high P.I. (with good one-minute I.R.) 3

[electricpete]

Can anyone provide information related to concerns that may be associated with the particular symptom of high Polarization Index P.I.?

For example, epoxy-mica medium voltage machine shows 20 G-ohms at one minute and 150 G-ohm at 10 minutes => PI = 150/20~7.5.
I am familiar with IEEE43 guidance that PI loses significance above 5 G-ohms one minute, since readings above this based on lower current can have error, but still interested in exploring possible meanings of this symptom).

I have heard various places this could indicate dry / brittle / resin-depleted insulation (thumbrule PI > 6 suggests this condition).

My two biggest questions:
1 - Does this "thumbrule" apply only to older Asphaltic systems?
2 - What is the physical reason why insulation systems which are dry / brittle / resin-depleted might have high PI?
3 - Has anyone seen high PI associated with insulation problems (on machine with good 1-minute IR)
4 - Any references?


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(2B)+(2B)' ?

 

[Addy71]

I have added it as a PDF now, should work.