Short Time Or Spot Test; Polarization Index (Pi) Test - Megger BM11D Manual Del Usuario

Application Notes

Short time or Spot Test

This is the simplest insulation test, giving a reading of insulation
resistance in MΩ. The test is applied for a short but specific
period of time, immediately after which a reading is taken. The
time is typically 30 or 60 seconds and the BM21 will allow you
to set this time and the test will finish automatically. The reading
will not necessarily be the maximum value but if the same time
is used each test, then the same point on the curve of
increasing apparent resistance is being compared. On
installation these readings will be compared to the required
minimum specification. Further readings taken for maintenance
purposes must be monitored for any trend that they show. The
readings are subject to variation from temperature and humidity
and these factors may require the insulation reading to be
corrected. Information on temperature correction is given in the
AVO publication 'A Stitch in Time'. Alternatively, a Polarization
Index (PI) test may be used.

Polarization Index (PI) Test

This test method is a particular
example of the time resistance
method. Time resistance tests
take successive readings at
specified times and are
independent of temperature.
They can often help in the
situation where past test
record frequency are limited.
Good insulation generally
shows an increase in
resistance over a 10 minute
period.
15
A
Insulation
Insulation probably good
probably good
M
M B
Moisture and dirt
may be present
Moisture and dirt
may be present
10 Minutes
0 TIME
10 Minutes
0 TIME
With contaminated insulation, absorption effects are masked by
high leakage currents and the readings are therefore fairly flat.
The PI test takes the ratio of the values at 10 and 1 minutes -
this is the Polarization Index. The PI test also has the
advantage of not requiring temperature correction. The value of
PI can give a rough guide to condition of insulation, although
the figures should always be interpreted in the context of the
equipment history and your experience.
PI TEST RESULT
<1,0
1,0 to 2,0
2,0 to 4,0
>4,0
There are two specific conditions to be aware of when
interpreting PI tests results, particularly if a history of the
A
equipment is unavailable.
1) Dry, brittle insulation (e.g. on windings) on can give a high
PI but fail under shock conditions.
B
2) If multi-layered insulation fails in one of the layers while the
others retain high resistances, the effect on the test current
will tend to increase the PI value, masking possible
problems from surface leakage caused by dirt and
contamination.
INSULATION CONDITION
Unsatisfactory
Dubious
Good
Very Good