21
3.5. dAr (dIELECtrIC AbSorPtIoN rAtIo) ANd PI (PoLArIzAtIoN INdEx)
In addition to the quantitative value of the insulation resistance, it is very useful to calculate the quality ratios of the insulation
(the DAR and the PI) because they can eliminate the influence of certain parameters likely to invalidate the “absolute” insulation
measurement. They also serve to predict the evolution of insulation quality over time.
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 if they gradually fade, they perturb the measurement at the start, for a more or less long time depending
on whether the insulation is sound or degraded.
These ratios therefore complete the “absolute” insulation value and reliably reflect the condition, good or bad, of the insulation.
In addition, long-term observation of the evolution of these ratios is a way to monitor the ageing of the insulation. For example,
that of a revolving machine or of a long cable.
The values of DAR and PI are calculated as follows:
DAR = R
1 min
/R
30s
(2 values to be noted during a 1-min measurement)
PI = R
10 min/
R
1 min
(2 values to be noted during a 10-min measurement)
The times of 1 and 10 minutes for the calculation of the PI and the times of 30 seconds and 1 minute for the calculation of the
DAR can be modified using the CONFIG key or in SET-UP (see §5), to adapt to particular applications.
3.5.1. MMEASUrEMENt
There are several ways of measuring the DAR and the PI:
In manual configuration
START/STOP
Press the START/STOP button again
to stop the measurement.
502 M
Ω
502 V
978 nA
Elapsed Time 00:10:10
DAR (30s/60s)
PI (1.0m/10m)
Capacitance
2.64
1.05
320 nF
BURN
START/STOP
Press the START/STOP button.
Wait one minute for the DAR or ten minutes for the PI (if the default
values are used).
10 100 1
10
10
10
100
1
100 1
k
Ω
M
Ω
G
Ω
T
Ω
499 M
Ω
502 V
978 nA
Elapsed Time 00:10:08
BURN
Set: 500 V
