John and Bud,
It may be counter-intuitive, but it is explanable. Briefly, there are
gradient fields produced whenever/wherever lightning strikes. In the case
where lightning scores a direct hit to the ground (either via a short tree
or post, or right to the grass), fields propagate through the ground at
rather shallow depths, but deep enough to cut across most buried wires.
The energy in the field propagates outward, just like RF from an antenna,
and can induce very large currents in buried conductors. This was
illustrated a few years ago when the home QTH of a co-worker took a direct
hit to the grass in the back yard, conveniently missing his nearby 90 foot
tower. Utilities to his house are buried and run through the back yard.
There was more than $6k damage to electrical/electronic items in the house,
including telephones, because of the induced energy from the gradient
fields.
Of course, there have been plenty of damage incidents caused when lightning
attaches to an overhead phone line at some distance from a residence and
the energy carries down the wires, even though the actual lines to the
residence may be buried. In my travels around the country, I have noticed
that these days, most telephone lines appear to be buried, even in rural
areas. That helps a lot, but still leaves them subject to gradient field
energy unless they are buried deeply or in metal conduits. (The usual
burial medium these days seems to be plastic tubing or pipe, not metal).
Hope this explanation helps.
73, Dale
WA9ENA
W0UN -- John
Brosnahan
<shr@swtexas.net> To
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rfi-bounces@conte cc
sting.com
Subject
Re: [RFI] DSL surge protection
02/24/2005 11:31
PM
At 06:04 PM 2/24/2005, W2RU - Bud Hippisley wrote:
>At 22:48 2005-02-24, W0UN -- John Brosnahan wrote:
> >What damage I have had here at the new QTH has been
> >from spikes coming in the phone lines and NOT the power
> >line. --W0UN
>
>Ditto. Four years ago I lost a laptop and most innards of three desktops,
>a few multi-line phones, etc., from a nearby lightning surge that came in
>via the phone line. I didn't lose any ham radio gear only because the PC
>I use for logging and rig control had no direct or indirect connection to
>the telephone line.
>
>Bud, W2RU
I should have added that the power lines are above ground and the
phone lines are buried. So it is counter-intuitive.
--John W0UN
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