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Thread: Value of Grounding Straps?

  1. #1
    Stormtrack supporter Jerry Prsha's Avatar
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    Default Value of Grounding Straps?

    As the temperature drops and we dry out for the Winter months, my wife's car especially has static discharge problems as we leave the vehicle. I plan on mounting a grounding strap on the frame of the vehicle to minimize the issue but the process made me start thinking about grounding my vehicle as well for other reasons.

    Because of our hobby, I'm guessing that our chances of a lightning strike are going to be higher than an average individual.

    That being the case...

    • I'm wondering of there's anything that we can do to ground the automobile any more than normal so that we don't lose our electronic equipment?
    • Antennas mounted on the vehicle are probably going to take the strike and is there anything we can do to better channel the energy so that it takes an easier ground path than through our equipment like we do on our homes?
    • Does a grounding strap lessen the chances of a direct strike?

    Has anyone studied this in any depth?

    TIA for any education you might provide...
    Jerry Prsha - St. Louis, MO
    "I contend that we are both atheists. I just believe in one fewer god than you do.
    When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours." Stephen Roberts

  2. #2
    Dan Robinson
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    The static discharges you experience when exiting the vehicle unfortunately are a result of charge residing on your body, which a grounding strap on the vehicle won't affect. Body charge from exiting a car usually results from the rubbing of your clothing against the upholstery on the seats.

    For lightning, a grounding strap that hangs from the frame will help protect your tires by giving the lightning a way to get to ground around them, but it won't help with electronics onboard.

    Induction-coil type arresters are used on some tower-based antenna installations, but I'm not sure how effective they would be in such a small application as on a vehicle. Surge suppression is really an 'all bets are off' thing when you're dealing with a direct strike - no measures will give you 100% protection in that case.

    Unfortunately there is no way to prevent a strike - all you can do it try to channel it where you want it if it does happen.

  3. #3
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    The other thing that should help is to power the sensitive devices with a loop inside the car rather than using the external frame/vehicle battery "ground" loop. Connect AC devices through surge protected outlets off an inverter. Use an auxiliary battery pack 12v supply for intermittent and small loads and float 12v into it through a constant current AC charger off a protected outlet. That way you may just get away with frying the inverter.

  4. #4
    Member Ryan McGinnis's Avatar
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    Ah, I get to dig this old gem out! I used to love Usenet -- you could find experts for everything there. Mr. Coffman touches on grounding straps here in terms of lighting -- in his opinion, it might help alleviate the typical damage caused to a car by a strike, though it may increase the likelyhood of being struck.

    --Begin FWD--

    From: "Gary Coffman" <ke4zv@bellsouth.net>
    To: "Ryan" <ryan@digicana.com>
    References: <142941283.20030629123915@digicana.com>
    Subject: Re: Lightning dangers from Faraday's cage compromise while storm chasing/spotting?
    Date: Sun, 29 Jun 2003 16:06:23 -0400

    > I've done a bit of back research on my question, and I think I have
    > much of it answered. Mainly, I'm concerned that I may be introducing
    > a lightning hazard into the passenger compartment of my car by using a
    > magmount scanner antenna. The antenna is mounted to the trunk of my
    > car and the coax is run through through the rear trunk gap, into the
    > trunk, up through a gap at the edge of the rear seat upholstery, along
    > the floorwell, and to a handheld scanner attached via velcro to the
    > dash on the passenger side.
    >
    > From what I've read, it seems that this pretty much defeats the
    > Faraday's cage, as I'd feared. Since I primarily use the scanner to
    > find severe storms which usually contain awe-inspiring displays of CG
    > lightning, this is a disconcerting thing to discover.

    Yes, that certainly does destroy the integrity of the Faraday cage.
    A hollow closed conductor will have all the current on the *outside*.
    But if you provide a penetration via a unsuppressed cable to the
    inside, it will bring a very high potential inside. Internal arcs then
    become a very real concern.

    > But I still wonder: how much of a risk is it to have this set up?
    > Assuming for a moment that the mag mount antenna is directly struck,
    > what is the most likely effect? Would the bulk of the current follow
    > the coax into the passenger compartment, or would the current likely
    > jump at some point from the coax to the car body? Seeing as my
    > handheld scanner is not grounded, whereas my car body sort of is, I
    > would hope so -- but part of me fears that by the time the actual
    > discharge is occurring, the charge may simply go as far as it can along
    > a conductive path (i.e., to my handheld scanner) and then find a new
    > path -- even if that means arcing around in the inside of my car and
    > blowing a hole through something not normally conductive.

    That's right. Understand that a direct strike entails about 20,000 amperes
    at millions of volts. Serious stuff. It wants to find a path to Earth, and
    it will
    explore every available conductor as a way to find one. The current will
    divide in inverse proportion to the impedances of the various paths. In
    other words, the largest part of the current will take the lowest impedance
    path. But every path will be electrified, and until a low impedance path is
    found, every path will be at a very high potential. So arcing is a virtual
    certainty.

    The eventual path to Earth will most likely involve arcing across the
    tires from the wheel rim to Earth. If the car does take a direct strike,
    it is likely the wheel bearings will be scored and will have to be replaced.
    If having the vehicle struck is considered likely, it might be worthwhile
    to install a light chain from the frame to Earth, ie a classic grounding
    strap. This will offer a lower impedance path to Earth, and may save
    the wheel bearings. OTOH, it may increase the likelyhood that the
    vehicle will be struck in the first place.

    > Honsetly, I could give a flip if the scanner was damaged or
    > destroyed. What I'm worried about is getting zapped. Is internal
    > arcing a reasonable possibility if my mag mount is directly struck? Is
    > there any way to minimize the danger? Mostly you seem to recommend
    > using polyphasers at the drilled coax entry point in order to give the
    > lightning enough resistance to cause it to opt to follow the body of
    > the car instead of the coax. But I'm not so keen on drilling holes in
    > the car body; if there were a way to effectively use a polyphaser
    > without drilling holes in the body, I'd do it in a heartbeat.

    There's really no alternative to maintaining the integrity of the
    Faraday cage. You don't have to use Polyphaser products, but you
    do have to provide a very low impedance path for the current to take
    to the car's outer skin. You can use a shunt fed grounded antenna
    (note I don't mean Earth ground), or a slot radiator, but that's going to
    entail drilling and bolting, or welding, to the car body too.

    Note that the scanner, and you, aren't the only things at risk. Your
    car's computers (modern cars have several) are also at extreme
    hazard. They are very expensive to replace, and the car won't run
    if one is damaged. So you have a large financial incentive to avoid
    leading large potentials inside the vehicle even if you don't care
    about your radio equipment.

    > I thought about trying a glass mount antenna, but then I figured
    > that that would be just as bad; I'm guessing lightning can easily jump
    > through a fourth inch of glass to get to a highly conductive coax
    > cable, and that's probably preferable to jumping a good foot through
    > air to get to the car body.

    Yeah, not to mention that glass mount antennas are poor antennas
    anyway.

    The big problem with any vertical rod antenna is that it is going to
    break into corona and streamer formation as the field intensity
    builds. This makes it the most likely target of a strike. In other words,
    it behaves as a classic Franklin lightning rod. So if you can avoid
    using a whip antenna, the probability of the car being struck by
    lightning is reduced.

    A shunt fed loop antenna, a slot antenna, or a patch antenna, all
    directly body grounded, are alternatives to the vertical rod. A patch
    antenna could be mounted to the inside of the glass. That would
    eliminate streamer formation and a penetration of the Faraday cage
    of the body. But a VHF patch antenna would be fairly large, about
    19 inches on a side. That would obscure vision from that window.
    A slot antenna would also be quite large at VHF. Both would also
    be directional if mounted in a window. That would be a problem
    as you changed directions.

    A DDRR loop could be mounted on the roof. It is very low profile,
    similar to a luggage rack in appearance, but it needs a very good
    electrical connection to the roof to form a vertically polarized omni
    pattern. So you're back to drilling and bolting, or welding, to the car
    body.

    Now I should say that lightning strikes on cars are fairly rare. I
    should also say that the car windows make the body less than a
    perfect Faraday cage anyway. But fixing a vertical whip antenna
    on the car does increase the odds of it being struck, and leading
    the signal inside the car with an unsuppressed coax does increase
    the likelyhood of an internal arc which could harm you, or the car's
    expensive electronics.

    If I were storm chasing, I'd give this serious thought. I'd want to
    use an antenna form that isn't as apt to break into streamer
    formation as a vertical whip, and I'd suppress the coax where it
    enters the vehicle. That means some holes in the car, but I consider
    that a minor issue compared to what could happen to the car, or
    you, if lightning were to strike the setup you now have.

    Gary

  5. #5
    Member John Erwin's Avatar
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    Thanks for that C&P Ryan, some good advice there.

    I've spoke here on this topic before, but I'll repeat a few items. I have extensive experience in the Aircraft industry which of course is very concerned about lightning strikes; most of the approach used there can also be applied to vehicles.

    - Bonding is extremely important. Most cars have a very poor bond between the main chassis/body and the doors, hood and trunk (and exhaust pipe). This can be improved by installing bonding straps/cables between these components and the body of the car. This will strengthen the "Farraday's Cage" as mentioned in the above thread. Basically the idea here is to allow the current to flow over the skin of the car and into the ground. Different potentials between components mean arcing and damage.

    - I've always been a proponent of permanently mounted antennas for this reason (and performance). Mag mounts are asking for trouble, unless you take them off during a storm or avoid them altogether. I have all my antennas either NMO or other hard type mount. I've never had a direct strike but several closeby ones which are more likely to be a factor for most folks.

    - I don't use a ground strap on my chase vehicle but have installed on my wife's car. For whatever reason you would always get a shock after getting out of her car, probably for the reasons Dan listed earlier. I installed a strap and presto, problem gone. I suspect touching the metal of the door prior to exiting the car discharges the body and thus no shock.
    John Erwin
    VE4WX

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