The protective relay is used to detect abnormal conditions within the electrical circuits by measuring the different electrical quantities constantly under normal as well as fault conditions. The electrical quantities which may vary in fault conditions are; current, voltage, phase. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. For example, unselective protection operation during a medium voltage network fault will cause an outage for an unnecessarily large number of consumers. This signal level is typically 5A nominal. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system.
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