Protec Equipment Resources has a wide variety of insulation resistance test sets, or meg-ohmmeters (meggers) in our equipment inventory from known manufacturers such as Megger and Metrel. Each test set is capable of performing the valuable Polarization Index or Dielectric Absorption test … but why are these tests of value?
An important property of insulation is that it “charges” during the course of a test. The polar DC field applied by the meg-ohmmeter causes re-alignment of the insulating material on the molecular level, as dipoles orient themselves with the field. This movement of charge constitutes a current. Its value as a diagnostic indicator is based on two opposing factors: the charging current dies away as the structure reaches its final orientation, while “leakage current” promoted by deterioration in the insulation passes a comparatively larger, constant current. The net result is that, with “good” insulation, leakage current is relatively small, and resistance rises dramatically as charging goes to completion. 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 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 (DA) 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.
Note that resistance readings alone are difficult to work with, as they may range from enormous values in new equipment down to a few meg-ohms just before removal from service. A test like the PI is particularly useful because it can be performed on a wide range of apparatus, and yields a self-contained evaluation based on relative readings rather than absolute values.