[TowerTalk] Porcupines and other wives tales

[Keith Dutson]

>The measured energy of the strike is not the point here. The point is to
try and prevent the charge build up to where the strike occurs.

Well, if you are going to prevent a strike, you basically have to bleed off
all of the strike energy.  I don't think that is possible.

A cloud that is charged by the conventional rain drop method does not have
the potential to form a strike.  This charge has been measured just prior to
a strike and found to be far short of the potential required to draw the
arc.  The latest theory is that random cosmic rays strike the cloud causing
a portion to be supercharged.  The question is now how fast does this
happen?  I certainly do not know the answer, but if it is a matter of
seconds, there is nothing that can bleed off the charge and stop the strike.

73, Keith NM5G

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Jarvis
Sent: Tuesday, July 04, 2006 1:59 PM
To: towertalk at contesting.com
Subject: [TowerTalk] Porcupines and other wives tales


I had tuned out on the water tower-omni antenna lightning protection, and
noticed the "porcupine" post.  Things morphed somewhat, I see.

To avoid forensic reading and proper attribution, let me say that some folks
have said some right things....and a few may underestimate the problem or
over-estimate the potential for remediation.

consider this exchange:

Just look up measured energy
> of
> a strike and compare it to the ability of the porcupine to dissipate 
> this

The measured energy of the stike is not the point here. The point is to try
and prevent the charge build up to where the strike ocurrs.

-0-
The point (no pun intended) is to bleed off the charge faster than the
ground charge builds, whether it be moving into the area, building up, or
both.  If that charge is building on the object faster than it can be bled
off and a feeder is produced that connects then the object will be hit.
When that strike ocurrs the current comes from an area considerably larger
than the tower and contains far more energy than would have been available
from the tower.

-0-

It's all well and good to try and bleed off charge via lightning rods...even
'fattened' ones like porcupines.  But those rods have a fixed ability to
handle the discharge of an approaching charged cloud.  Lifetime of a
porcupine with a direct strike is measured in nS.
[snip]