Field Failure Relay is used to protect main circuit or DC Shunt wound motor when field supply is not present or during open field conditions. When Shunt Wound DC motor does not receive any field supply, it starts running on increased speed. Theoretically speaking, in the absence of field supply, motor starts running at infinite speed. This can damage prime mover by breaking open the armature coil from the commutator When Field Failure Relay is used with field circuit in series it does not allow power being supplied to Armature It is only allowed when field circuit is complete. Owing to absence of magnetic storage and iron core,it has an actuating time of less than 1.2 milliseconds. Field Failure Relay, also known by the name of Field Loss Relay also has its application in DC clutches, brakes and solenoids as a safety guard.