Protective Relay

A protective relay is a device in an electric power system designed to detect a fault, such as a short circuit or an abnormal over-current, overvoltage, reverse power flow or frequency condition, and trip a circuit breaker to isolate the faulted section of the grid. The earliest protective relays were electromechanical devices using coils and moving parts to sense these abnormal conditions. Modern microprocessor-based digital protective relays emulate the functions of the original electromechanical devices while adding types of protection and supervision that were impractical before, and a single digital relay can often replace two or more older electromechanical devices, lowering both capital and maintenance cost. Because of their long service life, tens of thousands of older electromechanical protective relays are still in service protecting transmission lines and electrical equipment worldwide. Protective relays must generally respond and trip a breaker within a few thousandths of a second, with response times for particular fault conditions often set by standards such as ANSI C37.90 or the IEC 60255 series. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/