20' isn't all that much rebar. There's probably that much in a 3'x3'x4'
block for rohn 25/45 type base when its done right. And I would be
surprised if anyone could put such a limit on 'absorption' by a ufer ground
like that, sounds like something pulled out of thin air to me. The truth
is, concrete is a pretty darn good conductor in its own right, especially in
ground where it stays relatively moist. The whole reason for concrete
encased ground rods is to improve the conductivity and spread the current
out more so that the current density is lower when it gets into the
surrounding soil that is a worse conductor.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Roger D Johnson
> Sent: Tuesday, July 04, 2006 17:43
> To: Jim Lux
> Cc: towertalk@contesting.com
> Subject: Re: [TowerTalk] Grounding base slab and pier tower bases
>
> Jim Lux wrote:
> > At 08:47 AM 7/3/2006, Roger D Johnson wrote:
> >> Jim Lux wrote:
> >>> At 07:44 PM 7/2/2006, JC Smith wrote:
> >>> <various stuff about Ufer grounds snipped>
> >>>
> >>>
> >>> Jim Lux, W6RMK
> >> If my memory has not deteriorated too much, my understanding of a
> >> Ufer ground is that it approximates a capacitance in parallel with
> >> a resistance. The theory was that the capacitance would absorb the
> >> energy of the lightning strike which would then bleed off through
> >> the resistance of the concrete.
> >
> > Or, just a big lump of semiconductive material: concrete, which looks
> like
> > a lossy RC to a fast impulse.
> >
> >
> >> The Ufer ground seems to have morphed into a substitute for a ground
> >> rod in power distribution systems using relatively short conductors
> >> in the concrete. While this might have a low enough impedance for a
> >> safety ground, I don't think it would provide enough area (capacitance)
> >> for effective lightning protection.
> >
> > Depends... The resistance from a 20 ft long wire to the concrete isn't
> all
> > that high. You could calculate how much heat is going to be dissipated
> > from a notional 20 kA stroke. Say the resistance, overall, is on the
> order
> > of 5-10 ohms. 20kA into 10 ohms is about 4000 MW, but lasting only
> about
> > 2-20 microseconds, so 8-80 kJ or so, which isn't a huge amount of heat
> to
> > inject into 20 ft of wire and concrete. The capacitance of the block of
> > concrete to the surrounding soil will be fairly large and the resistance
> low.
> >
> > The NEC is definitely more focussed on dissipating transient energy
> (from
> > near strikes, etc.) and fault currents, not on dissipating a direct
> stroke,
> > so the 20ft encased in concrete thing might be insufficient for
> lightning
> > protection.
>
> I did find a site which said that 20 ft of rebar in concrete is capable
> of absorbing an 8 kiloamp surge. Since the average lightning strike is
> around 15 Kamps, it's obvious that these mini-Ufers don't offer much
> protection from lightning strikes. I've noticed that most of the sites
> that promote a Ufer ground made up of the tower base and rebar also
> advocate the use of a radial system. My feeling is that the radial
> system actually provides the lions share of the protection.
>
> 73, Roger
>
>
> --
> Remember the USS Liberty (AGTR-5)
> http://ussliberty.org/
> _______________________________________________
>
>
>
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