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.
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