06 May 2012

The home Faraday cage

This is one of those things that if you look at how our sun acts up giving us lovely Coronal Mass Ejections that play havoc with electrical systems or if you are worried about a nuclear EMP and wish to have some electronics working after the event, that you will want to make or build: a Faraday cage.  There are lots of other things you can do with such a object, but for modern home use it usually goes into one of those 'You know one of these events is likely to happen...' sorts of thinking.  If you are preparing for an unknown future and want some ability to do some of what we consider modern functions, you will look at utilizing such a thing as a Faraday cage.

A Faraday cage can be made of mesh or sheet metal that is conductive to electricity, and it acts to distribute electricity over its surface inducing an opposite charge on the other side of the conductor, so they cancel out, thus canceling out the wave that induced the current.  It is a hollow conductor and what is on the inside, so long as it is insulated from the conductor, will not receive any of the energy of the incoming wave or charge change event (like lightning).  The aluminum container that is an aircraft is a Faraday cage in that it conducts something like a lightning strike around its exterior surface so that the strike goes over the skin of the aircraft and comes out the bottom of it, not touching the electronics or passengers on the inside.  With that said there are still some openings in an aircraft that allows it to receive other electromagnetic waves, so it isn't a perfect example of this.  A car can be struck by lightning and the lightning will flow over the surface of it and head towards the ground, leaving the passengers (if present) unharmed.  I've actually witnessed one of those events first hand during a mild storm that didn't have any lightning associated with it, but that flash and boom a block away going around a car was an amazing sight.

There are lots of ways to create a Faraday cage, including using a 'space blanket' of aluminized mylar, to go around an object to protect it.  Make sure anything inside is insulated from the coating so that the charge flows around the surface of the protection.  You can also connect the container to a grounded object (say the little screw holding on the faceplate on a three prong electrical outlet) and you can drain off a charge to ground.   During a CME event, once the worst of the storm has passed, you may need to cut the ground due to induced ground currents flowing back up the wire... although your circuit breakers or fuses in the home will probably go first.

In your home are a number of Faraday cages, starting with the microwave oven.  It uses short wave radiation to induce movement in molecules and anything outside needs to be protected from this.  Thus anything on the inside of a microwave oven is in a Faraday cage.  A clothes dryer with closing metal door is also one.  All metal breadboxes with only tiny holes or mesh to allow air passage would also qualify.  Duct tape works passably well to create a Faraday cage, if push comes to shove, so something that might not be such a cage to start with could be turned into one so long as there is electrical conductivity around the surface of the object.

Since the cage is an enclosure that is aimed at stopping electromagnetic effects requires that any doors have lips to them or that their frame have interior lips to force an indirect passage into the interior.  You can use a flashlight to demonstrate this, and if you see no direct light from it when the doors are closed, then you have closed off the interior from electromagnetic waves.  A single 90 degree bend ought to do it.

To put one together I had some criteria and wasn't all that keen on doing sheet metal fabrication and welding... since I don't have any sheet metal brakes or welding equipment.  That meant using the next best thing: a purchased metal cabinet.  There are scads of designs, sizes, shapes, colors, and all sorts different manufacturers out there, and your needs will dictate sizing to you.  One of the prime criteria I had was that it fit a modestly tall, watercooler sized device in it, and that drove all later decisions.  If you don't care about looks, then big and cheap works just as well as anything else, and that is what I went with:

EMP_CME_ 003

I got that from Global Industrial and the manufacturer is Edsall, although it was only presented generically on the site.  It is spec'd at 130lbs for delivery, but it was strapped to a pallet and the entire thing was a beast to get up two sets of half-flights of stairs.  Much sweating and recovery time involved.  It arrived a bit dented, battered, scratched and in the general condition I expect industrial freight hauled stuff to arrive in.  I had to bet in the sheet metal top rear corners with a dead-blow hammer and do some tapping with a rubber mallet here and there, as well.  It is of the insert tab with a few machine screw bolts used to put it together and the doors are a beast to get on if you are doing it alone.  One person in good physical condition should be able to get it done in a day, two people in only an hour or two... alone and having some physical problems, it will take you a bit longer.

The prime problem with the slightly dented condition, particularly of some of the interior parts that were to match up, top to bottom, is that they didn't match up all that well and left gaps between the bent sheets.  If you have a tap welder, you can fix that up in no time at all... and any welding rig should be able to take care of other gaps from shipped condition.  I stuck a radio on FM inside and it still got reception... not great reception but not the static I expected.  AM gets through as it is just changes in amplitude of waves not variations in frequency... growing up I could go into the basement of a neighbors house, inside the enclosed root cellar that had a good half foot of dirt all around and still get AM signals.  You care more about the changes in frequency than slow variations on a wave, as it is those changes that induce a current.

Now to address the gaps and places where the slot/tab arrangement left openings, I could have used ever reliable, somewhat conductive from the right manufacturer, duct tape.  Instead I went with something made for the purposes of blocking out radio frequency changes:

EMP_CME_ 004

That non-grey shiny stuff is aluminum tape.  You can get copper tape, stainless steel tape, bronze tape... but aluminum is preferred as it doesn't oxidize at all quickly.  And if you need to make tinfoil hats, you can make one with adhesive properties to more properly affix it to your hair... which will come out with it, of course.  Wherever there were tabs or seams, I used this stuff.  I did the tops and bottoms of the doors to get a set of full 90 degree turns, and put little pockets around the lock and upper locking bar opening as well.  Along the top I did a 2/3 double over of the tape and used the last third to adhere it to the lip above the doors.  After I got done with the worst of it, I shut off the lights and used a powerful flashlight on the outside to find direct beams coming through the small places that I missed the first time.

I also spent time to put a small ground cable on that I attached to a bolt in back on the bottom and hooked up to the center of a nearby outlet:

EMP_CME_ 005

I did a voltage check between the ground wire and the third prong of the outlet to make sure there was no active voltage leaking through, then did an Ohm meter test and came up with 0.5 ohms of resistance between the exposed lead on the wire and the third prong.  That third prong is connected to the metal box behind the outlet, and the screw connects to the metal box and the lead wire connects to the screw.  Basically, within the limits of the cheap multi-meter I got free from Harbor Freight, there was little resistance between the lead wire and the ground.  Between the front of the cabinet and the end of the lead wire I got around 15 ohms of resistance, which was from the tape to the cabinet and then to the wire.  That isn't half bad, actually!

I put a radio in set to a strong local FM station and it was strong until it got part way in the cabinet, then started to get static and once I closed the doors it was all static.  I've been putting in some of the minor stuff that helps after a major EMP or CME event, and a few big pieces will go in the bottom next to the solar array... mainly its rectifier/battery arrangement which is made to run the AWG off-grid all on its lonesome.

Total cost for the cabinet, tape, and getting a decent dead-blow hammer from Harbor Freight, plus freight to get the cabinet to me disassembled was around $500.  Perhaps I could have done better by scouting around some of the local scrap yards or hitting up Salvation Army or Goodwill... but I don't have a lot of energy and what time I do have during the day needs to be put to good use.

This project has interposed itself in front of my woodworking projects due to priority.  Those projects are on hold until I can get a reliable outdoor shelter up to do the finishing of the wood.  So next up is the methanol chiller system to test out.  Once I get a piece from the folks at Shelter Logic for my shed-in-a-box, I'll get that up and get back to finishing my workbench and then start on a router table/workbench extension.  After that I need to make a few other pieces like miter saw stand, grinder stand, and a final finishing/small work/gun tinkering bench.  Somewhere in there I would also like to make a real gun cabinet, too...

There you go!

More of the stuff I do instead of blogging because I detest repeating myself, and I hate having gloomy future prospects turn into reality... and if that is going to happen, I'll need this other stuff to survive.  And if you want to extend from my terrorism and organized crime work, you might want to catch Rumors of War III.

2 comments:

Salvatore Aguilar said...

It’s quite unusual to have a Faraday cage in your home, but I think it’s okay to have one if thunderstorms often strike your area. You never know when it’ll be handy. It was nice sourcing out for a metal cabinet to use as your Faraday cage too.

Salvatore Aguilar

A Jacksonian said...

The use of the cabinet is what I call 'expedient repurposing'... I wish I could find a supplier for something better. Next up is using rivet shelving as a Faraday cage with aluminum screening doubled over. That will be just plain ugly, but no one makes side panels for stuff like rivet shelving, but those have been few and not meeting my size ranges. If such were available I'd get them, as it is screening is cheap and available.