William Stanley
New member
- Location
- Lakewood, Co.
Industrial control panels are required to be rated for available fault current per NEC 409 and labeled with the short circuit current rating. The NEC definition of short circuit current is the "prospective symmetrical fault current" or RMS value of fault current. UL 508 requires that equipment defined in article 409 must be rated for the instantaneous peak let through current rather than the RMS value. I do not understand why UL does not require that control panels be rated for the maximum RMS value of fault current instead of the instantaneous value. When a UL listed control panel has a nameplate rating of 5K SCCR, why can't the RMS let through value of a fuse be used to protect the control panel rather than the peak let through. In many cases, there are no fuses that will protect the equipment using the peak let through, but there are fuses that will protect the panels using the RMS values. There seems to be a conflict between UL 508 and the NEC. Any help would be appreciated. Thanks, Bill Stanley.