Letter Re: EMP Question

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Hugh,

I’ve asked Matt Bracken this question, and he didn’t know. I’ve read all of your EMP-related data, but none of them answer the question of whether batteries, particularly the small D, C, AAA, AA types need to be shielded to protect them from EMP. All authors make much of electronics in devices but never mention separate stores of batteries or the dangers of batteries stored in electronic devices like radios, sights, et cetera. Your advice would be greatly appreciated. – S.D.

Hugh Responds: There is much FUD in the online world (and in books) about EMP. Some of it is well deserved, and some of it is simply because of ignorance about how such forces work. There are two interrelated factors that are critical in determining how destructive an EMP would be towards electronics:

  1. Peak impulse energy
  2. Length of the reception antenna

The peak impulse energy is determined by how large the EMP device is (how much energy it produces) and how close the device is to “ground zero”. The antenna is determined by the length of conductive material attached to the device you are wondering about (and to some extent, the orientation of the antenna in relation to the energy burst). Both of these factors determine how much energy from the EMP makes it into the device. Most small electronics (handheld radios, cell phones, calculators, ipods, GPS, et cetera) have very small antennas. In the case of handheld radios, you can often store them with the antennas disconnected as well. All modern electronics have some ESD protection built-in because they are so sensitive to it, and because of this built-in protection and the small antenna structures small devices are not likely to be affected by an EMP. Of course, what good is your cell phone when the cell tower is non-functional? The batteries themselves have virtually no effective antenna as well.

The established infrastructures are a different story. Power lines are very long antennas as are copper phone lines, non-fiber cable TV, and other things that have very long conductive surfaces. How does this affect you? If you are charging your cell phone or hand-held radio when an EMP hits, it will probably be toast. If it is simply in your pocket, it will probably survive. (Remember, proximity to the burst is important. If you’re at ground zero, it probably won’t, but you have other things to worry about then… or maybe not.)

The path to ground is also important. Things that are isolated from ground will not be affected the same as things that are grounded. You can think of it like a bird landing on a power line: As long as the bird does not complete a path to ground, it is pretty safe. However, if it bridges a ground (like landing on a power transformer) it’s “toast”.

Faraday cages work by keeping the EMP energy on the outside surface of the cage, thus protecting the electronics inside. They do not need to be grounded to do this either. The better the cage, the more energy is kept outside and the safer the inside is. Since there are so many variables that are at work here, it never hurts to keep at least some of your backups in a Faraday cage to be sure. Given that the largest antenna structure on a small device is the charger, you should also unplug your chargers from the wall when they are not in use.

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