GB681779A - Improvements in and relating to apparatus for and method of testing wire insulation - Google Patents

Abstract

681,779. Detecting and indicating insulation faults. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 14, 1950 [Nov. 16, 1946], No. 27820/50. Class 37. The insulation of a wire is tested by producing R.F. corona between the wire and an adjacent electrode whereby the voltage therebetween acquires a relatively low frequency modulation component as a result of insulation variations as the wire is traversed relatively to the electrode, which is demodulated to operate devices for indicating, recording, or counting weak spots in the insulation. An insulated conductor 1 is earthed and continuously wound past a small electrode 4 by a motor driven takeup reel 5, and other wires 7, 8, 9 may be simultaneously tested by paralleled electrodes 10, 11, 12. A parallel resonant circuit 13 comprising an inductor 14 and series connected condensers 15, 16, 17, 18 connected between the electrodes and earth is energized at its natural frequency (e.g. 400 kc/s.) by an inductively coupled oscillator 20 to develop a voltage of the order of 5000 volts which produces R.F. corona between each electrode and the associated wire. The variations in the insulation of the wires traversing the electrodes set up corresponding variations of the corona intensity, which modulate the voltage across the resonant circuit at a frequency of the order of 10 kc/s., the modulation amplitude being small when the insulation is good and increasing on the occurrence of weak spots. A portion of the modulated resonant voltage is tapped off between condensers 17 and 18, gridleak rectified by a triode 28 whose anode circuit is tuned by an inductor 36 and parallel condenser 37 to e.g. 10 kc/s., and the resultant modulation signal is again gridleak rectified by the triode 40 whose anode circuit is supplied from source 32 through a resistance 47 in series with a gas discharge lamp 46 shunted by a capacitor 48. When the insulation under test is good, the signal input to triode 40 is small and the valve passes anode current sufficient to energize the lamp 46, but the passage of poor insulation increases the input, reduces the anode current and extinguishes the lamp, while the sudden voltage rise at point 49 indicating the arrival of weak insulation energizes the grid of the normally cut-off triode 51 to operate a control relay 56 to e.g. stop the passage of the wire, a recorder 60, and a counting device 62 which may stop the wire after a predetermined number of operations by weak spots. The sensitivity of the discharge lamp is adjustable by condenser 48.