Appendix - Theoretical Outline; Measurement Of Polarization Index (Pi) - HT PV-ISOTEST User Manual

11. APPENDIX – THEORETICAL OUTLINE
11.1.

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:
Some reference values:
PI Value
from 1.0 to 1.25
from 1.4 to 1.6
11.2. 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:
Some reference values:
DAR Value
from 1.0 to 2.0
from 2.0 to 4.0
PI 
>1.6
DAR 
< 1.0
> 4.0
Ins . re ( 10 min)
Ins . re 1 ( min)
Insulation condition
Not acceptable
Ins . re 1 ( min)
Ins . re ( 30 s )
Insulation condition
EN - 36
PV-ISOTEST
Good
Excellent
Dangerous
Questionable
Good
Excellent