Patent GB-451768-A (GB)
Title: Improvements in means for testing electric insulation
IPC: G01R31/18
Filed: 1935-01-05
Abstract: 451,768. Locating faults in insulation; cable winding machines. HENLEY'S TELEGRAPH WORKS CO., Ltd., W. T., 11, Holborn Viaduct, London, and SAVAGE, J. H., 13, Danson Mead, Welling, Kent. Jan. 5, 1935, No. 434. [Class 37] [See also Groups IX and XL] In apparatus for testing insulating-material by subjecting it to a voltage sufficient to 'produce a discharge of electricity at weak points in the material, the discharge sets up in a high-frequency circuit high-frequency currents which exercise a trigger action on a low-pressure gaseous discharge tube and start a flow of current through an operative circuit of which the tube forms part. The cable 1, the insulation of which is to be tested is passed from a reel 42 to a wind-up reel 30 through a tubular electrode 2, mounted in an enclosed chamber 21 and connected to a high frequency inductance 6, a tapping on which is connected to the grid of a mercury-vapour valve 8. The valve is given a variable grid bias by means of the resistances 68 .. 70 and the D.C. supply to the anode is taken through a switch 12, drop switch 72, fault counter 73, the coil of relay 9 and the switch 74 of the relay. In operation, both switches 12 and 72 are normally closed, and when a fault is detected a high frequency potential difference is established across the impedance 6 which causes current to flow through the valve 8. This operates the electromagnetic counter 73 and opens switch 74 to break the anode circuit of the valve to re-set the device. The relay 9 may also operate switches 76, 77, the former of which completes a circuit operating an alarm bell 80, a lamp 81 and, when switch 83 is closed, a solenoid-operated marking device 48. The switch 74 may be shortcircuited by a switch 174 when it is desired that the signals given may persist and in this case the machine is stopped when a fault is detected by the switch 77 which interrupts the trip circuit of a circuit-breaker 85 in the supply circuit of the driving motor 27 of the cable-winding machine. The switch 72 and a second switch 88 similar thereto serve respectively to break the anode circuit of the valve and to stop the motor 27 when the bare end of the cable passes the drop member 75. Switch 89 serves when closed to short-circuit switch 77 so that the motor is not operated at the detection of each fault but only at the end of a length of cable. In addition, a solenoid-operated braking device 45 for the supply reel 42 may be provided, which may be operated whenever the motor 27 is stopped. The valve 8 may be replaced by a neon lamp which is connected to a voltage supply of a value just below the normal discharge value of the lamp and which has an external controlling member to which is applied the high frequency voltage across the inductance 6. This controlling member may be a plate placed close to the lamp and connected to the tapping on the inductance 6 or may be a coil surrounding the lamp, in which case the inductance 6 may be used as the coil. The glow of the valve or lamp may serve to indicate the detection of a fault. The cable 1 to be tested is drawn from the reel 42 over a guide roller 23 through the chamber 21 by a draw-off device comprising a capstan wheel 24 driven from the motor 27 and an idle wheel 25 round which the cable is passed over a guide pulley 29 to the wind-up reel 30 which is also driven from the motor 27 through a slipping coupling which limits the tension of the cable between the draw-off device and wind-up reel. The guide pulley 29 is given a reciprocating axial motion by a wormoperated device, and both reels 30, 42, which are mounted in open bearings 41 for ready removal and replacement, are driven through dog clutches.