A thermal cutoff is a device used to break an electrical circuit when it reaches a certain temperature. By cutting off electrical power at certain temperatures, this type of device protects the other components of the circuit from damage. It is available in two varieties: thermal fuses and thermal switches. The thermal fuse is a consumable thermal cutoff that is used one time and then replaced. The thermal switch can be reset after it cools down to a safe operating range.
Thermal fuses are used to protect circuits that only overheat as a result of rare malfunction. They are commonly used in household devices, such as hair dryers and kitchen equipment. A thermal fuse usually consists of a pellet of metal and a spring encased in a class tube. As the circuit overheats, the pellet melts and the spring pushes the electrical contacts apart, breaking the circuit. The thermal fuse differs from a traditional fuse in that it is only activated by temperature and not by excess electrical current.
The thermal switch is used in circuits which are more likely to overheat as a part of their normal operation. This switch may also be used in circuits that overheat as a result of correctable user error. Common applications of the thermal switch include home electronics, workshop equipment, fluorescent light fixtures, and power supply units. The thermal cutoff device is also called a reset switch, because it can be reset after the device cools to a safe operating temperature.
As a component of an electrical circuit, the thermal cutoff device provides protection from breakdown of wiring insulation, component damage, and fire. Usually, this type of device is manufactured with exacting specifications that determine exactly where the particular thermal cutoff's shut-down point is. This shut-down point is called the device rating. The appropriate cutoff device rating for use in a system is decided by determining the highest safe operating temperature of the individual components. The lowest of these temperatures is the correct thermal cutoff rating.
In some instances, the thermal switch's ability to reset itself is intentionally used as part of the circuit's functional design. Flashing Christmas lights are one of the best known examples of this type of design. As the metal strips in the thermal cutoff heat up, the thermal switch opens, breaking the circuit. This causes the lights to blink off and cool, resetting the switch. This process is called cycling.