Thermal overload relays are overload relays that apply the principle of current thermal effect and use electrical thermal bimetallic sheets as sensitive components. The electrical thermal bimetal sheet is made by heating and rolling two alloys with large difference in linear expansion coefficient. When heated, the bimetallic sheet bends from the high expansion layer to the low expansion layer. When the current is too large, the element acts due to "heat", so that its linked moving contact cuts off the power supply of the controlled circuit and the protected equipment.