Re: nuclear EMP protection???

From: Robert Morein (nowhere_at_nowhere.com)
Date: 06/04/04 

Date: Thu, 3 Jun 2004 23:29:58 -0400

"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:c9onla$37v$1@hood.uits.indiana.edu...
> In article <Er-dnaEXlrEgByLdRVn-hA@comcast.com>,
> Robert Morein <nowhere@nowhere.com> wrote:
> >I want to implement a degree of protection from nuclear-induced EMP for
my
> >home electronics, principally my computer network.
> >
> >Although severe EMP effects require a Faraday shield, I'm aiming for a
> >modest amount of protection, ie., all I can do at modest cost, that might
> >improve survivability near the fringe of the effect area. So I'm
switching
> >to optical fiber for the network connections, and putting isolation
> >transformers in front of the UPS'es.
> >
> >I can't find details for the kind of powerline surges that are caused by
> >nuclear induced EMP. Is it a ringwave, or a DC pulse? How strong, how
long?
> >Common and/or normal mode?
> >
> >Comments/info appreciated.
>
>
>
> As I understand it, the EMP comes from the gamma flash stripping electrons
> from atmospheric molecules, and they can bounce around for many minutes
> before settling down.
>
> Hardening equipment against nuclear EMPs is sort of a specialized topic, I
> don't know much about it. But you should really design the equipment from
> the beginning with that in mind. And you can forget about just slapping a
> Faraday cage on to it. "Cage" won't work. Aluminum foil won't work.
> Think more in terms of alternating layers of aluminum and a magnetically
> permeable material like steel, mu-metal, or one of the MetGlas alloys.

I find no evidence in the literature that anything other than a Faraday cage
is required for complete protection, although it is possible that the
environment close to a nuclear blast might require what you describe.
You are probably reasoning that powerful magnetic fields could induce
potentials INSIDE the Faraday cage. However, although a Faraday cage is
permeable to constant or slowly changing magnetic fields, it rapidly becomes
impermeable as the frequency goes up, due to the skin effect. And the
induced EMF is proportional to the rate of change of the flux.

Check out http://www.tfd.chalmers.se/~valeri/EMP.html
One author, with uncertain credentials, claims that antennas less than 30"
in length, are "safe". The context is not given.

But in this case, I don't have a Faraday cage. I'm not trying to harden
against nearby nuclear explosions, because, as others have remarked, there
would be other things to worry about. Nevertheless, one can approach a
Faraday cage in little steps. By isolating a local system from long
antenna-like structures, such as power lines and unshielded network cables,
it is possible to reduce the potential difference that can be induced
between parts of the system. All this has been proven in terms of protection
from lightning strikes. Two computers in different parts of a structure but
connected by a network cable can have lightning damage over the data cable,
because of ground currents, some of them caused by transzorb-based surge
protectors.

In particular, this means that CAT-5 cable should be replaced by optical.
Any cable that can serve as an antenna should be shortened to the minimum or
bundled correctly. And I have isolated the cable modem with a 3' fiber optic
link.

Power protection is more vexing. Transzorb based protectors limit the spike,
but tend to "light up" the ground. Current-dumping protectors provide better
control of lightning transients, but in the presence of a prolonged surge
simply follow the waveform up.

Isolation transformers provide the best common mode suppression, but
typically incorporate rudimentary or no normal mode suppression. I have
added two isolation transformers in front of my UPS'es, which contain
tranzorbs. If I knew something about the kind of waveform induced by EMP,
I'd have a better idea how to optimize power protection.

If futile against EMP, my efforts are hardening my home network against the
occasional "superbolt", which is a particular kind of strike that has about
100X the energy of a typical lightning strike.

> It'll cost you. You'll have to check the literature for ideal thicknesses
> and spacings. Holes are bad, but if you need them, more small holes are
> better than fewer big holes. Chimneys on your holes would help.
>
> Do something about those antennas bringing the effects into your box, like
> the power cord, and cables to the keyboard and monitor. I'm not sure what
> it would take to filter that, but it may involve Zener diodes, engineered
> spark gaps, and other things of the sort. Peronsally, I would use spark
> gaps whenever possible, just on general principles. Fiber optics are
> good. Just do something about the holes that bring the fibers in and out.
>
> Any wiring layed out is an antenna, receiving flux proportional to the
> enclosed area. Loops are bad. Connections between components should be
> made with coaxial cable, or something of the like. The components should
> have voltage protection right on the pins, and if possible should be rated
> to much higher voltage and power than you'll ever need.
>
> And at this point I've really reached my limits.
>
> --
> "Don't try to teach a pig how to sing. You'll waste your time and annoy
> the pig."

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