Biddle MEGGER - Page 42

values good evidence has been obtained that the motor
in question is in reliable condition as far as its insula-
tion is concerned. If there is an appreciable difference
in the two, there is, on the other hand, good evidence
that more thorough reconditioning is called for. If the
insulation fails under the 2500 volt test, after following
steps 1, 2 and 3, we believe there is a likelihood that the
motor in question, which in this example is a unit con-
sidered to be classed in the 300 to 1000 volt range, would
fail in service even though an attempt were made to
recondition it on the basis of low voltage tests only.
The multi-voltage method can also be helpful in deter-
mining the presence of excessive moisture in the insulation of
equipment rated at voltages equivalent to or greater than the
highest voltage available in the multi-voltage Megger instru-
ment in use. In other words, even though the highest Meg-
ger instrument voltage available does not stress the insulation
beyond its rating, a 2-voltage test can, nevertheless, often re-
veal the presence of moisture. If the insulation resistance
is first tested on the short time reading basis—at one voltage,
e.g., 500 volts, and then at a higher potential, e.g., 2500 volts
(nominal in relation to the voltage rating of the equipment
being tested), a lower value of insulation resistance at the
higher d-c test voltage usually indicates the presence of mois-
ture. The applied voltages should preferably be in the ratio
of 1 to 5. Experience has indicated that a change of 25% in
the insulation resistance value, with a 1 to 5 ratio in test
voltages, is usually due to the presence of an excessive amount
of moisture.
This method is not based on a dielectric absorption phe-
nomena, but it does relate to the ‘““Evershed Effect”.
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