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Battling Elements and Animals

Rural Electrification Magazine, October 1995

by John Marks, Technical Editor

Non-visible insulator failure has plagued a number of utilities in recent years. When insulators fail, leakage current jumps to grounded assemblies sporadically, causing blinking lights, radio static and TV interference for consumers. A few years ago, Cloverland Electric Co-op in Dafter, Michigan, noticed a rapid increase in customer complaints, along with a steady increase in recloser operations. "We tried tightening all the pole line hardware, trimming trees more frequently and increased line patrolling all to no avail," says Dick Newland, area superintendent.

"Finally, we obtained an insulator tester from Hi-Test Detection Instruments that allowed linemen to 'meg' insulators while the line was energized. We ultimately tested about 35,000 insulators, and found and corrected more that 2,200 defective units." Then, he says, customer complaints and service difficulties decreased dramatically, saving the co-op $75,000 annually in trouble calls-an impressive savings for a total investment of $8,000 in two instruments.

John Farquhar of Hi-Test Detection Instruments says that while any bell-type porcelain insulator may crack, those with pins and caps made of aluminum (instead of malleable iron) are especially prone. This is because aluminum and porcelain expand and contract at very different rates. The resultant hairline cracks beneath the caps aren't visible during routine inspections. Eventually, enough heat builds up from leakage current that the insulator bell explodes. Even before that happens, the partial discharge causes radio and TV interference and, sometimes, pole fires.

Dick Disselhorst, line superintendent at Missouri Rural Electric Co-op in Palmyra, Mo., was so impressed with the tester that he ordered one while it was being demonstrated. Five of the co-op's poles had burned down because of faulty suspension insulators. "When you see the pole turn black, you know you've got a problem," says Disselhorst. "Before this happens, we look at the square washers and can see blackening (charring) around the washer. The instrument pays for itself if it prevents one failure, such as on a corner pole right out of a substation strung with 4/0 wire."

Disselhorst says that his insulators, 95 percent of which have aluminum hubs, have been going bad only after 15 years. Any insulator will fail eventually, and we've got some that have been up there almost 60 years." Disselhorst adds that he finds the Hi-Test instrument useful in salvaging good insulators from disconnected lines.

Jack Roemer, operations manager at Cherryland Electric Co-op in Grawn, Mich., reports that his co-op has used the Hi-Test instrument to test porcelain suspension-type insulators on 11 distribution feeders over the last year-and-a-half. "We replaced more that 400 failed insulators," he says, "and our complaints on blinking lights and radio interference have been reduced considerably. However, it will take us another five to eight years to complete the remaining 33 feeders. Meanwhile, we expect to see a steady reduction in overall system line loss." Roemer found that most of the failed insulators dated from 1977 to 1979, and all were aluminum ball-and-pin.

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