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What does a rise in leakage current indicate?

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    What does a rise in leakage current indicate?

    You are performing a dc overpotential test on a medium-voltage cable. As you apply the test voltage in steps, the leakage current continues to rise even though you have stabilized the voltage. What does this indicate?
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    If leakage increases after stabilizing voltage then that means the current is getting back to ground somehow. Investigate the cable insulation for nicks and cuts.
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    Not to be a smarta$$, BUT, what superhero are you hiring to inspect a half mile of MV cable routed through duct banks and/or cable trays?
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    What do you mean Arnold? Question came up on the level 2 practice exam and I am trying to understand the answer.

    Quote Originally Posted by ARNOLD ZIFFEL View Post
    Not to be a smarta$$, BUT, what superhero are you hiring to inspect a half mile of MV cable routed through duct banks and/or cable trays?
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    My comment was in regard to inspecting the jacket for visible nicks and cuts. On most MV cable installations you will come across only the last 6-8 feet is not enclosed in conduit or duct banks, and even then in many cases it is fire taped, again hidden.

    Leakage current that does not drop off and stabilize is a pretty good indication of an imminent problem. Make sure your test connections and clearances are done properly and record the results accurately. Let the big dogs decide whether to re-energize the cable, for example your company's engineer or a very experienced tech.

    D. IEEE 576 [11]

    IEEE Standard 576 [11] includes recommendations on highvoltage dc testing. Recommendations are limited to installation proof testing. Voltage levels are provided to be used after nstallation and before being energized. The test values are in line with other industry standards. IEEE 576 includes discussion of insulation leakage current, including how to interpret the test results.
    The test should be run for 15 min. Generally, the voltage is applied and leakage current is recorded after 15, 30, 45, and 60 s and at 1-min intervals thereafter.
    Considerable experience is needed to properly interpret dc test results. The shape of the leakage current curves is one of the most important things to watch. In general, the leakage current will start at a relatively high value and drop off rapidly, becoming constant at a lower value. The fact that the current becomes stable and levels off is more important than the actual magnitude of the leakage current. If the current does not drop or, if after dropping, it begins to rise again, a strong indication of trouble on the circuit is evident. The test can be stopped at this point before a failure or it can be continued until the cable fails. After failure, the weak spot can be located and repaired.

    http://www.southwire.com/support/DCF...oltageCbls.htm
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    For those of you who don't have X-ray vision, this should go without saying: What I meant was inspect the termination (if any) and any portion of the cable that is visible outside of the conduit. Most of the time it is just a poor termination job. I guess I could have been more clear on that. Very informative post by the way.

    Quote Originally Posted by ARNOLD ZIFFEL
    Not to be a smarta$$, BUT, what superhero are you hiring to inspect a half mile of MV cable routed through duct banks and/or cable trays? On most MV cable installations you will come across only the last 6-8 feet is not enclosed in conduit or duct banks, and even then in many cases it is fire taped, again hidden.
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    Quote Originally Posted by madMAX View Post
    For those of you who don't have X-ray vision, this should go without saying: What I meant was inspect the termination (if any) and any portion of the cable that is visible outside of the conduit. Most of the time it is just a poor termination job. I guess I could have been more clear on that. Very informative post by the way.
    I agree with you that most MV cable problems found during acceptance testing are a defective or poorly applied terminations. However I once found a bad cable in tray about 500 ft. in, that had a perfect impression of a philips screwdriver tip(good megger, failed hipot). It had to fall from the catwalk 50 ft. above. Another time found a fault 40 ft. back inside of a bus duct, caused by rolling the reel over a board with a nail sticking out maybe 1/4"(found during meg test). Also had cable reels show up on a job without the ends capped off during transit. The supplier took it back, and sent out what they swore were 3000 ft' reels of new cable. It meggered fine, passed hipot, and then failed 8 hours into no load energization. I'm sure they returned the same old cable, but that is still being fought over in court. All 3 very costly delays to jobs.

    Again report any suspicious test results, our job is to point out problems, not to keep someone else's job on schedule. If you document your findings and the customer decides to energize anyway, that is their problem.

    Maybe I live in small fish bowl, but generally I follow the same half dozen splicers, and I would not suspect the quality of their work first.
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    Good thread. Thanks for all of the help you guys!
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  17. dnigra is offline Junior Member
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    at this very moment i was looking that question up as well from the same practice exam

    thanks
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    Wow great posts!
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