>I have seen those pulses carried by 12ga copper wire with no visible
damage. Put that same 12ga wire across a battery and in milliseconds it is
melted.
That's simple physics. Move a lot of electrons along a conductor and get a
lot of heat caused by orbital transfer from atom to atom. For DC, this heat
continues to build as long as current flows. For a short duration
(microseconds), the heat raises the temperature of the wire just enough to
allow dissipation by radiation (cooling). Change to long duration
(milliseconds) and the accumulated heat raises the temperature to the
melting point. That black stuff that is usually found after the melt is
actually oxide formed when melted metal combines with air. :)
Keith
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of David Robbins K1TTT
Sent: Thursday, July 29, 2004 3:44 PM
To: 'Tower Talk List'
Subject: RE: [TowerTalk] Grounding, Lightning & corona discharge
Lightning is a complex waveform. Typical rise times can be 1-2 microseconds
with tails that decay from 10 to 100's of microseconds. A sine wave with a
1 microsecond rise time (1/4 of the cycle) would be 4MHz, significant
frequency components obviously go well above that but get weaker as you go
up. As a quick example compare the strength of static from a thunderstorm
say 50 miles away... the noise on 80m will be much louder than on 10m and up
into the vhf bands it gets progressively weaker.
However! Lightning can have a significant dc component also. As in the
stroke that w8ji commented on that melted coax shields and other wires and
magnetized CRTs. Large positive strokes can have peak currents of
100-300ka. But even worse, the tail of the stroke can last milliseconds at
levels of hundreds of amps. These are the strokes that burn down power line
shield wires, melt holes in steel, and do all sorts of nasty things. To
point out the difference between the potential power of these strokes
consider this... where I work we have a pulse generator that can put out
30-40ka pulses a few microseconds long, we use it for testing insulators,
soil characteristics, lightning arresters, and other odd things... I have
seen those pulses carried by 12ga copper wire with no visible damage. Put
that same 12ga wire across a battery and in milliseconds it is melted.
<snip>
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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