[TowerTalk] Fwd: Re: Earthing a tower

[Jim Lux]

On 12/30/12 9:17 AM, Patrick Greenlee wrote:
> I have a friend who is a ham and is EE with 30+ years of antenna design
> experience.  He tells me emphatically, "concrete is not a good
> conductor."

Perhaps it isn't a good conductor from an antenna/loss standpoint, but 
it's a pretty good lightning conductor.

Concrete is a MUCH better conductor than the average soil that the 
concrete is sitting in.  The point of concrete encased grounding 
electrodes (aka Ufer ground) is that the concrete provides a much better 
connection between the metal part carrying the current and the 
surrounding soil. It's in intimate contact with both metal and soil: 
large surface area with soil, so current density is low, conductor cross 
section is huge, so resistance is low.  The concrete is also solidly in 
contact with the rebar or embedded copper wire, and going to stay that 
way, regardless of the moisture content, or vibration, or whatever.



  Given this, I'd be trying to not depend on concrete as part
> of a grounding system.

As it happens, though, concrete is part of the grounding system for an 
enormous number of installations, to the point where it's almost 
required in some states (California). The NEC allows some other 
approaches, but it's pretty clear from the notes and explanations in the 
handbook that the Ufer ground is preferred, and the other approaches are 
for situations where it just isn't possible.

For lightning grounds (more the subject here), ring grounds are also 
popular: a very long buried copper wire around the periphery  also has a 
lot of contact area, and has the same "field leveling" effect as 
concrete (that is, within the ring, potential differences between 
different parts of the soil will be minimized)


  Rebar installed correctly in steel reinforced
> concrete is probably at least 2 inches under the surface of the concrete
> on any side, top, or bottom.  Weld plates installed when the concrete
> was wet (to give you something to anchor to) or bolts or other
> components set into the concrete are typically NOT in contact with the
> rebar cage inside the concrete (which is not a good conductor anyway.)

There have been a number of studies of concrete encased grounds 
comparing rebar vs the 20 ft of bare copper wire approach, and they work 
about the same.  There's even an interesting study where they looked at 
whether the rebar has to be all electrically connected, or whether just 
having it all in the same concrete block (and spread out..not as a big 
ol bundle cast in concrete) is as good. The results were that for the 
most part, you don't need to have the rebar bonded together by welding 
or some such. The rebar makes the "effective bulk" conductivity of the 
concrete low enough that it serves as an effective grounding conductor 
to the concrete/soil interface.