Polymeric positive-temperature-coefficient (PPTC) devices have long been used on low-voltage devices and systems to protect circuits from overcurrent (short circuits) and overtemperatures.
Now Raychem Circuit Protection, a subsidiary of Tyco Electronics, Menlo Park, Calif. (raychem.com), has developed versions that work with line voltages up to 240 V.
The solid-state components are made of a cross-linked polymer with a conductive matrix of carbon embedded in it. When heated, which can be due to overcurrent or overtemperature, the polymer expands quickly. This, in turn, breaks the vast majority of conductive carbon paths, so resistance becomes very high and current flow extremely low. Operations can reset the device by turning off the power. This lets the polymer cool, compress, and rebuild the carbon matrix. The devices can be tripped and reset at least 6,000 times with little loss of electrical properties. The devices make it possible to incorporate primary-side transformer protection and can be used to help protect motor windings from damage from fault conditions or application overloading.