I’m making a box to protect small devices out of cardboard and 2 layers of aluminum foil. Should these 2 layers be connected together electrically or isolated by the cardboard between them? Where would the connection be? The corners? The edges? Does the answer change if only the lid is made of 2 layer cardboard sitting tightly on a thick metal box?
Obviously the metal devices will be insulated with thick pvc from the inner layer of the box.
Are you talking about making a Faraday Cage? If so, do these instructions help?
I don’t understand why there are two layers but a Faraday cage needs to provide complete, unbroken conductivity around its entire volume to the limit of whatever half-wavelength of radiation you are trying to protect the contents against. Aluminum foil is an adequate conductor but it would need to be in intimate contact at all edges and corners such that there is no leakage.
It should also be noted that while Faraday cages protect against static (and quasi-static) charges, a very dynamic and high power electromagnetic pulse can still result in some electrodynamic response through differential charge around the surface of the cage, breakdown of the conductive materials, or induced magnetic fields that can penetrate through the cage. Appropriately grounding the cage will help protect against dynamic transients by giving them a preferred path away from the interior and for all practical purposes a high frequency electromagnetic pulse will exceed the 50 kV/m dielectric breakdown limit of the atmosphere before it will vaporize copper or aluminum but often the shields promoted as ‘EMP-proof’ are really little more than metaphorical tin-foil hats. Military specification electrical devices that are ‘hardened’ against EMP have very complex shielding provisions and connections to assure their ability to withstand attacks, and even they often have problems passing qualification test requirements for certification. Your foil-lined box is probably not as secure as you might think although if you make a completely gap-free layer of aluminum it’ll probably prevent a cell phone from getting a signal.
Stranger
Cheap aluminum foil is so thin I feel I can do better even if it’s a complete shield. By using double. Not just by using two sheets touching each other. Right?
So the question then is what range of wavelengths do you need to protect against? I assume a burst is a mix of all wavelengths, effectively (electronic equivalent of white noise?) but what range of wavelengths are strong enough to be destructive?
This will tell you the maximum tolerable gaps. I.e. tighter than just one foiled box lid on one foiled box with tiny air gaps.
Do you have a nuke to test with?
Are you aware that there are numerous commercial products? Looking on Amazon the favored design seems to be bags rather than boxes. The smaller ones seem inexpensive, and claim to be properly tested.
You don’t really need a nuke; you just need this:
They’re all junk; they may be good enough to shield an RFID tag or car ignition fob but they won’t protect against high altitude EMP.
Stranger
You can make something out of aluminum foil that will work, but it’s not possible to produce an effective commercial product for $30? That doesn’t seem consistent to me.
The commercial products I’ve seen just have some kind of fold-over flap with inconsistent contact. Real EMP-hardened devices will have bridging interfaces on connectors and hatches that closely intermesh the connections.
I doubt the o.p.’s solution will actually provide the kind of protection he thinks it will unless he is actually sealing it with conductive tape but at least aluminum is an unperforated sheet, so provided all of the edges or folded or otherwise put into intimate contact. The junk you find on Amazon is just mylar-lined pouches and boxes with little or no attempt at actually fully enveloping the package.
Stranger
Just talked to a physics PhD. I’ve got my MSEE so there is no shortage of education. 2 thin layers are better than one. The goal is to avoid current loops. So either connect them at one point, or all edges perfectly. Without a connection a capacitor is formed with the cardboard. An old school trash can with a tight fitting lid is ideal.
How about an all metal one gallon empty paint can with top?
Perfect but I want something bigger. May 5 cans for 5 separate devices?
This seems like a good idea. Google says they are usually tin-plated steel, both good conductors, and a tight seal at the lid.
(Good to know that all my old paint is safe from EMPs.)
The big question in all this is - how well sealed? Your old school trash can (as depicted in assorted recycle bin computer icons) will have seals all around the lid, but likely some tiny air gaps since the lid is not designed for a microfine fit. How good is good enough?
Were there any tests with that US government test facility against things like a laptop or SSD in a container?
I certainly hope that the side effect of a remote nuclear air burst is not all of us being electrocuted by the mattress bed springs in the middle of the night? As I understand, the effect of EMP is like a really really bad static electricity shock from a carpet, but induced in any sort of decent length wire to fry any CMOS or similar chips on an electrical connection. Presumably even a length of circuit board trace would be enough that a fatal shock would do in the chips attached?
It also occurs to me that the soft black conductive foam that a lot of sensitive computer parts come in, might be a useful way to ensure a good conductive seal around a lid to the box, if the air gaps in the foam are small enough to exclude most of the EMP.
Anti-static polyethylene form is conductive enough to prevent charge buildup leading to electrostatic discharge (ESD) but in no way adequate to protect contents from high altitude EMP. If you can’t use it to make a circuit to power a light bulb, it won’t work as a Faraday shield.
The best way to protect delicate electronics from EMP is actually to put them underground and not connected to an antenna or other conductor. Even a few inches of moist soil is enough to attenuate a pulse, and anything you put in an enclosed vault or subbasement will survive any atmospheric electrical activity.
Stranger
A trash can lid will make tiny current loops during an attack. Let’s say for example if contact is made every inch due to imperfections. It’s better to have something squishy that is also conductive. Like what?
Can only add to this by pointing out these kinds of perforated aluminum sheets are used to make Faraday cages with solid conductive angle and bar joining all of the pieces. I have also seen some kind of homemade plans using simple wire mesh ‘hardware cloth’.
I may have this wrong, but small gaps between metal cladding are a danger in protecting against high power EMF because sparks can cross those gaps.
If you are trying to protect against a nuke, you are probably going to need something like a box made out of copper plates, properly sealed at the corners, with “fingers” to seal up the door, plus a ground loop of thick copper both above and below the box, connected to a very solid earth ground (google “halo ground” and “ring ground”).
Personally, I think if you have a nuke going off overhead, keeping your electronics functional is probably going to be the least of your worries.
Is the goal here really to protect against a nuke?