Characteristics, Selection Guidelines, and Performance of Circuit Protection Devices for ASDs

The goal of all circuit protection devices is to mitigate and minimize collateral damage when a failure occurs to either the source (ac line) or the equipment. The circuit protection device is often given little attention, i.e., until erroneous trips hamper performance or unexpected damage occurs within the equipment. Then, the devices are often reviewed for performance characteristics and operating conditions. To meet this need, various devices have been designed for this purpose, and various standards have been written to provide guidance for the usage of those devices. The focus of this paper will be on the interplay between adjustable-speed drive (ASD) short-circuit current rating and protective device sizing, as well as the scaling of such systems, i.e., if a fuse sized for a 10-A circuit should have the same guidelines as that used for a 1000-A circuit. This paper will review the impact of Underwriters Laboratories 508A and 508C, and the National Electric Code on the ASD protection requirements. This paper evaluates a test setup scheme to measure the fuse performance. The peak current and I²t test results with both test schemes from three protection devices, a time-delay (TD) Class J fuse along with two non-TD Class J and T fuses, are presented and explained. A novel method of comparing the results is offered by viewing the collateral damage associated with a failure. This methodology has a minor impact at low power ratings, but it has major implications with high power ratings, where equipment is often field repairable.