DRSSTC Circ. Analy SS TC Theory Tesla Tips Whiteboard

Official Air Breakdown Voltage

from the Tesla List

Updated 7-29-06 & 6-4-11

Also see Breakdown V

Date:  Fri, 25 Nov 2005 17:27:07 -0700 Subject:  Official air breakdown voltage?

Original poster: "JT Bowles"

I have read all over the net, around 25KV/inch  <<I use this one I read on one place, around 30KV/inch Now i have just been told    around 30KV / cm  !!!!   Thatís 3 times the voltage I have been told It is common sense that air pressure, humidity, current, and time of day all affect air breakdown voltage, but: IS there any official way to determine exactly what air breakdown voltage is? ( such as KV/distance

Date:  Fri, 25 Nov 2005 17:43:41 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Terry Fritz

Hi Jason,

There is a "very official" way.  Up until recently, high voltages were "officially" measured with ball sparks gaps since that is all that worked scientifically and repeatably.  A pretty official chart is here:

http://hot-streamer.com/TeslaCoils/Misc/    [The file SGapVolt.jpg has been moved - 5-10-11 John C.]
3MV/Meter is also almost a "standard" value...

Problem is, voltage fields stretch, bend, and distort....  So given a particular situation, figuring out the breakdown voltage can get super messy!!!  There are fancy computer programs that can do it pretty well but it takes like an EE degree to run them...  Some Ph.D folks can do it on paper...  The functions are NOT straight lines at lower (~10kV) voltages either....

But all that is for "DC"......  With "AC", we can cut arc channels through the air and spoil all the of those rules anyway.  It still can be figures out by some high voltage AC gurus, but it is getting to the technical nightmare level at that point.  Over the decades, Tesla coilers have made "rules" based on observations like John Freau's formula that get "us" through without too much brain damage.

So for DC, the chart above does well.... (It works OK for 60Hz stuff too)  There are some "rules of thumb" out there....  Beyond that, there are very messy methods only a few can figure out.  Beyond that, the answer is pretty close to NO. - Cheers, Terry

Date:  Fri, 25 Nov 2005 19:53:13 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "Mike" <mike.marcum

When accuracy is important in labs I think it's set up where climate is controlled (stp and less than 5% humidity) and the electrode diameter is 5-10x the spark length (as close as practical to 2 infinite planes). Also the DC voltage is raised gradually (forget the exact rate, but nowhere near what would be considered impulse). Not cheap and easy in MV ranges. I think those conditions are where the 30kV/cm comes from. As you mentioned, change any of those by even a small amount and you'll get lower breakdown voltages for a given distance. 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. - Mike
 

Date:  Fri, 25 Nov 2005 19:55:16 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "Antonio Carlos M. de Queiroz"
Tesla list wrote:

>Original poster: "JT Bowles"
>
>I have read all over the net, around 25KV/inch  <<I use this one read on one place,           around 30KV/inch Now i have just been told    around 30KV / cm  !!!!   Thats 3 times the voltage I have been told It is common sense that air pressure, humididty, current, and time of day all affect air breakdown voltage, but: IS there any official way to determine exactly what air breakdown
>voltage is? ( such as KV/distance )

30 kV/cm at normal temperatures and pressures, but this the ELECTRIC FIELD intensity that causes ionization in the air, and does not mean that is 30 kV for each cm in a spark. The value is directly proportional to the pressure of the air and inversely proportional to the absolute temperature.  For two large parallel plates with rounded edges, the electric field between them is very uniform, and there are really 30 kV for each cm of spark.For an insulated ball far from other objects, the figure means that a ball with 1 cm of radius (and a well polished surface) requires 30 kV of potential (relative to distant objects) to ionize the air around it.  Two balls with 1 cm of radius cannot hold a voltage of more than 60 kV between them without corona or sparks, no matter what is the distance between them (less if they are close).

Antonio Carlos M. de Queiroz
 

Date:  Sat, 26 Nov 2005 12:32:02 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Jim Lux

At 04:27 PM 11/25/2005, Tesla list wrote:

Official?  In what sense?  The great state of Louisiana (I think) was contemplating legislating the value of pi to be 3 for the simplification of children's schoolwork.

But seriously...

There are tabulated values of breakdown voltage for spark gaps of various dimensions.  The handbook of Chemistry and Physics used to have them, or, you can look for a sphere gap table or a rod gap table (google is your friend).

There has been a lot of published research on the breakdown fields for air and a number of gases at a variety of frequencies, from Paschen in the late 1800's on to the present.  (e.g., right now, I'm involved in some work to quantify microwave breakdown in simulated Mars atmospheres)

If you want an equation:
http://home.earthlink.net/~jimlux/hv/paschen.htm - Paschen's Law
 

Date:  Sat, 26 Nov 2005 12:35:42 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles"

Got it.  Thanks, but 30KV/cm is still massive amounts of power for so little space.  Current definitely affect sparks the most, I'd say.
 

Date:  Sat, 26 Nov 2005 12:36:34 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "Mike"

Hi Terry, All,
I was doing further reading on this and hit on Google this article.
http://www.haefely.com/pdf/scientific/e1-19.pdf which is a study on humidity and under certain conditions. Also polarity seemed to be a player to the conditions; I was searching for more info on the AC breakdown aspects and such but when I got an interesting hit it was a "buy me" thing from an IOP paper site. Still, you may find the free one linked to above interesting, at least that one is available. There was another paper, also a buy me type, that had some method of detecting just before the gap fired and switching 7 us before then to a ball gap to avert the strike 3 us before it happened.  In that one, there was a 150 us rise time of the main charge (DC) to the gap, their mention of detection but the free part ended there.  I wonder if it was some pickup for audio near the gap, listening to corona, like you hear on a AM radio just before a nearby strike? I've heard those tones grow from a slow growl to a high pitch scream over a few seconds then a close by lightning strike.
 

Date:  Sat, 26 Nov 2005 12:27:06 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Ed Phillips

It is common sense that air pressure, humididty, current, and time of day all affect air breakdown voltage, but: IS there any official way to determine exactly what air breakdown voltage is? ( such as KV/distance ) "

Use spheres as terminals at both ends of the spark and use tables of spark distance vs voltage as a function of the radii of the spheres.  These are usually for low frequencies or DC of course. - Ed
 

Date:  Sat, 26 Nov 2005 12:34:02 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles"
Terry, thank you. It seems you have saved my a** once again (though I donít yet know how to read the chart) Very interesting what you typed.
 

 Date:  Sat, 26 Nov 2005 13:55:08 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "Antonio Carlos M. de Queiroz"

Tesla list wrote:

The 30 kV/cm figure for initial breakdown is valid for at least 300 MHz.  (At least, this is what my "Reference Data for Radio Engineers" says.)

Antonio Carlos M. de Queiroz
 

Date:  Sat, 26 Nov 2005 13:56:20 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles" <jasotb@hotmail.com>

Maybe this is why my my capacitor gap fires at 18KV, when my system is run at 8KV (My capacitor gap is make of two very pointy objects, .75 inch apart, and it fires very loudly and frequently.)

So basically, i need to use polished, smooth spheres for my safety gaps?
 

 Date:  Sat, 26 Nov 2005 13:56:59 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: William Beaty

On Fri, 25 Nov 2005, Tesla list wrote:

> Original poster: "JT Bowles"  >
 >
> I have read all over the net, around 25KV/inch  <<I use this one I read on one place,           around 30KV/inch

The e-field threshold voltage for air-breakdown is around 30KV/cm.  The other values above are wrong.

> Now i have just been told    around 30KV / cm  !!!!   That's 3 times the voltage I have been told

That's not a voltage!  You misunderstand.  The breakdown value for air is a value of e-field, not a voltage.  It's the value of e-field located at a point in the air.  It has little to do with the voltage between two wires, or the voltage at a tesla coil terminal.

For example, if you put 30KV across two wires, then hold them an inch apart, the e-field will NOT be 30KV/in in the space between the wires. Instead the e-field will be very strong near the surface of the wires, and very weak in the empty space between them. The strong field near the wires can trigger a spark, even though the breakdown field is 30KV/cm and the wires are an inch apart.
 >
> It is common sense that air pressure, humididty, current, and time of day all affect air breakdown voltage, but:
 >
> IS there any official way to determine exactly what air breakdown voltage is? ( such as KV/distance )

Yep, but there are three things you need to know.  First, to eliminate any tiny regions of strong field, use large, polished balls as electrodes.  Second, to create a parallel field, the maximum distance between the balls must be at least 10x smaller than the balls' diameter (meaning, use large balls with a tiny gap between them.)  And third, there is a voltage between the balls' metal surface and the empty space.  If you simply measure the breakdown field by measuring the breakdown voltage and measuring the gap distance, you'll be off by about 1KV.

See Jim Lux' page on this:

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

William J. Beaty                            SCIENCE HOBBYIST website

Date:  Sat, 26 Nov 2005 13:58:39 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: William Beaty
On Fri, 25 Nov 2005, Tesla list wrote:

> Original poster: "Mike"
> 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.

William J. Beaty                            SCIENCE HOBBYIST website

Date:  Sat, 26 Nov 2005 16:59:22 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Ed Phillips

Got it. Thanks, but 30KV/cm is still massive amounts of power for so little space.  Current definitely affect sparks the most, I'd say."

No current flows until the spark starts, so breakdown voltage can't be a function of it.  Brightness of spark, duration, different matter. - Ed

Date:  Sat, 26 Nov 2005 16:59:35 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: Ed Phillips

The 30 kV/cm figure for initial breakdown is valid for at least 300 MHz. (At least, this is what my "Reference Data for Radio Engineers" says.)

Antonio Carlos M. de Queiroz"

Never noticed that; it's one of my standard references (have several editions starting with #1, but never looked up breakdown voltage). - Ed

Date:  Sat, 26 Nov 2005 17:00:37 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Jim Lux
At 11:35 AM 11/26/2005, Tesla list wrote:
>Original poster: "JT Bowles"
>
>Got it. Thanks, but 30KV/cm is still massive amounts of power for so
>little space.
>Current definately affect sparks the most, I'd say.
Volts isn't power... it's easy to charge yourself up to several tens of kV and produce a nice air spark to a doorknob, but there's not a lot of energy.

Voltage is what gets the spark started, current is what makes it hot.

Date:  Sat, 26 Nov 2005 17:01:39 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: Jim Lux

At 12:56 PM 11/26/2005, Tesla list wrote:
>Original poster: "JT Bowles"
>
>Maybe this is why my my capacitor gap fires at 18KV, when my system
>is run at 8KV
>(My capacitor gap is make of two very pointy objects, .75 inch
>apart, and it fires very loudly and frequently.)
>
>So basically, i need to use polished, smooth spheres for my safety gaps?

Safety caps shouldn't be firing all the time, so the shape isn't super important (but read on), because you can just adjust it to where it fires at the right point (i.e. just not firing with the NST at max drive with no TC hooked up).  However, sharp pointy electrodes tend to have erratic breakdown voltages that are more subject to outside influences.  A loop of AWG10 solid copper wire is stiff enough to hold its shape and relative spacing.   Something like carriage bolt heads would work well as a safety gap (although they wouldn't be all that suitable for your "main gap" for a variety of reasons).  Brass drawer pulls make very nice rounded smooth electrodes, and they're cheap.

Your main gap needs to be something that can take the heat dissipation.  The classic "two bolts" type gap has real problems with getting the electrodes really, really hot, which then reduces the breakdown voltage (because the hot spot emits electrons) and erodes quickly.  A nice, cheap, static gap is a few pieces of 3/4" copper tubing spaced evenly. It's got nice radius of curvature, it's got good thermal conductivity, and lots of surface area for cooling.
 

Date:  Sun, 27 Nov 2005 09:43:52 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "D.C. Cox"

Off a standard toroid of 30 x 7 or 34 x 8.5, discharging to a 7 inch ground terminal, or most any terminal of reasonable size discharging to a well rounded ground terminal of 25-30% of the size of the main terminal, it works out to approx 26.5 kV/cm.  This is a single spark discharge (once per 10-20 second spacing) from a Tesla coil or Van de Graaff generator.

It's a reasonably accurate way to measure true TC output (peak discharge potential in single pulse mode).

Use a 50 kV diode string and approx 6-10 megOhm resistance on the primary side to set the firing rate.  This way the spark will not "grow" as it does at high rep rates and accurate measurements can be taken. - Dr. Resonance

>The 30 kV/cm figure for initial breakdown is valid for at least 300 MHz. (At least, this is what my "Reference Data for Radio Engineers" says.)
>
>Antonio Carlos M. de Queiroz"
>
>    Never noticed that; it's one of my standard references (have several editions starting with #1, but never looked up breakdown voltage). Ed 

Date:  Sun, 27 Nov 2005 09:44:07 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster:
>> The 30 kV/cm figure for initial breakdown is valid for at least 300 MHz. (At least, this is what my "Reference Data for Radio Engineers" says.)

>> Never noticed that; it's one of my standard references (have several editions starting with #1, but never looked up breakdown voltage).
I think some care, here, may be needed in the use of the of the term 'valid'.

Breakdown voltage varies muchly with frequency, even from DC to low audio.  (cf Peek, for one....).  Values presented in Handbooks are for a purpose, which may or may not be obvious in the presentation.  It may be that 'keep it under 30KV/cm' is valid for safe equipment desing [sic], while being useless for measuring, or for complex waveforms.

Most engineering specs need to be understood, in depth, especially at their 'edges'.

best, dwp -        (sample story on request....)
 

Date:  Sun, 27 Nov 2005 09:54:01 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles"

Carlos said:
"The 30 kV/cm figure for initial breakdown is valid for at least 300 MHz.(At least, this is what my "Reference Data for Radio Engineers" says.)

*Well, my coil resonates at i dont know...  My unloaded secondary is 240KHZ. With toroid....i have no clue

Date:  Sun, 27 Nov 2005 09:54:26 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "JT Bowles"

I say you are correct. Mostly, Voltage requirements for dielectric breakdown,  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.

Date:  Sun, 27 Nov 2005 10:03:41 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "JT Bowles"

exactly. a 12KV spark, at 30MA may only be 1/2 inch long. But, at thousands of amperes, it can stretched to over a foot

 Date:  Sun, 27 Nov 2005 10:04:05 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "JT Bowles"

Yes, my main gap is 6 X (5/8") Cu tubes, in series, each 2" long
 

 Date:  Sun, 27 Nov 2005 11:35:42 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Terry Fritz

Hi Antonio,

At 01:21 PM 11/26/2005, you wrote:
>>....
>The 30 kV/cm figure for initial breakdown is valid for at least 300 MHz. (At least, this is what my "Reference Data for Radio Engineers" says.)
>
>Antonio Carlos M. de Queiroz

Interesting!!  We always worry so much about high voltage RF breakdown distances.  RF tends to arc rather easily compared to DC.

However, "capacitive" leakage current is probably the real problem.  If we have say 100kV at 100MHz and there is just 0.1pF of capacitance to ground, the current is:

100000 x 2 x pi x 1e8 x 1e-13 = 6.2 amps!!!!  That will easily burn though any insulation!!

Good insulators at say 1e10 ohms will leak only 10uA at DC, but if you start oscillating the voltage, the leakage currents can get bad very quickly.

So I guess we should not worry so much about RF breakdown voltages, but should rather be concerned with high voltage RF capacitive leakage currents....Cheers, Terry
 

 Date:  Sun, 27 Nov 2005 12:39:32 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: Jim Lux

At 09:03 AM 11/27/2005, Tesla list wrote:
>Original poster: "JT Bowles"
>
>exactly. a 12KV spark, at 30MA may only be 1/2 inch long. But, at
>thousands of amperes, it can stretched to over a foot

Distinguish, here, between a spark and a "drawn arc".  Spark means jumping across the gap.  Drawn arcs are totally different.

Date:  Sun, 27 Nov 2005 12:40:04 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: Jim Lux
At 08:54 AM 11/27/2005, Tesla list wrote:
>Original poster: "JT Bowles"
>
>Carlos said:
>"The 30 kV/cm figure for initial breakdown is valid for at least 300
>MHz.(At least, this is what my "Reference Data for Radio Engineers" says.)
>
>*Well, my coil resonates at i dont know... My unloaded secondary is 240KHZ. With toroid....i have no clue

It would be a mighty unusual coil to have a resonant frequency that is higher with toroid than without.  And if 240 kHz is less than 300 MHz, anything lower than 240 kHz is also less than 300 MHz.

Bear in mind that is for "initial breakdown".. what you see emanating from your topload is usually not "initial breakdown", unless you're doing single shot tests as Dr. R suggested.
 

Date:  Sun, 27 Nov 2005 12:41:00 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: Jim Lux

At 08:54 AM 11/27/2005, you wrote:
>Original poster: "JT Bowles"
>
>I say you are correct. Mostly. Voltage requirements for dielectric breakdown, in this case atmosphere, are linear, at slow-pulse DC

Not precisely true (and on this list, being careful with your wording is important, to avoid confusion.)

Breakdown voltage for a uniform field gap (or one that closely approximates it, as in large spheres separated by a distance much less than their radius) is roughly linear as a function of the product of distance and density  (aka Paschen's law).

> 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.

Where have you been reading this?  I wouldn't conclude this, and neither would most other researchers in the field of HV breakdown, so that "we" might be a bit overstated.

First.. are you talking gas dielectrics or any?  Are you talking gases at atmospheric pressure?  Which gases? (recombination time varies with the gas).

There are some "rise time" effects.. If you shoot a real fast pulse in, it might not breakdown because the pulse is over before the spark has time to develop.   The same kind of thing is true for RF breakdown.. if the field reverses before the ionized atom/electron has time to collide with another to start an avalanche, then the spark might not progress very far.

If you are interested in really learning about spark breakdown.. I'd recommend you find a copy of "Spark Discharge" by Bazelyan and Raizer.  It will set you back perhaps $100 or so, brand new, so a library might be a better strategy.

Another good book might be Cobine, Gaseous Conductors, which talks about spark breakdown, although more oriented towards arcs with some stuff about glow discharges. this one was published by Dover in paperback for about $10, so it is cheaper on the used market.

The great lightning book by Martin Uman: "Lightning" also published by Dover (and, unlike Cobine, it's still in print) has a fair amount of stuff on discharge processes.

For HV stuff in general, a good "free" start is the High Power Microwave Transmitter report by William North, which is on hot-streamer.com.

http://hot-streamer.com/TeslaCoils/OtherPapers/NorthReport/ - <=This paper has been moved/removed 5-9-11 John C.

Interlibrary loans can be real useful. A bit of self education will go a long way to eliminating "foot in mount" disease. - Jim
 

Date:  Sun, 27 Nov 2005 15:03:43 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "Malcolm Watts"

On 27 Nov 2005, at 10:03, Tesla list wrote:

> Original poster: "JT Bowles"
 >
> exactly. a 12KV spark, at 30MA may only be 1/2 inch long. But, at thousands of amperes, it can stretched to over a foot

Stretch is not the same thing as jump. It is for precisely this reason that a lot of people used to think that their coils were putting out MV instead of 100's of kV. - Malcolm
 

 Date:  Sun, 27 Nov 2005 15:28:46 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "Malcolm Watts"

Hi Terry,

On 27 Nov 2005, at 11:35, Tesla list wrote:

I don't think the story ends there either. I've used 15kHz frequencies to test the insulation breakdown of polyesterimide-coated wire (my favourite :) and noted that at those frequencies, unlike DC, there is a lot of corona heating of the insulation going on. It is a rather brutal insulation test but a goody if you want to find the best quality insulation. - Malcolm 

 Date:  Sun, 27 Nov 2005 17:44:51 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: FIFTYGUY

In a message dated 11/27/05 12:17:53 PM Eastern Standard Time,

>exactly. a 12KV spark, at 30MA may only be 1/2 inch long. But, at thousands of amperes, it can stretched to over a foot

Almost everybody here has stretched a 12kV 30mA spark to about 3". Many have stretched a 12kV 500mA spark to well over a foot. So it doesn't take thousands of amps.  FWIW, I notice the arcs from my "pole-mounted power distribution transformer" are the same size and length at 14.4kV as from the 7.2kV configuration, provided the power in the arc is the same. - Phil LaBudde
 

Date:  Sun, 27 Nov 2005 18:08:50 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "JT Bowles"
You are right. I need to either phrase things correctly, or not comment at all.

And, i think i accidentally said the "AC requires much more power to generate sparks" in reverse.

Even I am confused now at what I have typed. I'll assume you are all correct.   :P
 

Date:  Sun, 27 Nov 2005 18:28:43 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles"

Oh, forgive me. I thought it said 300Khz, not MHz I am not saying a toroid increases frequency. Everyone, intelligent, knows adding capacitance lowers freq. 

Date:  Sun, 27 Nov 2005 18:29:26 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles"

Drawn arc, I do believe. If you start at 1' and use 12KV, with 5000amps of current, you wont get a spark at all. But if you begin with a drawn arc, you may get it to STRETCH to 1'.

Clear?
 

Date:  Sun, 27 Nov 2005 19:32:23 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "Mike"
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 poster: "JT Bowles"
>
>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.

Date:  Mon, 28 Nov 2005 00:20:36 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "David Rieben"
JT,

Hmm, 12 kV at 5000 amps, that's 60,000 kVA! I'd think that you could draw a plasma arc over 20 ft. with that kind of power, assuming that you didn't get FATALLY burned by being as close as would be required to pull this stunt! Think of the incredible flash arc plasma you see when a main primary power line gets a fault circuit short across it and that would be what you would experience by discharging continuous 12 kV, 5000 amp electrical source! It would be one heck of a site, but unless you were positioned  >50 ft. from the spark source, you most likely wouldn't live to tell about it!

David Rieben

Date:  Mon, 28 Nov 2005 00:22:17 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "Gerry  Reynolds"

Hi JT,

You are new to this field  (I'm gathering from your previous posts), so you won't be able to phrase things correctly probably for a while.  Commenting "not at all" is not the right path either.  I would suggest, instead, to ask questions, respect the time that people take in trying to answer them, and be more flexible and open minded in what they are trying to say.  There are a lot of professional people in this group that know a lot more than I do and I respect their opinions.  Many have Ph.D's and specialize in this field. They do know what they are talking about and we all can learn from them. Even at that, there is probably no one in this group that hasn't made a hasty statement that turned out incorrrect.  The more understanding we are toward each other, the better the learning experience and more useful the forum will be. - Gerry R.

>Original poster: "JT Bowles"
>
>You are right. I need to either phrase things correctly, or not comment at all. And, i think i accidentally said the "AC requires much more power to generate sparks" in reverse.
>
>Even I am confused now at what I have typed. I'll assume you are all correct.   :P 

Date:  Mon, 28 Nov 2005 00:23:28 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "JT Bowles"

Amperage determines how far an arc may be drawn. Cannot it be true that at 20KV, one inch may be stretched to 10 feet? (perhaps if the amperage is INCREDIBLY HIGH, millions of amps) Thank you mike- 20Kv is a common measurement

Date:  Mon, 28 Nov 2005 00:22:52 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: Jim Lux

At 06:32 PM 11/27/2005, you wrote:
>Original poster: "Mike" mike.marcum
>
>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.

Be aware that "required clearance distances" are assuming typical wiring and construction practices and very non-uniform fields (as in a wire strung between insulators?)
 

Date:  Mon, 28 Nov 2005 09:25:37 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: Jim Lux

At 11:23 PM 11/27/2005, you wrote:
>Original poster: "JT Bowles"
>
>Amperage determines how far an arc may be drawn. Cannot it be true that at 20KV, one inch may be stretched to 10 feet? (perhaps if the amperage is INCREDIBLY HIGH, millions of amps). Thank you mike- 20Kv is a common measurement

I don't think you could stretch a 20kV source quite that far.. with a lot of current behind it, perhaps a few feet. here's a picture of big Jacob's ladder at the Deutsches Museum
http://www.luxfamily.com/travel/austria/images/dm2.jpg

I think it's 60kV.  The squares in the mesh are probably 2" or 3", for scale, but they're a lot closer to the camera.  The distance at the top of the horn gap is around 50-60 cm, as I recall, maybe a bit bigger.  That's a many hundred kVA transformer driving it, by the way.
 

Date:  Mon, 28 Nov 2005 10:12:09 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "Mike"

That 33300 kVA spark that was posted awhile back was arcing probably 70 feet and was actually cut short. Wish they woulda let it run for a min or 2 to see how far it woulda grew. - Mike

Date:  Mon, 28 Nov 2005 17:57:44 -0700 - Subject:  Re: Official air breakdown voltage?

Original poster: "Mike"
Actually no where near millions of amps are needed. At 20kV probably a few amps or so will reach 10 feet if the conditions are right, maybe. Rule of thumb for that is around 3.5 kVA per foot of plasma trail, but this is assuming you crank up the voltage along with the current (you'd have better luck with 70kV at .5 A as opposed to 3500v at 10A). Otherwise the spark blows itself out from it's own magnetic field before it grows too far (at least that's my guess why). - Mike

Date:  Mon, 28 Nov 2005 18:07:02 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "David Rieben"

Hi Mike,

Yes, I remember that video clip. I believe Bert Hickman posted it on his Stonridge Engineering high voltage page.  IIRC, it was a 500 kV line (I assume ~289 kV single phase to ground), which is A LOT more than 12 kV. However, I, like you, would have been curious to see just how long, both time wise and length wise, that arc would have gone unhendered ;^), - David Rieben

Date:  Mon, 28 Nov 2005 18:14:48 -0700 - \Subject:  Re: Official air breakdown voltage?

 Original poster: Aaron Banerjee

I've seen anywhere from about 4Kv/cm for a point gap to 30Kv/cm for a spherical gap.  Once you get the spark going, I'm not sure how far it could be stretched -- I guess so long as you keep the "conductive" path, you could conceivably go quite a ways (depending on amperage I suppose).

If you're trying to calculate voltage from a spark, I'd suggest starting so far away that you don't get a spark, and then moving in until you first get breakdown.  Before doing this, of course, you will have taken appropriate safety precautions.

I'm thinking that the length you get will be somewhat closer to the "breakdown voltage" you'll see in a table.  Maybe that could be used to estimate output voltage?

Date:  Wed, 30 Nov 2005 13:18:43 -0700 - Subject:  Re: Official air breakdown voltage?

 Original poster: "Gerry  Reynolds"

Hi Mike,

Is that the 500KV transmission line switch that opened "hot"??  I just sent an mpeg of this to JT and I think Terry has it on www.hot-streamer.com/temp somewhere. - Gerry R

arcs are totally different.

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