Protection of certain electrical systems by use of PTC device

Claim 1

An electrical system comprising: (a) a power circuit comprising a load, a PTC device connected in series with the load and means for connecting the power circuit to a direct current or alternating current power source; and (b) a control circuit which includes at least one resistive heating element and the current through which becomes significant when a fault condition occurs in the power circuit; said PTC device being in a low temperature low resistance state under normal steady state operating conditions of the system and being thermally coupled to said resistive element by a metal strap which at least partially encircles said resistive element and extends over at least a portion of the surface of said PTC device, the PTC device and the resistive element being such that (1), when the power circuit is connected to a power source and a fault condition develops in the power circuit, current flowing through the control circuit causes heating of said resistive element, and the heat generated by said resistive element rapidly raises the temperature of the PTC device, causing the PTC device to change to a high resistance state thus substantially reducing the current through said load in the power circuit; and (2) when said fault condition is removed and the power circuit is disconnected from the power source, the PTC device cools to a low temperature, low resistance state, and reconnection of the power circuit results in restoration of normal steady state operating conditions.