Thoughts Regarding Faraday Cage Techniques For An EMP


As many of you know, an EMP (electromagnetic pulse) could end life as we know it in approximately one hundredth of a microsecond. Faster than the blink of an eye. While some may argue the likelihood of such an event, lets focus instead on the methods one might consider to protect their electronic equipment in a Faraday cage.

But first, briefly, an EMP in the context of this article is a pulse of considerable electromagnetic energy (with a wide range of frequencies and amplitudes) resulting from a nuclear detonation at (high) altitude in the atmosphere. As the pulse travels away from the burst point at the speed of light, the radiation can be ‘collected’ by metallic and other conductors at a distance. The energy of the radiation can then be converted into strong electric currents and high voltages. With sufficient energy, particularly from the high-frequency components of the EMP, electrical and electronic equipment connected to (or associated with) the collector may suffer severe damage from a strong current and voltage surge.

The way that the electromagnetic energy is collected from the EMP is complex, because much depends on the size and shape of the collector, its orientation with respect to the source of the pulse, and the frequency spectrum of the pulse. As a general rule, the amount of energy collected increases with the dimensions of the conductor which serves as the collector (or antenna).

Long runs of cable, piping, or conduit
Large antennas, antenna feed cables, guy wires, antenna support towers
Overhead power, telephone, and cable TV lines
Long runs of electrical wiring, conduit, etc.. in buildings
Metallic structural components (girders), reinforcing bars, corrugated roof
expanded metal lath, metallic fencing
Railroad tracks


Faraday Cage

One thing we can do to protect electronic devices is to use a Faraday cage.

What is a Faraday cage? It is an enclosure (shield) formed by conductive material or by a mesh of such material. Such an enclosure blocks external electric fields by channeling electricity along and around, but not through, the mesh, providing constant voltage on all sides of the enclosure. Since the difference in voltage is the measure of electrical potential, no current flows through the space. A Faraday cage operates because an external electrical field causes the electric charges within the cage’s conducting material to be distributed such that they cancel the field’s effect in the cage’s interior. Faraday cages are named after the scientist Michael Faraday, who invented them in 1836.

An ideal Faraday cage is a conductive metal box (on all sides). Contrary to what many believe, the Faraday cage does not have to be grounded to do its job. If you have trouble with that statement, do some research (web-search) and you will discover the physics. The Faraday cage keeps the charge (EMP) on the outside (during that fraction of a microsecond till its gone) while a grounded Faraday cage will simply bleed the charge to ground during that same fractional microsecond. In either case, the electronics inside are protected (to the extent of the construction properties of the cage itself).

Note: Faraday cages are intentionally grounded when the intended use is to operate an electronic device inside (e.g. a Faraday enclosed room) while the electronic device itself requires a proper ground (and proper isolation techniques from the incoming power source).

With that said, a covered metal trash can will act as a Faraday cage.

Note: Any conducting surface of the electronic devices that you wish to protect cannot be touching the metal of the Faraday cage. An easy solution is to place a non-conductive material such as cardboard on the inside between the devices and the cage itself. Or you may simply wrap the electronic devices in a non-conductive material and place inside…

Metal screening material will serve as a building material for a Faraday cage. For example you might choose to build a box (or larger) while using wood materials. You might then wrap all sides with conductive metal screening material to achieve the Faraday effect.

You might be wondering about the limitation of the hole size in the screen material as it relates to its effectiveness at stopping an EMP. A rule of thumb is that the hole diameter must be less than 1/2 the wavelength of the radiation frequency to adequately attenuate (sufficient dB) so as not to damage the electronics inside the Faraday cage.

The EMP radiation frequencies are very wide ranging, however research papers indicate that it is the very high frequencies that are of concern (apparently up into the microwave region) for electronic device damage. If one uses the frequency of a microwave oven (2.4 GHz) for example, the wavelength is just under 5 inches. Half that is 2.5 inches. The holes in typical screening material are even much less than that (resulting in an increased attenuation at those frequencies).

Excerpt from QST August 1986, “EMP and the Radio Amateur” and condensed from NCS TIB 85-10, “The electric field strength of an EMP remains fairly constant in the 10 kHz to 1 MHz band and it decreases by a factor of 100 in the 1 to 100 MHz band and continues to decrease at a faster rate for frequencies greater than 100 MHz”.

So, it appears that EMP field strength decreases by at least an order of magnitude for each decade of frequency above 1 MHz.

With all that said, it appears to me that ordinary screen mesh size is more than quite adequate for protection, while solid metal is obviously the best.

Note: The lower frequencies of an EMP are more easily ‘collected’ by long ‘collectors’ like power lines, etc…

Will a shipping container make a good Faraday cage? I happen to have a 40-foot container nestled behind some trees on the property which was here when I bought it last year — and I had presumed that it was ideal for EMP. The problem is that while all sides and the ceiling are heavy steel, and while the steel front doors are electrically ‘hinged’ to the rest of the container (even though the door closes onto rubber gasket material), the bottom (underneath the wood floor) is not solid steel. Instead it is made up of steel cross-member beams. Whoops… An EMP could theoretically enter through the floor. It was nearly impossible to see underneath to verify this – but I managed. Theoretically one would have to line the floor with metal and electrically connect it (or weld) to the existing structure.

Will an old microwave oven make for a good Faraday cage? Yes, although I would suggest to cut the power cord. I’m not sure of the attenuation specs (dB) in that frequency range, but I’m sure it’s ‘just enough’ given today’s engineering to maximize profit ;)

Will a metal ammo box be good Faraday cage? If it’s all metal, and if the metal cover is hinged via metal pins (electrical conductivity), then yes. If in doubt (regarding the conductivity of the hinges) then remove the rubber gasket if it exists. If its painted and you’re removing the rubber gasket, be sure to scrape the contacting surfaces of paint.

Will a galvanized steel trash pail make a good Faraday cage? Yes! Seems like a very simple and effective method.

Will a metal gun cabinet make a good Faraday cage? Yes it should, so long as the door is conductivity hinged with the rest of the metal frame (which it should be via the metal pins and hinge assembly). Additionally, most gun cabinets are felt-lined which eliminates the problem of isolating a electronic device from the interior metal walls.

Okay, lets hear from you and your own ideas on this subject…

(Some data sourced from “The Effects Of Nuclear Weapons” by the Dept. of Defense)

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    1. The surrounding materials of a chest freezer (or fridge) may or may not be 100-percent metal, and all-enclosed including the bottom (think ‘plastic’, etc..). All with no gaps (visualize screen mesh with regards to gap size). Additionally these appliances tend to have thick gaskets on the doors which would need to be removed. But then the door probably won’t close properly (a gap). It’s uncertain whether or not the hinge assemblies on these appliances will provide an adequate electrical conductivity connection through the hinge assembly to the door (maybe, maybe not).

      There are uncertainties, and it all depends on your model’s characteristics and ability to adapt to their short-comings.

  1. The question for me on this topic has always been whether to do a DIY of some sort or to find a manufactured solution that fit my purposes. I ultimately purchased a Black Hole Faraday Duffle bag online. I view this approach as a two-factor solution with smaller Faraday bags going inside to further ensure gear protection from signals or disruption.

    Sure, I could have gone the galvanized trash can route or DIY container, but imagine how well that works if you need to get up and bug out from your location. My intent for this bag was also serve as a electronics type of bug out bag. It looks like any other duffle and is pretty generic by design…

    The Black Hole Faraday Duffle bag is capable of shielding for the following ranges:

    800 MHz: 63 dB
    900 MHz: 67 dB
    1800 MHz: 68 dB
    1900 MHz: 65 dB
    2400 MHz: 62 dB

    The key thing for me is that this is actually tested to shield gear for IT Forensics experts. That’s an important differentiator for a topic where DIY can mean anyone can become an expert online.

    The bigger question for me is the task in building a redundancy of electronics gear to put in this bag in the event of an EMP. Not cheap and you really have to put some thought into this to do it right, i.e. backing up data and/or equipment.

    Ideally, I will create a built-in Faraday and grounded area in the home long-term, but this gets me going for now and is dual purpose in being able to bug out with what works…

  2. Speaking of EMP’s and Faraday Cages (I have some electronics in a home made large ammo can) I want to encourage my fellow preppers on MSB to watch this youtube video on the electronics going on at CERN (spooky stuff but really happening).

  3. Won’t know until after the fact if it works, but I have all my stored electronics (hand-held radios, rechargeable batteries and chargers, etc.), in zip-lock freezer bags inside metal Christmas popcorn cans. I buy a couple every year, eat the popcorn and re-purpose the cans for storage. Talked with a retired mathematician who worked on the Apollo Space missions (yes, a real rocket scientist) who assured me this was quite adequate.

    1. I have done this with 3 popcorn tins that were gifts years ago. When I saved back the tins, little did I know that one day they would become efficient Faraday cage units. One holds my old non-digital cameras, one holds some thumb drives for photos and info, a few DVD videos and music, and the other houses an extra Kaito radio. Without a computer, the data on drives and DVDs are useless except for posterity purposes. Can’t bring myself to store my laptop in another cage every day and haven’t rationalized the need to buy a second one since technology forces updates so rapidly.
      I wonder if many are protecting a computer somehow? Anyone?

  4. Will most likely need to tape aluminum foil over rivets underneath the lid, if your trash can is so equipped with a handle on the lid. Also over the rivets on the inside of the can for the handles on the sides of the can. Also roll up aluminum foil to use as a seal underneath the lid(between the lip of the lid and the can)when the lid is placed on the can. This helps to ensure a better seal between the lid and the can.

  5. A Faraday cage must be 100% conductive for its entire surface. Wrap gaskets, or seal around them with aluminum tape. No leaks.

    What we are trying to emulate is the skin effect.

    Grounding the simple Faraday cages that we an build is not a good idea. A path to ground will cause more electrons to flow to ground due the greater conductive path available. We don’t want more electrons. We want the charge to build up on the outside cage surface with the least amount of electron flow. We are dealing with 50,000 volts or more per square meter. Those electrons are very excited. Just let them bleed off into the air molecules.

  6. @small thought,
    Take a look at the Gauss surface integral for conductive bodies and see if you still agree with your statement about electron grounding. Even the surface integral of a sphere may change your reasoning. The electrostatic field internal to a continuous surface on a conductive body is Zero. That is why a Faraday body works and the surface voltage potential is not relevent even if it was 1,000,000 volts. Let’s not mislead the readers, there already too much of that.

    1. You spherical example requires a charge at the interior center of the sphere. Since the center is isolated from the sphere, Gauss’s law would be violated by the charge imbalance.

    2. No joke
      Wow you really have things mixed up. Simply put, you don’t ground a Faraday cage. Do you have any idea of how such a ground could be constructured?

      1. Faraday may have lived a long time ago, and there may well have been subsequent research that shows that grounding is not advisable BUT – Faraday’s whole theory was that you needed to put a grounded conductor between the field and the protected equipment, since there was no known effective insulator against magnetic fields. In common uses of shielding on things like sensitive communications cabling or electronic equipment, proper grounding is an important feature. So the question is, how sure are you that grounding in not necessary/recommended, and where is the technical justification for that opinion?

  7. Seriously would appreciate any thoughts and opinions on this:

    My gun safe has an electronic keypad powered by a simple 9 volt battery. Got a bad feeling I know the answer, but is it likely to be zapped by an EMP thereby rendering me unable to get into the safe?

    1. I recently changed my electronic lock with the SG manual lock. Wasn’t that hard but took some time. I can get into the safe in less than 13 seconds with practice. No more EMP or other failures.

  8. Folks, I’m a marine electronics tech. I highly recommend testing your ‘schemes’ for Faraday cage construction. Don’t rely on any – repeat ANY of the ideas presented here – as gospel.

    I’ve run tests using pretty easy to duplicate methods. All of the above ideas – chicken wire, trash cans, various steel boxes…. And all failed miserable at first.

    Simple to do. Take a wireless telephone; or, a cellphone – both if you have them. Put them in your container. Call the number. You can test other frequencies by simply putting an energized transceiver set to receive (with antenna of some sort where appropriate) in the container in the same manner. You can test most mobile devices – laptops, tablets by simply searching your network for them with an external device. If they appear present – not good enough yet a protection concept.

    What’s most probably going to happen is that you’ll hear through the ‘walls’ of the container the discouraging sound of the ringer. Or your network scan will reveal your ‘hidden’ tablet/computer. You see – any gap – no matter how small, a pin-prick…… is sufficient to allow a radio wave to penetrate the enclosure. The higher the frequency the wave, the smaller the opening that can allow such entry. BUT!!! – don’t think merely full-wavelength. Partial (1/2, 1/4, 1/8…..) also enter amazingly small openings.

    What is needed is a minimum of a double insulated container within a container. Take the garbage can idea. The outermost garbage can must be grounded. Use an insulator (like above – cardboard is fine)inside it and then make a non-conductive pedestal of sorts (I use styrofoam blocks) for the next container. A smaller metal can then with its own inner lining of an insulator. Even if only the device you wish to protect being tightly and triple-wrapped with foil will do. All seams – original gaps must be also foil-taped (like is used for aluminum flexible duct-work is fine). I recommend spaced metal conductive foil strips on the outside of the tape to the original metal of the container. See…. the tape has adhesive and whereas it does bond – it is not necessarily an electrically-conductive bond. The best method I found was to wrap with foil – triple roll the edges of the foil to make a pad and tape that wrap down with the tape. Again – only a double (or greater ‘Russian Doll’) containerized electronic item/device independently seated/inside will NOT be affected.

    Good luck. But, please – test your setup.

  9. @silver lodge
    If the battery is inside the metal safe and all the wiring is in the safe your safe is good to go. The door must make electrical contact with the body of the safe, perhaps thru the hinge. Check this with an ohm meter. It would be a good idea to take aluminum foil and fill the gap between the door and the safe body.

  10. I have solar panels. Can I expect them to be damaged by an EMP if they are exposed? If so and I store them in the cardboard boxes they came in and wrapped the boxes with aluminum foil, will that protect them? I appreciate the info. thanks

    Ps Ken, thanks for the info on surge protectors from yesterday but I don’t see how that will protect the circuits in the panel itself?

  11. @man on foot

    Solar panels exposed to an EMP would be destroyed. Solar panels wrapped all around with heavy alum. foil would be protected. Mak sure that there are no open spots(holes) in the foil. Also, I would use a heavy gauge foil( at least .010 inches thick). This is to protect from the inducted current in the foil. With these considerations your panels would be well protected.

  12. I plan on just partying like its 1899
    If all my stuff is fried (and i dont have much other than vehicles or my i pad or my Kestrel) i think we have some huge problems.

    My food and water dont require electric, nor do my protection devices, and from what i have read it would have to be one hella huge EMP and extremely well placed to completely fry cars and such, so, i think personally im going to just keep my preps as low tech as possible. My backup communication involves me going next door or up the road and talking to folks, not going to be worrying about whats going on half way across the world, will have hands full enough just dealing with my direct situation.

    1. I’m with you Kulafarmer. The most important items are food, water, and shelter. Although a radio would bring in information that we might need to know. I’m not going to fret over everything else. I think at this point I will protect a radio and the walkie talkies that we have on hand. We have a generator, but there is nothing I can do about that as it is too big. I don’t really care about all of the electronic gadgets. For the past year, all of my purchases have been focused on non electrical gadgets.

  13. New empty one gallon paint can for about 3 dollars makes a good EMP cage when lined on the inside with cardboard.

    1. I did this very thing. I have some radios, a solar charge controller, hand held GPS, 12 volt LED bulbs, and other miscellaneous small electronics stored in some cardboard lined paint cans. The lids are easy to remove if you ever need to use the items stored inside, and if you use an old messy can you don’t have to worry about some thief stealing them since old paint is not high on a thief’s list of desired items to steal.

      1. Bilge Pump McCoy

        why did you line the old paint cans with cardboard?

        (assuming they can fairly easily be washed/cleaned on the inside, am wondering if it was for some particular reason? or was it an alternate to cleaning paint out?)

        1. @ Anon
          Anything/Everything inside of a Faraday Cage “must” be insulated from the surrounding metal. If the equipment is in contact with the metal it most likely will not survive the EMP. This goes for any size F-Cage. Hence the Cardboard.

          1. NRP

            Thank you, much.

            sort of a “duh” for me, I guess. Still and all, appreciate the explanation, as who knows when it may come in handy for me to be properly informed.

          2. @ Anon

            My friend, we all had to learn this stuff at one point. Please never worry about asking any questions, 99% of the reason Ken runs this Blog is to help others…… The other 1% is to harass me…. HAHAHAHA

            Gata feel the LOVE… LOL


            PS, Ken has a LOT of great info here, would take years and years to gather all this on our own, I know how hard he has worked on this stuff..

  14. I have 2 one I converted from a semi truck tool box that I lined with thick plastic sheets and made a gasket from aluminum foil and a wooden structure 7ft high 7ft wide and 10ft long , covered it with aluminum window screen , I hope it was’nt all for not. no one know’s if these will actually work ,I pray it will. good luck in sept, God bless.

  15. One of the commenters above had it right. Multiple layers are better. The aluminized mylar food storage bags are good as well, and cheap. Put your electronic stuff in a mylar bag, tape it shut, put that in a cookie tin (or wrap with foil), and drop that in your galvanized trash can, or a filing cabinet. I have measured that each layer provides about 25dB-40dB of attenuation, and those add up. You need 80dB of attenuation to drop the 50,000 volt/meter field strength down to a safe 5 volts per meter. Also, remove any connected wires, cables, and antennas from the device before storage. Wires will not be damaged by EMP, but your charger might, so protect that, too, assuming it will be useful after the party starts!

    Grounding is not necessarily going to help, especially if you just use a single wire, which is less conductive at high frequencies due to skin effect. You’d need to use wide copper braid or the like, and then you need a good conductive earth ground, like one or more 8ft ground rods. Concentrate on the cage, not the ground and your stuff will ride through OK.

    See you on the other side!!

  16. I’m not sure if we’re all on the same page as to how an EMP works. Faraday cages are nice and all, but let’s see first what needs to be protected, and how an EMP actually operates.

    IF an appliance, vehicle or device is completely POWERED OFF, would it still be affected by an EMP?

    Not sure if I’ve ever heard of or read anyone who’s clarified this simple common-sense point.

    Whether the answer is yay, nay or maybe, have ANY TESTS or EXPERIMENTS been conducted and recorded by anyone, anywhere, to prove the answer?

  17. Can one create a Faraday cage to protect modern car engine areas with all the computer electronics inside?

  18. Most of the experts on this subject recommend doubling your EMP protection by putting items in a Faraday bag or other enclosure and then putting those items in your Faraday cage. I use a galvanized trash can and I keep most everything in separate Faraday bags. I also use metal tape to tape the lid to the can and eliminate any gaps in the seal. This may be overkill but I figure the items I am trying to protect are so crucial that I can’t take the chance of losing them due to laziness on my part.

  19. Thanks for the heads up on the metal cargo containers. I hadn’t thought bout the flooring.but from what I have seen in the ones sold here on the Mississippi coast,are solid on all six sides. But I am wondering , on most of the ones I’ve seen that gave rubber gaskets in the doors , are they needed to be removed And, does the paint around the pins on the hinges need to be removed

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