# Breakdown Voltage of Air at given Distance

#### imzack

Joined Nov 3, 2010
72
I have looked everywhere, but cannot find an equation that specifies and explains the following case...

I have two plates (A Capacitor), with no dielectric, only AIR.

Given the separation distance of the plates, at what voltage would I get a spark gap?

I don't know the voltage when Air Ionizes, but I am sure it changes based on temp and humidity, but I need somewhere to start.

Any help would be greatly appreciated!

Thanks,

Zack

#### imzack

Joined Nov 3, 2010
72
Is there a proper equation though?

I see something here on wiki.... but not sure if it only applies to parallel-plates, or if it is a generalized example...

"A parallel plate capacitor can only store a finite amount of energy before dielectric breakdown occurs. The capacitor's dielectric material has a dielectric strength Ud which sets the capacitor's breakdown voltage at V = Vbd = Udd. The maximum energy that the capacitor can store is therefore"

#### BR-549

Joined Sep 22, 2013
4,938

#### dl324

Joined Mar 30, 2015
12,924
Seems to be 30KV/cm

#### dl324

Joined Mar 30, 2015
12,924
Is there a proper equation though
Seems like simple arithmetic to me.

If you have plates 1cm apart air will ionize at 30KV. At 0.5cm, 15KV; at 0.1cm, 3KV...

And as you mentioned; temperature and humidity will affect arc voltage.

#### BR-549

Joined Sep 22, 2013
4,938
Don't worry about the temp or humidity. For air, it's the distance.
It only takes a 200 - 300 V. Even less.

#### DerStrom8

Joined Feb 20, 2011
2,390
The voltage in order to "jump" a certain distance greatly depends on temperature, humidity, air quality, altitude, etc. The general ratio is 1.1kV per millimeter (or 11kV per centimeter), but there is no accurate way of determining it. Your location and environment are huge factors.

#### dl324

Joined Mar 30, 2015
12,924
The general ratio is 1.1kV per millimeter (or 11kV per centimeter)
Do you have document pointer? I've used 25KV/inch (which is close to 1.1KV/mm) for many decades, but can't recall where I got that number. When I googled, 30KV/cm came up most often.

I was so unsure of my original number that I deleted the post

#### DerStrom8

Joined Feb 20, 2011
2,390
Do you have document pointer? I've used 25KV/inch (which is close to 1.1KV/mm) for many decades, but can't recall where I got that number. When I googled, 30KV/cm came up most often.

I was so unsure of my original number that I deleted the post
I can't remember where I learned that. Just another thing I picked up over the years. It's hardly worth remembering, though, because it is so easily affected by a wide variety of factors.

#### dl324

Joined Mar 30, 2015
12,924
I can't remember where I learned that. Just another thing I picked up over the years.
Same here. Guess I should just be thankful for what I can remember...

#### cmartinez

Joined Jan 17, 2007
7,368
Your location and environment are huge factors.
I'd also consider altitude, since I'm sure air density plays a role too

#### Lestraveled

Joined May 19, 2014
1,946
In Florida we used 10KV per inch. When you get above 30KV that rule goes away. Geometric shapes plays a big part breakdowns (arcs). Every edge MUST be rounded. I am currently a consultant to a chemical company that is developing resins for high substrates. Yesterday we tested a coupon to 67KV. It arced around the coupon. (information withheld.)

Please believe me, there is no equation that accurately predicts the electrical breakdown of air.

#### alfacliff

Joined Dec 13, 2013
2,458
it depends also on the shape of the electrodes. a sharp point takes less voltage and a large round sphere takes more. there is info in the ITT electronic data for radio engineers book, unfortunatly mine is at home.

#### DerStrom8

Joined Feb 20, 2011
2,390
I'd also consider altitude, since I'm sure air density plays a role too
Yep:

The voltage in order to "jump" a certain distance greatly depends on temperature, humidity, air quality, altitude, etc.
it depends also on the shape of the electrodes. a sharp point takes less voltage and a large round sphere takes more. there is info in the ITT electronic data for radio engineers book, unfortunatly mine is at home.
Another excellent point!

#### nsaspook

Joined Aug 27, 2009
8,515
it depends also on the shape of the electrodes. a sharp point takes less voltage and a large round sphere takes more.
Yes, it does as this video demonstrates. The 90kV potential is on a plate designed to be a Corona ring but someone placed a terminal backwards outside the radius of the ring. It's a 100mA supply so this was actually very loud.

But captured with no audio so it was edited in later.

#### alfacliff

Joined Dec 13, 2013
2,458
there are ignition testers that have two electrodes in a plastic tube you connect to the coil and pull apart to test the high voltage output the farther, the higher.

#### wayneh

Joined Sep 9, 2010
17,153
In Florida we used 10KV per inch.
I don't know when and where I learned it, but 10kV per inch is also the number in my head. It's probably something I looked up once long before the internet came along.

#### WBahn

Joined Mar 31, 2012
26,398
The electric field strength at which air (STP) breaks down is usually taken to be 3 MV/m (30 kV/cm). But that, IIRC, is for non-ionized air and is the field strength needed to ionize air molecules directly. In practice, there will always be some ions and free electrons (due to cosmic radiation, if nothing else) and so it will take a smaller field strength to initiate avalanche breakdown. My guess (I don't know) is that the 11 kV/cm value is based on empirical data for the "normal" radiation environment at the surface of the earth and possibly for some particular humidity level.

The actual voltage across a gap that will produce the breakdown field strength can't be determined just by using the voltage and the distance, since that would require a homogeneous electric field. If the electrodes have any curvature, that will affect the field intensity near the surface. Any sharp protrusions will greatly concentrate the field and result in discharge initiation at a much lower voltage than would otherwise be the case.

#### WBahn

Joined Mar 31, 2012
26,398
Don't worry about the temp or humidity. For air, it's the distance.
It only takes a 200 - 300 V. Even less.
Do you even begin to understand how self contradictory this is? You say it depends only on the distance, and then give a voltage without regard to any distance whatsoever.

#### ian field

Joined Oct 27, 2012
6,539
I'd also consider altitude, since I'm sure air density plays a role too
Around about the time of WW2, they started getting aircraft up to altitudes where the ignition spark could happen pretty much anywhere *EXCEPT* the plug gap.