COMBITEST422
EN - 97
polarization. Because of the polarized molecules, a polarization (absorption) current
generates, which lowers the total value of insulation resistance.
Parameter
PI
consists in the ratio between the value of insulation resistance measured
after 1 minute and after 10 minutes. The test voltage is maintained throughout the whole
duration of the test and, at the end, the instrument provides the value of ratio:
min)
1
(
.
min)
10
(
.
re
Ins
re
Ins
PI
=
Some reference values:
PI Value
Insulation condition
from 1.0 to 1.25
Not acceptable
from 1.4 to 1.6
Good
>1.6
Excellent
12.2.2. Dielectric Absorption Ratio (DAR)
Parameter
DAR
consists in the ratio between the value of insulation resistance measured
after 30s and after 1 minute. The test voltage is maintained throughout the whole duration
of the test and, at the end, the instrument provides the value of ratio:
)
30
(
.
min)
1
(
.
s
re
Ins
re
Ins
DAR
=
Some reference values:
DAR Value
Insulation condition
< 1.0
Dangerous
from 1.0 to 2.0
Questionable
from 2.0 to 4.0
Good
> 4.0
Excellent
12.3. CHECKING CIRCUIT SEPARATION
A
SELV
system is a zero-category system or safety extra low voltage system
characterized by power supply from an independent (e.g. batteries, small generator set) or
safety source (e.g. safety transformer), protective separation from other electrical systems
