On 7/18/13 6:29 AM, Grant Saviers wrote:
"extrapolation" may have been a poor word choice, but my reasoning is
that copper pipes are not buried without special precautions for
domestic water service because of corrosion. Another reason is that the
galvanic cells and/or stray currents that exist in soils can cause
electrolytic erosion. Copper clad ground rods are known to have finite
life, sometimes quite short in certain soils. Some concrete mixtures
were a disaster with buried radiant heating copper tubes and that
application now very rarely uses copper in concrete. There are
discussions on this reflector that AM broadcast station copper ground
screens disappear over time. Copper alloys in marine environments are
protected with sacrificial anodes or impressed voltage systems. Copper
in air lasts a long time, in an electrolytic or hostile chemical
environment not so.
So, why expose a Ufer ground lead to what might be an environment that
significantly shortens its life, particularly if it isn't possible to
inspect it? As I have read the Ufer literature and codes, steel rebar
is the conductor, not buried copper wire, and the connection is to a
protected rebar exit point from the concrete.
wikipedia re Ufer: "Ufer's original grounding scheme used copper encased
in concrete. However, the high pH of concrete often causes the copper to
chip and flake. For this reason, steel is often used instead of copper."
see http://www.psihq.com/iread/ufergrnd.htm and
http://en.wikipedia.org/wiki/Ufer_ground
I'd be interested in seeing a reference for the "chip and flake"
I'm not too impressed by the Wikipedia entry. For one thing, they don't
cite Ufer's paper, nor any of the subsequent ones on the subject. And
that comment about "flash into steam", as you can imagine from this
discussion, is one that I would not agree with. Unfortunately, the cited
reference is too general to actually find it.
Ufer's original paper
IEEE CP 61-978 (Conference Paper), Herbert G. Ufer
Then there's all of these, among others..
Herbert G. Ufer "Investigation of footing-type grounding electrodes for
electrical installations" IEEE Trans. Power Apparatus and Systems, Vol.
83 p. 1042-1048 October 1964
Fagan, E.J. and Lee, R.H. (July/August 1970). The Use of
Concrete-encased Reinforcing Rods as Grounding Electrodes", IEEE Trans.
on Industry and General Applications, Vol. IGA-6, No. 4, pp. 337-348
Harding, G. and Harris C.A. (1970). Some Engineering Objections to Using
Reinforcing Steel as Grounding Electrodes", IEEE IGA Conference
Proceedings, pp. 181-187. Discussion by Lee, R.H.
Preminger, Julius. (Nov./Dec. 1975). "Evaluation of Concrete-encased
Electrodes", IEEE Trans. on Industry Applications, Vol. IA-11, No. 6,
pp. 664-668.
Kawai, M. (1965). "Studies of Tower Footing Resistance on Transmission
Lines", IEEE Conference Paper No. 31 CP 65-704.
Grant KZ1W
On 7/17/2013 6:26 PM, David Gilbert wrote:
Why is that a "reasonable extrapolation"? Copper wire won't corrode,
and rust/corrosion is the ONLY reason that codes require rebar to be
fully embedded in the concrete. Please explain ...
Dave AB7E
On 7/17/2013 12:52 PM, Grant Saviers wrote:
Building codes now REQUIRE Ufer grounds in many jurisdictions for new
foundations. Codes also REQUIRE that rebar be covered with concrete,
usually a minimum of 3" to prevent corrosion ingress along the rods.
So it is reasonable to extrapolate that ground wires connected to the
rebar should not exit the concrete below grade ... <snip>
Grant KZ1W
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