A Faraday cage is a metallic enclosure that prevents entry or escape of electromagnetic fields, including electromagnetic pulses (EMP). Survivalists care about this because nuclear weapons detonated high in the atmosphere cause EMPs that can affect (destroy) modern electronics, but not older electronics using tubes. This includes everything from communication to the computers that have been in most cars since the mid-1980s. The book One Second After deals with the aftermath of an EMP attack on the U.S.
Storing electronic items you don’t use daily – hand held radios, energy radios, laptop computers, GPS, other spare electronic equipment, etc. – in a Faraday cage/box is probably a good idea. It’s one of those low probability/ high risk scenarios; if you store those items in a Faraday cage and nothing happens, no harm. If there is an EMP, you’ll have that critical gear.
If you believe the potential threat is overstated, you can read a report by a government commission charged with investigating the issue. If you distrust the government because they tend to treat us as sheeple, this is one time there is not sugar-coating; see the Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack, April 2008 (PDF, 7MB). An excerpt from the preface:
The electromagnetic pulse generated by a high altitude nuclear explosion is one of a small number of threats that can hold our society at risk of catastrophic consequences. The increasingly pervasive use of electronics of all forms represents the greatest source of vulnerability to attack by EMP. Electronics are used to control, communicate, compute, store, manage, and implement nearly every aspect of United States (U.S.) civilian systems. When a nuclear explosion occurs at high altitude, the EMP signal it produces will cover the wide geographic region within the line of sight of the detonation. This broad band, high amplitude EMP, when coupled into sensitive electronics, has the capability to produce widespread and long lasting disruption and damage to the critical infrastructures that underpin the fabric of U.S. society.
Because of the ubiquitous dependence of U.S. society on the electrical power system, its vulnerability to an EMP attack, coupled with the EMP’s particular damage mechanisms, creates the possibility of long-term, catastrophic consequences.
Building a Faraday cage: There is a lot of information on the net about Faraday cages, some of it conflicting, none seemingly very authoritative. For example:
An ideal Faraday cage consists of an unbroken, perfectly conducting shell. This ideal cannot be achieved in practice, but can be approached by using fine-mesh copper screening. For best performance, the cage should be directly connected to an earth ground. [bold mine]
Note this says for “best performance” and not that it is a requirement. Other sites note that grounding is needed only for static discharge and is not necessary to protect the electronics inside the cage. Also see here (comments #13 and #23) and here (comment at 11:11am). Again, none of it authoritative.
Erring on the side of caution, I suggest grounding Faraday cages – but am seeking advice on this and will update if I get an answer. If you life in an apartment, finding an appropriate ground might be an issue. Note that anything put inside the can must be inside a non-conductive container such as plastic, rubber, wood, or cardboard. For more on building your own Faraday cage, see this.
Update: Also, test your Faraday cage. I purchased a small galvanized can – enough for a laptop, GPS receivers, emergency and hand-held radios, power inverters, and a few other electronics – thinking I would be set. However, both cell phone and GMRS radio signals passed through the galvanized can, grounded or not, and after ensuring the lid had a tight seal. The same is true of metal ammo cans, perhaps due to the gap created by the rubber seals, (though others still refer to this as a viable solution – search on “Faraday”). My own testing shows both method do not work for radio signals.
What does work? Aluminum foil. I tested both a cell phone and GMRS radio wrapped in common aluminum foil, and the did not receive signals if tightly wrapped. If the foil was loose, signals did get through. The video below also demonstrates wire mesh will work (poor OPSEC with phone numbers, though). In the near term I plan on constructing a Faraday cage and will post photos and video, updates to follow.
Update 2, 25 April 2010: I finally have a combination that will at least block radio signals (cell phone and GMRS/FRS). A shoebox carefully wrapped in foil did not work. Then I tried a slightly larger box (that the shoebox could fit into) and wrapped it with aluminum screen. Neither box alone blocked the signals, but nesting the foil covered shoebox inside the screen covered box blocked most signals. Placing the two into steel filing cabinet drawer completely blocked all radio signals.
But the signals aren’t the same as an EMP so I don’t know if it will actually work for that. Still it seems that demonstrating the ability to block at last some signals is needed.
It shouldn’t be as hard as it is to find clear, and especially credible, information on creating Faraday cages. I’ve still not found anything conclusive on the need for grounding but will go with what I have. In the off chance there ever is an EMP and it does work, I’ll have a pleasant surprise.