Abstract: This invention relates generally to underground electric power distribution systems. In particular, it relates to underground loop feed circuits which include reclosers and transformers and a plurality of high frequency actuated normally closed isolator switches for automatically locating and sectionalizing portions of the loop circuit in which faults originate. The actuator switches between which a fault occurs are armed in response to a fault current and only those switches which are armed are opened thereby isolating the faulted section of cable. The remaining actuator switches remain closed.

Abstract: A solid state inverse overcurrent relay having a long slender wire connected to respond to a current which is proportional to the line current in a power system. An infrared detector or phototransistor is positioned to sense a predetermined color of radiation from the wire and is connected to fire a silicon controlled rectifier which is connected in series with a trip coil for a circuit breaker. The trip coil and silicon controlled rectifier are connected to an independent DC power source. The long slender wire on energization responds to the current which is proportional line current with the same time current characteristic as a fuse. In an alternate embodiment of the invention the relay is connected to the power system through a bridge rectifier which provides the DC current for operating both the heat responsive wire as well as the infrared detector.