A thermal overload relay is an overload relay that applies the principle of current thermal effect and uses electrical thermal bimetal as a sensitive element, also known as a thermal relay. The so-called electrical thermal bimetal is made of two kinds of alloys with large differences in linear expansion coefficients that are heated and rolled. When heated, the bimetallic sheet bends from the high expansion layer (active layer) to the low expansion layer (passive layer). When the current is too large (exceeds the set value), the element acts due to "heat", so it's linked moving contact cuts off the power supply of the controlled circuit and the protected equipment. Thermal overload relays are used for motor overload protection, and there are many types. The thermal overload relay is easy to install and use, has complete functions, and is low cost. It has been proved by practice that it can reliably protect the motor, so it has always played an important role.