Faraday Cage

[belizesailor]

We are having an especially active lightening season here on the Rio (at least near my place). Last night, a bolt struck very near my house and generated enough EMP energy to damage my stereo (it is a car stereo unit).

Since I live off the grid, my house is set-up very much like a boat. My small electronics (stereo, VHF, Sirrius Radio, etc) are all installed inside a small wooden cabinet. I am thinking of modifying this cabinet to in effect make it a Faraday Cage. Appears easy enough: line it with metal (likely just use good old sheet metal), no contact between devices and metal, bond all metal pieces together (for example, the door to the cabinet). I plan to use sheet metal, seal any seams with aluminum coated tape, and bond the sheet of metal on the inside of the door to the others with a wire and/or maybe via the hinge mechanism too.

If it works out for the house, then maybe build one for the boat too.

Anyone ever done this? Suggestions?

[S/V Alchemy]

No, but I have a steel boat. The whole thing's a Faraday cage. On fibreglass boats, those in the know unplug their electronics and throw their handheld VHFs and a GPS in the microwave, which is similar to what you are trying to make.

[Paul L]

I've read in some places that you also have to ground the Faraday cages.

[belizesailor]

Originally Posted by S/V Alchemy

No, but I have a steel boat. The whole thing's a Faraday cage. On fibreglass boats, those in the know unplug their electronics and throw their handheld VHFs and a GPS in the microwave, which is similar to what you are trying to make.

Certainly one upside of a metal hull. The microwave or oven trick I suppose works OK for small handheld stuff, but I would like to have all the permanently installed stuff already inside a Faraday Box -- just close it and go.

Should be pretty easy to do here at the house....a bit more complicated on the boat (with SSB and such).

[belizesailor]

Originally Posted by Paul L

I've read in some places that you also have to ground the Faraday cages.

Yes, I plan to ground it.

I've read that it depends upon the size of the cage and expected force of electrical charge, but I don't see how grounding it could be a bad idea -- especially since I have no control over the potential load.

Hmmm...maybe I should build one around the bed too -- that strike sure was close last night!

[SunDevil]

I'm not sure you need to ground it. What if it is your ground that gets hit by the lightning?

I would like to make or find a Pelican 1200 case with a faraday cage built into it.

[micheck]

I think that your hassle may be the cables that come across the Faraday Cage from the outside (antenna/power/control...); maybe that is a use for lightning arrestors (sp)

[Paul L]

Originally Posted by SunDevil

I'm not sure you need to ground it. What if it is your ground that gets hit by the lightning?

I would like to make or find a Pelican 1200 case with a faraday cage built into it.

There's some comments on grounding the cage in the WiKi article
Faraday cage - Wikipedia, the free encyclopedia
Including

If the cage is grounded, the excess charges will go to the ground instead of the outer face, so the inner face and the inner charge will cancel each other out and the rest of the cage will retain a neutral charge.

[belizesailor]

Originally Posted by micheck

I think that your hassle may be the cables that come across the Faraday Cage from the outside (antenna/power/control...); maybe that is a use for lightning arrestors (sp)

Yes, I think so, next part of the project to figure out. I assume anything which passes through the side of the Faraday Cage reduces its effectiveness. I suppose some kind of "surge suppressor" or quick disconnect might work...will have to research further...open to suggestions.


Aha, found this in the Wiki article:

To a large degree, though, they shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are significantly smaller than the wavelength of the radiation.

Now, just need to find the wave length of lightening produced EMP...

[Paul L]

Originally Posted by belizesailor

.......
Now, just need to find the wave length of lightening produced EMP...

It is listed in these articles Lightning Protection

[LakeSuperior]

How will you prevent the induced current from the magnetic part of the pulse? I don't think the Faraday cage will help you here.

[Adelie]

Originally Posted by LakeSuperior

How will you prevent the induced current from the magnetic part of the pulse? I don't think the Faraday cage will help you here.

If the cage is steel, iron, Nickel or other ferro-magnetic materials, the magnetic field will flow thru the skin of the cage like the electrical field without penetrating into the cage.

Aluminum, magnesium etc. will work for the electrical component of a strike but not the magnetic. Stainless depends on the alloy.

[belizesailor]

Originally Posted by Adelie

If the cage is steel, iron, Nickel or other ferro-magnetic materials, the magnetic field will flow thru the skin of the cage like the electrical field without penetrating into the cage.

Aluminum, magnesium etc. will work for the electrical component of a strike but not the magnetic. Stainless depends on the alloy.

Aha, someone who clearly knows more about the subject than I do (which ain't much!).

So, to confirm my understanding of what you are saying: A ferros metal cage should shield from both the electro and the magnetic components of the Pulse -- correct?


Thanks.

[SunDevil]

I thought they used copper to make the mesh? I really don't know much about them, but I should know more. I would like to make a water proof, lockable, surge protected and lightning resistant case for a USB hard drive I have.

I think this subject will get a lot more attention in September with the TV show NBC has.

[Jentine]

The simplest Faraday cage aboard is the oven. Open the door, insert electronics and close the door. The only disadvantage is that you can't bake cookies at the same time.

Jim