A master trip relay is a type of electrical protection relay that is used in electrical substations to trip (disconnect) the entire substation in the event of an emergency or fault condition. It is designed to provide a quick and reliable means of shutting down the entire substation to protect equipment and personnel. The master trip relay is typically connected to other protection relays within the substation, such as overcurrent relays and earth fault relays, which detect specific fault conditions and send a signal to the master trip relay to initiate the trip. The master trip relay will disconnect the entire substation from the power supply when a fault is detected, this helps to prevent equipment damage, power outages, and potential hazards to personnel.