TeraOhm 5kV
14
Time dependence test – Diagnostic test
POLARISATION INDEX
The purpose of this diagnostic test is to evaluate the influence of the polarization part of
insulation (Rpi, Cpi).
After applying a high voltage to an insulator the electric dipoles distributed in the
insulator align themselves with the applied electrical field. This phenomena is called
polarization. As the molecules polarize, a polarization (absorption) current that lowers
the overall insulation resistance occurs.
The absorption current (I
PI
) typically collapses after a few minutes. If the overall
resistance doesn’t increase
,
this means that other currents (e.g. surface leakages)
dominate the overall insulation resistance.
-
PI is defined as the ratio of the measured resistances in two time slots. The most
typical ratio is 10 min value to 1 min value but not as a rule.
-
The test is typically performed at the same voltage as the insulation resistance test.
-
If the one-minute insulation resistance is greater than 5000 M
Ω
, then this
measurement may not be valid (new modern types of insulation).
-
Oiled paper used in transformers or motors is a typical insulation material that
requires this test.
In general, insulators that are in good condition will show a “high” polarization index
while insulators that are damaged will not. Note that this rule is not always valid. Refer to
Metrel’s handbook
Insulation Testing Techniques
for more information.
General applicable values:
PI value
Tested material status
1 to 1.5
Not acceptable (older types)
2 to 4 (typically 3)
Considered as good insulation (older types)
1 (very high insulation resistance) Modern type of (good) insulation systems
Example for minimum acceptable values for motor insulation (IEEE 43):
Class A =1.5, Class B = 2.0, Class F =2.0, Class H =2.0.
DIELECTRIC DISCHARGE
An additional effect of polarization is the recovered charge (from Cpi) after the regular
discharging of a completed test. This can also be a supplementary measurement for
evaluation of the quality of insulating material. This effect is generally found in insulating
systems with large capacitance Ciso.
DD value
Tested material status
> 4
bad
2 - 4
critical
< 2
good
