[TowerTalk] Grounding base slab and pier tower bases

[Roger D Johnson]

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


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