EMP protection

[wagspe208]

Sure, a little off topic maybe. I drive a lot, so I have time to think about strange stuff.
I was wondering if a metal building if properly grounded would serve as a Faraday Cage. Or if you have to do the expanded metal/ chain link fence thing??
Oh, of course this means would a metal building protect against an EMP? N. Korea just annouced they have a functioning one. What can go wrong?
Wags

[BioHazard]

I too am curious if a standard simple metal building might provide protection? I've heard that a simple metal galvanized garbage can will. There was also some talk about steel containers, I can't remember if they would make a EMP safe place or not...

For example those cheap metal shed kits you can buy in about 10x12 size for relatively cheap...could that create a faraday cage?

[mike90045]

The long seams at each panel need to be bonded 90% or so, the painted metal, 4 screws, and the slats the screws attach to, wont do a lot.   Yeah, bummer. 

Metal containers are solid and welded, except for the doors and wooden bottom.

[veggie]

According to various articles I read, the items stored inside the metal container should be isolated from the metal structure.
(Perhaps stored on wood shelving).
Also, the building shell should be connected to a ground spike by a heavy ground strap.

[rcavictim]

According to various articles I read, the items stored inside the metal container should be isolated from the metal structure.
(Perhaps stored on wood shelving).
Also, the building shell should be connected to a ground spike by a heavy ground strap.

Inside a proper Faraday cage there is no field and whether the cage is floating or gounded electrically matters not to what is inside.  An aircraft in flight is an example of an ungrounded Faraday cage.  Inside the cage, since there is no electric field there is no need to isolate objects inside from the metal structure of the cage.

[veggie]

yes....but a metal container is not a proper Farday cage, so would it not be prudent to adequately ground the building and isolate the contents ?

veggie

[rcavictim]

yes....but a metal container is not a proper Farday cage, so would it not be prudent to adequately ground the building and isolate the contents ?

veggie

Grounding a metal box that is not perfectly sealed will have absolutely no benefit nor increase risk to the contents in the event that electro magnetic fields enter the box as radio waves through the imperfections (slots or holes) of the surface of the box. 

The use of a simple conductive coper strap attached to a conductive metal rod pounded into the ground will help adjacent structures from being zapped if the box is hit by lightning, but it won't make any difference to stuff or living creatures inside that box, even if imperfect.  The ground strap will also be frequency selective.  Higher frequency currents, those phenomena which occur with very fast transients will not be grounded at these higher frequencies unless the earth connection is distributed, thus lowering the effective inductance of the path to earth. All lengths of wire including ground rods and straps have inductance and inductance represents increasing electrical resistance to alternating currents of increasing frequencies. For example.  You could improve the ground for higher frequency transients by placing more than one ground rod and strap.  How far do you want to take this?  You could place a ground rod and copper strap every 6 inches all around the circumference of the base of your metal container.  This would protect the box from voltage rise above earth potential for frequencies up to about 500 MHz or more, but would not make any difference for objects inside the box.

A high voltage pulse event like a lightning hit is not the same thing as an EMP, but a lightning pulse WILL cause an EMP to be generated in it's vicinity.  Circuits carrying the lighning impulse that have been hit can generate and radiate a local EMP.  Lightning strikes (short duration voltage pulses at high current and rapid risetime flowing in a metallic conductor)  and airborn RF EMP's are both capable of causing damage to electronics and electrical circuits but the protection methods are different.  Lightning protection involves low resistance conductors to earth and ground rods to drain the voltage impulse to earth in the shortest and most direct path possible with large diameter conductor capable of carrying up to 10's of kiloamps for up to ~2 milliseconds (extreme case) without vaporizing.

As far as EMP goes, the only way to protect the objects inside the box from RF pulse or also continuous wave threats is to purposely engineer the slots or discontinuities in the walls of the box according to the wavelength of the threat. It is both simple and somewhat complicated to explain.  You could study the system known as a "waveguide below cutoff frequency".  Basically explains that an electromagnetic wave (radio wave) cannot penetrate a hole or slot in a continuous conducting sheet if the hole is smaller than approximately 1/2 wavelength of the radio wave.  For a given size hole or slot there will be a specific critical frequency where wavelengths longer (lower frequency) will not penetrate and shorter wavelengths (higher frequencies) will penetrate.  Protection against EMP threats therefore becomes more difficult as the wavelength of the threat is decreased.  Satellite receiving dishes can be solid metal or perforated metal mesh to reduce weight and windload.  As long as the perforations are small compared to a half wavelength of the satellite signal frequency the radio wave will see the perforated reflector as a solid conductive sheet of metal.

In the case of a steel shipping container with a contiguously welded steel floor and no broken seams the dimension of the end door opening will stop EMP from entering the box regardless of whether the steel doors are closed or wide open below a critical frequency determined by the width and height of the door opening.  The only way to prevent all RF entry is to make continuous electrical connection around the perimeter of the door which electrically connects the door to the frame of the container, effectively bringing the maximum length of the slot discontinuity in the door seal to zero.  You could do this by welding the door to the frame with a continuous bead but that would render the door kind of inconvenient to use.  You could install conductive wiping contacts commercially available for this purpose made of strips of spring brass or copper fingers all the way around the door frame and the door which engage to make a continuous metallic contact all around the gap when the door is closed.  This is the only practical way to make a wideband RF tight door seal.  For threats occuring at only one specific frequency a choke joint can be designed as is employed in the RF door seal of common domestic and commercial microwave ovens.

If you have wires from the outside world coming into the metal box through a deliberate opening in your metal box then you have a totally different scenario and grounding may help depending on where that wire comes in from.  Some form of engineered decoupling and bypass filter at point of entry will also be required.

This is all pretty complicated stuff and too much to make clear in a single post.  I hope what I have written makes more sense than confusion.

[veggie]

Thanks rcavictim,
That is perhaps the best explanation of EMP threat protection that I have read on the entire web.
I would like to nominate you as the EMP "goto-guy" for the MicroCoGen forum 

veggie

[rcavictim]

Thanks rcavictim,
That is perhaps the best explanation of EMP threat protection that I have read on the entire web.
I would like to nominate you as the EMP "goto-guy" for the MicroCoGen forum 

veggie

You're welcome.  I don't like to be seen out in public wearing my tinfoil hat but this time I thought I'd make an exception. 

[rl71459]

Hey RCA, Can you make me one of them foil hat's too?

[Henry W]

Here is somthing that might work for small items. Get 4" or 6" Dia. pvc pipe and cement one end with a PVC cap.

Than cement a cleanout cap on the other end. load them up with important diodes for generators or what ever.

Put some moisture removal crystals what ever they are called than silicone the threads before screwing the cap on than thread the cap on and tighten snug.

Seal all joints with silicone.

Get a galvanized steel pipe that is threaded on both ends large enough to slide the PVC capsule in than seal the steel capsule up with threaded caps.

Go out in the back yard and bury it. How far? I don't know.

Henry

[Henry W]

Another thought. Would throwing the water tight capsule in a pond work?

[rcavictim]

Wayyy to much trouble.  If you want to protect small components like semiconductor diodes just wrap them in aluminum foil and toss 'em in a drawer.  If putting the foil pouch inside a paper envelope makes it easier to place identification on the item then do that too.

[veggie]

Get an old microwave from the dump.
I doesn't need to be in working order.
Microwaves cookers are essentially a faraday cage except they are built to keep waves from getting out.
Put your stuff into it and close the door.

Don't plug it into the wall ! 

veggie

[rcavictim]

Get an old microwave from the dump.
I doesn't need to be in working order.
Microwaves cookers are essentially a faraday cage except they are built to keep waves from getting out.
Put your stuff into it and close the door.

Don't plug it into the wall ! 

veggie

Veggie,

I specifically mentioned the door seal technology on microwave ovens as an example of a frequency specific filter.  You might be surprised to try this test.  Unplug your microwave oven.  Place your cell phone inside, close the door and then try to phone your cell.  Inside that shielded box it will ring!  The cell phone does not operate in the same band as the 2450 MHz of the microwave oven.  So much for shielding!  An EMP, and these are broadband by nature, will penetrate that oven.