The thermal switch may be a bimetallic strip, often encased in a tubular glass bulb to protect it from dust or short circuit. Schematic symbol for a thermal overload switchĪ 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.
#THERMAL FUSE SERIES#
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. 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). 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. Thermal fuses are usually found in heat-producing electrical appliances such as coffeemakers and hair dryers. The elements are conductive and usually consist of binary or ternary fusible alloy of tin, bismuth, antimony, indium, lead, and other metals. The difference against electrical fuse is using the surrounding temperature, instead of the temperature generated by ohmic heating of the fusible element. Īnother mechanism is more similar to electrical fuse, a fusible element that melts when subjected to temperature above its threshold. The Tamura LE series, NEC Sefuse SF series, Microtemp G4A series, and Hosho Elmwood D series, for example, may use thermoplastic pellets that lose strength or melt at specific temperature. When the pellet melts, the spring is released, separating the contacts and breaking the circuit.
One mechanism is a small meltable pellet that holds down a spring.
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. 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 protecting the windings of a small motorĪ thermal fuse is a cutoff which uses a one-time fusible link.
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