posted on Sep, 5 2010 @ 12:42 PM
A faraday cage does not need a ground strap. The ground strap is for the safety of the people outside the cage, not for things in the cage. A cage can
obtain a significant potential above ground due to static electricity buildup and the like. Someone that walks up to it and touches it, may get a rude
Electrical objects in a cage do not need full isolation from the cage. If the cage is a full solid cage, then the EMF running though the cage matches
the voltage across any part of the cage. The contact potential between any metal object inside the cage and the side of the cage will always be zero
volts. If the cage has 100v induced in it from an outside field, then the metal conductor running from one side of the cage to the other side also has
exactly 100volts across it. Zero volts difference in contact potential at both ends, means, no current will flow.
That is what they mean by “RF current flows on the outside of a conductor". When you induce voltage across the conductor, the inside of that
conductor is in a uniform voltage field that is generated by the metal around it. Lack of contact potential with anything, means that no current flow
can be generated, so the center of the wire can not actively partake in any current flow that the wire is delivering. The outside of the conductor
forms a faraday cage for the center of the wire at high enough frequencies. No current, no damage.
Anything with a solid metal cage with no penetrations, will be shielded.
Anything in a metal cage with penetrations that tolerate an EMP threat level, are going to be fine.
Anything in a solid metal cage where all wires leading to it are fully shielded, will be fine.
If you have components, or penetrations that can not tolerate an EMP. Most likely, the only parts that will be damaged, will be the parts directly
connected to the wires coming in.
A computer in a tight fitting case, with nothing plugged into it, is 100% EMP resistant. No computer in that situation will be bothered.
A computer in a solid case that has shielded peripheral cables, power cable, and an Ethernet connection/phone connection. The only things that could
possibly be damaged is the power supply, Ethernet card/telephone card, and monitor.
The peripherals that may be damaged are the keyboard. The spike from the keyboard may be able to travel to the computer. If you have a wireless
keyboard and mouse, that will protect the computer from any threats from it’s peripherals.
A plug in Ethernet controller that can easily be changed would be able to restore that functionality. A wireless network would avoid that problem all
A replacement power supply will bring back computer function if the one in the computer is damaged.
Some modems have a gas surge arrestor. Some have thyristors. They won’t be damaged from anything except a direct lighting strike. You can keep a
spare modem if you want to.
Some old monitors have full metal shielding, Those won’t be bothered. Some LCD’s have full metal shell around the active components. Those most
likely won’t be bothered.
The easiest insurance is to keep everything intact on your old computer and just put it in a closet, with nothing plugged in. After an EMP or what
ever, all you have to do is pull it out and plug it in, and you are good to go. That old stack of computers in the closet may be a gift after an
The same thing goes for all other electronics. If it has a solid metal case, then it’s internals won’t be bothered. If that metal case has
penetrations (antenna, power) then the things that will be bothered are the stuff directly connected to those penetrations. Small portable radios are
pretty resistant if they don’t have their antenna deployed. If you have a radio reciver and know how to change the first RF preamp then it may be a
good idea to keep a replacement MOSFET to replace the one in the radio. That is the most likely thing to be damaged. Power supply diodes and
regulators are another thing to stock. They could be damaged by spikes coming in from the power line.
You don’t even need a solid metal cage. A metal plate will reflect any RF wave that has a wavelength that is significantly smaller than its length.
It will create an un-illuminated(by the RF field) area behind it. The only wavelengths that can pass through are ones that are long as/longer than the
Any wavelength that is significantly longer than an object will not induce a (appreciable) voltage in an object.
So, if the threat is coming from directly overhead. A metal roof will provide a safe zone for small electronic devices directly underneath it. If you
are in a concrete building with reinforcing mesh in the concrete, the mesh will provide shielding (attenuation) from the full force of the EMP
A metal storage building with a metal roof and sides, will provide protection from all sides, except the bottom. No grounding required. If the EMP
threat is coming from below the building, then EMP is the last of your worries.
[edit on 5-9-2010 by Mr Tranny]