Lightening Rod for Vehicle protection?

[J Tyler]

A member of the Xterra forum was driving when his truck was struck by lightening. It hit his radio antenna, blew out his cowl, and the truck died. 3 1/2 months later it is still at the dealership, $11,500 in repairs paid for by his insurance company changing out EVERYTHING electrical, and still not fixed. http://www.clubxterra.org/forums/showthread.php?t=38344

SO, with my Xterra sitting outside with metal antennas mounted all over the roof, I begin thinking.......

I'm putting together a 2 1/2 foot mast to get my anemometer higher (out of the slipstream), and was using plastic so as to not be a big lightening rod. BUT, reading the story on Xterra forum has made me rethink.....Why not go ahead and switch to aluminum for that rare, offchance that I get hit one day. Then again, am I making my Xterra a big target by doing this? LOL! I would LOVE to get you guys to comment.

Having a 3 foot aluminum rod on the roof to mount my anemometer.....What do you think? I would rather THAT be struck than the antenna for my Ham Radio (besides killing the radio, it is wired into my electrical system just like the other guys factory radio). If it travels down the anemometer cable, it would kill the little battery powered display.

Thanks for your thoughts, guys.

 

[Dan Robinson]

Unfortunately there are no guarantees when the vehicle is hit directly. A tall conductor with 1/4" braided cable attached and dragging on the ground under the vehicle would channel most of the current to ground (and probably save the tires). But Ohm's law is a pain when it comes to lightning - everything's going to get some current pulse, either direct or inductive. And that's if the lightning hits the mast - no guarantees that it will be the connection point over one of the antennas, even if the mast is slightly taller.

 

[David Wolfson]

The lightning rf signature is, I think, equivalent to a very powerful multi-MHz pulse rich in harmonics. That means a strike very close to a radio antenna induces a big pulse to the front end of the device it's attached to. Using the best in-line arrestor is certainly a good idea to help. The second issue is, I think, that all the electrical devices in the vehicle are grounded to the body, which can be of uncertain quality. FWIW, I've grounded my ham antenna with a short, rather heavy wire from a washer right on the mount direct to the firewall. Third, an additional measure would be to power (and ground) devices you'd like to try to save through an auxiliary battery loop internal to the vehicle entirely separate from the vehicle system. I'd certainly seek more expertise on this, but some basic rf arresting in the loop together with the skin effect should block any dangerous power from making it inside the vehicle.

 

[Skip Talbot]

Not to diminish the dangers or damage potential of lightning, but it seems like the chances of obtaining vehicle damage from lightning are so low, it's probably not worth doing anything other than protecting the wiring for your electronics. I'm not sure a few ounces of aluminum are going to make you more of a lightning "magnet" or have any influence on where the bolt strikes as you are already sitting in two two tons of steel. The height of an antenna or anemometer mast is insignificant compared to the distance lightning travels to make that connection.

 

[STexan]

I ran across this video on another forum I frequent many months ago. it's kind of cool ... How many of us stand out in the middle of nowhere next to a fence, with our body and several thousand dollars worth of photography equipment at PEAK lightning periods, tempting the lightning God's???

I tend to want to agree with skip ... when it's your time to get hit, it's going to happen regardless what you do or don't do, and the only question is what affect it's going to impart. I think too that a lot will depend on if it's a minor 'ding' or a major strike. Perhaps in a small fraction of cases, pro-active measures might alleviate some of the damage that may occur.

Watch video >

 

[Timothy Finn]

Something else to consider....

Cars nowadays are designed to be a Faraday Cage in case of lightning strike, and the car should channel the energy around the structure of the vehicle, diverting it to the ground.

In the case of the Xterra, that is really super rare. You said it hit the radio aerial, and that is what I suspect did the damage. The aerial is a grounding point for the stereo, and since the power was channeled into the vehicle's electronics, as well as ground, it possibly sent a lot of that energy through the car's instrument panel bus, and ground points, frying everything hooked up to it.

That's just a one in a million shot.

Check out this video.....

Watch video >

Tim

 

[STexan]

I also suspect the strike on the xterra was probably a multi-stroke hit. I've counted 4 strokes before and I'm to understand as many as 10 strokes can occur. But certainly, there are many different types and severities of CG lightning strikes.

 

[J Tyler]

I tend to want to agree with skip ... when it's your time to get hit, it's going to happen regardless what you do or don't do, and the only question is what affect it's going to impart. I think too that a lot will depend on if it's a minor 'ding' or a major strike. Perhaps in a small fraction of cases, pro-active measures might alleviate some of the damage that may occur.

[/video]

Very interesting stuff, guys. Especially this video. Notice that you had big metal light poles in the parking lot, but the lightning chose the rig with the long flatbed metal trailer. I'm guessing the driver had the strike go thru him (burning him) because the cab of the rig was fiberglass?

SO.....It looks like the school of thought is, your chance of being hit in the car is extremely small to start with. Whether you use a metal or plastic 2 1/2 foot mast should make very little difference (if at all) in whether you are struck or not. But, if you DO use a metal mast for something like the anemometer, it might direct some of the strike into it rather than into the antennas which are wired into the vehicles electrical system.

For me, its 6/half dozen which material I use, so what the heck. I think I'll go ahead and go with the aluminum mast.

Thanks for the thoughts.

 

[David Wolfson]

From what I've read, most of the cases, including this truck driver, where people inside a vehicle are hurt, they're hurt on their arm and down their outward facing side by a non-lethal fraction of the current. I think this is the skin effect in action, where the shoulder and arm look like part of the skin of the vehicle, since the window is highly insulating. In the truck video, note the skin effect in action even for what is called the "ground", where the earlier strike is seen skittering across the parking lot in front of the camera (unless that was a reflection, which it could have been, IMHO).

I would add my voice to the opinion that a few feet of mast, and whether it's metal or plastic make just about no difference in whether a vehicle gets struck, and only a slight effect on where the strike current goes afterwards.

 

[STexan]

Which begs the question, why can the guy (and electronics) in the repeated controlled demonstration not be affected (hurt) but others in a real lightning storm sometimes are?

But back to the original question .. personally, I'd go with whatever is easiest to work with and that would meet the demands, and I wouldn't be concerned with perhaps an additional step leader attracter. If the anemometer was "insulated and isolated" with PVC, a lightning strike would probably hit something else on the vehicle anyway.