These NEMA thermal overload relays prompt contactors to break electrical connections with downstream equipment once a set temperature is reached to protect connected loads from overheating and causing damage. Thermal units, also known as heaters, contain either bimetallic strips or melting alloy solder materials. When the current exceeds a predetermined threshold and temperatures rise, the material bends or liquefies, causing the thermal unit to break the circuit and cause the contactor to break its power contacts. As the materials cool down, they will return to their normally closed position and the circuit can be reset automatically or manually.