This is the first draft of a new page on grounding for electromagnetic pulse (EMP) protection.
On the other EMP pages on this site, I have warned against grounding for EMP protection unless you have a considerable amount of experience in grounding in high-intensity, high-frequency electromagnetic fields. On this page I will try to explain the dangers of the wrong type of EMP grounding, and explain what types of grounding might be useful.
First, consider the following thought experiment: Imagine that you have a long copper rod vertically imbedded into the ground, but with 10 feet of the rod protruding vertically above the ground. The copper rod is electrically well-grounded. Now, suppose the copper rod is hit with a nuclear electromagnetic pulse that has an electric field strength of 20,000 volts per meter in the vertically polarized direction. This means that the 10-foot (3.048 meter) section of the copper rod that is protruding from the ground will have more than 60,000 volts induced on the top of the rod. It is true that the voltage will induce a current that will travel through the rod toward the ground at nearly the speed of light, but that will take about 10 nanoseconds. The pulse is not an instantaneous, infinitely narrow spike, so as the current begins traveling down the rod, additional voltage is being induced at the top of the rod. Therefore, depending upon the exact characteristics of the pulse, this well-grounded rod has a very high voltage pulse induced at its upper portions that will last for several nanoseconds. The voltage pulse at the top of this grounded rod doesn't contain much total power, but it is far beyond the voltage necessary to punch through a semiconductor junction, and completely destroy any transistor or integrated circuit. Even if there is no direct connection between the top of the vertical copper rod and the transistor or integrated circuit, the voltage at the top of the rod could induce voltages in nearby wiring sufficient to destroy the circuit components.
If the vertical rod were replaced by a well-grounded copper wire, with a horizontal run, the situation could be worse since the horizontal component of the pulse is often of a much higher electric field strength. If the 10-foot (a little more than 3 meters long) wire were exposed to a 50,000 volt per meter pulse, a voltage of 150,000 volts could be induced on the end of the wire for several nanoseconds. If this wire were used to "ground" the shield around sensitive electronics equipment, it is easy to see that what you are actually doing is inducing a very high voltage spike on the entire shield, which is then transmitted very effectively to the equipment inside. So this method of "grounding" an equipment shield can be a very good way of assuring the destruction of the equipment that you are trying to protect. (If the "ground" wire is curved or curled up in some way, the pulse induced on the equipment shield will be even worse!)
This is why EMP shielding can be very difficult. It is also why nested shields (or nested faraday cages) are so important. Even if you apply what you will learn later on this page about grounding, that does not reduce the importance of having layers of protection. Alternating layers of a good conductor and a good insulator are important. If you are trying to protect small equipment like radios, wrap the radio inside an insulator (such as a plastic Zip-loc® freezer bag), then wrap that insulated bag in aluminum foil, then wrap that in another layer of plastic, then use another layer of a good conductor (aluminum foil or aluminum or copper screen), and continue to add more layers if you need additional protection.
The same principal applies if you are building a shielded room. For a room, you may be able to use an air space between the inner and outer shields as an electrical insulator between the inner and outer shields (depending upon your construction technique), but it is always much better to have at least two layers of shielding. separated from each other by an insulating material (or by air).
In order to have an effective ground, the wire that you are using as a ground must be shielded in order to prevent it from becoming an "accidental antenna" for the EMP. The central ground wire and the outer shield must both be connected together underground, but the outer shield must not be connected to anything at the top end. The central wire can be used to ground an equipment shield as long as it does not protrude more than about 30 inches (about 76 cm.) from the shield around the central ground wire.
Of course, even this method of using a shielded ground wire is less than perfect since the current from the pulse going down the outer shield of the ground wire is close enough to the central ground wire to induce a small pulse on that wire, but it is much better than the idea of using an unshielded ground wire.
A large-diameter coaxial cable might be usable for making a shielded ground wire, but not the ordinary coaxial cable that is usually used for things like connecting video equipment. There are several practical problems even with a large-diameter coaxial cable. One problem is that the ground wire should really be larger that what is available even with large diameter coaxial cable (unless you are considering using something really robust like Andrew Heliax®). Another problem with even the largest coaxial cable is that (even though some cables are designed for intact underground burial) they are not designed to have an underground connection between ground and shield. You have to be careful about connections between conductors that are to be buried underground. Corrosion and other chemical processes can degrade the buried connection.
The best way to construct a shielded ground wire is to use heavy electrical wire for the ground wire and flexible copper tubing for its shield. No connection of any kind should be made to the copper tubing except underground. The central wire should have a good electrical connection to the copper tubing at the underground part of the wire. Although soldering can be used for the connection, it is definitely not the best option. A brazed or Cadwelded® connection for the underground connection are better options.
I'll have some photos of a proper grounding system later. For many people, this will seem all too complicated; and it does get complicated, even for experienced electrical engineers. This is why the best option is often just to forget the grounding of the shield, and concentrate on using multiple layers of shielding.
If you are grounding an equipment shield, it is best to ground only the outer shield. Professionally designed and maintained EMP shielded rooms often have a ground connection to all of the shields, but doing this properly is completely out of the reach of individuals unless you happen to be both wealthy (with access to some very expensive and specialized test equipment) and a knowledgeable engineer.
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