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lightning rod question

Discussion in 'Electronic Basics' started by izzi4, Oct 19, 2004.

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  1. izzi4

    izzi4 Guest

    Hello, I was wondering why there is a ball on lighting rods? example
    http://www.period1.com/lightni.jpg I was just wondering what the purpose of
    the ball is, I know that the point creates a concentrated electric field at
    the tip but i can't imagine the balls purpose. Thanks in advance
     
  2. Bert Hickman

    Bert Hickman Guest

    Decoration. Most of these balls are colored glass.

    -- Bert --
     
  3. Uncle Al

    Uncle Al Guest

    A metal rod sticking up into the air is both a silly thing and nearly
    indestructible. By adding apical decoration it is transformed into an
    asethetic statement, the price goes up, and there is something to be
    damaged and repaired/replaced.
     
  4. Sam Wormley

    Sam Wormley Guest

    Oh the irony--you're standing in the front yard when the lightning strikes
    the lightning rod... potentially saving your life. The thermal shock
    of the strike fractures the glass ball, half hurled striking your head
    killing you! Oh the irony.
     
  5. Don Kelly

    Don Kelly Guest

    Still decoration. At one time (early 30's or late 20's ) there were rods
    made with a ball at the top based on some half baked idea that came into
    vogue (to raise the price, probably) that it would provide better shielding.
    To sum up- it didn't work. There have been, are, and will be a variety of
    devices made and sold to provide lightning shielding. Typically they don't
    do as well as simply sticking a bunch of old bedspring on the roof and
    grounding them well - but they are shinier and more expensive.
     
  6. Vermin

    Vermin Guest

    That would appear to be an "early streamer" type lightning rod. i.e.
    it is supposed to attract lightning better than a Franklin rod, by
    some means of ionizing the air around it.

    There is a lot of debate as to the effectiveness of these devices, in
    my opinion they are no better than snake oil and the Australian
    standard at least is unsupportive of them (despite appreciable
    pressure from manufacturers of these devices).

    V.
     
  7. Doug

    Doug Guest

    If you get killed by this sequence than it was definitely your time to go!
     
  8. rayjking

    rayjking Guest

    Hi,

    The ball is usefull in spreading heat from a direct strike. A #8 wire can
    carry any known strike but it will bern at the point of the strike without
    more thermal mass.

    Ray
     
  9. Gordon Youd

    Gordon Youd Guest

    What's lightning, life just gets darker with age.

    ----------------------------------------------------------
     
  10. izzi4

    izzi4 Guest

    wouldn't the head produced be caused by the current through the wire, or is
    there another method of heating in lightning strikes i'm not aware of?
    Wouldn't this spread the heat through the region of the ball but not farther
    along the path of the wire? I hadn't considered the heat involved before,
    intresting.
     
  11. rayjking

    rayjking Guest

    The heat generated is much more in the air above the wire. The power
    generated causes the vapor in the air to turn to steam
    and the rapid expansion of the steam creates the thunder.

    Ray
     
  12. Bert Hickman

    Bert Hickman Guest

    Not exactly... thunder does not require the presence of water in any
    form. Like all sparks, the rapid expansion of the spark channel
    creates a shock wave that ultimately creates thunder. For lightning,
    the exact mechanisms are subject to some debate, but thunder simply
    does not require the presence of water in either vapor or liquid form.

    -- Bert --
    --
     
  13. Tom Biasi

    Tom Biasi Guest

    I didn't catch the original post but I thought that this may be a good time
    to mention that lightning rods are/were not designed for direct hits. The
    purpose is to keep the accumulated charge below a safe level.
    Tom
     
  14. Don Kelly

    Don Kelly Guest

    ---------------
    Actually this is extremely questionable. Design that I know of is based on
    the probability of the stroke striking the rod rather than the protected
    area. Granted, in some situations, particularly with tall structures, such
    draining does occur (possibly increasing the chance of side flashes-another
    problem) but this is a bonus, not the basis for design.
    I am aware of a purveyor of lightning protection that claimed that his
    protection works on the basis of charge dissipation. I have seen no evidence
    that it does. I have no idea if he is still is in business.

    Generally the source of the charge is several miles overhead -the rod
    doesn't get seen by it. When a leader gets near a rod then it may be a
    preferred target for the next step and if it is, then the main stroke will
    be to the rod-if not something else gets hit. Design is based on it being
    such a target for higher current strokes (but not necessarily lower current
    strokes). This is true for protective systems for transmission lines as
    well as structures. Catch the damaging strokes know that some of the little
    ones will get by.

    References:
    Moussa & Shrinivasta, "Shielding of Tall Structures Against Direct Lightning
    Strokes" Canadian Conference on Electrical and Computer Engineering,
    Vancouver, BC, 1988
    Energy Systems Journal, Vol. 11, N0.1 1991
    EPRI Transmission Reference Book, 345KV and Above.
    I believe that IEEE changed its standards to reflect the concepts involved
    in these references.
    Sorry that I do not have more recent references but I have been retired for
    some time.

    I agree with Vermin on the lack of usefulness of the ball. There appears to
    be no rational basis for its use. Note that its thermal mass is generally
    pretty small and by the time it starts to dissipate heat, any damage is
    done.
     
  15. Tom Biasi

    Tom Biasi Guest

    Thanks for the info, I'll see what else I can find. Probably the foremost
    expert on lightning in this group is Mark Kinsler. I believe he did his
    doctorate on the subject. I have not seen posts from him in a long while.
    Tom
     
  16. w_tom

    w_tom Guest

    A best source of information in this discussion are both
    posts from Don Kelly. Especially where he defines what makes
    a lightning rod effective: earth ground. We tend to
    rationalize only upon what we see. We see the lightning rod
    and assume IT is the protection. We then get all hyped over
    blunt rods verses pointed rods verses one with a ball. All
    irrelevant once the facts are considered. Since we don't see
    earth ground, then we tend to forget the most important
    component of a lightning protection system.

    Don also cites another well regarded expert on the subject -
    Dr Abdul Mousa. Dr Mousa's IEEE reviewed papers contain good,
    scientific language. But to summarize into laymen's terms -
    Early Streamer Emission (ESE) protectors that are suppose to
    discharge the air are scams. Air terminals provide the best
    or more conductive path from cloud to earth. Which again
    demonstrates what makes lightning rods effective - quality of
    that earth ground system.

    Don't worry about the ball on the end of a lightning rod.
    Worry about what makes that lightning rod effective - earth
    ground. Lightning seeks earth ground. Either it obtains
    earth ground via the building or it obtains earth ground via
    the lightning rod. Which is the better conductor - a then
    damaged building or a blemished lightning rod? Lightning
    protection is about the electrical connection from cloud to
    earth which is why protection is only as effective as the
    earth ground system and connection to that earthing system.
    The most important component is the one we forget about
    because we do not see it - earth ground.
     
  17. Bob Masta

    Bob Masta Guest

    I'm no expert on this, but I do recall my undergrad Electromagnetic
    Fields instructor coming down on the draining side of this issue.
    His examples were opposite to yours: He pointed out that the
    size of the ground wires on the typical barn or home lightning rod
    was far too small to withstand a direct hit. He noted that tall
    buildings, which do sustain repeated hits, have massive ground
    conductors to handle the current.

    That was probably the only meaningful thing I recalled from that
    class. All the rest was curls, dels, and other funny symbols.
    (That was over 35 years ago. A lot of synapses could have
    drained to ground since then!)


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  18. Rich Grise

    Rich Grise Guest

    Hm. And here, I thought it was for catching ball lightning.

    ;-)
    Rich
     
  19. Rich Grise

    Rich Grise Guest

    The water vapor in the air is already steam. That's what steam
    is, is water vapor.

    The thunder is the sonic boom from the explosive expansion from the
    ionization.

    Y'know, in a dry climate, how you can shuffle across the carpet and
    draw a spark? Snap! Well, thunder is that Snap!, but from a spark
    three miles long!

    God shuffles across the carpet in Heaven, see, and ....

    Cheers!
    Rich
     
  20. Rich Grise

    Rich Grise Guest

    That's what I was going to say! :) :)

    Actually, I was going to guess, "It's a world globe - note the wind
    vane"

    At one time (early 30's or late 20's ) there were rods
    Cheers!
    Rich
     
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