[TowerTalk] tower base grounding

[Gary Schafer]

By not bonding the rebar you run the risk of an arc inside the foundation
with a lightning strike. Part of the strike will go through the bolts that
the tower is bolted to in the concrete. Even if you have a good external
ground.

73
Gary  K4FMX

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of Jim Jarvis
> Sent: Sunday, January 29, 2006 12:48 PM
> To: wc1m at msn.com; towertalk at contesting.com
> Subject: [TowerTalk] tower base grounding
> 
> 
> Dick, (TT):
> 
> You raise several questions:
> 
> "The (inspector's) note said all rebar must be bonded to the ground grid,
> and inspection
> will be required prior to backfill. "
> 
> I think an argument can be made for NOT bonding the rebar.  Rather, a
> robust
> copper-strap
> system should run OVER and OUTSIDE the foundation, and connect to your
> planned lightning
> dispersal ground system.  My recommendation would be to meet with the
> inspector or planning
> officer and discuss the technical merits of each approach...find out why
> they want to route
> a strike through the foundation, rather than around it...and then go with
> what is required.
> 
> "A bigger concern is that I was going to try to dig the 2.5' x 2.5' x 4'
> hole
> by hand in order to get a nice square hole exactly to the required
> dimensions and pour the concrete against undisturbed soil. "
> 
> Depending on your soil, and flood factors, this is a major consideration.
> The ability of the foundation to withstand overturning moment is highly
> dependent
> upon the soil density.  Over-excavation and backfilling will diminish that
> substantially, unless you layer the backfill and use a power-tamper as you
> go.
> I'd vote for hand digging, so long as it's YOU and not I who are digging.
> heh.
> On the other hand, a guyed tower's foundation basically supports vertical
> compression
> loads, not overturning moments...particularly true where you have a pin
> base.
> Personally, I'd excavate and tamp the refill, just to minimize the
> cosmetic
> sinking
> over the next couple of years.
> 
> " Do building codes address non-AC cable runs? Has anyone
> out there been required to install conduit just for coax/hardline and
> low-voltage control cables?"
> 
> As I understand it..and I am NOT a student of the NEC, the code DOES
> address
> low voltage wiring.  My recommendation would be to indicate to the code
> officer
> that you're NOT going to use conduit,  that the nature of amateur
> experimentation
> requires flexibility with multiple low-voltage cables, and NO AC
> POWER...and
> that
> you're using direct-buriable hardline for RF feed, as is commonly done in
> broadcast
> installations, and the local cable company.  If you decide to use conduit,
> it will
> only be for cosmetic reasons, to keep the cables out of sight.  The 4'
> bury
> depth
> is to avoid personnel safety for folks like cable and telco, and
> irrigation
> systems,
> who use vibratory plows to pull stuff in 18" below the surface.
> 
> Someone with knowledge of code specifics may want to address this point,
> but I think it's finessable.
> 
> 
> n2ea
> jimjarvis at ieee.org
> jimjarvis at verizon.net
> 
> 
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