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Re: [TowerTalk] ground rod depth problem - and understanding ground rods

Subject: Re: [TowerTalk] ground rod depth problem - and understanding ground rods
From: David Robbins <>
Date: Wed, 14 Nov 2012 09:38:32 -0600 (CST)
List-post: <">>
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: (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
into it.

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, 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.

Jerry, K4SAV

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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