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High voltage arc gap design

Discussion in 'Electronic Design' started by David Frantz, May 29, 2008.

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  1. David Frantz

    David Frantz Guest

    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

    Side View

    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.
  2. ScadaEng

    ScadaEng Guest

    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.
  3. John

    John Guest

    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
  4. Jamie

    Jamie Guest

    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.."
  5. Ecnerwal

    Ecnerwal Guest

    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).
  6. Tom Bruhns

    Tom Bruhns Guest

    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.

  7. mpm

    mpm Guest

    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?
  8. ScadaEng

    ScadaEng Guest

    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 -
  9. Don Kelly

    Don Kelly Guest

  10. James Arthur

    James Arthur Guest

    All else failing, you could just measure the arc-over voltage: ramp up
    high voltage through a resistor and see when and where she snaps.

    That'll give you an answer that covers both questions of gap-setting,
    and of gap & surface conditions.

    Why wonder when you can measure?

    James Arthur
  11. James Arthur

    James Arthur Guest

    Oh, another question: any fried bugs piled up beneath this thing?

    James Arthur
  12. Tom Bruhns

    Tom Bruhns Guest

    ;-) At that power level, they tend to vaporize rather than just fall
    down dead. Used to have a problem with flies getting into the air-
    variable neutralizing capacitor in a 1kW AM broadcast transmitter, and
    vaporizing themselves while drawing a pulse of current large enough to
    engage the over-current relay. Things got more exciting one day when
    the protection circuits didn't. One of the results was a track melted
    in the ceramic core of a 200W wire-wound resistor, molten ceramic
    dripped down on the deck below. Didn't find the fly carcass, though.

  13. James Arthur

    James Arthur Guest

    Molten ceramic? That's good stuff! (But don't show this to Tim
    Williams ;-)

    James Arthur
  14. JosephKK

    JosephKK Guest

    Ouch! That close, did that person live through that/et seq.?
  15. Guest

    | From reliable soft failure analog to crappy on/off digital.
    | TV is the first frontier...

    It is the tradeoff we get for more efficient use of the spectrum.
  16. Guest

    | Feh. Examine the remains of three 20 kV lightning arrests mistakenly
    | connected across a 66 kV transmission line.
    | The fireball looked pretty cool too. But not to the lineman who was
    | standing practically underneath them closing the switch.

    Wish you had videos/pictures.

    Which kind of mistake was make? Wrong type of protectors? Wired the wrong way?
  17. But how efficient is it to have several hundred megahertz of on-air
    spectrum used for selling soap, anyway? Couldn't there just be one
    station in each district running all the commercials and we could get by
    with much less spectrum, power consumption and overall cost. I'm
    surprised TV has lasted this long - every time I visit someone with
    cable, a random flip through the available channels lands on a
    commercial more than half the time, it seems. Maybe people like watching

  18. Tim Williams

    Tim Williams Guest

    They have that, it's called QVC. And it's still not enough. ;-)

  19. JosephKK

    JosephKK Guest

    Wow. Only one reclose? Or was some system monitor person really on
    the ball?
  20. Fred Bartoli

    Fred Bartoli Guest

    Paul Hovnanian P.E. a écrit :
    You mean they resurrect it (the transmitter) every 33 year?
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