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DC Testing to sectionalize faults – Warranty Impact

April 14, 2010

DC Testing to Sectionalize Saults—Warranty Impact

Q:

We use a 10kV hipot DC to sectionalize URD faults. We actually use this on several sections at a time so it will go through good sections of cable until it reaches the bad section. We split the run of cables fed by a common fuse in half and hipot one way (could be 4 or 5 sections of cable) and if it is not found we hipot in the other direction. Do we have to wait 120 days after this and if so how do we track it?

A:

There are no record retention requirements imposed by our warranty. The individual who submits a warranty claim might query the operations folks to learn if there has been any fault activity over the past 4 months in the adjoining circuits. It is well established in the literature and in IEEE standards, that applying 10kV DC on aged polymeric cables is inherently destructive. If that body of literature is not easily available to you, I can provide you with the appropriate references. There are less destructive ways to sectionalize faults. For example, if you use the same approach, but at 1kV there is no issue with our warranty. The amount of damage from imposing DC is proportional to the voltage squared times the time, so 1kV is 100 times less damaging than 10kV imposed over the same length of time. There are also sectionalizing arc reflection TDRs that can identify the failed section with a few very brief pulses.

The best idea is to avoid this situation entirely. Treat or replace 100% of the at-risk cable segments in a loop and the probability of a failure in the loop will be dramatically lower. Avoid rejuvenating only the easy-to-treat cable, as half-steps will only get you to half-reliability.