Thermal Fuses
These are the least versatile devices available, but valuable as an added safety back-up to a primary device. The construction of the most common form of thermal fuses basically uses a spring encapsulated into a wax pellet. The pellet is formulated to melt at a given maximum temperature. As the wax melts the spring stretches until it breaks, breaking the circuit in the process. Other types of thermal fuses incorporate the use of a specially formulated solder that effectively melts when exposed to given maximum temperature. These devices are non-resettable and react primarily to changes in temperature. There is some self-heating effect for these devices under higher current loads which may cause the operating characteristics to change over time.