MI 3200 TeraOhm 10 kV
Measurements
25
Diagnostic test is a long duration test for evaluating the quality of the insulation material
under test. The results of this test enable the decision to be made on the preventive
replacement of the insulation material.
DIELECTRIC ABSORPTION RATIO (DAR)
DAR is ratio of Insulation Resistance values measured after 15s and after 1 minute. The
DC test voltage is present during the whole period of the test (also an Insulation
Resistance measurement is continually running). At the end, the DAR ratio is displayed:
s
DAR
R
R
iso
iso
15
min
1
Some applicable values:
DAR value
Tested material status
< 1.25
Not acceptable
< 1.6
Considered as good insulation
> 1.6
Excellent
Note: When determining Riso (15s) pay attention to the capacitance of the test object. It
has to be charged-up in the first time section (15s). Approximate maximum capacitance
using:
V
s
F
U
t
C
10
3
max
.
where:
t ........... period of first time unit (e.g. 15s)
U ......... test voltage.
To avoid this problem, increase the
DIAG. Starting time
parameter in
CONFIGURATION menu, because start of timer in the DIAGNOSTIC TEST functions
depends on the test voltage. The timer begins to run when test voltage reaches the
threshold voltage, which is product of the
DIAG. Starting time
and nominal test voltage
(Unominal).
Using filters (fil1, fil2, fil3) in the DAR function is not recommended!
Analysing the change in the measured insulation resistance over time and calculating
the DAR and PI are very useful maintenance tests of an insulating material.
POLARIZATION INDEX (PI)
PI is the ratio of Insulation Resistance values measured after 1 minute and after 10
minutes. The DC test voltage is present during the whole period of the measurement (an
Insulation Resistance measurement is also running). On completion of the test the PI
ratio is displayed:
