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Re: Official air breakdown voltage?

Original poster: "Mike" <mike.marcum@xxxxxxxxxxxx>
Well, 60Hz is weird and definitely not linear. Haven't seen a breakdown table like DC, but it's around 20kV for 1", 100kV for 10-12", and 11 feet for 500kV (from a lineman that works on 'em hot and needs to know that distance). It's roughly following the rule of thumb 5x the voltage=10x the distance with current controlling how far it grows after that. 

Mike
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, November 27, 2005 11:54 AM
Subject: Re: Official air breakdown voltage?



Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
I say you are correct. Mostly. Voltage requirements for dielectric beakdown, in this case atmosphere, are linear, at slow-pulse DC
From what I have been reading lately, AC voltage requirements for the same dielectric are often much higher. Thus, we can conclude ac requires a much higher voltage to generate, say 1' arcs, than dc does. 



From: "Tesla list" <tesla@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: Official air breakdown voltage?
Date: Sat, 26 Nov 2005 13:58:39 -0700

Original poster: William Beaty <billb@xxxxxxxxxx>

On Fri, 25 Nov 2005, Tesla list wrote:

> Original poster: "Mike" <mike.marcum@xxxxxxxxxxxx>
> The 25KV/inch figure varies widely, but a good starting
> point for hobby level stuff where 50-foot+ discharges are seldom
> encountered and high accuracy isn't needed.

25KV/inch isn't about air breakdown.   Instead it's an attempt to answer
this question:

    I've just created a big long spark.  What voltage caused it?

The answer is:  "who knows?"   There is no way to calculate voltage from
spark length.

Lots of people want to be able to calculate voltage from spark length.
Too bad.  Things don't work that way, so you'll have to measure the
voltage if you want to know its value.  A one-inch spark can be created by
any voltage between about 2KV and 75KV.  For very long sparks, the wattage
of the power supply becomes more important than it's voltage.

On the other hand, if you jumped a 2mm spark between 50cm polished brass
spheres...  And you used well-filtered DC, and slowly brought the two
spheres together... then it's possible to get a rough idea of the voltage
involved. But if one of the spheres had a microscopic scratch, or if the
air was slightly dusty, or if you're in a basement with a bit of Radon
gas, then again you'll have little idea of what voltage caused the spark.


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