It is common practice to perform resistance-to-ground (RTG) measurements to verify the integrity of the Insulation System in a motor. It is also common practice to re-test a motor, when there are changes in the readings compared to previous test results or if the present reading is lower than expected. In many cases, the second test is taken immediately after the first one, giving a higher RTG reading. This is not an indication of motor health, but rather a higher polarization level.
As a rule of thumb, it takes about four times the charge time for the charging currents to fully dissipate. Therefore, if we perform a one-minute RTG test, it will take about four minutes for the charge to fully dissipate. If we perform a ten-minute Polarization Index test, then the discharge time would be approximately 40 minutes.
To ensure the most accurate test results possible, it is recommended to wait a minimum of four times the charge time, before taking a second RTG test.
Tip provided by PdMA Corporation