Insulation Tester
Before an Insulation Test
17
Polarization Index (PI)
As part of the insulation test, the Tester measures and stores polarization
index (PI), when appropriate. A polarization index test requires 10minutes to
complete. Therefore, the Tester will start a countdown at 10minutes. When an
insulation test reaches 10minutes, the polarization test is complete. The
results are available for display during a test or when you store the test results
and view the record PI field. The field is identified by:
Dielectric Discharge (1537 only)
The dielectric discharge (DD) test, also known as the re-absorption current
test, is a measurement of the current during dielectric discharge of the
equipment being tested.
The measurement principle is as follows:
1. Equipment to be tested is first charged for 30 minutes to reach a stable
state (capacitance charging and polarization are completed and the only
current flowing is the leakage current).
2. Equipment is then discharged through a resistor inside the mega-
ohmmeter and the current that flows is measured. This current is made up
of the capacitance charging current and the re-absorption current that
combines to give the total dielectric discharge current. This current is
measured after a standard time of 1 minute. The current depends on the
overall capacitance and the final test voltage.
The value DD is calculated with this formula:
I = current after 1-minute discharge
V = voltage after 1-minute discharge
C = measured object capacitor
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