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An insulation test is not intended to be a destructive test. The test voltage used must be restricted to a value commensurate with apparatus voltage rating and condition of insulation being tested. This is particularly important in the case of small, low-voltage transformers or those units containing an excessive amount of moisture.

b. Polarization index (PI) test. A PI test or dielectric absorption test is a continuation of the insulation resistance test in which the voltage is applied for a longer period of time. For good insulation, the resistance values will increase with time. The polarization index is the ratio of the lo-minute to the 1-minute readings. An index below 1 indicates poor insulation. An index between 1 and 2 indicates that the insulation is questionable. An index of a 2 and higher indicates good insulation.

c. Power factor test. The power factor of an insulation is a measure of the energy components of the charging current. The test indicates the power loss caused by leakage current through the insulation. The equipment to be tested should be disconnected and all bushings should be cleaned and dried. The test should be conducted when the relative humidity is below 70 percent and the temperature is above 32 degrees F (0 degrees C). On transformer tests, the power factor of each winding with respect to ground, and each winding with respect to its other winding, should be measured. Evaluation of the data obtained should be based on comparison of data with any previous tests on the same transformer or on test data from similar units.

d. High-potential test.

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