The polarization index test (PI test) is performed to quantitatively measure the ability
of the ground wall insulation to polarize. The PI test is the most confusing HVDC test in
use due to the subtleties in the interpretation of the results. When an insulator polarizes,
the electric dipoles distributed in the insulator align themselves with an applied electric
field. As the molecules polarize, a “polarization current”, also called an absorption
current, is developed that adds to the insulation leakage current. The test results become
confusing when attempting to attribute variations in the PI value to the polarizability of
the insulator or other affects such as humidity, moisture and instrument error.
The PI test is typically performed at the same voltage as the megohm test and takes 10
minutes to complete. The PI value is calculated by dividing the insulation resistance at 10
minutes by the resistance at 1 minute as shown below: