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Diagnostic Testing – Should I do it?

November 12, 2010

Q:

Our engineering department is having a debate about testing our cables. I plan on testing my URD cables in advance of injection. I am hoping to use the test results to help me determine which cables need injection, to prioritize the work and keep costs to a minimum. I was told that the testing won’t necessarily provide me with useful data. Can testing be used effectively for this purpose? I was also told that cable companies are not in favor of their cables being exposed to multiples of rated voltage. The testing can go as high as 2.5 times the rated voltage. What are the potential risks of testing to these levels?

A:

To put it bluntly, diagnostic testing of URD cables is a waste of time and money. The Novinium specialists have tried diagnostic testing, followed the developments closely, and been disappointed by the results.

The attraction of diagnostic testing is understandable. If it were possible to distinguish the good from the bad, for a reasonable price, of course we all would be diagnosing like crazy. And for many feeder cables, the economics of diagnostic testing might make sense. But the economics preclude the use of most dielectric diagnostic technologies for lateral underground residential distribution. (For more on the subject, see Diagnostic Testing of Stochastic Circuits, by G. J. Bertini, IEEE Electrical Insulation Magazine, March/April 2009).

All offline diagnostic testing is destructive. Subjecting old cables to voltages or frequencies other than operating voltages or frequencies will accelerate or cause failures. Just taking a cable offline can be a damaging perturbation.

Here is a fact that you can take to the bank: 99 percent of the cables that have been rejuvenated are still in service after more than two decades. Over 99 percent of those were never tested before being treated. The capital cost of rejuvenation is only slightly more than the O&M cost of testing. The longevity of rejuvenation is measured in decades; diagnostic testing has to be repeated every few years. It’s simply cheaper to treat.

The most reliable diagnostic is also the least expensive. For more information, refer to Diagnostic Testing of Stochastic Circuits.


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