A thermal cutoff is an electrical safety device that interrupts electric current when heated to a specific temperature.
A thermal fuse is a cutoff which uses a one-time fusible link. Unlike the thermal switch which automatically resets 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. A thermal fuse is most useful when the overheating is a result of a rare occurrence, such as failure requiring repair (which would also replace the fuse) or replacement at the end of service life.
One mechanism is a small meltable pellet that holds down a spring. When the pellet melts, the spring is released, separating the contacts and breaking the circuit. The NEC Sefuse SF/E series, Microtemp G4A series and Elmwood D series, for example, use pellets that contain Copper, Beryllium, and Silver.
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.
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. Such an arrangement may be found in a surge protector. The thermal fuses are wired in series with the varistors; when the varistors conduct, the fuse heats up and fails, which eliminates the risk of fire which can occur when the varistors are overloaded.
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 from dust 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.
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.