now you are in my area of kind of expertise... yes, the ground response to
high current lightning is very non-linear.
classically it has been represented by an equation put forth by Weck, search
for 'weck ground resistance' and you'll
get lots of references, one quickie:
http://www.ipst.org/techpapers/2011/papers/39.pdf (see page 2, equations 6 and
where i work we have done full scale ground tests and improved on those basic
models a bit for the software products
i work on. and yes, measuring it is hard, it is fun to watch streamers go
across wet grass in the morning when you really
want it to go down the rod. the basic result is that resistance goes down
quickly as the current goes up, as a quick example
take a 2 rod ground system measuring 25 ohms with standard instruments, it will
drop to 15 ohms or less with 20kA going
encasing the rod in concrete increases the effective diameter and thus the
starting resistance... but it reduces the ionization
so the reduction during the stroke is less... in fact for ufer type grounds we
do not model any non-linearity. but since it starts
at a lower resistance the ionization isn't that important. damage to concrete
encased rods or foundation rebar is rare
because concrete is a good conductor and the rod/rebar has significant surface
area to spread out the charge. encasing
ground rods in concrete or bentonite is used in extreme cases, usually where
towers are on solid rock, but it is expensive
so it is only done in rare cases. there is(was? haven't heard from them in a
while) also a company that was producing a
special conductive concrete that had some kind of additive in it that made it
even better for grounding, and so conductive
they were trying to market it as a resistive heating material for runways or
there are lots of answers out there, but also lots of myths... and many people
end up over doing the grounding. my
recommendation for tower grounding (unless you live in central florida) is to
do the minimum required by local code and
call it done... spend more time on the single point entrance ground.
Nov 14, 2012 09:53:57 AM, RadioIR@charter.net 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
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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