A valuable property of insulation, but one that must be understood, is that it “charges” during the course of a test. The polar DC field applied by the tester causes re-alignment of the insulating material on the molecular level, as dipoles orient themselves with the field. This movement of charge constitutes, of course, a current. Its value as a diagnostic indicator is based on two opposing factors: the current dies away as the structure reaches its final orientation, while “leakage” promoted by deterioration passes a comparatively large, constant current. The net result is that, with “good” insulation, leakage current is relatively small, and resistance rises dramatically as charging goes to completion. This changing resistance is exactly what an experienced technician wants to see. Deteriorated insulation will pass relatively large amounts of leakage current at a constant rate for the applied voltage. This will “flood out” the charging effect.
Time-Resistance Methods, as they are known, take advantage of this charging effect. Graphing the resistance reading at time intervals from initiation of the test yields a smooth rising curve for “good” insulation, but a “flat” graph for deteriorated equipment. The ultimate simplification of this technique is represented by the popular Polarization Index (PI) and Dielectric Absorption tests, which requires only two readings and a simple division. Performing the PI test the one-minute reading is divided into the ten-minute reading to provide a ratio. In DA the time values are typically 30 seconds and 60 seconds. Obviously, a low ratio indicates little change, hence poor insulation, while a high ratio indicates the opposite.
References to typical PI values are common in the literature, which makes this test very easy and readily employed. Note that resistance readings alone are difficult to work with, as they may range from enormous values in new equipment down to a few megohms just before removal from service. A test like the PI is particularly useful because it can be performed on even the longest of cables, and yields a self-contained evaluation based on relative readings rather than absolute values.
Protec Equipment Resources rents and sell the very popular Megger MIT series of insulation resistance test sets. Contact us today to discuss your testing needs.