Simplified Guide To Understanding Short-Circuit Current …

1 Simplified Guide ToUnderstandingShort- circuit Current RatingFIND IT, FIX IT, FORGET ITWhat Is A Short-Circuit Current Rating(SCCR)?SCCRs on components and equipment represent themaximum level of Short-Circuit Current that the component or equipment can withstand and is used fordetermining compliance with NEC This ratingcan be marked on individual components or assemblies. The new SCCRs represent the maximumamount of fault Current that the assemblycan withstandunder fault conditions. Assembly ratings take intoaccount all components contained within the equipmentrather than just the main overcurrent protective common mistake is to assume that the interruptingrating of the overcurrent protective device protectingthe circuit represents the SCCR for the entire ratings, used for compliance with NEC , apply solely to the overcurrent protective is the characteristics of the overcurrent protectivedevice ( opening time, let-through energy)

2 Thatneed to be used in determining compliance with NEC , not the interrupting 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 [ ] Industrial Machinery Electrical Panels [ (A)] Certain HVAC Equipment [ (B)] Meter disconnect switches [ (3)] Certain Motor Controllers [ ]Effective April, 2006, all equipment listed to UL508A willbe required to be marked with an assembly and installers need this information in orderto ensure compliance with NEC Equipmentinstalled where fault Current levels exceed their Short-Circuit Current limit can be hazardous to persons andproperty.

3 SCCRs marked on components and equipment make it easier to verify proper protection forcomponents and equipment for specific applications whether it be the initial installation or relocation 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 method is nowavailable.

4 Any method used, whether UL508A or another approved method, should be based on the weakest link approach. In other words, the assemblyshould be limited to installation where fault levels donot exceed the withstand rating of devices with the lowest SCCR. The marking determined should represent the limits of the assembly for a safe installation. Current -limiting overcurrent protectivedevices can be used in the feeder or branch circuits toincrease the assembly SCCR where lower rated components are used.

5 When Current -limiting devices(that limit fault Current levels to within the lower ratedcomponents' ratings) are used, they provide protectionfor the assembly provided the Current -limitingOvercurrent Protective Device (OCPD) is used withinits You Ready For The New SCCR Marking Requirements?CAUTION: Short-Circuit Current ratings (SCCRs) aredifferent than interrupting ratings marked on overcurrent protective Is AffectedHow Are They AffectedFacility/Process Engineers/Need to specify end use equipment with ratings adequate for available Consulting Engineersshort- circuit Current .

6 Need adequate ratings so equipment has flexibility to be moved Panel Builders/Need to determine and mark the assembly Short-Circuit Current rating on the Machine Builders/equipment they build. Need to market equipment with ratings as least as high HVAC Manufacturersas their ContractorsNeed assurance that the equipment they install is adequate for the available Short-Circuit Current at the point of installation, to avoid red tags, and lost time and InspectorsNeed to assure that the available Short-Circuit Current where equipment is being installed does not exceed the rating marked on the this table to initiate the process of identifying the Short-Circuit Current rating of your components and devices in power circuits.

7 For further information contact your local Cooper Bussmann sales engineer, Cooper Bussmann Application Engineering,or visit disconnect and OCPDF eeder circuit OCPD and ComponentsBranch circuit OCPD and Components*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+Combination Rating is used for Sweep Sweep 2 IR=5kA (Default Value)UL98 Non-FusibleDisconnectSwitchesUL1429 PulloutMolded Case SwitchUL98 FusibleDisconnectSwitchesUL489 CircuitBreakersBranch CircuitFusesUL98 Non-FusibleDisconnectSwitchesPower DistributionBlocksFuse HolderUL98 FusibleDisconnectSwitchesUL489 CircuitBreakersUL98 FusibleDisconnectSwitchesBranch CircuitFusesBranch CircuitFusesUL98 Non-FusibleDisconnectSwitches200kA*UL489 CircuitBreakers5kA200kA*10kA**Magnetic MotorStarter5kA**Type E SelfProtectedStarter5kA**5kAInstantaneou s

8 TripCircuitBreakers+Molded CaseSwitch200kA*5kA200kA*10kA**10kA200kA *200kA*5kA200kA*10kA**10kA5kAFuse HolderBusbarMolded Case Switch200kA*UL508 ManualMotor Starter5kA**UL508 Switch5kA**Drives5kA**Contactors5kA**Rec eptacle10kAGFCI Receptacle2kA10kA5kADefaultValueYourValu eGlossary 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 , and control relays with associated wiring, terminal blocks.

9 Pilot lights and similar Rating - The highest Current at ratedvoltage that a device is intended to interrupt understandard test Current Rating The prospective symmetrical 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 electric signals directing the performance of a controller, andwhich does not carry the main power circuit . A controlcircuit is, in most cases, limited to 15 circuit Conductors and components of supply, branch, and feeder circuits, supplying main linepower to loads such as heaters, motors, lighting,receptacles, power transformers, and appliances.

10 FIND IT Look 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.