In melting alloy thermal overload relays, the motor current passes through a small heater winding. Under overload conditions, the heat causes a special solder to melt allowing a ratchet wheel to spin free thus opening the control circuit contacts. When this occurs, the relay is said to “trip”. To obtain appropriate tripping current for motors of different sizes, or different full load currents, a range of thermal units (heaters) is available. The heater coil andsolder pot are combined in a one piece, nontamperable unit. Melting alloy thermal overload relays must be reset by a deliberate hand operation after they trip. A reset button is usually mounted on the cover of enclosed starters.