Post installation cable test: a practical approach

Damon Mount - UK power manager

An essential stage in the commissioning of any power cable is testing prior to energisation. Properly performed, this test will provide the information needed to ensure that any problems that have occurred during the manufacture or installation can be corrected before they lead to costly and disruptive faults. But what is the best way to approach post-installation cable testing?

Without any shadow of doubt, the first step is to get the preparation right and, if you are using a contractor to carry out the work, this means choosing one that can unequivocally demonstrate that they know what they are doing. This may seem obvious but there are good reasons to be wary and, although this sounds paradoxical, one of the main underlying reasons is that modern cable test sets, such as those in the Megger range, are very easy to use.

This is undoubtedly a good thing overall, but a possibly unexpected side effect is that contractors with little or no cable test experience may believe that if they hire one of these test sets and quickly peruse the manual, they’re ready to offer cable testing services. They are not. There is much more to cable testing than simply knowing how to operate the test set, especially if the work is to be carried out safely and to deliver accurate results.

An experienced contractor like Elmeridge Cable Services will first formulate a safe method of work, and will manage the test area effectively and efficiently. It is important to keep anyone who is not involved with the testing away from the area, remembering particularly that there’s no visual difference between an energised cable and a de-energised cable.

This doesn’t just mean keeping members of the public away as colleagues acting as onlookers can be even more hazardous – they are the ones who are most likely to poke at the test set in an attempt to be helpful, and to offer distracting comments and suggestions.

The next step is to prepare the cable for test and, in particular, to prepare the cable ends so that, when carrying out a sheath test, there will be no flashover between the metallic sheath or screen and the semiconducting oversheath. Checking that the semiconducting material has been properly removed by carrying out a quick and simple test using an ordinary handheld insulation tester is always worthwhile.

It is now necessary to remove any cross-bonding links that are used in the cable system and it is essential to allow sufficient time for this. It is by no means unusual for a single link box to contain 30 or even 45 links. Removing these prior to testing and refitting them after testing will involve hours of work, something that those setting deadlines for testing often fail to realise.

With preparations complete, testing can commence, but there is another essential factor that needs to be considered – the right test equipment must be available on site. This doesn’t just mean the test set that will be used for carrying out basic sheath testing, but also the additional equipment that will be needed to diagnose and locate the fault, if one should be found.

Few cable operators will be happy about the delays involved in returning to base to fetch more test equipment – it needs to be on site already, so that faults can be dealt with quickly and effectively.

This is another area where experienced cable testing contractors score. Since they carry out cable testing regularly, it is worth their while investing in a full range of test equipment and, just as important, in training their staff to use it safely and effectively. Companies that carry out cable testing relatively infrequently are unlikely to be able to match this level of investment and commitment.

The test voltage should, of course, be applied in line with the appropriate standards and specifications, and a good suggestion is that the test results should be written down on paper, even if they are stored in the instrument. The paper records may hardly ever be needed, but  they are a simple and inexpensive precaution against stored results being accidentally erased or overwritten.

If all has gone well, it only remains to discharge the cable, issue the test certificate and, in the case of a contractor, send the client an invoice. Never rely on the test set alone, however, to discharge the cable! Even if the test set is indicating zero volts it takes only a few seconds to make sure the cable really is discharged by using a discharge stick. Failing to do so could, in the worst case, lead to a fatality.

What about those cases where a fault is found? The first thing to do is to STOP and assess the situation. Turn off the test equipment, discharge the cable and check that all connections at the local and distant ends of the cable have been made correctly. Re-test and, if the fault persists, decide on a testing strategy that will allow the fault to be diagnosed and located with the minimum risk of causing further damage to the cable.

This will typically involve the use of TDR to pre-locate the fault, and then a pin-pointer to find its exact location. Once again, it’s important to keep the crowds away, especially when using an acoustic pin-pointer, as footsteps in the vicinity can make it much more difficult to determine the precise fault location.

Accurate, dependable results are essential because, for the fault to be remedied, the cable will need to be excavated. This is a costly and time consuming task, so the risk of digging in the wrong place must be eliminated as far as possible.

It is always worth bearing in mind that post-installation sheath tests are relatively inexpensive to carry out and they are an excellent way of guarding against potentially costly future problems. A vital point, however, is to resist the temptation of opting for cheap solutions.

If you carry out the work yourself, make sure that you have the right equipment, that it’s calibrated and in full working order and that your staff are properly trained. Allow plenty of time for the work to be carried out – rushing work significantly increases the risk of making mistakes and, in cable testing mistakes can be fatal.

If you subcontract the work, choose your contractor with care – opting for the lowest price may well turn out to be false economy. Ensure that your chosen contractor has a proven track record in cable test and, bearing in mind the points discussed in this article, check that they will be able to cope efficiently and safely, even if the work turns out not to be straightforward.

Post-installation cable testing, properly carried out, is an excellent and very affordable form of insurance. But as with any other form of insurance it pays to check the details carefully and to avoid automatically opting for the cheapest option, unless you’re really sure that it meets your needs.