GB1351654A - Cable fault location - Google Patents

Abstract

1351654 Pulse-echo cable testing systems M A McFERRIN 1 Oct 1971 [1 Oct 1970 9 Aug 1971] 45818/71 Heading H4D In a method of locating a fault in an insulated cable a voltage sufficient to initiate and maintain ionization of insulating material in the fault region is applied to ensure complete breakdown; and a D.C. pulse, having a short rise time and of short or long duration, is applied to the cable concurrently with a point in the region of the peak of the current due to the ionizing voltage, the time interval between transmission of the pulse and reception of the fault reflection pulse being utilized to determine the fault location. Alternatively if the propagation velocity of the pulse is not known the above time interval may be compared to the time interval for reception of a reflecting pulse from the far end of the cable. Ionizing voltage is supplied from 14 and passed to a current limiter 35. A controller 38 closes solenoid switch 37 to apply the ionizing voltage to cable 10 via fuse 33. A current detector 49 signals when the current reaches a level indicating ionization and causes the controller 38 to enable pulse generator 20 to apply a D.C. pulse to the cable. A coupling network 58 and balance network 61 ensure that the pulse is not fed into the receiving circuitry 41. The pulse enters the cable via network 32. The pulse generator also applies a signal to start counter 43, delay 60 ensuring that the count does not start until the pulse has entered the cable. The signal also triggers an oscilloscope 44a. The reflection pulse is passed to switch 31 which may be in the position shown or may be coupled to compare network 39. In the latter position like reflections due to topping &c. from cable 10 and the comparison cable cancel out so that only the fault reflection pulse is passed to the receiver. However the fault reflection pulse may still be distinguished with the switch as shown. The reflection pulse passes through a circuit serving to protect the receiver from over voltages, a high pass filter to eliminate low frequency noise and a pulse shaper. The pulse is applied to counter 43 through gate 42. The gate opens after a predetermined time interval after sending the D.C. pulse in order to ignore early reflections. The count is stopped by the pulse and may be read from display 44. Are - set pulse is also fed to controller 38 thus de-energizing solenoid 36. In addition the reflected pulse may be observed on oscilloscopes 44a. Oscillators 45, 46, 47 are provided so that different cables in each of which the D.C. pulse has a particular velocity may be tested.