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Old 25-07-2016, 11:43   #16
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Re: Chart table faraday cage?

Sure line your chart table or drawer with thin sheet metal or screen. The sheet metal will likely be less trouble I suppose it's a good thing but the real risk is all your installed and high $ stuff.
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Old 25-07-2016, 12:06   #17
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Re: Chart table faraday cage?

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Originally Posted by buzzstar View Post
Now I am somewhat more confused than usual, or at least uncertain: Will a Faraday Cage protect a wired device located inside the cage? I'd always thought a Faraday Cage was effective against loose electrical charges (EMP, lighting, etc. looking for a route) but not that which was carried by wire into the Faraday Cage. Can someone with both actual and theoretical knowledge please answer, rather than those folks like me speculating based upon experience, intuition, and /or partial information.

The Faraday cage will carry magnetic and electrical fields around the enclosed volume provided there are not holes in the cage same size as the wavelength of the field. I did research on this a while back and forget what the critical wavelength was. I remember that it was moderately small and consequently had a preference for sheet metal rather than mesh.

If there are wires penetrating the enclosure then protection for the items connected is erased and protection for other items is compromised.

The one thing I was uncertain of is will any metal do? For electrical fields any conductor will do. For magnetic fields it was unclear whether the cage needed to be ferrous. Consequently my preference would be mild steel

A steel sheet metal cash box seems like a good cost effective solution. I would sand the mating surfaces where the lid and box meet so there's good contact but I'm not sure this is necessary.


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Old 25-07-2016, 14:38   #18
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Re: Chart table faraday cage?

Hi, folks. I'm brand new, so if I screw up any way, please don't hesitate to inform me. On Faraday Cages, I do have some knowledge as I've been a pilot (airplane) for over 50 years. An airplane is a Faraday Cage, as is a car and an elevator (that's why your cell phone often fails to work in an elevator). Aircraft take lighting strikes fairly often with no major effect. Forget Hollywood movies to the contrary. The Air Force once sent an F-106 into thunderstorms intentionally seeking strikes. The plane sustained over 6,000 lightning strikes and was still flying the last I heard. Grounding the Cage is unnecessary, but can be done if one so chooses. A microwave oven is a Faraday Cage, even those with glass. The glass contains a mesh to complete the Cage. The tighter woven the mesh, the better. Solid metal is best. As to my qualifications for making the above statement, besides being a pilot, I hold an MSEE. I've been lurking here for some time now, and the Faraday question caused me to register. You people are GREAT!
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Old 25-07-2016, 14:42   #19
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Re: Chart table faraday cage?

Lots of bits and pieces here, making a whole story of it is the fun part.
Yes, we do shield things with wires poking out. Think of a military communications centre - it is shielded and it connects to the outside world. Yes, the comms side uses fibre and optical couplers, but the power side still uses copper.
The trick is to know what you are "shielding" against.
Fundamentally, Lightening is one of the laziest buggers around. Bend a cable and it says - "S.. That" and jumps the corner. Treat it sweetly, use long sweep bends in the conductors and it roars around the corner quite happily. Most boats with a listening conductor do not sweep the grounding wires appropriately, so the listening jumps the designed earth path and takes a short cut.
Lesson 1 - Large grounding conductors and earth plate properly installed.
Next an issue of a Faraday cage. Any conductive metal box will do. Does not need to be fully sealed, but openings are weak points, so make them small, engage them and check the conductivity. Yes, a kettle will do, the spout is long and thin and swept, so it will block most EMF. If you fabricate a box, may as well use copper, it will last a heck of a lot longer and is easy to work. Stainless is a pain in the butt and mild steel will corrode. Clamp the lid, may as well ensure a good seal, but a good overlap will also work. Use long spigots where wires enter and add a fuse block as well - stops any stray pulse doing further damage.
Lesson 2 - build something that is useable, not so damn restrictive it will not get used.
Microwave ovens can work well - if they are unplugged!!
Plan ahead. If lightening is about, take precautions, do not rely on the safety measures alone. We are often our own worse enemies in this regard.
Hope that helps
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Old 25-07-2016, 15:17   #20
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Re: Chart table faraday cage?

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When I read this I questioned our plan to use our Dickinson diesel stove as a cage. It has no window and the oven handle actually locks down forming a seal. Does that meet the requirement as you know them? Hoping not to rethink this. Sorry for kidnapping the thread.
Any kind of hole or gap can function as a slot antenna.

https://en.m.wikipedia.org/wiki/Slot_antenna

Cookie tins and ammo boxes have a gap. My understanding is that you need to seal these gaps with a metal foil tape for them to work as faraday cages.
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Old 25-07-2016, 15:24   #21
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Re: Chart table faraday cage?

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......

So if something isn't connected to the boat wiring, which hand-held gear typically isn't, then the likelihood of damage is virtually nonexistent. This is the same as you being safer inside the boat during a lightening event than on the deck!
Not sure where you get your virtually zero statistic, but I think it is false. Here's one example from an Ocean Navigator article
Quote:
.
In the course of my survey, I realized that some things not directly linked to the boat wiring were damaged. Undoubtedly my EPIRB was fried. The same for handheld VHF radios, autofocus binoculars, weather radio and, yes, even my high-power spotlight. 100,000 volts flying around seems to have a way to mess up a lot of stuff.

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Old 25-07-2016, 15:29   #22
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Re: Chart table faraday cage?

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Lesson 2 - build something that is useable, not so damn restrictive it will not get used.
Microwave ovens can work well - if they are unplugged!!
Plan ahead. If lightening is about, take precautions, do not rely on the safety measures alone. We are often our own worse enemies in this regard.
Hope that helps
Thanks, great info! Your point in lesson 2 is why I was thinking the chart table faraday cage in the first place. Why not shield the place where you're going to store that type of stuff in the first place. I'm thinking of laying fine copper mesh on the inside of all surfaces of the chart table, and then put copper finger stock along all the edges to ensure a good conductive "seal" when the lid is closed. I can add more protection, not just against lightning but also moisture, but putting my portable electronics inside anti-static ziplock bags with silica gel desiccant packets inside.

-David
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Old 25-07-2016, 15:35   #23
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Re: Chart table faraday cage?

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Thanks, great info! Your point in lesson 2 is why I was thinking the chart table faraday cage in the first place. Why not shield the place where you're going to store that type of stuff in the first place. I'm thinking of laying fine copper mesh on the inside of all surfaces of the chart table, and then put copper finger stock along all the edges to ensure a good conductive "seal" when the lid is closed. I can add more protection, not just against lightning but also moisture, but putting my portable electronics inside anti-static ziplock bags with silica gel desiccant packets inside.

-David
Ensure you have a bonding strap at the hinge (both sides for security, and redundancy) and at the open side. I would also add a locker catch arrangement, to physically "lock" the lid closed when there is a storm around, as it ensures there is no gap and improves the contact at the join
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Old 25-07-2016, 15:41   #24
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Re: Chart table faraday cage?

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Not sure where you get your virtually zero statistic, but I think it is false. Here's one example from an Ocean Navigator article


Yes, it will mess with a lot of stuff.
One issue is the direct hit. It will burn. Most people who get hit by lightening die because of internal burns! Wet bodies actually help there as less current flows through the body.
The other is the induced current. If it runs parallel to a cable, it will induce a current in it, so seemingly disconnected gear can still get fried.
Best solution is to avoid it, use the conductive path that is available. Not been there myself, but I would be inclined to adopt the old fishermen's trick and attach cable or chain to shrouds and hang it in the water as well
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Old 25-07-2016, 15:53   #25
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Re: Chart table faraday cage?

Thats all and fine for hand stuff, but how do you protect all the mounted gear you have?
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Old 25-07-2016, 16:04   #26
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Re: Chart table faraday cage?

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Thats all and fine for hand stuff, but how do you protect all the mounted gear you have?
I'm specifically interested in protecting the handheld stuff, you know, the backup gear. If your primary and backup gear both get fried you're really in a pickle. With the backup stuff protected, you still can communicate and know where you are.

-David
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Old 25-07-2016, 17:33   #27
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Re: Chart table faraday cage?

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I have never actually talked to anyone experiencing failure of handheld equipment due to a lightning strike. So I was curious that you might be the first.

One obvious answer is to get a metal box with a lid that fastens with screws (no hinges). Thin steel is a good choice of material. The screws should be about 4 inches (10 cm) apart. When leaving the boat put all the handheld stuff in the box and put the screws in the lid.

A fancier box can be made if the lid seals with electrical contact strips sometimes called finger stock. This eliminates the need for lots of screws.

In my experience failure of powered off handheld equipment due to lightning is mostly the stuff of Internet legends.
May be a lot of internet legends out there, but I can assure you that hand held equipment can get fried. We seen a friend's Roper 65ft same height mast schooner get the fore mast blasted by lighting 200 yards from us. It blow off VHF aerial and wind indicators which was on that mast, we went out to the boat and boarded with a strange uneasiness, when opening the companion way a strange smell arose and there was a mixed bag of non working electrical's including new and old hand held devices.
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Old 25-07-2016, 20:18   #28
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Re: Chart table faraday cage?

unplugging all the cables would save the equ... the power spikes come in the cables.

you could build a cage for EMP's though if you are really worried about the world ending and sailing away.

or maybe move. I don't think I've seen lighting in 15+ years.
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Old 25-07-2016, 20:43   #29
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Re: Chart table faraday cage?

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The one thing I was uncertain of is will any metal do? For electrical fields any conductor will do. For magnetic fields it was unclear whether the cage needed to be ferrous. Consequently my preference would be mild steel.
Hi Adelie, I'm building out rooms right now for 6 scanning and transmission electron microscopes. Our EMI shielding is all aluminum plate. In the past I worked on a linear accelerator and the control room was protected by copper mesh. I have not worked with galvanized sheets but our situations (and budgets) are very different.

Most everything on this thread is good advise, just build some sort of box to throw your portables (mobile, gps, iPod, laptop) into and if you can access your mounted electronics, then try to disconnect all the wires, antennas and feeds as much as possible. Of course it's not practical to do that daily, but if it's summer in Tampa Bay and you're leaving the boat for a month, then it would be a good precaution.
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Old 26-07-2016, 05:46   #30
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Re: Chart table faraday cage?

Probably the "best" thing you can do for a VHF handheld is remove the antenna and battery when away from the boat. Those two things are where most of the energy is "received" from a nearby lightning strike. Most high quality handhelds are built with a metal enclosure around the sensitive bits.
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