Higher withstand MCC for better selective coordination

Abstract

Underwriters Laboratories 845 standard for low voltage motor control centers requires a short circuit withstand rating on the bus for only 3 cycles. However, in power distribution systems today low voltage motor control centers are increasingly fed from low voltage power circuit breakers located in low voltage switchgear ahead of or in a motor control center line up; these low voltage power circuit breakers are typically rated for up to 30 cycles short delay which can be greater than the UL 845 short circuit withstand test requirements. The trip settings on these low voltage power circuit breakers may include short-time delay that can result in the motor control center enduring fault currents for much more than the 3 cycles for which it is rated. In this paper it is proposed that motor control center bus designed and tested for higher number of cycles of withstand can address the above potential issue by offering better coordination and safer power distribution design options such as: 1) The instantaneous setting on the upstream low voltage power breaker can be eliminated to provide for increased coordination at the motor control center level. 2) Low voltage power circuit breakers (or where applicable larger frame molded case circuit breakers with electronic trips) can be used in a motor control center line up both as mains and feeders when increased coordination is required with downstream loads. 3) When selective coordination is required by National Electric Code for motor control centers being used in emergency, legally required standby or health care essential systems, motor control centers with increased withstand ratings can be provided, that are compatible with higher over current protective device short time delay time settings. 4) Better system design flexibility by allowing motor control center assemblies to be integrated in to a 30 cycle rated low voltage switchgear line up.