Bill et al,
Grounding /earthing for large buildings is not made with a driven
ground rod, contrary to popular thought.
In this instance the ground is called a 'made electrode' and
consists of all the rebar (reinforcing bar) in the footings welded together
and a connecting strap brought out to connect to the electrical system when
it is installed.
Concrete is a reasonable conductor. At current levels expected
(including fault currents) the current density to the earth thru this metal
& concrete structural part is low but the total current can be high.
Although the current carrying capability of the earth is quite variable it
is hardly ever zero and this system works well to establish a local
equipotential for the electrical environment of the building.
In the US, ground rods are driven for individual electrical
installations - homes, for instance. The impedance/resistance to earth is
measured. If it is inadequate a second ground rod is driven and
electrically tied to the first. The resistance is not measured the second
time; this is defined as sufficient.
There are places where the ground conductivity is inadequate and the
earth is not used for that purpose. Norway, built upon a large piece of
granite, comes to mind and they have special rules for earthing there. From
John's comment the moon seems similar and 'mooning' would have to be done a
special way.
Oh, how I enjoy these discussions.
br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
[email protected]
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