Thermal Fuse

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A thermal cutoff is an electrical safety device that interrupts electric current when heated to a specific temperature.
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 is a bimetallic strip, often encased in a tubular glass bulb to protect it fromdust or short circuit. Unlike the thermal fuse, it is reusable, and is therefore suited to protecting against temporary situations which are common and user-correctable. Thermal switches are used in power supplies in case of overload, and also thermostats in heating and cooling systems.

Another type of thermal switch is a PTC thermistor, these thermistors have a 'switch' temperature at which the resistance suddenly rises rapidly, limiting the current through the circuit.

Thermal switches are included in some light fixtures, particularly with recessed lights, where excessive heat is most likely to occur. This may lead to "cycling", where a light turns off and back on every few minutes. Christmas lights take advantage of this effect. Flasher bulbs interrupt power when heated. Twinkle/sparkle mini-bulbs momentarily shunt current around the filament.

Thermal switches are part of the normal operation of older fluorescent light fixtures, where they are the major part of the starter.