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Fan the Strands

October 26, 2010

Fan the Strands

Fan the Strands
Q:

I have a question regarding a statement I heard one of your colleagues make at an ICC meeting recently. Would you comment on the criticality that wire brushing of the conductor has when installing a connector?

A:

You are indeed Xcel-lent because you are not afraid to ask the tough questions. And I am Xub-erent, because I love to dispel myths. When connectors are qualified to ANSI C119.4 do you think the manufacturers use old corroded conductors? If you answered yes, stop reading here. If you answered no, read on.

You’re smart too! Question number 2: Do you think the manufactures’ wire brush the strands? If you answered no, stop reading here.  If you answered yes, read on.

Now classify these four statements as true or false …

  1. Our line teams will ignore any pronouncement that mandates cleaning the strands.
  2. We never operate our conductors at or above 90°C.
  3. We never splice old cables; we always use brand new cables.
  4. Reliability is not really important to my firm.

If any of these 4 statements is true for you, stop reading here. Only if all four of these statements are false, should you read on.

You might be amazed at the number of people who don’t make it this far. You are truly unique! Are you ready for the bottom line? We doubt that there would be many aluminum conductor, medium voltage cables that have been in the ground for more than two decades that would pass an ANSI C119.4 test with common connectors and common crimping or swaging techniques that have not been wire brushed. Hold on, it gets worse. Wire brushing the outermost layer of strands is not enough; you have to remove the patina off close to 100% of the strands!  Here’s the rub … that is not easy to do.

The skeptic replies, “This sounds a little bit like Chicken-Little, claiming the sky is falling. Our guys, never wire brush and we are not having a zillion failures.” The skeptic’s point is not without merit. Because most connectors never get anywhere near 90°C there is not a thermal runaway, at least until the conductor does get close to or above 90°C. Small diameter URD cables with sub-par compression connections may provide decades of reliable service simply because they are never loaded to near their nameplate ampacity. Feeder cables are a different story. Sooner or later they are quite likely to be fully loaded. It will probably happen when loads are at their peak and perhaps when a parallel feeder has failed. Could you think of a worse time to discover this wisdom? Are splice failures during these emergencies traced back to compression connection issues? Does anybody look at the inter-strand interfaces and measure the resistance? Or is the conclusion simply that the splice failed or that there was a thermal runaway? I know the problem is a lot bigger than reported.

One data set and report are available at …

Thermal Testing of Novinium Feeder Cable Splices

…, but this report is the tip of the iceberg of the effort we have made at Novinium. We may publish results of our extensive internal tests at a future date.  In the mean time, strand preparation best practices are available at …

NRI 500-599: Craftwork