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what is PI value and its significance for insulation
testing?
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Answer Posted / maheshwar prasad
Polarisation index is the ratio of megger value taken for 10 minutes to the megger value taken for 1 minute. The significance of polarization index test. Let I be the total initial current during polarisation index test or PI test. IC is the capacitive current. IR is resistive or conductive current. IS is surface leakage current. IP is polarization current of the insulator. Value of insulation resistance test or IR value test, i.e. value megger reading just after 1 minute of the test, is- Megger value of 10 minute test, is Therefore, result of polarization index test, is From the above equation it is clear that, if the value of (IR + IS) >> IP, the PI of insulator approaches to 1. And large IR or IS or both indicate unhealthiness of the insulation. Th value of PI becomes high if (IR + IS) is very small compared to IP. This equation indicates that high polarization index of an insulator implies healthiness of insulator. For good insulator resistive leakage current IR is very tiny. It is always desired to have polarisation index of an electrical insulator more than 2. It is hazardous to have polarisation index less than 1.5.
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