Chapter 1

Introduction and General Philosophies

1.1 Introduction and Definitions

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, 1989).

Relays are used in all aspects of activity: home, communication, transportation, commerce, and ...

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