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Old 18-01-2012, 13:15   #16
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Re: Building an effective faraday cage to protect against lightning strikes

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Originally Posted by Viking Sailor View Post
How about a small steel fire safe. I've seen them for under $75. And, it would protect against EMP, fire, and theft. A thought.

I believe the secret is in the mesh...
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Old 18-01-2012, 13:41   #17
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

hei, I found an very interesting article about your problem.

it seems you should not ground a small faraday cage, but see for yourself...
is a bit long though

Electromagnetic Pulse Protection - EMP - Futurescience.com
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Old 18-01-2012, 14:28   #18
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

How about lining a locker with metal foil (copper, aluminum, etc.)
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Old 18-01-2012, 15:29   #19
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

Thanks for the explanation afstfm.

Free electrons repel each other like crazy, so their behavior is rather predictable. They travel on the outside of conductors because that is where it is easiest for them to move and they can get the furthest away from each other. For round conductors, the smaller diameter the conductor the more electrons that can fit per surface area.

To illustrate this to yourself, draw two circles, one with a circumference of one inch and one with a circumference of two inches. Now get a bunch of dots from your paper hole puncher and set them around the outside of the circles just touching each other, and just touching the line of the circles. Does the larger circle have exactly twice as many dots as the smaller circle? No, it has fewer per inch. And if you repeat the experiment putting dots on the line Inside the circle you will see that many fewer fit inside the same size circle than did outside.

This is part of why the wires for your lightning ground system cannot have curves with radius tighter than eight inches, nor bends greater than 90 degrees. When the electrons are forced to bunch up or turn away from their heading to ground they jump off the conductor onto an easier path to ground and that is called a Side Flash.

Lightning ground FYI: Copper braid is not permitted for lightning ground systems - the electrons must make many direction changes, and copper ribbon is not permitted because so few electrons can fit on the surface as compared to the same amount of stranded wire. Also, both are prone to surface corrosion, which causes massive resistance. If you ground your shrouds be sure to ground the CHAIN PLATE and not the backing plate - grounding the backing plate does not work because the current must make two 90 degree turns through the bolt at two very inadequate connections.

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hei, I found an very interesting article about your problem.
Electromagnetic Pulse Protection - EMP - Futurescience.com
Very interesting - thank you for sharing that! I see Mr Emanuelson states that good conductors such as Cu or Al must be used to construct the Faraday cage - but his brief address of the difference between a Faraday cage and an electromagnetic shield cleared up my confusion about protecting electronics from induced current - for lightning protection you want an electromagnetic shield, not a Faraday cage to protect your laptop/electronics. Be careful about your interpretation of the article - he uses "Faraday cage" interchangeably with "electromagnetic shield" - too bad, because otherwise the article would make more sense to people without a background in physics.

But I *still* don't know whether I am supposed to have the shiny side of the tin foil on the inside or outside of my hat.
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Old 18-01-2012, 18:12   #20
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Re: Building an effective faraday cage to protect against lightning strikes

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... what about using your oven? Or is that an old wives tail? ... Mrs. Rain Dog
This is what we do, but I don't think it works very well. A friend put put computers, GPS, cell-phone in his oven & shortly after that his phone rang! So much for keeping out electromagnetic radiation!

We have survived one lightning event - probably only a near miss, as all we lost was our radar, autopilot control head (not the brain) & our LED masthead light. About $4K lost, but I think we were VERY lucky. Friends have said that anything with an inductor in it blew up when they were hit, plugged in or not.
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Old 18-01-2012, 18:13   #21
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

Why not a steel boat?
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Old 18-01-2012, 18:40   #22
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Re: Building an effective faraday cage to protect against lightning strikes

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Your shrouds, (grounded) are already a medium to poor Faraday cage. Running even more wires will probably not help much. To come much closer to accomplishing your goal of keeping your electronics from getting wiped out because of a lightning strike, you would have to physically disconnect all your electronics, which of course is not practical and at times not safe.

We had multiple "unplugged" items fried in a lighting strike. We also have shrouds. One GPS was wrapped in tin foil, in a zip lock bag with desiccant and inside a waterproof ditch box. Still fried... I think a Faraday cage needs to be pretty robust if you want it to work.. Electronics today are very sensitive to EMP...

My neighbor had wireless TacTic stuff fried too and it was not plugged into anything.
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Old 18-01-2012, 18:42   #23
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

Why not just get an Aluminum brief case. Make sure the two sides are electrically connected through the hinges and latches (use your DVM). Now you have a strong, safe, easy to carry, good looking brief case and your spares are protected. If you need something larger Google Zarges Aluminum Cases (they make brief cases too).
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Old 18-01-2012, 19:38   #24
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Re: Building an effective faraday cage to protect against lightning strikes

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Originally Posted by Maine Sail View Post
We had multiple "unplugged" items fried in a lighting strike. We also have shrouds. One GPS was wrapped in tin foil, in a zip lock bag with desiccant and inside a waterproof ditch box. Still fried... I think a Faraday cage needs to be pretty robust if you want it to work.. Electronics today are very sensitive to EMP...

My neighbor had wireless TacTic stuff fried too and it was not plugged into anything.
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David-

This is ony to protect the backup electronics (laptop, external drives, GPS) that are already disconnected. Even if not connected to anything, the EMP associated with a lightning strike can cause induced currents that can fry circuit boards. I'm pretty much resigned to losing all the primary equipment in the event of a lightning strike- this is hopefully to protect the backup stuff that we'll need once our main stuff is reduced to a smoldering heap.....
I said in order to come closer to your goal of eliminating the chance, and not eliminating the chance. I never said eliminating the chance was a possibility.
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Old 18-01-2012, 19:51   #25
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

Just some lateral thinking here and to assist in my own knowledge. Why would it not be effective to keep your spare instruments in a solid wooden box? As far as I know wood does not conduct electricity if it is kept completely dry. Yes I know that trees hit by lightening are blown to pieces but I always thought that the reason for this was that apart from their height this was due to the amount of water and resins in the tree which acted as a conductor. Ok, shoot me down, but I thought it was worth asking to those who are more knowledgeable on this subject.
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Old 18-01-2012, 19:58   #26
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

Does anyone remember the all aluminum Geek Suitcases? Look for one on EBAY.

If you want to see a faraday suit in action - this clip is mind-boggling. Makes ocean crossing seem totally tame. You can see that the closure need not be absolutely complete or perfect (mans face showing)

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Old 18-01-2012, 21:59   #27
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

we were hit by lightning last summer. sailing in the sea of abaco we came up on a tight little thunderstorm. being that the sea is only about ten feet deep i decided to just drop the sails, put out an anchor, and ride out the brief storm. that part worked fine. the admiral was down below and i was standing in the companionway, under the bimini, and keeping an eye on the storm and our position (with a handheld gps).
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Old 18-01-2012, 22:20   #28
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

part 2. (somehow my wrist hit the post button and i sent half the story)

suddenly there was the loudest bang i have ever heard, kind of like standing in front of a 155mm howitzer when it is being fired. i was facing forward at the time and saw a blue light surrounding the mast. it disappeared and i realized that we had just been struck by lightning. i did a quick check all around to make sure we suffered no hull damage and was relieved to see we weren't likely to sink. the storm passed and i began checking for damage.

first, my boat is not grounded. it's a 37 foot cutter rigged fiberglass boat. it has seven bronze throughulls and an unused dynaplate in the hull, but nothing at all is tied together electrically. my damage consisted of -

the alternator was fried.

the only electronics i have that are run off the main breaker panel are a vhf radio, a fishfinder, and a navtex receiver. the vhf and depthsounder were fried. the navtex was not, and i wonder if it wasn't for it's unusual fuse setup. on most electronics only the positive wire is fused. on the navtex, both positive and negative wires had their own fuses. the fuse on the negative wire was blown, but not the positive wire. i just replaced the one blown fuse and had no further problem with it. my three handheld gps's and my two handheld vhf's were not harmed. they were all down below just lying around at the time except for the gps i was holding.

my fridge, cpt autopilot, and solar panel were not harmed. they are all wired directly to the house battery bank, with fuses. none of the fuses were blown.

all of my navigational lights were blown. some of my interior lights were also blown. other electronic gear such as laptop computer and tv were not affected.

total damage was under five hundred dollars.
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Old 18-01-2012, 22:24   #29
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

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Originally Posted by Nicholson58 View Post
Does anyone remember the all aluminum Geek Suitcases? Look for one on EBAY.

If you want to see a faraday suit in action - this clip is mind-boggling. Makes ocean crossing seem totally tame. You can see that the closure need not be absolutely complete or perfect (mans face showing)

These are pretty cool videos. The reason they are not killed is because they are not connected to ground. It is the same reason birds can sit on wires at thousands of volts. Once the connection is established, no more current flows. If he were to touch the tower he would die. It is freaky to watch...that's for sure.
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Old 18-01-2012, 22:27   #30
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Re: Building an Effective Faraday Cage to Protect Against Lightning Strikes

How about learning to use a sextant? I know that I'm new to all of this but nothing replaces a paper chart and a tool.
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