Member
Posted October 31, 2016
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?
NETA Level IV
Posted October 31, 2016
Pro Subscriber
What size dry type? What was your final value?
Originally Posted by cjones09
Member
Posted October 31, 2016
225kVA and the final value was 157 gigohm after 10 min.
NETA Level IV
Posted October 31, 2016
Pro Subscriber
I know some megger values decrease when you physically get close to the test leads. 100+ Gig Ohms, I wouldn't fail it.
Member
Posted October 31, 2016
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.
Member
Posted October 31, 2016
Did you have the transformer completely isolated?
Member
Posted October 31, 2016
Yes the transformer was isolated. I'm wondering if maybe I have a bad test lead.
Member
Posted November 1, 2016
Do an open air test. The megger should max out pretty much right away
Junior Member
Posted November 1, 2016
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.
Member
Posted November 1, 2016
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.
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