What is it used for?
The Hi pot test is a test of the dielectric strength of the insulation to ground. It is used to determine if the ground wall can handle an over-voltage situation. An over-voltage is a voltage higher than the peak operating (line to line) voltage of the device under test (DUT). The peak voltage is 1.41 x the RMS voltage.
Hipot tests are used for everything from very low voltage devices to high voltage equipment. For medium to high voltage rotating equipment, DC Hipot tests called Step Voltage or Ramp tests are used to see if the start of an insulation breakdown can be detected as the voltage is raised. If a breakdown is starting the test can be terminated before there is an arc.
There are two types of Hipot tests, DC Hipot tester and AC Hipot. In addition, AC Hipot comes in different power frequencies from very low frequencies to 50Hz.
The Electrom iTIG III Winding Analyzer includes the DC Hi pot test and can include automatic Step Voltage and Ramp tests reviewed in the Test Technology section.
AC hipot can be added to the iTIG III motor tester through a separate unit that is controlled by the iTIG III.
How DC Hipot Tester works
(for more details see Hipot Tests under Test Technologies):
A DC over-voltage potential is applied to the windings, and the current to ground is measured with a resolution of 1 pA for the iTIG III and 10nA for the Power Pack III. This current includes leakage from the windings as well as other currents such as surface currents on the outside of the windings. For more information on the various leakage currents see Insulation Resistance under the Test Technology section.
The test voltage used depends on the condition of the DUT, and typically ranges from about 2E to 3.4E+1700V where E is the RMS operating voltage of the DUT.
The iTIG III tester has 3 possible failure modes that will shut off the test:
- The over-current trip-out level is exceeded; this level can be set in µA by the user.
- An arc between the windings and ground is detected.
- The current to ground is accelerating beyond the rate of change (ROC) level set as a limit for a step voltage test.
Can DC Hi pot test be destructive?
DC Hipot tests are done at a voltage higher than the operating voltage of the DUT and hence referred to as over-voltage tests. They are not destructive if done properly. The main reason for this is that the test voltage is far below the design voltage of the insulation. Furthermore, the energy available is determined by the voltage applied and the capacitance of the system. This capacitance is relatively low. Should an arc occur because of weak insulation, the relatively low energy involved in the arc will not damage the insulation, unless the test is done under conditions that are not recommended for over-voltage tests.
For more details and an example of a Hipot test, see DC Hipot Test under Test Technologies.