Application Of Test Techniques; Short Time Or Spot Test; Polarization Index (Pi) Test; Dielectric Discharge (Dd)Test - Megger BM25 Guia Del Usuario

APPLICATION OF TEST TECHNIQUES

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 Megger 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.
MΩ
Time 10 Minutes
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 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 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 problems from surface
leakage caused by dirt and contamination.

Dielectric Discharge (DD)Test

The Dielectric Discharge (DD) Test is a diagnostic insulation test that
allows ageing and detorioration of insulation to be assessed. The result is
dependent on the discharge characteristic, so the internal condition of the
insulation is tested, largely independent of any surface contamination. The
charge that is stored in the insulation material under test, is measured
INSULATION CONDITION
Unsatisfactory
Dubious
Good
Very Good
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