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Designing For A LowResistance

Earth Interface(Grounding)

by

Roy B. Carpenter, Jr.Joseph A. Lanzoni

An LEC PublicationRevised July 1997

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DESIGNING FOR ALOW RESISTANCE EARTH INTERFACE

(GROUNDING)

Roy B. Carpenter, Jr. and Joseph A. LanzoniLightning Eliminators and Consultants, Inc.

Boulder, Colorado, USA

Introduction

Grounding (or earthing) is the art of making an electrical connection to the earth. Theprocess is a combination of science and “art” as opposed to pure science. This process isrequired because it is necessary to go through a process of “testing the options,” asopposed to calculations made via some formal process. The options for each site must bedetermined through visualization and evaluation, individually, using a related analyticalprocess.

The earth must be treated as a semiconductor, while the grounding electrode itself is apure conductor. These factors make the design of an earthing system complex, notderived from a simple calculation or the random driving of a few rods into the soil.Knowledge of the local soil conditions is mandatory and is the first step in the designprocess. This includes its moisture content, temperature, and resistivity under a given setof conditions.

Evaluating the Soil Conditions

Accurate design of a grounding system requires an accurate assessment of the site’s soilconditions. However, even a small site will often have widely varying soil resistivityfrom one spot to another. Many measurements must be made, and samples of the soilmust be taken from several test locations and analyzed for both moisture and temperature.The actual measurement technique using the four-point tester is illustrated in Figure 1.Note that at least 10 measurements are recommended to properly assess the site soilresistivity. Large areas require more measurements, but 10 should be the minimum.Only soil to a depth of 10 feet, or 3 meters, needs to be tested in most situations. In veryunusual situations, more specifically in very dry areas or under extreme conditions, referto the test meter instructions for the procedure required to assess resistivity as a functionof depth. Table 1 lists some common soils and their resistivity.

When the measurements are completed, the average resistivity should be calculated, thetemperature measured, and the moisture content assessed. Moisture content is assessedby taking soil samples at depths of about 1 foot, or 1/3 meter, and putting it in a plasticbag immediately. Weigh the sample first, dry it out completely, and weigh it again.Express the difference in percentage. The result is the percent moisture by weight.

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Figure 1: Ground resistance testing

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Figure 2: The influence of moisture content

Figure 3: The influence of temperature

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The temperature measurement and the percent of moisture should be compared withFigures 2 and 3, respectively, to determine the actual soil resistivity under optimum andworst-case conditions. This will permit a calculation of the range in grounding resistanceachievable in that soil with the final system design. The required design data has nowbeen defined, and the design process can start from a solid foundation; i.e., the requiredparameters. The initial calculations should be based on the measured resistivity, and thefinal system design must take into account the extreme moisture and temperaturevariations. Within the USA, variations of about 250 percent are normal for conventionalsystems.

Design Step 1: Calculating the Requirements with Conventional Rods

From work performed by many experts, we know that the resistance of any groundingelectrode R1 may be estimated from:

Where:ρ = Soil resistivity in ohm-metersL = The electrode length in feetd = The electrode diameter in inches

If the soil resistivity averaged 100 ohm-meters, then the resistance of one ¾-inch by 10-foot electrode to true earth would be found to be 0.321p or 32.1 ohms. Obviously, that ishigh and most likely not acceptable. The next step is to determine how many of theserods are required to achieve a given goal.

Design Step 2: Calculating the Required Number

Where:R1 = Resistance of one rodK = The Combining Factor =N = The Number of Rods Required (when they are properly deployed)

actual design process. Since making an electrical connection to earth involves aconnection between a conductor and a semiconductor, it is not point-to-point contact butconductor-to-area contact. That is, making an electrical contact with earth requires asignificant volume of that earth around the conductor to complete the connection. Thiscan best be illustrated by considering the implications of the data presented by Figures 4

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L

din English units1 1915

961= −

ρ.

ln ( )

RR K

NN = 1

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Figure 4: Measured resistance change as a function of distance from the rod

Figure 5: Shells of resistance (around a vertical rod)

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Figure 6: The interfacing hemisphere

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and 5, which illustrate the result of measuring the change in resistance of equal segmentsof earth along any radial from a driven rod. Notice that the change in measuredresistance decreases exponentially with distance from the road, as illustrated by Figure 4.

Notice also that at about 1.1 times the length of the rod in earth, the change in resistancebecomes negligible. This indicates that its connection to earth is nearly complete.Actually, about 95% + 2% of the connection has been completed.

From these data, we know that for every rod driven into earth, an interfacing hemisphereof that earth is required to complete the electrical connection. The diameter of thathemisphere is approximately 2.2 times the length of the rod (L) in earth, as illustrated byFigure 6. When more than one rod is required, they should be spaced no closer than 2.2times the length of that rod in any direction. If multiple rods are driven too closetogether, those connections are considered incomplete; because all rods do not have acomplete interfacing hemisphere, and the effectiveness of those additional rods arereduced proportionately and, in reality, wasted.

To illustrate, consider Figure 7. Using those data, if we assume that one 10-foot rodprovides a resistance to earth of 100 ohms, then 10 rods at 5-foot intervals reduces theresistance to about 28 ohms. At 10-foot intervals, it is about 18 ohms; and at 22-footintervals (2.2 times their length), it is down to only about 8 ohms. There are the samenumber of rods, but properly spaced.

One other factor of concern is length of the grounding electrode. It is common practiceto keep extending the length of the electrode into the earth to lower its resistance. Thispractice is not recommended for most situations, as will become apparent from anevaluation of the data offered by Figure 8. An analysis of these data show that as theelectrode is extended into the earth, the percent reduction in resistance to earth per unitlength of rod becomes exponentially less with each increment of length. For example, toreduce the resistance of a 10-foot rod in a given soil to half the 10-foot value, it requiresextending that rod to 100 feet in that same soil. Further, it is unusual for the soil toremain constant as a function of depth. Most often, resistivity increases with depth,compounding the problem. A reasonable conclusion from these data is this: Many shortrods (six to ten feet in length) are usually more productive than a few long ones inachieving a given resistance to earth. Remember: the longer the rod, the greater theinterfacing hemisphere diameter.

NOTES:

1. Often one or a few very long rods are tried, and because of measurement errors,the user thinks he has achieved a low resistance when, in fact, he probably hasnot.

2. More often than not, grounding systems are subjected to transient phenomenawhere the di/dt can exceed 100 kA/microseconds. In these situations, the surgeimpedance is the important factor, not the DC resistance. Using short, large-

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diameter rods such as the Chem-Rod is far more effective in reducing the surgeimpedance. Use of Ground Augmentation Fill (GAF) reduces the impedance further.

Figure 7: Rods too close

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Given the foregoing guidelines, the design process can start with the proper spacingcriteria, using the expression given to calculate the resistance of one rod plus that for Nrods (RN) as above. When there is limited space, as there usually is, that must be takeninto account. To optimize the calculations, a personal computer may be programmed to:

1. Vary electrode length and the size of the interfacing hemisphere in concert.

2. Vary the location of these interfacing hemispheres to gain maximum use of availableland area.

3. Conduct a trade-off analysis between the number, length, and location of theelectrode-hemisphere combination.

When the lowest resistance combination has been found and yet is too high to satisfy therequirements, other factors must be considered. Limits have been reached which areestablished by the area of land available and the soil resistivity. Doubling the rods in thatarea will reduce the resistance by no more than about 10%. To reduce the groundingresistance, either more land is required or the resistivity of the available land soil must belowered. Soil resistivity can be lowered, and the related cost is usually much less thanthe cost of more land.

Ground Rod Resistance vs. Length0.75 Inch Diameter Ground Rod

0.00

200.00

400.00

600.00

800.00

1,000.00

1,200.00

1,400.00

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Rod Length, Feet

Res

ista

nce,

Ohm

s 100 ohm-meter soil

500 ohm-meter soil

1000 ohm-meter soil

Data Calculated from IEEE Single Ground Electrode Equation

Figure 8: Ground rod resistance versus length

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Dealing With High Soil Resistivity (Soil Conditioning)

Soil resistivity is a function of several factors. These include the type of soil, moisturecontent, temperature, mineral content, granularity, and compactness. Usually, moistureand mineral content are the only factors that can be influenced by any practical controlconcept. Figures 2, 3, and 9 illustrate the influences of moisture, temperature, andmineral content, respectively. Controlling temperature is usually not practical, butreducing sensitivity to temperature is. Moisture can be controlled where required, but themineral content has the most dramatic influence, as illustrated by Figure 9. The highermineral content also reduces soil sensitivity to moisture content. It is, therefore, obviousthat increasing the mineral content is the first step to be considered in soil conditioning.

Soil conditioning is the process of adding the right amount of metallic salt into the soil—uniformly —to achieve the required conductivity. Various methods have been attemptedto accomplish this objective. In Table 2, the results of some examples are contrasted andcompared with a conventional ¾-inch by 10-foot conventional rod in five different soils.The top line is the conventional rod alone; the second and third lines involve aconventional rod in soil that was hand mixed with salt (NaCl) and measured after one andthree years. The fourth is a two-inch diameter copper tube filled with NaCl, providedwith air breathing holes at the top and leaching holes at the base. It extracts moisture from the air (if it has any) and forms a saturated solution of the metallic saltswhich are leached out as the solution is formed. In dry areas where conditioning isneeded the most, it is about as effective as the equivalent length of empty two-inch pipe.

In 1984, LEC introduced the Chem-Rod to the marketplace. Its design results in a moreuniform distribution of the metallic salts throughout the electrode interfacing hemisphere.It absorbs moisture from the soil and air and leaches the metallic salts out at all levels,conditioning most of the interfacing hemisphere and using the available soil moisture.The metallic salts are selected on the basis of application and location.

From the Table 2 data, two factors are evident:

1. The Chem-Rod provides a much lower resistance to earth than any other optionavailable.

2. That resistance is much more stable; it varied by only 40%, while the other optionsvaried from 200 to 250%.

NOTE:

The Chem-Rod resistance is dependent on the conditioning process. When the metallicsalts migrate slowly through the soil, it may take up to six months for the process tostabilize at the lower resistance.

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The higher the soil’s resistivity, the longer the conditioning process takes. The next stepin the design process is to calculate the resistance R1 using the Chem-Rod data, againusing the same variables as before but with the Chem-Rod parameters. The resistanceof one Chem-Rod after conditioning is completed is:

*Can vary between 0.5 and 0.05% but normally does not exceed 20%after six months. The higher the initial soil resistance, the greater thepercentage of reduction. High-density soil will take much longer tostabilize.

If this still does not meet the objective, or if time is a critical constraint, then moreextensive steps are in order. Still, we must deal with the soil within the interfacinghemisphere (IH).

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LCR = −

ρ1915

96

2 6251 0 2

.ln

.( . *)

Figure 9: The influence of salt on soil resistance

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Replacing Soil in the Interfacing Hemisphere (IH)

Since about 95% of the grounding resistance of a given electrode is determined by thecharacter of the soil within the IH, it is obvious that replacing that soil with a moreconductive soil could achieve the desired objective. However, that action may proveimpractical. A more practical action may be to replace only that part of the soil thatexercises the greatest influence on the ultimate grounding resistance and to use the lowestresistivity “soil” available. Figure 10 presents a plot demonstrating the influence of thesurrounding soil as a function of the radius of what we choose to call the “CriticalCylinder” (that is, the amount of backfill to be placed around the grounding electrode).Notice that 52% of the connection is completed by a 12-inch-diameter Critical Cylinder,and 68% of that connection is completed by a 24-inch Critical Cylinder. The mostproductive option is therefore expected to be between these two diameters.

The next step is to select the proper backfill or soil to replace that within the CriticalCylinder. The options include top soil that is 10 ohm-meters, betonite that is 2.5 ohm-meters, and various forms of special mixes. LEC has chosen to use a special mix toovercome the negative effects of various conductive clays such as betonite and provide a

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Figure 10: The influence of soil within the critical cylinder

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very conductive backfill. The volume of bentonite varies by 300% between wet and dryand does not absorb or pass through metallic salts easily. The LEC backfill is calledGrounding Augmentation Fill (GAF). The resistivity of the GAF is between 0.4 and 0.8ohm-meters and is very osmotically conductive, thus assuring the required moisturesupply. Refer to Table 3 for a list of options and their parameters.

Then one needs to calculate the impact of using GAF backfill within the previouslydefined Critical Cylinder (Figure 10). Installation involves first augering the appropriatehole in the interfacing hemisphere, inserting the Chem-Rod, then backfilling that holewith 100% GAF, wetting it as it is installed. The resulting immediate resistance (beforeconditioning) of one ¾-inch by 10-foot rod to earth may be calculated from thefollowing:

The 12-inch cylinder: R1 = .321 (.52ρGAF + .48ρ)The 24-inch cylinder: R1 = .321 (.68ρGAF + .32ρ)

The resulting long-term resistance should approach about 0.2 R1 for the average site.

As an example, consider a 10-foot Chem-Rod in a 24-inch hole, backfilled with GAF,when the soil in the area is 100 ohm-meters. That resistance (R1) is now:

RCR-10 = .321 [(.68)(.8) + (.32)(100)] = 10.5 ohms before the Chem-Rod

conditions the local soil and no more than 2.2 ohms after an average ofthree to six weeks of conditioning. Some dense soils require a muchlonger period of time.

Again, if one rod does not achieve the desired goal, multiple rods must be considered asbefore (refer to prior section).

Making Up the Required Moisture

As indicated by the formerly referenced Figure 9, if there is too little moisture in theinterfacing hemisphere for any electrode, the resistance will be proportionately higher, tothe point where the connection is virtually non-existent. If the connection is needed andlittle or no moisture is present, moisture must be provided. This can be accomplishedvia:

1. The LEC Autonomous Automated Moisturizer shown in Figure 11, if there is no localsource of water, or

2. Using the local water source for automatic water injection and control, as illustratedby Figure 12.

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Either of these sources can be used to inject the required moisture into the Chem-Rod.The Chem-Rod will add moisture and minerals both to the soil that surrounds it and thenout beyond the GAF. Both of these options are in use and have been proven to satisfy therequirements.

Permafrost and Temperature Problems

Frost levels and freezing temperatures have always been a problem situation forgrounding systems. It is true that controlling temperature may not be practical; however,it is possible to minimize the effects. Using permafrost as the “worst-case”, consider thefollowing situation: Figure 3 illustrates the impact of low temperatures on resistivity, based on conventional soils. Tests performed by the U.S. Corps of Engineers in Alaskahave proven that the resistance of a simple conventional electrode can be lowered byfactors of over twenty (i.e., 1/20). The treatment involved simply replaces some of the

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Figure 11: LEC autonomous solar motivated moisturizer

Figure 12: The utility motivated moisturizing concept

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soil in close proximity to the electrode. The Critical Cylinder of a ground rod isillustrated by Figure 13.

The tests performed by the Corps of Engineers, using a treated Critical Cylinder, yieldeddramatic results as illustrated by Figure 14. Notice that the resistance of a singleuntreated rod reached about 20,000 ohms, while the GAF-treated rod reached only 1,000ohms, one-twentieth of the conventional driven rod.

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Figure 13: The critical soil cylinder within an interfacing hemisphere

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Assessing the Results

Finally, it is essential that the end results are measured correctly. All too often, it hasbeen assumed that a single measurement may be made from one point of a groundingsystem, regardless of its size, using the components that the manufacturer provides withhis tester. In fact, that is seldom correct. The instrument usually is supplied with thecomponents to test one or two rods only, provided the rods are not too long. For longrods and large grounding systems, it is necessary to move out away from the site asignificant distance, sometimes up to one or more kilometers. The Fall-of-Potentialmeasurement technique is the most common and the most accurate technique availabletoday, if it is implemented properly. Figure GT-1 illustrates the technique for single rodsand small systems. Figure GT-2 illustrates one technique for large systems.

Conclusions

This paper has presented a logical approach to the design of a required earthing system,approaching it step by step. It starts with the site soil assessment and proceeds asfollows:

1. Estimate the resistance of one conventional rod.2. Estimate the resistance of multiple rods that will fit.3. Estimate the resistance of one Chem-Rod after conditioning.4. Estimate the resistance of multiple Chem-Rods after conditioning.5. Review the moisture content and temperature ranges and modify as required.

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Figure 14: Ground rods in permafrost: Point Barrow, Alaska

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Rev. 7/97

Figure GT-1: Ground resistance testing

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Figure GT-2: Ground resistance testing

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TABLE 1

SOIL RESISTIVITIES(Approximate Ohm-Meters)

Description Median Minimum Maximum

Topsoil, loam 26 1 50Inorganic clays of high plasticity 33 10 55Fills – ashes, cinders, brine wastes 38 6 70Gravelly clays, sandy clays, silty clays, lean clays 43 25 60Slates, shales 55 10 100Silty or clayey fine sands with slight plasticity 55 30 80Clayey sands, poorly graded sand-clay mixtures 125 50 200Fine sandy or silty clays, silty clays, lean clays 190 80 300Decomposed gneisses 275 50 500Silty sands, poorly graded sand-silt mixtures 300 100 500Clayey gravel, poorly graded gravel, sand-clay mixture 300 200 400Well graded gravel, gravel-sand mixtures 800 600 1,000Granites, basalts, etc. 1,000 --- ---Sandstone 1,010 20 2,000Poorly graded gravel, gravel-sand mixtures 1,750 1,000 2,500Gravel, sand, stones, little clay or loam 2,585 590 4,580Surface limestone 5,050 100 10,000

Notes: 1. Low-resistivity soils are highly influenced by the presence of moisture. 2. Low-resistivity soils are more corrosive than high-resistivity soils.

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TABLE 2

GROUNDING RESISTANCE OF VARIOUS ELECTRODES

GROUNDINGELECTRODE

MEASUREDSOIL

RESISTIVITY(OHM-METER)

ELECTRODERESISTANCE

(OHMS)

VARIATIONOVER A YEAR

Copper-Clad 9 7.2Rod 62 22(3/4”x10’) 270 65

3.7K 43030K 10K 2.5

Rod in Manually 9 2.3Salted Soil - 62 18First Year 270 44

3.7K 35030K 1.5K 2.0

Rod in Manually 9 5.0Salted Soil - 62 30Third Year 270 80

3.7K 40030K 3K 2.0

Air-Breathing 9 0.5Rod 62 9

270 223.7K 24030K 2K 2.0

Chem-Rod 9 0.262 2

270 103.7K 9030K 1K 0.4

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TABLE 3

SOIL ENHANCEMENT OPTIONS

1. Conductive Concrete30 to 90 ohm-metersSubject to ice and corrosive effects

2. Bentonite2.5 ohm-metersHighly variable with respect to moisture (300%)

3. Carbon-Based Backfill Materials0.1 to 0.5 ohm-metersWater-retention capability inferior to clays

4. Clay-Based Backfill Materials (GAF)0.2 to 0.8 ohm-meters depending on moisture contentHigh water-retention capability

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LightningEliminators& Consultants, Inc.

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www..lightningeliminators.com

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