OK, lets clear up some things about ground rod testing. To test you drive 2 test rods in the ground. The 1st one is called "Z" and is refered to and the current rod, should be placed about 100ft or so from the rod under test. The 2nd rod is called "Y" and is refered to as the potential rod should be placed 62% of the distance of "Z".
The test set creates a circuit from the rod under test through "Z", through earth, and back to the test set. The voltage at "Y" is measured and used to determine the resistance to earth of the rod under test.
Now, the rod under test and "Z" will have a sphere of influence that may cause your voltage reading at "Y" to be inaccurate. So a "Y" gets moved to 52% of Z and 72% of Z and the results are plotted. If the readings of the 3 tests are similar you know you have a good test, if there is a difference then the spheres of influence are affecting your readings and "Z" must be moved further from the rod under test.
