GB118902A - Improvements in Detective and Protective Devices for Electric Cables. - Google Patents

Abstract

118,902. Beaver, C. J., Richards, A. F. W., and Claremont, E. A. Sept. 22, 1917. Protective arrangements for alternating- current systems, in which the cable contains an insulated auxiliary or test conductor surrounding the main conductor and concentric with t h e outer covering, comprise means for connecting the test conductor to earth through a battery and circuitbreaker trip coils in series when relative disturbance of the currents in the main conductors occurs, so that should a fault allow leakage between the test conductor and the outer covering, the trip coils are energized. Means are also provided for connecting the test conductor, at will, to earth through instruments for testing the insulation between the main and test conductors. The test conductor is preferably in the form of flat metal tape spirally wound over the insulated main conductor and preferably insulated from the outer covering in the manner described in Specification 22355/12, [Class 36, Electricity, Conducting &c.]. The circuit-breakers are of such construction and adjustment as to operate only by passage of a direct current of predetermined strength. At each end of a feeder section, a balancing-transformer g<1>, g<3> has its primary coil connected to the main conductors, and its secondary h<1>, h<3> in series with a solenoid i, i<3> adapted to close a switch connecting the test conductor d to earth through the main circuitbreaker trip coil l', l<3> and a battery m<1>, m<3>. The batteries m<1>, m<3> are in opposition, so that no current flows by merely earthing the ends of the test conductor. Should a fault allow leakage between the test conductor and the outer covering, the coils l<1>, l<3> are energized and the circuit-breakers w<1>, w<3> operated. Reactance coils may be included in the circuits between thetest conductor and earth, or may be situated at o<1>, o' and arranged to constitute primary coils of transformers, the secondaries of which are in circuit with solenoids p<1>, p<3> and signalling-devices v. Alternating-current leakage to the test conductor thus gives a signal and energizes the solenoids p<1>, p<3>, which operate switches to close circuits, including the batteries and trip coils, so that'the circuit-breakers w<1>, w<3> are operated. The ends of the test conductor may be connected to earth through plate gaps n', n<3> arranged to break down in the event of the potential of the test conductor rising above a predetermined value by leakage from the main conductors. Switches q, r are provided by which the test conductor may be earthed through an electrostatic volt-meter s, volt-meter or ammeter t, or shunted galvanometer u.