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  #1  
Old 12-16-2006
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Question Faraday Cage

I have a small extra icebox space in my galley about 2 cubic feet. I am considering lining the interior with coppersheet on all sides, bottom and bottom of the lid to make a Faraday cage. I would store backup GPS, VHF, 2 meter handheld Ham, Cell phone, Personal ERIB and a spare autopilot control head. Does anyone see a problem with this? Do you think it would be effective in event of a lighting strike or close call? I am reluctant to pull a Benjamin Franklin to test the effectiveness! But I think if the Faraday cage works this would be the ideal electronics backup. Any ideas? Should the cage be grounded or should it be isolated (my bet)?
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Good idea. Sure couldn't hurt, and it might even work in a lightning strike or anything short of a nuclear EMP blast :-)

RE: grounding the cage itself, there's them that say do, and them that say don't. I, like you, would probably not ground it, on the theory that as much energy is likely to come UP the ground wire in a close strike, as be routed down it from energy collected on the cage itself.

Bill
WA6CCA
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One might suspect that regardless of construction, any lightning strike close enough for the cage to be effective, would also be close enough for the owner to not enjoy the benefits of the preservation of his gear!
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Wink faraday

Quote:
Originally Posted by sailaway21
One might suspect that regardless of construction, any lightning strike close enough for the cage to be effective, would also be close enough for the owner to not enjoy the benefits of the preservation of his gear!

I would suspect that would depend on what you were touching when the lighting hit!
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Well, I know what I'd be holding on to!
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Old 12-17-2006
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In all the reports I have ever heard of boats being struck by lightning, I have never heard of one where any of the crew was injured (doesn't mean it doesn't happen, I'm just saying). The boats seem to take a lot of damage, and I recall reading one story where a boat was hit repeatedly and the discharge path went through one of the quarter berths - twice. Even then, no one was killed. The danger seems to be primarily to electrical and electronic gear, and to hull structure if there is insufficient capacity in the grounding path to prevent explosive arcing.

To prevent damage to my gear in a lightning strike, I would try to avoid getting hit by lightning. That means getting the boat to be at the same potential as the surroundings, thereby making it "invisible" to lightning. Typically, this involves grounding the stays to a grounding plate using heavy gauge wire with as few bends in it as possible, using a charge dissipator on the masthead (fuzzy metal fingers which break up the ion concentrations that initiate a strike) and making sure there are no floating grounds anywhere on the boat when operating in an electrical storm. Only once I had done those things would I worry about building myself a Faraday cage. People have reported that gear stored in their ovens (a simple Faraday cage) was destroyed during a strike, so I'm not sure how much confidence I'd place in a copper foil lined box. Don't forget, the voltage and current involved in a lightning strike are tremendous.
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Get a tin box like what the Christmas cookies come in. If the idea is to surround electrically sensitive things with equal potential on every side, that should do it. There will remain the chance that a direct strike will destroy the tin, but then you may not need the contents anyway!

BTW, since the contents will not see any potential difference on said inside walls, there should be no need to ground it.
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The real problem is making sure that the lid to said space is well-connected to the rest of the copper box. I wouldn't ground it, as the EMP associated with lightning strikes is due to induced voltage, and grounding it wouldn't change anything IIRC. Everything inside the box should be at the same voltage, provided the lid is connected to the rest of the box properly.

Whether to lightning bond/ground a boat or not is a huge subject of debate. It is said that grounded/bonded are hit more often than ungrounded/unbonded boats, but that the damage is usually more severe on the ungrounded/unbonded boats.
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Well, Texas, I think you'd be better off just installing a small safe. Already all metal, and probably ferrous metal so it protects you against magnetic fields and inductance, which plain copper won't. (Plus, it can be used as a safe.) A "gun cabinet" would be made from sheet steel instead of heavy-wall "safe" material, also available pre-made.

You'll need good contact springs to ensure the lid is firmly connected to the walls, you can often scrounge these from computers and other gear that have been junked, spring copper (beryllium copper?) "combs" that press against the lid and clamp onto the other half.

Might want to jumper some heavy ground braid past the hinges and keep a maul & chisel on board, because if it IS struck...the lid may be welded shut.
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My concern about the attachment of the lid/door is that if it is not attached properly, the cage isn't closed, and then the faraday cage is effectively useless.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
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