An electrothermal relay is made up of bimetallic (made up of two metals having different thermal expansion coefficients) strip.
When the current flow through the conductor, it produces heat. Due to which the temperature of the bimetallic strip rises and expands. The metal having high thermal expansion coefficient expands more than the other metal. Due which the strip bends & closes the contacts to usually activate the trip circuitry.
Thermal relays are usually used for electric motor protection by setting Current less than FLC of motor.
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