Frank,
Your test current of 5 amps, comes out of the ground at a
second ground, the ground at the transformer. The 5 amp current is running thru two ground rods. To get the resistance of the ground rod being tested as you suggest, one has to already know the resistance of the other ground connection at the transformer. Use Ohm's law to calculate the resistance of the circuit 120 V รท 5 A = 24 Ohms. Then subtract the resistance of the ground connection at the transformer and you will be left with the resistance of the ground rod you are testing.
Don,
The IEEE Green Book quote is a much better summation than the way I say it with my words. But it says the same thing I'm trying to say.
Over in General Discussion, in
Frank's thread \"Gnd. rod resistance tester\"[/b] BJarney references
[b]http://www.tradeport.on.ca/Appli...Ground Resistance Testing\"[/i] . In AEMC's document, on page 7, section (C) and Figure 9 show what the Green Book describes when saying:
IEEE Green Book
Around a grounding electrode, the resistance of the soil is the sum of the series resistances of virtual shells of earth, located progressively outward from the rod. . .As the radius from the rod increases, the incremental resistance per unit of radius decreases effectively to zero.
"Effectively to zero" says the Earth has zero resistance once the current gets outside of the "effective resistance area" of the ground rod. AEMC states:
Understanding Ground Resistance Testing
Eventually, adding shells at a distance from the grounding electrode will no longer noticeabley affect the overall earth resistance surrounding the electrode. The distance at which this effect occures is referred to as the effective resistance area and is directly dependent on the depth of the grounding electrode.
In AEMC's document, Figure 14 & 15, on page 12, show graphs of the resistance between two points when close together and far apart. In Figure 15, two ground connections have their own seperate resistances. Figure 15 shows the simplest state to consider with respect to Frank's test procedure.
Al
