Application Of Test Techniques; Short Time Or Spot Test; Polarization Index (Pi) Test - AVO MEGGER BM25 Guide De L'utilisateur

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 BM25 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
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.

Application of Test Techniques

Insulation
probably good
are
MΩ
M
Moisture and dirt
may be present
0 Time TIME
0
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
A
interpreting PI tests results, particularly if a history of the
equipment is unavailable.
1) Dry, brittle insulation (e.g. on windings) on can give a high
B
PI but fail under shock conditions.
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
10 Minutes
10 Minutes
problems from surface leakage caused by dirt and
contamination.
INSULATION CONDITION
Unsatisfactory
Dubious
Good
Very Good
19