fusible link

Electricity can be a very dangerous thing if the electrical current short-circuits. It can start fires and cause unwary people to be inadvertently electrocuted.

This is why people usually put a fuse in any circuit. Sometimes, however, the current in a circuit is too great to use a simple fuse. A fusible link is the mack daddy of all fuses. It is essentially a piece of braided wire that melts and disrupts the circuit before it can cause anything really bad to happen.

You often find them in the ignition systems of automobiles.

Fusible links are also employed in ventilation systems for fire protection, as part of a device called a fire damper. Where an air duct penetrates a rated wall or fire barrier (or where a pesky building code official may require it), a fire damper will usually be installed. These dampers consist of several spring-loaded hinged blades across the cross section of the duct, held open by a fusible link. When the air temperature gets hot enough (i.e. there is a fire), the link melts and the blades snap shut, closing off the duct and maintaining the integrity of the fire barrier.

Fusible links used for this purpose are generally larger and have much lower melting points than the type used for overcurrent protection as described in the above writeup; they typically fail at around 160 to 210 degrees Fahrenheit (71 to 98 degrees Centigrade). Most of the ones I’ve seen are made out of some type of ceramic material.