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Re: [TowerTalk] my long lightning story (was RE: lightning strike)

Subject: Re: [TowerTalk] my long lightning story (was RE: lightning strike)
From: "Roger (K8RI)" <>
Date: Fri, 30 May 2008 04:47:32 -0400
List-post: <">>
Dick Green WC1M wrote:
> For 10 years I thought I could protect my station from anything but a
> superstrike, but that was before I put up a 110' tower, and before the
> summer thunderstorms around here became more violent (Climate change? Who
> knows?)
Something to remember about lightning, conductive paths, induced 
voltages, and interconnected equipment.

First, it sounds to me like you may have had one of those "super strikes".
Second, you don't necessarily need to have  continuous electrical paths 
to trace out problems.

Although most damage is normally caused by spikes coming in on either 
the electrical mains or telephone lines, it's not necessary for any 
outside connection when a powerful nearby strike is involved.

According to the NWS literature a strike a mile away can induce voltages 
as high as a 1000 volts per meter in a piece of wire. As the inverse 
square law still holds, imagine what a strike a 100 or 200 yards out can 
do.  That means in these cases even relatively short interconnecting 
wires between equipment may be long enough for damaging  and sometimes 
lethal voltages to be induced.
Even with every thing floating a spike with a steep rise time can create 
substantial differences in voltage between the ends of relatively short 

I believe in keeping everything grounded but I've seen some strange 
things happen.  I use a SPG and the grounds to the equipment are pretty 
close to equal length.  All the coax cables coming in go through a 
grounded bulkhead. Two of those are for  my 144 and 440 arrays which are 
at the top of the system at 130 feet and on a single cross boom (they 
are vertically polarized).  Those lines went to a Alpha Delta two coax 
switch with a center position that grounds all.  I had one of the cables 
disconnected and laying on the desk about 10" to a foot from the 
switch.  There was a nearby strike, although not a powerful one.  There 
was a brilliant flash and a sound like a shotgun blast as an arc flashed 
between the outside of the disconnected PL259 and the coax switch.  Now 
both lines AND the ground cable to the switch are about 6 feet long, 
give or take and they all go back to that grounded bulkhead. I had 
three, six foot cables tied to a ground and two of those had apparently 
close to a 100 KV between them.  That could only be due to the rise 
time.  None of the cables or fittings were damaged and the antennas 
worked fine when I checked (after the storm had passed)

So even with all the equipment disconnected from the mains and antennas 
there is a strong likely hood that a powerful near by strike could 
induce enough voltage in the interconnecting cables to still do a good 
deal of damage.  I mentioned my one neighbor having a lot of damage. One 
piece was a stereo that was disconnected.  Apparently the speaker wires 
were long enough to pick up a substantial enough charge to destroy the 

Although it's possible for substantial damage with everything 
disconnected under some conditions, it's still a good idea to do so as 
the majority of the time it will make a difference as will a  properly 
grounded system. Each step  improves your chances, but they never reach 
zero and particularly when one of those super strikes is involved.  BTW, 
although still rare,  *apparently* the super strikes are more common in 
the NE compared to the rest of the US.  They are also more likely to 
occur in cooler weather as compared to the hot humid weather of the 
central part of the country in Tornado Alley.

Roger (K8RI - ARRL Life Member)
N833R (World's oldest Debonair)

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