090724-1942 EST
crossman:
Your experiments are useful information for you. When I have commented on putting 120 V on an isolated ground rod for this type of experiment there have been numerous responses that this is not safe. The experiment needs to be done where you have complete control of the environment around the experiment, you have some idea of the voltage gradients that may occur, and you are protected relative to these gradients.
Your DC resistance measurement with the Simpson 260 on the low ohms range only uses the 1.5 V cell in the meter. There is an 11.2 ohm resistor between the 1.5 V cell and one meter terminal. The other cell end goes to the other meter terminal. The meter with some series resistors is across the 11.2 ohm resistor as a voltmeter. When the meter leads are shorted for zero calibration full cell voltage is applied to the voltmeter and a 10,000 variable resistor is used for the zero adjustment in the series resistor string. At half scale on the meter the external resistance under test should be 11.2 ohms. This you can verify by looking at the meter scale for the 50% point on the DC range. Mine looks like it points to 12 ohms. Thus, besides the 11.2 ohm resistor Simpson must assume another 0.8 ohms internal resistance in the leads inside the meter.
Your 2 ohm reading corresponds to about (1-0.84)*1.5 = 0.24 V across the 2 ohm resistance being measured. If for any reason there is an external voltage source, dissimilar materials in an electrolyte in the 2 ohm path, then you can have an error in the resistance reading. This is because the source voltage is not 1.5 anymore.
A DC measurement of soil resistance or a conductive liquid is not a good method and can result in large errors. Try this glass of tap water test. Nearly fill the glass. Insert two pieces of bare #14 copper wire in the water on opposite sides of the glass. Mount so they do not move much. In my experiment I used the Rx100 range on the Simpson 270 (same as a 260). Connect the Simpson ohmmeter to the two electrodes. My initial reading was about 1500 ohms. This quickly changed to about 5000 ohms. Now switch the selector back and forth between + and - and the measurement characteristic will repeat. This results from gas bubbles on the electrodes or other electrolytic action. Using AC for the excitation produces a lower resistance reading. At 1 kHz on my bridge I read about 900 ohms.
When you make earth resistance measurements always do a voltmeter check between the electrodes with no excitation current. This is to look for stray currents that might cause an error in your readings.
To get more controlled results I believe you should use AC excitation. I use 60 Hz because it is convenient. The areas where I have run tests the stray currents produce less than 0.25 V. Using a 28 V source the residual stray current effect it insignificant compared to the accuracy required.
If you are really as low as 2 ohms, then at 28 V you need a transformer with 20 A capability.
Screwdriver probes with a high impedance meter are adequate for measuring voltages in the earth at the surface from stray currents.
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