A thermal overload relay, or thermal electrical relay, is an economical choice of electromechanical protection device that operates based on the principle of current thermal effects. The sensitive element is a bimetallic strip produced by heating and rolling two kinds of alloys with large differences in coefficients of linear expansion. When heated, the bimetallic strip bends from the high expansion layer (active layer) to the low expansion layer (passive layer). If the current exceeds the set value, the bimetallic strip is heat up and bent to close normally open contacts, which will energize the trip coil of the circuit breaker.
Thermal overload relays are vital devices used to protect electric motors form overload, phase imbalance, and rotor jamming. They come in various types, are easy to install and use, and serve multiple functions at a low cost.