"Reg Edwards" wrote

I am not very familiar with USA power-grounding regulations and they
probably vary from state to state. But I am under the impression that in
some simple circumstances, perhaps domestic, a single ground rod is
considered adequate for safety purposes.

Under similar circumstances a maximum ground electrode resistance (
whatever
its construction ) of 50 ohms is specified.

A 'standard' ground rod is 8 feet long and 1 inch in diameter.

An average soil resistivity is 500 ohm-metres ( = 2 mS ). See Eznec?

The calculated resistance of a single standard rod in average soil is 194
ohms. ( I'm sure the calculating formula can be found somewhere on the
IEEE
shelves. There ought to be greater use made of it. )

Now I don't expect the alarm bells to be rung from Washington to LA. But
isn't there a serious inconsistency somewhere?

As a matter of interest, the resistances to ground of 2, 3 and 4 rods in
parallel, spaced 6 feet apart in average soil, are as follows -

2 Rods = 113 ohms
3 Rods = 86 ohms
4 Rods = 70 ohms

all of which exceed the specified maximum of 50 ohms.

Quite a low average soil resistivity of about 130 ohms is needed to bring
50
percent of installations within specification.

If my starting data is wildly adrift then disregard my waffle.
----
Reg, G4FGQ


Yes Reg, sorry the figures are (thankfully) in error. Especially since 25ohm
is required by our U.S. NEC, and that's easily attained in some soils, not
so easy in others. This is being changed in the 2004 code changes, but it
was paraphrased below:

The NEC does not specify a maximum earth resistance for the grounding
electrode system required under Article 250-81. The only place that does
specify earth resistance is under Article 250-84, for "made" (rod, pipe, and
plate) electrodes. Here the NEC specifies a resistance to ground of 25 Ohms
or less for a single electrode. If the electrode does not meet 25 Ohms, it
must be supplemented by one additional electrode. However the combination of
the two electrodes does not have to meet the 25 ohm requirement!

http://www.cpccorp.com/deep.htm

Neither should anyone be overly concerned with acheiving low DC-resistivity
of a grounded electrode. In spite of some interesting comments about their
perceived successes in the white paper I cited above, the grounding issue is
usually overstated - the ground for lightning is the real issue, as any 10'
5/8" copper rod earth ground works for 60hz electrical safety. For safe
termination of lightning downconductors and mast grounds etc, the best
ground you can reasonably achieve is going to be enough - as long as the
principles of bonding are adhered to religiously. That means as close to
equipotential as possible, and it does not assume a good ground, or even any
ground at all in certain cases. We surely want the best ground reasonably
attainable, and if you can sink 30' of connected rods, great. If you sink a
few 8-foot 5/8" or a couple of 1"x10', even better. See the data in that
paper above for some sample resistance measurements which they consider
"average" (ha - Richard Clark's laughing ;-), rightfully so.

Best regards,

Jack