HT EASYTEST - Measurement of Polarization Index (PI); Dielectric Absorption Ratio (DAR)

EASYTEST-COMBI519
EN - 99
12.2.1. Measurement of polarization index (PI)
The purpose of this diagnostic test is to evaluate the influence of the polarization effects.
Upon the application of a high voltage to insulation, the electric dipoles distributed in the
insulation align in the direction of the applied electric field. This phenomenon is called
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(.
reIns
reIns
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(.
sreIns
reIns
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