DC Hipot Test
This page is an overview of the iTIG DC hipot test. For details about hipot test methods, including types of hipot testing and iTIG hipot test measurement techniques, see DC Hipot Test Methods.
Why is a Hipot test used?
The high potential (hipot) test is a test of the dielectric strength of the insulation to ground. The hipot test is used to determine whether 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). Standard peak voltage is 1.41 x 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 whether the start of an insulation breakdown can be detected. As the voltage is raised during the test, if a breakdown starts, the test can be terminated before there is an arc.
How DC Hipot Tests Work
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 and 10nA for the Power Pack. 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 about the various types of leakage currents, see Insulation Resistance Measurement Methods.
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 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 a DC Hipot test be destructive?
DC hipot tests are done at a voltage higher than the operating voltage of the DUT and therefore referred to as over-voltage tests. The tests not destructive if done properly. The main reason the tests are not destructive 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. If an arc occurs 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 Methods.