Patent GB-556884-A (GB)
Title: Improved means for the location of faults in electric cables
IPC: G01R31/08
Filed: 1942-09-17
Abstract: 556,884. Locating faults; current and voltage measurements. HENLEY'S TELEGRAPH WORKS CO. Ltd., W. T., and SAVAGE, J. H. Sept. 17, 1942, No. 13114. [Class 37] Means for locating a fault between the conductor and the metal covering of a cable comprises a source of continuous current with movable connecting means between it and two places a short distance apart on the metal covering of the cable, a resistance connected between the conductor and covering at one end of the length of cable, and a measuring device for indicating the voltage or current in the resistance. The apparatus is made up in two units, a power unit movable along the cable XYZ and a detector unit connected to one end of the cable. The power unit consists of a battery Bland two short conductors CTI, CT2 from its terminals each provided with means for making connections with the metal covering of the cable. A current limiting resistance Rl may be inserted in one or each of the conductors and an indicating lamp 1-1 may be connected across this resistance to show when current is passing. An additional set of connections CT3, CT4 with resistance R2 and lamp L2 may also be provided, a switch S1 being arranged to change over from one pair of connections to the other. A reversing switch S2 is also provided. Connection with the metal covering may be made by pressure point contacts, or by contact wheels or slides. The detector unit may be a high resistance voltmeter, preferably a valve voltmeter as shown. Two similar valves VI, V2 are arranged so that their cathodes are connected together and their cathode-anode paths form two adjacent arms of a bridge, the other two arms of which are formed by equal resistances R9, R10. A battery B5 is connected between the iunction of the resistances R9, R10 and the junction of the cathodes of the valves. A galvanometer G, connected across a resistance R11. adiustable for sensitivity variation, is connected across the anodes of the valves. The input to the detector is applied across a resistance R4 between the cathode and grid of the valve V1. A battery B2 supplies heating current for the cathodes. Across this is connected an adjustable resistance R6 in series with a potentiometer R7, the slider of which connects the resistance R4 to the cathode. The other end of this resistance is connected to the grid of the valve V1 through a bias battery B3. Precise adjustment of the bias is effected by means of the resistance R6 and potentiometer R7. A resistance R8 and bias battery B4 are connected between the cathode and grid of the valve V2. An input filter circuit consisting of a series resistance R3 and a shunt condenser C1 is provided between the cable and the resistance R4 to bypass alternating current. A stabilizing earth connection is provided through a resistance R5. After the connections have been made to the metal covering of the cable but before the current is switched on to the power unit, balance is obtained by adjusting the bias on the valve V1. The switches S1, S2 are then closed and a deflection obtained on the galvanometer G. This may be balanced out by further adjustment of the potentiometer R7 and the tests are repeated until a large deflection is obtained, indicating that the position of the fault has been passed in the movement of the connections along the cable. For the final stage the connections to the metal covering are brought closer together and the current is increased by inserting the resistances R1, R2. which were removed durine the preliminary tests so that the current was limited by the lamps L1, L2.