The thermal relays work on the principle of the thermal effect of electrical current. This relay consists bimetallic strip which is small in sizes and is heated by heating coils or strips supplied through a current transformer. Insulated liver arms carrying a contact are pivoted and is held in contact with the trip with the help of spring S. The spring tension can be varied by rotating the sector-shaped plate A.
Under normal operating condition the spring remains straight, but under the action of fault current the strip is heated and bent, and the tension of the spring is released. Thus the relay contacts are closed which energizes the trip circuit.
The construction of the bimetal element consists of two nickel alloyed steel strips welded together. These strips have a high heat resistivity and are free from the thermal secondary effect ageing.
Thermal relays are most widely used for protection of low-voltages squirrel cage induction motors or DC motors of lower output ratings. The limitation of such a relay is the short time overload withstand. Its heating effect is usually designed to withstand short-time overload periods say up to 6 to 7 times full loads current.
The thermal relay is not suitable for operation on the short circuit as it will heat the element adequately since the strip may defect so as to close the contacts. This type of relay is used in conjunction with instantaneous short-circuit relays of high setting or suitably graded time limits fuses.