Extended Term Analysis
These methods incorporate comparisons of resistance values
recorded at different points of time (up to ten minutes). They
can provide useful information about the condition of your
equipment even if PM records are not available.
Generally speaking, the resistance measured at the end of 5
or 10 minutes should be higher than it was at one minute. The
best way to determine a good or bad reading for your specific
application is to solicit information from the manufacturer or
evaluate new and progressively older equipment.
60-30 Testing
The ratio of a reading recorded at 60 seconds compared to
that recorded at 30 seconds is one method that gives you
a Dielectric Absorption Ratio (DAR). This ratio provides you
with the Polarization Index (PI) when you divide the reading
observed at the longer term by that of the shorter. The rule
of thumb regarding this index (60-second reading divided by
30-second reading) is that it has to be higher than “one” to
be acceptable. Anything that has a ratio under 1.25 should be
watched carefully and anything over 1.4 is good. Because of
the time-frames specified, this test can be difficult to perform
and is not commonly used.
10-1 Testing
The methods of obtaining the ratios and index numbers are
the same in this test method as used in the 60-30 test, but the
duration of testing is extended. A measurement is recorded
at 1 minute and another one recorded at 10 minutes.
This Polarization Index table applies to both test methods:
Insulation Condition 60-30 Test PI
10-1 Test PI
Bad
Below 1.0
Below 1.0
Unreliable
1.0 to 1.25
1.0 to 2.0
OK
1.4 to 1.6
2.0 to 4.0
Excellent
Above 1.6
Above 4.0
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