The bimetallic disc reacts to excess temperature, or current, by flexing and disconnecting the compressor from the power supply. Some protectors have a resistor in series with the disk to aid in opening the contacts in situations of excessive increase in electrical current. In this case, the protector has three terminals, and two when the resistor is absent.
Each thermal protector is specified to operate at a certain temperature and when a current is applied to its terminals, it must open at a specific time, according to the manufacturers' table.
