Faults on underground power cables are almost always disruptive and costly to fix. So there’s a very big incentive to minimise the risk of such faults occurring. One of the most effective ways of doing this is to use partial discharge (PD) testing, which looks for the characteristic electromagnetic signature of produced by small electrical discharges in air gaps, voids and cavities in cable insulation, and in joints and splices. These discharges typically start long before these problems develop into a fault, so detecting them early gives time for action to be taken to avert outright failure.

A large percentage of failures on new cables are attributable to poor installation work. PD testing before a new cable is energised will reveal many of the issues, such as improper or incomplete stripping of insulation, improper shrinking of insulating sleeves, or nicks and cuts in the insulation. The faults can then be rectified, almost always at the installation contractor’s expense, before the cable enters service. PD testing is also an invaluable way of revealing similar issues after repairs have been carried out on a cable.

Regular PD testing on in-service cables provides useful data that can be trended or compared with data from similar cables to flag up changes that need further investigation. It is also an excellent tool for evaluating suspect cables.

While no test method can reveal every incipient fault, PD testing is undoubtedly the best form of “insurance” against cable faults that’s currently available. And, considered in the light of the financial impact of a typical cable fault, it’s insurance well worth buying.

Author: Tony Walker, Applications Engineer for HV Cable Diagnostics and Fault Location at Megger

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