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CaptainHarley's avatar

Faraday Cages ... anyone here know much about them?

Asked by CaptainHarley (22429points) January 5th, 2012

I’ve been reading up on Faraday Cages as a means of protecting electronic equipment against Electro-Magnetic Pulses ( EMP ). The literature online is mostly explanatory about how the cages work, but not so much about the particulars: can cables pass through the cage without allowing the equipment inside to be affected by an EMP; can you drape an EMP cloth over a small building, or must it completely surround the building on the bottom as well; why, if a car is a faraday cage by construction, does it need a ground, etc.

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28 Answers

XOIIO's avatar

An EMP cloth would just me thin copper wire in a grid, it has to be metal. Wired can’t go through because electric pulses, singlas or interference wuld get through and an EMP would go into the machines inside, unless you used a shielded cable and fuses/dioded to stop too much power coming though, dioded would be to send a signal out but not let any in.

A faraday cage needs to be sealed with metal on all sides, and works best with metal on the bottom. A car would never work bcause of all the parts and electronics touching and the windows.

Cars are insulated from the ground as well because of the rubber on the tires.

jerv's avatar

Passing a cable through would negate the effectiveness for the equipment that the antenna is attached to, though other equipment would probably be safe. I have my misgivings about shielded cables merely because I doubt the connection would be shielded all the way from one end to the other.

I think a van might be able to protect the contents of it’s cargo area if done properly, but there would be no practical way to keep EMP from frying the engine controls (one reason I am glad I have a carb and distributor :D ).

CaptainHarley's avatar

Thanks, @jerv

Its my understanding that if you attach a chain to the frame of a vehicle, that will provide a ground for the EMP-generated electicity to flow around the contents and engine compartment. Is that accurate?

HungryGuy's avatar

I investigated Faraday cages here on Fluther about a year or so ago to shield my servers from EMPs should the Arabs burst a nuke high in the atmosphere (Armageddon may come, but there’ll still be internet porn, dagnabit!). There are companies that sell Faraday cages for this precise purpose, but they’re ridiculously expensive—> http://www.faradaycages.com/ and http://www.shieldingsystems.eu/.

You can build your own Faraday cage by surrounding a wooden shelf with fine copper screening (which isn’t cheap either) and then grounding it, but as others have said, cables passing through will compromise it. You’ll have to rig some sort of shielded cable (like TV cable) for power and networking where the cable shield is attached to the copper mesh, but that’s tricky.

Axemusica's avatar

I’d agree with @jerv. The cage is nearly pointless if there’s a direct route to something you’re trying to protect.

jerv's avatar

The engine is not completely enclosed though, so i am not sure how well that would work. Best to just get an older rig that doesn’t rely on solid-state electronics.

If memory serves, that is part of why Russia used vacuum tubes for so long after everybody else went to transistors in their military radios; no school like old-school :D

CaptainHarley's avatar

@HungryGuy

Precisely what I am trying to do. I have a concept for enclosing a small building that involves a concrete pad, a fine mesh floor on top of that, with the walls and roof enclosed by the same fine mesh which is then bound to the floor, and a ground from the outer wall to the ground.. Think that might do the trick?

CaptainHarley's avatar

I have also heard ( though I have no way of testing this ) that a short chain welded to the car’s frame will provide a ground for the charge on the outer shell of the car.

HungryGuy's avatar

@CaptainHarley – You want to shield a whole building? Why? Protecting a prized sports car, or something like that? Well, it is feasible. If you don’t need to breach the copper mesh for power cables, it should be straightforward (though expensive to build a cage that big).

You could build a cage big enough for a car and have the garage lighting outside the cage, so no wiring has to breach the cage. The hardest problem will be ensuring that the edges of the screen rolls where they seam together are absolutely perfect.

Now, it kind’a sounds like you’re trying to create a mobile Faraday cage out of a vehicle. I doubt that’ll work. You’d have to cover the windows with copper mesh and solder every strand to the metal body. Even the gaps of the doors, boot, and bonnet are too wide. Plus install screening completely underneath the car. And the axles would have to pass through the mesh and/or you’ll have to mold mesh around the tyres (which will fray the moment you try to drive it). Plus, lots of body parts aren’t even metal, they’re fiberglass and other composites. In the end, your car will be undriveable after this treatment.

If you want a mobile Faraday cage to survive Armegeddon, best to get an older cargo van type vehicle that has no electronics, and just build a conventional Faraday cage inside the cargo space.

CaptainHarley's avatar

@HungryGuy

But I thought that a car automatically WAS a Faraday Cage! One of the guys giving out advice online for this sort of thing said that all you have to do is weld a short chain to the frame so that it conducts the charge to ground. [ confused ]

HungryGuy's avatar

@CaptainHarley – I’ve heard the same stories where being inside a car will shield you from lightning attacks and such. But a lightning attack isn’t the same thing as an EMP.

A lightning attack is like a “short circuit” where the electricity wants to flow through the path of least resistance, of which one point is the top if your head if you’re standing out in the open. Inside a car, you’re insulated from the ground (not perfectly, but quite well) because the rubber-like tyres are a good insulator. So you’re (relatively) safe from electrocution inside a car (provided no metal dangly bits are touching the ground).

An EMP, on the other hand, is more like a radio wave that propagates symmetrically from its point of origin and will pass though any gap wider than its wavelength. So an EMP could easily pass through the glass windows or even the gaps in the doors.

A good test to check if your Faraday cage is EMP-proof is to put a radio inside it. If the radio receives a signal, your Faraday cage has a “leak.”

jerv's avatar

Most cars that get hit by lightning suffer severe damage to their electrical system. The passengers are safe, but the power surge through the electronics as the electricity finds it’s way to ground fries them. My distributor doesn’t mind so much, but integrated circuits can be fried by currents too small to feel.

I can think of a way to “harden” the electronics, but it is neither cheap nor easy.

HungryGuy's avatar

@jerv – The only way I know of to “harden” electronics is to shield them. I.e., the chip has a shield completely surrounding it the way a TV cable has a shield surrounding the signal wire. And all signal traces need to be shielded as well.

@CaptainHarley – To summarize… To protect yourself from lightning, you want to be shielded and insulated. But to protect yourself from an EMP, you want to be shielded and grounded.

jerv's avatar

@HungryGuy Yep! Like i said, neither easy nor cheap.

CaptainHarley's avatar

Well, I could just buy replacement components and put them in a Faraday Bag. : )

jerv's avatar

…or you could just get an older rig. EMP doesn’t affect carburetors :D

CaptainHarley's avatar

We just bought a new Hyundai, so that’s right out.

jerv's avatar

I know of Corvette owners who retrofit carbs to their engines. And NASCAR mods modern engines that way all the time since they don’t allow fuel injection.

The ignition system is a little trickier, but if you have any machinist friends it shouldn’t be hard to make a distributor and get rid of the electronic ignition. The camshaft is already geared to go half the speed of the crankshaft so you could run it off the cam instead of having to rebuild the engine to run it off the crank.

Keep a spare battery, coil, and alternator in a safe, EMP-proof place and you could get your car running less than half an hour after an EMP pulse!

HungryGuy's avatar

@jerv – I’m not a gearhead, but I think modern cars are so utterly computerized these days that you can’t EMP-proof a car just by putting in a mechanical carburetor and distributor cap and having a spare battery on hand.

jerv's avatar

@HungryGuy You will lose some accessories like the power mirrors, heated seats, and in-dash GPS, but the only electrical parts of the engine are the fuel delivery and ignition systems. The ignition system of a gasoline engine of any vintage also includes the ignition coil, and that is one component that actually is sensitive to EMP.

I forgot about the fuel pump too, but if you go carb, you only need about 5 PSI instead of 35–90 like most injected systems need, and that should be easy to rig up if you are enough of a gearhead to do the other stuff, and if Cap is anything like any of the other bikers I know, I am sure he could figure it out if he put his mind to it.

True, you will lose a little power and a bit of MPG plus the tolerance of environmental conditions that EFI cars have compared to carbs (my old Toyota hates wet weather) but it will be more roadworthy than something that has no way to deliver fuel or spark to the cylinders at all.

Another option is an old Diesel. No ignition system at all, and the older ones use mechanical fuel pumps and injection.

HungryGuy's avatar

@jerv – That sounds like the way to go, then: an old diesel with no electronics. In fact, older Freightliner tractors use air pressure for the “luxuries” like power mirrors and windows etc.

If Cap has a CDL, he can use an old Freightliner cab as his “car.” :-p

jerv's avatar

@HungryGuy Or a diesel bike :D

CaptainHarley's avatar

@HungryGuy

If it gets to the point where I have to drive an old tractor rig, I don’t think whether or not I have a CDL is going to matter much! Heh!

jerv's avatar

One point that was just brought to my attention was that while an EMP pulse doesn’t have to be terribly large to kill an integrated circuit, any that is capable of screwing up an ignition coil would also probably cause problems with the human nervous system. I am not sure of the specifics (I’ll have to do a little digging) but common sense dictates that the operational status of your car is a moot point if the EMP fries your brain.

Build a garage onto your underground bomb shelter and I think your car will be as safe as you are though XD

XOIIO's avatar

@jerv And line the walls with copper mesh

CaptainHarley's avatar

@jerv

Groan! I refuse to go to those lengths to guard against something that is a relatively remote possibility! : P

jerv's avatar

Seriously though, if you have a garage that is a well-built Faraday cage, a car or motorcycle parked inside will be fine without modification anyways. And you’re right that is is remote; you have more to worry about from looters.

CaptainHarley's avatar

@jerv

Oh, I GOT sumfin fer dem looters! MWahahahaha! : D

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