[TowerTalk] Ufer Grounding

[Grant Saviers]

If you Cadweld to the rods, I think you need to use weldable rebar, 
otherwise the exothermic reaction temperature is going to create a weak 
spot.

IMO, the wire ought to route directly to and tied to perimeter rods, and 
welding probably isn't needed. The difference in conductivity of the 
larger diameter rebar vs #2 copper probably doesn't matter. Multiple 
straight parallel paths from every tower leg to the perimeter rods may 
be a better way to spend the wire, lowering inductance and spreading the 
current more evenly inside the concrete.

Adding rods and wire in a radial pattern would also improve the RF 
properties.  Since you needed 24 ft deep rods in the prior installation 
that would allow one rod per radial, maybe two with the recommended 
separation and usual maximum radial length.

Lay #2 in every trench below every conduit run to the shack.  I have 
more or less "average soil" and could measure 110' of bare wire in the 
trench as helpful as well as it is needed to tie each tower to the entry 
panel ground.

My favorite guide is at 
https://www.timesmicrowave.com/documents/resources/protectbrochure.pdf
I think it is the best written with solid engineering explanations.

Just my thoughts, haven't been "hit".

Grant KZ1W

On 7/14/2016 19:48 PM, Paul Christensen wrote:
> It's looking more probable that the remote Internet site built last year by
> N4CC and myself will require a complete move about a half-mile down the
> road.  Assuming this is the case, I want to pay particular attention this
> time to Ufer grounding of the two tower bases.  These are large Pirod
> self-supporting towers with substantial concrete piers.  The relevant
> portion of Motorola's R56 standard is copied and pasted below.  The soil in
> this area is very sandy and at the current site, we ended up having to drive
> down a total of four 24 ft. rods to get adequately low earthing resistance.
> That was a rough day of work even with a hammer drill.
>
> My thought is to "spiral wind" solid #2 AWG wire, beginning the bottom rebar
> layer.  Cadweld it at the start point, and then periodically Cadweld at
> random points where the wire crosses other rebar sections.  The wire would
> exit at the top of the pier using a small-diameter PVC pipe as shown in one
> of the R56 diagrams.  As the wire nears the top, possibly it could branch
> out into two additional directions for bonding to the tower base and
> perimeter ring.
>
> Any issues with this plan?
>
> Paul, W9AC
>
> Motorola R56 Ufer Guidelines:
>
> . Concrete-encased electrodes shall be encased by at least 51 mm (2 in.) of
> concrete, located within
> and near the bottom of a concrete foundation or footing that is in direct
> contact with the earth.
>
> . Concrete-encased electrodes shall be at least 6.1 m (20 ft.) of bare
> copper conductor not smaller
> than 25 mm2 csa (#4 AWG) or at least 6.1 m (20 ft.) of one or more bare or
> zinc galvanized or other
> conductive coated steel reinforcing bars or rods at least 12.7 mm (0.5 in.)
> in diameter.
>
> . Concrete-encased electrodes shall be bonded to any other grounding
> electrode system at the site.
> See "Common Grounding (Earthing)" on page 4-5.
>
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