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

[Jim Lux]

On 11/14/12 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

No.. it's not because of "ground saturation", it's because putting two 
rods close together isn't much better than one rod.  You can solve the 
analytic equations for resistance in a uniform medium and come up with a 
chart of distance/length ratio and resistance ratio.  Such a chart is in 
the IEEE grounding book.  When you grind through it all, it turns out 
that if you separate by twice the length, you're within a few percent of 
half the resistance of one rod.

 From such things that are easy to remember are rules of thumb born.



and the need to spread the
> charge over a larger area.

And to reduce the "per rod" current, so you don't get "smoking rods", 
particularly in utility fault causes, where you might have a high 
current source that lasts a long time before some upstream breaker trips.




  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.

There's some data out there.. but really, what you do is not worry so 
much about the ground rod, but worry about making sure that all your 
equipment is tied together, so there's small voltages between wires.




>
> 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.

Concrete is a fairly good conductor, and since the electrode is "cast" 
into it, it makes very good contact (much better than a driven rod will 
ever do.. most soil, unless waterlogged, has little air spaces between 
the grains).




  I would guess that it might explode if there
> were insufficient ground rods in the system.

Nope.. explosion takes steam, steam takes liquid water in a big enough 
space to boil.  A properly done concrete encased electrode doesn't have 
any gaps between rod and concrete. Now, if you have some ratty old bolt 
that was rusty when you poured the concrete, and then you wiggled it 
around as the concrete was setting....




  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?

They DO use Ufer grounds.  That big monopole is bolted to the foundation 
and that foundation has rebar, etc.

Lots of commercial installations do belt and suspenders, drive rods, 
bury a ring, use a Ufer, bury a DX QSL card, etc.  The cost differential 
isn't much, and do you want have to spend time explaining WHY you didn't 
follow the 50 year old handbook they gave you?


  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.

It really is the code, not just a guideline.  (apologies to Pirates of 
the Caribbean).  The NEC describes what you need to do for a Ufer ground.

Ufer's papers (and lots since) describe the physics and why it works.

There is no substantive question about which works better, rod in soil 
or concrete encased grounding.. the latter is better.



>
> Lots of questions and nowhere to go for answers.
>

There are two IEEE standards to take a look at for starters..
IEEE 1100 aka Emerald Book is good on grounding in general
IEEE 142 is the ground electrode book

You can probably get them on interlibrary loan.

The references in IEEE 1100 will give you all the backup information on 
the electrode designs, etc.