Dielectric absorption testing
In a dielectric-absorption test, a voltage supplied from a source of constant potential is applied across the insulation. The test voltage used may have to be signed if significantly higher than the insulation resistance test in order to obtain measurable current readings. The potential source can be either a meg-ohmmeter, as described above, or a high-voltage power supply with an ammeter indicating the current being drawn by the specimen under test. The voltage is applied for an extended period of time, from 5 to 15 minutes, and periodic readings are taken of the insulation resistance or leakage current.
The test data is evaluated on the basis that if insulation is in good condition, its apparent insulation resistance will increase as the test progresses. Unlike the insulation-resistance test, the dielectric absorption test results are independent of the volume and the temperature of the insulation under test. For the dielectric absorption test, the values recorded at each one minute interval are plotted on log-log paper with coordinates for resistance versus time. The slope of the resulting curve gives a good indication of the insulation condition. A good insulation system will have a slope that is a straight line increasing with respect to time. The characteristic slope of a poor insulation system will be a curve that flattens out with respect to time.
Polarization index testing
The polarization index is a specialized application of the dielectric absorption test. The index is the ratio of insulation resistance at two different times after voltage application, usually the insulation resistance at 10 minutes to the insulation resistance at 1 minute. The use of polarization-index testing is usually confined to rotating machines, cables, and transformers. A polarization index of less than 1.0 indicates
that the equipment needs maintenance before being placed in service. References are available for polarization indexes for various types of equipment as well as in table 1-2. Acceptance testing specifications for Electrical Power and Distribution Equipment and Systems NETA 1999 and Recommended Practice for Testing Insulation Resistance of Rotating Machinery (IEEE Std. 43-2000) are references available for polarization indexes for various types of equipment. The polarization index test lasts for 10 minutes. The insulation resistance is recorded after 1 minute, then again after 10 minutes. The polarization index is the quotient of the 10-minute and 1-minute readings as shown in the following equation:
PI = RIO/R, (dimensionless) Where:
PI = polarization index
R = resistance.
For polarization indexes in transformers, an acceptable value would be 2 or higher, values between 2 and 1 indicate marginal condition, and values below 1 indicate poor condition. After insulation resistance readings have been made, the test voltage is returned to zero and the insulation is discharged.