In a recent new building I put a Ufer ground in the perimeter footing
which yielded about 1000 sq ft of concrete in contact with ground.
Since the slab is over a vapor barrier and insulation, there is no good
electrical contact. My towers have Ufer's also, about 250 sq ft of
concrete in soil contact. You can do the arithmetic about how many
ground rods and buried connection wires that will take to equal the soil
contact area. My to do project list has on it measurement of the
conductivity among all three.
There has been thread discussion in the past about how concrete reacts
to strike currents. My logic is that if enough rebar is well connected
to the Ufer ground point then the energy is spread through a lot of
volume, current density is less and the concrete is not damaged. A bad
situation might be rebar spaced small distances so the current density
has hot spots which can boil the water in the concrete with potentially
damaging consequences. However, most any rebar design for structural
purposes has plenty of steel tied together to distribute the current. A
single ground rod in concrete in comparison might be a bad idea.
I think current UBC recommends or requires Ufers. My inspector had the
electrician add two rods at the service entrance, but I think they are
useless in comparison. I'll measure the conductivity between them and
One other data point is my new primary transformer by Puget Sound Energy
has a Ufer ground point on the buried concrete vault and no ground
rods. I'd guess there are about 8 sq ft of concrete in contact with the
soil. I think you will see Ufers on most power line and cell tower bases.
On the web, one company in the midwest advertises drilled ground field
systems, e.g. 30 to 60' depth as the solution to manage very frequent
strikes on very tall BC towers. A pipe cased well is one alternative if
you have one.
One other observation from the top of a 3500' ME mountain from a hike
there. It is all bare granite at the top. Many wires were just strung
on the surface to get the area increased. I didn't stick around to
observe them in action.
On 11/14/2012 6:53 AM, K4SAV wrote:
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 everything.
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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