ECN Forum Forums Code Related Discussion NEC & other Code issues Pashchen's Law

OK, if you haven't yet read this IEEE paper, do so now, before proceeding:
http://www.combinationafci.com/resources/doc_ieee_combination_afci.pdf

Now, for the point of this thread:

The author refers to a 'law of physics' which, he asserts, proves that a series arc cannot form between copper electrodes (think: broken wire) at household voltages.
(See page 7 of the paper)

The graph included in the paper as "Figure 5" does not support this. It is a graph of voltage vs. air pressure, with the electrodes kept 1" apart at all times. He posted the wrong graph.

I am looking for links, or other DATA, that directly address his assertion.

Any takers?

http://en.wikipedia.org/wiki/Paschen%27s_law

http://home.earthlink.net/~jimlux/hv/paschen.htm

http://www.highvoltageconnection.com/articles/paschen-curve.html

(I don't know why it is highlighting one link and not the other two)

This would apply for a sustained gap, where no re-contact between conductors would happen to re-draw the arc. And in reality, we can see these series arcs happen with very very narrow (smaller than a millimeter) gap. In addition to the air gap breakdown voltage, there is a voltage drop characteristic, much like seen in semiconductors, whenever you have transitions between different metals and the arc. I have only found a small amount of science on these aspects of arcs. But they do play some role in gas-discharge lighting.

"Using Paschen's Law, the minimum breakdown in air is calculated to be 327 V at standard atmospheric pressure. This occurs at a distance of 7.5 µm."

That's the statement we need to explore.

As I read it, that statement claims that you cannot have a sustained arc at household voltages- and the voltages are even higher for larger gaps. 7.5 microns is a very small gap. Microscopic, even.

Yet, we've all encountered 'damage from arcing' at contactor points and panel busses.

Can anyone find a 'voltage vs. gap distance' curve?

Arc damage is cumulative and it tends to be self perpetuating. Arcs damage points and damaged points arc more.
Once you have a high resistance contact it will just get hotter, making the resistance higher, rinse, repeat.

Greg .... are you saying the damage we see is from a momentary spark? As opposed to a sustained arc?

Are we to believe that 240v. can 'spark' but not 'arc?'

A spark is an arc unless you are really splitting hairs. I agree with the author who says it is hard to maintain an arc at normal household voltages and currents but I have a buzz box welder that can maintain an arc at some pretty low voltages.

Greg:
Would I be correct that you have to start the weld by contact? And if you loose the arc, you have to make contact to restrike the arc?

How low can you drop the voltage to?

Yup ... anyone who has welded knows you can do an awful lot with 1500 watts .... even at a modest 12v setting. That's car battery voltages.

FWIW, I've drawn some pretty good arch simply by jump-starting cars. Available energy? Well, enough to detonate one such cars' battery. Yet ... could the arc have been maintained for any length of time? I have not tried it.

"Peer review" is what we're doing here. We are checking the assumptions, assertions, and logic of Mr. Engle. That means finding his sources.

Mr. Engle also makes statements in his paper that infer the composition of the electrodes matters. For a broken wire, that's a pair of copper electrodes. He asserts that there is a difference between and arc and a spark. He asserts there is a not enough energy to light cotton. He asserts that there is a difference between a 'series' arc and a 'parallel' arc.

Yet, the biggest assertion is that the laws of physics precludes arc creation. He refers us to Pashchen's Law. Well .... does the law really say that?

I would say that this matter must be resolved before the AFCI debate can proceed.

In the real world the arc path is facilitated by dust/ particulates, etc.

So Pashchen's assumptions are NOT valid.