Thermal cutoff
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A or thermal cutoff is an electrical safety device that is designed to interrupt electrical current flow when heated to a specific temperature.
[edit] Thermal fuse
A thermal fuse is a cutoff which uses a one-time fusible link. Unlike the thermostat which automatically resets itself when the temperature drops, thermal fuses are more like their cousins, the electrical fuse: a single-use type of device that cannot be reset and must be replaced when it fails or is triggered. These are most useful when the overheating is a result of a rare occurrence, such as the need for repair (which would also replace the fuse) or the end of service life.
Thermal fuses are usually found in heat-producing electrical appliances such as coffeemakers and hair dryers as a safety device to disconnect the flow of current to the heating element in case of a malfunction (such as a defective thermostat) that allows the temperature to rise to dangerous levels, possibly starting a fire.
Unlike electrical fuses or circuit breakers, thermal fuses only react to excessive temperature, not excessive current, unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature.
[edit] Thermal switch
A thermal switch (sometimes thermal reset) is a device which normally opens at a high temperature (often with a faint "plink" sound) and the re-closes when the temperature drops. The switch is a bimetallic strip, often encased in a tubular glass bulb to protect it from dust, or from touching another object which could bend it and cause malfunction 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 some light fixtures, particularly with recessed lights, where excessive heat is most likely to occur. This may lead to "cycling", where a light turns on and off every few minutes. Thermal switches are also used in power supplies in case of overload, such as in the power packs of model trains.
They are also part of the normal operation of older fluorescent light fixtures, where they are the major part of the starter. Christmas lights also have flasher bulbs which interrupt power when heated, or twinkle/sparkle mini-bulbs which momentarily shunt current around the filament.
