[TowerTalk] Re: Static, Lightning, and protection

[Didier Juges]

At 09:50 AM 3/22/2004, David Robbins K1TTT wrote:
> >
> > Good point. At higher altitude, the dielectric strength of air is
> > less, so the voltage cannot build up as much before the strike
> > starts, so the clouds cannot build as much energy as they
> > can in Florida.
>
>statistically the change is relatively small.  In a typical area of
>central Florida the median stroke current is about 25kA, where in
>central Colorado it is about 20kA.  In the highest current range
>(200-300kA) there are actually more strokes in Co .03% vs .02% in Fl.

Maybe the generally dryer air in CO offsets the altitude's effect on 
dielectric strength. I agree that the change is fairly small. I design HV 
TWT supplies for, among other things, military planes (radar and 
countermeasures), and some of what I do is qualified up to 90,000 feet 
(most go to 65,000). I also do some commercial equipment. We typically 
consider 3,000 feet elevation to be ground level. Most commercial equipment 
is certified to 10,000 feet and does not require special precautions, 
considering everything we do is potted in silicone compound. Air insulated 
equipment is another story.

A note on observing single storms.  I have also seen the phenomena
>mentioned where lightning seems to stop before it reaches my station and
>then starts again on the other side.

I agree. There are many documented examples of strikes in a valley between 
two ridges. Assuming the storm is a static thing that moves across terrain 
and only hits the highest point is a mistake.

>A good analysis of strokes to
>an area as small as a ham station would take many years of data,
>probably much more than is available in the current u.s. dataset.  Even
>for detailed studies of power lines the minimum size data block we
>normally use is 10 minutes on a side (about 10mi) and we average the
>data over a 10 year period.

There you go again, trying to confuse us with data :-)


>David Robbins K1TTT
>e-mail: mailto:k1ttt at arrl.net
>web: http://www.k1ttt.net
>AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

73,
Didier KO4BB