On 1/19/15 5:29 PM, Brian Carling wrote:
The advice varies about this considerably. This week is the first
time I've even heard of UF ER or conductive concrete!
It's not anything special concrete-wise. All concrete is pretty
conductive, unless you take special efforts to make it an insulator
(often by adding fly ash to the aggregate, apparently). It tends to
absorb moisture from the surrounding soil, and concrete is more
conductive than rock or most soil (unless you're living in the fabled
top-band saltwater swamp.. like the Canadian Shortwave Broadcast station
at the top of the Bay of Fundy)
The professional experts that I know recommend putting a 20 to 30
foot ground rod into the ground at each corner of your house and
connecting heavy gauge copper conductors up to lightning rodsup on
the roof.
It kind of depends on what the concrete looks like. If you're doing a
retrofit of an existing building, driving new rods or doing a perimeter
ring might be more practical. A lot also depends on how much disruption
is allowed. Trenching all the way around for a ring ground might be
easy if you're looking at new construction, but if you're going to be
digging through the prize rose bushes in the landscaping, a localized
rod driving might be more acceptable.
It seems like if the only thing you need is a large area of this
allegedly conductive concrete stuck in the ground, why not ground
everything to the concrete slab your house sits on!!
which, in fact, is what they do these days. The lightning protection
system in the building is tied to the structural steel which in turn is
tied to the rebar in the foundation/slab. This is very common in large
multistory buildings, for instance.
In residential construction in Southern California (a very low lightning
area, though) a Ufer ground is required. So, in fact, everything is
grounded to the slab my house sits on. Nothing beats hundreds of square
feet of contact area for a good reliable ground.
I've worked in buildings in New Mexico which were essentially large
steel sheds (slab poured, structural steel supports bolted on, then
steel panels screwed onto the structural steel). The lightning
protection earth connection came up as a stranded AWG 2 (or thereabouts,
I didn't measure it..) cable from the slab. It was bolted with a lug to
the steel. The air terminals were connected to cables that ran down the
outside of the building and were connected to the same cable that came
out of the slab.
Interestingly, they did not depend on the bolts holding the steel to the
concrete on that building for electrical connection. I think it's
because the bolts were chemical anchors (epoxied into holes) installed
long after the slab was installed, but the engineer wasn't around to ask
when I was thinking about it.
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