Advanced Guide To Understanding Assembly Short-Circuit Current ...

Find It, Fix It...Forget ItHow To Get A Higher SCCR So An AssemblyIsn’t Limited?Once these weakest links are found, and if thedesired assembly short-circuit current rating is notachieved, there are options to fix these weak links. Byfixing these weak links, higher short-circuit currentratings can be achieved. In general there are threeoptions that can be used to fix weak links encounteredin a panel.Options For Fixing Weak Links1. Use Overcurrent Protective Devices WithHigher Interrupting RatingsOvercurrent protective devices with low interruptingratings often become the weakest link and limit theshort-circuit current rating of the assembly. Theinterrupting ratings of the overcurrent protective devicesused directly affects the assembly rating. The only fix isto use devices with higher interrupting ratings.CAUTION: Combination ratings are often limited bythe interrupting ratings of the overcurrent protectivedevices used.3. Buy More Expensive Components WithHigher RatingsA 65kA circuit breaker will more than double the costcompared to a 14kA circuit breaker.The fusible solution provides a rating as high as 200kA,at a comparable or lower cost to an existing design,making it the most economical solution.Circuit breakers add unnecessary cost, while fuses are economical.2. Use Current Limitation To Fix LowRated ComponentsIt only takes one component with a low short-circuitcurrent rating 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 using currentlimitation. Since current-limiting fuses reduce high levelshort-circuit currents to a minimal level they can allowthe use of these lower rated devices while providing ahigher assembly short-circuit current rating. Type 2protection, using fuses, can be used to achieve highbranch SCCR.Modern current-limiting fuses have:• The highest interrupting ratings of anyovercurrent protective devices, and• Current limitation to fix low-rated components.Example of the use of current limitation to increase a low short-circuitrated component.© 2006 Cooper BussmannPage 6