2_chrisholstpaperfinal

Session Two: Fundamentals of Earth Electrode Design

Earthing, Lightning & Surge Protection Forum IDC Technologies 1


Author: Rohit Narayan Presenter: Chris Holst

ERICO

Abstract: This paper captures the fundamental principles of earthing electrode design, earthing system resistance and soil resistivity measurements and computations. It will form a basis for understanding the reasoning behind incumbent earthing practices and will act a guideline to engineers trying to grasp the essence of earthing system design.

Introduction

The fundamentals of earthing electrode design, are seldom taught as part of the curriculum in universities and colleges. These days we are also seeing a diminishing level of formal training offered in the work place on this subject. As such, most new engineers would have to self-educate on this subject. A lot of information is available within various standards about earthing electrode resistance and soil resistivity testing. A remarkable amount of information is also available in older publications from Power Utilities around the world. This paper attempts to cover the fundamentals of earthing electrode design in a single document and is good starting point for someone who is a novice on the subject but also for engineers who are looking for a quick reference. It will cover:

1. Explanation of what soil resistivity and earth resistance is. 2. Effect of soil resistivity and electrode dimension on the earth resistance. 3. Calculation of earth resistance from known dimensions and soil resistivity

measurements

4. Discussion on parallel earth electrodes and calculation of resistance for parallel ground rods.

5. Methods of testing soil resistivity and earth resistance. 6. Post installation measurement of step and touch voltages 7. Discussion on the advantages and disadvantages on common earth

improvement techniques including the use of soluble compounds, Bentonite, GEM and chemical earth rods.

8. Examples of how modern software can be used to make complex computations of step and touch voltages.

9. Graphics showing commonly used grounding components including ground rods, couplers, earth mats and connectors.



1) Distribution of Voltage in Ground- Sheath Theory To understand grounding principles the first thing that we will consider is how the voltage is distributed in the mass of earth when a current is injected into a vertical ground rod. The intuitive understanding of this will enable us to better appreciate why electrode designs are done a certain way. For example, this will help us to understand why we would use deeper earth electrodes or radial electrodes.

The soil is not uniform in its conductivity and this factor must be accounted for in the design of the earth electrode system. However, to develop an understanding of the principles of the current flow and the voltage distribution in the soil we will use a graphical model assuming uniform soil. This is called the sheath theory of expanding soil conductivity. In Figure 1 the hemispherical sheaths depict imaginary equipotential lines formed in the soil when a current is injected into a vertical ground rod.

2) The Electrode Resistance

The electrode resistance is that resistance offered to the flow of current into the ground down to the expanse where the resistance of the ground becomes so low that it becomes negligible.

Consider the cut away section of the sheaths surrounding the earth electrode in Figure 1. In simple terms this resistance can be explained by the following relationship.

R 1/A

where R is the resistance and A the area of each of the sheaths.

As the distance from the ground rod increases, the surface area of the sheaths, get larger. This means that at some distance, the additional soil area has negligible effect on the ground resistance.

It is for this reason, when measuring earth resistance to a remote earth, the test only needs to be confined to few tens, perhaps a few hundred of metres. For example: When testing a single 2-metre electrode, the test is referenced to remote earth at



distance of about 40 metres. Any greater reference distance than this would add insignificantly to the resistance. Testing of earth resistance is discussed in more detail later in this paper.

It is easier to see which dimensions of the earth electrode will have a greater impact on the electrode resistance, if we consider what happens to the area of the hemispherical sheaths. In Figure 2, we see that when the electrode is made longer, the area increases significantly. Hence 1/A reduces giving us a reduction in the earth resistance. However, if the diameter of the ground rod is increased, this offers very little change in the area of the hemispherical shells and hence little changes in the resistance.

This intuitive understanding can be extended to horizontal electrodes. It can be seen in Figure 3 that making a horizontal electrode longer will increase the surface area of the sheaths surrounding it. Hence longer electrodes rather than deeper electrodes, will give a greater reduction in the electrode resistance.

Another factor that will have an impact on the earth resistance is the conductivity or resistivity of the soil. It is this factor that makes it impossible to have a one size fits all earthing design for different sites.

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3) Soil Resistivity

Soil resistivity is another name for the specific resistance of the soil. It is measured in ohm-metres or ohm centimetres. An ohm-metre is that resistivity of the soil when it has a resistance of 1 ohm between opposite faces of a cube with 1 m sides.

Resistance is directly proportional to soil resistivity. This relationship is not as easy to compute in real life as it may sound, because soil resistivity will inevitably vary with depth. The second difficulty in dealing with different locations is that the resistivity can vary greatly across a large site and from one site to another.

The tables below give an idea of the resistivity of several mediums that are of interest for the design of grounding system.

MATERIAL TYPICAL RESITIVITY

Copper 1.72 x 10-8 ohm.m

GEM, Material 0.12 ohm.m

Bentonite 2.5 ohm.m

Concrete 30 to 90 ohm.m

Factors that will affect the resistivity of the soil are the soil type, compactness, chemical composition, temperature and water content. Figure 4 shows the effect of moisture content and temperature on soil resistivity.



4) Measurement of Soil Resistivity

There are several methods of measuring the soil resistivity. These include

1) Wenner Array 4 point Method 2) Schlumberger Array 3) Driven Rod Method

The Wenner Array method is discussed in this paper because this is the most common method of measuring soil resistivity. The scope of this document does not allow detailed discussion on other soil testing methods.

Using the Wenner Array method, four small electrodes (auxiliary probes) are placed in a straight line at intervals of a, to a depth of b. A current is passed through the outer two probes, and the potential voltage is then measured between the two inner probes. A simple Ohms Law equation determines the resistance. From this information, it is now possible to calculate the resistivity of the local soil. For most practical circumstances, a is twenty times larger than b, where we can then make the assumption that b=0.

Then the Resistivity () is given by: = 2 a Re

Where: = resistivity of the local soil (-m) a = distance between probes (m) b = depth of probes into the ground (m) Re = resistance value measured by the testing device ()

These values give an average resistivity of the soil to a depth a. It is recommended that a series of readings be taken at different values of a, as well as in a 90o turned axis. It is a good practice to tabulate or plot the results because that gives a good idea of how the resistivity is changing with depth and will indicate the best type of earthing electrode system to design for the subject site.

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For example, if the resistivity is very high at the top 3 metres but drastically drops after that depth, the designer would specify electrodes that are driven or drilled to deeper than 3 metres. Conversely if the resistance does not improve beyond a certain depth, say 2 metres, then horizontal electrodes may be considered in the earth electrode design because installing deeper electrodes would yield diminishing returns.

Figure 6 shows a typical record sheet for resistivity measurements. Experience has shown that many people performing soil resistivity tests do not have a full appreciation of the extent to which the test needs to be carried out. It is often noted that only a single or a handful of values are measured. It is recommended that for the design of an earthing electrode system a comprehensive set of results in the range from 2-40metres should be requested.

SPACING a Measured Value of Re

Resistivity R = 2 a Re

2 4 6 8 10 12 14 16 18 20 25 30 35 40

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5) Calculating the Earth Electrode Resistance of a Single Rod.

The resistance from the electrode to remote earth can either be calculated using empirical formulae, by using nomograms or by the use of software.

Examples of available formulae are contained in AS1768 Lightning protection Standard, Appendix C. The formulae listed below, extracted from AS1768, are the two most commonly used.

Traditionally software programs have been able to carry out earthing electrode system designs using two-layer models of soil resistivity. That means that resistivity measured had to be averaged out to two values with corresponding depths. Modern software can take multiple layer resistivity values as inputs.

In fact, the real value of software is not so much in computing resistance values for single or a few electrodes as this can be done easily with formulae. Software can be powerful tools for calculating resistance of multiple electrodes, step and touch voltages and also simulating fault current injection.

Another method of calculating the resistance of a single earth rod, when the dimensions and the resistivity are known is using nomograms. In the example in Figure 7, a 7m earth rod, of diameter 10mm will produce a resistance of 7.6 ohms if the reading from the Wenner 4 point test is 1 ohm.



6) Calculating the Earth Electrode Resistance of Multiple Ground Rods

When electrodes are used in parallel it may seem at first that the resistance could be calculated by simple equation 1/R = 1/R1+ 1/R2+ 1/R3. However when one takes a closer look at the sheath theory discussed earlier, it becomes evident that the spacing between the electrodes may have some impact on the combined resistance. This is because the hemispherical sheaths of the electrodes will overlap each other and the overlap area has to be considered in the calculations. In the extreme case, if two electrodes are as close together as physically possible the size of the sheath offered by them will be similar to the sheath offered by one electrode. The combined resistance of the two electrodes will be similar to that of one electrode if they are installed very close together.

Figure 8 Parallel Ground Rods

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Rules of thumb and utilisation factors are used in everyday calculations to quickly compute parallel resistances without excessive analysis.

For example when two electrodes are placed 1 electrode length apart, 85% utilisation of their parallel resistance is achieved. When these electrodes are 2 electrodes apart, 92 % utilisation is achieved. We sometimes see a rule of thumb used in practice that states that the electrode spacing needs to be at least twice the electrode depth, based on this utilisation.

Prior to the existence of software to carry out calculations, nomograms were the incumbent method of calculating resistance of multiple electrode systems. There is no reason that these cannot be used today for quick calculations.

Figure 9 shows a nomogram that can be used to design a multiple electrode system if the resistance of one electrode was known through calculation or measurement. This has been extracted from the Handbook of the Electricity Authority of NSW, 1973.

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The calculation of the electrode resistance for multiple ground rod systems is quite easy when using modern software. It is essentially a matter of inputting the soil resistivity, electrode dimensions and the grid size layout and it will churn out a number without too much fuss.

7) Measurement of Electrode Resistance

When an electrode system has been designed and installed, it is usually necessary to measure and confirm the earth resistance between the electrode and true Earth. The most commonly used method of measuring the earth resistance of an earth electrode is the 3-point measuring technique shown in Figure 10. This method is derived from the 4-point method, which is used for soil resistivity measurements.

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The 3-point method, called the fall of potential method, comprises the earth electrode to be measured labelled E (electrode) and two other electrically independent test electrodes, usually labelled P (potential) and C (current). These test electrodes can be of lesser quality (higher earth resistance) but must be electrically independent of the electrode to be measured. An alternating current (I) is passed between electrodes E and C through the soil while a voltage measurement is taken between E and P. The resistance from E to true earth, remote earth or general mass of earth is simply calculated using Ohms Law; Rg = V/I, internally by the test equipment.

When performing a measurement, the aim is to position the auxiliary test electrode C far enough away from the earth electrode under test so that the auxiliary test electrode P will lay outside the effective resistance areas of both the earth system and the other test electrode (see Figure 11). If the current test electrode, C, is too close, the resistance areas will overlap and there will be a steep variation in the measured resistance as the voltage test electrode is moved. If the current test electrode is correctly positioned, there will be a flat (or very nearly so) resistance area somewhere in between it and the earth system, and variations in the position of the voltage test electrode should only produce very minor changes in the resistance figure.



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Measurement accuracy can be affected by the proximity of other buried metal objects to the auxiliary test electrodes. Fences, buildings, buried metal pipes or even other earthing systems can interfere with the measurement and introduce errors. Often it is difficult to judge, merely from visual inspection of the site, a suitable location for the tests stakes and so it is always advisable to perform more than one measurement to ensure the accuracy of the test.

Fall of Potential Method This is one of the most common methods employed for the measurement of earth resistance and is best suited to small systems that dont cover a wide area. It is simple to carry out and requires a minimal amount of calculation to obtain a result.

The outer test electrode, or current test stake, is driven into the ground a good distance away from the earth system, This distance will depend on the size of the system being tested and the inner electrode, or voltage test stake, is then driven into the ground mid-way between the earth electrode and the current test stake, and in a direct line between them.

Maximum dimension across earth system, m

Distance from electrical centre of earth system to

voltage test stake, m

Minimum distance from electrical centre of earth

system to current test stake. m

1 15 30

2 20 40

5 30 60

10 43 85

20 60 120

50 100 200

100 140 280

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The Fall of Potential method incorporates a check to ensure that the test electrodes are indeed positioned far enough away for a correct reading to be obtained. It is advisable that this check be carried out, as it is really the only way of confirming the accuracy of the test results.

To perform a check on the resistance figure, two additional measurements should be made; the first with the voltage test electrode (P) moved 10% of the original voltage electrode-to-earth system separation away from its initial position, and the second with it moved a distance of 10% closer than its original position, as shown in Figure 13.

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If these two additional measurements are in agreement with the original measurement, within the required level of accuracy, then the test stakes have been correctly positioned and the DC resistance figure can be obtained by averaging the three results. However, if there is substantial disagreement amongst any of these results, then it is likely that the stakes have been incorrectly positioned, either by being too close to the earth system being tested, too close to one another or too close to other structures that are interfering with the results. The stakes should be repositioned at a larger separation distance or in a different direction and the three measurements repeated. This process should be repeated until a satisfactory result is achieved.

The Slope Method

This method is suitable for use with large earthing systems, such as sub-stations. It involves taking a number of resistance measurements at various earth electrode to voltage electrode separations and then plotting a curve of the resistance variation between the earth and the current. From this graph, and from data obtained from tables, it is possible to calculate the theoretical optimum location for the voltage electrode and thus, from the resistance curve, calculate the true resistance.

It is similar to the fall of potential method but several readings are taken by moving the inner test electrode (P) from very close to the earth grid to the position of the outer test electrode. The readings obtained are then plotted on a graph. Figure 14 shows and example of the graph obtained. It can be observed that at approximately 60% of the distance the slope is the gentlest and

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the resistance corresponding to this is the true resistance of the electrode being measured. In this case it is 20 ohms.

For full details of this method, refer to paper 62975, written by Dr G.F. Tagg, taken from the proceedings of IEE volume 117, No 11, Nov. 1970.

8) Soil Improvement Techniques

In areas of very high soil resistivity, low resistance cannot be achieved through the use of buried and driven earth electrodes alone. There are four techniques commonly used for the improvement of soil. These include:

Use of soluble additives like common salt or copper sulphate Use of bentonite or bentonite-gypsum mixture Use of GEM, Ground Enhancement Material Use of chemical ground rods.

a) Soluble salts will often give good short-term results and are sometimes necessary. These are soluble in nature and it is envisaged that these will leach away into the soil over time. No long-term studies have been done to show their performance over extended periods.

b) Bentonite and Bentonite-Gypsum mixes are relatively cheap in Australia and available in abundance. Some caution needs to apply when these are used in very dry soil conditions as exhibited in large parts of Australia. The IEEE Std 80 2000 Section 14.5 States that

Use of bentonite, a natural clay containing the mineral montmorillionite, which was formed by volcanic action years ago. It is non-corrosive, stable, and has a resistivity of 2.5

m at 300% moisture. The low resistivity results mainly from an electrolytic process between water, Na2O (soda), K2O (potash), CaO (lime), MgO (magnesia), and other mineral salts that ionize forming a strong electrolyte with pH ranging from 8 to 10. This electrolyte will not gradually leach out, as it is part of the clay itself. Provided with a sufficient amount of water, it swells up to 13 times its dry volume and will adhere to nearly any surface it touches. Due to its hygroscopic nature, it acts as a drying agent drawing any available moisture from the surrounding environment. Bentonite needs water to obtain and maintain its beneficial characteristics. Its initial moisture content is obtained at installation when the slurry is prepared. Once installed, bentonite relies on the presence of ground moisture to maintain its characteristics. Most soils have sufficient ground moisture so that drying out is not a concern. The hygroscopic nature of bentonite will take advantage of the available water to maintain its as installed condition. It may not function well in a very dry environment, because it may shrink away from the electrode, increasing the electrode resistance.

c) GEM, or Ground Enhancement Materials usually are poured into drilled holes as a slurry backfill for the electrodes and set hard over time. This material contains a small percentage of bentonite and cement, but mostly constitutes a conductive form of carbon. Formulae used for calculating the resistance for concrete encased electrodes but using the resistivity value of GEM, can be used to calculate or predict resistances. It is often not possible to predict the resistance when using other types of soil improvement techniques. Studies have shown that GEM can produce



excellent long-term results in a variety of soil conditions. The IEEE Std 80 2000 Section 14.5 States that

Ground enhancement materials, some with a resistivity of less than 0.12 m (about 5% of the resistivity of bentonite), are typically placed around the rod in an augured hole or around grounding conductors in a trench, in either a dry form or premixed in a slurry. Some of these enhancement materials are permanent and will not leach any chemicals into the ground. Other available ground enhancement materials are mixed with local soil in varying amounts and will slowly leach into the surrounding soil, lowering the earth resistivity.

d) Chemical rods are sometimes used in extreme cases of very high resistivity. Chemical-type electrodes consist of a copper tube filled with a salt. Holes in the tube allow moisture to enter, dissolve the salts, and allow the salt solution to leach into the ground. These electrodes are installed in an augured hole and typically back-filled with soil treatment. Periodic refilling is required otherwise the electrode will become ineffective.

9) Step and Touch Voltages

Step and touch voltages are a concern that needs to be incorporated into the electrode design and testing regime at substations, transmission towers, areas frequented by people and major sites like wind farms. Figure 15 shows how touch and step voltages can develop in the near vicinity of a structure, which encounters a fault current. It also depicts how this voltage can be mitigated, by designing a ground grid in the area of concern.

IEEE Std 80-2000 in Annex B, C and D provides the methodology and the calculations for computing the maximum step and touch voltages using formulae.

It has become a common practice these days to compute maximum and step voltages, using software programs, most of which use the formulae from IEEE Std 80. Modern programs can use two layer soil models or multi layer soil models referring to the variation in soil resistivity used as a possible input into the software.

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The example below has been generated by commercially available software and provided by PhysElec Solutions, courtesy of Dr Franco DAllesandro.

Grounding Design Example: Generated by PhysElec Solutions

Type:

The earth grid being simulated is made of horizontal conductors (Grid size is 50 x 50 m with a compression ratio of 0.5, buried at a depth of 0.5 m below ground level) and 3 m ground rods at all node points.

Soil model used:

LAYER TYPE REFLECTION RESISTIVITY THICKNESS No. COEFFICIENT (ohm-meter) (METERS) ----- ------ ------------- ------------- -------------

1 Air 0.00000 0.1E+11 Infinite 2 Soil -0.999990 100.000 3.00000 3 Soil 0.666667 500.000 20.0000 4 Soil 0.818182 5000.00 Infinite

Soil type: HORIZONTAL MULTILAYER

Fault current specified: 2000 amperes




-30

-18

-6

6

18

30Y AXIS

(METERS)

-30 -18 -6 6 18 30

X AXIS (METERS)

48

36

24

12

0

-12

Z AX

IS (M

ETE

RS)

SOIL SURFACE

3-D View of Conductors

Grounding Grid (3D - View) [ID:Scenario1]

Results:

Resistance of Electrode System: 6.68 ohms

Main Electrode Potential Rise (GPR max.): 13360 volts

1334813308132671322613185131451310413063130221298212941129001285912819127781273712696126561261512574




Safety report:

MAXIMUM MAXIMUM ALLOWABLE COMPUTED OUTCOME VALUE (V) VALUE (V)

---------------------------------------------------------------------

--

TOUCH VOLTAGE (on grid): 542 490 ACCEPTABLE STEP VOLTAGE (on grid): 1700 364 ACCEPTABLE

All computed voltages in the observation zone are below the maximum allowable values.

Touch voltage profile

1524 .. 16081440 .. 15241356 .. 14401272 .. 13561188 .. 12721104 .. 11881020 .. 1104936 .. 1020852 .. 936768 .. 852684 .. 768600 .. 684516 .. 600432 .. 516348 .. 432264 .. 348180 .. 26495 .. 18011 .. 95


Step voltage profile



345 .. 364326 .. 345307 .. 326287 .. 307268 .. 287249 .. 268230 .. 249211 .. 230192 .. 211173 .. 192154 .. 173135 .. 154116 .. 13597 .. 11678 .. 9759 .. 7840 .. 5921 .. 402 .. 21

10) Conceptual Design of an Earth Electrode System for a Substation

The IEEE80 Standard for Safety in AC Substation Grounding is a most common document used to assist with the design of an earthling system. Other documents like the Australian ENA Earthing Guide and guidelines from local authorities should be used in conjunction with this standard. Conceptual analysis of a grid system usually starts with inspection of the substation layout plan, showing all major equipment and structures. To establish the basic ideas and concepts, the following points may serve as guidelines for starting a typical grounding grid design:

a) A continuous conductor loop should surround the perimeter to enclose as much area as practical. This measure helps to avoid high current concentration and, hence, high gradients both in the grid area and near the projecting cable ends. Enclosing more area also reduces the resistance of the grounding grid.

b) Within the loop, conductors are typically laid in parallel lines and, where practical, along the structures or rows of equipment to provide for short ground connections.

c) A typical grid system for a substation may include 120mm2 bare copper conductors buried 0.30.5 m below grade, spaced 37 m apart, in a grid pattern. At cross-connections, the conductors would be securely bonded together. Ground rods may be at the grid corners and at junction points along the perimeter. Ground rods may also be installed at major equipment, especially near surge arresters. In multilayer or high resistivity soils, it might be useful to use longer rods or rods installed at additional junction points.

d) This grid system would be extended over the entire substation switchyard and often beyond the fence line. Multiple ground leads or larger sized conductors would



be used where high concentrations of current may occur, such as at a neutral-to-ground connection of generators, capacitor banks, or transformers.

e) The ratio of the sides of the grid meshes usually is from 1:1 to 1:3. Frequent cross-connections have a relatively small effect on lowering the resistance of a grid. Their primary role is to assure adequate control of the surface potentials. The cross-connections are also useful in securing multiple paths for the fault current, minimizing the voltage drop.

Figure 16 shows the typical layout of an earth electrode for an electrical substation.

11) Post Installation Measurement of Step and Touch Voltage Measurements.

Most good quality ground testers can be used as an aid to measure step and touch voltages at an installed site. The measurements are made directly by the use of test probes supplied with the equipment. A reference probe is connected to the main earth bar of the facility when these tests are being done. For example the touch voltage test can be done between a piece of switchgear and a distance of 1 m from the equipment. Or the tests for step voltages can be done at several locations with the spacing of 1 m in the ground, for example, outside the fence line. These readings are measured in ohms and need to be multiplied by the maximum permissible fault current to obtain the step and touch voltages.

These values should then be compared to maximum permissible values for a specified body mass.

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12) Earthing System Connections

IEEE Std 80 makes the following statement regarding the choice of connectors for use in ground grids for electrical substations:

All connections made in a grounding network above and below ground should be evaluated to meet the same general requirements of the conductor used; namely, electrical conductivity, corrosion resistance, current carrying capacity, and mechanical strength. These connections should be massive enough to maintain a temperature rise below that of the conductor and to withstand the effect of heating. The connections should also be strong enough to withstand the mechanical forces caused by the electromagnetic forces of maximum expected fault currents and be able to resist corrosion for the intended life of the installation.

IEEE Std 837-1989 provides detailed information on the application and testing of permanent connections for use in substation grounding. Grounding connections that pass IEEE Std 837 1989 for a particular conductor size range and material should satisfy all the criteriaelectrical conductivity, corrosion resistance, current carrying capacity, and mechanical strengthfor that same conductor size range and material.

Exothermic connections (CADWELD) meet all the requirements of IEEE Std 837 and are the most common method of connection in electrical substations and telecommunications grounding systems.

Other connection methods, including U-Bolt clamps, Hammerlok, compression connectors and acorn clamps are available and can be used for other applications like distribution transformer earthing, switchboard earthing to AS3000 or control equipment earthing systems.

13) Materials for Earthing Conductors and Electrodes

Copper is almost universally used as the conductor although some legacy use of galvanized conductors is observed from time to time.

Copper theft has become an endemic problem in recent years and there is a growing use of alternate conductors, including copper coated steel and composite conductors that have tinned copper in the centre and steel outer.

The most commonly used ground rods are copper bonded steel. In high salinity soil conditions, stainless steel ground rods are used. Copper bonded ground rods are effective in a wide range of soil conditions. Copper Bonded ground rods that comply with international standards BS7430, and UL467 should be selected to ensure longevity in the soil.



14) Telecommunications Facility Earthing Electrode System Design

The design of an earthing electrode system for a telecommunications facility is covered in AS3015-2004.

The calculation and testing of the earthing electrode resistance is done in exactly the same manner as described previously in this paper. However step and touch voltages are not usually calculated. This is because risks of step and touch voltages are traditionally associated with large fault currents in power systems and these risks do not exist at telecommunications sites.

It is recognized that high earth voltages can occur due to lightning strikes on or near telecommunications towers. It has not been possible to calculate these ground voltages due to the complex nature of lightning impulses and the lack of a simple and consistent method of calculation. Some software programs can simulate lightning impulses and calculate the resulting step and touch voltages.

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One consideration that has been incumbent in telecommunications earthing electrode design is the need to handle high frequency noise and lightning strikes. As such one may find discussions on the need for low impedance earthing at telecommunications sites as opposed to just low resistance.

While earth impedance measurements can be performed, there is no uniformity in the test equipment used to measure this and no benchmark on what is a good impedance. As such earth resistance measurements are done far more commonly and earth impedance measurement is almost never done. A more common impact of the high frequency consideration on the electrode design is an intuitive one and is exhibited in the widespread use of flat tape (commonly 25 x 3mm copper) as the grounding conductor. Horizontal tape conductors will provide high capacitive coupling to the ground and hence lower impedance when compared with round or stranded conductors.

Figure 18 shows the typical layout of the earthing electrode system at an outdoor telecommunications site.

Conclusion

The process for the design of the earthing electrode system starts with careful planning. A scaled site drawing showing where equipment, structures and building will be installed is a good start. The soil resistivity tests should be completed and results recorded. If it is not possible to access the site for soil resistivity testing, then it may be possible to have a series of design scenarios or a scaleable design based on an assumed range of resistivity values.

The key target criteria in the design are the system resistance to remote earth and the step and touch voltages. The understanding of the sheath theory of conductivity helps us decide upon the geometry of the earthing electrodes. Empirical formulae, nomograms and software programs can be used to carry out the calculations. IEEE Std 80 used in conjunction with country specific guides like the ENA Earthing Guide in Australia form an excellent reference set for the design of earthing electrode systems for electrical substations. AS3015-2004 is an excellent reference for the design of telecommunications earthing electrode systems.

The earthing grid resistance can be measured using several methods. Fall of potential methods may be used for small earth grids. Larger grids will require testing using the slope method or more sophisticated methods. The advantage of the slope method is that it minimises the chances of error and provides a more accurate result for larger earthing electrode systems.

Post installation step and touch voltages should be measured where step and touch voltages are part of the design criteria. The resistances and the step and touch voltages should be measured after installation and compared with the design values.

Ground improvement materials can be used in soils with high resistivity to reduce the earth resistance. IEEE Std 80 2000 notes the advantages of cement-based ground improvement materials like GEM. Care should be taken if using bentonite materials in dry environments.



The most comprehensive test standard for the pre-qualification of connectors used in earthing electrode systems is IEEE Std 837-2000. Exothermic (CADWELD) connections pass the test regime stipulated in this standard c-crimps and bolted connections do not.

Discussing the choice of conductor size is outside the scope of this paper. Various standards can be referred to for guidance on this. Copper is the most widely used conductor material for earthing systems. The high incidence of copper theft has led to the development and introduction of other materials suitable for use as grounding conductors if consideration is given to their electrical properties relative to copper.



Bibliography

1) NSW State Electricity Earthing Handbook, 1973. 2) Standard IEEE 80, - 2000: IEEE Guide for Safety in AC Substation Grounding 3) Standard IEEE 837 IEEE Standard for Qualifying Permanent Connections

Used in Substation Grounding 4) Telecom Training Centre Fiji; Course Notes, Earthing Principles PS001 5) Earthing Training Manual ERICO 2001 6) 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

7) Photos and Images are Courtesy of ERICO Inc. 8) 3 D Graphics of Earth Simulation. Images Courtesy of Dr Franco DAllesandro 9) Paper 62975, written by Dr G.F. Tagg, taken from the proceedings of IEEE

volume 117, No 11, Nov. 1970 10) Grounding Design Example: Generated by PhysElec Solutions, Dr Franco

DAllesandro 11) Standard AS3015 Electrical installationsExtra-low voltage d.c. power

supplies and service earthing within public telecommunications networks



Commonly Used Grounding Equipment

The images in this section show some commonly used grounding equipment and their applications.

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Figure 21 Grounding Plates used to Make Effective Bonds to

Reinforcement Steel

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