexible cable is damaged (for example bya lawn mower) and exposes liveconductors. Here again the RCD providesprotection of anybody who comes intocontact with the exposed live conductors.

The extra protection provided by RCDs isnow fully appreciated and this isrecognised in BS 7671 Regulation 415.1.

It must be stressed, however, that theRCD should be used as additionalprotection only and not considered as asubstitute for the basic means of directcontact shock protection (insulation,enclosure etc).

3.6 RCDS IN REDUCED ANDEXTRA-LOW VOLTAGEAPPLICATIONS

In normal use, dangerous touchvoltages should not occur on electricalequipment intended for use with, andsupplied from, an extra-low (notexceeding 50 V AC) or reduced voltage(not exceeding 63.5 V to earth in three-phase systems or 55 V to earth in single-phase systems) source. Such circuits areknown as:

Separated extra-low voltage (SELV), inwhich the circuit is electrically separatedfrom earth and from other systems.

protective extra-low voltage (PELV), asSELV except that the circuit is notelectrically separated from earth.

Functional extra-low voltage (FELV), anextra-low voltage system in which notall of the protective measures of SELVor PELV have been applied.

Reduced low-voltage system a voltage system in which all exposed-conductive-parts are connected toearth and protection against indirectcontact shock is provided by automaticdisconnection by overcurrent protectivedevice or RCD.

SELV, PELV and reduced low voltagesystem arrangements involve electricalseparation of the final circuit normallyby means of a safety-isolatingtransformer. In normal use, thetransformer prevents the appearance ofany dangerous touch voltages on eitherthe electrical equipment or in thecircuit. Although extremely rare, a faultoccurring within the safety isolatingtransformer may result in a dangeroustouch voltage, up to the supply voltage,appearing within the circuit or on the

electrical equipment. Where additionalprotection against this risk is required, orin the case of a reduced low voltagesystem, an RCD with a rated residualcurrent of 30 mA or less, can beinstalled in the primary circuit to achievea 5 s disconnection time.

In PELV, FELV and reduced low voltagesystems an RCD can, if required, beconnected into the secondary circuit ofthe transformer. This will provideadditional protection against electricshock under all conditions:

• Shock protection if there is a failure of the transformer and mains voltage appears on the secondary side

• Protection against indirect contact from the low voltage secondary voltage

• Additional protection against direct contact from the low voltage secondary voltage

It must be remembered that, since aFELV circuit is not isolated from themains supply or earth, it presents thegreatest risk from electric shock of allof the ELV methods.

An RCD can also provide this additionalprotection in a SELV circuit and itselectrical equipment but in this case adouble-fault condition, which need notnormally be considered, would have tooccur before the RCD could operate.

Manufacturer’s guidance should alwaysbe sought when applying RCDs in extra-low and reduced voltage applications,to confirm that devices will operate atthese voltages. This is particularlyimportant with respect to the testbutton since its correct operationdepends on the supply voltage.

3.7 RCDS IN ELECTRICVEHICLE CHARGING

Particular care must be taken in theselection of the type of RCD to be usedin electric vehicle charging installations.

• BS 7671 does not permit the use of Type AC RCDs for this application

• Type A may be used where any smooth DC fault current is less than 6 mA or appropriate equipment that ensures disconnection of the supply in case of smooth DC fault current above 6 mA (i.e. RDC-DD) is installed

• Otherwise, Type B is required.


4FIRE PROTECTION

4.1 BACKGROUND

Electrical fires continue to be asignificant issue in uk installations.Electricity is major cause of accidentalfires in uk homes – over 20,000electrical fires each year. fire statisticsfor 2011/12 identify that 89 % ofelectrical fires are caused by electricalproducts, 11 % (circa 2,200) of whichare caused by faults within installationsor by people not using installationsproperly.

More recent statistics from 2013/14attribute 12 % of fires to electricaldistribution (wiring, cabling, plugs).These statistics demonstrate thatelectrical fires occur and can causeinjuries, deaths and damage or destroysignificant amounts of property.Electrical fires can be a silent killeroccurring in areas of the home that arehidden from view and early detection.

Source: Department for Communitiesand Local Government, Fire Statistics2011/12

Household electricity supplies are fittedwith fuses or circuit-breakers to protectagainst the effects of ‘overcurrents’(‘overloads’ in circuits which areelectrically sound and ‘short-circuitfaults’ due to contact between liveconductors in a fault situation.) RCDsprovide additional protection against theeffects of earth leakage faults whichcould present a fire risk.

4.2 PROTECTIVE MEASURESAS A FUNCTION OFEXTERNAL INFLUENCES

It is widely accepted that RCDs canreduce the likelihood of fires associatedwith earth faults in electrical systems,equipment and components by limitingthe magnitude and duration of currentflow.

The ability to provide additionalprotection against the risk of fire isrecognised in BS 7671, for example:

• Regulation 422 defines the precautions to be taken in ‘Installations where Particular Risks of Danger of Fire Exist’. Regulation 422.3.9 requires, in TN and TT systems, that wiring systems, with the exception of mineral insulated cable and busbar trunking systems, are protected against insulation faults to earth by an RCD having a rated tripping current not exceeding 300 mA.

• Section 705 defines the particular requirements that apply to ‘Agricultural and Horticultural Premises’. Regulation 705.422.7 requires, for the protection against fire, an RCD having a rated tripping current not exceeding 300 mA. The RCD is required to disconnect all live conductors.

Research commissioned by theDepartment of Trade and Industry in1997, established that a common sourceof earth faults is surface tracking oninsulation. The report confirms thatcurrents as low as 50-100 mA havebeen found to be sufficient to causeignition and fire as a result of trackingand that at these currents, an RCD ratedto provide protection against electricshock, would also have preventedignition. Attention is drawn also to thefact that minimising the presence ofelectrically conducting dust or liquids,which may arise due to leakage orspillage, can reduce the onset of surfacetracking.

Again, in BS 7671, Chapter 42 setsrequirements to prevent the wiringsystems and electrical equipment beingexposed to the harmful build-up ofmaterials such as dust or fibre likely topresent a fire hazard.

ELECTRICITY IS MAJOR CAUSEOF ACCIDENTAL FIRES IN UKHOMES – OVER 20,000ELECTRICAL FIRES EACH YEAR.


5INSTALLATION RISKS

5.1 BACKGROUND

It is clear that increased use of correctlyselected rcds, in addition to goodwiring practice, can reduce the effectsof electric shock and the possibility offire risk significantly. rcd protectionalso provides an additional level ofprotection where the wiring complieswith Bs 7671 but the integrity of thewiring system has been damaged.

5.2 TYPICAL RISKS

Mechanical damage to cablesThe risk of people cutting through livecables is well known. Examples includethe following:

penetration of cable insulation in wallsand beneath floorboards. This is acommon occurrence during DIy work inthe home. The main danger arises whensomeone comes into contact with livecables either directly or indirectly,resulting in an electric shock.

Cutting the supply lead or an extensionlead with an electric lawn mower orhedge trimmer. This is anothercommon occurrence and can result ineither a serious electric shock or deathwhen bodily contact is made with theexposed live conductor.

Trapped or poorly maintainedextension leads. The effects here aresimilar to those described above.

vermin. It is surprisingly common formice and other vermin to chew throughcables, exposing the live conductors.

In all the above situations, even ifbodily contact does not occur, damageto the cable insulation can result in afire risk which is significantly higher ifRCD protection is not used.

Locations containing a bath or showerThese locations present a much higherrisk because a wet body presents amuch easier path for an electric currentto flow to earth. Consequently BS 7671prohibits the use of electricalequipment, other than shaversconnected through an appropriateshaver supply unit, within 3 m of thebath or shower basin. Nevertheless,tragedies have occurred as a result ofpeople using extension cables to supplyportable electrical appliances in theselocations.

Fire risk associated with fixed electricalappliancesFaulty electrical appliances increase therisk of fire. For example, fire can occurwhen the insulation on an electricmotor breaks down due to deteriorationor external damage.

This can result in the ignition of anyflammable material, including dust, inthe vicinity of the non-insulated ‘live’parts.

Bad wiring practiceAlthough all new and/or modifiedinstallations must comply with thecurrent edition of BS 7671 it is possiblethat a person may incorrectly erect orsubsequently incorrectly modify aninstallation.

Examples of the risks of electric shockand fire resulting from incorrectly wiredsystems include the following:

• Inadequate earthing or bonding

• Wires trapped during installation

• Insulation damaged during or after installation

• Bad system design

RCDs are not a substitute for goodwiring practice. However, correctlyinstalled RCDs will continue to providea high degree of protection against therisks of electrocution and fire evenwhen an installation deteriorates dueto poor maintenance or lack ofcompliance with BS 7671.

FAULTY ELECTRICALAPPLIANCES INCREASETHE RISK OF FIRE.


The term RCDs covers a rangeof products some of which arelisted below:,

• rccB (residual current operated circuit-Breaker without Integral overcurrent protection)

• rcBo (residual current operated circuit-Breaker with Integral overcurrent protection)

• srcd (socket-outlet incorporating a residual current device)

• fcurcd (fused connection unit incorporating a residual current device) prcd (portable residual current device)

• cBr (circuit-Breaker incorporating residual current protection)

• Ic-cpd (In-cable control and protective device for mode 2 charging of electric road vehicles)

• Mrcd (Modular residual current device)

6RCD SELECTION

this chapter is designed to help thespecifier, installer and end user todecide on the appropriate residualcurrent protection.

where it is intended to protect thewhole or part of the fixed electricalinstallation by an rcd, the layman isstrongly advised to seek expert advice.

Portable residual current devices(PRCDs) are available for use by thenon-specialist where normal socket-outlets are not protected by RCDs. Theymay be high sensitivity RCD adaptors,which plug into the socket-outlet, orextension units which include a plug, ahigh sensitivity RCD and one or moresocket-outlets.

A PRCD is not part of the fixed electricalinstallation and only protects theequipment that is supplied through it.

It should be noted that BS 7671Regulation 411.3.3 requires additionalprotection by means of an RCD.

In practice there may be specificprotection issues which are not coveredin this handbook. For additionalguidance regarding the suitability of aparticular RCD for specific applicationsit is recommended that readers consultany of the BEAMA RCD manufacturerslisted at the beginning of thispublication.

6.1 RCD SELECTIONCRITERIA

6.1.1 Sensitivity

For every RCD there is normally achoice of residual current sensitivity(tripping current). This defines the levelof protection afforded. Protection isdivided into two broad categories:

personal protection (additionalprotection of persons or livestock

against direct contact) This is ensuredwhen the minimum operating current ofthe RCD is no greater than 30 mA andthe RCD operates to disconnect thecircuit, within the specified time, in theevent of an earth leakage.

Installation protection This isassociated with devices that are used toprotect against the risk of fire caused byan electrical fault. RCDs which operateat residual current levels up to andincluding 300 mA provide this type ofprotection.

6.1.2 Residual Current Devices (RCDs

Table 2 aims to identify RCD usetogether with the benefits provided.However, before looking at Table 2there are two other classifications ofRCD that need to be considered –general and time-delayed operationeach having Type AC, A, F or Bcharacteristics.

6.1.3 General and Time-Delayed RCDs

RCCBs to BS EN 61008: Specification forresidual current operated circuit-breakers without integral overcurrentprotection for household and similaruses (RCCBs) and RCBOs to BS EN61009: Specification for residual currentoperated circuit-breakers with integralovercurrent protection for householdand similar uses (RCBOs) may bedefined by the time they take to operateas follows.

WHERE IT IS INTENDEDTO PROTECT THEWHOLE OR PART OFTHE FIXED ELECTRICALINSTALLATION BY ANRCD, THE LAYMAN ISSTRONGLY ADVISED TOSEEK EXPERT ADVICE.


For RCCBs complying with BS EN 61008and RCBOs complying with BS EN61009 the time delay feature isindicated by the letter ‘S’.

Selectivity is achieved between RCDswhen:

• the upstream RCD is of selective type (type S or time-delayed type with appropriate time delay setting), and

• the ratio of the rated residual operating current of the upstream RCD to that of the downstream RCD is at least 3:1.

In the case of RCDs with adjustablerated residual operating current andtime delay, reference shall be made tomanufacturer’s instructions forselectivity.

It is not possible to achieve selectivitywith two S type RCDs in series.

6.1.4 Types AC, A, F and B RCDs.

Residual current devices may also beclassified as Type AC., Type A, Type Fand Type B as follows:

RCD Type AC: RCD tripping onalternating sinusoidal residual current,suddenly applied or smoothlyincreasing.RCD Type A: RCD tripping onalternating sinusoidal residual currentand on residual pulsating direct current,suddenly applied or smoothlyincreasing.

NOTE 1: For RCD Type A tripping is achieved

for residual pulsating direct currents

superimposed on a smooth direct current up

to 6 mA.

RCD Type F: RCD for which tripping isachieved as for Type A and in addition:for composite residual currents,whether suddenly applied or slowlyrising intended for circuit suppliedbetween phase and neutral or phaseand earthed middle conductor; forresidual pulsating direct currentssuperimposed on smooth direct current.

NOTE 2: For RCD Type F tripping is achieved



to 10 mA.

RCD Type B: RCD for which tripping isachieved as for Type F and in addition:

• for residual sinusoidal alternating currents up to 1 kHz;

• for residual alternating currents superimposed on a smooth direct current;

• for residual pulsating direct currents superimposed on a smooth direct current;

• for residual pulsating rectified direct current which results from two or more phases;

• for residual smooth direct currents whether suddenly applied or slowly increased independent of polarity.

NOTE 3: For RCD Type B, tripping is achieved



to 6 mA.

Product standards for RCDs for use inDC supply sytems are currently underdevelopment.

Bs 7671:2018 regulation 531.3.3states 'different types of rcds exist,depending on their behaviour inpresence of dc components andfrequencies. the appropriate rcdshall be selected.

General RCDs operate‘instantaneously’, i.e. they do nothave an intentional delay in operationand thus cannot be guaranteed to‘discriminate’. This means that wherethere are two or more general RCDsinstalled in series in an installation;more than one device may trip in theevent of an earth leakage current.This would result in healthy circuitsbeing disconnected even though theinitial fault occurred in a different partof the installation. Selectivity isessential in installations where it isimportant to ensure that a completesystem is not ‘shut down’, forexample in domestic installations toensure that lighting and other circuitsare not disconnected if an earthleakage occurs in a power circuit.

Time Delayed RCDs provideselectivity in circuits where RCDs areconnected in series. It is essential toinstall devices which incorporate atime delay upstream of the generaldevice, so that the device nearest afault will trip. RCDs with built in timedelays should not be used to providepersonal protection.

TYPE AC, TYPE A, TYPE FAND TYPE B RCDS ARE NOTSUITABLE FOR USE IN DCSUPPLY SYSTEMS.


rcd Examples of type of equipment / load

Type AC

Type A

Type F

Type B

Type B+

Resistive, Capacitive, Inductive loads generally without any electronic components, typically:

• Immersion heater

• Oven/Hob with resistive heating elements

• Electric shower

• Tungsten & halogen lighting

Single phase with electronic components, typically:

• Single phase invertors

• Class 1 IT and Multimedia equipment

• Power supplies for Class 2 equipment

• Appliances such as a washing machine that is not frequency controlled e.g. d.c. or universal motor

• Lighting controls such as a dimmer switch and home and building electronic systems LED drivers

• Induction hobs

• Electric Vehicle charging where any smooth DC fault current is less than 6 mA

Type A is also suitable for Type AC applications.

Frequency controlled equipment / appliances, typically:

• Some washing machines, dishwashers and driers e.g. containing synchronous motors*

• Some class 1 power tools

• Some air conditioning controllers using variable frequency speed drives

Type F is also suitable for Type AC and Type A applications.

Three phase electronic equipment typically:

• Inverters for speed control

• UPS

• Electric Vehicle charging where any smooth DC fault current is greater than 6mA

• Photo voltaic

Power Electronic Converter Systems (PECS) typically:

• industrial machines

• cranes

Type B is also suitable for Type AC, Type A and Type F applications.

Type B+ RCDs are not recognised in BS 7671 and do not have an international or harmonised (BS EN) standard.

* Manufacturer’s instructions should be taken into account.

power electronic converter pEC

device or part thereof for the purpose of electronic power conversion, including signalling,

measurement, control circuitries and other parts, if essential for the power conversion function

power electronic converter system pECS

one or more power electronic converters intended to work together with other equipment

For PECS, if a Type B RCD is required, the product will be marked with the symbol .

The instructions shall include a caution notice highlighting that where an RCD is used for

protection against electrical shock, only an RCD of Type B is allowed on the supply

side of this product.

In all cases equipment / appliance manufacturers instructions must be considered when selecting the Type of RCD.

TABLE 2 – APPLICATION EXAMPLES OF TyPES AC, A, B AND F RCDs


RCD Type A

Type AType A Type B Type FRCD RCD RCD

S

Wh

A Type F RCD should not be fittedupstream of a Type B RCD as the loadcharacteristics that a Type B RCD hasbeen selected for could impairoperation of the Type F RCD.

FIGURE 13 – ExAMpLE InstALLAtIon ArrAngEMEnt

A Type AC RCD should not be fittedupstream of a Type A, F or B RCD as theload characteristics that the Type A, F orB RCD has been selected for couldimpair operation of the Type AC RCD.

A Type A RCD should not be fittedupstream of a Type F or B RCD as theload characteristics that a Type F or BRCD has been selected for could impairoperation of the Type A RCD.


Suitable for Industrial &Commercial Applications

Earth LeakageSensitivity mA (2)

Suitable forDomestic Applications

Suitable as a MainIncoming Device (CU)

Suitable as an OutgoingDevice on a CU, DB,PB or SB (5,7)

Part of the Incomeron a CU, DB, PB orSB (5,7)

Provides PersonalProtection

Provides ProtectionAgainst ElectricalFire(8)

Protection to SocketOutlets 20A or less

Fixed Wiring Protection

Portable ApplianceRated 20A or Less

Can be used toDiscriminate withInstantaneousDownstream Device

(6) (6) (6) (6) (6)

DEVICE TYPE RCCB RCBO SRCD FCURCD PRCD CBR MRCD

TABLE 3 – rcds for dIffErEnt AppLIcAtIons

Notes:(1) Only if used in conjunction with suitable overcurrent protection (e.g. Fuse/circuit-breaker).(2) 10 mA RCDs are associated with highly sensitive equipment and high risk areas such as school laboratories and in hospital areas.(3) yes provided 30 mA or less, but not normally used.(4) With time delay.(5) CU – Consumer unit to BS EN 61439-3.(6) Must provide double pole isolation(7) DB – Distribution Board; PB – Panel Board; SB – Switch Board(8) For agricultural and horticultural premises, the RCD is required to disconnect all live conductors


TRANSFORMER

MAININCOMER (CB)

MAINSWITCHBOARD

PANELBOARD

DISTRIBUTIONBOARD (DB)

OR CONSUMERUNIT (CU)

Earth fault protectionassociated with theincoming circuit isprovided by CBR/MRCD.(Could also provide earthleakage monitoring system).

Individual outgoing wayscan be protected by CBRs.

Choose RCD protectionin line with Figures 14-18.

Earth fault protectionassociated with theincoming circuit isprovided by CBR/MRCD.

For each stage ofthe system:

ENSUREEFFECTIVE

SELECTIVITY

PROTECTION LEVEL CONSIDERATIONS

6.2 RCD SELECTION GUIDES

The following selection guides are intended to help the specifier or installer decide on the most appropriate solution tocommon installation arrangements.

6.2.1 Commercial/industrial system RCD protection options (figure 14)

6.2.2 Sub distribution and final circuit RCD protection options (figures 15– 19)

FIGURE 14 –COMMERCIAL/INDUSTRIAL SySTEM RCD PROTECTION OPTIONS

DISTRIBUTION BOARD(DB) OR CONSUMER

UNIT (CU) Individual circuit protection eg: suppliesto outbuilding/portable equipment.

Personal protection provided if RCDis 30 mA.

RCD protection limited toone circuit only.

Can be retro fitted atminimum cost.

RCCBRCBOPRCDSRCD


FIGURE 15 – OUTGOING CIRCUIT RCD PROTECTION, SEPARATE FROM THE DISTRIBUTION BOARD


FIGURE 16 –SPLIT LOAD PROTECTION (A)

FIGURE 17 – SPLIT LOAD PROTECTION (B)

FIGURE 18 – DUAL SPLIT LOAD PROTECTION (C)

FIGURE 19 – THE MOST COMPREHENSIVE OPTION – INDIVIDUAL OUTGOING PROTECTION ON ALL WAyS

PROTECTION LEVEL CONSIDERATIONSSPLIT LOAD CONSUMERUNIT (CU) OR DISTRIBUTION

BOARD (DB) WITH MAININCOMING SWITCH –

DISCONNECTORAND RCCB(S), TO PROTECTA SPECIFIC GROUP(S) OF

CIRCUITS

Commonly used to provideRCD protection to a group(s)of circuits e.g. Socket-Outletssupplying portable equipment.

Personal protection provided ifRCD is 30 mA.

Fault on one of the RCDprotected circuits will trip outthe supply to all associatedRCD protected circuits.Installation partially RCDprotected.


SPLIT LOAD CONSUMERUNIT (CU) OR DISTRIBUTION

BOARD (DB) WITH MAININCOMING RCD AND

SECONDARY RCCB(S), TOPROTECT A SPECIFIC

GROUP(S) OF CIRCUITS

Main incoming RCD will provideprotection to complete installation.(Typically 100 mA Time Delayed).

Intermediate RCCB(s) commonlyused to provide RCD protectionto a group(s) of circuits e.g. Socket-Outlets supplying portableequipment.

Personal protection provided ifRCD is 30 mA.

Installation is fully RCD protected.

Main incoming RCD can be selectedto provide fire protection for thecomplete installation.

Intermediate RCCB can be selected toprovide personal protection onhigh risk circuits.

Fault on one of the RCD protectedcircuits will trip out supply to allassociated RCD protected circuits.

Correct selection of devices for themain incoming RCD and intermediateRCCBs will provide selectivitybetween devices.

DB - RCCB/CBRCU - RCCB

PROTECTION LEVEL CONSIDERATIONSSPLIT LOAD CONSUMER

UNIT (CU) OR DISTRIBUTIONBOARD (DB) WITH MAIN

INCOMING SWITCHDISCONNECTOR AND

RCCB(S), TO PROTECT ANUMBER OF SPECIFIC

GROUPS(S) OF CIRCUITS

Main incoming SwitchDisconnector to isolate allcircuits. 30mA RCDswill provide protection togroups of circuits.

Personal protection and fireprotection is provided toall circuits.

Installation is fully RCD protected.Fire protection and personalprotection provided for thecomplete installation.

Meets 18th Edition requirements forprotection of Socket-Outlets andcables concealed in walls andpartitions.

A fault on one circuit will causethe upstream RCD to operatedisconnecting the supply to allcircuits associated with that RCD.Only a section of the installationis a!ected.


DISTRIBUTION BOARD (DB) ORCONSUMER UNIT (CU) WITHINCOMING MAIN SWITCH -

DISCONNECTOR ANDINDIVIDUAL RCBO OR CBR

PROTECTION ON OUTGOINGCIRCUITS

Outgoing circuits with individuallyRCBO or CBR protection will operate without a!ecting other circuits.

Personal protection provided if RCBOs are ≤ 30 mA.

Installation is fully RCBOprotected. Fire protection andpersonal protection provided forthe complete installation.

Meets 18th Edition requirementsfor protection of Socket-Outletsand cables concealed in wallsand partitions.


7OPERATION ANDMAINTENANCE

7.1 TESTING BY THEEND USER

All rcds should be tested at least onceevery six months to ensure that theyare still operative. this can be carriedout by the end user. It involvesoperating the test device (normally apush button) marked ‘t’ or ‘test’.this should cause the rcd to trip,disconnecting the supply to theprotected circuit. reinstate the supplyby reclosing the device or pressing the‘reset’ button as appropriate.

If the rcd does not switch off thesupply when the test button is pressed,the user should seek expert advice.

7.2 TESTING BY THEINSTALLER

7.2.1 Time/current performance test

BS 7671 requires a test independent ofthe RCD test button facility to beapplied to ensure that the RCD satisfiesthe disconnection times required forfault and additional protection asdetailed in Chapter 41.

The test parameters detailed in Table 4are in accordance with the requirementsof the relevant product standards –BS EN 61008 series 1 and BS EN 61009series 1 which satisfy theserequirements.

All tests must be performed with allloads disconnected using anappropriate calibrated test instrumentconnected as close to the RCD aspossible.

7.2.2 Functional test

Upon completion of the installation anoperational check of the RCD should beundertaken by pressing the RCD test

button as described in 7.1 above. If theRCD fails to trip, investigate inaccordance with the ‘Trouble shooting’chart (Figure 20).

7.2.3 Insulation tests

When insulation tests are carried out onan installation, the applied voltageshould not exceed 500 V DC (RCDs aredesigned to withstand this voltage).

An RCD in circuit may affect insulationresistance test results. It may benecessary to disconnect RCDs for thepurpose of these tests.

7.2.4 Earth loop impedance testing

Some earth loop impedance testers aredesigned to inject an a.c. test currentthrough line and earth conductors of upto 25 A. This current will trip all RCDs. Toavoid tripping the device during the testsome instruments have the facility to testwith a 15 mA test current. Others use a DCcurrent to desensitise the RCD for theduration of the test however this type oftester only works on RCDs that aresensitive to AC faults alone and does notprevent many types of RCBOs fromtripping. Type A, Type F and Type B RCDs(designed to the product standards BS EN61008, BS EN 61009 and BS EN 62423)will trip upon detection of the DCdesensitising current.

Earth loop impedance figures forinstallations which contain RCDs sensitiveto both AC and DC fault currents (i.e. typeA, type F and type B devices), should bedetermined either by calculation or byusing a tester having a test current belowthe device trip threshold. Alternatively, testmethods can be used which will not tripthe RCD. One such method is to measurethe earth fault loop impedance on thesupply side of the RCD and add this to thevalue of the combined resistance (R1+R2)on the load side of the RCD. This methodalso checks the continuity of theprotective conductor.

Type

General Any value Any value

S ≥25 >0,030

RatedCurrent

InA

RatedResidualCurrentI∆nA

a For RCCBs and RCBOs of the general type with I∆n≤ 0,030 A, and RCBOs of the generaltype incorporated in or intended only for association with plugs and socket-outlets, 0.25 Amay be used as an alternative to 5I∆n

TABLE 4 – STANDARD VALUES OF BREAK TIME AND NON ACTUATING TIME

Standard values of break time (s) and non-actuating time (s)

at a residual current (IΔ) equal to:

I∆n 2I∆n 5I∆na

0,3 0,15 0,04

0,5 0,2 0,15

0,13 0,06 0,05

Maximum break times

Maximum break times

Maximum non actuatingtimes


7.3 TROUBLESHOOTING

7.3.1 Troubleshooting for the end user

In the event of a trip occurring on anrcd it is most likely to be caused by afault in a piece of equipment suppliedby a socket-outlet. the flow chart

below gives a simple guide to actions tobe taken to identify the source of thefault. If in doubt, consult a qualifiedelectrician.

7.3.2 Troubleshooting for the electricalcontractor/instructed person

Potential causes of unwanted tripping: Supply side (upstream of the RCD)

• Mains borne disturbance

• Site machinery/plant

• Lightning strike

• Equipment or faults external to the installation e.g. cable joints breaking down causing sporadic unwanted tripping to one or more premises, a fault in an adjacent installation

Load side (protected ‘downstream’ sideof the RCD)

• Wrongly specified RCD

• Wet plaster / condensation

• No 'selectivity between RCDs

• Crossed neutral on split load board

• N – E fault

• High standing earth leakage currents caused by:

• Surge Protection Devices (SPDs)

• Too many items of current using equipment containing filter circuits

• Excessive length of mineral insulated cables

• Heating elements (e.g. cookers)

• Householder / DIy faults (e.g. nails/picture hooks)

• Moisture ingress (appliances, sockets etc.)

For assistance in faultfinding, a step-by-step trouble shooting flow chart isgiven below.

FIGURE 20 – TROUBLE SHOOTING FOR THE END USER

No

RCD TRIPS?

RCD TRIPS?

Last circuit switched-onhas earth fault.

Seek expert advice.

No

RCD TRIPS?No

Yes

Yes

Yes

RCD Trips

Switch o!/Unplug allappliances. Reset RCD.

all fuses. Reset RCD.

fuses one at a time.

Switch o! all MCBs/remove

Switch on MCBs/replace

Next MCB/Fuse.

Next appliance

Last appliance reinstatedhas earth fault.

Submit appliance for repair.

Plug in/switch-on appliances one at a time.


7.4 DETAILED FAULT-FINDING IN RCD PROTECTED INSTALLATIONS

RCD faultyChange RCD

RCD withinspecification

Retest with known

good RCD test set

No

Note 3

Note 2

If trip time greaterthan 40ms disconnect

ALL line and neutralconnections and retest

Note 1 N from neutral bar. L switch o! all CB’s or remove fuses.Note 2 Minimum insulation resistance 2 MΩ.Note 3 Some test sets are influenced by voltage and certain loads.

at RCD terminals

Trace and correct fault.Reconnect circuit

Replace each circuitafter test.

At 5 x rated residualtripping current

At rated residualtripping current

Isolate distribution boardfrom all incoming live

conductors. Secure isolation.

RCD TRIPS

Disconnect RCDoutgoing conductors(Note 1) Reset RCD.

Switch on all equipmentisolators and replace plugs one

at a time, until RCD trips

Disconnect circuits individuallyand test with 500V insulation

tester until faulty circuitis found.

Faulty equipment identified.Locate fault in equipment.

RCD TRIPS

Switch o! all equipmentisolators. Remove all plugs

from socket-outlets.Reset RCD.

Check RCD connections

Use RCD test setto test RCD

No

Yes

Correctly located interminals & tight?Correct polarity? etc.

On TNCS supplycheck for N to E

fault close to RCD

No

Yes

No

RESULTOK

RESULTOK

RCD testcircuit fault

FailPass

RCD TESTEROK

No

RESULTOK

Yes

NoNo

Yes

Yes

will not resetRCD TRIPS

For intermittenttrips see list ofpossible causes.

500VTEST

RCD DOES NOT TRIPwhen test button is pressed

TROUBLESHOOTINGRCD INSTALLATIONS

FIGURE 21 – TROUBLE SHOOTING FOR THE ELECTRICAL CONTRACTOR/INSTRUCTED PERSON


8.1 VOLTAGEINDEPENDENT RCD

voltage independent rcds use theenergy of the earth fault current to tripthe mechanism directly. In this type ofrcd the output from the sensing coiloperates a specially constructedmagnetic relay and so releases the rcdmechanism, independently of themains voltage.

Voltage independent RCDs normally usea polarised (field weakening) relayconstruction. This operates bycancellation of the permanent magneticflux (which holds the relay ON) by theexcitation flux (produced by the faultcurrent). This can only occur in onehalf-cycle of the AC supply because themagnetic flux will be reinforced in theother half cycle. Operating times canvary from 20 to 120 ms at rated trippingcurrent.

8.2 VOLTAGEDEPENDENT RCD

Voltage dependent RCDs generallyemploy an electronic amplifier toprovide an enhanced signal from thesensing coil to operate a trip solenoid orrelay (Figure 23). RCDs of this type aredefined as ‘voltage dependent ‘ becausethey rely on a voltage source, derivedfrom the main supply, or an auxiliarysupply, to provide power to theamplifier. The basic principle ofoperation is, however, the same asvoltage independent RCDs.

8RCDCONSTRUCTION

FIGURE 22 – VOLTAGE INDEPENDENT RCD DESIGN

FIGURE 23 – VOLTAGE DEPENDENT RCD DESIGN


An rcd will detect and trip not only ona line to earth fault and may also tripautomatically on a neutral to earth faultdepending on the design. the majorityof earth faults occur in appliances,particularly portable appliances andtheir flexible cables. this means that inmany installations, faults can be locatedeasily by unplugging all appliances andthen plugging them in again. the rcdwill trip when the faulty appliance isreconnected.

Faults on the fixed wiring are oftencaused by nails or screws drivenbetween the neutral and earthconductors, reversed neutral and earthconnections or a neutral conductortouching an earthed mounting box.Withdrawing a fuse or tripping a circuit-breaker in a final circuit does notnormally interrupt the neutral and maynot prevent an RCD from tripping. Sucha condition could occur whilst alteringthe circuit wiring. Cutting through acable could cause the RCD to trip butthis may not be noticed at the time andduring fault finding, the trip may not beassociated with the cable being cut.

The most effective way of testing forearth faults in the wiring or equipment isby measuring the insulation resistancebetween line and neutral conductorsand earth using a 500V d.c. insulationresistance tester.

Before commencing insulationresistance testing it is essential to ensurethat the distribution board or consumerunit is completely isolated from thesupply voltage and all OvercurrentProtection Devices (OCPDs) areisolated.

Safe isolation procedures must beadopted and where necessary themeans of isolation should besecured.

It is important to ensure that there areno time-switches, contactors etc.isolating any part of individual circuitsfrom the test equipment whilst the testsare carried out. Care should also betaken to ensure that equipment will notbe damaged by the tests.

It is also important to disconnect orisolate current using equipmentwherever possible. Very often it is notpractical to isolate lighting equipment,in which case to avoid the equipmentbeing damaged by the test voltage,lighting circuit’s line and neutralconductors should be connectedtogether for the duration of the test. Forother circuits line earth faults arerelatively easy to find since the lineconductors can be isolated bywithdrawing the fuse or by switching offthe circuit-breakers. Each circuit shouldthen be tested separately and the faultycircuit can then be identified.

In the case of neutral to earth faults,neutral conductors should bedisconnected from the neutral bar oneat a time and tested individually. Thefaulty circuit will then be readilyidentified without necessarilydisconnecting all neutral conductors.Where RCBOs are installed load cablesshould be disconnected from thedevice.

It might be assumed that any standingprotective conductor current below thetrip level of the RCD could be ignored.Unfortunately, this is not so because theRCD sensitivity is effectively increased tothe difference between the RCD tripcurrent and the standing protectiveconductor current. For example, anRCD with a rated residual operatingcurrent of 30 mA will have a typical tripcurrent of 22 mA; if the standingprotective conductor current is 10 mA itwill only take an earth fault current of12 mA to trip the RCD. This could leadto unwanted tripping.

It is often possible to obtain ameasurement of standing protectiveconductor current in final anddistribution circuits using a milliampclamp meter, the circuit live conductorsbeing encircled by the jaws of theinstrument. With this test the instrumentsees the same current as the RCD.

A high impedance neutral to earth faultmay not pass sufficient fault current totrip the RCD. From a safety point of viewa neutral to earth fault with little or nocurrent flow through it does not presenta danger. However, when any protectedload is switched on, some of the loadcurrent will travel through the neutral toearth fault; when the load current islarge enough the RCD will trip. Becausethe fault is load-dependent, tripping canappear to be random. Switching on alarge load connected to another sub-circuit that is healthy may still trip theRCD. This is because part of the loadcurrent may flow to earth through theneutral block and the neutral to earthfault. The symptoms of a partial neutralto earth fault are very similar tounwanted tripping and are discussed inmore detail later.

9.1 MAINS BORNETRANSIENTS AND SURGES

Although the overall reliability of RCDsis excellent, in a number of casesconditions can occur within an installationthat can cause an RCD to trip when noapparent fault condition can be found.This type of unwanted tripping is oftenincorrectly referred to as ‘spurious’ or‘nuisance’ tripping and can be a source ofconsiderable frustration for a contractorwho attempts to trace this elusive fault.However, once the reasons for unwantedtripping are understood, and it is realisedthat it is attributable to the installationconditions and not the RCD, then amethodical course of action willovercome the problem with a minimumof effort.

9DETAILED FAULT-FINDING ON RCDPROTECTED INSTALLATIONS BYCOMPETENT PERSONS


Two main causes of unwanted trippingcan be identified:

• Transient surge currents between phases or between phase and neutral within the installation

• A combination of supply network transient overvoltages, and capacitance to earth within the installation

9.1.1. Tripping due to surge currents

In theory, tripping due to surge currentsbetween phases or between phase andneutral should not occur but in practiceany magnetic device such as an RCDwill have leakage flux. If the load currentis large enough, this leakage flux willinduce sufficient secondary current totrip the RCD. For example, BS EN61008-1 Clause 9.18 requires that anRCCB should not trip when one secondsurges of six times rated current flow. BSEN 61009-1 requires that RCBOs do nottrip when subjected to 0.5 µs/100 kHzring wave with a current surge of 200 A.Experience in the field has shown thattripping due to surge currents is not themajor cause of unwanted tripping.

9.1.2. Tripping due to transient overvoltage and capacitance to earth

A transient overvoltage can be definedas a temporary surge, of limited energy,caused by a sudden change in powerrequirements. Sources of transientovervoltage include reactors of anytype, e.g. motors, transformers,contactors, power factor correctioncapacitors etc. They are also caused byarcing at switch, contactor, relay andcircuit-breaker contacts.

It is known that lightning strikes can bea source of unwanted RCD tripping.K.M. Ward (‘Lightning Damped’,Electron, 23 January 1979) stated: ”theinitial surge on an 11 kV line, due to astrike, may be of the order of 240 kV atthe point of impact and will reach apoint two miles from the point of originin 3 µs by which time it will have fallenexponentially to some 140 kV.”

Some of these transient overvoltagescould be expected to be transformeddown and would appear on the 230 Vmains. Hence, unwanted tripping canoccur some considerable distance (i.e.several kilometres) away from the pointof impact of a lightning strike. Ward alsostates that secondary distributionsystems may carry transientovervoltages of up to 3.3 kV.

The blowing of a simple rewireable fusecan also cause transients. One of thepioneers of fuse technology, H.W. Baxter(Electric Fuses, 1950) stated: “It isnoteworthy that, given sufficientinductance, the peak voltage with a 10inch (copper) fuse wire reached 6000 V(approximately 29 times the circuitvoltage).”

It has been reported that overvoltagesof 6000 V can be reached with BS 1363plug fuses used in inductive circuits. Itwould not be unreasonable (based onBaxter’s research) to get a peak voltageof 2 kV from a two inch copper fusewire subjected to a prospective faultcurrent of 1.2 kA, or a peak voltage of1.1 kV from a two inch copper wiresubjected to a prospective fault currentof only 400 A.

Although significant transients can arisewithin an installation they would onlyoccur under fault conditions. Theymight, however, travel to otherinstallations where they could causeunwanted tripping of an RCD.

Discharge lighting can be a majorsource of transient overvoltages.Discharge lighting is distinct from otherequipment in that high voltage pulsesare produced deliberately to initiate thedischarge. Because of the inductivenature, and hence lagging power factorof the control circuits of dischargelamps, a capacitor is frequently used forpower factor correction. This isconnected directly across the supplyterminals of each lamp. Thecharacteristics of discharge lamps andtheir control gear also produceconsiderable third harmonic current(approximately 20 % of phase current).This is not reduced by the power factorcorrection capacitor. As a result, thepercentage of third harmonic current isgreater in a high power factor circuitthan a low power factor circuit.

When a discharge lamp is switched on,a surge of many times rated current (i.e.several hundred amps) may occur forseveral microseconds due to thecharging of the power factor correctioncapacitor. Alternatively, if no powerfactor correction capacitor is included,the opening of the supply switch willcause voltage surges of several kV.Either situation can cause unwantedRCD tripping.

Some types of discharge lighting (e.g.high-pressure sodium lamps and metalhalide lamps) use external igniters,which produce a series of high voltagepulses, which cease when the lampstarts. These pulses are of short durationbut range from 3 kV to 4.5 kV for high-pressure sodium lamps. Metal halidelamps are ignited by applying 9 kVpulses at 10 ms intervals for up to 7seconds directly on to the lamp. Thevery large number of discharge lamps inuse (particularly for street lighting)makes it likely that this is a major sourceof transient overvoltages.

From the foregoing it will be seen thatunwanted tripping may be caused bytransient overvoltages in the mainssupply, originating from outside theinstallation.

The question of how these transientovervoltages trip an RCD has not yetbeen discussed. Transients can appear inthree possible forms:

• Between phase and neutral and of opposite polarity with respect to earth

• Between phase and neutral but of the same polarity with respect to earth

• Either on phase only or on neutral only with respect to earth

Tests on installations, with transientsurge suppression connected acrossphase and neutral, have shown no reduction in the amount of unwantedtripping.

For a transient overvoltage to trip anRCD, it must cause a current imbalanceby either:

• Causing a flashover to earth due to breakdown of insulation or

• Allowing sufficient high- frequency earth leakage due to the capacitance to earth.

If the former were happening, then aflashover from a low-energy transientwould be followed (at least in somecases) by a mains flashover. Noinstallations investigated so far haveshown this type of damage.


C =

5000Hz4 x 50 x 10-6

20000Ω1000

0.05Xc

Frequency =

= 1.6nF2π x 5000 x 20000

1

1

In practice, an isolated transient doesnot occur. There will be several hundredsuch pulses and they may well havepeak voltages greater than 1 kV. Thecumulative effect of their fast rise times,coupled with their fast repetitionfrequency, could produce sufficientearth fault current to trip an RCD ifsufficient capacitance to earth exists.

9.2 CAPACITANCE TO EARTH

The capacitance of 1.0, 1.5, and 2.5 mm2

flat thermoplastic insulated twin and earthcable is approximately 150 pF per metre. Itwould not be unusual for a domesticinstallation to have 100 m of 2.5 mm2

cable and 250 m of 1.0 or 1.5 mm2 cable,which would result in a capacitance toearth of up to 52.5 nF. This would allow astanding protective conductor current of11 µA/m or a cable leakage current ofnearly 4 mA for the whole installation (at230 V, 50 Hz).

The capacitance to earth of 2.5 mm2

mineral insulated (MI) cable is higher,approximately 400 pF/m. This would allowa standing earth leakage of 30 µA/m. Acommercial or industrial installation couldcontain 500 m of cable, which couldresult in a capacitance to earth of up to200 nF. This would allow a standingprotective conductor current of nearly 15mA while providing a very low impedancepath for any transient overvoltages.

The advantages of mineral insulated (MI)cable over plastic insulated types are in noway disputed, but the higher capacitanceof MI cable can present the contractor

with additional earth fault problems. Theseshould be taken into account during theearly design stage when MI cable andRCD protection are to be used together.

Another major source of capacitance toearth is radio frequency interference (RFI)suppression components. It is commonpractice to connect capacitors betweenlive and neutral, live and earth and neutraland earth. These capacitors are usuallysupplied on a single ‘delta-connected’ unitto the BS EN 60939 series of standards.The BS EN 60939 series has noprescriptive requirements concerningmaximum values of capacitance butcovers safety and testing requirements ingreat detail. The capacitors in these filtersshould be Class y (i.e. a capacitor suitablefor use on 230 V mains systems wherefailure of the capacitor could lead to risk ofan electric shock (see BS EN 60939-2).

RFI suppression units, particularly thoseallowed in non-household appliances,could cause significant standing protectiveconductor currents and will provide a pathfor currents resulting from transientovervoltages.

The large value of neutral to earthcapacitance, allowed in permanentlyearthed equipment, would not normallycause a problem. It would becomesignificant if transient overvoltagesoccurred between neutral and earth. Also,double pole switching would cause anRCD to trip since up to full mains voltagecould suddenly appear across thiscapacitance during switch off. (SeeSection 9.3).

All this means that there are no clearguidelines limiting the amount ofcapacitance to earth that a manufacturerfits into his equipment. BS EN 60335-1puts maximum limits on the 50 Hz, 230 Vprotective conductor current forhousehold appliances to 5 mA forstationary Class 1 appliances with heaters(e.g. cookers), 3.5 mA for motor operatedClass 1 appliances, 0.75 mA for other Class1 appliances and 0.25 mA for Class 2appliances. BS EN 60598 place limits onthe maximum protective conductorcurrents allowed in luminaires. BS EN60335-2-90 covers the safety aspects ofmicrowave ovens and specify a maximumprotective conductor current of 1 mA.

The Electricity Safety, Quality andContinuity (Amendment)

Regulations 2009 (ESQCR) came intoforce on 6th April 2009. Effective from31st January 2003 the Electricity Safety,Quality and Continuity Regulations 2002replaced the Electricity Supply Regulations1988. There are no requirements in theESQCR which specify any maximumallowable value of protective conductorcurrent in an installation. Until 1988 themaximum allowable value of protectiveconductor current in an installation wasgiven by the Electricity Supply Regulations,1937, and was limited to one ten-thousandth part of the maximum currentto be supplied to the installation. Therelevant Clause (Clause 26) was referred toin Regulation 13-9 of the 15th Edition of

other British standards which arerelevant to earth leakage or rfIsuppression, but which make noreference to maximum allowableearth leakage currents orcapacitance to earth, are:

BS EN 62368-1:2014: Audio/video,information and communicationtechnology equipment. Safetyrequirements

BS EN 55011: Industrial, scientificand medical equipment. Radio-frequency disturbancecharacteristics. Limits and methodsof measurement

BS EN 55013: Sound and televisionbroadcast receivers and associatedequipment. Radio disturbancecharacteristics. Limits and methodsof measurement

The capacitance to earth required to cause a current flow of 50 mA can becalculated. Assuming an isolated pulse with a 50 µs rise time and a peak voltageof 1 kV, say:


the IEE Wiring Regulations. The 1937Regulations were replaced by theElectricity Supply Regulations 1988, sincewhen there has no longer been anylimitation on protective conductor currentwithin the regulations for the supply ofelectricity.

An installation is “deemed to comply” withthe safety requirements of the regulationsfor the supply of electricity if it complieswith BS 7671 Requirements for ElectricalInstallations IET Wiring Regulations.

BS 7671:2018 Regulation 531.3.2 statesthat RCDs shall be selected and erectedsuch as to limit the risk of unwantedtripping. In addition to other requirements,including sub-division of circuits, it alsolimits accumulation of protectiveconductor and/or earth leakage currentsto not greater than 30 % of the ratedresidual operating current of the RCD.

9.3 CABLES ANDOVERHEAD LINES

There are indications that the problem ofunwanted tripping occurs more frequentlyin installations supplied by overhead linesthan by those supplied by undergroundconcentric cable. An analysis of thecapacitance and inductance of these twotypes of conductor shows that:

• The capacitance of a non- armoured cable is significantly greater than that of an equivalent overhead line.

• The inductance of an overhead line is greater than that of an equivalent non-armoured cable .

Table 5 gives typical values of inductanceand capacitance for overhead lines andcables obtained from the ElectricalEngineer’s Reference Book. They agreewith values, which can be derived from other sources;

• The Calculation and Design of Electrical Apparatus W. Wilson.

• The Switchgear Handbook,Vol. 2.

Another significant fact is that, in general,the characteristic impedance of a cable isvery much lower than that of an overheadline. Only a small fraction of any voltagesurge that travels down a line would betransmitted down an equivalent cable. Forthis reason equipment at the end of anoverhead power line is sometimesconnected to the line by a short length ofsurge-minimising cable. A cable is, by itsvery nature, a good attenuator of transientovervoltages.

The distance between the installation andthe sub station will be significant since theamount of attenuation between theinstallation and the transient overvoltagesources (e.g. sub station transformers,street lighting, tap changers etc) will be a

function of cable length. Also, anoverhead line behaves as a good aerial toradio-frequency signals and noise while anunderground cable, by both its nature andlocation, is less susceptible to this type ofinterference.

The foregoing analysis indicates thatunwanted tripping of an RCD is less likelywithin installations supplied byunderground steel-armoured cables, dueto the large inductance and capacitanceof the cable, and more likely oninstallations fed by overhead lines due tothe low capacitance of the line. The factthat TN-C-S cables are not generally steel-armoured, suggests that unwantedtripping is more likely on TN-C-Sinstallations than on non TN-C-S systems.

Overhead Line Cable

Inductance (mH/mile)

Capacitance (µF/mile)

1.8 to 2.1

0.015

TABLE 5 – APPROXIMATE VALUES OF INDUCTANCE AND CAPACITANCE FOUND IN PRACTICE

0.8

0.27 to 0.5


9.4 NEUTRAL TO EARTHFAULTS

Although neutral to earth faults do notnormally fall under the heading of‘unwanted’ tripping, they can result inintermittent effects within an installationthat appear illogical and very similar tounwanted tripping. This is particularly truewhere the neutral to earth faultimpedance is significant or where theinstallation is part of a PME (ProtectiveMultiple Earthing) TN-C-S system. This isaggravated by the fact that it is difficult toelectrically isolate parts of the neutralwiring of an installation.

The detection of a neutral to earth fault byan RCD depends on either:

• The existence of a neutral potential above earth caused by the voltage drop along the neutral or

• The existence of a load connected into the protected circuit. Part of the load current then flows back via the earth return thus tripping the RCD. This load current will also cause a voltage drop along the neutral

In a non TN-C-S system, the existence ofsuch a neutral potential above earth isalmost inevitable due to all the consumerloads connected to that neutral. In a TN-C-S installation the neutral potential aboveearth can depend on that one consumerload, due to the TN-C-S link. Therefore, ina TN-C-S installation with no load, or avery light connected load, an RCD cannotdetect a neutral to earth fault. This canalso happen in a non-TN-C-S systemwhere all consumers on the same neutralare taking virtually no load or in aninstallation that happens to be close to thesub-station. In practice these last effectsare rare.

Where the neutral to earth faultimpedance is significant, then the earthleakage current will be insufficient to tripthe RCD. The fault may now be describedas ‘unwanted tripping’ since the detectionof this fault becomes totally load-dependent and the RCD may trip atrandom times. By itself this condition isnot serious since, if no current is flowing,the neutral to earth fault is not a dangerbut as soon as a load is connected, andthe earth leakage current reaches adangerous level, the RCD will trip. Theuser or installer may be baffled by all theseeffects and will often describe the fault asspurious or nuisance tripping.

9.5 DOUBLE GROUNDING

‘Double grounding’ is a phenomenonwhich occurs when two earth faults – aphase to earth fault and a neutral to earthfault – occur simultaneously in a circuitprotected by an RCD. Where these earthfaults do not present large impedances,and therefore the earth fault current islimited, nothing out of the ordinary is likelyto happen. However, when the earth faultimpedance of both faults is significant,then a situation can occur where thephase to earth fault current can cancel theeffect of the neutral to earth fault currentsince the two currents flow in oppositedirections through the RCD. Furthermore,‘double grounding’ can render the RCDtest circuit inoperative due to the neutralto earth fault current cancelling the effectof the test circuit current.

‘Double grounding’ phenomena rarelyoccur in practice. They virtually neveroccur on non-TN-C-S systems due to theneutral to earth voltage. They have beenknown to occur on TN-C-S systems butonly when the systems were unloaded.Problems have occurred on TN-C-Sinstallations during the commissioningstage where it is common for no load tobe connected.

9.6 CONCLUSIONS

Confusion can be caused by thecombination of installation conditions thatcould lead to random tripping on TN-C-Sinstallations. Often, the RCD is blamed forbeing “too sensitive”, “unreliable” etc.,when it is the installation conditions whichare to blame.

Where phase to earth and neutral to earthfaults occur on the same circuit, it is easierto locate the phase to earth fault first.Tripping of the circuit-breakers or removalof fuses in the sub-circuits can disconnectthe live circuits easily. After elimination ofthe live to earth fault, the neutral to earthfault will be easier to locate.

In all cases, especially TN-C-S Systems, theeffect of load current must be borne inmind. A heavy load should be applied to allsub-circuits before assuming that all earthfaults have been eliminated.

Flowcharts for fault-finding on RCDprotected installations, are shown insection 7.3 (Figs. 20 and 21). They includeall the possible faults discussed in thissection. They provide a logical approachto diagnosing an earth fault and theircareful use should allow any earth fault

problem to be solved. Sections 7.3 and 9.2show that the effects due to protectiveconductor currents can be quite complex.

Capacitance to earth is frequently thecause of unwanted tripping and can easilyreach significant levels due to thecumulative effect of cables and RFIsuppression components. Limits onprotective conductor currents are nolonger set by the electricity supplyregulations (The Electricity Safety, Qualityand Continuity (Amendment) Regulations2009) and BS 7671 does not set maximumlimits for installations. Protectiveconductor currents and capacitance toearth limits set by British Standards arelarge enough to allow a build-up ofprotective conductor currents amongappliances, luminaires and RFI suppressioncomponents to a level that may trip anRCD.

With the arrival of the EMC Directive, thislack of control has resulted in an increasein the use of the protective conductor forfunctional purposes since electromagneticcompatibility (EMC), and not the provisionof additional protection by RCDs, is thepriority of equipment manufacturers. Theobjectives of good RFI suppression and ofearth fault protection create a conflict ofinterests, which have yet to be resolved.

Residual current devices from BEAMAmanufacturers are designed to overcomemany of these problems.


10ANNEX

10.1 FIRE PROTECTION –EXTRACT FROM DTI REPORT

the following information is extractedfrom the department of trade andIndustry report residual currentdevices; added value for home safetyby kind permission. It forms the basisfor the information in section 4 andunderlines the role of rcds in fireprevention.

10.1.1 The Incidence of Fires inHousehold Electrical Appliances

Table 6 gives details of the averageannual number of fires to which firebrigades are called and which havebeen identified as associated withfaults in electrical appliances (Firestatistics UK).

Product safety standards seek tominimize the risk of fire ignition and toensure that if fire ignition does occurthen the fire is contained. In practice,

these provisions for fire safety aresupplemented by overcurrent andsometimes earth leakage protectiondevices in the supply installation.

For Class II appliances under normalconditions, no earth fault current pathwill be involved and no addedprotection against fire would beexpected if an RCD was used.

For Class I appliances, RCDs would beexpected to provide closer protectionand limit the duration of current flowand energy transfer to insulation in theevent of fault currents to earth andthereby reduce risk of fire ignition. Inthe list of appliances shown in Table 6,Class I appliances predominate.

10.1.2 Electrically Induced Fire Ignitionand propagation

Fires in electrical wiring systems andelectrical equipment are usually theresult of arcing or overheatingassociated with current carryingconductors.

When electrical conductors are subjectto arcing or overheating and areadjacent to insulation the chemicalprocesses of combustion can occur asfollows:

• An initial heating of the insulation – the resulting temperature increase will be rate dependent on the amount of heat generated, the specific heat of the product mass, the thermal conductivity of the material and the latent heats of fusion and vaporisation where these procedures occur.

• Degradation and decomposition of the material.

• Flame ignition – this depends on the availability of oxygen, the flash points of the materials and their limits of flammability.

Washing machines

Blankets, bedwarmers

Other

Electric cooking appliances

TV

Dishwashers

Tumble/Spin drier

Electric water heating

Refrigerators

Electric space heating

Lighting

Central heating

Other wiring

Irons

Plugs, socket switch

1747.00

640.00

477.00

445.00

368.00

320.00

305.00

263.00

253.00

234.00

183.00

76.00

52.00

16.00

16.00

I

I

I

I

I

I

I

I

I

I

II

II

II

Appliance TypeProbable Construction

Class

Average incidence of firesattended by the fire brigades

attributed to faults in appliances

TABLE 6 – AVERAGE ANNUAL NUMBER OF FIRES IN THE UK ATTENDED By FIRE BRIGADES WHERETHE FIRES ARE ATTRIBUTED TO FAULTS IN ELECTRICAL APPLIANCES, LIGHTING OR WIRING


Product safety standards will generallyrequire low flammability materials to beused where insulation is touching orsupporting electrically live parts.However, even low flammability fireresistant plastics can supportcombustion if a high temperature ismaintained for a sufficient length oftime. Materials classified as lowflammability may also support a localflame for a short time. To avoid flamespread, it is important that designs allowadequate separation between thesematerials and other high flammabilitymaterials which may be present. Flamepropagation sometimes occurs as aresult of distortion or melting of plasticparts which allows them to come intocontact with, or to drop onto, a heatsource.

BS 7671:2018, the IET WiringRegulations, recommend the use ofAFDDs conforming to BS EN 62606 as ameans of providing additionalprotection against fire caused by arcfaults in AC final circuits. See BEAMAGuide to AFDDs.

10.1.3 RCD protection Against FireInduced by Surface Tracking AcrossInsulation

Surface tracking is a common causeof insulation failure. It arises from thegrowth of conducting paths at thesurface. These may be due toconducting deposits from theatmosphere and the presence ofmoisture. When the path carries enoughcurrent, it will become thermallyunstable resulting in a permanentlyconducting state. The action isprogressive and ultimately a conductingpath will bridge the insulation.

Surface tracking can occur at voltagelevels well below the intrinsicbreakdown strength of the dielectric.An established track between twoconductors can produce localtemperatures sufficient to igniteflammable vapour released from theinsulation by the heating produced inthe track or adjacent materials.

The rate of growth of tracks in practiceis slow until a conducting path has beenestablished. A standard test has beendeveloped to compare the resistance tosurface tracking of different materials ina short time span. The test is detailed inBS EN 60112 (See Section 10.1.6).

In the tracking test, if a current of 500mA or more flows for at least twoseconds in a conducting path betweenthe electrodes on the surface of thespecimen, or if the specimen burns, thematerial has failed the test.

Although the test is designed to be usedonly for comparative purposes, it is clearthat if the final results of the test arerepresentative of the long term effectsof normal levels of pollution on thetracking resistance of materials, and thespacing between the test electrodes isrepresentative, then an RCD with a tripcurrent of 500 mA would providesuitable protection against fire formaterials which pass the test. RCDprotection would only be effective fortracking paths to earth and not fortracking paths between phase andneutral supply conductors.

10.1.4 Surface Tracking Induced byFluid Contamination of Insulation

The tracking test is designed to simulatethe long term effects of surfacecontamination and moisture on thetracking resistance of insulation.However, the test can also provide aninsight into the effect of fluidcontamination on insulation and theability of RCDs to halt or preventtracking.

A diagram of the tracking testconfiguration and operation are shownin Section 10.1.6

During a standard tracking test it can beestablished that the liquid conductivityin the first phase gives a current of theorder 2-5 mA. In the second phase,surface discharge activity occurs atcurrent levels of the order 2-5 mA.When discharge activity ceases, thecurrent is of the order of 2 mA. Whenthe liquid is present, the current flowwould be sufficient to trip a 30 mA or100 mA RCD. In the period wheresurface discharge activity occurs priorto a low resistance tracking path beingestablished, an RCD would not beexpected to act to completely eliminatedeterioration of the surface and theformation of incipient tracks.

Further tests have been made todetermine the current levels needed tocause flame ignition by tracking. Asample of a printed circuit board havinga good resistance to tracking was usedfor these tests. Using a standard tracking

test solution, a track across theinsulation between adjacent conductorsdeveloped at a current of 80 mA. As thecurrent increased to 90 mA, the trackglowed red and a yellow flame ignitedalong the track. The flame height wasapproximately 8 mm. The current levelsobserved in these tests indicate that a30 mA RCD would have interrupted thisprocess before flame ignition. In asecond similar experiment with a 30 mARCD in circuit, it was not possible todevelop a track between the conductorsor to cause flame ignition.

Some household detergent fluids have ahigh conductivity compared with thestandard test fluid used in the trackingtest. Tests were carried out using onecommon fluid which has a conductivityof approximately five times that of thestandard tracking test fluid. Using aliquid having a higher conductivitywould be expected to accelerate theonset of tracking failure.

The current levels measured in thesetests were in the range 8-84 mA. It wasobserved that the nature of the solutionappeared to play a more dominant rolein the failure process in these teststhan when the standard test solutionwas used. During the tests, a pinkcoloured flame 2 mm high wasobserved, apparently associated withdecomposition of the fluid.

It is clear from the tests carried out, thatif contamination by conductive fluidbridges insulation between a supplyconductor and earth, and produces ahigh conductivity path, then this is likelyto trip an RCD having a sufficiently lowtrip current level. However, if theresistance of the fluid is such thatheating will cause evaporation atcurrent levels below the threshold forRCD operation, then tracks can form.

The above tests suggest that RCDs canbe sensitive enough to trip due to thepresence of conductive fluidcontamination, spillage or spray inappliances as a result of earth currentflow and may arrest the progress oftracking before flame ignition occurs. Ifappliances may be subject to fluidcontamination of insulation, for exampledue to deterioration of seals inappliances or spillage, RCDs can provideprotection at current levels whereovercurrent protection would not beexpected to operate.


10.1.5 Electrical Equipment Faults andFire Hazard Limitation

In this section, consideration will begiven to the role which RCDs andovercurrent protection devices can playin reducing the risk of fire associatedwith potential faults in home electricalwiring systems and componentscommon to different types of electricalhousehold appliances.

10.1.5.1 Wiring Installations andEquipment

The major fire risks in fixed installationsare overheating of connections andsustained arcing. Modern PVC insulatedwiring, if properly installed, can beexpected to outlast the lifetime of theproperty. The wiring must be protectedagainst short-circuit or sustainedovercurrent by the use of fuses orovercurrent circuit-breakers. Also, thecurrent rating of the circuit must not beexceeded in the event that the circuit islater extended.

In the event of overheating ofconnections, neither overcurrentdevices nor RCDs would protect againstfire ignition unless a secondary eventoccurred such as contact with anotherconductor which might produce a highovercurrent, or contact with an earthedconductor.

Surface tracking may occur in wiringinstallation accessories such asdistribution boxes, switches and socket-outlets due to environmental pollutionand moisture. Condensation is likely tooccur particularly in areas such ascellars and where wiring is routed intobuildings. An established track canproduce a localised temperatureincrease sufficient to ignite flammablevapour released from the insulation as aresult of heating produced in the track.Whereas an RCD should provide someprotection against tracking between thephase or neutral conductors and earth,no protection would be providedagainst a phase to neutral track.Overcurrent devices would not provideprotection against fire ignition bysurface tracking.RCD protection wouldalso operate in respect of earth leakagecurrents due to damaged insulation onwiring conductors in metal conduit andat entry points in metal wall boxes.

Arc Fault Detection Devices (AFDDs) aredesigned to detect parallel arcs (line to

line, line to neutral and line to earth) andseries arcs (arcing within one of theconductors). Unlike a circuit breakerwhich detects overloads and shortcircuit currents and RCDs which detectcurrent imbalance, an AFDD utiliseselectronic technology to analyse thesignature (waveform) of an arc todifferentiate between normal arcing andarcing faults. Upon detection of anarcing fault, the AFDD disconnects thefinal circuit from the supply. Furtherdetails can be found in the BEAMAGuide to AFDDs.

10.1.5.2 Motors

The principal causes of fire ignition inmotors are arcs or sparks ignitinginsulation or nearby flammable material.Such events can occur when the motorwinding short-circuits or grounds orwhen the brushes operate improperly.Overheating can occur when theventilation is restricted or the motor isstalled. Bearings may overheat becauseof improper lubrication. Sometimesexcessive wear on bearings allows therotor to rub on the stator. The individualdrives of appliances of many typessometimes make it necessary to installmotors in locations and underconditions which are injurious to motorinsulation. Dust that can conductelectrically such as brush material maybe deposited on the insulation, ordeposits of textile fibres may preventnormal operation and obstruct coolingvents.

Motors may be provided withovercurrent protection to limitoverheating should the motor stall orfail to start. In addition, anovertemperature cut-out may beprovided. RCDs can provide additionalprotection in respect of fault currents toearth when basic insulation between thewindings and an earthed housingbecomes contaminated by dust, cracksor fails due to, for example, thermalstress, mechanical stress or ageing.

10.1.5.3 Transformers

The primary cause of fire withtransformers is overheating ofconductors and insulation. Fusing isprovided to prevent overheating underoverload fault conditions and may besupplemented by overtemperaturecut-outs.

In many applications, parts of the

transformer may be connected to earthand, in the event of a failure to earth, anRCD can provide protection by limitingthe current flow and the consequentheating effect.

10.1.5.4 Switch and Relay Contacts

Failures of contacts may occur due toweak springs, contact arcing, sparkerosion and plating wear. Failures due tocontamination can also occur. Surfacedeposits, particularly carbon or ferrousparticles, cause electrical failures andinsulation breakdown. High resistancecontacts often due to the deposition ofnon-conducting or semiconductingmaterial at the contact surfaces willcause local overheating which mayresult in fire. These faults will not bedetected by overcurrent or RCDprotection devices.

Where contamination or tracking acrossinsulation provides a conductive path toearth, RCDs can offer additionalprotection.

Vibration will accelerate mechanicaldeterioration of contacts and othermoving components.

10.1.5.5 Internal Wiring andConnections

There are two types of faults in electricalwiring. These are open circuit faults,where a conductor has parted, andshort-circuit faults where a conductingpath exists between one conductor andanother conductor or earth. A fault canbe a combination of both an opencircuit fault in a conductor and a short-circuit fault.

Open circuit faults, such as poor wiringconnections due to contact ageing, arean important cause of local overheatingand are unlikely to be detected by RCDsor over current protection devices.Arcing which results from conductorfailure in a flexible cord, althoughpotentially a fire hazard, will not bedetected by RCDs or overcurrentprotection devices unless a short-circuitfault exists at the same time due to, forexample, the broken end of theconductor piercing the insulation.AFDDs provide protection against seriesand parallel arcing (see section 10.1.5.1)

Connections will be sensitive to factorssuch as load cycling, the initial integrityof the contact interface, vibration,mechanical disturbance, the effect of


environmental contamination andgrowth of tarnish films at the contactinterface. Where connections are madeto components, surface tracking mayoccur as a result of conductive surfacedeposits and moisture. Under thesecircumstances where the tracking is toan earthed surface, protection may beprovided by an RCD.

10.1.5.6 Heating Elements

Heating elements may have an earthedsheath. RCDs will provide early warningof breakdown of insulation and will alsodetect pin holes in sheathing when usedto heat water.

10.1.5.7 Summary

From the above considerations, it isclear that although RCDs andovercurrent devices have a role in

reducing the risk of fire in electricalequipment, they will not respond tomany of the failure modes likely toinitiate fire ignition. Particular problemsarise in detecting overheating ofconnections and in-line wiring faultswhich are a common cause of fires,however AFDDs provide protectionagainst series and parallel arcing (seesection 10.1.5.1). The role RCDs can playin providing additional protection isillustrated by Table 7.

TABLE 7 – POTENTIAL FAULT CONDITIONS FOR CIRCUIT PROTECTION DEVICES AND ELECTRICAL COMPONENTS

Surface contamination ofinsulation:carbon tracking.

Item PotentialFaults

RCDAdded Protection

Over currentProtection

Over temperatureCut-out

Motors

Transformers

Switch andRelay Contacts

& Controls

HeatingElements

Wiring

Connections

WiringAccessories

ElectronicCircuits

RCD’s will trip at low values of earth leakagecurrent due to:tracking or contamination; cracks or faults in insulation caused by thermo-mechanicalstress or mechanical damage; arcs or sparkswhen the motor winding short-circuits orgrounds or when brushes operate improperly.

Will respond to overheatingif the motor fails to start, provided the operating current is set close with running current.

Will respond to overheatingcaused by lack of ventilationor conductor overheatingwhile running.

Surface contamination ofinsulation: overheating.

Where there is a failure of insulation betweenthe primary winding and earth, RCDprotection will operate.

Fusing is usually provided to prevent overheating underfault conditions.

Over temperature cut-outsmay be fitted.

The rating and performancecharacteristics are notsuited to the duty-cycle.Tracking or contamination.

RCD’s can provide protection where trackingor contamination provides a conductive path to earth.

Needed for live-neutral faultprotection.

Pin holes in metal sheathingof mineral insulatedelements allowing moistureto penetrate.

RCD’s will provide an early indication ofbreakdown of insulation.Note. Certain elements may not be sealed bydesign and at switch on, significant levels of earth leakage current can occur due tomoisture ingress.

Needed to protect againstlive-neutral insulation failure.

Open circuit faults onflexible cords. Short circuitdue to insulation damage.

RCD’s will detect any loose wires whichcontact an earthed surface. They will alsodetect insulation damage in metal conduct.

Required to preventoverheating in the event of afault, insulation damage andlive-neutral failure.

RCD’s will detect connections loosened byfor example, vibration which come free andtouch earthed surfaces.

Vibration loosening.Mechanical disturbance:deterioration of contactinterfaces and overheating.connections not dimensioned in respect oftheir heating.

Will protect against highcurrent live-neutral orlive-earth contact ifthe connection becomesfree.

RCD’s will respond to condensation leadingto liquid build up in enclosures and trackingacross insulation.

May be subject to condensation in humid areaswhich are subject to widetemperature fluctuations.Contacts can overheat, dueto vibration, poor insulationor surface oxidation.

Contamination Local protection in the formof fusing is appropriate.

May be appropriate for somecomponents.


10.1.6 The Comparative Tracking IndexTest (BS EN 60112)

The diagram above shows the trackingtest configuration. The tracking testoperates as follows:

a) A standard contaminant liquid having a conductivity of 2.4 Siemens is fed as a single drop to fall between the two electrodes which are set to a test voltage,

b) The heat developed by the passage of current through the liquid evaporates the liquid and heats the specimen.

c) In the final stages of evaporation, discharges can be observed on the surface of the insulation which are known as scintillations and these create sites which develop into a tracking path. Different materials will require a different number of drops of the test solution or a different test voltage to produce tracking sufficient to form a sustained conduction path between the electrodes.

The test is continued to 50 drops of thetest solution. A failure has occurred if acurrent of 500 mA or more flows for atleast 2 s in a conducting path betweenthe electrodes on the surface of thespecimen, thus operating anovercurrent relay; or if the specimenburns without releasing the relay.

10.1.7 practical Tests on SurfaceTracking Induced by FluidContamination of Insulation andRCD protection

10.1.7.1 Background

Using standard tracking test equipmentthe liquid conductivity in the first phasegives a current between the electrodesof the order of 100 mA. In the secondphase, surface discharge activity isassociated with currents of the order2-5mA. When discharge activity ceases,the current is of the order of 2mA.Clearly, when the liquid is present thecurrent would be sufficient to trip a 30mA RCD. However, during the dischargeperiod before the establishment of a lowresistance tracking path, an RCD wouldbe insensitive to the level of currentneeded to prevent deterioration of thesurface of the insulation.

10.1.7.2 Tracking tests

Further tests at ERA have been made toassess the effects of contamination ofinsulation by conducting fluids and theeffectiveness of RCD protection inpreventing fire ignition by tracking.

standard test solutionA 0.01 ml drop of the standard trackingtest solution was applied to the

insulation between two conductors ona printed circuit board (gap 3 mm) anda voltage applied between theconductors. When the voltage appliedwas 80 V, bubbles were seen to form atthe electrode interface with the fluid.The measured current was 4.2 mA. At10 V the liquid evaporated.

In a second test at 250 V, thephenomena observed were similar tothose in a conventional tracking test rig.Scintillations were observed as the liquidevaporated and eventually a trackformed which extended approximatelyhalfway across the insulation. Thecurrent measured was 37 mA.

A second drop was applied at the sameposition on the insulation. Further tracksformed and the current level increasedto 65 mA. On the fourth drop, thecurrent increased to 80 mA and a trackwas formed between the conductors.As the current increased to 90 mA, thetrack glowed red and a yellow flameignited along the length of the track.The flame height was approximately8 mm. The current observed in thisexperiment allows the inference that anRCD with a trip current of 30 mA wouldhave interrupted the process beforeflame ignition occurred. An RCD with atrip current of 300 or 500 mA would nothave been effective.

In a second similar experiment with a32 mA RCD in circuit, tracks wereformed but it was not possible to ignitethe material. Nine attempts were made.A factor in this experiment was that onlya small quantity of liquid could beapplied between the conductors toavoid causing the RCD to trip.

household liquid solutionSome detergent fluids have a highconductivity compared with thestandard test fluid used in the trackingtest. A test was carried out using onecommon fluid which has a conductivityof approximately 5 times that of thestandard tracking test fluid.

A drop of solution was applied to theinsulation between two conductors ofthe printed circuit board (gap 3 mm) anda voltage applied between the tracks.When the voltage applied was 40 V,bubbles were seen to form at theelectrode interface with the fluid. Thecurrent measured was 8 mA. Within atime of the order of seconds, the

ELECTROLYTEDISPENSINGNEEDLE

PLATINUMELECTRODE

SPECIMEN

4+ 0.1mm

60°

FIGURE 24


bubbles spread to form a central pathbetween the printed circuit boardconductors.

In a second test at 250 V, thephenomena observed were similar tothose in a conventional tracking test rigwith scintillations occurring as the liquidevaporated and eventually a trackbetween the conductors wasestablished. The order of currentobserved during the test were 8-84 mA.Following complete evaporation of theliquid and the cessation of dischargeactivity, the resistance between theprinted circuit board conductors wasmeasured as greater than 400 MΩ.During the test, a pink coloured flame 2mm high was observed apparentlyassociated with decomposition of thefluid. The currents involved in thisexperiment suggest that RCD operationwould have interrupted the process.However, the dominant cause of the

high conductivity was the presence ofthe fluid or its effect in maintaining acontinuous conductive path along thetrack. Examination of the specimen afterthe test suggested that the fluid played adominant role in the failure processrather than the intrinsic properties of theprinted circuit board insulation.

A third test was made to establish theprogress of events with a 30 mA RCD incircuit. In the presence of a drop of thefluid across the insulation, the RCDtripped due to the high conductivity ofthe fluid. With less liquid present,scintillations occurred and the RCD didnot trip. Following a second applicationof the liquid, tracking developed acrossthe insulation. A period followed inwhich the track glowed red then theRCD tripped to halt the process.

It is clear from the tests that ifcontamination by conductive fluid can

bridge insulation to earth and produce alow conductivity path, this is likely to tripan RCD. However, where the filmresistance is such that heating will causeevaporation, tracks will form below thethreshold for RCD operation. RCDs willprovide no protection against trackingor fire ignition when live to neutralinsulation is bridged unless there is anassociated path to earth.

The above test shows that highsensitivity RCDs will trip when thepresence of conductive fluidcontamination spillage or spray inappliances results in earth current flow.In such cases, the RCD may arrest theprogress of tracking before flameignition of insulation occurs. Althoughonly a limited amount of testing hasbeen carried out in the present work, itis clear that RCDs have the potential toreduce the incidence of fire due tosurface tracking.


10.2 REFERENCES

10.2.1 Documents to which reference is made in this Guide

Bs 4293: Specification for residual current-operated circuit-breakers (Superseded by BS EN 61008 series)

Bs 7671: Requirements for Electrical Installations. IET WiringRegulations Eighteenth Edition

Bs En 55011: Limits and methods of measurement of radiodisturbance characteristics of industrial, scientific andmedical (ISM) radio-frequency equipment

Bs En 55013: Sound and television broadcast receivers andassociated equipment. Radio disturbance characteristics.Limits and methods of measurement

Bs En 60065: Audio, video and similar electronic apparatus.Safety requirements

Bs En 60112: Method for the determination of the proof andthe comparative tracking indices of solid insulating materials

Bs En 60335-1: Specification for safety of household andsimilar electrical appliances. General requirements

Bs En 60335-2-90: Specification for safety of householdand similar electrical appliances. Particular requirements.Commercial microwave ovens

Bs En 60598: Luminaires

Bs En 60939: Passive filter units for electromagneticinterference suppression – Part 1: Generic specification

Bs En 60939: Passive filter units for electromagneticinterference suppression – Part 2: Sectional specification:Passive filter units for which safety tests are appropriate– Test methods and general requirements

Bs En 60939: Passive filter units for electromagneticinterference suppression – Part 2-1: Blank detailspecification – Passive filter units for electromagneticinterference suppression – Filters for which safety tests arerequired (assessment level D/DZ)

Bs En 60939: Passive filter units for electromagneticinterference suppression – Part 2-2: Blank detailspecification – Passive filter units for electromagneticinterference suppression – Filters for which safety tests arerequired (safety tests only)

Bs En 60947: Specification for low-voltage switchgear andcontrolgear

Bs En 61008 series: Residual current operated circuit-breakers without integral overcurrent protection forhousehold and similar use (RCCBs).

Bs En 61009 series: Residual current operated circuit-breakers with integral overcurrent protection for householdand similar uses (RCBOs).

IEc 60479: Effects of current on human beings and livestock

IEc 61140: Protection against electric shock – Commonaspects for installations and equipment

IEc 62955: Residual direct current detecting device (RDC-DD) to be used for mode 3 charging of electric vehicles

10.2.2 Other relevant documents not specificallymentioned in the text

IEt guidance notes

health and safety Booklet hsr25: Memorandum ofGuidance on the Electricity at Work Regulations 1989

10.2.3 Associated Directives and Statutory Regulations

Electricity Supply Regulations 1988. Replaced by: TheElectricity Safety, Quality and Continuity (Amendment)Regulations 2009

Electricity at Work Regulations 1989

Health and Safety at Work etc. Act 1974

European Directives

The Low Voltage Directive (LVD) (2014/35/EU)

The Electromagnetic Compatibility (EMC) Directive

(2014/30/EU)


10.3 TERMS AND DEFINITIONS

Circuit-breakerA device capable of making, carrying and breaking normalload currents and also making and automatically breaking,under pre determined conditions, abnormal currents, suchas short-circuit currents.

Residual Currentr.m.s. value of the vector sum of the instantaneous values ofthe currents flowing through the main circuit of the RCD.

RCD – Residual Current DeviceMechanical switching device or association of devicesdesigned to make, carry and break currents under normalservice conditions and to cause the opening of the contactswhen the residual current attains a given value underspecified conditions.

Note: See Section 1.3 for definitions of different types ofresidual current device.

EFR – Earth Fault RelayA device incorporating the means of detection of an earthfault current, of comparison of its value to the earth faultcurrent operating value and of giving a signal to anassociated switching device to open the protected circuitwhen the earth fault current exceeds this value. Relays canbe directly connected or fed from a separate toroid.

Note: Although not necessarily an RCD, this type of device isused in conjunction with other devices to provide protectionof the total installation against the effects of high earth faultcurrents.

CU – Consumer Unit(may also be known as a consumer control unit or electricitycontrol unit).

A particular type of distribution board comprising a typetested coordinated assembly for the control and distributionof electrical energy, principally in domestic premises. It willnormally incorporate manual means of double pole isolationon the incoming circuit and an assembly of one or morefuses, circuit-breakers, residual current operated devices,signalling and or other control devices.

DB – Distribution BoardAn assembly containing switching or protective devices (e.g.fuses, circuit-breakers, residual current operated devices)associated with one or more outgoing circuits fed from oneor more incoming circuits together with terminals for theneutral and protective circuit conductors. It may also includesignalling and or other control devices. Means of isolationmay be included in the board or may be provided separately.

pB – panelboardAn assembly containing switching or protective devices (e.g.circuit-breakers or fusegear typically in accordance with BSEN 60947-2 and/or BS EN 60947-3) associated with one ormore outgoing circuits fed from one or more incomingcircuits together with terminals for the neutral andprotective circuit conductors. It may also include residualcurrent protection systems, signalling and or controldevices. Means of isolation may be included in the board ormay be provided separately.

SwitchboardAn assembly of switchgear with or without instruments.Theterm however, does not apply to groups of local switches infinal circuits.

Class I EquipmentEquipment which relies on connection of exposed-conductive-parts to a protective (earth) conductor in thefixed wiring of an installation, in addition to basic insulation,for protection against electric shock.

Class II EquipmentEquipment in which protection against electric shock doesnot rely on basic insulation only, but in which additionalsafety precautions such as supplementary insulation areprovided in the absence of any means of connectingexposed-conductive-parts to a protective conductor.


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