Don,
The idea of the earth as a conductor tricks my mind into thinking of a really fat wire, with a cross section of a couple square feet runing from the service to the xfmr pole, and since dirt intuitively doesn't conduct well, I think it must be the source of the resistance. Well, I turned my minds perspective to a half sphere, with the ground rod being at the center of the radius (for simplicity, I'm treating the ground rod as a point). At 10 feet distance from the rod, the conductor formed by the earth has the surface, or cross section, of a half sphere, or ½(4Pir²) where:
Pi = 3.14...
r = 10
And the cross section of the earth conductor is about 628 ft². At 20 feet from the rod the earth conductor cross section becomes 2,512 ft². The conductor becomes huge quickly, and is thought of as perfect. What we, as electricians, measure is the resistance of the connection to that huge earth conductor.
The earth is simultaneously an infinite source and an infinite sink of electrons. The electron that leaves the ground rod will almost certainly not be the electron that goes up the pole to the xfmr, but the effect is the same as if it is.
The resistance that we measure with a ground resistance test set tells us how good (or bad) the connection is between the GEC and the infinite conductivity of the Earth itself. If we test the ground rod we will get one measurement. Then if we take the test set to the xfmr ground and test it, we will get another, different, reading depending on how well that ground is made.
Since what goes in, must come out, there are two contact points, at the minimum, for Frank's circuit to be complete. Each of the contacts to earth (ground rod and xfmr ground) have their own resistance. 5 Amps flowing says the total resistance in the circuit is 24 Ohms. The two big pieces are the two ground connections, not the earth itself.
Al
[This message has been edited by ElectricAL (edited 08-20-2002).]
