High voltage arc gap design

I am an engineer working for a local radio station. One of our transmitters has a repeating problem. For no apparent reason, it shuts
itself down, and the fault monitor indicates that an arc has occurred at the transmitter's built-in arc gap. These appear to be nuisance problems, as a thorough search has revealed no faulty components, and the transmitter resets and goes back on the air when an operator acknowledges the error. It will then run for several hours before the problem repeats.
I have noticed that the transmitter uses an unusual arc gap design, unlike any of our other units. The other transmitters use the traditional round ball style arc gap. This transmitter uses a pipe and point arc gap. I have constructed these diagrams to better illustrate the problem.
Top View http://bayimg.com/hajGBaAbm
Side View http://bayimg.com/HAjgAAabM
It should be noted that the dielectric is ordinary atmospheric pressure air, of varying temperature and humidity. The arc gaps are set to the original manufacturer's specifications. Unfortunately, the manufacturer has since stopped supporting this model transmitter.
Is the arc gap distance too small? Or is this arc gap design prone to problems? None of the other transmitters have this problem. Any help would be very greatly appreciated. Thank you for your time.
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Are there any signs of pitting, uneven surface area? If it ligitimately arced once and pitted it may become more subceptable to lower voltages. I would expect to see some sort of pitting showing there was in fact an arc, otherwise I would suspect the arc detection circuitry. If it is pitted you might try carefully restoring the surface with fine sandpaper or equivilant.
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If all else fails, replace the tecnology! Ditch the present spark gap and install a proven type arestor. Check the usual vendors that utilities use, G.E., Westinghouse to name a few. Buy an arrestor rated for the same voltage, 32KV and all is good. No second thoughts of an original miss - design.
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Air breaks down at about 40kV per inch so you would expect a 3/4" spark gap to flash over at about 30kV, so the 3/4" gap for a 32kV supply would seem too small.
What does the transmitter manual say for setting up the spark gap?
Does this spark gap protect the power supply or is it at the base the antenna?
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David Frantz wrote:

is the point smooth and rounded like it should be?
Do you have Debris flying around like dust particles
is it possible you have rodents running around getting in there and tripping it off? :))
We deal with over 1 Mvolt units at work and one the problems is sharp edges due to a valid arc at some point. Also, air quality has a lot to do with it.
The round tip designed are more prone to this problem via the ball tipped types. Static build up in the air can cause a build up of electrons on the tip and discharge much like how it does on an antenna which is why they use ball tips to help reduce the static noise.
Just a thought..
http://webpages.charter.net/jamie_5 "
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In article

Not using a dry gas certainly adds to the variables.

You might check for any surface quality issues on the electrodes - any pitting, surface defects, or tendency to be pointy rather than rounded and smooth will reduce the holdoff voltage. As drawn, it does not look too terrible ("points" not actually pointy).
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David Frantz wrote:

Hi David,
The other posters have already mentioned cleaning/polishing the surfaces of the gap.
However, because the smaller radius electrode is positively charged, this design will also tend to more easily form positive streamers, and will thus be more prone to flashovers, than a simple sphere gap that uses a pair of larger equally sized electrodes. If your drawing was approximately to scale, the positive electrode diameter appears to be about 1/2 - 5/8" in diameter, and the radius of curvature (ROC) is perhaps only ~0.75 cm. When operated at 30 kVDC, the E-field at the tip of the positive electrode is well above threshold to initiate positive corona, even if the electrode is polished.
I suspect that the design is causing the problem. A sphere gap, using a pair of 2" diameter electrodes will provide more consistent performance. You can use a standard spark gap table to calibrate estimated breakdown voltage such as or IEEE 4-1995 or the table below:
http://home.earthlink.net/~jimlux/hv/spherev.htm
Good luck,
Bert
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------------------- You have hit it right on the head.
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I'm curious--what's the radius of the tube and of the "points"? Where are you (what elevation above sea level)? What's the temperature of the air around the gaps? At sea-level and 25C, 0.75" is not enough for 30kV and needle-point gap, and could be a problem if the radius on the tips is too small; and of course, at higher elevations and higher temperatures things get worse. Sams' "Reference Data for Radio Engineers" has a section on gaps in the "Miscellaneous Data" chapter.
Cheers, Tom
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Are you sure the arc event is happening at the base of the tower? I am assuming this is an AM Station? What make & model transmitter do you have? What type of antenna?
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David Frantz wrote:

Well, i would not worry about it if the transmitter was in the US as the FCC Gestapo has outlawed it; drop dead date coming up fast.
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Robert Baer wrote:

Then you know something no one else does. Tell us when the FCC is shutting down terrestrial radio broadcasting.
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Michael A. Terrell wrote:

From reliable soft failure analog to crappy on/off digital. TV is the first frontier...
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Robert Baer wrote:

That spectrum had value for other uses. A 1 Mhz slice at the bottom end is fairly useless for anything other than local radio service. It also has very different propagation, and is reserved for radio by international agreement, because it can easily cross multiple borders. With the high background noise in most areas you need a kilowatt or more to cover a small town. The only time I've had clear reception in years was when the whole region was without electricity for 30 miles or more, right after the hurricanes a few years ago.
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"Michael A. Terrell" wrote:

What spectrum is that?
I read a thread some time ago on an hdtv group about what TV stations would be doing after the date for dropping analog. Some stations will keep their secondary UHF frequencies as their new primary broadcast frequencies. But many plan to switch their digital broadcasts to the frequency they now use for analog. In other words, it appears that TV stations will be spread across the VHF/UHF bands more or less as they are today.
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"Paul Hovnanian P.E." wrote:

700 Mhz to 800 Mhz. In other words, everything above Ch 52.

Not likely. They already lost channels 70 to 83, and Ch 14 in some areas ( Chicago?) for UHF land mobile service. The DTV conversion is shaving 17 more channels for the original UHF TV spectrum. Ch 1 was lost over 50 years ago to land mobile fire and police radio VHF High band service.
In less that a year there will only be 51 of the 83 channels assigned over the years.
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wrote: ...

You're quite sure about that, are you?
Cheers, Tom
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Tom Bruhns wrote:

I don't think it was lost, They just didn't use it because it falls on the IF freq of the receiver or close to it.
But then again, I could be incorrect there.
http://webpages.charter.net/jamie_5 "
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On Jun 2, 3:08 pm, Jamie

There's a nice, though I don't know how accurate, history of USA's Channel 1 at http://www.anarc.org/wtfda/channel1.htm . Back that early, I don't know if there were any TVs using a 45MHz IF; early ones used 21MHz, or thereabouts, much to the consternation of amateur operators wanting to transmit on the 15 meter band.
Now how did we get to this from "high voltage arc gap design"??
Cheers, Tom
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Tom Bruhns wrote:

I meant VHF low band, 30 to 50 Mhz. What do you expect when I can't sleep more than a few hours a day? :(
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