Answer
The polarization index (PI) is a metric used to evaluate the health of electrical insulators based on their Insulation resistance (IR) measurements.
PI is determined by dividing the IR value measured after 10 minutes by the IR value measured after 1 minute. A PI greater than 2 indicates healthy Insulation, while a PI less than 1.5 suggests potential hazards.
An ideal PI approaching 1 signifies significant leakage currents, which could imply unhealthy conditions in the Insulation
The PI test helps identify the presence of contaminants, such as moisture,
and assesses the overall quality of the insulating materials.
Unlike IR alone, PI is relatively insensitive to temperature variations,
making it a more consistent method for monitoring Insulation integrity
However, PI is not applicable to situations where the total leakage current stabilizes quickly,
such as in motors with minimal absorption current
.The polarisatn index should be more than 2
If less than 1.5 it means the Insulation is unhealthy ,not good and the equipment should not be used as per Rajeev.
PI <1 is hazardous
1<Pi < 1.5 bad
1.5<PI<2. Doubtful
2<PI<3 is adequate
3<PI <4 is good
PI >4 is excellent.
From cargo operating manual.
Why do we take for 10 minutes ?
There is something called polarisation current that decays over 10 minutes.
The Insulation material is hygroscopic in nature I.e the molecules in the Insulation consume energy , I.e. electrical current for aligning the molecules as per the electrical field this takes about 10 minutes .
You have to give Insulation resistance about 10 minutes for this current To decay. Then you will get this .
Then you take for 1 minute the 1 minute will have surface resistance , capacitance and polarisatn value. Then you take the ratio.
https://www.youtube.com/watch?v=iUi_BmDE0IY&pp=ygUS cG9sYXJpc2F0aW9uIGluZGV4