Protection of Motor Branch-Circuits

To protect the motor branch-circuit against short-circuits, overload relay protection must be coordinated with protection provided by the short-circuit protective device (SCPD). The SCPD may be a fused switch or a circuit breaker. The figure Typical Coordination Curves shows the critical point (I_c) in this coordination.

Operation of the Melting Alloy Overload Relay As heat melts the alloy, a ratchet wheel is free to turn. The spring then pushes the contacts open.

Typical Coordination Curves

At current values greater than I_c, the SCPD reacts quicker than the overload relay. At current values less than I_c, the overload relay reacts quicker. Refer to Articles 110 and 430 of the NEC for guidance in selecting the SCPD. These articles help with coordinating the components of a motor branch-circuit (such as determining the location of point I_c).