Chauvin Arnoux C.A 6545 Notice De Fonctionnement page 54

machines or very long cables, for example.
On this type of component, the measurement is disturbed at the beginning by parasitic current fl ows
(capacitive load current, dielectric absorption current) which decrease with time. To measure exactly
the representative leak current of the insulation, it is therefore necessary to perform long duration
measurements, to by-pass the parasitic currents present at the beginning of measurements.
Then the PI or DAR ratios are calculated:
PI* = R R
10 min / 1 min
DAR = R R
1 min /
The quality of the insulation is a function of the results.
DAR
< 1,25
< 1,6
> 1,6
For multilayer insulation, if one of the layers is defective but if all the others show strong resistance,
the calculation of the PI and PAR ratios is not suffi cient to show up this type of problem. It is therefore
necessary to supplement the PI and DAR indications by a dielectric discharge test used to calculate
the DD term.
This test measures the dielectric absorption of heterogeneous or multilayer insulation while ignoring the
parallel-surface leak currents. It consists of applying a test voltage for a period suffi cient to electrically
"charge" the insulation to be measured (a typical value is the application of a 500 V voltage for 30 minutes).
An ordinary insulation measurement or a programmed-time test is done for the desired duration and at
the desired test voltage. When ending the insulation measurement the test object is discharged fast and
the capacitance is measured. After one minute the residual current through the test object is measured.
The term DD is then computed from the equation below:
DD =
The indication of the insulation quality as a function of the resulting value is as follows:
Note: The dielectric discharge test is particularly suited for the insulation measurement of rotating
machines and in general for the insulation measurement of heterogeneous or multilayer insulation
containing organic materials.
(2 values to be noted during a 10-min. measurement)
(2 values to be noted during a 1-min. measurement)
30 s
PI Condition of insulation
< 1
< 2
< 4
> 4
current measured after 1 minute (mA)
test voltage (V) x capacitance measured (F)
DD value Insulation quality
7 < DD
4 < DD < 7
2 < DD < 4
DD < 2
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Insuffi cient or even
dangerous
Good
Excellent
Very bad
Bad
Doubtful
Good insulation