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Interpretation-Minimum Values
Insulation resistance of electrical equipment is affected by many variables
such as the equipment design; the type of insulating material used, including
binders and impregnating compounds; the thickness of the insulation and
its area; cleanliness, moisture and temperature. For insulation resistance
readings to be a conclusive measure of the condition of the equipment
being tested, these variables must be taken into consideration.
After the equipment has been put into service, factors such as the design of
the equipment, the kind of insulating material used, and its thickness and
area cease to be variables, allowing minimum insulation resistance values
to be established within reasonable tolerances. The variables that must be
considered after the equipment had been put into service, and at the time
that the insulation resistance measurements are being made, are cleanliness,
moisture, temperature and mechanical damage (such as fractures).
Good Housekeeping
The most important requirements in the reliable operation of electrical
equipment are cleanliness, and the elimination of moisture penetration
into the insulation. This may be considered as good housekeeping, and
is essential in the maintenance of all types of electrical equipment. The
very fact that insulation resistance is affected by moisture and dirt, with
due allowances for temperature, makes the Megger insulation tester the
valuable tool that it is in electrical maintenance. It is at once a gauge of
cleanliness and good housekeeping as well as a detector of deterioration
and impending trouble.
What Readings to Expect – Periodic Tests
Several criteria for minimum values of insulation resistance have been
developed and are summarized here. They should serve as a guide for
equipment in service. However, periodic tests on equipment in service will
usually reveal readings considerably higher than the suggested minimum
safe values.
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A STITCH IN TIME