On Fri, 02 May 2008 16:56:08 +0700, Bruce in Bangkok

I think Martin is generalizing incorrectly about the skin effect, which
applies only to high frequency AC. It's a phenomenon that becomes important
at radio frequencies. At DC, or at low frequencies, conduction is uniform,
or nearly so, across the whole section of a conductor.

Of course, copper has something like 10 times the conductivity of steel, so
you have to take the thickness of the copper cladding into account, too.


Most of the copper coated ground rods I see are electroplated and the
plating is maybe 0.0005 thick. I don't believe it carries much
current.

Lightning events do have high-frequency content so skin effect does
come into play. But you're right, .0005" of copper won't make much
difference.

Consider that even steel has far higher conductivity than the earth
into which it's driven. Making a ground rod more conductive than steel
would serve no useful purpose. A better course for low-Z ground is to
use more ground rods, because surface area is what determines
effectiveness.

The diameter can be smaller and resistance can be higher than the
cables coming out of a welder because it only handles high current
during very short transient events, and even tens of volts of drop are
acceptable during lightning transients.

Residential utility ground current should be very low, well under an
amp and more like milliamps. There is a substantial neutral wire
running back to the pole transformer. The reason for a groud
reference is to prevent the system from attaining a common-mode
voltage significantly above ground potential --e.g., 120 volts line
to neutral but neutral is 400 volts above ground due to static buildup
somewhere so the terminal voltages w.r.t. gnd are 400 and 520. Zap!
Copper is a better choice for corrosion protection because zinc is
very vulnerable to even weak acids. Slightly acidic soil would eat
the zinc off a galvanized steel rod rather quickly.

You could get a better conduction path by stringing a bare copper wire (say
8 ga.) next to the steel rod, than by plating the rod. 'course, that
doesn't provide any corrosion protection, but my point is that doesn't do
anything for you electrically. As far as a lightning strike is concerned,
the current is enormous, producing a voltage drop so high that lightning may
even jump LATERALLY from a tree trunk to a person standing under the tree.
In order to get to the ground rod, the current will have traveled from the
roof somewhere, probably along several paths.

I believe that the ground rod is mainly there to establish that the house
neutral is not floating, as Don Foreman said (more eloquently.)