[Elecraft] OT: Ground rods and concrete

Tue Apr 18 22:30:25 EDT 2017

Let me inject an experience that occurred while I was a field engineer 
for the phone co. working in Miami. We had a buried cable that ran 
diagonally across an open green area. About 1/3 of the way across was 
what we called a 'hand hole', a shallow pit where a splice or repair 
had been done. It is back filled and, once the grass recovers, no one 
knows it is there. Lightning struck in the vicinity. The telco cable 
has the nice aluminum sheath and is well grounded. The lightning hops 
onto that in preference to running through the ground. The water in 
the hand hole exploded and left a small crater. The hole was about 2' 
deep and 3' x 5'.

This makes perfectly good sense to me. You have water constricted into 
a small enclosure. The lightning electrode, in this case the telco 
cable, gets very hot very quickly. The water turns to steam and 
explodes. Nothing surprising about that.

Let me also mention that the experts will tell us to not use solder on 
any grounding conductors as the solder will melt. Well . . . water 
boils at an appreciably lower temp than solder melts. What has 
happened to the water in the vicinity of the lightning electrode 
before the solder has had time to melt?

As for assertions that exploding Ufer grounds are a myth, I can tell 
you for certain that the crater in Miami was not a myth. I kept a 
photo of it on my desk for a few months to show those curious.

Experience tells me that Ufer grounds are, in general, a terrible 
idea. There is very little reason to risk a building or tower 
foundation to save the few $$$ of installing a correct ground system. 
Ufer grounds may be OK, indeed preferred, in places like West TX, NM, 
AZ, and parts of So CA, or in other very dry locations. But to use or 
recommend them in locations with much moisture is asking for trouble. 
In any location with much moisture, concrete will have the same water 
content as the surrounding soil.

73 de dave
ab9ca/4

On 4/18/17 6:39 PM, Ken G Kopp wrote:
> I was the first to use the term "exploding tower base" in this discussion.
> The term "exploding" was probably not correct.  As several others have
> phrased it, an instant expansion of steam is more correct.  Concrete is
> never totally "dry" in the context of this conversation.
>
> I witnessed lightening blowing apart the two tower bases I spoke of in
> person, directly, and in real-time. It happened in the mid-sixties at the
> St. Petersburg, FL Coast Guard base.  As someone else has mentioned;
> the Tampa Bay region has the highest incidence of lightening in the
> Western Hemisphere.  To the one of you who accused my of lying ...
> I was there ... you weren't.
>
> At the time I was an ET aboard the USC&GS (Now NOAA) Oceanographic
> Survey ship Hydrographer/WTEI and we were in the area deliberately
> attracting lightening with balloon-hoisted cables.  I -do- know something
> about the infinite uncertainty of lightning.
>
> I'm a retired electric power company two-way radio tech and have probably
> dealt with more types of towers than most of you.  Large electric
> transmission
> line towers are almost always set on four concrete piers, and are grounded
> with (usually) copper straps cad-welded to each tower leg and connected to
> ground rods a bit away from the cement.  Each of us can search long enough
> to find "facts" that support our various positons, especially on today's
> Internet.
>
> Here in the mountains of the West ... as well other regions ... finding and
> maintaining a "good" ground at a radio site atop a mountain can be a
> "challenge".  It's almost never done with ground rods.  My power company
> employer has several hundred mountain-top microwave and/or radio sites.
> Been there, done that, as they say.
>
> One responder spoke of fitting a copper pipe with a garden hose fitting and
> "flushing" it into the ground.  That works very well, and is how some of my
> 14 grounds are installed.  It helps if the downward end is partly
> flattened, BTW.
>
> A much bigger problem overall than lightening is water collecting in tower
> legs,
> especially in climates where it can rust (undetected) from the inside and /
> or
> freeze and split one or more legs.  There's an accepted way to avoid this.
>
> 73!
>
> Ken Kopp - K0PP
>
>
> On Apr 18, 2017 at 4:30 PM, <w5sum at comcast.net> wrote:
>
>> some 40 years ago, maybe longer I put up a 50' rohn 25G tower. Dug the
>> hole
>
>     and set the tower base in it alone with a 12' 3/4" ground rod and
> poured the
>     cement, left about 6" of ground rod protruding. I bonded to that rod
> and grounded
>     the tower. 3 years later I had a huge lightning strike on my tower and
> yes.. it cracked
>     that base.
>
>     never again
>
>     Ronnie W5SUM
>
>> ______________________________________________________________
>>
>>
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