Simplified Guide To Understanding Short-Circuit Current Rating

Simplified Guide ToUnderstandingShort-Circuit Current RatingFIND IT, FIX IT, FORGET IT

Are You Ready For The New SCCR Marking Requirements?What Is A Short-Circuit Current Rating(SCCR)?SCCRs on components and assemblies represent themaximum level of short-circuit current that thecomponent or assembly can withstand and is used fordetermining compliance with NEC ® 110.10.This ratingcan be marked on individual components orassemblies. Assembly ratings take into account allcomponents contained within the equipment.A common mistake is to assume that the interruptingrating of the overcurrent protective device protectingthe circuit represents the SCCR for the entire circuit.Interrupting ratings, used for compliance with NEC ®110.9, apply solely to the overcurrent protective device.It is the characteristics of the overcurrent protectivedevice (e.g. opening time, let-through energy) thatneed to be used in determining compliance with NEC ®110.10, not the interrupting rating.CAUTION: Short-circuit current ratings (SCCRs) aredifferent than interrupting ratings marked onovercurrent protective devices.Who Is Affected By The SCCR MarkingsThe 2005 NEC ® has requirements for anyone buildingequipment listed to the 2005 NEC ® or to UL508A andrequires the following to be marked with an SCCR:• Industrial control panels [409.110 ]• Industrial machinery electrical panels [670.3(A)]• HVAC equipment [440.4(B)]• Meter disconnect switches [230.82(3)]• Motor controllers [430.8 ]Effective April, 2006, all equipment listed to UL508Awill be required to be marked with an assembly SCCR.Inspectors and installers need this information in orderto ensure compliance with NEC ® 110.10. Equipmentinstalled where fault current levels exceed their shortcircuitcurrent can be hazardous to persons andproperty. SCCRs marked on components andassemblies make it easier to verify proper protectionfor components and assemblies for specificapplications — whether it be the initial installation orrelocation of equipment.How Is SCCR Determined?For meter disconnect switches and motor controllers,this withstand level or SCCR is often determined byproduct testing. For assemblies, the marking can bedetermined through product listing or by an approvedmethod. With the release of the UL508A IndustrialControl Panel standard, an industry approved methodis now available. Any method used, whether UL508A oranother approved method, should be based upon theweakest link approach. In other words, the assemblyshould be limited to installation where fault levels donot exceed the withstand rating of devices with thelowest SCCR .The marking determined shouldrepresent the limits of the assembly for a safeinstallation. Current-limiting overcurrent protectivedevices can be used in the feeder or branch circuits toincrease the assembly SCCR where lower ratedcomponents are used.Who Is AffectedFacility/Process Engineers/Consulting EngineersControl Panel Builders/Machine Builders/HVAC ManufacturersElectrical ContractorsElectrical InspectorsHow Are They AffectedNeed to specify end use equipment with ratings adequate for availableshort-circuit current.Need to determine and mark the assembly short-circuit current rating on theequipment being built. Need to market equipment with ratings at least as highas their competition.Need assurance that the equipment installed is adequate for the availableshort-circuit current at the point of installation, to avoid red tags and lost timeand labor.Need to assure that the available short-circuit current where equipment isbeing installed does not exceed the rating marked on the equipment.

Use this table to initiate the process of identifying the short-circuit current rating of yourcomponents and devices in power circuits. For further information contact your localCooper Bussmann sales engineer, Cooper Bussmann Application Engineering,or visit www.cooperbussmann.com.Supply Disconnect and OCPDBranch CircuitFusesUL489 CircuitBreakersUL98 FusibleDisconnectSwitchesUL98 Non-FusibleDisconnectSwitchesUL1429 Pullout200kA* 5kA 200kA* 5kA ‡ 10kA 5kAMolded Case SwitchFeeder Circuit OCPD and ComponentsBranch CircuitFusesUL489 CircuitBreakersUL98 FusibleDisconnectSwitchesUL98 Non-FusibleDisconnectSwitchesPower DistributionBlocks200kA* 5kA 200kA* 5kA ‡ 10kA 200kA*Fuse HolderBranch Circuit OCPD and Components200kA*Branch CircuitFusesUL489 CircuitBreakersUL98 FusibleDisconnectSwitches5kA 200kA* 5kA ‡UL98 Non-FusibleDisconnectSwitches5kA***Magnetic MotorStarter5kA***Type E SelfProtectedStarter5kAMolded CaseSwitch+Instantaneous TripCircuitBreakers*Class CC, J, T, R, and L UL248 Branch Circuit Type Fuses.**When used in conjunction with Class CC, J, T, R, and L UL248 Branch Circuit Type Fuses.***50HP or less 480V‡Default value increases to 10kA when protected by fuses+Combination Rating is used for Sweep 1.For Sweep 2 IR=5kA (Default Value)

DefaultValueYourValueGlossary of TermsIndustrial Control Panel - An assembly of asystematic and standard arrangement of two or morecomponents such as motor controllers, overload relays,fused disconnect switches, fuses, and circuit breakersand related control devices such as pushbuttonstations, selector switches, timers, switches, andcontrol relays with associated wiring, terminal blocks,pilot lights and similar components.Interrupting Rating - The highest current at ratedvoltage that a device is intended to interrupt understandard test conditions.10kABusbar5kAMolded Case SwitchShort-Circuit Current Rating – The prospectivesymmetrical fault current at a nominal voltage to whichan apparatus or system is able to be connected withoutsustaining damage exceeding the defined acceptancecriteriaControl Circuit – A circuit that carries the electricsignals directing the performance of a controller, andwhich does not carry the main power circuit. A controlcircuit is, in most cases, limited to 15 amp.Power Circuit – Conductors and components ofsupply, branch, and feeder circuits, supplying main linepower to loads such as heaters, motors, lighting,receptacles, power transformers, and appliances.Fuse HolderUL508 ManualMotor StarterUL508 SwitchDrivesContactorsReceptacleGFCI Receptacle200kA*5kA***5kA***5kA***5kA***10kA2kA

Find It, Fix It, Forget ItFIND IT – Look for... the Weakest LinkThe SCCR marking for the assembly will be basedupon the lowest short-circuit current or withstand ratingof any power circuit component contained within theassembly — or the “weakest link”. Many componentsused in industrial control panels, industrial machineryand HVAC equipment may be the weakest link,including:• motor controllers• definite purpose contactors• supplemental protectors• circuit breakers (with low interrupting ratings)• power distribution blocks.FIX IT — How To Get A Higher SCCR SoEquipment Isn’t LimitedBy fixing these weakest links, higher SCCRs can beachieved. In general, there are three options:1. Use overcurrent protective devices withhigher interrupting ratings — Overcurrentprotective devices with low interrupting ratings oftenbecome the weakest link and limit the SCCR of theassembly. The interrupting ratings of the overcurrentprotective devices used directly affects the assemblyrating. The only fix is to use devices with higherinterrupting ratings.2. Use current limitation to fix low ratedcomponents — It only takes one component with alow SCCR to limit the entire assembly. Many of thecomponents used in industrial control applications havelow short-circuit current ratings. Fix these low ratedcomponents and the assembly rating by usingcurrent limitation. Since current-limiting fuses reduce highlevel short-circuit currents to a minimal level, they allowthe use of these lower rated devices whileproviding a higher SCCR.3. Buy more expensive components withhigher ratings — A 65kA circuit breaker will morethan double your cost compared to a 14kA circuitbreaker. The fusible solution provides an interruptingrating as high as 200kA, at a comparable cost to yourexisting design, making it the most economical solution.FORGET IT — Use Current-Limiting FusesNot only can modern current-limiting fusible solutionsfix undesired weak links and obtain a high assemblyshort-circuit current rating, they can also provideenhancements to your design in the areas of flexibilityof installation, increased workplace safety, and reliableprotection through the life of your equipment. Theseenhancements take care of the hassle associated withequipment with limited ratings so you can FORGET IT!Benefits of Using Cooper Bussmann Current-Limiting FusesStraight Voltage Ratings:• Straight voltage ratings allow for more installations whereequipment can be installed. Straight voltage ratedequipment can be installed in any type of electrical system.With a straight voltage rating you can ship it anywhere andnot have to worry.High Assembly SCCRs:• High SCCRs allow for more applications where equipmentcan be installed. With a high SCCR you can ship itanywhere and not have to worry.• A wider range of fault current levels provides assurance thatthe equipment will be installed correctly and safely.• Meets the UL508A and NEC ® requirements for SCCRmarking.• Marketing advantage over competitive equipment with alower rating.Increased Flexibility In Panel Use and Installation:• Valuable time that was spent gathering information forproper application is drastically reduced with fuses becausemodern current-limiting fuses can provide high assemblySCCRs and straight voltage ratings since they have:- High Interrupting Ratings of 200,000A or more- Excellent current limitation to protect low ratedcomponents at high short-circuit current levels- Straight voltage ratings so they can be installed in anytype of application independent of the grounding schemeused, unlike devices with a slash voltage rating which arelimited to installation in ONLY a solidly grounded wyetype system

Cooper Bussmann Products AndTechnical Support Delivered WorldwideCustomer AssistanceCustomer Satisfaction TeamThe Cooper Bussmann Customer Satisfaction Team isavailable to answer questions regarding CooperBussmann products and services. Calls should be madeMonday – Friday, 8:00 a.m. – 4:30 p.m. Central Timefor all US time zones.The Customer Satisfaction Team can be reached via:• Phone: 636-527-3877• Toll-free fax: 800-544-2570• E-mail: busscustsat@cooperbussmann.comEmergency and After-Hours OrdersTo accommodate time-critical needs, Cooper Bussmannoffers emergency and after-hours service for next flightout or will call. Customers pay only standard price forthe circuit protection device, rush freight charges and amodest emergency fee for this service. Emergency andafter-hours orders should be placed through theCustomer Satisfaction Team. Call:• Monday – Friday, 8:00 a.m. – 4:30 p.m.Central Time 636-527-3877• After hours 314-995-1342Application EngineeringApplication Engineering assistance is available to allcustomers. The Application Engineering team is staffedby degreed electrical engineers and available by phonewith technical and application support Monday – Friday,8:00 a.m. – 5:00 p.m. Central Time.Application Engineering can be reached via phone, faxor email:• Phone: 636-527-1270• Fax: 636-527-1607• E-mail: fusetech@cooperbussmann.comOnline ResourcesVisit www.cooperbussmann.com for the followingservices:• Product cross reference• Arc-flash calculator• SCCR calculator• Training modulesYour Authorized Cooper Bussmann Distributor is:©2005 Cooper Bussmann • St. Louis, MO 63178636-394-2877 • www.cooperbussmann.comPrinted in USA Reorder #7009 1-06-5M