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A 60 amp breaker in our circuit breaker box serves a sub-panel in our barn. That panel serves 5 120 volt lighting and convenience circuits. The sub-panel has two bus bars, each attached to one phase of the buried 240 volt cable from the house, which consists of three separate conductors (two hot and one neutral)in a trench. For the first two years or so of operation, both phases of the drop were energized. Now, for some unknown reason, only the breakers connected to one bus bar are energized. I can find no problems with any connections so must assume that one conductor of the buried cable is broken. I have two questions. First, is there some way to definitively confirm that a break is the problem? Second, if it is, how can I determine where along the 80 feet or so of cable the break has occurred?

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To determine if there is a break.

  • Turn off the breaker in the main panel.
  • Remove the conductors from the breaker, and connect them with each other.
  • In the barn panel, disconnect the ungrounded feeder conductors.
  • Test for continuity between the ungrounded conductors in the barn.

If the test shows open (no continuity), then there's a break in one of the lines. If not, then something else is wrong.

If the lines test as not broken, test for continuity through the feeder breaker.

  • With the conductors still disconnected, turn the breaker in the main panel on.
  • Test for voltage at each of the terminals.

If they both show voltage, the breaker is not the problem. If either does not show voltage, replace the breaker.

If the lines do test as broken. You can either dig them up and look for the damage, replace them (abandoning the existing cable), or contact a local Electrician to see if they can locate the break for you.

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I would suspect the lines were buried without benefit of conduit, or, water may have seeped into the conduit. In both cases, water will cause the insulation to fail, and the copper wil be eroded away.

Get an electrician to check for voltage at the feed point, and again at the breaker box. If the bus feeding one side of the breakers is not fed, you get no power on that side.

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  • I appreciate both your answers. Some of the tests you describe have already been done and there is every indication that one of the conductors is the problem. The issue is where in the line the problem can be found. I don't want to dig up the entire line if the break can be isolated. On the other hand, if the problem is water inside the jacket on the conductor, then the problem may be extensive and not isolated to one spot. In any event, is there a way, perhaps with resistance tests, to show how many feet from the panel the break occurs? – user13781 Jun 18 '15 at 1:41
  • There are various methods but none are a silver bullet and most of them require specialised equipment (even the brute force method of cutting up the cable into sections requires you to know how to rejoin it safely afterwards). hackersdelight.org/hdcodetxt/divDouble.c.txt – Peter Green Jul 27 '16 at 21:20

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