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Air breakdown voltage variability

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Proud Mary

Mon Aug 10 2009, 05:33AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

blackgrunge wrote ...

And as for "extremely straight forward".....get owned

I am not quite sure what you intend by this remark, which I had not heard before, but I think I get the gist of it.

Trying to understand the exact nature of a spark discharge is at the very heart of what we do here!

This is a scientific-technical website, where many of our members are anxious to increase their knowledge of high voltage phenomena, rather than be spoon-fed the watered-down factoids of populist edookayshun.

blackgrunge

Mon Aug 10 2009, 06:52AM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

Harry wrote ...

blackgrunge wrote ...

And as for "extremely straight forward".....get owned

My point exactly. I was just commenting on an earlier post that claimed this was a straight forward concept when clearly its not given the explation you provided.

Herr Zapp

Mon Aug 10 2009, 07:35AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Harry -

"......rather tha be spoon fed........"

Well said!

Herr Zapp

Proud Mary

Mon Aug 10 2009, 07:41AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Aha, I see!

Perhaps the Swine 'Flu has made me even more dense and irritable than usual!

I'm sure you'll agree then then that the short, very basic, account I tried to give of the spark discharge process was if anything over-simplified. I might have added that the traditonal forumulation of Paschen's Law as taught to school children is incorrect to the extent that it uses air pressure as a term, when strictly speaking - in my view - it should be air density. Moreover, for gaps on the micrometre scale the law fails altogether - a field intensity of ~50E6V per metre is needed to arc a 5 micrometre gap, whilst for a one metre gap itself only ~3.6E6V is needed, a seeming anomaly we also find with extremely thin dielectric solids.

The counter-intuitive fact that dielectric strength increases by a factor of ten and more when two conductors are almost but not quite touching, will surely one day find exciting applications in nanotechnology if it hasn't done so already. I am really only a beginner here, and can't offer a satisfactory explanation for this phenomenon, but I'm sure other members further along than myself will be able to provide it.

Poor old Kilovolt has probably heard more about spark discharge anomalies than he ever wanted to know, so we should reassure him that Paschen will do very well for a common or garden safety spark gap. Line voltage (230V) safety spark tubes are hermetically sealed, immune to ambient moisture and changes in air pressure and cost only a few Euros, each, so there's little point in making them yourself except for the fun of it. . For higher voltages, you can obtain variable spark gap tubes - effectively a small trigatron - where the breakdown voltage is set by applying grid bias - there are plenty of them around as so-called 'New Old Stock' for a few roubles each. I bought a dozen of them last year (because they were extremely cheap) but haven't got round to needing any of them yet, so I'll happily send one to Kilovolt if it might help him out.

HV Enthusiast

Mon Aug 10 2009, 11:56AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

blackgrunge wrote ...

Harry wrote ...

blackgrunge wrote ...

And as for "extremely straight forward".....get owned

My point exactly. I was just commenting on an earlier post that claimed this was a straight forward concept when clearly its not given the explation you provided.

Firstly, you should read posts more thoroughly and understand the context they are answered before opening your mouth and inserting foot.

Secondly, as I said before, in the context of the original poster's question, Paschen's Curve provides a general enough relationship to provide insight into the variability of arc breakdown between electrodes. And as i said before, Paschen's Curve is an extremely straightforward relationship and easy to understand.

Sure, as Harry pointed out, voltage breakdown in gases is in reality very complex, but again, in the context of the first person's question, Paschen's Voltage law is very straightforward relationship to show variability of breakdown voltage vs. temperature and pressure. Both calculations and experimental data have been strongly correlated with this relationship for both short and long gaps under the range of pressures for which this relationship is applicable.

A good paper to read on the subject which shows both experimental and calculated correlation is one that was published in Physics Review (Issue 56) in 1948 by D. H. Hale.

Proud Mary

Mon Aug 10 2009, 01:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I'm afraid it really is all my fault for adding layers of complication to what should have been a simple answer, and my confusion over American slang, though I did try to make up for my digressive spiel in my last post in this thread.

Of course it's correct to say that Paschen's formula will give a highly reliable prediction of the breakdown voltage of air in any ordinary practical engineering problem such as the design of a safety gap, and I'm sorry if my rambling into obscure and usually minute variables has confused the issue.

When a spark gap is a safety feature, then it should always be very conservatively designed. In practice, this means making the gap significantly smaller than that needed for circuit protection, so it will surely fire before any harm can be done.

Kilovolt doesn't mention the voltage range which must be protected, but in good quality designs at line voltage it's usual to see multiple defences using MOV anti-surge resistors and spark gaps and sometimes LF chokes in combination, so that if the MOVs fail the spark gaps will fire as a last resort.

PS: A lecturer is someone who talks in other people's sleep.

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