Patent GB-723977-A (GB)
Title: Improved method and apparatus for the testing of electric cables
Inventor: KING GEORGE HARRY
IPC: G01R31/08
Filed: 1951-10-25
Abstract: 723,977. Locating insulation faults. SOUTHERN UNITED TELEPHONE CABLES, Ltd. Jan. 23, 1953 [Oct. 25, 1951], No.25022/51. Class 37. [Also in Group XL (c)) A method of locating faults in cable insulation, particularly between the two conductors of a co-axial cable consists in applying an increasing D.C. potential between the conductors until the insulation fails, producing steep-fronted waves travelling to both ends of the cable, the interval between their respective times of arrival enabling the position of the fault to be calculated. The system consists of two pulse-shaping channels, one producing a flat-topped pulse which commences a finite and controllable interval of time after the arrival of the wave front at one end, while the other produces a steep-fronted pulse with an exponential tail commencing at the same time as the wave front arrives at the other end. These two pulses are fed to an indicator unit, which shows when the two pulses start at the same instant, the delay of the first pulse being adjusted until coincidence occurs. Pulse shaping and delaying circuits, Figs. 2, 3.-In the circuit shown in Fig. 2, the incoming wave-front is converted to a positive pulse voltage as shown in Fig. 2A at the anode of V1, which pulse is sharpened by differentiation in the grid circuit of V2. Valves V2 and V3 act as a single shot multivibrator to produce a square pulse, the front of which coincides with the incident pulse, and which is longer than the maximum time difference between arrival of pulses at opposite ends of the cable. This waveform is integrated in the Miller integrator V4, and then applied to the cathode of a diode V5 variably biased from potentiometer 3 so that the output waveform is delayed by an adjustable amount with respect to the input. This delayed waveform is differentiated in V7 and again by the circuit between the anode of V7 and the grid of V8, to give a short pulse at the anode of V8, which is applied to a second multivibrator V9 and V10 giving a flat-topped pulse delayed relative to the incident wave front. In the circuit shown in Fig. 3 the other wave front is fed to a single shot multivibrator V11 and V12 which produces as shown in Fig. 3A a long negative-going pulse at the anode of V12 which is differentiated and inverted by valve V13 to give a short undelayed pulse sharpened in the cathode-follower stage V14. Pulse comparing circuit, Fig. 4.-Each of two similar stages comprises a pentode, whose suppressor and control grids, to which the outputs 4, 5 of the circuits shown in Figs, 2, 3 are respectively connected, are normally biased beyond cut-off. The pentode V17, V18 feeds a negatively-biased diode V16, V19, the output of which is applied to one of the grids of a double triode trigger circuit V15, V20, so biasing the other grid to actuate a relay in its anode circuit. If the pulse from 5 lies within the pulse from 4 pulses are produced in both diodes to actuate both relays, while if they do not overlap neither diode conducts. The two diodes are biased to give outputs with pulses of different magnitudes so that if the trailing edge of the undelayed pulse intersects the front of the delayed pulse between certain heights only one relay is operated. Potentiometer 3 in channel A is adjusted until the delay produces' coincidence of the pulses.