AEMC 1000N User Manual page 27

Megohmmeter Model 1000N/1100N
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Insulation resistance values are a function of the type of insulating
material. The actual value you read may vary greatly and is not as
important as the trends of the values over time. This is why the
resistance measurement must be taken in a greater context. Some other
factors to consider are:
Previous Testing Results
These are very important, since they will indicate the decline in the
insulation resistance over time. All new equipment should be tested and
documented to serve as a benchmark for future testing.
Careful Visual Inspection
By taking a very close look at the equipment you are testing it may be
possible to see cracks, excessive moisture, burn marks, etc., which may,
over time, cause catastrophic equipment failure.
Manufacturers' Recommendations for Specific Equipment
The definitive source for information on a specific piece of equipment is
its manufacturer. Most manufacturers will provide basic information
about the insulation resistance which may be encountered during testing.
Comparisons with Similar Equipment
Similar equipment should provide similar insulation resistance values.
This would also remain true when testing cables. For three-phase
systems, it would be very useful to compare resistive values between the
phases.
By performing insulation resistance tests regularly and recording the test
results, it may be possible to predict failure by detecting a downward
trend in the resistance. Careful notations should be made as to
time/date, temperature, applications, etc.
The information contained in this manual is intended only as a guide to
acceptable procedure; it is not intended to be used as a test specification
for specific electrical equipment.
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