Q and A: Cable fault location
Q: Can I detect all types of cable fault with a Megger insulation tester?
A: No! Cables can and do fail in many different ways. Some of the faults can be detected with a straightforward insulation test, but many cannot. An insulation tester will not provide useful information about short-circuit, high resistance and flashing faults. To see why, think of a 33 kV cable with a high voltage flashing fault that has a breakdown voltage of 13 kV – that is, the fault only appears if the cable is subjected to a voltage of 13 kV or more. It’s clear that an insulation tester, which will typically have an output voltage of 5 kV or 10 kV, depending on the model, will not be able to detect this fault. In a case like this, a DC hipot test is needed to determine the breakdown voltage.
Q: I have a full short-circuit fault and I’m trying to use Digiphone to pinpoint it but I can’t hear the surge in the headset. What should I do next?
A: Not being able to hear the surge definitely makes it more difficult to pinpoint the fault, but there are several things you can do about this. The first is to use an audio frequency tracing unit. With this, as you follow the route of the cable and pass the location of the fault, you should notice a change in the characteristics of the received signal. Typically, the signal will quickly become weaker or the instrument’s depth display will show a sudden change.
Another approach is to use a Digiphone but look at the electromagnetic signal it receives rather than searching for the sound produced by the surge. Even if the sound is non-existent, the electromagnetic pulse can usually still be detected as the current discharge into the cable will create an electromagnetic field around it. Once the location of the fault has been passed, this electromagnetic field will be much weaker, as there will be little or no current in the cable after the fault. It is easy to see this change as the Digiphone shows the received electromagnetic field strength directly on its screen.
Finally, in some short-circuit faults, the outer jacket of the cable is damaged. In these cases, the normal sheath-fault pinpointing methods can be used.
Q: I have many PILC cables in locations where there’s a lot of moisture in the ground. I find that most of the time the arc reflection method (ARM) of fault location doesn’t provide satisfactory results with these. What’s the best method to use in such cases?
A: The reason for this problem is that damping – a result of the high conductivity of the moisture surrounding the cable – attenuates the returning TDR pulse from the fault location so much that it becomes impossible to detect. Transient methods are more likely to be successful. An example is the ICE or impulse current method, which works by monitoring HV reflections rather than the relatively small pulse produced TDR. Alternatively, Megger cable fault location instruments and Megger test vans support unique techniques such as double surge and ARM Plus. These techniques are demonstrably superior to the standard ARM method and are much better able to identify and locate ‘difficult’ faults.
