Open cable locating for sheathed cables

摘要

Open circuit faults in a cable are located by applying a step function DC voltage to one end of a conductor along the cable, for example the metallic shield along a fibre optic cable. The DC current along the conductor is measured over a sampling period and integrated to determine the capacitive charge on the cable. The distance between the end of the conductor and the open circuit fault is computed from the calculated charge and the known capacitance per unit length of the conductor. The DC current in the conductor is measured by sampling the current at a predetermined sampling frequency. The sampling frequency is preferably an integral multiple of the local AC mains power frequency, either 60 Hz or 50 Hz to eliminate the effect of induced AC voltages in the calculations. To eliminate the effects of a resistive fault, the method may include the steps of determining the conductor resistance and subtracting from the calculation of capacitive charge a DC fault current that is calculated from the conductor resistance and the DC voltage. An apparatus for carrying out the method is also disclosed. <IMAGE>