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Bushing insulation testing

In bushing insulation test there are three major current elements which any of those could affect the test result. These current elements are Capacitive current, leakage current and creepage current. In Tan Delta or power factor test the affective current element to the test is called creepage current which needs to be filtered. The affect of this current element could specially be seen in bushing insulation trending in which you see the Tan Delta grows very fast. Using the megger tester with three leads (I mean L, E and G) and measuring Polarization Index is very helpful. In this case the surface of bushing comes in contact with G lead. The power factor testing which is obtained will be real, If Polarization Index is about 1. The power factor test should be repeated, if Polarization Index is more than 2.

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