Megohmmeter Model 1000N/1100N
- 23 -
Remember that a DC insulation test may be used for acceptance testing,
but is more commonly used to check the gradual deterioration of
equipment over its lifetime. Consult your equipment manufacturer for
specific test or test voltage if not known.
Insulation resistance decreases with moisture, temperature and age, and
should be recorded over time at a given temperature and corrected.
DIELECTRIC ABSORPTION TEST
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 =
IEEE Std. 43-1974, -
Recommended Practice for Testing Insulation Resistance of Rotating Machinery
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