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Should I fail this transformer?

    #11
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    Thank you for helping to clarify, Zog!

    Quote Originally Posted by Zog View Post
    Once you get up in the Gig range the PI ratios are not very important, there is a note somewhere regarding this, I will see if I can find it.
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  2. #12
  3. quantum is offline Junior Member
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    Quote Originally Posted by cjones09 View Post
    So today I re did the test and the problem was not the test leads but the jumper cables I was using to short the windings together. I did the test without the jumpers and the readings went to full scale. Most PI values were well over 4. I've been taught that jumpers help reduce voltage stress on the windings but it clearly has a big effect on the readings.
    When using jumpers it is really important to make sure they are hanging in free air or if that's not possible, they are at least only touching insulated parts. If you think about it, say you use chopped up car battery jumper cables that are not rated for even a 1kv megger test and these are energized and leaning against the transformer core or ground. At this point you are really testing the insulation of your jumper cables.
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  4. #13
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    Thanks quantum! This is exactly what happened. How necessary would you say are jumpers when PI testing transformers? I've always been taught to use them but it obviously effects the reading - maybe even in free air?

    Quote Originally Posted by quantum View Post
    When using jumpers it is really important to make sure they are hanging in free air or if that's not possible, they are at least only touching insulated parts. If you think about it, say you use chopped up car battery jumper cables that are not rated for even a 1kv megger test and these are energized and leaning against the transformer core or ground. At this point you are really testing the insulation of your jumper cables.
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  6. #14
  7. nscdrgs is offline Junior Member
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    Quote Originally Posted by cjones09 View Post
    I am PI testing a small dry type transformer and my reading was 157 gigohm after 10 min but PI is less than 1.

    Neta says investigate valuess less than 100 megohms but PI "shall not be less than 1.0"

    Should I fail this transformer?
    Hi,
    I had this issue before when the Megger battery was low.
    I charged and I retested and was fine.

    Another time the weather just changed during the 10 minutes test from cloudy to rain. I retested in same conditions (rain).
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  8. #15
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    If you think about why a PI less than 1 is failing, it makes more sense. That would mean that the insulation is breaking down under test. The reason it isn't as relevant at such high readings, is that the measurement is extremely close to 0 current flow. Any tiny change in conditions could cause a massive change in results. The insulated leads touch, test set gets moved, air moves around the test area; all of those things will cause an extremely small change in current draw, thus decreasing the measured insulation resistance.

    Jumpers aren't necessary when performing insulation resistance on a transformer. The windings are interconnected, and thus all energized. See the attached diagrams of the windings for a Delta Wye transformer. If you were to energize 10kVDC H1 with the positive lead without the connections tied together, you're still going to have 10kVDC on H2 and H3.
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    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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