Understanding EMPs & Faraday Cages

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    This post is meant to be a beginner’s guide for preppers to protect their emergency electronics from an EMP.

    EMP Electro-Magnetic Pulse
    LEMP Lightning EMP
    HEMP High-altitude EMP
    SREMP Source Region EMP (Low Altitude EMP)
    HPM / HMP High-Powered Microwave Pulse

    So, what’s the big deal? Why would EMPs hurt us? Well, they don’t hurt us. They hurt electronics (devices with Integrated Circuits). When these waves come close to conductive materials (metals, wires, circuitry) they induce a current. The currents are high enough to burn out the circuits in electronics. Of non-electronic devices, only the most fragile devices like a light bulb filament will be damaged by a regular strength EMP—and light bulbs are iffy. i.e. They may not be affected. The rule of thumb is – the smaller the element, the more susceptible it is. Larger items like motors, generators, household wiring, etc. probably won’t be effected by EMPs. I say probably because a powerful enough EMP will fry anything. But as we see below, most likely scenarios won’t hurt those larger items.

    Unfortunately, virtually every modern electrical device has some control or other that has some IC in it. So, even if the device itself is intact, the switch may not make it unless the switch is shielded.

    The earth does block EM waves. EMP therefore only effect line-of-sight (some accounting for refraction is required). So if a nuclear bomb were detonated in San Fransisco at an altitude of 1 mile, we would not feel any effects here in eastern Colorado because the Rocky Mountains would block it. Similarly if Washington DC were attacked by a SREMP, it would be blocked by the Appalachians (do they go that far up north?). You would need to also take into account the blast radius when calculating the altitude and so forth.

    A nuclear explosion will have an EMP effective frequency up to 100 Mhz (radio wave range). They are extremely powerful but potency varies greatly with altitude. Hence we have the low altitude SREMP, and the high altitude HEMP.

    MYTH: Nuclear explosions themselves create EMPs that will destroy electronics.

    Not exactly. Actually, the EMP generated by the blast itself is quite minimal. The effective radius is so small (3 to 8 km outside ground zero– * see RANGE below) that you often have other things to worry about. In this region, you will still see a lot of damage from the explosion. If you survive the blast, but are affected by the SREMP, you were really cutting it close.

    The powerful EMP is an HEMP. The explosion of radioactivity reacts with the earth’s magnetic field which generates the powerful EMP that we usually think of when discussing a nuclear induced EMP.Update: There are some sources of HEMP that are not necessarily a nuclear device. But the common danger that is anticipated is from a nuclear device.

    The SREMP is the result of a nuclear bomb detonation at or near the surface of the earth. The HEMP comes from a nuclear explosion in space. To give you some numbers to throw around, the earth’s radius is about 6,400 km. The atmosphere is considered to end at an altitude of about 100 to 120 km. The earth’s magnetic field on the daylight side is more powerful than the night time side. Therefore, an explosion on the night side of the earth will not have nearly the same EMP potency or range. So a HEMP would be ideally detonated on the sunny side of the planet.

    A famous test detonation of a 1.44 megaton (6 PJ) warhead was made 400 km above the Pacific Ocean. It affected some streetlights and other minor electronics almost 1500 km away in Hawaii. But it didn’t shut down the Islands. While the explosive effects are much diminished, the EMP effects are greatly enhanced. They theorize that since the mainland US has a greater magnetic field than the Pacific Ocean, there would be an even greater effect if exploded somewhere over the contiguous 48 states.

    HMPs come from E-Bombs. These are non-nuclear devices that are specifically designed to create massive amounts of microwave energy. E-bombs require tremendous amounts of energy. But being dependent on non-nuclear energy, they are limited to a very small effective range. Because much information on these weapons is classified, it is difficult to say what their capabilities are. But based on available intel, we are looking at E-bombs with a range of a few hundred yards, or a couple of miles at most.
    The advantage is that HMPs go to much higher frequencies than nuclear EMPs. Some go up to 11 Ghz. The higher the frequency, the tighter the shielding required to protect against it. And the higher the frequency, the easier it is to short out exposed circuitry.
    I’ve heard apocryphal references to more powerful “super weapons”. But my nose says “urban legend” when I read about them. I haven’t been able to verify their existence, much less their actual capabilities.

    Lightning is known to create an EMP with a similar frequency range as the EMP created by a nuclear blast. But the range and energy of a LEMP is so minimal that you often don’t need to worry about it. If you have a lightning rod on your house, be sure to route the grounding and bonding away from outlets as best you can. Plan your house layout so that you keep your computer center and entertainment center away from where your cable runs.

    Shielding is the primary means of protecting our equipment from an EMP. There are supposedly devices that “harden” your equipment from an EMP. I don’t really know what this is. But from what I’ve read, most of these are for all practical considerations ineffective. And most of us won’t be getting all our electronics hardened anyway.

    So what is shielding? You may have heard of the term “Faraday cage”. But you may not know what it is or how it works. It is merely a cage made of conductive material. A Gaussian sphere is a cage that is complete as opposed to an open “cage”. The process was actually discovered by Benjamin Franklin and, later, made famous by Michael Faraday—the physicist after whom the “Farad” and the “faraday cage” were named. One wonders why no unit was named the “Franklin”.

    It will act as a conductor for any electric current. But it will protect anything inside from electricity.
    With EM waves it has a slightly different effect. When the wave induces a current in an element of the cage, it has to absorb some of the wave’s energy. The more conductivity the cage elements provide, the more energy is absorbed. The more energy is absorbed, the less energy penetrates. As a rule of thumb, most EMPs that I have studied are not so powerful compared to the conductivity of most common metals. So, the choice of metal makes little difference for practical purposes.

    Obviously, the difference between a wall of metal (a Gaussian sphere) and a cage of metal (Farada cage) is that a cage has openings. The size of any openings in a faraday cage is of particular concern for shielding. The rule is that the opening cannot be as big as the wavelength of the pulse. We discussed frequencies. This is inversely proportional to the wavelength.

    HEMP = up to 100 Mhz = 3 m wavelength minimum.

    We always want to apply a factor of safety because waves and fields are weird. 3 is usually a good number. All openings should be 1 m or smaller in diameter.Update: See updated message 30 Sep 2011

    HMPs = up to 11 Ghz = 2.72 cm.Update: See updated message 30 Sep 2011

    So, if you have a screen with a grid smaller than 1 cm, you should be safe from microwave weapons.
    This means that a car will most likely be protected from EMPs from lightning or a nuclear blast. A HMP may get to the stuff in the cab of the car, but probably won’t affect the items under the hood. But if some items in the cab are wired to electronics under the hood, it may cause damage to the stuff under the hood (see surge protection below). I’m afraid I don’t know enough about a car’s electrical system to determine how things might be affected.

    Higher frequency waves like gamma waves have such short wavelengths, they can actually penetrate the spaces between atoms. Thus only very dense or very thick layers of material can block them. Hence, lead is used as shielding.

    Thickness is also an issue. There is a concept in EMP discussions called “skin thickness”. Without getting into the math and physics, the thicker the material, the stronger the pulse it can take. So, aluminum foil may be fine if the detonation is far enough away. But a cage made of #12 wire would be more effective. Just make sure the space between wires of the cage are small enough compared to the wavelength. (see update on my post Sept 1, 2011 below).
    Update: See updated message 30 Sep 2011

    Does it need to be grounded? Does it need to go all the way around? These are related questions. if the cage goes all the way around, it does NOT need to be grounded. But if it is an open cage (top or bottom are open, but not in the direction the EMP comes from) then the cage can still work if it is grounded properly. But it is safer and much more practical to create a screen of wire for the bottom and lift off the cage off the top and replace it with each use.(see update on my post Sept 1, 2011 below).

    It might be nice if you had a ham radio inside your faraday cage. But you wouldn’t be able to get any signals inside a properly designed cage. So you need to run an antenna from outside through an insulated opening to your equipment.(see update on my post Sept 1, 2011 below).

    The problem with this idea is that an EMP would burn out the radio through the antenna cable. So, you need to install a surge protection device on the antenna cable. Ideally, you should have a surge protector or circuit breaker that will switch the signal to ground instead of absorbing it. But I don’t know of any devices that do this. But you can imagine what it would be like if they did…

    You would also apply this principle to any devices that require some outside signal.

    A common question is “what kind of range do I need to worry about?” That is a very good question. There are many variables. But you could consider 15 miles as the rule of thumb for SREMPs. This is based on a 1 megaton bomb. I don’t know what common sizes are. But in all the internet discussions, this seems to be the baseline that most people talk about.

    *The term “ground zero” has multiple meanings. So the SREMP range was difficult to pin down in my research. The estimate of 3 to 8 km might be outside the few miles of greatest impact, or the radius of 90% fatality, or from the actual point of impact (radius = 0 m). I guessed it would be outside the range of 90% fatality. For a 1 megaton bomb it is about 10 to 13 miles. So, the EMP could go around 15 miles.

    Since my house is out in the country, I’m not too worried about an HMP. Since I’m fairly far away from any major target, I’m probably not going to be effected by an SREMP. Buckley AFB and DIA are both around 10 miles away. So, an SREMP might be an issue if those are targeted. But if they are targeted, I’m toast anyway.

    It is the HEMP that would be a problem. If this happens, the entire country and surrounding regions would be affected. Any preparedness measures should keep this condition in mind. If only four 1 megaton bombs spaced for maximum effectiveness are detonated, most comms would be affected. Based on data from the Pacific Ocean detonation, it seems a little extreme to believe the entire US could be reduced to the 1800s by a single bomb. It would, however, be enough to create a lot of chaos for a while.

    As always hope for the best. Prepare for the worst.

    NOTE: I am not a physicist. I’m an engineer. So a few items written above may not be perfect. But most of this is accurate enough for the standard prepper.

    There were requests for more information on nuclear explosions. Begin with the DHS website and go from there.

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