Making a Faraday Cage, by Rourke

Saturday, Dec 10, 2005

Electromagnetic pulse (EMP), is now a commonly know effect thanks to movies showcasing the nuclear version such as Goldeneye (James Bond) and the remake of Ocean’s 11 which shows a non-nuclear version. The real question for survivalists is: what is EMP actually going to do to my valued equipment and what can I realistically do about it? Let’s start with lightening since it is very well understood. We know that lightening will tend to seek out the highest conductive point that is grounded and then seek to flow through a conductor, often back and forth a few times, until the electrical charge imbalance between the cloud and ground is neutralized. To protect buildings, lighting rods have been erected on them historically to channel the electrical current around the building and into the ground.
With EMP the concept of channeling the energy away from what we wish to protect is somewhat similar. EMP is a pulse of energy, and travels through everything. The idea is to shield something from it my placing it in (surrounding it with) a conductive (metal) box that is then grounded (preferably into the earth). The conductive box thus channels a large amount of the electrical energy passing into it down the ground, sparing what is inside of it from the full force of the electro magnetic pulse.
The good news is such a Faraday Cage or Faraday Shielding need not be expensive. A metal file cabinet or inexpensive light weight gun safe will work. I bought a used heavy metal box that I think was originally used to keep coal or sand in. What you want is a complete and connected conductive metal (steel) exterior surface, not a grid or cage with gaps (this is why the term Faraday Cage is actually not a good term since with a cage people think of bars). [JWR Clarifies:  Actually, a cage-like structure would work, but the largest gap in the mesh would have to be less than 1/4 of the wavelength of the expected pulse. Hence, 1/2" wire mesh should be sufficient.]  Ideally you should connect a very heavy gage electrical wire (monster cable or 220 volt heavy electric wire) to a stand-alone dedicated ground (usually a copper rod drilled in below your basement floor). Some people say connect it to your copper plumbing or steel pipes. The problem with this is that the pipes could act as an antenna and actually channel energy to the Faraday shield. Just talk to an electrician about putting in a ground.
Although the Faraday principal says the electrical items are safe inside as long as they are not connected or touching the exterior shield, I think you still have to worry about the spark gap. Thus my advice is to put your expensive electronic stuff (computer, laptop, night vision, digital camera, radio equipment, etc.) into plastic tubs with plastic lids, and that then into the Faraday Cage. Plastic of course was developed as an insulator for radar in WWII, which due to it’s high voltage had to have an insulator between metal parts so the electricity didn’t just spark through the air (spark gap). Mineral oil has also been used as an insulator in high voltage equipment.
Remember, having anything plugged in, or hooked to an antenna defeats the purpose of shielding around it as you have provided an electrical highway [an unintentional antenna] right in. The use of a [lightning protection fast] fuse on such necessary plugged in equipment is probably your best bet as I fear a circuit breaker is not going to react quickly enough to save delicate equipment. However, I am not an electrical engineer nor am I claiming to be an EMP expert here. I am just trying to apply practical solutions to the problem and welcome the constructive criticism or correct by others who are more knowledgeable and can offer better solutions to the problem. - Rourke

More on the subject:
Understanding EMP:  
EMP Bomb: 
Faraday Cages: 
Duncan Long on EMP:

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