Short Circuit Rating

[schan05]

When we talk about SC rating of a piece of electrical equipment, such as switchboard, it is not quantified with a maximum allowable time duration. However, when we apply the insulated power breakers in low voltage power distribution switchboard, those power breakers could withstand SC up to 0.5 or 1 second. Is it given that switchboards are constrcuted to withstand published SC level up to one or more seconds? I couldn't find anything on NEMA regarding this. Thanks.

 

[alehman]

The requirement for switchboards built to UL891 standards (most common in the U.S.) is 3 cycles.

 

[rbulsara]

There is UL 1558 also for switchgear using ANSI rated breakers, which have a short time rating of 1 or 3 seconds...You need to look up those standards..Look up websites of reputed switchgear manufacturers.

 

[schan05]

Thanks for the helps.

 

[alehman]

And power circuit breakers without instantaneous trip elements can only be used to protect UL1558 (aka ANSI) switchgear because of the difference in the bus bracing.

Molded case circuit breakers can be used to protect UL891 switchboards or UL1558 switchgear because they are required to have instantaneous trip.