Core idea: A relay uses one electrical signal to switch, isolate, interlock, alarm, or command another circuit. Protective relays can be classified based on their operating principle, construction, or function: 1. For example, unselective protection operation during a medium voltage network fault will cause an outage for an unnecessarily large number of consumers. Engineering use: Relays are used in control panels, motor circuits, PLC interfaces, alarms, breaker trip circuits, and power system protection schemes. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions.