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ResidualCurrentdevices

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ResidualCurrentdevices(RCD's&RCCB's)

PanelRCDSocketStyleRCDFuseBoardwithRCDprotection

Aresidualcurrentdevice(RCD),orresidualcurrentcircuitbreaker(RCCB),isanelectricalwiringdevicethatdisconnectsacircuitwheneveritdetectsthattheelectriccurrentisnotbalancedbetweenthephase("hot")conductorandtheneutralconductor.Suchanimbalanceissometimescausedbycurrentleakagethroughthebodyofapersonwhoisgroundedandaccidentallytouchingtheenergizedpartofthecircuit.AlethalshockcanresultfromtheseconditionsRCDsaredesignedtodisconnectquicklyenoughtomitigatetheharmcausedbysuchshocks.

IntheUnitedStatesandCanada,aresidualcurrentdeviceisalsoknownasagroundfaultcircuitinterrupter(GFCI),groundfaultinterrupter(GFI)oranapplianceleakagecurrentinterrupter(ALCI).InAustraliatheyaresometimesknownas"safetyswitches".

PurposeandoperationRCDsoperatebymeasuringthecurrentbalancebetweentwoconductorsusingadifferentialcurrenttransformer.Thedevicewillopenitscontactswhenitdetectsadifferenceincurrentbetweenthelineconductorandtheneutralconductor.Thesupplyandreturncurrentsmustsumtozero,otherwisethereisaleakageofcurrenttosomewhereelse(toearth/ground,ortoanothercircuit,etc.).

RCDsaredesignedtopreventelectrocutionbydetectingtheleakagecurrent,whichcanbefarsmaller(typically530milliamperes)thanthecurrentsneededtooperateconventionalcircuitbreakersorfuses(severalamperes).RCDsareintendedtooperatewithin2540milliseconds,beforeelectricshockcandrivetheheartintoventricularfibrillation,themostcommoncauseofdeaththroughelectricshock.

IntheUnitedStates,theNationalElectricalCode,requiresGFCIdevicesintendedtoprotectpeopletointerruptthecircuitiftheleakagecurrentexceedsarangeof46mAofcurrent(thetripsettingistypically5mA)within25milliseconds.GFCIdeviceswhichprotectequipment(notpeople)areallowedtotripashighas30mAofcurrent.InEurope,thecommonlyusedRCDshavetripcurrentsof10300mA.

Residualcurrentdetectioniscomplementarytoovercurrentdetection.Residualcurrentdetectioncannotprovideprotectionforoverloadorshortcircuitcurrents.

RCDswithtripcurrentsashighas500mAaresometimesdeployedinenvironments(suchascomputingcenters)wherealowerthresholdwouldcarryanunacceptableriskofaccidentaltrips.ThesehighcurrentRCDsservemoreasanadditionalfiresafetyprotectionthanasaneffectiveprotectionagainsttherisksofelectricalshocks.

Insomecountries,[1]twowire(ungrounded)outletsmaybereplacedwiththreewireGFCIstoprotectagainstelectrocution,andagroundingwiredoesnotneedtobesuppliedtothatGFCI,buttheoutletmustbetaggedassuch.TheGFCImanufacturersprovidetagsfortheappropriateinstallationdescription.

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ExampleInternalmechanismofRCDThephotographdepictstheinternalmechanismofaResidualCurrentDevice(RCD).Thedevicepicturedisdesignedtobewiredinlineinanappliancepowercord.Itisratedtocarryamaximumcurrentof13amperesandisdesignedtotriponaleakagecurrentof30mA.ThisisanactiveRCDthatis,itdoesn'tlatchmechanicallyandthereforetripsoutonpowerfailure,ausefulfeatureforequipmentthatcouldbedangerousonunexpectedreenergisation.

Theincomingsupplyliveandthegroundedneutralconductorsareconnectedtotheterminalsat(1)andtheoutgoingloadconductorsareconnectedtotheterminalsat(2).Theearthconductor(notshown)isconnectedthroughfromsupplytoloaduninterrupted.

Whentheresetbutton(3)ispressedthecontacts((4)andhiddenbehind(5))close,allowingcurrenttopass.Thesolenoid(5)keepsthecontactsclosedwhentheresetbuttonisreleased.

Thesensecoil(6)isadifferentialcurrenttransformerwhichsurrounds(butisnotelectricallyconnectedto)theliveandneutralconductors.Innormaloperation,allthecurrentdowntheliveconductorreturnsuptheneutralconductor.Thecurrentsinthetwoconductorsarethereforeequalandoppositeandcanceleachotherout.

Anyfaulttoearth,forexamplecausedbyapersontouchingalivecomponentintheattachedappliance,causessomeofthecurrenttotakeadifferentreturnpathwhichmeansthereisanimbalance(difference)inthecurrentinthetwoconductors(singlephasecase),or,moregenerally,anonzerosumofcurrentsfromamongvariousconductors(forexample,threephaseconductorsandoneneutralconductor).

Thisdifferencecausesacurrentinthesensecoil(6)whichispickedupbythesensecircuitry(7).Thesensecircuitrythenremovespowerfromthesolenoid(5)andthecontacts(4)areforcedapartbyaspring,cuttingofftheelectricitysupplytotheappliance.

Thedeviceisdesignedsothatthecurrentisinterruptedinafractionofasecond,greatlyreducingthechancesofadangerouselectricshockbeingreceived.

Thetestbutton(8)allowsthecorrectoperationofthedevicetobeverifiedbypassingasmallcurrentthroughtheorangetestwire(9).Thissimulatesafaultbycreatinganimbalanceinthesensecoil.IftheRCDdoesnottripwhenthisbuttonispressedthenthedevicemustbereplaced.

RulesandregulationsUseandregulationsdifferwidelyfromcountrytocountry.InEurope,theUKhasonlymandatedtheuseofRCDsinnewinstallationssinceJuly2008.Incontrast,GermanyrequirestheuseofRCDsonallsockets

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upto20Awhichareforgeneraluse.ThisrulewasintroducedinJune2007(DINVDE0100410Nr.411.3.3).IntheU.S.,theNationalElectricalCoderequiresGFCIsinbathrooms,kitchens,garages,outdoorareas,crawlspaces,unfinishedbasements,nearwetbars,swimmingpools,andspasinresidentialconstruction.

UseandplacementInmostcountries,notallcircuitsinahomeareprotectedbyRCDs.IfasingleRCDisinstalledforanentireelectricalinstallation,anyfaultwillcutallpowertothepremises.NormalpracticeindomesticinstallationsintheUKistouseasingleRCDforallRCDprotectedcircuitsbuttohavesomecircuitsthatarenotprotectedatall(socketsusuallyareontheRCDlampholdersusuallyaren'tothercircuitsvarybywhoinstalledthesystem).GFIreceptaclesintheUSAhaveconnectionstoprotectdownstreamreceptaclesforexample,alloutletsinaroommaybeprotectedbyoneGFI.

ResidualcurrentandovercurrentprotectionmaybecombinedinonedeviceforinstallationintotheservicepanelthisdeviceisknownasaGFCIbreakerintheUSandasanRCBOinEurope.IntheUS,RCBOsaremuchmoreexpensivethanRCDoutlets.

MorethanoneRCDfeedinganotherisunnecessary,providedtheyhavebeenwiredproperly.OneexceptionisthecaseofaTTearthingsystemwheretheearthloopimpedancemaybehigh,meaningthatagroundfaultmightnotcausesufficientcurrenttotripanordinarycircuitbreakerorfuse.Inthiscaseaspecial100mA(orgreater)tripcurrenttimedelayedRCDisinstalledcoveringthewholeinstallationandthenmoresensitiveRCDsshouldbeinstalleddownstreamofitforsocketsandothercircuitswhichareconsideredhighrisk.

TestingRCDscanbetestedwiththebuiltintestbuttontoconfirmfunctionalityonaregularbasis.RCDsifmiswiredmaynotoperatecorrectlyandaregenerallytestedbytheinstallertoverifycorrectoperation.UseofasolenoidvoltmeterfromlivetoearthprovidesanexternalpathandcantestthewiringtotheRCDsuchatestshouldbeperformedoninstallationofthedevice.Thetestshouldberepeatedateveryoutlet"downstream"oftheRCDtoensurethatthedownstreamoutletsarealsowiredcorrectly.

LimitationsAresidualcurrentcircuitbreakercannotremoveallriskofelectricshockorfire.Inparticular,anRCDalonewillnotdetectoverloadconditions,phasetoneutralshortcircuitsorphasetophaseshortcircuits.Overcurrentprotection(fuseorcircuitbreaker)mustbeprovided.CircuitbreakersthatcombinethefunctionsofanRCDwithovercurrentprotectionrespondtobothtypesoffault.TheseareknownasRCBOs,andareavailablein1,2,3and4poleconfigurations.RCBOswilltypicallyhaveseparatecircuitsfordetectingcurrentimbalanceandforoverloadcurrentbutwillhaveacommoninterruptingmechanism.

AnRCDwillhelptoprotectagainstelectricshockwherecurrentflowsthroughapersonfromaphase(live/line/hot)toearth.Itcannotprotectagainstelectricshockwherecurrentflowsthroughapersonfromphasetoneutralorphasetophase,forexamplewhereafingertouchesbothliveandneutralcontactsinalightfittingadevicecannotdifferentiatebetweencurrentflowthroughanintendedloadfromflowthroughaperson.

WholeinstallationsonasingleRCD,commonintheUK,arepronetonuisancetrips.Nuisancetripscancausesafetyproblemswithlossoflightinganddefrostingoffood.

RCDsalsocausenuisancetripswithapplianceswhereearthleakageiscommonandnotacauseofinjuryormortality,suchaswaterheaters.

HistoryandnomenclatureIntheearly1970smostGFCIdeviceswereofthecircuitbreakertype.HoweverthemostcommonlyusedGFCIssincetheearly1980sarebuiltintooutletreceptacles.Theproblemwiththoseofthecircuitbreakertypewasthatofmanyfalsetripsduetothepooralternatingcurrentcharacteristicsof120voltinsulations,

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especiallyincircuitshavinglongercablelengths.Somuchcurrentleakedalongthelengthoftheconductors'insulationthatthebreakermighttripwiththeslightestincreaseofcurrentimbalance.

TypesAResidualCurrentBreakerwithOverload(RCBO)isacombinationofanRCDandaminiaturecircuitbreaker(MCB).

InEuropeRCDscanfitonthesameDINrailastheMCBs,howeverthebusbararrangementsinconsumerunitsanddistributionboardscanmakeitawkwardtousetheminthisway.IfitisdesiredtoprotectanindividualcircuitanRCBO(ResidualcurrentCircuitBreakerwithOvercurrentprotection)canbeused.ThisincorporatesanRCDandaminiaturecircuitbreakerinonedevice.

ItiscommontoinstallanRCDinaconsumerunitinwhatisknownasasplitloadconfigurationwhereonegroupofcircuitsisjustonthemainswitch(ortimedelayRCDinthecaseofaTTearth)andanothergroupisontheRCD.

ElectricalplugswhichincorporateanRCDaresometimesinstalledonapplianceswhichmightbeconsideredtoposeaparticularsafetyhazard,forexamplelongextensionleadswhichmightbeusedoutdoorsorgardenequipmentorhairdryerswhichmaybeusednearatuborsink.OccasionallyaninlineRCDmaybeusedtoserveasimilarfunctiontooneinaplug.ByputtingtheRCDintheextensionleadyouprovideprotectionatwhateveroutletisusedevenifthebuildinghasoldwiring.

ElectricalsocketswithincludedRCDsarebecomingcommon.IntheU.S.thesearerequiredbylawinwetareas(SeeNationalElectricalCode(US)fordetails.)

InNorthAmerica,RCD("GFCI")socketsareusuallyofthedecorasize(asizethatharmonizesoutletsandswitches,sothatthereisnodifferenceinsizebetweenanoutletcoverandaswitchcover).Forexample,usingthedecorasizeoutlets,RCDoutletscanbemixedwithregularoutletsorwithswitchesinamultigangboxwithastandardcoverplate.

Active/passivelatching/nonlatchingRCDsmaybeobtainedthathavedifferentbehavioursifthecircuittheyareprotectingisdeenergised.[2]

Onetypewilltriponpowerfailureandnotremakethecircuitwhenthecircuitisreenergised.Thistypeisknowasnonlatching[2]oractive[3][4].Anothertypewillremakethecircuitwhenthecircuitisreenergised.Thistypeisknowaslatching[2]orpassive[3][4].Thefirsttypeareusedwhenthepowerdrawingequipmentisregardedasasafetyhazardifitisunexpectedlyreenergisedafterapowerfailuree.g.lawnmowersandhedgetrimmers.

Thesecondtypemaybeusedonequipmentwhereunexpectedreenergisationafterapowerfailureisnotahazard.AnexamplemaybetheuseofanRCDonacircuitprovidingpowertoafoodfreezer,wherehavingtoresetanRCDafterapowerfailuremaybeinconvenient.

MaincharacteristicsThefollowingkeyparametersdeterminetheRCD:

Numberofpoles[2Por3Por4P]Ratedcurrent[inA]Sensitivity[inmA]Type[ACorAorB]Breaktime[inms]Surgecurrentresistance[inA]

Numberofpoles[2P,3Por4P]RCDsmaycompriseoneortwopolesforuseonsinglephasesupplies(twocurrentpaths),threepolesforuseonthreephasesupplies(threecurrentpaths)orfourpolesforuseonthreephase&neutralsupplies(fourcurrentpaths).

Ratedcurrent[inA]TheratedcurrentofanRCDischosenaccordingtothemaximumsustainedloadcurrentitwillcarry(iftheRCDisconnectedinserieswith,anddownstreamofacircuitbreaker,theratedcurrentofbothitemsshallbethesame).

Sensitivity[inmA]RCDsensitivityisexpressedastheratedresidualoperatingcurrent,notedI?n.PreferredvalueshavebeendefinedbytheIEC,thusmakingitpossibletodivideRCDsintothreegroupsaccordingtotheirI?nvalue.

Highsensitivity(HS):61030mA(fordirectcontact/lifeinjuryprotection),Mediumsensitivity(MS):1003005001000mA(forfireprotection),Lowsensitivity(LS):31030A(typicallyforprotectionofmachines).Itshouldbeborneinmindthatnameplateratingandrealtripcurrentarenotnecessarilythesame.ForexampleUK30mARCDsmusttripatanimbalancecurrentlowerthan30mA.

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Type[ACorAorB]StandardIEC60755(Generalrequirementsforresidualcurrentoperatedprotectivedevices)definesthreetypesofRCDdependingonthecharacteristicsofthefaultcurrent.

TypeAC:RCDforwhichtrippingisensuredforresidualsinusoidalalternatingcurrents.TypeA:RCDforwhichtrippingisensured:forresidualsinusoidalalternatingcurrents,forresidualpulsatingdirectcurrents,forresidualpulsatingdirectcurrentssuperimposedbyasmoothdirectcurrentof0.006A,withorwithoutphaseanglecontrol,independentofthepolarity.TypeB:RCDforwhichtrippingisensured:asfortypeA,forresidualsinusoidalcurrentsupto1000Hz,forresidualsinusoidalcurrentssuperposedbyapuredirectcurrent,forpulsatingdirectcurrentssuperposedbyapuredirectcurrent,forresidualcurrentswhichmayresultfromrectifyingcircuits,i.e.:threepulsestarconnectionorsixpulsebridgeconnection,twopulsebridgeconnectionlinetolinewithorwithoutphaseanglemonitoring,independentlyofthepolarity.

Breaktime[inms]Therearetwogroupsofdevices:

G(generaluse)forinstantaneousRCDs(i.e.withoutatimedelay)Minimumbreaktime:immediate.Maximumbreaktime:200msfor1xI?n,150msfor2xI?n,and40msfor5xI?nS(selective)orT(timedelayed)forRCDswithashorttimedelay(typicallyusedincircuitscontainingsurgesuppressors).Minimumbreaktime:130msfor1xI?n,60msfor2xI?n,and50msfor5xI?n.Maximumbreaktime:500msfor1xI?n,200msfor2xI?n,and150msfor5xI?n

SurgecurrentresistanceThesurgecurrentreferstothepeakcurrentanRCDisdesignedtowithstandusingatestimpulseofspecifiedcharacteristics(an8/20simpulse,namedafterthetimeconstantsoftheriseandfallofcurrent).TheIEC61008andIEC61009standardsimposetheuseofa0.5s/100kHzdampedoscillatorwave(ringwave)totesttheabilityofresidualcurrentprotectiondevicestowithstandoperationaldischargeswithapeakcurrentequalto200A.Withregardtoatmosphericdischarges,IEC61008and61009standardsestablishthe8/20ssurgecurrenttestwith3000ApeakcurrentbutlimittherequirementtoRCDsclassifiedasSelective.

ImportantnoteregardingTNCearthingsystemAccordingtoIEC60364:

RCDshouldnotbeusedinaTNCsystem(perdefinition,RCDwouldnotprovidenecessaryprotectionincircuitswithLPENwiring)!WhenanRCDisusedinaTNCSsystem,aPENconductorshouldneverbeuseddownstream(instead,separatePEandNwiringisrequired).

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