Selective Coordination

The Underwriters Laboratories (UL) 845 standard for low-voltage (LV) motor control centers (MCCs) requires a short-circuit withstand rating on the bus for only three cycles. The LV 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 LV power-circuit breakers may include short time delay that can result in the MCC-enduring fault currents for much more than the three cycles for which it is rated. In this article, it is proposed that MCC 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.