Megger testing devices under vacuum. 4

[bobo480]

Numerous HVAC & transformer technical manuals warn not to Megger equipment under vacuum. Why?

A vacuum is close to a perfect insulator. What is the reasoning for the caution?

Thanks,

 

[Skogsgurra]

Have a look at Paschen's Law.

A perfect vacuum is a perfect insulator. But there are also not-so-perfect vacuums. They ionize easily. Especially if you put one thousand volts there.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

I think there are a number of factors at work.

If you have a vacuum, then you have no oil. Oil is an insulator.

When you apply a vacuum, it tends to draw water vapor out of the insulation. In the process the water vaporizes. Little bubbles of vapor within the insulation are points of voltage stress intensification.

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[c4xpower]

Apparently, stressing across an open Vacuum Insulated Circuit Breaker with a DC voltage can produce X-Rays!
Most manufacturers testing procedures give a minimum distance to maintain from the equipment under test.
Don't think that this applies to AC pressure testing though.

 

[waross]

I would be wary of AC also. The old shoe X-ray machines that were once ubiquitous in shoe stores across North America applied AC to the X-ray tube and let the tube rectify the current. With high voltage AC you may have two X-ray sources instead of one.

Bill
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"Why not the best?"
Jimmy Carter

 

[bobo480]

Excuse my ignorance, what purpose does the vacuum serve in a "vacuum circuit breaker" non CF6 charged? (other than preventing oxidation of the contacts) Whould it not ionize the gap & flash over at a lower potential?

Bosko

 

[isquaredr]

What's to ionize?
The vacuum interupters are to do exactly that, interupt normal and fault current. Sometimes on MV units the vacuum bottles may be surrounded with sf6.

 

[dpc]

These vacuum bottles have a very high vacuum and provide an excellent means of arc quenching since there is no air to ionize and sustain an arc. Some material is vaporized from the electrode when an arc is drawing, but this generally re-deposits. The arc is interrupted very quickly - sometimes so quickly that transient voltages can be an issue.

 

[waross]

As Skogs pointed out, a perfect vacuum is a good insulator. Meggering across a less than perfect vacuum can cause flash-overs and if the test voltage is high enough, X-rays.
Also, the OP was about meggering HVAC equipment and transformers. Two examples where the term "Vacuum" may mean a reduced pressure that is susceptible to flash-over rather than a perfect vacuum.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DanDel]

Actually, it's AC voltage (or half-wave rectified DC) across a vacuum that will produce X-rays, not pure DC.

 

[bobo480]

OK, going back to my initial question on Megger testing devices under vacuum. The breaker must be evacuated to much lower level to achieve its dielectric properties.

I would then assume Vacuum Insulated Circuit Breakers emit X-Rays while in their open state with rated voltage applied to the line terminal, unless electron streaming is inhibited due to the high vacuum?

Bosko

 

[bobo480]

Sorry,

The question was answered while I posted my last reply.

Thanks to all for the help!

Bosko

 

[Compositepro]

Actually X-rays are produced by the decelleration of electrons striking a target.