Thermal Unit, Type S, Type A, melting alloy, Class 20

Thermal units are intended to protect motors, controllers, and branch circuit conductors against excessive heating due to prolonged motor over currents up to and including locked rotor currents. Protection of the motor and the other branch circuit components from higher currents, due to short circuits or grounds, is a function of the branch circuit fuses or circuit breakers. Class 20 thermal units respond to overload current conditions according to trip curves. These thermal units will trip in 20 seconds or less on overload conditions. The proper thermal unit is selected using the motor full load current and a selection table. These thermal units have a one piece construction. The motor current passes through a small heater winding within these thermal units. Under overload conditions, the heat causes a special solder to melt allowing a ratchet wheel to spin free thus opening the control circuit contacts and opening the motor contacts.