GROUND MEASURING TECHNIQUES:

ELECTRODE RESISTANCE TO REMOTE EARTH & SOIL RESISTIVITY

Elvis R. Sverko

ERICO, Inc. Facility Electrical Protection, U.S.A.

Revision Date: February 11, 1999

Abstract: This paper presents different concepts andprocedures for ground measuring techniques. A few ofthe techniques that are discussed are the measuring ofresistance to remote earth for a single earth electrode orfor an entire earth electrode system, and also theresistivity of the local soil using an earth system tester.

1. Introduction

A good grounding system (also known as an earthelectrode system) is important for the protection of anoverall system facility. There are many factors thatdetermine how well a grounding system performs. Twomajor parameters are its resistance to remote earth andthe resistivity of the local soil. Each of these values canbe measured to help determine and design the bestsolution for the grounding system.

The resistance to remote earth of the grounding systemneeds to be at a minimum in order to sustain itseffectiveness. A few of the components that make upthis resistance are the physical properties of thematerial used to make the electrode and conductor, allconnections made, contact resistance between theelectrode and the soil, and the soil resistivity. Acomplete grounding system might include only oneearth electrode, an entire group of electrodes with agrounding grid, or anything in between and beyond.The earth electrodes from any of these types ofsystems can have their resistance to remote earthdetermined.

2. Theory vs. Practice

Theoretically, the resistance to remote earth of an earthelectrode can be calculated. This calculation is basedon the general resistance formula:

R = (ρ x L) / A

where:

R = resistance to remote earth (Ω)ρ = soil resistivity (Ω-cm)L = length of conducting path (cm)A = cross-ectional area of path (cm)

This general formula is a simplified version of somecomplex formulas (derived by Professor H. B. Dwight ofMassachusetts Institute of Technology) used tocalculate the resistance to remote earth for a groundingsystem. The assumption in the general formula is thatthe resistivity of the soil is constant throughout theconsidered area, or averaged for the local soil.

In the practical (real) world, soil resistivity is notconstant, properties of electrodes and their connectionsvary (except with CADWELDTM), and complexequations just don’t cut it. Therefore, an actualmeasuring technique is necessary. This technique isdone with an earth resistance tester, often called amegger. One example of the earth tester is the ERICOEST301Universal Earth System Tester, seen in figure 1,(please refer to the ERICO EST301 OperatingInstructions manual for detailed instructions on its use.)This type of instrument can be used at various stagesin the life of a grounding system, once duringinstallation to see if it meets all specifications, andanytime thereafter to observe any possible changes.

Figure 1: ERICO EST301 Universal Earth System Tester

3. Earth Electrode Measurement (Single Electrode)

There exists different measuring techniques forresistance to remote earth of a grounding system. Onesuch technique is the 3-pole earth electrodemeasurement for a single electrode. This techniqueuses the electrode under test (EUT), a reference probe,and an auxiliary probe, set in a straight line. Figure 2shows the single electrode measuring method, andfigure 3 shows the single electrode measuring setup forthe ERICO EST301 earth tester.

Figure 2: Single Electrode Measuring Method

Figure 3: Single Electrode Measurement Setup

After applying a voltage at the designated location inthe circuit, the current between the EUT and auxiliaryprobe is measured, and the voltage between the EUTand the reference probe is measured. Using Ohm’sLaw, the resistance can now be calculated. This routineis completed by the earth tester. It has been determinedthat the distance D be approximately 100 feet, and thedistance between the EUT and reference probe be 0.62times the value of D. These values have been testedusing the fall of potential method to give optimal results(see figure 4.) But due to the large amounts ofuncertainty in all properties of probes and soil, thevalue of D should be varied for each individual testuntil reasonably consistent values appear; however, itis recommended that the value of D stay greater than 80feet. It is also necessary to take a second set ofreadings at 90o to the original in case of interferencefrom overhead power lines or any undergroundelectrical equipment or metal objects.

Fall of Potential

0

10

20

30

40

50

60

0 20 40 60 80 100

Distance between earth electrode and ref. probe (ft.)

Res

ista

nce

(Ohm

s)

Figure 4: Fall of Potential

4. Earth Electrode Measurement (Multiple ProbeSystem)

A second earthing resistance measuring technique isthe process of measuring the resistance to remote earthof a single earth electrode in a multiple electrodegrounding system. This technique is used when theearth electrode cannot be disconnected from the rest ofthe grounding system such as a communication towerinstallation. When the earth electrode can bedisconnected, the previous 3-pole single electrodetechnique may be used. Caution should always betaken when disconnecting any earth electrode. The same principles apply to this technique as they didfor the single probe technique, the only difference isthat for the multiple electrode grounding system, thecurrent is measured with a current transformer aroundthe EUT, (see figures 5 and 6.) After the proper voltageand current values are measured, a simple Ohm’s Lawequation determines the electrode’s resistance toremote earth.

Figure 5: Multiple Electrode Measuring Method

Figure 6: Multiple Electrode Measurement Setup Once the resistance to remote earth for each electrodein the entire grounding system is determined, one cancalculate the resistance to remote earth for the entiregrounding system in one of two ways. The firstapproach is in understanding that the electrodes are inparallel with each other (through the grounding gridand ground itself.) Because they are in parallel, the rulefor parallel resistances can be used.

Rs = 1 / [ (1/R1) + (1/R2) + (1/R3) + …(1/Rn) ]

where

Rs = resistance to remote earth for entire grounding system (Ω)

R1,2,3…n = resistance to remote earth for each individual electrode (Ω)

However, this rule is not completely accurate in thisapplication because of the extra resistance to remoteearth through the grounding grid. The second approach used to calculate the resistanceto remote earth for the entire grounding systemassumes that the resistances to remote earth for eachelectrode are equal. Then, for the entire groundingsystem, the resistance to remote earth is 40% lower fora system with only two electrodes, 60% lower for asystem with three electrodes, and 66% lower for asystem with four electrodes (compared to the resistanceto remote earth for one of the equal electrodes.) Thesevalues are slightly larger than the values given by theparallel resistance rule. 5. Soil Resistivity

The second major factor in determining how well agrounding system performs, is the resistivity of thelocal soil. Soil resistivity is the resistance measuredbetween two opposing surfaces of a 1m3 cube ofhomogeneous soil material (see figure 7,) usually

measured in Ω–m, or Ω–cm. Soil resistivity has adirect effect on the resistance of the grounding system.The evaluation of the resistivity of the local soil candetermine the best location, depth, and size of theelectrodes in a grounding system, and can also be usedfor many other applications. A geological survey usesthe soil resistivity to locate ore, clay, gravel, etc.beneath the earth’s surface. Depth and thickness ofbedrock can also be determined. The degree ofcorrosion of the local soil also can be obtained from itsresistivity value. Due to these many reasons, it isnecessary to measure the resistivity of the local soil.

Figure 7: Soil Resistivity Definition Cube

Many different factors have a direct effect on theresistivity of the local soil. A large factor is the type of

soil. The resistivity range can go from 1 Ω−cm to the

upwards of over 1,000,000 Ω−cm (see figure 8.)Moisture content can be a large factor in determiningthe resistivity of the local soil. The drier the soil, thehigher the resistivity. The soil resistivity remainsrelatively low (and constant) if the moisture content ofthe soil is greater than 15% (by weight,) and skyrocketsfor lower values of moisture content. Another largefactor in the determination of soil resistivity is thecontent of minerals, such as salts or other chemicals.For values of 1% (by weight) salt content, the soilresistivity remains low (and constant,) and skyrocketsfor lower values of salt content. Finally, compactnessand temperature can set the resistivity of the local soil.With temperature, the colder the soil is, the higher theresistivity. Due to seasonal changes where thetemperature can change drastically for a particular area,the resistivity of the local soil can also changedrastically (see figure 9.) Many of these factors(moisture content, mineral content, compactness, andtemperature,) of the local soil can change during the lifeof the grounding system, and therefore change theresistance to remote earth of that grounding system.

Soil Resistivity (ΩΩ -cm)lower upper

Surface Soils 100 5,000Clay 200 10,000Sandy Clay 10,000 15,000Moist Gravel 5,000 70,000Dry Gravel 70,000 120,000Limestone 500 1,000,000Sandstone 2,000 200,000Granites 90,000 110,000Concrete 30,000 50,000

Figure 8: Resistivity range for different types of soil

Typical year

05

1015

2025

3035

0 2 4 6 8 10 12

Month

Ear

th R

esis

tanc

e (O

hm)

Figure 9: Typical electrode resistance to remote earth in a year

Like the resistance to remote earth of an electrode,measuring the resistivity of the local soil can be donewith a specific metering device. The process is

sometimes referred to as the four pole (or four-terminal)method (see figures 10 and 11.)

Figure 10: Soil Resistivity – Four Pole Method

Figure 11: Soil Resistivity – Four Pole Setup

Four small electrodes (auxiliary probes) are placed in astraight line at intervals of a, to a depth of b. A currentis passed through the outer two probes, and thepotential voltage is then measured between the twoinner probes. A simple Ohm’s Law equation determinesthe resistance. From this information, it is now possibleto calculate the resistivity of the local soil by using theEqually Spaced (or Wenner Arrangement) method.

ρ = [ 4 x π x a x R ] / [ 1 + ( ( 2 x a ) / SQRT ( a2 +4 x b2 )) – ( a / SQRT ( a2 + b2 ) ) ]

where

ρ = resistivity of the local soil (Ω-cm)a = distance between probes (cm)b = depth of probes into the ground (cm)R = resistance determined by the testing

device (Ω)

For most practical circumstances, a is twenty timeslarger than b, where we can then make the assumptionthat b=0, and the formula becomes simply:

ρ = 2 x π x a x R

These values give an average resistivity of the soil to adepth of the value of a. It is recommended that a series

of readings be taken at different values of a, as well asin a 90o turned axis, so that the measuring results arenot distorted by any underground pieces of metal(pipes, ground cables, etc.) These final values shouldbe plotted, so that a consistant value be determined.

6. Improving the Grounding System

If it is determined that a grounding systems resistanceto remote earth is not low enough, there are severalways that it can be lowered. The first approach wouldbe to lengthen the electrodes, which puts them deeperinto the ground. If it is difficult to drive ground rodskeep into the ground, a horizontal “crows foot”approach using copper tape, may be used. Anotherapproach would be to use multiple ground rods. TheNational Electrical Code (NEC 1996) states that if oneelectrode is being used and it’s resistance to remote

earth is greater than 25Ω, then only one additionalelectrode needs to be installed. A final way to helplower the resistance to remote earth for a groundingsystem, is to treat the soil with some type of groundenhancing material (i.e. GEMTM, salt, chemical rod, etc.)The thickness of the electrode used plays only a smallpart (and it is not very economical) in helping to reducethe total resistance to remote earth of the groundingsystem.

7. Conclusion

It is important for a facility to have a good groundingsystem. The safety of all personnel and equipment is atstake. In order to be sure that a good groundingsystem is in place, it is necessary to maintain a lowresistance to remote earth of all the electrodes, and alow resistivity of the local soil. There are differentmethods for obtaining these measurements. Due tovariations in electrodes and soil, a number ofmeasurements should be taken and evaluated for aconsistancy.

It is the intent that this paper be used as a guide fordescriptions and definitions of different groundmeasuring techniques for electrode resistance to remoteearth and soil resistivity. Detailed instructions on theuse of any earth testing device should be fully read andunderstood before they are used. There always existsthe possibility of ground faults in a system, and stepand touch potentials. It is important to note that beforeattempting to test any grounding system, all safetyprecautions be taken.

References

Earley, Mark W., Editor in Chief, National ElectricalCode Handbook, Seventh Edition, 1996

Elmes, UNILAP GEO X Earth Tester OperatingInstructions, LEM

Getting Down to Earth, Manual on Earth-ResistanceTesting for the Practical Man, Fourth Edition,Biddle Instruments, 1982

IEEE Guide for Measuring Earth Resistivity, GroundImpedance, and Earth Surface Potentials of aGround System, ANSI/IEEE Std 81-1983

Simmons, J. Philip, Soares Book on Grounding,International Association of ElectricalInspectors, Fifth Edition, Editor1996

Superior Grounding,http://www.superiorgrounding.com/soils.html

Switzer, W. Keith, Practical Guide to ElectricalGrounding, ERICO

Understanding Ground Resistance Testing, AEMCInstruments, Workbook Edition 6.0

Author

Elvis R. SverkoApplications EngineerERICO, Inc. Facility Electrical Protection34600 Solon RoadSolon, Ohio 44139U.S.A.Phone: (440) 248-0100Fax: (440) 519-1675email: esverko@erico.com