Overload relays are specialized circuit breakers used with industrial motors to protect the motors from damages caused by overload or electrical faults. Single-phase and multi-phase power systems typically include an overload relay for interrupting power in the power conductors when a fault condition occurs, such as a ground fault, phase loss, overcurrent, or undercurrent condition. An overload relay is essentially a sensor that is connected between the power supply and the load and upon the detection of a current overload condition, disconnects the load from the power supply. An overload relay is usually designed to operate over a wide range of values and the user must set the trip current based upon the specifications of the motor in use. In the past, overload relays have been of the thermal type wherein a bimetal actuator is heated directly or indirectly as a function of the motor current and operates to effect interruption of the motor circuit under overload conditions. The coupling length of a latch released by the thermal deformation of the bimetal plate is adjustable according to a kind of a load to facilitate adjustment of time from the start of flow of an overload current through the load until the disconnection of the load from its power supply. More recently, overload relays have been designed to utilize electronic circuitry responsive to signals derived from the secondary windings of current transformers whose primary windings carry the motor phase currents.
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