ABSTRACT
What is a relay; more specifically, what is a protective relay? The Institute of
Electrical and Electronic Engineers (IEEE) defines a relay as ‘‘an electric device that is designed to respond to input conditions in a prescribed manner
and, after specified 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).
