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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