[TowerTalk] Water tower omni

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Responses below:

bthorson at 4smps.com wrote:

>Can you tell me more about the 'Rolling Ball' concept and lightning? 
>I've never heard of it.
>  
>
Read Uman and Rakov for info re the rolling ball concept of protected 
volume.

>Someone, I think it was in the forum, said that lightning protection is 
>like an insurance policy. The more you spend, the better your 
>protection, but you couldn't guarantee that you weren't going to get 
>taken out. Of course, there's the people who say that's silly, that 
>professional broadcasters have to stay on all the time, through 
>lightning hits etc. Of course their equipment is a bit heavier and a lot 
>more expensive the typical equipment I install.
>  
>
You are correct; whether or not a specific installation will be damaged 
is a probability issue. The nearest you can get to 100% protection is to 
be inside a metal sphere. Rakov and Uman present probabilities 
associated with several sizes of both rolling ball and cones of protection.

>The side tank mounting has been, at least so far, a relatively safe 
>method. I think we've had one side tank mounted antenna taken out at 
>most. (Actually the antenna wasn't taken out, but the coax as it was an 
>old installation that wasn't properly grounded, if I remember correctly.)
>
>  
>
Lightning isn't generally destructive of modest sized metal conductors. 
Uman cites a study in Poland in the 1950s in which #12 iron wire reduced 
lightning induced fires 90%. #12 copper wire will survive most direct 
hits. A Hi-Gain vertical here survived a direct hit with no visible 
damage, though it was a while before I could hear again and I was 
surprised no windows in the house broke. Broadcast towers and the 
equipment on them take lots of hits.

The peak current in a lightning stroke used to be cited as 18,000 Amps 
or less for half of lightning events. Uman and Rakov state that current 
observations put it at 30,000 Amp in the first stroke; subsequent 
strokes are usually half or less of that. The upper limits is 150,000 
Amp. Fortunately, as Jim noted, the duration of a stroke is measured in 
microseconds. However, the strikes that often start fires exhibit 
continuing current up to 200 Amp between strokes.

Uman and Rakov cite studies showing the futility of depending upon 
"porcupines." The molten metal spraying from them adds to the potential 
for igniting flammable material.

The following is a summary of lightning’s properties, derived from 
information in the book by Professors Rakov and Uman:

Cloud to ground voltage is estimated to range from 50 to 500 MegaVolts.

A lightning flash, which is the name applied to a complete lightning 
strike or event, includes a stepped leader, one or more return strokes 
that are separated by dart leaders, and sometimes includes continuing 
current between strokes.

The voltage at the tip of the stepped leader is tens of MegaVolts.

The current in the stepped leader, which connects from cloud to ground 
before the first stroke, may be 1,000 Amperes.

The peak current in the first stroke is typically 30,000 Amperes, 
revised upward from the 18,000 Amperes figure that was previously 
published. Current in second and later strokes is typically half or less 
than half of that in the first stroke.

Current in a stroke may reach 150,000 Amperes.

A stroke lasts for tens of microseconds. Time between strokes is 
typically tens of milliseconds.

A lightning flash typically includes 3 to 6 strokes. The observed range 
is from 1 to 26 strokes.

Some flashes include continuing current between strokes. Continuing 
current is typically 100 to 200 Amp but may range from 10 Amps to 
hundreds of Amps.

The total time of a flash is typically 200 to 400 milliseconds, ranging 
from 10s of milliseconds to 2 seconds.

Total charge lowered to earth by a flash is typically 25 to 30 Coulombs, 
but ranges across tens of Coulombs. A Coulomb is the charge moved by a 
current of one Amp in one second.

25 Coulombs in 200 milliseconds is an average current of 125 Amp. A 
modest size metal conductor can conduct that current for that fraction 
of a second. A study in Poland in the 1950s showed that #12 iron wire 
strung over thatched roof cottages and grounded at each end reduced 
lightning induced fires 90%. #12 copper wire will survive many times 
better than the iron wire.

Over 50% of flashes terminate at multiple points on earth, thus 
spreading the current and energy among several targets.

It is an informative book, written by two professors who have engaged in 
the study of lightning for a few decades. They don't claim to be the 
originators of the information. They cite references from researchers 
all over the world.

73 de WOØW