AEMC 5070 User Manual page 25

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NOTE
1.
In the case of multilayer insulation, if one of the layers is defective but the resistance
of all the others is high, neither the quantitative insulation measurement nor the
calculation of the PI and DAR quality ratios will reveal the problem.
This makes it important to perform a dielectric discharge test so that the DD Ratio
can be calculated. This test measures the dielectric absorption of heterogeneous or
multilayer insulation and disregards parallel-surface leakage currents.
It involves applying a test voltage for long enough to electrically "charge" the
insulation to be measured (typically, a voltage of 500 V is applied for 30 minutes).
At the end of the measurement, the instrument causes a rapid discharge, during
current circulating in the insulation is measured.
The DD Ratio is then calculated as follows:
The insulation quality rating, as a function of the value found, is as follows:
DD > 7
DD between 7 and 4
DD between 4 and 2
DD < 2
The dielectric discharge test is especially well-suited to insulation measurements on
rotating machines, and in general, to insulation measurements on heterogeneous or
multilayer insulations containing organic materials.
2.
of certain parameters likely to invalidate the "absolute" insulation measurement.
The most important of these parameters are:
Temperature and relative humidity with which insulation resistance varies to a
quasi-exponential law.
The leakage currents (capacitive charging current, dielectric absorption
current) are created by the application of the test voltage. Even though they
that depends on whether the insulation is in good condition or degraded.
the insulation layers are in good or poor condition.
In addition, changes in these ratios over time can be observed and used for
preventative maintenance (e.g. to monitor the aging of the insulation of a
population of rotating machines).
Megohmmeter Model 5070
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