Overcurrent Protection Within the Motor Branch Circuit at Low Fault Currents

The maximum ratings of devices acceptable for the overcurrent protection of the components within the motor branch circuit are defined in the National Electrical Code.[1] Motor starters with thermal relays usually provide this protection at the normal operating overcurrents, while fuses or time-limit circuit breakers provide protection at overcurrents above the permissible operating values. The time-current characteristics of the components used for the protective function should be related in order to obtain the maximum degree of protection against overheating and the resultant damage due to excessive currents.