Residential lightning strikes, a statistical question

Discussion in 'Amplitude Modulation' started by WB2CAU, Mar 27, 2021.

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  1. WB2GCR

    WB2GCR Ham Member QRZ Page

    Eric,

    Maybe listening to the "Lonely Guys" net is giving you bad dreams? :)

    My recollection is that rooftop TV antennas were generally poorly grounded regardless of
    their having been on "apartment houses" or private houses. Most back in the day used TV twin
    lead, and therefore no ground. The masts were just hung on the brick, either parapets or chimneys,
    or tripods or similar mounts off wooden roofs or framing. So, no ground.

    Coax often ran down, when it was used, to a balun going to twin lead space lugs.

    So, I doubt it had a significant effect. May have bled some charge.

    I have read from several sources that stainless steel chimney brushes, large size, work as well as do
    the more expensive commercial porcupine type contraptions. If I were putting up a tower now,
    I'd buy a few of them for the tippy top of my tower!

    But really, my concern would be more with what I could afford to do on the ground WRT putting in
    a proper "ring" type ground system... including the bonding to the tower structure, etc.

    I think if it were feasible, I'd do the same around the QTH, but frankly I'm not sure how to not sink
    an awful lot of $$ into trenches, rods and heavy copper cable... :(

    _-_-
     
  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    The “cone of protection” does not refer to preventing lightning. It refers to a zone (actually hemispherical) in which the upward leader from the air terminal (lightning rod) will be effective in meeting the downward leader. Once they connect, the stroke current is conducted to ground with minimal or no damage to the structure, assuming a properly designed, installed, and maintained lightning protection system.

    Charge dissipation devices don’t work as such because it’s impossible to discharge a large enough volume of air to have any effect on the progression of the lightning bolt. They can work as air terminals however.

    Expensive infrastructure uses grounded air terminals. For example, take a close look around the pad next time you see a video of a rocket launch. You’ll see towers surrounding the pad with thick cables strung between them. Those are the lightning air terminals. You will not see dissipation devices.
     
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  3. W3SLK

    W3SLK Ham Member QRZ Page

    Many years ago when I was a member of LARC in Lynchburg, VA., we had a member who (like many other members), worked for GE/Ericcison. He was an RF engineer but became involved with (commercial) repeater equipment damage caused by lightning. Originally this was taken up as a curiosity but over the years it became his obsession. To the point he was teaching technical classes about it at Mary Washington University. One night at our club meeting, he gave us the "Reader's Digest Condensed Version" of his class. A couple of things I remember taking away from that night: 1)Tie 3 knots in the power cord to your equipment to prevent lightning from ruining your gear via the power line; 2) These 'ionizing gas tubes' will do little to shunt a direct strike. The best advice was during a storm, disconnect your equipment from all your antennas and ground it and the aerial as much as possible; 3)No matter what you do, how much money you spend, there is always the 'grand-daddy of strokes' out there lurking, and you can't do nothing about the damage it will cause!
     
  4. AA4PB

    AA4PB Ham Member QRZ Page

    I had one of those wire "porcupine" devices at the top of my tower for several years. When I took it down, the tips of all the wires were melted down into a small ball so it was taking a lot of current from somewhere.
     
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  5. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Corona discharge.

    Generally one wants to avoid that near antennas because it generates noise. The sharp tips raise the electric field nearby, causing the air there to ionize and a small current to flow. Antennas typically have spherical “corona balls” on the top to reduce corona discharge.

    Air terminals (lightning rods) are deliberately somewhat sharp to promote a local high electric field and encourage an upward leader to safely start there rather than elsewhere on a structure.
     
    Last edited: Mar 28, 2021
  6. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    From the National Weather Service. "Note that lightning protection systems do not prevent lightning from striking the structure, but rather intercept a lightning strike, provide a conductive path for the harmful electrical discharge to follow (the appropriate UL-listed copper or aluminum cable), and disperse the energy safely into the ground (grounding network)."

    https://www.weather.gov/safety/lightning-rods


    A fun read is The Lightning Rod Man by Herman Melville. http://www.melville.org/lrman.htm
     
    Last edited: Mar 28, 2021
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  7. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Apparently my 160m radial ground system works for lightning as well. About 3 miles of #12 bare copper, cut into 120 ea. 133'4" lengths and buried just beneath the sod. Lightning gap across the base insulator. It's been there since 1983, and no significant damage to antennas or equipment in the shack. Although some lightning "experts" claim to the contrary, I believe a radial ground system is the best protection, since due to the short rise time of the pulse from a nearby lightning strike, it's more akin to RF than to 60~ a.c. or DC. I read somewhere that the predominant frequency of a lightning strike is about 10 MHz, so a radial system for that frequency would be adequate.

    Someone told me that 3 or 4 rods driven into the ground would be better than my radial system as far as lightning is concerned. But RF pulses in the 10 MHz range would not penetrate very deeply into the ground, so most of those 8' ground rods would serve no purpose, whereas my radial system, before I installed it, was a bundle of copper wire about 5" in diameter. Even though it's spread out in the form of a 267' diameter disc, that would still take a lot more current than would 3 or 4 8' steel rods each less than an inch in diameter, connected via short pieces of #4 or #6 solid copper.
     
  8. NC5P

    NC5P Ham Member QRZ Page

    I had (actually still own but daughter lives there now) a house just outside Albuquerque, NM. It was in front of railroad tracks used by commuter trains, Amtrak, and some spur service. There had been several strikes to the property, mostly without incident. One evening I was reading a book on the bed and a bolt struck the utility pole (blew up the transformer) out by the tracks. Current came down the power drop, into my power panel on the bedroom wall, jumped to my knee, came out the ankle, then into the metal bedframe. I had entrance and exit wounds on my leg and it hurt (seemed like the bone) for six months. My neighbor who had lived there much longer told me he thought the tracks attracted lightning. I don't know if that was true but there did seem to be a higher than normal incidence of them striking in the near vicinity. We were not exactly on high ground, about a quarter mile east was a high embankment where a storm channel was and just to the east of that was the Balloon Fiesta Park, all higher than my house.

    I did get struck a second time but that time in Tempe, AZ. I was riding my bicycle on the trail by Tempe Town Lake when lightning struck the railroad bridge and something got me in the shoulder. I had my hand on a metal part of the frame so I only had an entry wound on the shoulder. I could feel it going down my arm. It hurt for a while but not nearly as long as my leg did from the first incident.
     
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  9. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Wow! I’m glad you survived both to tell us about it!
     
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  10. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Lightning RF spectra are very broad but most energy is at 1 MHz and below.
     

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