More on Grounding...and Concrete [RE-wrenches]

[ASAP POWER!]

I don't know all of you except through this list but I want to say that I
appreciate everybody.  This is such a great list, even cross-compared to
other industry groups.  So now we have some solid history and current code
and an alternative on this subject.  Well done!

Ignoring all the structural integrity issues for the moment between seepage
freeze, actual strikes, etc affecting the rebar within the concrete, here's
what has happened to me twice but only involving "retrofitted" concrete
grounds on residential pads.  Inspector could not determine if Ufer
grounding was done, and if he could, if it was done at proper length/depth
in the pad or not, and also, if it's just tied to the water pipe, how far
that pipe travels  before it's PVC or something else, and therefore you
still have to punch in the 8ft. copper rod.

Does anybody know what happens with residential construction grounding
systems generally?

Peter D

-----Original Message-----
From: matthew tritt [mailto:solarone at charter.net]
Sent: Friday, April 25, 2003 8:33 AM
To: RE-wrenches at topica.com
Subject: Re: More on Grounding...and Concrete [RE-wrenches]


David,

One of the reasons why an Ufer ground can be a good solution to tower
grounding, is that many really great wind sites seem to be located in really
rocky ground. Another method I like (but it's expensive!) is a horizontal
ditch filled with a special type of mineral salt and a chemical salt filled
copper tube. The best make of these is Chem Tube, which is capable of
providing a grounding resistance of <2 Ohms (versus 35 Ohms for the best
concrete encased electrode system). Chem Tube claims that one of their rods,
which range from 4 feet to 20 feet in length, can replace 20 conventional
rods. The rod end has a cap to allow replacement of salts, which get used-up
over time.

The great thing about this type of ground is that it works equally well in
dry soil and rock as in wet soil.

Matt T
----- Original Message -----
From: "David Blecker" <blecker at seventhgenergy.org>
To: "RE-wrenches-topica.com" <RE-wrenches at topica.com>
Sent: Friday, April 25, 2003 6:34 AM
Subject: RE: More on Grounding...and Concrete [RE-wrenches]


Allan, Travis, Don, all

Thanks for the feedback.  The PSI article on Ufer Grounding was
particularly interesting.  It seems that the Ufer concrete ground "can" be
an effective method, but only if done right.

 From the NEC

250.52 Grounding Electrodes.
(A) Electrodes Permitted for Grounding.
(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2
in.) of concrete, located within and near the bottom of a concrete
foundation or footing that is in direct contact with the earth, consisting
of at least 6.0 m (20 ft) of one or more bare or zinc galvanized or other
electrically conductive coated steel reinforcing bars or rods of not less
than 13 mm (½ in.) in diameter, or consisting of at least 6.0 m (20 ft) of
bare copper conductor not smaller than 4 AWG. Reinforcing bars shall be
permitted to be bonded together by the usual steel tie wires or other
effective means.

So, a typical pier and pad foundation with a 5- 6 foot square pad would not
meet NEC's requirement to have a 20 ft minimum length of rebar.      It's
not clear from the code whether or not the 20 feet of Cu ground wire would
be in compliance if it were snaked or threaded back and forth in a 5-6 ft
square concrete pad.

Another concern is the structural integrity of the foundation.  Rohn says
all rebar has to be covered by at least 3 inches of concrete but the Ufer
method would have one piece of rebar extending up and out from the
foundation to provide an attachement point for the ground wire that then
attaches to the tower leg.  Would the exposed rebar create a path for
moisture that would lead to cracking from the freeze/thaw
cycle?     Probably no more so than the embedded
reporters...errrr...J-bolts for the tower.   Perhaps if we added an extra
piece of rebar specifically for this purpose, then it would be ok.

If we build a foundation that allows a 20 foot rebar rod, I may be tempted
to try the Ufer method.  (see HP#75 Page 11 for one such example) but I
would probably sink conventional ground rods too.  For a regular pier and
pad foundation, it seems like ground rods are the way to go.

Dave

----------------------------------------------------------
David Blecker, P.E., Director
Seventh Generation Energy Systems
608-424-1870 (ph)  424-1810 (fax)

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