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[TowerTalk] Lightning Protection Components

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Subject: [TowerTalk] Lightning Protection Components
From: (Chris R. Burger)
Date: Thu, 10 Apr 1997 15:19:57 +200
YO3CTK wrote:

> We were surrounded by dozen of apartment buildings,
> 10-story high, with plenty of antennas on the roofs. Why the lightning
> has stricken this particular tree, when all surrounding buildings were
> made of reinforced concrete and (presumably) well grounded, is a mystery
> for me. Maybe someone will come up with a theory.

In South Africa, thatched roofs are relatively common.  They look 
good, and they offer excellent insulation against the weather.  In 
some areas, they are also cheap, as you can harvest the building 
materials where you build the house!

We also have a very high lightning ground strike density.  The 
highest strike density in the world occurs in Swaziland, less than 
four hours from here by road.

It goes without saying that virtually every thatch roof has a 
lightning arrestor (or more) associated with it.  Most of these are 
simply poles, made from galvanised steel, with sharp tips.  They are 
typically over 20 m tall.

The recommendation is that all flammable components must be contained 
within a 45 degree angle of the tip of the rod.  The term used in the 
lightning protection industry is a "cone of protection", with an apex 
angle of 90 degrees.  The angle is probably determined primarily from 
anecdotal information, but insurance companies do accept it as 
adequate protection when they have to underwrite your mansion.

Us humans tend to exaggerate vertical dimensions, probably as a self-
preservation measure.  30 m is a lot when you're looking down from the 
top of your tower, but not a lot if it represents a horizontal 
distance.  I would not want to be 30 m from a charging bull elephant, 
for example!  If you consider that a 10 storey building is typically 
only 30 m tall, you can understand that a 10 m tall tree at a 20 m 
from the base is at risk.

You didn't mention how tall the tree was, or how far it was from the 
buildings, but this effect will probably help to explain what 

When I was at university, lightning destroyed a 50 year old tree 
around 5 m from my res window.  I was studying at the time, and my 
books were covered in sawdust.  The tree had simply disintegrated for 
something like 300 mm above ground level, and the upper part of the 
tree toppled.  There was no visible damage to the upper part of the 

As an aside, I grew up with the admonition that I had to stay away 
from trees during thunderstorms.  In fact, if you're out on the 
grasslands, the safest thing you can do is to lie down until the 
storm has subsided.  You just don't want to be the highest point in 
the area!

Chris R. Burger

PS  The tree didn't collapse within 70 mm of the base, but then it 
wasn't guyed!

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