ABSTRACT
What is a relay; more specically, what is a protective relay? The Institute of Electrical and Electronic Engineers (IEEE) denes a relay as “an electric device that is designed to respond to input conditions in a prescribed manner and, after specied conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits.” A note adds: “Inputs are usually electric, but may be mechanical, thermal, or other quantities or a combination of quantities. Limit switches and similar simple devices are not relays” (IEEE C37.90, 1989).
