There is a difference between a house foundation and a tower foundation.
The biggest difference being size.
A house foundation may have 3000 square feet of contact with the soil
for a UFER ground. Your tower may have 36 or 180 (if you assume 6x6x6
and count the sides). The amount of energy per square foot going
through the concrete to the soil is a lot different.
I would attach the rebar cage to the tower during the pour for strength
and some energy dissipation, but I would depend on the ground rods
attached to straps above the concrete for lightning. And no contacts
through the concrete to the ground under ground. (How can you check it
in 5-10 years?)
Some energy will go through the concrete to the soil. But you want most
of the energy going to the ground rods outside the concrete base.
I also think 3 ground rods are the minimum necessary. More is better,
especially if your soil is not very conductive. Get that lightning
energy away from the tower and into the ground.
Tying the ground rods together with a ring of wire or strap can only
help. If one of your ground rods does not conduct to the ground as well,
the energy can be spread through the other ones.
Remember we are talking about lots and lots (and lots...) of energy in a
lightning strike. Anything you can do to split it up and reduce the
resistance and inductance to ground is good.
On Wed, 2008-05-28 at 17:34 -0500, Richardson, Ed wrote:
> Thanks for the feedback Kip. How do they do a UFER griound in TX? I do am
> worried about Cu coming out below ground providing a path in for moisture or
> Practically the rebar cage should be grounded as well as the tower legs. Just
> how to do it right?
> -----Original Message-----
> From: Kipton Moravec <firstname.lastname@example.org>
> To: Richardson, Ed
> Sent: Wed May 28 16:33:29 2008
> Subject: Re: [TowerTalk] Tower Ground Question
> On Wed, 2008-05-28 at 13:20 -0500, Richardson, Ed wrote:
> > Just getting set to pour the foundation for my new tower - finally. I am
> > not sure in which manner to proceed with the final grounding step.
> > I have tied my rebar all together securely, ran a #2 cable all around
> > the four sides of the cage just to make sure there is continuity across
> > the corners.
> What kind of cable? I was under the impression that copper and concrete
> don't play well together. In the houses in Texas, when they bring copper
> through the foundation, it is wrapped in duct tape before the concrete
> is poured where it touches concrete.
> > My first and preferred option was going to be to install a #2 solid
> > tinned Cu cable from each tower leg through the concrete to the closest
> > rebar side and continue out of the concrete below ground level to the
> > nearest ground rod. There will be three ground rods spaced about 6 feet
> > from the tower all connected together with 1.5" Cu strap ring.
> > The second option recommended on a couple web sites is to connect the
> > rebar in the concrete out the concrete below ground to the nearest
> > ground rod and run a second cable above ground from the tower leg just
> > above the concrete to the ground rod.
> > Is there a preference to either option? What have others done?
> > Ed, VE4EAR
> You do not want water getting into your rebar and rusting it, so you do
> not want metal coming out of the concrete underground. The concrete
> completely surrounds all rebar.
> You attach straps from each tower leg above ground to the ground rods.
> The ground rods should be spaced as far apart as they are long. For 8'
> ground rods, make sure they are at least 8 feet apart.
> Tying the ground rods together with a wire or strap is a good idea.
> Don't forget to run wires to the house service ground and phone ground
> from the tower. If you get a lightning strike you want everything in
> the house to go up and down together. When you have voltage differences
> that is when equipment gets destroyed.
Kipton Moravec KE5NGX
"Always do right; this will gratify some people and astonish the rest."
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