The decay method
Cable fault pre-location method
Most faults on medium- and even high-voltage cables can be traced using the available surge voltages of up to 32 kV. However, with intermittent faults (i.e. voltage-dependent faults), this voltage may not be sufficient to trigger a breakdown, meaning that the fault location cannot be determined in this way.
In such cases, the decay method can achieve the desired result. In this procedure, the cable is connected to a test voltage source and the capacitance of the cable charged up until the voltage causes a breakdown.
In the decay method, the time domain reflectometer evaluates the voltage wave that oscillates between the fault location and the voltage source after the breakdown. A capacitive voltage divider is used for decoupling.
The evaluation is as simple as that for the ICM method and is made easier by the IRG. The operator marks two sequential positive voltage peaks, signal edges or zero crossings, for example, in the evaluation diagram and reads off the distance. The difference between these two values, divided by 2 and minus the test leads, gives the distance to the fault location.
Since, due to the high output impedance at the voltage source, the voltage is only reversed at the fault location, the device calculates the displayed distances.
Overview of cable fault location
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