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Test Mode Summary
Insulation Resistance Testing
This test mode measures insulation resistance continuously at the selected voltage. The test is
applied for a short but specific duration, immediately after which a reading is taken. Test duration is
typically 30 or 60 seconds and the S1-5010 will allow you to set this duration, and the test will finish
automatically. The reading will not necessarily be the maximum value but if the same duration 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.
Example of IR Graphic display
Polarization Index (PI) Test
Time resistance tests are successive insulation readings at specified times, taking the ratio of
dielectric absorption into account, and are independent of temperature. Good insulation generally
shows an increase in resistance over a 10 minute period. With contaminated insulation, absorption
effects are masked by high leakage currents and the readings are therefore fairly flat.
The PI test is a particular example of the time resistance method, and takes the ratio of the insulation
values at 1 minute (designated T1) and at 10 minutes (designated T2) - this is the Polarization Index.
The PI test also has the advantage of not requiring temperature correction.
Example of PI display
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