GB753557A - Improvements in the testing of the insulation of electric conductors - Google Patents

Abstract

753,557. Testing insulation resistances. SOUTHERN UNITED TELEPHONE CABLES Ltd. May 26, 1953 [May 23, 1952], No. 36972/54. Divided out of 737,746. Class 37. Apparatus for testing the insulation of an insulated conductor by applying a known test voltage to the conductor through a resistor of known value, comprises timing means to determine the initial charging period of the conductor and to operate, at the end of that period, switching means for transferring the circuit from the conductor to the test voltage through a second resistor of equal valve to the first resistor and for completing a circuit to an output signal terminal, and means responsive to the potential relative to earth at the junction of the second resistor and conductor to prevent the completion of the circuit to the output signal terminal when that potential is below a predetermined valve. In the Figure conductor 1 receives the test voltage firstly via resistors R1, R2 and secondly through a single resistor R3 equal to R1+R2 and voltage drawn from the R3/wire 1 insulation resistance junction is applied to the grid of electrometer valve V2 and the V2 output goes into diode V4 through amplifier V3 and if the voltage across R3 is high enough due to the insulation on conductor 1 being below the standard required V4 conducts and triggers dual state valve V5 so that its second anode 15 conducts and operates EX. When the test voltage charges conductor 1 via R1, R2 a multi-vibrator V1 is actuated so that relay AX operates and locks followed by slugged relay BX whose delay period determines the period of charging the. conductor 1 since BX operates quick-acting relay CX which is effective in transferring conductor 1 into connection with R3. The actuation of BX also energizes a slugged relay DX to release BX and CX and if EX has not operated then when DX falls back an output voltage 7 is available and used to operate a relay (not shown) which connects the next conductor to be tested. If EX has operated no output signal is available and the process stops and fault lamp 16 lights up. The operator releases EX by closing RS. The slugged relay BX may be replaced by a known form of electronic timer. The relay arrangement described in Specification 753,471, [Group XIX], may be used for connecting in several insulated conductors to the test circuit in sequence.