Chauvin Arnoux C.A 6549 Bedienungsanleitung Seite 74

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 infl uence 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 according to a
quasi-exponential law.
the disturbance 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 ratios complete the "absolute" insulation value, and reliably refl ect whether the insulation is 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 ageing 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 min 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 modifi ed in SET-UP to adapt to a possible change in a standard or to
the needs of a specifi c application.
A capacitance in parallel to the insulation resistance extends the settling times of the
measurements. This can affect or even inhibit the measurement of DAR or PI (depending on
the time set for recording the fi rst resistance value). The following table shows typical values
for the capacitance in parallel to the insulation resistance at which a successful DAR or PI
measurement is still possible (at the default time for recording the fi rst resistance value).
100 k
50 V 40 µF
100 V 40 µF
500 V 20 µF
1000 V 5 µF
2500 V 2 µF
5000 V 1 µF
(2 values to be noted during a 10-min measurement.)
(2 values to be noted during a 1-min measurement.)
1 M 10 M 100 M
40 µF 20 µF 10 µF
40 µF 20 µF 10 µF
20 µF 10 µF
5 µF 5 µF
2 µF 2 µF
1 µF 1 µF 0,5 µF
74
1 G 10 G
1 µF 0 µF
1 µF 0 µF
5 µF 2 µF 1 µF
2 µF 2 µF 1 µF
1 µF 0.5 µF 0 µF
0.5 µF 0 µF
100 G
0 µF
0 µF
1 µF
1 µF
0 µF
0 µF