This term also applies to a polymeric positive
temperature coefficient (PPTC) device, or as it is more commonly known, a self-resetting
fuse. The fuse can take any shape, as it is made from a
polymer compound. This compound is impregnated with
conductive particles.
In operation, a PPTC fuse conducts electricity through the conductive particles in contact with one another in the polymer matrix. When the current rises to a preset level, the heated polymer expands to the point where the conductive particles are no longer touching one another, cutting off the flow of electricity. When the fuse cools, the circuit is restored.
This is most suitable for devices that often experience excessive heating under extreme usage conditions, such as motor-driven devices, control and monitoring circuits, or in test equipment (where where one must protect against faults in the circuit being tested.) In each case, the device itself does not have an internal fault, but must be kept off until the conditions behind the power surge or high-heat state are removed.