stated thus: Insulation resistance should be
approximately 1 megohm for each 1,000 volts of
operating voltage with a minimum value of 1 megohm.
For example, a motor rotated at 2,400 volts should have
a minimum insulation resistance of 2.4 megohms. In
practice, megohm readings normally are considerably
above this minimum value in new equipment or when
insulation is in good condition.
By taking readings periodically and recording
them, you have a better basis for judging the actual
insulation condition, Any persistent downward trend is
usually fair warning of trouble ahead, even though the
readings may be higher than the suggested minimum
safe values. Equally true, as long as your periodic
readings are consistent, they may be all right even
though lower than the recommended minimum
Common Test Voltages
Commonly used dc test voltages for routine
maintenance are as follows:
DC TEST VOLTAGE
up to 100 volts
440 to 550 volts
4,160 volts and above
(See selector switch
settings on Megger© in fig.
100 and 250 volts
500 and 1,000 volts
1,000 to 2,500 volts or
1,000 to 5,000 volts or
Use care in applying test voltage to the
component to be tested. Do not use a high-test
voltage on low-voltage equipment or
Do not exceed the commonly used test voltages
mentioned above unless you are following the
equipment manufacturers instructions to do so. On the
other hand, a test voltage lower than the operating
voltage of the component to be tested may not reveal a
problem that the test should indicate. If the test voltage
is too low, you may get no more than a resistance
reading such as you would get with an ohmmeter.
Causes of Low Insulation
Insulation resistance varies with the temperature.
The effect of temperature depends on the type of
insulation, the amount of moisture in and on the
insulation surface, and the condition of the surface.
The amount of moisture in insulation has a great
effect on its resistance. For meaningful results, tests of
insulation resistance should be made under as nearly
similar conditions as practical. Long cables can be
exposed to a variety of conditions along the cable route
at the same time. A comparison of readings may not
indicate a change in insulation condition.
An accumulation of things like dust, dirt, and
moisture can cause low-resistance readings. A motor
stored or kept idle for a while may have to be cleaned
and dried out before being installed and placed in
Records should be kept where tests are performed
periodically. The frequency of the tests should be based
on the importance of the circuit. One test each year is
usually adequate. Records of each circuit or component
may be compared.
Trends may indicate a future
problem, and corrections may be made in time to
prevent future problems in cables or components, like
motors or transformers.
Effects of Temperature
If you want to make reliable comparisons between
readings, you should correct the readings to a base
temperature, such as 20°C (68°F), or take all your
readings at approximately the same temperature
(usually not difficult to do). We will cover some general
guidelines to temperature correction.
One rule of thumb is that for every 10°C (50°F)
increase in temperature, you halve the resistance; or for
every 10°C (50°F) decrease, you double the
resistance; for example, a 2-megohm resistance at
20°C (68°F) reduces to 1/2 megohm at 40°C (104°F).
Each type of insulating material will have a
different degree of resistance change with temperature
variation. Factors have been developed, however, to