Keith's comment below reminded me of a question I had about horizontal
ground rods on lightning protection systems. Current wisdom seems to
suggest that if it is impossible to drive ground rods vertically, that
laying them out horizontally underground is the next best thing. This
sounds reasonable, but I am wondering what the difference is between a
horizontal ground rod and the thick bare conductor leading up to it. Both
are thick copper conductors running horizontally. Another way to ask the
question is, if I substituted a 10 foot piece of #8 wire for a 10 foot
horizontal ground rod, what would be the difference? If there is no
difference, then just bury thick wire radials and be done with it. It would
save on all the worry about joints except where the wire is bonded to
whatever is being protected.
I do see the difference when it comes to vertical ground rods, since it's a
better way to deliver copper down into the packed earth. But that
horizontal case puzzles me.
Dudley - WA1X
----- Original Message -----
From: "Keith Dutson" <kjdutson@earthlink.net>
To: <towertalk@contesting.com>
Sent: Monday, January 17, 2005 9:32 AM
Subject: RE: [TowerTalk] There's 'ground', and then there's 'ground'
>
> I read Polyphaser's technical note before installing my system. There
> is
a
> lot of discussion about soil conductivity and how adding more rods will
> improve conductivity. Do you have an idea of HOW MUCH impedance would be
> reduced?
>
> Keith NM5G
There's a heck of a lot of details that can go into this, but, it's easy to
establish the BEST that you can do, in a relative sort of way.
If the rods are "far" apart (basically on the order of 2 rod lengths), then
they're essentially independent, so the resistance of N rods in parallel is
the resistance of one rod divided by N. (assuming same soil conditions,
etc.)
If the rods are closer together, you get less improvement, culminating in
the situation where the rods are right next to each other, when the
resistance isn't much lower than a single rod.
The other factor to consider is the inductance of the wire(s) going to the
rod(s). 1 microhenry per meter is a nice rule of thumb number to use for an
isolated conductor... it might be twice or three times that, or half or a
third, but it probably won't be 10 uH/meter or 0.1 uH/meter..
You can also bury rods horizontally... To a first order, it's surface area
in contact with the soil that's the important thing.
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