How to Build a Faraday Cage to Protect Your Electronics

An artist's impression of an EMP explosion.
An artist’s impression of an EMP explosion.

One of the things that gives us the most troubled sleep of all is the risk of, and outcomes from, an EMP attack on the US.  In case you’re not fully up to speed on this draconian danger, we discuss EMP attacks – what they are, how fearsome their impacts would be, and how easy they are to stage – in several articles here.

Our sense is that the danger of an EMP event is steadily increasing.  To be blunt, the world is becoming an increasingly unfriendly place, and with growing sophistication of both nuclear weapons and their associated delivery systems (ie missiles) by both North Korea and Iran (as well as other countries that aren’t being quite so public about their actions) and some threats that translate quite clearly to ‘if we need to, we’ll use an EMP device to bring your country to its knees’, the thought of an EMP attack is far from impossible to countenance.  At the same time, our lives continue to become more and more dependent on electronics for everything we do.

We are increasingly of the opinion that it is prudent to maintain a spare set of essential electronic items in a protective Faraday cage so that if/when an EMP occurs, you have a backup set of equipment to turn to.

One consideration when planning for this.  There is no point in keeping backup equipment that relies on other equipment or services owned/operated by other people/organizations, unless you are certain that these other parties will also be able to continue to provide services after an EMP.  For example, what is the point of having a backup cell phone if all the cell towers and network infrastructure by the wireless companies is fried by EMP, and also if the cell phones owned by most of your contacts are also fried!

So, with that as introduction, let’s continue…..

As you may already know, and as our other linked articles explain, an EMP attack destroys electronics by creating high voltage surges in them.  These high voltage surges are induced by electromagnetic radiation – a fancy way of saying ‘radio waves and similar type things’.  Because the voltage is induced by electromagnetic radiation, there is no need for electronic objects to be connected to anything – they wirelessly ‘receive’ these voltage surges, whether they want to or not, the same way that radios receive radio waves, televisions receive broadcast tv signals and cell phones receive phone calls.

Worst of all, perhaps, switching off your devices doesn’t protect them from these voltage surges.  You can unplug your devices and take the batteries out, but they are still at risk of being ‘fried’ by the electromagnetic radiation caused by an EMP device.

Note that while EMP effects are a problem to your at-home electronics, solar flares and storms are not a problem, assuming your electronics are not plugged into utility power (or possibly internet connections).  Unplugged, and battery operated, devices will not be affected by the different type of EMF radiation generated in a solar storm.

Back to EMP risks and counter-measures.  There are several ways to protect your electronics.  Some are impractical, at least for us, because they involve a redesign and ‘hardening’ of the electronic items when they are designed and built.

Others are impractical for other reasons, such as burying our electronics at least 10 ft underground.

The easiest approach for most of us would be to select items we wanted to protect and save for use after an EMP, and place those items inside a special type of container, known as a Faraday Cage.  This device is named after the English scientist Michael Faraday, the discoverer of electromagnetic fields, and deemed the inventor of these protective containers, back in 1836, although in truth it was Benjamin Franklin who first observed the properties of such containers, in 1755.

If you think about it, there’s something slightly strange about using a device first observed 260 years ago, to protect against a modern type of risk only developed 60 years ago.  But progress is a funny thing, right?

What is a Faraday Cage

The chances are you may have already experienced the protective effects of a Faraday cage, and without even realizing what was going on.  If you’ve ever been on a plane that was struck by lightning, the reason you lived to tell the tale – and the reason the plane wasn’t destroyed – is because the entire plane acts as a super Faraday cage, protecting itself and its contents.

A Faraday cage is simply an electrically conductive metal container that completely surrounds its contents.  When electromagnetic radiation reaches the container/cage, it has two choices for what it does next – it can either travel through the container, and thereby exposing the contents inside to its damaging effects, or it can travel around the outside of the container on its conductive exterior.

It is ‘easier’ for the radiation to travel on the conductive exterior, and indeed, the conductive exterior works so as to in effect compel the radiation to take this route rather than to go inside, through, and outside the container again.

So, think of a nice old-fashioned metal biscuit tin with a close-fitting lid.  Instant Faraday cage!  Any type of metal container, of most reasonable shapes and sizes, will work perfectly well as a Faraday cage.

Tight mesh metal screens have sometimes been used as radio frequency shielding too.  We don’t recommend this approach because the wave-length of the energies released by an EMP tend to be shorter, and therefore might be small enough to ‘fit through’ the holes in the mesh.  Plus, unless you have excellent electrical connections at each node where the screen mesh running in one direction intersects with the screen mesh running at sort of right angles to it, you’ll end up with invisible gaps in your radiation blocking.  Best to play it safe and stick to solid metal.

As implied by the comment on screens and their lesser effectiveness, nothing is an absolute in this world, and a Faraday cage – even made out of thick 100% copper, won’t necessarily eliminate 100% of all the radiation.  A little bit might still ‘leak’ through into the inside.  But a well constructed Faraday cage will reduce the radiation inside itself to perhaps one millionth or less of the level of the radiation outside the container, and that is probably sufficient reduction (the technical term is ‘attenuation’) as to reduce the radiation level from a dangerous level that risks the integrity of your devices to a trivial level that poses no threat to them.

Faraday Cage Construction 1 – The Need for a Total Metal Enclosure

Don’t think that a container that has metal on some of its surrounding surfaces – maybe even as many as five of the six sides of a cube shape – will protect its contents.  A typical example of that would be some sort of metal container with a plastic lid.

Such a shape is not a cage at all – rather it could become a ‘wave guide’ which might funnel radiation through it and maybe even concentrate it in some essentially unpredictable manner.

The container needs to have as close to complete metal coverage as possible.  Small holes are okay, but the electromagnetic radiation can travel through holes and other cuts and slits in the container.  The bigger the hole, the more radiation that can go through it, exactly as you’d think if designing a container to block out light (which is also electromagnetic radiation) or to be waterproof.

So don’t, for example, go to Harbor Freight and buy a ‘stainless steel tool chest’ that actually only has some very thin pieces of stainless steel in the center of some of the tool chest’s sides, and plenty of plastic elsewhere.  It might be a great way to carry tools, but it is not useful as a Faraday Cage.

Faraday Cage Construction 2 – The Need for Good Electrical Joins

So maybe you’ve ended up with an enclosure that is metal on all sides.  That’s good, but there’s more to it than that.  Are all the sides connected to each other electrically?  If they are not, you again have a wave guide or possibly a radiation ‘concentrator’ rather than some sort of blocking cage, and might end up with worse outcomes than without an enclosure at all.

Make sure there is no varnish or other ‘invisible’ insulation on each of the metal surfaces.  And have good connections on all sides, as much as possible, so that the shortest physical distance from any point to any other point on the enclosure is also the same as the shortest electrical distance.  As soon as you start to require the radiation to go ‘the long way around’, you start to tempt it to take ‘a shortcut’ through the container rather than around it.

In simple terms, if you have a cube type shape, that means that each of the six sides of the cube should be physically and electrically connected to other sides of the cube on each of its four edges.  A ‘press fit’ is acceptable only if the two surfaces are each clean and not corroded (both dirt and corrosion are usually insulators rather than conductors).  You’ll of course have welded or soldered joins on most of the sides of the enclosure, with just its ‘lid’ being openable in some form, and you need to be sure that all four edges of the lid provide a positive seal and electrical connection.

An Essential but Usually Overlooked Requirement with Metal Tape

Now for an important consideration.  If you are sealing some sort of container, you probably know or can guess that you should use a metal foil type of tape to seal the container with.  Okay, but there’s a trick that many people don’t think through.  The typical adhesive on generic metal foil tape acts as an insulator.  It insulates the tape from whatever you are taping it onto, and also insulates the tape from itself (if you have tape overlapping itself).  This makes the metal foil tape close to useless.

Be sure to get tape that has conductive adhesive on it.  Needless to say, Amazon is your friend, and offers a wide range of different types of metal foil tape with conductive adhesive on it, as you can see from the link.  Better still, the material isn’t very expensive.

Do You Need to Ground Your Faraday Cage?

This is one of the most widely misunderstood aspects of Faraday Cage design and construction.  Faraday cages do not need to be grounded, indeed, as best we vaguely remember our advanced college physics classes, they should not be grounded.

You’ll see many prepper sites that say you should ground a Faraday Cage, but they either don’t say why, or say ‘to bleed off the charge’, or perhaps work on the semi-stated assumption that you ground other electrical things for safety and so, therefore, you should also ground a Faraday Cage.  This is all wrong.  Faraday Cages do not accumulate charge.  They simply allow a charge to pass from one side of them to the other, without passing ‘through’ the inside of the cage.

That process also points to why you should not ground a Faraday Cage.  If you ground the cage, it is no longer an isolated part of normal space that just so happens to be better conducting than the space inside it.  Now it has become an antenna of sorts, and even can be considered as a ‘magnet’ for the radiation.  Think of the process like a lightning rod – a lightning rod actually works by attracting lightning – it ‘pulls’ the lightning to itself rather than allowing the lightning to semi-randomly choose anywhere else to land.  The last thing you want is to change from a neutral Faraday cage to an active receiver of the radiation!  Instead of the EMF passing around your object, it now goes onto the object and travels along it and down to the ground.

Like all antennas, a grounded Faraday cage ceases to be a conductive material and instead starts to become a resonating material with inductive and capacitive properties, with some parts of its length having high resistance (impedance) and thereby potentially defeating the major concept of the cage – its ability to more effectively transfer EMF around its exterior rather than through its middle.  It will be itself absorbing and re-radiating some of the EMF it is now ‘receiving’ and conducting, and while we’re not exactly sure, we fear some of that transmission may be into itself and whatever is inside.

Think of it this way.  Planes get struck by lightning dozens of times every day, but nothing happens to them because the planes are essentially Faraday cages.  The lightning strikes the plane, travels around/through it, and then keeps on going.  But imagine if the plane had a big long antenna trailing off the end of it, all the way down to where it was dragging along the ground.  Now the plane has changed from being an electrically neutral thing that the lightning doesn’t ‘see’ or expend energy on, and instead becomes a huge big ‘magnet’ that draws lightning from miles around to it.

‘Nuff said?  Don’t ground your cage.  Not only don’t deliberately ground it, but also keep it away from any ‘accidental’ grounds – for example, don’t have it resting on solid earth or attached to any metal beams that might lead to the ground, and – whatever you do – please don’t have any electric wiring feeding into it!

Insulating the Contents from the Cage Walls

Something we often see overlooked in articles about Faraday cages is the need to insulate the contents from the cage walls.  When the cage is actually ‘doing its job’, those walls – both inside and outside the cage – are going to be alive with energy, and that energy will be eager to find anything and everything to flow into and fill.

If you think about what would happen if you had the contents actually touching the walls, there’s a chance you’d end up creating an electrical path through the cage that would be easier or almost as easy for the radiation to travel along as it is for the radiation to go around the outside of the cage.  The last thing you want to do is to encourage the radiation to come into your cage and move around.

We suggest you simply line the inside of your cage with thin foam – perhaps 1/4″ – 1/2″ thick.  That’s all you need to do – have some sort of light simply stand-off that keeps your electronics off the walls of the container.

Testing Your Faraday Cage

There are several ways you can test your Faraday Cage once you’ve constructed it.  Go somewhere with a very strong cell phone signal (such as right next to a nearby cell tower) and confirm you’ve all the bars on your phone showing max strength.

Put your phone in the cage and try calling it.  If you hear it ring, you know you’ve got a problem.

Next, place a call from your phone to someone else, then put the phone in the cage and close it up.  Does that cut off your call?  If it doesn’t, you again know you’ve got problems.

Try this with the phone and cage oriented in different directions (and with the phone either vertical or horizontal inside), in case radiation can get in one way but not another way.

Another test is to check for electrical continuity all the way around the cage.  With an ohm meter, set it on its lowest/most sensitive ohm scale and first calibrate it to zero ohms.  Then stick the probes on random parts of the cage, and ensure that everywhere you place the probes, you’re getting under a 1 ohm resistance reading.  Test a range of combinations from any side to any other side.

Those are the two easiest types of test to do.  Happily, there’s not really much of a trick to building a cage, and so there’s every good chance your cage will pass these tests.


Build yourself a Faraday cage – perhaps out of plumbing ductwork type thin metal sheeting that is easy to work and not too ridiculously heavy.  Follow the design considerations discussed above – perfect electrical and physical seal, nothing touching inside, outside not grounded, and you should have an effective way of protecting an essential set of spare electronic devices for WTSHTF.

17 Replies to “How to Build a Faraday Cage to Protect Your Electronics”

      • enough

        actually NO, not really. The bottom line is EMPs are a BROAD blast of enegry pulses accross a huge spectrum of EM frequencies. And its those frequencies that are the problem…..I’ll explain. I built a Faraday cage using a large steel ammo box INSIDE a large garbage can. The inside of BOTH cans where insulated with bubble wrap so the radios inside the cans couldn’t touch the sides of the cans. BOTH can were sealed with a layer of alumni mum foil between the lids and and cans and BOTH can lids were sealed with metal tape . I had inside the inner most can a FM radio, a cell phone, a shortwave radio and a GMRS radio. The FM, cell phone and SW radios were blocked from receiving any signals. HOWEVER, the GMRS STILL received signals. This was very disappointing especially since GMRS band is very low and not much power. Theoretically, my cans should have blocked all signals, but the did not, I had no gaps in insulation anywhere on the 2 cans so honestly my opinion now is, IN A REAL EMP WE ARE SCREWED. Maybe burrying them underground would be better, I dont know. All I know is, id DID EVERYTHING EVERYONE recommends to make a Faraday cage, and it failed.

        • David Spero


          Thanks for sharing your experience.

          The thing about a Faraday cage is that it isn’t a 100% block. It reduces but doesn’t eliminate the passage of radio frequency energy into the cage. It doesn’t guarantee the safety of everything in side, but it sure does massively increase the chances.

          Think of it a bit like driving in a car with safety belts and air bags. You might still be killed in an extremely severe crash, but you’ve a much better chance of survival, and will get fewer injuries, if you do have a belt and bags than if you don’t.

          The good news is that two of your devices would no longer receive signals. As to why the GMRS still did, more data is required to answer that question, but I’ll wager it was not receiving a full 5 by 9 signal inside the double cage. Maybe – probably – the attenuation in signal it experienced would also be enough to prevent its circuitry being fried in an EMP event.

  1. Julie Morales

    Hi, thanks for the new article…it’s been a while!Julie

    Glad to hear metal ductwork will do; I’m always reading about how great those metal ammunition boxes are for this purpose, but since so many people have heard that, they tend to be pretty hard to find, at least for a fairly low price. I take it those metal Christmas cookie tins would also work, for smaller items. Does it make a difference if it is a round tin, or a square tin?

    For lining, I’ve heard the “egg crate” type foam rubber is good…what about bubble wrap, would that work?

    Finally, I know this would be a bit more difficult, but would it be possible to have a metal building that would work effectively as a Faraday cage? I’m thinking of something akin to a metal silo (grain type), or a shed whose walls & roof are made of that corrugated metal. Wondering, what one would have to do to make the outer part completely conductive, yet everything on the inside insulated…would being mounted on concrete blocks or piers mean that it is not grounded? I’ve seen a lot of metal sheds like that where I live.

    • David Spero

      Hi, Julie

      Yes, time flies when you’re ‘enjoying yourself’, doesn’t it. Hope all is well with you, too.

      The shape of a Faraday cage is very very mildly a factor, but for all intents and purposes, doesn’t really matter at all (smooth and round is better than sharp and spikey). Cookie tins are great, albeit small.

      Bubble wrap is fine as an insulating surface.

      I’m writing about making whole buildings as Faraday cages in a second article, probably coming out soon.

    • David Spero

      That is a good idea, but two thoughts. First, make sure there is a good electrical connection between the door and the frame. I suspect that with many safes, the polish/varnish/paint would interfere with the electrical connection.

      Second, if you have any holes at all – ie for an electrical lead in to a de-humidifier, you’ll need to seal those.

      And third (yes, I know I promised only two!) – gun safes are awfully expensive! There are cheaper ways of creating a Faraday cage! A regular metal cabinet with good electrical seal on the doors would be just as good. It doesn’t need to be as strong as a safe.

  2. Illini Warrior

    Something that doesn’t get mentioned is the continuing need for the Faraday cage after that initial EMP or Carrington Effect wave …. there’ll be follow-up waves of possibly even greater intensity …. you’ll be cracking open that Faraday cage to access your communication gear and then need to re-seal adequately for another possible wave ….

    • David Spero

      Hi – I touch on one aspect of this in the second article I’m currently writing.

      But can you point to a source for your expectation that there will be follow-up waves of possibly even greater intensity? I’ve never seen that mentioned in any of my research (which is not to say it isn’t correct, perhaps I merely didn’t research thoroughly enough!).

      The closest I’ve found is the E3 pulse, which comes anywhere from 1 – 100 seconds after the E1 pulse, but which is orders of magnitude less powerful.

  3. Rod Roberts

    I bought 2-30gal all metal garbage cans. I have lined the inside with two layers of heavy cardboard as an insulator. Of course, both of these cans have solid metal lids and they are both lined, but they make metal to metal contact all the way around the top of the cans. I have put 2 layers of 1 inch foam rubber in the bottom to keep anything from coming in contact with the metal. Will this work as a Faraday cage?

    • Julie Morales

      I think the general principle is similar to insurance: yes, I could get hit by a truck tomorrow & killed, but just in case I don’t get killed, I have medical insurance. Like any insurance, its all a matter of how much money or effort you want to put into it, some people are content to take whatever comes & spend little to nothing on insurance, other people try to cover as many contingencies as they can (or can afford to.)

    • David Spero

      I totally disagree (with Stock, not you, Julie – sorry, this note appears as a reply to you, not him). But perhaps you could first explain why you believe that would be so. It is hard to disprove a ‘nothing’.

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