Megohmmeter Models 5000N/5100/5110
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Polarization Index
Method
This test is based on the comparison of absorption characteristics of
good insulation vs. the absorption characteristics of humid or otherwise
contaminated insulation. During the test, a test voltage is applied for an
extended period, usually 10 minutes. The operator takes a reading every
10 seconds for the first minute, and then every minute up to 10 minutes.
A curve is drawn showing the insulation resistance value versus time.
Test Duration
10 minutes
Interpreting the Results
If the results were plotted on a graph, the slope of the curve would
indicate the condition of the insulation under test. A good insulation will
show a continual increase in resistance for typically 10 to 15 minutes.
Contaminated, moist, or cracked insulation will produce a relatively flat
curve.
A ratio known as the
polarization index
can be obtained by dividing the
value from the 10-minute reading by the value from the one-minute
reading. This polarization index is indicative of the slope of the curve.
A low polarization index usually indicates excessive moisture and
contamination. On large motors or generators, values as high as 10 are
commonly expected.
10-minute reading
1-minute reading
The IEEE Std 43-1974
2
lists the following minimum values for the
polarization index for AC and DC rotating machines:
Class A: 1.5
Class B: 2.0
Class C: 2.0
Polarization index =
2
IEEE Std. 43-1974, -
Recommended Practice for Testing Insulation Resistance of Rotating Machinery
Summary of Contents for 5000N
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