Diagnostic Testing – Should I do it?
Dear Ms. Frog,
Your wisdom is needed to help with an ongoing debate in our engineering department regarding testing of our cables. I am planning on testing my URD cables in advance of injection. I am hoping to use the test results to help me determine which cables are in need of injection, to prioritize the work and keep costs to a minimum. I am being told that the testing will not necessarily provide me with useful data. Can testing be effectively used for this purpose? I am 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 2.5X rated voltage. What are the potential risks of testing to these levels?
Testy in the NE
Diagnostic testing of URD cables is a waste of time and money. Believe me, my colleagues and I at Novinium have tried. We have followed the developments closely and have been disappointed by the results. The purveyors of such testing have a seductive song. If one were able to distinguish the good from the bad, for a reasonable price, of course we all would be diagnosing like crazy. There are plenty of feeder cables where at least the economics might make sense, but the economics preclude the use of most dielectric diagnostic technologies for lateral underground residential distribution. One of my learned colleagues wrote an excellent paper on the subject, titled “Diagnostic Testing of Stochastic Circuits” published in the Spring issue of IEEE Electrical Insulation Magazine. You can read that article at …
With the exception of online testing, all offline diagnostic testing is destructive. No matter how hard you wave your flippers, subjecting old cables to voltages or frequencies other than operating voltages or frequencies will accelerate or cause failures. Heck, taking a cable offline can be a damaging perturbation. The actual destructiveness of the proposed off-line test is best estimated by how many times the sales guy proclaims the test is non-destructive!
The most reliable diagnostic is also the least expensive. You have to read the “Diagnostic Testing of Stochastic Circuits” paper to learn that secret.
Did you ever wonder why the testers always find that about 70% of your cables are good and 30% are bad? The reason is simple. If they told you 90% of your cables are bad, then you would say, “Why do I need you?” No repeat business. If they told you that 10% of your 40-year-old cables were bad, you would wonder what they had been smoking. The answer has to be 30% bad or there is no business case for testing! No revenue. No job.
Here is a fact you can go to the bank with: 99% of the cables that have been rejuvenated over more than two decades are still in service. Over 99% of those were never tested before being treated. Since we know that at least 30% had to be bad (wink; wink), rejuvenation has effectively stopped 29% of the entire population of cables from failing. Rejuvenation’s capital cost is only slightly more than testing’s O&M cost. Rejuvenation’s life is measured in decades; diagnostic testing has to be repeated every few years. It’s cheaper to treat.
One last thought, so I can tell some of my friends, “I told you so!” Some utilities are capitalizing the costs of diagnostic testing … that will work until the first FERC audit. What respectable accountant would dare to suggest that diagnostics are anything other than maintenance?
Test the water before you jump in,