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Should I fail this transformer?

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    Should I fail this transformer?

    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?
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  2. #2
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    What size dry type? What was your final value?

    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?
    Reply to Post • 

  4. #3
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    225kVA and the final value was 157 gigohm after 10 min.

    Quote Originally Posted by Test Tech from DMV View Post
    What size dry type? What was your final value?
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  6. #4
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    I know some megger values decrease when you physically get close to the test leads. 100+ Gig Ohms, I wouldn't fail it.
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  8. #5
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    I agree that the value is good but since NETA-ATS specifically says "value shall not be less than 1.0" does that mean it fails no matter what the value is until PI is greater than 1.0? Thanks for the reply.

    Quote Originally Posted by Test Tech from DMV View Post
    I know some megger values decrease when you physically get close to the test leads. 100+ Gig Ohms, I wouldn't fail it.
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  10. #6
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    Did you have the transformer completely isolated?
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  12. #7
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    Yes the transformer was isolated. I'm wondering if maybe I have a bad test lead.

    Quote Originally Posted by lester mcmanaway View Post
    Did you have the transformer completely isolated?
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  14. #8
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    Quote Originally Posted by cjones09 View Post
    Yes the transformer was isolated. I'm wondering if maybe I have a bad test lead.
    Do an open air test. The megger should max out pretty much right away
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  16. #9
  17. Zog is offline Junior Member
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    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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  18. #10
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    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.

    Quote Originally Posted by lester mcmanaway View Post
    Do an open air test. The megger should max out pretty much right away
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