Importance

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Abstract

A self-protecting current limiting positive temperature coefficient (PTC) resistive element arrangement protects a load circuit coupled to a power line via an actuator. Two or more PTC sections having temperature sensitive resistances are thermally coupled and electrically insulated. The PTC sections have low electrical resistance at low temperature. At least a first of the PTC sections is in series with the load and is resistively heated by excessive load current. At least one other section, thermally coupled to the first, controls the actuator by the change in its resistance induced by heat from the first section. In a three phase embodiment, five PTC sections are stacked. The first, third and fifth sections are in series with the power line phases and the load. A current fault causes excessive current flow to the load circuit, heating and increasing the resistance of the first, third and/or fifth PTC sections. This tends to block current flow by increasing the resistance across the power line, and shifts the voltage drop from the load circuit to the protective PTC. The second and fourth interleaved current limiting PTC sections are heated by conduction. The second and fourth PTC sections are electrically coupled to control the motor starter or other actuator. Load current responsive heating of any of the first, third or fifth sections increases the internal resistance of the second and/or fourth sections by thermal conduction, causing a transition of the second and/or fourth sections into the high resistance state to disable the actuator and protect against further heating.

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First Claim