My understanding of ground rod performance characteristics during a
strike leaves a lot to be desired, and I can't find any information to
answer those questions either. We have rules that specify distance
between rods because of' ground saturation and the need to spread the
charge over a larger area. I don't understand exactly what happens with
the underground plasma that takes place around a rod during a strike,
and what that does to the ground rod impedance, and how that affects
ground saturation. I would guess that the impedance of that ground rod
during a strike is a huge non-linear function, not even close to what
you might measure with any instruments under normal conditions.
Besides, if I had that information I could do an accurate model of a
ground system instead of having to ballpark and conservatively estimate
Then if you encase the ground rod in concrete, how does that effect the
underground plasma and the rod impedance during a strike. Also what
happens to the concrete. I would guess that it might explode if there
were insufficient ground rods in the system. I wonder how many would be
sufficient. If the impedance of the ground rod is much lower when
encased in concrete, why don't the commercial cell tower companies use
concrete around the rods? I wonder if they have tried it. Would
concrete be better than packing the hole with bentonite? I know there
is some information on Ufer grounds but those are just guidelines and
really don't answer the details of how things work.
Lots of questions and nowhere to go for answers.
On 11/14/2012 7:00 AM, Jim Lux wrote:
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 it's hygroscopic.
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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