11. May 2011 19:44
Middle East Query – Diagnostic Testing Timing
Dweller of the Desert asked 22 questions in his post …
Middle East Query – 22 Questions.
In this installment I address question 13.
13. Can the customer test the cable right after injection? Could it be done immediately? If not, after how many days, weeks or months?
You are not going to like my answer, but this frog is incapable of subterfuge. There are no cable testing methods that will tell you when a cable will fail, which is what you really want to know. At best, testing will provide you a number you can track over time. An extensive U.S.A. study, the Cable Diagnostic Focused Initiative (CDFI) led by NEETRAC (The National Electric Energy Testing Research & Application Center at Georgia Tech’s College of Engineering) of commercialized cable testing methods came to this conclusion—see CDFI slide 41. The CDFI is the largest and most comprehensive study ever undertaken. All cable testing methods, except online PD testing, can cause damage to the cable insulation and shorten cable life. Some cable testing methods such as Tan Delta or Power Factor are not comparable before and after injection, because cable injection alters the chemistry and physics of the cable, changing the measured parameters in ways counter to the claims of the diagnostic supplier. See my 2010, September 10, post, “Cable Rejuvenation Impact on Loss Factor (tan-Delta).” In one example from that post, the tan-Delta at 0.1Hz increased after treatment, even though dielectric strength increased substantially – just the opposite of what the proprietors of the test predict. Because available diagnostics do not provide useful information and/or the tests are inherently destructive, Novinium discourages cable testing before or after cable injection. If a circuit owner chooses to test its cable despite overwhelming evidence of futility and counter productivity, before or after injection, Novinium will suspend, but not extend, any warranty for 120 after the test. Make sure you check out my 2010, November 12 post: Diagnostic Testing – Should I do it? One sneaky way to test the veracity of diagnostic firms’ claims is to request a warranty for cables which test as good, but fail. And when I say warranty, I don’t mean a credit for future diagnostics – I mean money back. In a three-year experiment undertaken as part of the CDFI to measure the accuracy of online partial discharge testing, false negatives were about 9.5% and 19.5% for accessories and cable respectively. False positives were 69% and 56% respectively. I can do better than that with a role of the dice!
For now, Ma’a salama (مع السلامة/Good bye)
T. B. Frog