Protective relays are power system protection devices that monitor current, voltage, frequency, impedance, or differential quantities and command circuit breakers when faults or abnormal conditions occur. 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. Definite time delay means that the protection operate time dose not change or depend on the. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system reliability. The relays are in round glass cases.