Chauvin Arnoux C.A 6549 Mode D'emploi page 59

This makes it judicious to perform a dielectric discharge test, from which the DD term 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 500V is applied for 30mn).
At the end of the measurement, the instrument causes a rapid discharge, during which the capacitance of
the insulation is measured; 1mn later, the residual current circulating in the insulation is measured.
The DD term is then calculated as follows:
DD = current measured after 1 minute (mA) / [test voltage (V) x measured capacitance (F)]
The insulation quality rating as a function of the value found is as follows:
Value of DD
DD > 7
7 > DD > 4
4 > DD > 2
DD < 2
Note: 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.
§
What are the DAR (Dielectric Absorption Ratio) and the PI (Polarization Index)?
It is useful to calculate insulation quality ratios in addition to the quantitative insulation resistance value,
because they can be used to eliminate the influence 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 spurious currents (capacitive charging current, dielectric absorption current) created by the application
of the test voltage. Even though they gradually vanish, they perturb the measurement at the start, for a length
of time that depends on whether the insulation is in good condition or degraded.
These ratio complete the "absolute" insulation value, and reliably reflect whether the insulations are in good
or poor condition.
In addition, changes in these ratios over time can be observed and used for predictive maintenance, e.g.
to monitor the aging of the insulation of a population of rotating machines.
The DAR and PI ratios are calculated as follows:
PI = R 10 min / R 1 min
DAR = R 1 min / R 30 sec
Remark:
Note that the times of 1 & 10 mn for the calculation of PI and 30 & 60 seconds for the calculation of DAR
are those currently used and programmed as defaults in the instrument.
They can however be modified in SET-UP to adapt to a possible change in a standard or to the needs of a
specific application.
(2 values to be noted during a 10-mn measurement.)
(2 values to be noted during a 1-mn measurement.)
59
Quality of insulation
Very poor
Poor
Doubtful
Good