Sub station earthing

I have searched the electrical power and this forum for information on the methodology of testing an earth ground. My specific question is how the "Fall of Potential" and related processes work when all the resistance is attributed to the "ground under test" and none to the sensor probes that are about 14 inch long and probably have a maximum of 10 inches of earth contact. I am familiar with the term "hemisphere of influence", but this seems to have no relationship to the size of the electrodes and different soil types.

As far as free online information is concerned, there is a limited description in:

The aemc.pdf article is more thorough than others I have found, but it presents a slightly different theory on how resistance is calculated. Page 8, Figure 9 in that article states the resistance is based upon concentric shells. On page 2 of the following reference, it states that the theory (fall of potential) is based upon a hemisphere and I have seen that expressed slightly differently in other references, which used the term "sphere of influence".


Old telegraph engineers used to say earth conductivity (near zero resistance) was because the earth acted as an infinite number of parallel resistors. The the use of concentric shells or hemispheres to explain low resistivity is a "new" twist on that theory. Atmospheric physicists present a different theory, but I don't see any "earth conductivity measurement" articles using their approach.

The best info on grounding I have ever come across is in MIL-HDBK-419A. Search the web and try to obtain a copy.

Found MIL-HDBK-419A on the net, but very big, 9769 KB, and will take awhile for a std modem download.


The conductive quality of the ground in Australia must be quite good in that they run a 12.7 KV 50 HZ power transmission line with just a single-wire earth return (SWER).


I found the next article interesting in that the design criteria used for a SWER system assumes a 0.05 ohm/km for the earth return resistance, page 16.


If the earth resistance is only 0.05 ohm/km, there must be something wrong with the "fall of potential" ground tests that struggle to get a 5 ohm resistance with less than a 100 foot spread between the ground rod and the sensors.