Hello Members,

What is duration of DI-Electric Strength Test on Cables/Conductors.

Thanks
Syed

 

Depends on the operating voltage of the cable/conductors

Rgds

Arifeen

 

Replying to Syed Zafar's Wed, Sep 26, 8:41 am, query:

GENERAL INFORMATION
Insulation resistance measurements, using a "Megger-type" instrument at 500 Volts (dc) or 3,000 Volts (dc), can provide accurate indication of the condition of 'new' insulation materials. The results are also meaningful if periodic measurements are made over a number of years and
then compared. However, a one time measurement does not assure high dielectric strength. Thus, insulation resistance tests can be made for
extended time duration, i.e., 30-seconds, one-minute, ten-minutes, and even 15-minutes. Then resulting measurements are ratio-ed, and
evaluated. These tests are called:

A) POLARIZATION INDEX
The ratio of the ten-minute to the one-minute measurement is known as the Polarization Index. In general, a high ratio is indicative of 'good'
insulation.

B) DIELECTRIC ABSORPTION
Carrying out a 10-15 minute test will yield some meaningful information. Clean, dry insulation will show an increasing resistance reaching a
final steady-state level, after 10 to 15 minutes. Wet or dirty insulation will reach a steady state value much sooner.

CAUTION
Results will vary greatly greatly because they are influenced by:
a) insulation class.
b) surface contamination.
c) presence of moisture or chemical contaminants.
d) insulation temperature.

RECOMMENDATION
Use someone qualified to test and evaluate cable's insulation integrity, specially if working with high-voltage cables.

STORY
A client's motor (11,000 Hp, 11 kV) failed. My assignment was to evaluate the insulation integrity of three remaining motors of about the
same age. Using a hi-potential dc voltage source (0-10 kv). I carefully, and slowly, raised the test voltage, expecting to stop at 10 kV. Never
reached it! The winding "popped" at 6 kV. The client angrily questioned my "expertise!" My credibility in tatters, I explained that the
remaining motors would probably have failed on start-up.

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Phil Corso, PE wrote:
<clip>
>STORY
>A client's motor (11,000 Hp, 11 kV) failed. My assignment was to
>evaluate the insulation integrity of three remaining motors of about the
>same age. Using a hi-potential dc voltage source (0-10 kv). I carefully,
>and slowly, raised the test voltage, expecting to stop at 10 kV. Never
>reached it! The winding "popped" at 6 kV. The client angrily questioned
>my "expertise!" My credibility in tatters, I explained that the
>remaining motors would probably have failed on start-up.
<clip>

To explain Mr. Corso's point a bit further, a normal "hi-pot" test on a motor is intended to find weak insulation, not necessarily insulation that has already failed. A motor may fail a hi-pot test, but be able to run normally because the amount of current which is actually leaking may be only a few milli-amps. However, the weak spot in the insulation will normally get
worse, and eventually fail. It is difficult to predict when this will happen, except to say that it won't be at a convenient time.

A motor manufacturer will normally test their motors at a much higher voltage than the actual operating voltage. However, repeatedly
testing coils at higher voltages will itself damage the insulation. You should be sure you know what you are doing when you perform this sort of test.

A "hi-pot" test will find insulation defects between a coil and ground, or between two other points which are supposed to be isolated from one another. It won't find shorts between adjacent turns of the same coil for obvious reasons. There are other tests which can detect this, although I don't know if they are applicable to all types of motors.



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Michael Griffin
London, Ont. Canada
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