Appendix U

Fall of Potential Method


The receiver portion of the MiniIRes is completely separated from the transmission portion of the MiniRes. You can not damage the MiniRes by connecting the wrong terminals together

There are at least three methods for measuring the electrical resistance of an existing grounding electrode. In the following it is assumed the existing electrode is a single 10-foot rod.


The triangulation or “three electrode” method has the disadvantage that small errors in measuring or in positioning can generate large errors in the final answer. Two temporary electrodes are driven in the ground to form a triangle with the existing electrode as the third corner of the triangle. The temporary electrodes should be at least 15 feet (5 meters) and preferably 25 feet (8 meters) meters away from the electrode to be tested. Resistances are read between each of the pairs of electrodes. The resistances of the three electrodes (a, b, and x) are Ra, Rb, and Rx

                                                   (Rx+ Ra)  +  (Rx + Rb)  -  (Ra + Rb)

                                Rx     =       ___________________________




The Direct  method is some times referred to as the two-terminal method  It is straight forward but subject to many sources of error. On the MiniRes the P+ terminal is connected to the C+ terminal and from there connected to the existing electrode  The P- terminal is connected to the C- terminal and from there connected to the metallic water system or other metallic systems in good contact with the ground. Be careful of insulating couplings or systems isolated from ground by plastic moisture barriers or foam thermal insulating covers.


The “Fall of Potential “ method is the most common method for measuring the electrical resistance of an existing grounding electrode. This method is some times referred to as the three terminal method. The various models of the MiniRes have been used extensively for the grounding of cell phone towers, electrical transmission line towers and, most recently, wind turbine towers.

First, be sure that the grounding electrode to be measured MUST be disconnected from the NEUTRAL connection during the grounding test. If the grounding electrode remains connected to NEUTRAL during your testing, the results will be erroneous

Connect the P- terminal of the MiniRes to the C- terminal and from there over to the grounding electrode that is being measured. That leaves the C+ terminal to connect to the far electrode and the P+ terminal to connect to the intermediate electrode. The distant electrode should be at a distance of at least ten times the length of the grounding electrode. All three electrodes should be in a straight  line. Take readings with the intermediate electrode at numerous measured positions. Plot the readings. If the central quarter of the plot is relatively horizontal, you can use this value for the resistance of the ground electrode.

If the electrodes are too close together, there will not be a level “plateau” in the graph. You must expand the array until you do have a “plateau”. The value should be taken where the graph is about 62% of the distance from the electrode being tested and the far (C-) electrode.

If the ground electrode is not a single rod then the following would apply (the following is quoted from Brewer (1987)):

  1. When the grounding system contains two or more electrodes along a straight line, the maximum dimension is the distance between the first and last electrode.
  2. The maximum dimension for a grounding system containing electrodes in a circular ring configuration is the diameter of the circle.
  3. The diagonal distance across a grounding system having a square or rectangular form is the maximum dimension.
  4. The maximum dimension for a grounding system containing electrodes in a star configuration is twice the length of a single branch.


For sites where the grounding electrode system has not been installed as yet, the most common method of obtaining the necessary parameters for the designer is the Wenner technique. It is explained in another section of this manual. There are also some ASTM standards that can be obtained directly from ASTM over the internet.


All of the above methods assume lateral homogeneity and it is seldom the situation found. The designer of the system should make ample use of “the safety factor”.




Brewer, Myron L., (1987), Measurement of resistance to ground, Chapter 12. Practical Grounding, Theory and Design (published by abc TeleTraining, Inc. Geneva, IL.


Tagg, G.F.  Earth Resistances (1964), Pitman Publishing, London

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