Field Failure Relay, also known as Field Loss Relay protects the D.C. Shunt wound motor or main circuit, under open field conditions, or in the absence of the field supply. Speed of the Shunt Wound D.C. Motor increases as the field supply is weakened. Theoretically motor runs at infinite speed if run without field supply. This break opens the armature coils from the commutator and totally damages the prime mover. The FIELD Failure Relay, when used in series with the field circuit, ensures that no power is supplied to the armature unless Field circuit is complete. Field Failure Relay is a truly current operated device. As there is no iron core, there is no magnetic storage and actuating time is less than 1.2 mili-seconds. This is also used as safety guard in D.C. Clutches, Solenoids and Brakes.