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teneicm · 18-01-2012, 07:49

Hi Folks-

Based on a concern for protecting electronics from a lightning strike, I'm thinking about building a faraday cage to house the backup laptop, backup GPS unit and other small electronic devices.

My approach will be to utilize a mid-size pelican type case (hard sided and waterproof) and insert a 'cage' of either aluminum or copper mesh between the inside of the case and the foam. Assuming I can build the cage out of one piece and effectively solder a few points where the corners overlap, I should have a reasonably good conductive cage that will help dissipate the EMP associated with a lightning strike.

One of my concerns is that I may need to "ground" the cage in order for it to effectively dissipate any charge that builds up on it. Back in my lab days, we would ground the faraday cages surrounding our equipment (because it was easy to do)- but I never bothered to see if it had any practical effect. If I need to ground the case/cage, I can pretty easily install a waterproof plug through the case that will connect the mesh inside to a wire/alligator clip on the outside. If I connect the clip to any given metal through-hull, I think it should be grounded at that point. I don't think this set up raises any galvanic corrosion concerns (especially if my clip to through-hull connection uses the same metal), but honestly I'm not sure.

Anyone have any thoughts on this?

Ocean Girl · 18-01-2012, 08:25

I'm sure you have heard of this but what about using your oven? Or is that an old wives tail?

Considering how we are more and more reliant on electronics I'm surprised someone hasn't started a thread about this already...Gord?

Whatever you do please post results, my husband and i will be working via computer while we cruise so the input would be very appreciated.

Mrs.Rain Dog

ShipShape · 18-01-2012, 08:28

Can a Faraday cage block the EMR from nearby current? It is my (foggy) recollection that they were used for static protection. Hmmm, I guess the magnetic field would set up current in the Faraday cage itself. Your idea sounds reasonable. Most people use their oven as a Faraday cage, although I have never heard whether or not that works.

If your cage was exposed to the air I think it would not be necessary to ground it since if it is storming that means rain, which means high humidity, and excess electrons on the cage can quickly dissipate. However, having your screen inside the case I would say definitely ground it.

Using a thru-hull to ground your case is fine, be sure it is at least six feet away from your main conductors if you have not installed a proper lightning ground. In order for the connection to be of galvanic current concern there has to be a circuit. Outside the boat salt water completes the circuit, but you won't have any galvanic problems inside the boat as long as the connection stays dry. Of course, do not connect aluminum to your bronze thru-hull!

Both AL and Cu are fine conductors, but Cu is sooo expensive right now and Al corrodes much too quickly in marine environments. It sounds like you will not be able to easily/regularly inspect the screen ... why not use bronze or stainless steel? I think the conductivity is less important than corrosion for this application.

Matt Johnson · 18-01-2012, 08:30

I just plan on putting the stuff in the microwave... seems like a good use of the energy waster

teneicm · 18-01-2012, 08:36

Ovens and microwaves may work, but I want a dedicated place for these backup items to "live", as you never know if you'll be around or awake to put the items into the oven... and I certainly wouldn't want to make the mistake of turning on the oven having forgotten my laptop in there.

Shipshape- you may be right about the SS screen instead of aluminum- I'll need to check prices. Cu is ridiculously expensive. The screen will be inside the (presumably airtight) case, so even aluminum MAY be ok.

**David M** · 18-01-2012, 08:38

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.

teneicm · 18-01-2012, 08:45

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.....

jackdale · 18-01-2012, 09:12

Quote:

Originally Posted by teneicm

and I certainly wouldn't want to make the mistake of turning on the oven having forgotten my laptop in there.

You have to open an oven to light it. You would see a laptop or anything else in there. Or just put a tag on the oven door handle with a note.

St. Elsewhere · 18-01-2012, 09:24

I think that you could apply sheet metal, or even wire screen material, to the six sides of an existing drawer to create one. The top does not have to fit tightly, but it does need to be electrically bonded to the other sides.

ShipShape · 18-01-2012, 09:36

Quote:

Originally Posted by teneicm

the EMP associated with a lightning strike can cause induced currents that can fry circuit boards. ....

It sounds like you are more up on your physics than I am - so the magnetic pulse does set up current in the Faraday cage? If so, what would it matter how good of a conductor the cage is, and why ground it? You would ground it to give excess electrons a place to quickly escape, and current doesn't create excess electrons. If there is a reason to dump the induced current in the cage then you would need a circuit. What about just hook some kind of heat dump to the cage, a light bulb, for example?

(This is much more fun than doing the paperwork for my taxes.

)

Jesse · 18-01-2012, 09:59

Perhaps a crab pot with the sides,top and bottom bonded would do the trick?

teneicm · 18-01-2012, 10:16

Quote:

Originally Posted by ShipShape

- so the magnetic pulse does set up current in the Faraday cage?

(This is much more fun than doing the paperwork for my taxes.

)

Not certain- I'm not an EE and know just enough to be dangerous. My understanding is that the Faraday cage 'rearranges' the charge on the exterior to make sure that the net charge on the interior is ZERO. This gets into all sorts of theory on hollow conductors and the like, but I'm not certain if the 'rearranging' of the exterior charge requires a ground or not. One of the problems is that googling this leads mostly to crackpot survivalist sites that are convinved that we're all about to be hit with an EMP pulse from a nuclear blast and that we should wrap everything in tinfoil.

More fun than taxes, but about as confusing....

afmstm · 18-01-2012, 11:06

Hi,

EMP damage is caused by currents induced primarily by the high electric fields. A Faraday Cage uses the fact that electric fields cannot exist inside conductors - even if the conductor is hollow. This characteristic of electric fields does not rely on a ground connection, only on a continuous conductive surface. As you point out, you do need to make sure all sides of your cage are electrically well connected. I suggest you leave the lid connected to one side and allow it to bend to close.

In practice, you do need to be concerned about gaps in your shield. Radiation does not "see" holes in conductors that are much smaller than the radiation's wavelength. So, gaps in your cage should be smaller than the smallest energetic wavelengths in the pulse. 1mm corresponds to a frequency of 3 x 10^11 Hz - 300GHz. This is quite high. I don't know what the spectrum of energy for a lightning flash is but it's likely most energy will be in wavelengths longer than this. So, what this means is you can use metal sheet with 1mm holes in it, and have 1mm gaps. 1mm is probably conservative.

Incidentally, this is why your microwave oven has a window that is covered with a fine metal mesh - short wavelength light can pass through the holes so you can see in, but the microwave radiation cannot get out - its wavelength is around 10cm.