WHY A DC HIPOT TEST IS IMPORTANT
The DC Hipot test, or high potential test, can provide important information about several subjects, and is a test feature that will not cause damage or degradation to the device under test (DUT), if done correctly. For more information on why the DC Hipot test is nondestructive, Click Here.
Below is a list of the most common failure modes and weaknesses found with the DC Hipot test.
- Early warnings of weak groundwall insulation.
- Dielectric strength to ground.
- The dielectric strength of the phase to phase insulation.
- When the megohms measured in an IR test are lower than expected, a DC Hipot Step Voltage test can indicate if the motor or device under test (DUT) is dirty and/or moist, or if the insulation is breaking down. Since Insulation Resistance tests usually are done at one voltage and do not provide this information, a Hipot Step Voltage test can be very valuable.
- The IEEE 95 standard (for AC electric machinery 2300V and above) states that the following insulation problems may be detected by a controlled direct-voltage test:
- Surface contamination
- Cracks or fissures
- Moisture absorption
- Uncured resin
Click here for information on IEEE and access to standards.
For low voltage motors, the DC Hipot test is normally a 1-minute test at a specified DC voltage that is higher than the peak lead to lead operating voltage. The test is referred to as an over-voltage test since the voltage is higher than what the motor normally sees (Peak voltage = line to line RMS x 1.414).
AUXILIARY EQUIPMENT GROUNDED OR DISCONNECTED DURING A HIPOT TEST
Before starting an over-voltage test like the DC High Potential Test, the following components should be shorted to ground:
- Stator resistance temperature detectors or thermocouples
- Other devices associated with the stator windings
- Current transformer secondary windings
- Rotor windings (both terminals) and the shaft
- Objects close enough to become charged
- Motor tester chassis ground
Surge arresters and surge capacitors:
These must be disconnected prior to any over-voltage tests. Surge arresters have resistive elements and, Surge capacitors have discharge resistors. They are in parallel with the winding under test and will invalidate the current measurements.
For single phase motors, start and run capacitors should be disconnected.
The following subjects are covered:
- How DC Hipot works, and the different currents involved
- DC Hipot Test Pass/Fail Analysis
- Hipot Step Voltage Test
- Ramp Test
- Hipot Test Voltages
- Why DC Hipot tests are not destructive