A thermal cutoff is an electrical safety device (either a thermal fuse or a thermal switch)—a device designed to keep components safe in situations when an excessive temperature occurs. When the rated operating temperature of the thermal cutoff is exceeded, the device will fail to open in the electrical circuit and cut off the current. These devices may be for one-time use (a thermal fuse) or reset manually or automatically (a thermal switch).
A thermal fuse is a cutoff that uses a one-time fusible link. Unlike a thermal switch which may automatically reset itself when the temperature drops, the thermal fuse is more like an electrical fuse: a single-use device that cannot be reset and must be replaced when it fails or is triggered.
Thermal fuses are usually found in heat-producing electrical appliances such as coffeemakers and hair dryers. They function as safety devices to disconnect the current to the heating element in case of a malfunction (such as a defective thermostat) that would otherwise allow the temperature to rise to dangerous levels, possibly starting a fire.
A thermal switch (sometimes thermal reset or thermal cutout (TCO)) is a device which normally opens at a high temperature (often with a faint “plink” sound) and re-closes when the temperature drops. The thermal switch may be a bimetallic strip, often encased in a tubular glass bulb to protect it from dust or short circuit. Another common design uses a bimetallic shallow dome-shaped cap.
Bimetal thermostats use two different kinds of metal to regulate the temperature setting. When one of the metals expands more quickly than the other, it creates a round arc, like a rainbow. As the temperature changes, the metals continue to react differently, operating the thermostat. This opens or closes the contact opener, turning the electricity on or off as needed. Accuracy and efficiency are important for bimetal thermostats. Bimetal Thermostats which regulate the temperature inside heating pads, blankets, and medical pads, are used in any case where the applications need control for a low temperature.