GB1334892A - Insulation testers - Google Patents

Abstract

1334892 Protective arrangements SPETSIALNOE PROEKTNO-KONSTRUKTORSKOE I TEKHNOLOGICHESKOE BJURO PO ELEKTROBURENIJ 20 July 1971 33887/71 Heading H2K An insulation testing system for a power circuit, e.g. for a submersible motor 1, which circuit includes a string of earthed pipes 2 as one of its conductors, comprises first and second switching arrangements 4, 5, connected in series with the pipes, each switching arrangement including a rectifier 7, 11 and a controlled rectifier 8, 12 connected in parallel opposition, control means for sensing the current through the pipes and operative to cause the first switching arrangement 4 and thereby the second arrangement 5 to isolate the pipes for a predetermined period. and resistance measuring means operative during that period to measure the resistance of the other conductor or conductors to earth. The control means may comprise, a current transformer 6 and a semiconductor switching circuit 10 which produces an output at half the mains frequency so that the controlled rectifier 8 is only rendered conductive on alternate cycles. The conduction of the controlled rectifier 12 is initiated by a control voltage from the secondary winding of a current transformer 13 during the previous negative half cycle and conduction is sustained by the positive half cycle, so that when rectifier 8 is non- conductive the rectifier 12 is non-conductive. During the half cycle in which these rectifiers are non-conductive a unit 29 can determine the insulation resistance. This unit comprises a differential relay 32 and an ohmmeter 33 connected for energization by a voltage transformer 34 in accordance with the line voltage and by a voltage-amplitude sensing device 31 in accordance with the voltage across the non- conducting rectifier 8, this latter voltage being a function of the insulation resistance.