Protecting A-C motors with low-voltage air-circuit-breaker series trips

LOW-VOLTAGE air circuit breakers are sometimes used alone as motor starters, since they provide in a single device the various elements of a motor-branch circuit as required by the National Electrical Code. The purpose of this paper is to discuss the effectiveness of the direct-acting series trips used on such breakers as a means of protecting squirrel-cage motors in the event of: 1. Locked or stalled rotor, either during starting or running. 2. Overheating due to prolonged moderate overloads. 3. Short circuits in the motor or in connections to it.