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

[Jim Lux]

On 11/14/12 9:12 PM, K4SAV wrote:
> Jim, I think maybe you misunderstood the reason for my questions.  I
> wasn't asking what the rules and regulations say.  I have that data
> already.  I'm not constructing anything and want to know what to do.  I
> have done that already and it has proven to work very well for many
> strikes, with no damage so far.  It's the basis for some of the
> regulations I wanted to know more about.

For NEC and NFPA 780, you can find the committee discussions where they 
propose changes to the code, and in those the theory and experiment to 
justify the changes get mentioned, although for the grounding electrode 
thing, I suspect it's old enough that it's not online.


The IEEE docs I cited have a lot of the theory and data that you're 
looking for.  What's not in there is probably in the references for 
those docs.




  Maybe those regulations are
> based on experimental data, I'm not sure but I suspect they are, however
> I would think by now some have tried to make some sense of them
> analytically.

That's in the IEEE docs.. both the analytical stuff and the experimental 
data.



  Anyone that wants to write some software to analyze some
> of this stuff is going to have to understand the mechanisms.  I was
> looking for that kind of information (not that I'm developing software,
> but I'm very curious).  Grounding regulations don't describe the
> mechanism of ground rod underground plasma, when it exists, and what
> that does to the impedance of a ground rod.   Dave Robbins gave some
> excellent data references to help with that understanding.  Thanks Dave.
>

You're right.. something like the NEC doesn't describe mechanisms and 
theory (although the NEC handbook often has additional explanations 
beyond what's in the straight code).

However, the IEEE docs *do* talk about what you're looking for.



> Then the thought of encasing a ground rod in concrete came up, and I
> don't think I have ever heard of anyone doing that.  Ufer grounds are
> not exactly the same thing.  Rules for Ufer grounding aren't very
> similar.

Oh... you're talking about taking a standard 10 foot rod and encasing 
the required buried 8 feet in concrete?   That's not a Concrete Encased 
Grounding Electrode (what NEC calls it) or Ufer (what everyone else 
calls it).

The actual construction of a CEGR/Ufer (they are the same thing) has a 
fair amount of variability:  the code has a "minimum", but in practice 
people build all sorts of things with varying lengths of wire and 
varying whether you use a copper wire or use the rebar or whatever. 
There's some IEEE journal papers out there which I can't remember off 
the top of my head where people built various kinds of Ufers and tested 
them.  For instance, there's a whole discussion about whether the rebar 
has to be one continuous piece, or whether it can be sections, so 
someone went out and measured a bunch of cases.  There's also some 
papers where steel columns bolted to the foundation slab were tested.

The way I find them is by starting with the references in the IEEE 1100 
doc (because it's most up to date), and then working my way back through 
their references, etc.  A lot of the work was done in the 60s and 70s, 
as I recall.  IEEE Xplore access is probably a necessity for finding it all.


As far as modeling goes, there are papers out there where people use NEC 
(the antenna modeling code) to model lightning strokes and the victim 
structure (for induced currents and such).  They run the NEC model at a 
whole raft of frequencies to get the transfer function vs frequency. 
Then, they take the Fourier transform of the lightning current waveform, 
run that through the model, then transform the victim current and 
voltage spectrum back to time domain.

There are a fair number of papers on lightning propagation within soil 
and rock, as well. This is a topic of some concern for mining and 
drilling.  I don't recall if J.R. Wait wrote any papers on this, but 
since he probably wrote hundreds of papers on EM propagation in soil, 
it's a good bet.


  I know some use bentonite around a ground rod but what would
> happen if someone used only one ground rod encased in concrete as the
> ground for a tower?  Yeah that's probably dumb, but what would happen?

It would work.  No worse than the same rod just driven into the soil.

> Would it explode?

Not likely.

  A strike can go right thru a concrete wall.  What's
> the difference? (It also goes right thru many other building materials,
> including tile, if those are in the path where the strike wants to go.)
> There doesn't seem to be any rules for this kind of grounding, or any
> data or description to help understand what will happen.  I can guess
> what would happen, but I would rather not.


Spark propagation is a very, very complex topic, particularly when 
talking about through dielectrics.  It's complex just going through air. 
  I would suggest Bazelyan and Raizer "Spark Discharge" as one really 
good reference.

The book by Rakov and Uman  on lightning is also a really good source of 
information.  They might be a good place to start to look for references 
on "lightning effects".
http://books.google.com/books/about/Lightning.html?id=TuMa5lAa3RAC

Uman's book on "The Lightning Discharge" is probably work looking at.

His book on The Art and Science of Lightning Protection is also worth 
looking at

http://books.google.com/books?id=KO7fVcqispQC&source=gbs_similarbooks

>
> And I agree that the way the ground connections are made is the most
> important part of a grounding system, but that subject is easy to
> understand, and not so difficult to analyze.
>
> I realize most people don't ask these kind of questions.  As long as
> they know what to do to solve the problems, most don't care how it
> works.  Guess I'm just different.



Curiosity is good..

There's quite a bit of literature out there.. You just need to get 
started.

If you had to buy a first book on lightning, I'd get the $12 Dover press 
edition of Uman's Lightning

  http://www.amazon.com/Lightning-Dover-Books-Physics-Martin/dp/0486645754


>
> Jerry, K4SAV
>
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