In past blogs we discussed methods for localizing and pinpointing faults on medium and high voltage electric power cables. These methods focused on the use of high voltage surge generators (thumpers), high voltage radar techniques (arc reflection) plus electromagnetic and acoustic pinpointing. These techniques work well for medium and high resistance faults on shielded cable structures, but do not lend themselves to low voltage cables or pilot cables (signal and control lines).
There are a few reasons for this:
- The maximum voltages that may be applied to these cables do not enable sufficient surge energy to create a strong flashover that can be pinpointed by means of the acoustic method.
- Low voltage cable structures are mainly unshielded and do not provide controlled return paths for the high fault currents created during the “thumping” process.
The Step Voltage, also referred to as Voltage Gradient, method presents itself as an efficient and safe technique for locating faults on direct buried low voltage and pilot cables.
When a fault occurs on a low voltage cable the fault in most cases will flow into the surrounding soil, hence a “ground fault.” The Step Voltage method applies a controlled voltage to the conductor of the faulted cable. When a breakdown voltage is reached a fault current will flow through the fault into the surrounding soil and will travel back to the transmitter. This current flowing through the soil (soil resistance) will create a voltage gradient that can be measured on the surface of the ground. Earth probes and a detector are used to sense the voltage gradient and indicate direction to the fault.
This technique is also very useful for locating sheath to ground faults on jacket medium voltage cables, telephone and CATV cable structures.
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Editor’s Note: This post was originally published April 2012 and has been updated freshness, accuracy and comprehensiveness.