Steve,
Have a look
here , where I described the method we normally use in England to measure the impedance of a ground rod.
Following on from that description, when we measure the voltage between electrodes X and Z, then move Z closer and farther away, we're making sure that electrode Z is outside the voltage gradient of the rod being tested (X).
If Z remains outside the voltage gradients of X and Y, then the reading will stay the same on all three tests. If the readings differ, then Z must be within the voltage gradient.
I once heard of power distribution to some very remote parts of Australia being a single HV line with a return path by just a local ground rod. With enough current flowing, there could be enough of a gradient to get a p.d. of several hundred volts over a space of a couple of feet. I wouldn't want to be standing barefoot near that rod!
[This message has been edited by pauluk (edited 05-20-2002).]
