On 11/13/12 2:32 AM, Mike Reublin wrote:
On Nov 12, 2012, at 7:58 PM, Jim Lux <firstname.lastname@example.org> wrote:
It's all about contact area... Ufer grounds work well because a 20 foot wire in
a chunk of concrete has a huge contact area (a footing 20 feet long and 1x1
foot is 80 square feet, right?)
Wouldn't that be 20 cubic feet, or one long surface would be 20 square feet?
volume isn't the important metric.. surface area is.. a bar 20 feet long
and 1x1 foot cross section is 82 square feet in cross section.
I guess, though, the top of the footing isn't usually buried, so
probably 60 or so square feet..
Concrete is almost always higher conductivity than the soil surrounding
it (unless you're using some exotic low conductivity concrete) because
So instead of a contact area between conductor (rod) and soil (probably
not even a square foot), you have a fairly good contact that can't be
disturbed between wire and concrete, and then a very large contact area
between soil and concrete, along with the "current spreading" from the
concrete, so the current density at the concrete/soil interface is low.
In fact, for RF and transients, the *capacitive* coupling from the
concrete to the soil is pretty good.
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