Withstand Ratings

Equipment withstand ratings are linked to branch-circuit protection. The same parameters that affect the trip point of a given protective device, also contribute to how much (or how little) let-through energy the device can be exposed to and still function after the clearing of the fault. Withstand does not explicitly show up in Operation of the Melting Alloy Overload Relay or Typical Coordination Curves.

Traditional melting alloy and bimetallic overload relays have been the “weak link” in motor branch-circuit withstand ratings. Since these devices employ sensing elements directly in the current path, electrical faults leading to mechanical stresses are a concern. These devices typically contain small mechanical parts that can quickly become out-of-spec when exposed to let-through energy exceeding their withstand capability.

If the coordinated protection for the circuit operates properly (and the SCPD protects the circuit), the motor and the controller will be protected. The withstand rating of a branch-circuit must account for the withstandability of the lowest rated component in the circuit.