Understanding Short Circuit Current Ratings - c3controls

SCCR stands for Short-Circuit Current Rating, which is defined in the 2017 NEC (National Electric Code), Article 100 as: “The prospective symmetrical fault ... Close Close Home WhitePapers UnderstandingSCCRUnderstandingSCCR c3controlsMiniatureCircuitBreakershavea10kASCCRrating! c3controls'UL489miniaturecircuitbreakersandUL1077supplementaryprotectorsareratedat10kASCCRandofferB,C&Dtripcurvesofupto63amps!ShopMiniatureCircuitBreakers!WhatisSCCR?Whenitcomestoindustrialmachinery,it’scriticaltoensurethatelectricalpanelsaredesignedandbuiltwiththeproperSCCRtomaintainthesystem,eliminatedowntimeandincreaseworkersafety.Thegoalofthispaperistoexplainwhyitisimportant,andhowtofindthecalculationsneeded.SCCRstandsforShort-CircuitCurrentRating,whichisdefinedinthe2017NEC(NationalElectricCode),Article100as:“Theprospectivesymmetricalfaultcurrentatanominalvoltagetowhichanapparatusorsystemisabletobeconnectedwithoutsustainingdamageexceedingdefinedacceptancecriteria.”Insimplerterms,SCCRisthemaximumshort-circuitcurrentanelectricalcomponentcansafelywithstandwithoutcausingashockorfirehazard.Ingeneral,theSCCRratingforanelectricalpanelisbasedonunderstandingeachelectricalcomponentsSCCRratinginthatpanel.SCCRhasbecomeanincreasinglyimportanttopic,regardingtotheinstallationofindustrialmachineryandelectricalpanelsatcustomerfacilities.WhyIsSCCRImportant?BecausetheSCCRrepresentsthehighestshort-circuitfaultcurrentthatequipmentsuchasindustrialcontrolpanelscansafelywithstand,exceedingtheSCCRmaycausecatastrophicandviolentequipmentandcomponentfailure.2017NECARTICLE670INDUSTRIALCONTROLPANELS409.22ShortCircuitCurrentRating.Installation.Anindustrialcontrolpanelshallnotbeinstalledwheretheavailableshort-circuitexceedsitsshort-circuitratingasmarkedinaccordancewith409.110(4)Therefore,it’snecessarytoidentify(calculate)themaximumshort-circuitcurrentthatcanbepresentwheretheindustrialcontrolpanelorotherequipmentisinstalled.Theavailableshort-circuitcurrentmustbelessthanthemarkedSCCRoftheindustrialcontrolpaneltocomplywiththeNEC.It’simportanttounderstandshort-circuitcurrentratingsforindustrialcontrolpanels. Industrialcontrolpanel isageneraltermthatdescribesanassemblyoftwoormorecomponentsthatareenclosed.Theassemblycanbepower,controlorboth,butdoesn’tincludethecontrolledequipment.In ,the  andtheNationalElectricalCode (NEC®)thestandardrequiresindustrialcontrolpanelsthatcontainpowercircuitcomponents(supplymainlinepowertoloadssuchasmotors,heating,lighting,appliancesorreceptacles)tobemarkedwithanSCCR,previouslyreferredtoasa withstandrating.Asreferencedin UL508A andthe NEC,industrialcontrolpanelsthatcontainonlycontrolcircuitcomponentsdonothavetobemarkedwithanSCCR.Iftheindustrialcontrolpanelcontainsapowercircuitbranch-circuitovercurrentprotectivedevicesupplyingthecontrolcircuit,thentheSCCRmustbemarkedbasedontheinterruptionratingoftheovercurrentprotectivedevice.PowerCircuitvs.ControlCircuitPriortothereleaseofthelatesteditionofUL508A,SupplementSBdatedDecember20,2013,SCCRappliedonlytothepowercircuitofacontrolpanel.Thepowercircuitsupplieselectricitytotheworkersthatdotheheavylifting,poweringlargemotors,forexample.Thisistypicallysomewherebetween240VAC~600VAC,3phasepower.Thecontrolcircuitislowerpower,aslowas5volts.Asthenameimplies,itcontrolstheoperationofthepowercircuit,butisisolatedfromit.Thisisimportantsoastoisolatetheuserfromthehighvoltagesandcurrentsofthepowercircuit.Thinkofthecontrolcircuitasthebrainsandthepowercircuitasthebrawnofacontrolpanel.CalculatingShort-CircuitCurrentRating(SCCR)Today,themajorityofelectricalcomponentshavetheSCCRmarkedonthecomponentitselfbythemanufactureroracombinationratingontheULwebsite.IftheSCCRisnotavailable,adefaultvaluecanbeassignedbyreferencing TableSB4.1 inthe UL508A standard.Takingthesevaluesandthetimeneededtocompletelyunderstandthecircuitdesignbeingused,yourealizethatbasicallythecomponentwiththesmallestSCCRincertaincircuitsdeterminestheelectricalpanelSCCRrating.1.Identifyallthecomponentsinthepowercircuit.Theseinclude,butmaynotbelimitedto:DisconnectswitchesCircuitbreakersFuseholdersOverloadrelaysMotorcontrollers(contactors)Terminalblocksorpowerdistributionblocks2.DeterminetheSCCRvalueforallcomponentsinthepowercircuit.SCCRisusuallymarkedonthecomponent,combinationratingsareavailableontheULwebsite,orwiththemanufacturer’sinstructions.Ifnotavailablefromthemanufacturer,theSCCRvaluecanbedeterminedbyusingULTableSB4.1(seeAppendix).3.DeterminetheSCCRforeachbranchcircuit.ThelowestcomponentSCCRistheequivalentbranchcircuitSCCR4.DeterminetheSCCRforthefeedercircuit.5.TakeintoaccountfeedercircuitOverCurrentProtectionDevices.6.Determinethelowestvalueforanycomponentorbranchcircuit.ThisistheSCCRforthecontrolpanel.WhySCCRisImportantforInstallationSincethereleaseofthe2005NEC,industrialmachineryelectricalpanelsarerequiredtobemarkedwithanSCCRrating.Priortothis,panelmanufacturersonlyhadtosupplytheinterruptingratingofthepanelovercurrentprotectivedevice,ortheamperageatwhichthemaindisconnecttothepanelwould“trip”.Thismeansthatsincethe2005NECrelease,industrialmachineryelectricalpanelmanufacturershadtostartnotonlycalculatingSCCRratingsfortheirpanels,butalsounderstandtheavailableshort-circuitcurrentthatissuppliedatthecustomerfacilitywhereinstallationofthepanelwillbe.Iftheavailableshort-circuitcurrentsuppliedtothepanelishigherthantheSCCRratingmarkedontheratinglabelofthepanel,installationofthepanelisprohibitedbyNEC,670.5.Thisrequiresindustrialmachineryelectricalpanelinstallerstoverifytheavailablefaultcurrentattheircustomerfacilities.2017NECARTICLE670INDUSTRIALMACHINERY670.5ShortCircuitCurrentRating(1)Industrialmachineryshallnotbeinstalledwheretheavailableshort-circuitcurrentexceedsitsshort-circuit ratingasmarkedinaccordancewith670.3(A)(4).SCCRComplianceTosummarize,threethingsareimportanttorememberwhencomplyingwithSCCRratings.TheSCCRforthecontrolpanelisdeterminedthelowestSCCRvalueforanycomponentorbranchcircuit.Theavailablefaultcurrentmustbedeterminedatthecustomerfacilitywheretheelectricalpanelwillbeinstalled.Thisdeterminationcanbedonebythemanufacturerortheinstallationcontractor.TheSCCRratingoftheelectricalpanelmustbeequaltoorgreaterthantheavailablefaultcurrentdetermined.Disclaimer:Thecontentprovidedinthiswhitepaperisintendedsolelyforgeneralinformationpurposesandisprovidedwiththeunderstandingthattheauthorsandpublishersarenothereinengagedinrenderingengineeringorotherprofessionaladviceorservices.Thepracticeofengineeringisdrivenbysite-specificcircumstancesuniquetoeachproject.Consequently,anyuseofthisinformationshouldbedoneonlyinconsultationwithaqualifiedandlicensedprofessionalwhocantakeintoaccountallrelevantfactorsanddesiredoutcomes.Theinformationinthiswhitepaperwaspostedwithreasonablecareandattention.However,itispossiblethatsomeinformationinthesewhitepapersisincomplete,incorrect,orinapplicabletoparticularcircumstancesorconditions.Wedonotacceptliabilityfordirectorindirectlossesresultingfromusing,relyingoractinguponinformationinthiswhitepaper. 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