Thermal Fuses

Thermal fuses are a form of circuit protection device. Rather like an electrical fuse, a thermal fuse is a one-time, single-use device which cannot be reset but must be replaced if it is triggered or fails. A small pellet, made of copper, silver or beryllium, holds down a spring. When the pellet melts because of excessive heat, the spring is released, separates the contacts thereby breaking the circuit and causing the cut off of power. Unlike electrical fuses, thermal fuses react to excessive temperature rather than to a surge of current. As a result, they are extremely effective in preventing electrical fires caused by product malfunction and/or overheating. There are two main designs of thermal fuses. An axial thermal fuse has the thermal element enclosed in a ceramic body while a radial thermal fuse has a phenolic body. The choice of which to use depends on the specifications of the device. Thermal fuses are found in appliances which produce heat as part of their function such as coffee makers and hair dryers. They can also be a component in surge protectors where they are wired in with the varistors. When there is a power surge, they heat up and disconnect the power.