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HiPot Test
A HiPot test is performed using a test voltage that is substantially higher than the Megohm test; however, it should
be based on the motor's operating voltage and the appropriate standards/company guidelines.
During HiPot testing, look for unusually high leakage currents, a leakage current that does not stay constant, or a
leakage current that intermittently jumps up and down.
Breakdowns or high leakage currents are indications of damaged ground wall insulation.
Inspect the motor's slot liner, wedges, conductors between the junction box and the coils, and so on.
No further testing is necessary until the reason for an unacceptable HiPot reading is found and corrected.
Step Voltage Test
The Step Voltage test is used for predictive maintenance and field-testing. This DC test is performed at a voltage level
of what the motor typically experiences during starting and stopping. The test voltages are governed by IEEE or other
industry-accepted standards organizations such as NEMA, EASA, and IEC.
The DC voltage is applied to all three phases of the winding, raised slowly to a predetermined step level, and held for
a predetermined time period. This is continued until the target test voltage is reached.
Because the test is most stable at the end of each step, data is logged at that time. If at that time the change
in leakage current (IµA) more than doubles, insulation weakness is indicated and the test should be stopped. If
the change in leakage current (IµA) increase is consistently less than double, the motor insulation is likely in good
condition.
NOTE: You should only evaluate the doubling if leakage current is greater than 0.05 µA.
A Step Voltage test applies DC voltage in incremental steps, with each step held for 60 seconds by default. During
the holding period, the voltage is constant and the current should drop from the higher charging current value to a
lower leakage current value. If at any time the current exceeds approximately 85% of scale, the test will stop due to
an "overcurrent trip" condition.
If a rapid change in voltage is detected due to an arcing fault, an "overcurrent trip" condition will again be flagged.
An ideal change in leakage current will have an asymptotic decay to some minimal value. This minimal leakage
current at the end of the step is recorded and used to calculate the final resistance value for that step. The value
is then temperature compensated and compared to the minimum allowed insulation resistance. If the corrected
resistance falls below the limit threshold, the test will stop due to a "low Megohm" condition.
Following the holding period, the tester will ramp the voltage to the next increment and again hold for 60 seconds.
The final step will be at the standard test voltage and the final resistance calculation is recorded as the HiPot
resistance result.
The Step Voltage Test Setup Wizard can be used to customize the voltage, step times, and minimum resistance trip
values.
www.megger.com
Test Procedures
81
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