[TowerTalk] Lightning Advice/Ground Potentials

[Bill Turner]

ORIGINAL MESSAGE:

On Tue, 11 Sep 2007 20:32:08 +0000, kb9cry at comcast.net (Phil Camera)
wrote:

>OK, so now lets bond everything together, outdoors.  Now when we ge the induced energy, everybody (grounds) all rise and fall in that voltage together and there is no voltage difference inside of your equipment and voila, no current and no zapped electronics.
>
>Quite simple if you think about it.

------------ REPLY FOLLOWS ------------

Simple, but wrong. 

There is no such thing as "no voltage difference" when you are talking
a million amps or so. 

Let's say your tower takes a direct hit. Lightning wants to go to
ground. It doesn't want to go to your house, but it will if there is a
voltage difference and a path. You may think the base of your tower
has a zero impedance path to ground, but at a million amps, I
guarantee the impedance is small, but not zero. Even a perfectly
straight tower leg or piece of ground wire has some inductance and
resistance. Because your coax and rotator lines come off the tower at
some point above earth ground, even if only a few inches, they are
going to see a potential difference, and it can be quite significant
depending on your particular configuration. This potential difference
is going to seek its own path to ground, and that path is through the
coax/rotor wires running off to your house. At the point where those
wires enter the house, you need to provide a second path to ground
lest the voltage pulse get into your house and equipment and find its
own path to ground. 

And this is why the concept of a "Single Point Ground" makes no sense
to me. In the case above, there must be at least two grounds or there
will be trouble.

If there is an error in my example, please point it out.

Bill W6WRT