June 01, 2005
8 Motor Testing and CBM
AC and DC High Potential Tests
While the Insulation Resistance, Polarization Index and Dielectric Absorption tests are methods of evaluating general insulation to ground condition, the high potential test is a method of evaluating insulation to ground integrity, or dielectric strength. There are two basic types of high potential tests: AC and DC, of which we will cover both in today’s presentation.
VP, Electrical Reliability Group
T-Solutions, Inc.
AC High Potential
The AC high potential test is a pass/fail test of the ground wall insulation only. In effect, if the winding does not ground during this test, it passes. If it does fail due to winding defects, moisture or contamination, the AC potential will cause carbon tracking throughout the insulation system. In new machines, the applied voltage is twice the motor rated voltage plus 1,000 volts. In field tests of used machines, the voltage should be limited to 125-135% of motor rated voltage. However, it is common practice in motor repair shops, on low voltage machines, to apply the 2V+1000 rule to ensure condition before making a rewind decision. One of the most commonly used AC high potential tester in the repair industry is the Crown tester which also contains an overload and reset that is designed to cut out should there be a spike in charging current. This feature is more of a personnel safety device.
The AC high potential test is considered dangerous to the machine and personnel and should only be used sparingly for potentially destructive tests.
DC High Potential
The DC high potential test is considered less potentially destructive than the AC high potential test. The applied voltage is 1.717 x (2V + 1000V) for new insulation systems with a multiplier of about 0.75 to 0.85 times that value for used insulation systems. This type of test is normally performed on medium voltage and higher motors (>1,000 Volts) and is occassionally used for trending. The best method for using a DC high potential test is to perform what is called a ‘step test.’
The normal practice for testing a medium voltage motor is to disconnect the wye-connection and ground two of the phases. Then test the third phase to ground using the step-test method. This practice measures the insulation integrity between the tested phase and the groundwall, but also between the coils of the two other phases to the phase under test. The step test involves increasing the applied voltage in increments of 500 to 1,000 volts, then observing the charging current until it settles. A sudden increase in charging current indicates failure.
The most common manufacturer of the DC high potential meter is HiPot.
Did You Know?
Did you know that one of the required tests for many power utilities was to perform a high potential test on a new medium and high voltage (VPI’d) winding while it was submerged? This concept is sometimes still used today, but the submerging tanks tend to be replaced with a water spray.
