Thermal protection

A thermal cutoff is an electrical safety device that interrupts electric current when heated to a specific temperature. These devices may be for one-time use or may be reset manually or automatically.

A thermal fuse is a cutoff which 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. A thermal fuse is used 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 Tamura LE series, NEC Sefuse SF series, Microtemp G4A series, and Hosho Elmwood D series, for example, may use alloy pellets that contain copper, beryllium, and silver to melt at a precise temperature.

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 and not to excessive current (unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature). For example, in a surge protector thermal fuses may be wired in series with the varistors; when the varistors conduct, the fuse heats up and disconnects the power, which eliminates the risk of fire which can occur when the varistors are overloaded.

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