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DC Testing

September 1, 2011

Q:

We have been using DC testing for centuries without any problems. How do you explain that?

A:

DC testing of medium voltage AC cables comes up only occasionally. The warranty impact was discussed in an April 14, 2010 post, titled “DC Testing to sectionalize faults—Warranty Impact.”

I am actually not an expert on DC testing, so I have to rely on those that are. One way I can be pretty sure that DC testing is inherently destructive is that the folks that manufacture and sell DC testing equipment croak along on the same chorus. To wit, HV Diagnostics, Inc., one of the most respected suppliers of high voltage test equipment pronounces in its brochure:

“On Medium Voltage Extruded (XLPE, PE, EPR) cables, DC is no longer recommended by most international standards. DC has been found to be both destructive, causing premature failure of aged MV cables, and/or ineffective in detecting many types of serious pending insulation defects in new and old cable installations.”

You have to take that kind of pronouncement pretty seriously, because the supplier sells DC testing equipment. As for your anomalous experience, I would recommend that you compile a comprehensive data set and use randomization and suitable controls. Anecdotal results are inherently misleading. Also make sure that the equipment operator’s job does not depend upon the results—the guys that do testing for a living seem generally to be proponents of testing.

I don’t reflexively follow the crowd, but the consensus is that DC testing is destructive and does not provide useful data anyway!