Faraday Cage: To Ground or Not to Ground

The key to saving data from harm is to have backups on external hard drives in different locations...You are much more likely to lose data to a virus than lightning...If you have permanent files that you want to save, back them up on an external hard drive and keep it in your safe deposit box, and one at home to use...periodically upgrade your backup.... Faraday cages work best with metal that is good conductor, copper or aluminum, and need not be solid metal, screen works just as well...and yes ground it with typical ground wire....

http://en.wikipedia.org/wiki/Faraday_cage

The ground connection allows accumulated charge to bleed off, but whether you do or not does not alter the effectiveness of a Faraday cage. It will still exclude electric fields from its interior whether grounded or not. Consider this diagram of the world's largest, 1.5-million-volt Van de Graaf generator, now at the Boston Museum of Science.

Nesting cans is not necessary.

There have been many posts on lightning in the past. You might want to read through a search of all lightning related posts. It could take a while. Frequently I have had difficulty in finding some of the better ones that I liked because there are so many.

A Faraday cage seems to be a rather extreme solution but if you are in an area that is prone to electrical storms, for instance at the base of a mountain ridge or just in Florida, then it may work just fine. In all the bizarre videos of music made with indoor (man made) lightning, the cages were grounded to the generator. It helps complete the circuit.

With electrical storms, your best form of protection is a ground grid connected with a cable (typically a half inch copper multi-stranded conductor) up to some lightning rods about 12 to 18 inches long and mounted to the highest point of your roof. This provides a preferred path should you be in the range of a strike. But, if your trees are taller, it may go for them instead.

You may be better off investing in some whole house surge suppression connected to your circuit breaker panel. They provide a temporary short circuit path if the electrical lines are hit. They don't last forever, but it really depends on how much protection they provide. I've seen a demonstration of how effective they are and they are well worth it. Eaton makes them, and their power lab is in Philadelphia, PA. It is worth a visit if you have the time.

A really simple and sometimes effective method of protection (you have nothing to lose if it works) is to tie your power cord with one knot. The loop creates an inductive short circuit just in case only one side of the power line is hit. The indication that it worked will be a melted power cord up to the knot. Cheap insurance. If lightning hits both conductors at the same time(common mode), the damage will all be outside your house. If you use surge suppression, you don't need the knots and the surge suppression is more likely to work.

As far as grounding the Faraday cage to protect you from lightning...well it won't prevent lightning from taking it into its path. Anything inside it will reach the same potential but a high static charge will leak into the air so I don't know if the cage would offer a perfect solution. Air is usually a pretty good insulator until it becomes ionized.

Even with lightning rods and a good grounding system, you'll still have to deal with the inductive effects due to the high currents involved. That conductor from those rods on the roof may carry in excess of ten million amperes for a few milliseconds or more during a strike, producing a large electromagnetic pulse which will induce high currents in other conductors nearby. Best to keep sensitive electronics away from that end of the house, if possible.

Speaking of ground conductors in lightning protection systems, it is best to use not a round conductor, but a flat, wide copper ribbon, to reduce inductance and provide a large surface area for the high-frequency components (which travel only on the conductor surface, a phenomenon called 'skin effect'). You definitely don't want the conductor to act like an RF choke if at all possible. That lightning will follow some other route if it has to.

To the op: if you really want to build a super-duper Faraday cage, build two cages, one inside the other: Construct the the outer cage from silver- or silver-plated copper sheet, and the inner cage from mu-metal sheet which is electrically isolated from the outer one by means of insulated standoffs. This construction will protect your stuff from EMP in general, whether lightning-generated or thermonuclear. Tip: Win the lottery first.

Note: a microwave oven makes a great Faraday cage which will exclude RF as well. Just make sure the oven is unplugged if you don't want to accidentally bake the contents, or this could happen to you.

Sort of related to this thread is the need for some to avoid cell-phone calls at various times or from various people, but without offending the caller. If you shut the phone off, it sends a swan song to the network before shutting down indicating that it was, in fact, turned off rather than just having lost the signal. You can tell by the way the network responds when you call the number. If it lost signal, the network says you're number cannot be reached and you get voicemail right away.

If you want to simulate losing signal ("What can I say, Dahling, I was in a dead zone!"), just chuck your mobile in the microwave and shut the door - without shutting off the phone first. Just don't turn on the microwave unless you want the 'loss of signal' to be permanent. (I can imagine wanting this on a regular basis). Best to unplug the microwave just in case.

If the phone was shut off, it rings a number of times before playing your voicemail greeting. Shutting off the phone is an act of volition and will be seen as such by other callers. Losing signal?

"It happens." -- Forrest Gump

For your requirement I think Wal probably has a good answer. Possibly add to it the requirement that the plastic bag be similar to a "Zip Lock" so that it is somewhat air tight and water tight. In case the storage container is ever blasted with water from a fire hose you will want the bag water tight.

Since your disks seem to be pretty valuable I would suggest putting the packaged disks in a metal firesafe.

Note that a tight bag also protects the disks from the firesafe. I had a safe that either had some flux or some "wash down acid" left in the seams. After a couple of years one of the seams was corroded. In addition, most media in the safe had surface damage and could not be used.