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Spark gap physics.
开云体育Hello All Richard Andrews wrote:
The required voltage could be 2-3 times that due the compression ratio of 8.5 to 1 or higher. I worked a decade as instructor and maintenance contractor at a heavy industry that performed smelting. Lots of 138kV switch gear. At 138kV an air blaster is required to blow out the arc that ensues whenever a switch opens. Assume a 100A breaker rated for 150kV and the 138kV supply conductors have 10? resistance. At 100A the supply conductors will dissipate about 1000W over a distance of maybe several 1000's of feet. The load power at close to 100A is about 13.8MW. Now a 138kV to 13.8kV transformer supplied by that 100A breaker fails presenting a 50m? load. The current shoots up to around 13.7kA and the 10? supply conductors now drop 137kV. This leaves about 1kV to be dropped across the ensuing arc as the switch blades start to open. As the gap widens the arc current will start to decrease because the arc resistance will increase with distance. As the current decreases, less voltage will be dropped across the supply conductors, increasing the voltage across the arc. Now the arc has destroyed the blades in the switch and is starting to eat up the pinions that the blades once rotated upon. As the gap increases, the arc current decreases, and the arc voltage increases. Now the arc has eaten clear through the bottom of the switch gear and is continuing into underfloor conduits melting copper all along the path. The arc length might be 10 feet long now and the arc current has dropped to maybe 5kA and the arc voltage has risen to 88kV Ultimately the arc might extend as far as 100 feet dropping 137.5kV at a current of 50A before the ionized channel quenches. I was not able to see the actual fireworks. I was teaching a class at the time. I did get to see what was left of the switch gear later that day and it wasn't much. They had to cut the concrete pad the switch gear was mounted on (outdoor switch gear) and then back hoe a bunch of parking lot to get to good copper. It was a very expensive mess that took several weeks to fix. They lost about $3000 per hour for every hour that furnace had no power. Plus the cost to have the switch gear replaced. All due to the failure of one little air blaster. All for now |
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Re: Neon
开云体育The difference is gas pressure.
Obviously, the spark gap has atmospheric pressure, about 100
000 Pa, but the pressure in a neon lamp is much lower: 100 Pa
to 3000 Pa. ======================================================================================
Best wishes John Woodgate OOO-Own Opinions Only Rayleigh, Essex UK I hear, and I forget. I see, and I remember. I do, and I understand. Xunzi (340 - 245 BC) On 2023-07-13 22:27, Andy I wrote:
One thing I was never clear on, is the difference between a spark gaps and neon discharge lamps.? Are they the same principle?? They seem so widely different.? Spark gap discharge is very hot, leading to eventual destruction of the spark's electrodes.? Neon lamps are "cold" discharge.? Are they the same principle, just orders of magnitude apart?? Or different like night and day, which coincidentally end up with similar electrical properties? |
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Re: Neon
Maybe off topic, but there is some discussion of gas mixture in different neon lamps (many no longer made) in the 1965 GE Glow Lamp? manual I mis-named earlier. There is some discussion of purpose of radioactivity (maybe altering ignition/extinction characteristics).
I also remember CFL fluorescents had a significant amount of Neon along with typically Argon (plus small amount of liquid Mercury) in their mix. The older T8 & T12 1"/1.5" linear tubes typically didn't have Neon). I don't know about T5 or CCFL (higher ignition voltage in absence of heater/space charge properties).? I think the Neon suited the current density and shorter length. That consultant I worked with simply modeled fluorescent lamps by their known (plasma column) voltage vs. the impedances used for voltage dropping. He had other models to handle instant and preheat start and end-of-life current shifts. No Spice modeling. I wonder if spark gaps age, based on number of arcs endured and the energy involved (electrode shape change?). I think they have other uses than just as protection devices, but maybe not commonly. Murray -- Murray |
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Re: Spark gap physics.
开云体育That law is about hot-cathode
diodes and probably doesn't apply at all well to spark gaps. ======================================================================================
Best wishes John Woodgate OOO-Own Opinions Only Rayleigh, Essex UK I hear, and I forget. I see, and I remember. I do, and I understand. Xunzi (340 - 245 BC) On 2023-07-13 21:46, Richard Andrews
via groups.io wrote:
In my studies I found this:The Child-Langmuir law is a differential equation that describes the current through a spark gap as a function of the voltage and the gap spacing. |
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Re: Spark gap physics.
Thanks for the further clarification of purpose.?
It's not enough to get any real purchase on the problem, but OK that's your business not mine. I'm just saying that the tolerances on real spark gaps, and the many unknowns that a good model might erm, model, do suggest that it's not the easiest of approaches.? You might get 20% repeatability.?? There are many established methods for measuring very small currents at very high voltages -? stick to the path if you can, it avoids solving inessential problems.? |
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Re: Spark gap physics.
开云体育Do you mean that you are using
the spark gap as an EHT voltmeter? LTspice will simulate a
1Gohm/volt conventional analogue peak voltmeter, which would
seem to be simpler and more accurate. ======================================================================================
Best wishes John Woodgate OOO-Own Opinions Only Rayleigh, Essex UK I hear, and I forget. I see, and I remember. I do, and I understand. Xunzi (340 - 245 BC) On 2023-07-13 20:45, Richard Andrews
via groups.io wrote:
The spark gap is important to me because I am simulating an ion thruster I created. A proper spark gap will allow me to measure thrust. |
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Re: Spark gap physics.
开云体育This sounds like a very interesting project! ? Out of curiosity, how do you measure thrust with a spark gap? ? Dave ? From: [email protected] <[email protected]> On Behalf Of
Richard Andrews via groups.io
Sent: Thursday, July 13, 2023 12:45 PM To: [email protected] Subject: EXTERNAL: Re: [LTspice] Spark gap physics. ? The spark gap is important to me because I am simulating an ion thruster I created. A proper spark gap will allow me to measure thrust. |
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John Woodgate
murrayatuptown
Bonkers
Bell, Dave