GB1234942A - Improvements relating to the location of faults in the insulation of electric conductors - Google Patents

Abstract

1,234,942. Electric tests. BRITISH INSULATED CALLENDER'S CABLES Ltd. Aug.23, 1968 [Aug.25, 1967], No.39280/67. Heading G1U. Faults in the insulation of an electric cable 10, Fig. 1, screened with a semi-conductive sheath are detected by passing the cable lengthwise through an elongate insulating vessel 1 filled with an insulating liquid 4, containers 5, 6 filled with an earthed conductive liquid 7 being provided at each end of the vessel to earth the screen. The cable conductor is earthed and a high voltage is applied to annular electrodes 2, 3, disposed near the centre of the vessel 1, through resistors R1, R2, Fig.2. The electrodes are also connected through respective capacitors Cb1, Cb2 to the ends of a transformer primary winding L3, L4 whose centre tap is earthed. Resistance and capacitance are connected in parallel with each half of this winding and adjusted so that the circuit is normally balanced and no voltage induced in the secondary winding L1. A void in the insulation of the cable will initiate electric discharges as that portion of the cable passes through each electrode in turn, and this will unbalance the circuit and cause an indication to be given on an indicator 16 connected through an amplifier to the secondary winding. Alternatively the balanced circuit described in Specification 1, 071, 050 may be used. The vessel 1, Fig. 1, may be a plastics tube provided at each end with a liquid-tight semi-conductive seal 11, 12, made, for example, of a conductivelyloaded elastomer. The insulating liquid 4 may be deionized water and the conductive liquid 7 mercury, sodium chloride solution or tap water. A chain earth electrode may supplement the liquid earth electrodes 7.