Spring 2002 1

The basic procedure for testing ground electrodesis described by The Institute of Electrical andElectronics Engineers in IEEE Standard #81, IEEE

Recommended Guide for Measuring Ground Resistance and

Potential Gradients in the Earth. A grounding electrodeis defined as “a conductor embedded in the earth, usedfor maintaining ground potential on conductors con-nected to it, and for dissipating into the earth currentconducted to it.” It should be obvious that the effi-ciency of the electrode is inversely proportional to theresistivity of the surrounding earth. Ideally, it wouldbe nice to have a resistance of “zero,” but that is notpractically realizable. Values of a fraction of an ohmcan be achieved, however, with appropriate diligenceapplied to the design. A reliable means must, there-fore, be available to test the efficiency of such an in-stallation. Note that while grounding electrodes canvary from a single rod to a grid or array that is several

Measuring Ground Resistance —The Fall of Potential Method

by Jeff JowettAVO International

hundred feet across,the fundamentals oftesting are the same.There are some com-paratively minor con-siderations in tech-nique and procedurethat apply to specific types of electrodes, but the simi-larities far outweigh the differences in testing acrossthe spectrum of electrode designs.

The most fundamental and general of proceduresis called “fall of potential.” There are others, gener-ally devised with an eye toward simplification, butthey suffer from notable limitations that are includedin their IEEE 81 descriptions. The most thorough andreliable method is fall of potential. Its shortcomingsare largely psychological, so it is worthwhile for theoperator to understand what the test is doing and why.

The Official Publication of the InterNational Electrical Testing Association Winter 1999-2000

Tech Tips

Spring 2002

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When an electrode is buried and the electrical system tied to it, anelectrical connection has been made with the entire Planet Earth. Thissounds like a tall order! But, in fact, one need only be concerned with theimmediate environment. It is like a Physics 101 question: if you fire agun, does the Earth recoil? Yes, it does, but to such an infinitesimal de-gree that it is of no consequence, except possibly to musings on the theo-retical level. Grounding acts much the same way. Fault current going toground encounters resistance from the immediate environment, but be-cause it is free to spread out in all directions, attenuation is rapid. If thiswere not so, the whole concept of grounding would be ineffective! In aconventional circuit, the gauge of the wire holds this parameter constant.But in a ground circuit, the current path is rapidly expanding with dis-tance from the electrode. The net effect is that any significant resistanceis concentrated in the area immediately around the buried electrode. Be-yond a critical distance, the rest of the planet offers so little additionalresistance as to be, like the recoil from a rifle, of no practical consequence.

This phenomenon can be thought of like the dirt around the roots of aballed tree. A certain critical volume is required to maintain the tree. Simi-larly, a critical volume of soil surrounding the electrode determines itscapabilities. In fairly standard environments, with moist, water-reten-tive soil, this volume is small, perhaps on the order of twenty feet or so.But in difficult areas, with dry, sandy, or rocky soil, it can extend hun-dreds of feet. Further, a temporal effect superimposes upon the basicrelationship of electrode design to surrounding soil. Rainfall contractsthe field of influence and lowers resistance. Dry conditions and freezingexpand and increase it. A test method, therefore, must be flexible, adapt-able, and understood.

Figure 1

Lab technicians generally workin a controlled environment inwhich the technician is master ofthe situation. Circuitry and com-ponents are laid out according to aschematic which rigorously guidesthe investigation. Measurementstend to be predictable or, if not,point the way toward their correc-tion. A ground test, on the otherhand, is being performed on planetearth. Any alteration of results islikely to involve some form of ex-cavation. This can be intimidatingto someone trained on a bench.Field workers are used to “think-ing on their feet” in atypical situa-tions. But the rigors of a fall of po-tential test tend to seem impracti-cal, and they would like to havesomething more cost-effective.

Remember, a ground test is notbeing performed on a circuit in theconventional sense, where currentis being neatly directed from pointto point by human design. Rather,it is governed by nature, in theform of local soil and geologicalconditions, and it is the humanwho must adapt. That does not sitwell with many operatives. But itis precisely what the fall of poten-tial test was designed to address.Current going to ground via a bur-ied electrode is not constrained tofollow a straight-line path in theway we commonly think of elec-trical current. This is a critical dif-ference in the nature of a groundtest. Fault current radiates in all di-rections from the electrode. It is theability of the surrounding soil toaccommodate this pattern of dis-persal that determines how goodor poor the ground connection isand to which the test procedureand instrumentation must beadapted.

Spring 2002 3

A fall of potential test is the most general of methods for meeting theabove criteria. It may also be referred to colloquially as a “three-point”test, although this term is also applied to a separate test that is likewisedefined in IEEE 81. With reference to fall of potential, three points ofcontact are made with the soil. One is the connection to the electrodeunder test. The other two are probes that the operator places in the soil,one for current and the other for potential. The test set acts as a currentsource, and the current probe establishes a circuit through the soil viathe electrode under test. The potential probe then senses the voltage gra-dient established by the test current against the local soil resistance. Withcurrent and voltage drop accurately measured, the test set simply usesOhm’s law to calculate and display the resistance.

Simple, right? Yes, from the standpoint of theory of operation, it is. Toavoid polarization effects, the test sets commonly employ an alternatingsquare wave current. IEEE suggests this be at or near power frequency. Aslight offset from multiples of the utility frequency enables the test set tobase its reading on its own signal without interference from utility har-monics. Lightning strikes occur at high frequencies, and both those andground faults can involve enormous currents. Some test sets do operatewith high frequencies and currents, but these are dedicated toward spe-cialized investigations. For everyday practical use, microprocessor sen-sitivity permits test sets to be portable and simple to operate while af-fording a reasonable approximation of the test ground’s capabilities whencalled on-line.

Figure 2

The complication that manyfind frustrating is the procedure.Technicians are used to hooking upand taking a reading. With groundtesting, it is not that simple becauseone is not working on an isolatedcircuit. A common error is to sim-ply run the leads provided by themanufacturer out to their fulllength and read accordingly. If thisresults in an accurate test, it ispurely by luck because the infinitevariability of earth defies the estab-lishment of set routines. To be reli-able, the potential probe is“walked” at regular intervals anda series of readings taken. Whengraphed, the result should be a ris-ing curve where the probe iswithin the influence of the test elec-trode and then a leveling off. SeeFigure 1. Within the influence ofthe current probe, an additional re-sistance is superimposed, so thecurve will rise again. It is the valuerecorded at the level point that isthe measurement of the test item’sresistance. Readings taken withinthe test ground’s sphere of influ-ence (on the rising curve) only re-flect the resistance to that point, notthe resistance that will be felt by afault current. If there is insufficientspacing between the electrode un-der test and the current probe,there is no location where the po-tential probe is outside the influ-ence of the other two probes andthe graph does not become hori-zontal. See Figure 2. Test resultscan be readily manipulated bymoving the potential probe closeto the test ground, but that wouldnot be the true resistance of theelectrode.

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Five instruments to choose from — three-terminal models: DET62D and the 250260;four-terminal models: DET2/2, DET5/4R and DET5/4D.

Ground Resistance Testers available from AVO International

Applications DET62D, 250260 DET5/4R, DET5/4D DET2/2

Utility pole grounds • • •Multiple ground electrodes • •Distribution transformers • • •Residential wiring systems • • •Computer/communication • •System grounds •Lightning arresters • • •Lightning protection systems • • •Soil Resistivity • •

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Installation and service contracts are often writtenwith a clause requiring that a full fall of potential graphbe constructed and submitted as the test result. If thecurrent probe was not located sufficiently distant, thetwo spheres of influence would coincide. Resistancewould continue to rise with each probe placement,and there would be no way to tell from the graph howmuch resistance was associated with the test ground.The “proof” of the result, therefore, is included in theprocedure. If it does not graph out with the character-istic shape, the current probe must be moved fartherout and the procedure repeated. Tables appear in theliterature relating current probe spacing to electrodedimensions. These are intended only as a help, not asa rule. They provide a fairly good chance of havingan acceptable test on the first try. But, if for reasons ofpracticality, a shorter distance is used, and it yields acoherent graph, the result is good.

Numerous other methods exist. Some are indepen-dent, but many are based on simplifications or spe-cial adaptations of fall of potential. Understandinghow and why this test method was devised providesa foundation for the whole field of ground testing.

In the next issue, we look at the slope method.

Jeffrey R. Jowett is Senior Applications Engineer for AVO In-ternational in Valley Forge, PA, serving the manufacturing linesof Biddle®, Megger®, and Multi-Amp® for electrical test and mea-surement instrumentation. He holds a BS in biology and chemis-try from Ursinus College. He was employed for 22 years withJames G. Biddle Co. which became Biddle Instruments and is nowAVO International. He served the last 10 years in the marketingdepartment for distribution sales. Jeff has written trade journalarticles, conducted training of distributors’ sales staffs and cus-tomers, and given seminars and talks to various electrical societ-ies, most recently to NETA and the National Joint ApprenticeshipTraining Committee (NJATC).