I am wondering whether an altoids tin would act as a Faraday cage and protect, eg, a Photon microlight inside it. Also whether the aluminium body of a Surefire torch would protect its circuitry and possibly its LED.

Nope, the body of a surefire is part of the circuit. So, if EMP could/would/will blow it out, then it would travel through the flashlight body. The altoids tin on the other hand might.

Right, a faraday screen only works if it has somewhere to drain the energy off to, specifically a ground. If there isn't a huge welding wire connected to your altoids tin on one end and a copper rod driven deep into the ground on the other end, sorry, no protection. Same goes for cars, portable radios, flashlights, bicycles, cordless phones, cordless power equipment.... you get the idea.

While an EMP has a localized destructive effect on solid state circuits, it has a much broader temporary interference effect on same. You may be 50 miles or more from a major EMP event, and suddenly your flash mem stick is wiped clean, or your computers bios is gone, or your ipod has lost all your favorite songs.

Lightning is also an EMP, just not as strong as a thermonuclear detonation. Large meteors coming down through the atmosphere can also cause an EMP, though like the thermonuclear device ones big enough to blow circuits generally have much worse direct effects.

A really big problem we had with electronics aboard ship and up on hilltops was static precipitation. It is a localized type of EMP caused by the movement of dry air across the surface of the antenna. One way to isolate your antenna system without using those big honking diodes is to install a ferrite bead at the coax connection to the radio, or even inside the radio in some models. Isolators also work pretty good, though they can be defeated by static discharge.

Gosh, there are so many things that can be done. I guess them old tube jobs do still have some purpose. <img src="/images/graemlins/laugh.gif" alt="" />

EMP Electro Magnetic Pulse
By Rich “Raspy” Shawver

The way you make electricity require 3 things a conductor [Wire] a magnetic field and relative motion. A lightning strike generates an expanding [the moving] magnetic pulse or field that passes over power lines [the wire] generating a voltage spike in incoming services. The size of the spike is dependent on the strength of the field and the length of the wire involved. The resulting spike only has a duration of a very small fraction of a second but that is more than enough to damage electronic components. Circuit breakers are of no help. The spike is so short that it hits and does the damage long before the breaker has a chance to react and open. The spike resulting from a lightning strike can be 4 to 5 times as high the normal line voltage. The EMP of a nuclear weapon is exactly the same mechanism but with a much greater intensity. Depending on your distance from the detonation the field can be strong enough to generate damaging voltages in the internal wiring of the equipment.

If your equipment is hooked up but turned off, an incoming spike on the electrical power lines [and other attached lines, cable and phone] can arc over the switch if large enough. Also most electronic equipment now have circuits that have power running through them even when turned off for remote operation [even if it's not remote controlled]. Rather than building separate boards for each model companies have found it is cheaper to include many of the features on a common board. They just turn on the special features on the cheaper models.

The only safe way to keep equipment from being damaged is to enclose it in a metal structure that is grounded. The equipment has to be totally isolated from the cage and any outside connections. A structure of this type is called a Faraday Cage. A metal container [such as an ammo box] in which equipment can be stored if insulated from the container and the container is grounded will act as one. The way a Faraday cage works is the moving magnetic field strikes the cage. This starts to generate an electrical charge. As the electrical charge builds this in turn starts generating its own magnetic field that pushes back or repulses the original magnetic field. This then causes the original field to flow around the cage rather than through it. This is what keeps the EMP pulse from effecting the equipment inside the cage. While the best design for a cage is solid sheet it can be a fine mesh grid. The size of the openings in the grid is somewhat critical. If the openings or mesh size is too large the field the wires counter generate does not build up fast enough to block the field pulse. Something the size of chicken wire would be so large that there would be some leakage. The field on the cage has to build up fast enough to block the holes in the mesh. The cage does need to be grounded to dissipate the resulting electrical charge.

There are devices called lightning arresters among other names. That are sold on the market that are designed to stop or at least minimize the effects of such spikes. Many are good devices. But they are limited to the amount of protection they can supply. A large enough spike can overwhelm their capacity. While you can not shield wiring outside your own home. Inside it can and should be shielded. Some wiring is designed with shielding. This is a metallic or braided sheath surrounding the wiring. Another way is to run the wiring through EMT conduits. Both methods can be combined for better results. This shielding needs to be grounded. Just remember that this wiring is attached to wiring that is outside your control and probably not shielded.

The best way to build your shelter is to line the inside or outside with sheet metal. [Door included] The metal of the door must make electrical connection with the rest of the sheeting. For more reliability Line both inside and outside independently. A double Faraday cage. You do not have to do this for your entire house but should be done for a shelter, panic room, communication shack or a room that is a combination of any of the three. The best metal for making the cage is copper because it conducts better but any metal will work. [Gold and platinum would actually work even better but who could afford it.]

If you do run electrical or electronic conducting lines into your structure have them shielded but do not have your equipment connected to them except during actual use. I do not mean disconnected by a switch but physical distance such as pulling the plug out of the wall socket.

Remember if the moving magnetic field is strong enough the internal wires of the equipment itself can cause a voltage spike that will damage it. That is why most cars will be knocked out because the voltages would knock out any electronics. [Most cars are totally controlled by chips] That is why older vehicles [pre chip] are a better BOV. The voltages can be high enough to actually burn out the wiring itself.

I can see the question in your eyes. A car is basically a metal box wouldn’t that act like a faraday cage? If that is so how can a car be effected? Simple the box that is a car has some huge holes in it electrically speaking. That and many modern vehicles have gone to composite material mostly plastic for sheet material rather than steel to reduce weight. The engine compartment, that has much of the wiring running hither and yon to operate the engine. Plus you have the main power supply cables from the battery and alternator and they either hook into everything that uses electricity or that item does not have power. Look under the hood sometime even in the tight compartments today there are places you can see the ground. The passenger compartment has windows some real big holes in the box. Yeah, but the wires are under the dash. How much of the dash is plastic? Look at all the holes for gages and dials. In most cases the entire underside is open. Out your car in the bottom of your swimming pool. Just kidding. But if you did would the water be able to get to the wiring? If so then so could the pulse. While you could close off the engine area covering the windows with plate or mesh just wouldn’t be a workable solution. It would be difficult to say the least to see out and drive. But the local constabulary might have a comment or two.

If your only choice for a BOV does rely on electronics there is a remedy. On any car you could pull the entire wiring harness and up grade it. This would be with heavier gage wiring and using shielded wire. Get a spare set of the control modules needed for you car. Not just the computer control boxes but also the sensors and the items that are controlled by the computer. [They are also susceptible to pulse damage.] Wrap them in insulation [Electrical not temperature type I.E. rubber] and store them in a metal box. Attach the box to the frame so that it makes good electrical contact. Then do as they do to movie stunt cars. Attach a grounding strap to the frame. The strap is usually a braided copper cable that then drags on the ground. This is to dissipate and charge that builds up on the vehicle. The strap needs to be checked regularly and replaced as needed.

Yes the entire national power grid would be knocked out. While the most of the grid components them selves would be able to withstand such a pulse all the electronic control equipment would be fried. The means of operating and monitoring the system. Local power may be restorable but an inter linked grid would take a long, long time to restore.

Will your car or home equipment be affected? Mainly it will depend on luck. How far you are from the source and intervening obstacles. Even the angle that the local grid or house wiring is to the source can have an effect.

Sorry to be so long winded but it is not a simple subject and needs some explanation on the hows and whys.

Here are a couple of web sites that have a more technical explanation of how the field is generated.
http://www.geocities.com/CapeCanaveral/5971/emp.html
http://www.geocities.com/HotSprings/Falls/1984/emp1.htm

I am convinced that local release EMP devices are BS - the fantasy of paranoinacs and survivalists, in the worst sense of that term.

However, from some documents which I have obtained in litigation and cannot provide to you, I have learned that the detonation of a small nuclear (nucular, in the pronunciation of our fearlful leader) will result in third degree burns to exposed skin at a distance of 64 MILES.

I was born in '57. I went to a private schoolin Abilene, Texas with the kids of a bunch of SAC pilots from Dyess AFB. Their dads said, 'In case of air raid, get down in a comfortable possition, put your head between your legs, and kiss your ass goodbye.'

I am not frieghtened by the 64 mile stat - it just gives me one less thing to worry about. I'll be dead.

So, what's the difference between "nucular" and:

"fearlful", "possition" and "frieghtened"?

In the words of the great Steve Martin: "Yeah, I remember when I had my first beer...."

Vince

Got me that itme

> the body of a surefire is part of the circuit. So, if EMP could/would/will blow it out, then it
> would travel through the flashlight body.

I've come to the same conclusion. I think it would be fairly easy to design a hardened flashlight, though. It'd need a metal "lens cap" to shield the bulb/LED, and it'd need to not make the shell part of the circuit. But it's probably not worth it: cheaper and as effective to keep it in a metal biscuit tin when it's not being used. Don't let it touch the sides.


> The altoids tin on the other hand might.

Yep. I'm now fairly sure of this. The metal is opaque to the EMP and will sheild anything inside it. The EMP will induce current in the tin itself and in anything touching the tin, but something like a Photon Microlight which has a plastic shell keeping it away from the tin ought to be OK. The tin doesn't need to be thick, nor do small holes in it matter much, but it does need to be made of a conductor with the lid shut.

The controversial issue is whether a Faraday shield needs to be earthed. I've found several references which say it does (including some in this thread), but I don't believe them. I've also found references which don't mention it and some which flatly say earthing is not needed. I'd need to see more argument and more physics before I believe earthing is needed.

Earthing provides a favourable route to ground. If you don't earth the tin, the current will reach ground by some other route. If you are holding it, it might go to ground through you. But whatever the effect that has on you, it's not going to affect what's inside the tin. A charge may build up and take a while to dissapate, and the tin might be dangerous to touch meanwhile - but again that's about protecting people outside the tin, not the contents (and of course the danger, if any, applies to touching anything big and metal and unearthed). Aircraft can be EMP hardened and presumably are not earthed.

Some references: This was given earlier. It doesn't say earthing is required. Duncan Long is very clear about Faraday cages, says earthing is not required, and makes a lot of sense to me.

Overall, I reckon EMP shielding is an advantage for building a PSK in a tin instead of a plastic pouch. And for keeping other electrical equipment in a tin when it's not being used. Torches, GPS devices, your spare watch, I suspect simple magnetic compasses too.

Those stat.s are for the czar bombe, the largest nuc (soviet) ever test detonated (thank you History Channel), not a "small ' nuclear device, but why split hairs, if the major players start throwing nuc.s back and forth, we're all done for any way. If it comes to that, give me a couple of LARGE bottles of good wine, let the fireworks begin, and set one down right in my lap... I don't want to have to linger through rad. poisoning before I check out. On a side note, anybody else seen "Dirty War", it was an HBO movie I caught on DVD last weekend... very well researched, and VERY unsettling.

Troy

Brangdon while it is true that a faraday cage does not have to be grounded. It functions better if you drain the build up of charge to ground.

A hardened aircraft will build up a charge from an EMP burst. But they can also build up charges just from flying. These either slowly discharge to the air or when the plane lands.

The charge built up on something as small as an Altoids tin would disipate through the body. But would be so slight it probably would not even be noticed or at most a slight tingle.