Biddle MEGGER - Page 44

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INTERPRETATION—MINIMUM VALUES
Insulation resistance of electrical equipment is affected by
many variables such as the equipment design, the type ol in-
sulating material used, including binders and impregnating
compounds, the thickness of the insulation and its area, cleanli-
ness (or uncleanliness) , moisture, and temperature. For insula-
tion resistance measurements to be conclusive in analyzing the
condition of equipment being tested, these variables must be
taken into consideration.
Such factors as the design of the equipment, the kind of in-
sulating material used, and its thickness and area cease to be
variables after the equipment has been put into service, and
minimum insulation resistance values can be established within
reasonable tolerances. The variables that must be considered
after the equipment has been put into service, and at the time
that the insulation resistance measurements are being made,
are uncleanliness, moisture, temperature, and 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 con-
sidered as good housekeeping and is essential in the main-
tenance 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 which 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 here summarized. They should
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