MCB-MCCB-ELCB-RCCB

MCB/MCCB/ELCB/RCCBMARCH20,201133COMMENTSMCB/MCCB/ELCB/RCBO/RCCB:MCB(MiniatureCircuitBreaker)Ratedcurrentnotmorethan100A.Tripcharacteristicsnormallynotadjustable.Thermalorthermal-magneticoperation.MCCB(MouldedCaseCircuitBreaker):Ratedcurrentupto1000A.Tripcurrentmaybeadjustable.Thermalorthermal-magneticoperation.AirCircuitBreaker:Ratedcurrentupto10,000A.Tripcharacteristicsoftenfullyadjustableincludingconfigurabletripthresholdsanddelays.Usuallyelectronicallycontrolled—somemodelsaremicroprocessorcontrolled.Oftenusedformainpowerdistributioninlargeindustrialplant,wherethebreakersarearrangedindraw-outenclosuresforeaseofmaintenance.VacuumCircuitBreaker:Withratedcurrentupto3000A,Thesebreakersinterruptthearcinavacuumbottle.Thesecanalsobeappliedatupto35,000V.Vacuumbreakerstendtohavelongerlifeexpectanciesbetweenoverhaulthandoaircircuitbreakers.RCD(ResidualCurrentDevice)/RCCB(ResidualCurrentCircuitBreaker):Phase(line)andNeutralbothwiresconnectedthroughRCD.Ittripsthecircuitwhenthereisearthfaultcurrent.Theamountofcurrentflowsthroughthephase(line)shouldreturnthroughneutral.ItdetectsbyRCD.anymismatchbetweentwocurrentsflowingthroughphaseandneutraldetectbyRCDandtripthecircuitwithin30Miliseconed.Ifahousehasanearthsystemconnectedtoanearthrodandnotthemainincomingcable,thenitmusthaveallcircuitsprotectedbyanRCD(becauseumitenotbeabletogetenoughfaultcurrenttotripaMCB)Themostwidelyusedare30mA(milliamp)and100mAdevices.Acurrentflowof30mA(or0.03amps)issufficientlysmallthatitmakesitverydifficulttoreceiveadangerousshock.Even100mAisarelativelysmallfigurewhencomparedtothecurrentthatmayflowinanearthfaultwithoutsuchprotection(hundredofamps)A300/500mARCCBmaybeusedwhereonlyfireprotectionisrequired.eg.,onlightingcircuits,wheretheriskofelectricshockissmallRCDsareanextremelyeffectiveformofshockprotectionLimitationofRCCB:StandardelectromechanicalRCCBsaredesignedtooperateonnormalsupplywaveformsandcannotbeguaranteedtooperatewherenonestandardwaveformsaregeneratedbyloads.Themostcommonisthehalfwaverectifiedwaveformsometimescalledpulsatingdcgeneratedbyspeedcontroldevices,semiconductors,computersandevendimmers.SpeciallymodifiedRCCBsareavailablewhichwilloperateonnormalacandpulsatingdc.RCDsdon’tofferprotectionagainstcurrentoverloads:RCDsdetectanimbalanceintheliveandneutralcurrents.Acurrentoverload,howeverlarge,cannotbedetected.ItisafrequentcauseofproblemswithnovicestoreplaceanMCBinafuseboxwithanRCD.Thismaybedoneinanattempttoincreaseshockprotection.Ifalive-neutralfaultoccurs(ashortcircuit,oranoverload),theRCDwon’ttrip,andmaybedamaged.Inpractice,themainMCBforthepremiseswillprobablytrip,ortheservicefuse,sothesituationisunlikelytoleadtocatastrophe;butitmaybeinconvenient.ItisnowpossibletogetanMCBandandRCDinasingleunit,calledanRCBO(seebelow).ReplacinganMCBwithanRCBOofthesameratingisgenerallysafe.NuisancetrippingofRCCB:Suddenchangesinelectricalloadcancauseasmall,briefcurrentflowtoearth,especiallyinoldappliances.RCDsareverysensitiveandoperateveryquickly;theymaywelltripwhenthemotorofanoldfreezerswitchesoff.Someequipmentisnotoriously`leaky’,thatis,generateasmall,constantcurrentflowtoearth.Sometypesofcomputerequipment,andlargetelevisionsets,arewidelyreportedtocauseproblems.RCDwillnotprotectagainstasocketoutletbeingwiredwithitsliveandneutralterminalsthewrongwayround.RCDwillnotprotectagainsttheoverheatingthatresultswhenconductorsarenotproperlyscrewedintotheirterminals.RCDwillnotprotectagainstlive-neutralshocks,becausethecurrentintheliveandneutralisbalanced.Soifyoutouchliveandneutralconductorsatthesametime(e.g.,bothterminalsofalightfitting),youmaystillgetanastyshock.ELCB(EarthLeakageCircuitBreaker):Phase(line),NeutralandEarthwireconnectedthroughELCB.ELCBisworkingbasedonEarthleakagecurrent.OperatingTimeofELCB:ThesafestlimitofCurrentwhichHumanBodycanwithstandis30masec.SupposeHumanBodyResistanceis500ΩandVoltagetogroundis230Volt.TheBodycurrentwillbe500/230=460mA.HenceELCBmustbeoperatedin30maSec/460mA=0.65msecRCBO(ResidualCircuitBreakerwithOverLoad):ItispossibletogetacombinedMCBandRCCBinonedevice(ResidualCurrentBreakerwithOverloadRCBO),theprincipalsarethesame,butmorestylesofdisconnectionarefittedintoonepackageDifferencebetweenELCBandRCCB.ELCBistheoldnameandoftenreferstovoltageoperateddevicesthatarenolongeravailableanditisadvisedyoureplacethemifyoufindone.RCCBorRCDisthenewnamethatspecifiescurrentoperated(hencethenewnametodistinguishfromvoltageoperated).ThenewRCCBisbestbecauseitwilldetectanyearthfault.Thevoltagetypeonlydetectsearthfaultsthatflowbackthroughthemainearthwiresothisiswhytheystoppedbeingused.Theeasywaytotellanoldvoltageoperatedtripistolookforthemainearthwireconnectedthroughit.RCCBwillonlyhavethelineandneutralconnections.ELCBisworkingbasedonEarthleakagecurrent.ButRCCBisnothavingsensingorconnectivityofEarth,becausefundamentallyPhasecurrentisequaltotheneutralcurrentinsinglephase.That’swhyRCCBcantripwhenthebothcurrentsaredeferentanditwithstanduptoboththecurrentsaresame.Boththeneutralandphasecurrentsaredifferentthatmeanscurrentis