GB888327A - Improvements in means for measuring self and earth impedance of an electricity supply system - Google Patents

Abstract

888,327. Impedance testing ; measuring instruments. BAGGOTT, A. J. June 12, 1958 [June 13, 1957], No. 18666/57. Class 37. Apparatus for measuring the self and earth impedances of an A.C. supply system comprises a hot or cold cathode gas-filled electric discharge tube across line and earth or line and neutral which is normally inoperative but is triggered automatically to pass pulses of heavy current from the supply for discrete intervals of time at a regular frequency equal to or a sub-multiple of the mains frequency; the current therein developing a voltage which is integrated and measured to indicate the impedance between the test connections. In Fig. 2, terminals L, N, E are respectively connected to consumer's line, neutral, and earth cables, and L, N are connected through changeover switch S3 and heavy wires to capacitance C10 feeding the anode of a normally inoperative but intermittently triggered discharge tube V5, and a switch enabling the cathode thereof to be connected over resistance R16 and resonance swamp inductance L, to N, E, or N and E in parallel. Capacitance C10 is shunted by interpulse discharge resistance R15, and rectifier REC 3 provides a further discharge path. The pulsed voltage drop across R16 is integrated by capacitance C14, resistance R20, and rectifier REC5 to charge capacitance C9 to a unidirectional voltage representing the integrated discharge tube current value independently of the waveform thereof, which capacitance discharges through resistor R21 and scale control rectifier REC6 into a milliammeter indicating the test impedance value. A supply transformer energized across L, N over switch S 1 supplies a stabilized anode supply rectifier circuit, and on negative halfcycles charges capacitance C15 over resistor R31 and rectifier REC7 to excite the anode of thyratron V4 through the primary of transformer T2, whose secondary is connectible to two of the moving contacts of three pole two-way switch S2 and in one switch position excites a gas-filled decade counter tube V3 and thyratron V6, and in the other position is applied between the grid and cathode of discharge tube V5 whose screen is excited also from the supply transformer over resistance R18. In the first switch position, decade discharge tube V3 is triggered on each operation of thyratron V4 through transformer T2 to transfer its discharge from one cathode to the next, and the cathodes are connected to trigger thyratron V6 on each fifth operation of the thyratron, whereby V5 is correspondingly triggered, while in the second switch position V5 is triggered directly once per operation of thyratron V4. The grid of the latter is excited from a network of resistances R6, R7, P1, R29, capacitance C 4 , and rectifier bridge REC4 energized from the supply transformer, whereby the voltage at the junction of R6, C4 is similar to that at the anode of the discharge tube, and the slider of potentiometer P1 is adjusted so that V4 fires at about 180 v. on positive halfcycles to compensate mains voltage variation. Diode V7 in series with resistance R32 across charging capacitance C15 stabilizes the thyratron anode voltage. Resistance R23 in series with RC network R24, C11 between neutral and cathode of V5 protects the latter when its discharge circuit is completed through a faulty earth connection. The Provisional Specification describes a form of the invention wherein the decade gasdischarge tube is operated direct from the supply transformer once per positive half-cycle to trigger a thyratron once per operation (or once per sub-multiple of operations) to trigger the test discharge tube, the integrated voltage across whose cathode resistance is diode rectified, integrated, and the resultant voltage potentiometrically compared with a variable. stabilized voltage source.