Hi everybody,

I would like to know:

- Are Polarisation Index and Dielectric absorption sufficient to judge if the insulator is good or bad?

- Can we get the same results with DC test and AC test?

Regards.

 

When you say an insulator is "good" or "bad", what do you mean? If you mean Resistance, as indicated by the subject line of your posting, then dielectric absorption is not a measure of insulation resistance. It is possible to have almost infinite resistance, but still have lots of dielectric absorption. Whether or not this is "bad" depends on your application. As for AC or DC tests, high enough frequency AC will test dielectric absorption, but it will be hard to separate the effect from pure capacitance. Dielectric absorbtion can be tested for by placing a high DC voltage on the cable, then shorting it out to zero volts briefly, then watching for a delayed rise in voltage after the short is removed. That is dielectric absorbtion. DC tests will only test insulation resistance.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

Replying to Sam Bones' Aug 2, query.... if a historical record is unavailable, the answer is "yes" to part 1; a guarded "yes" to part 2.

Polarization-Index:
If your goal is to assess insulation 'condition' rather than 'quality', then the dielectric test is useful. However, a high 'PI' alone is not always an indication of "good" dielectric strength. Conversely, a low 'PI' is often indicative of "bad" dielectric strength.

DC or AC Over-Potential Testing:
Either over-potential test is riskier! That is, you must weigh the risk of insulation breakdown during a test vs the greater consequence of breakdown during operation.

If an actual case involving machines that were less than 2-years old, is helpful, let me know.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Hello again,
I mean by good or bad : if the insulation tested is dammaged or not, is there excessive moisture and contamination and if the equipement under test is still operational?

Regards

 

Hi,
Thank you Mr Phil Corso and Robert Scott.

Best Regards
Sam Bones