On 8/8/15 3:15 PM, Kim Elmore wrote:
I'm a researcher prologue that the National Severe Storms Lab in Norman, OK.
There's a fair bit of lightning research done here, mainly on the physics of
lightning. I know a few of the guys that do the research though I'm not
involved in any of it. That said, I'm not sure where hundreds of thousands of
amps number comes from. The highest-current strikes are single-stroke positive
flashes. They tend to peak around 50 kA or so. I'm not sure what the
distribution of peak current looms like, but 100 kA is rare. According to
Martin Uman's book, only about 6% of all flashes produce currents larger than
60 kA; the median peak current is 15 kA and the highest current recoded was 218
kA. While these numbers are admittedly old, they are certainly representative.
Yes, the vast majority of strokes are in the <30kA, however, there are
larger (very rarely). I was using Uman's data..
I used 100kA as a sort of "outer bound" to show that even with some
gigantic stroke, the temperature rise in a Ufer is trivially small. In
the past, when I've used "average" stroke currents, someone raises the
"but what about the really big ones?" question.
(esp since power dissipation goes as the square of current)
Kim N5OP
"People that make music together cannot be enemies, at least as long as the music
lasts." -- Paul Hindemith
On Aug 8, 2015, at 16:57, Jim Lux <jimlux@earthlink.net> wrote:
On 8/8/15 2:33 PM, N3AE wrote:
Concerning lightning damage to a concrete foundation .... ran across the
example in the link below. In this case a concrete guy anchor was damaged. But
looking at the pictures, this might have been caused by moisture entry due to
surface cracking with the lightning strike just finishing it off as the current
pulse turned the moisture into steam. Hard to say.
and this quote covers why "commercial" installations (where labor is a big cost) are different than
"ham" installations (where labor is often free); "the conductors are really a small portion
of the overall cost of the job"...
"Deep-driven rods in combination with heavy copper cabling have proven to be an effective, reliable
and durable solution. And don’t skimp on the copper. Use large conductors, not ‘Code
Minimum’ sizes. The conductors are really a small portion of the overall cost of the job.
“Install ring grounds and radials if there’s room, and bond all connections properly. In a major
lightning strike, you’re potentially dealing with tens of thousands to hundreds of thousands of amps.
Copper is cheap insurance compared with the equipment damage that could occur if the system is inadequate. In the
case of the KPTH-KMEG tower, another strong hit might have led to very expensive repairs … or worse.”
I think the real question is if a lightning strike can cause internal damage to
a tower concrete foundation that you can not easily detect.
N3AE
http://www.copper.org/applications/electrical/pq/casestudy/a6137/a6137.html
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