From: swanee@megadata.mega.oz.au (Geoff Swan)
Subject: Re: Thunderstorms   (was: Re: Night and Winter)
Organization: Megadata P/L, North Ryde, Sydney, Aust.
Date: Wed, 7 Oct 1992 23:40:05 GMT

> In article <2AD0E0AD.21246@news.service.uci.edu> cortese@skid.ps.uci.edu (Janis Maria Cortese) writes:
> >In article <BvM9nK.4M6@ddsw1.mcs.com> karl@ddsw1.mcs.com (Karl Denninger) writes:
> >>Lightning is one of those things you really can't protect well against.
> >Yes, you can.  If an electrical storm comes up, GET OFF YOUR COMPUTER
> >AND UNPLUG THE DAMNED THING.
> >Just unplug everything you can.  Nothing is better protection than that.
> 
Indeed it is possible to protect against lightning near strikes and line
strikes (direct strikes are another problem altogether). For example, I
design Remote Telemetry Units here at work which operate in all sorts of
places, from electricity switchyards and substations, water pumping stations
and pipeline monitoring across the desert. These places get quite a lot
of lightning activity (as you can imagine) as well as huge induced voltages
during a substation fault. No one rushes out to all the sites to unplug
the computer control equipment when a thunderstorm is near!! :>
Surge protection is fairly simple if you know what you are doing. Generally
combinations of varistors and gas discharge devices suffice, but you must
be careful to use the correctly rated devices. Varistors are like a pair
of zeners and will clamp the voltage transient (transorbs are similar but
faster devices that do the same thing). They will clamp a certain amount
of energy before self-destructing due to overheating. Gas discharge devices
are like neons in effect, except better tuned for surge protection. When
a gas discharge device fires, it presents a short circuit across the line
until the following voltage zero (great for AC, but don't shove one of
these across a DC line). Another thing to consider are distances (and
inductance) between the protection circuit and the circuit being protected.

At home I install a pair of 275LA20A (GEC number) varistors (we have a
240VAC line), one from active to earth and one from neutral to earth in
a plug or piggy-back plug and just plug it into an outlet in the house
near the outlet you are trying to protect. A few of these spaced around
the house is generally a good idea. They protect against spikes from
motors (washing machines etc) as well. For line strikes nearby (like just
outside the house) a 3 terminal gas discharge device (A-E-N) at the first
outlet from the meter box should do the trick. Remember the impedance of
the incoming line does help lose a fair amount of the energy from a strike
before it reaches the house, as the lightning pulse duration is normally
fairly short. The protection devices simply snub out the remaining energy.

    Sla/n go fo/ill,
                    Risky B.
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Geoff Swan (research & development)   _--_|\         swanee@megadata.oz.au
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