Working Principle of Thermal Relay
A thermal relay works depending upon the above-mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over the current of the system, it bends and makes normally open contacts.
Construction of Thermal Relay
The construction of a thermal relay is quite simple. As shown in the figure above the bimetallic strip has two metals – metal A and metal B. Metal A has a lower coefficient of expansion and metal – B has a higher coefficient of expansion. One heating coil is would on the bimetallic strip. When overcurrent flows through the heating coil, it heats up the bimetallic strip.
Due to the heat generated by the coil, both of the metals are expanded. But the expansion of metal B is more than an expansion of metal A. Due to this dissimilar expansion, the bimetallic strip will bend towards metal A as shown in the figure below.
The strip bends, and the No contact is closed which ultimately energizes the trip coil of a circuit breaker.
The heating effect is not instantaneous. As per Joule’s law of heating, the amount of heat generated, 
the overcurrent flowing through the heating coil of a thermal relay.
R is the electrical resistance of the heating coil. t is the time for which the current I flows through the heating coil. Hence from the above equation, it is clear that the heat generator by the coil is directly proportional to the time during which the over-current flows through the coil. Hence there is a prolonged time delay in the operation of the thermal relay.
That is why this type of relay is generally used where overload is allowed to flow for a predetermined amount of time before it trips. If an overload or over current falls down to normal value before this predetermined time, the relay will not be operated to trip the protected equipment.
A typical application of thermal relays is the overload protection of an electric motor.
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