Medium-Voltage Circuit Breakers and Contactors Are Not the Same: Neither are Their Protection Methods

Many standards and specifications related to medium-voltage (MV) motor and feeder circuits continue to center around the improper generic use of IEEE Device 52 as a circuit identifier for the switching device used within various points of power systems. IEEE Device 52 is defined as a device that is used to close and interrupt an ac power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions. While many of the present standards include detailed protection and coordination recommendations that are centered only on the protective and control settings for circuit breakers (CBs), the protection schemes for CBs and fused contactor-controlled motors are documented in IEEE Standard 3004.8. This article clarifies some key differences between MV contactors and MV CB ratings. It also compares the appropriate protection variations and requirements between the two devices, highlighting the dangers to personnel and equipment if misapplied. Recommended protective device functions for motor protection for fused contactor-controlled or breaker-controlled motors are presented with companion drawings.