Thermal fuses are safety devices installed in electrical appliances which are designed to interrupt the circuit if the temperature rises to a dangerous level. Heat-producing devices such as stoves, microwaves, and hair dryers often have thermal fuses, and they can also be installed in a wide variety of other electrical circuits, such as light switches. These devices are usually intended as failsafes which will kick into place when other electrical safety measures such as circuit breakers fail to operate.
The design of a thermal fuse includes a component which responds to heat, such as a plastic pellet. When the component is activated, the circuit is pushed open, preventing power from reaching the appliance. Thermal fuses are single use safety devices, which means that they need to be replaced after they have activated, and they should never be replaced without determining the reason the fuse failed. This may necessitate the services of a repair person to examine the appliance and address the problem, and in some cases, it may be time to discard the appliance altogether.
When a thermal fuse fails, in addition to examining the appliance to look for problems which could have led to overheating, it may be advisable to examine the circuit the device is connected to as well. An electrical problem in the wiring of a house, for example, could trigger overheating of an appliance without necessarily tripping the circuit breakers, and the problem could develop into a more serious one if it is not addressed. If appliances plugged into the same circuit repeatedly fail, this suggests that there is a problem with the circuit, not the appliances.
Replacing thermal fuses is generally very easy, with most hardware stores stocking a range of thermal fuses for this purpose, and they can be clearly seen in the wiring for the appliance. For people who are not comfortable with electrical work, it is a good idea to ask an electrician to do the work, as he or she can also check the appliance while replacing the thermal fuse. As always when working with electrical components, people should make sure that power to the circuit they are working on is cut off while they work.
An alternative to a thermal fuse known as a thermal switch or thermal reset is designed for multiple uses. In this case, the device opens the circuit when the temperature is too high, and closes it when the temperature drops back down. Certain types of circuits can grow warm naturally, making a thermal switch more useful as a safety device.