ABSTRACT
The phase overcurrent protection can be stand-alone for small systems, and is always provided as a backup protection for large systems too, which have unit protection. The elements in a protective system include relays, direct-acting solid-state trip devices, trip programmers for the low-voltage circuit breakers, and electromagnetic trip devices. Coordination is not a substitute for inadequate system planning and design. The protective device data and characteristics are built into the computer database. The software vendors have kept pace with the introduction of new devices and multifunction overcurrent relays with programmable characteristics and keep expanding databases. A graphical representation of the time-current characteristics is an accepted method, though it is possible to determine selectivity by comparing at the most three critical values of the fault currents and ascertaining the relay operating times. The thermal elements have given way to better digital relays for low-voltage motor protection.
