The thermal relay works on the principle of the thermal effect of electrical energy. The bimetallic strips, heating coils and the current transformers are the important parts of the thermal relay.
The current transformer supplies the current to the coils of the heater. The thermal energy of the heater coils heated the bimetallic strips. The bimetallic strips are made of the alloy of nickel and steel. The alloy of the nickel and steel has high steel resistivity, and also they are free from thermal ageing.
The insulated liver arm is connected to the trip coil along with the spring and the bimetallic strips. The tension of the spring is varied by the help of the sector-shaped plate.