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/
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|