Ground electrode system

.Introduction

In the earlier sections, we learnt about the need for connecting the power source neutral point to ground (system ground). We also discussed the requirement for a grounding connection at the consumer end, the equipment grounding. In both cases, the connection to ground or 'ground' requires the use of a system of ground electrodes. The effectiveness of grounding depends on obtaining as low a resistance as possible between the ground electrode system and the groundmass.

In this section, we will learn about the design of grounding system and the materials used for this purpose. The practices adopted in different countries follow the national standards/codes that are specified by the appropriate authority and can be significantly different. We will limit our discussion to the general principles involved in the design of earth electrode system. Specific examples of electrode system design are given in Section A for those who are interested. We will also discuss the methods of measuring the resistance of grounding systems.

Grounding electrodes

The construction of grounding electrodes depends on the local codes applicable. The purpose however is common. It’s to establish a low-resistance (and preferably low-impedance) path to the soil mass. It can be done using conductors that are exclusively meant for this function or by structures/conductors used for other functions but which are essentially in contact with soil. However, while using the latter category, it must be ensured that the ground connection is not inadvertently lost during repair works or for any other reason.

Factors contributing to ground electrode resistance

The resistance of a ground electrode is made up of the following components:

• The resistance of electrode material

• Contact resistance of the electrode with soil

• Resistance of the soil itself.

The values of the first two are quite low compared to the last and can be neglected. We will discuss the third, namely resistance of the soil, in detail further.

Soil resistance

Though the ground itself being a very large body can act as an infinite sink for currents flowing into it and can be considered to be having very low resistance to current flow, the resistance of soil layers immediately adjacent to the electrode can be considerable.

Soil has a definite resistance determined by its resistivity that varies depending upon the type of soil, presence of moisture and conductive salts in the soil and the soil temperature. Soil resistivity can be defined as the resistance of a cube of soil of 1 m size measured between any two opposite faces. The unit in which it’s usually expressed is ohmmeter.

FIG. 1 Soil resistivity

Resistance of the sample of soil shown in FIG. 1 can be arrived at by the formula:

=/ R LA ?

Where

R: is the resistance between faces

P and Q Ohm

A: Area pr face

P and Q (mz)

L: Length of soil sample in meter

Omega: Soil reisistivity Ohm-m

Soil resistivity for a given type of soil may vary widely depending on:

• The presence of conducting salts

• Moisture content

• Temperature

• Level of compaction.

Conducting salts may be present naturally in the soil or added externally for lowering the resistivity. Chlorides, nitrates and sulfates of sodium, potassium, magnesium or calcium are generally used as soil additives. However, the addition of such salts can be corrosive and in some cases undesirable from the environmental point of view. Especially, the presence of calcium sulfate in the soil is detrimental to concrete foundations and in case it’s to be used for electrode quality enhancements, it should be limited to electrodes situated well away from such foundations. Also, over a period, they tend to leach away from the vicinity of the electrode. Moreover, these additive salts have to first get dissolved in the moisture present in the soil in order to lower the resistivity, and provision should be made for addition of water to the soil surrounding the electrode to accelerate this process particularly in dry locations.

Moisture is an essential requirement for good soil conductivity. Moisture content of the soil can vary with the season and it’s advisable for this reason to locate the electrodes at a depth at which moisture will be present throughout the year so that soil resistivity does not vary too much during the annual weather cycle. It’s also possible that moisture evaporates during ground faults of high magnitude for long duration. The electrode design must take care of this aspect. We will cover this in more detail later in this section.

Temperature also has an effect on soil resistivity but its effect is predominant at or near 0ºC when the resistivity sharply goes up. Similarly, compaction condition of the soil affects resistivity. Loose soil is more resistive in comparison to compacted soil.

Rocky soil is highly resistive and where rock is encountered, special care is to be taken.

One of the methods of increasing soil conductivity is by surrounding the electrode with bentonite clay, which has the ability to retain water and also provides a layer of high conductivity. Unlike salts mentioned earlier, bentonite is a natural clay, which contains the mineral monmorillionite formed due to volcanic action. It’s non-corrosive and does not leach away as the electrolyte is a part of the clay itself. It’s also very stable. The low resistivity of bentonite is mainly a result of an electrolytic process between water and oxides of sodium, potassium and calcium present in this material. When water is added to bentonite, it swells up to 13 times of its initial volume and adheres to any surface it’s in contact with. Also, when exposed to sunlight, it seals itself off and prevents drying of lower layers.

Any such enhancement measures must be periodically repeated to keep up the grounding electrode quality. A section later in this section describes about electrodes, which use these principles to dramatically lower the resistance of individual electrodes under extreme soil conditions. Such electrodes are commonly known as 'chemical electrodes'. The IEEE 142 gives several useful tables, which enable us to determine the soil resistivity for commonly encountered soils under various conditions which can serve as a guideline for designers of grounding systems. These are shown in FIG. 2 and 3.

Measurement of soil resistivity

Soil resistivity can be measured using a ground resistance tester or other similar instruments using Wenner's 4-pin method. The two outer pins are used to inject current into the ground (called current electrodes) and the potential developed as a result of this current flow is measured by the two inner pins (potential electrodes)

(refer FIG. 4).

FIG. 2 Effect of moisture content on soil resistivity

Resistivity (in ohmmeter) Moisture Content (%) Top Soil Sandy Loam Red Clay

FIG. 3 Effect of temperature on soil resistivity : Temperature (°C) Resistivity (in ohmmeter)

The general requirements for ground resistance testing instruments are as follows:

• The instrument should be suitable for Wenner's 4-pin method. It should give a direct readout in ohms after processing the measured values of current injected into the soil and the voltage across the potential electrodes.

• The instrument should have its own power source with a hand-driven generator or voltage generated using batteries. The instrument will use an alternating current for measurement.

FIG. 4 Soil resistivity measurement

• Direct reading LCD type of display is preferable. Resistance range should be between 0.01 and 1999 ohm with range selection facility for 20, 200 and 2000 ohm for better accuracy.

• Indications should preferably be available for warning against high current through probes, high resistance of potential probes, low source voltage and excessive noise in the soil.

• A minimum of four (4) steel test probes of length 0.5 m and sectional area of 140 mm^2 along with the necessary insulated leads (a pair of 30 m and another pair of 50 m) should be supplied with the instrument.

All the pins should be located in a straight line with equal separating distance between them and the pins driven to a depth of not more than 10% of this distance. Care should be taken to ensure that the connections between the pins and the instrument are done with insulated wires and that there is no damage in the insulation.

The resistance of the soil between potential electrodes is determined by Ohm's law (R = V/I) and is computed and displayed by the instrument directly. The resistivity of the soil is given by the formula: 2 SR ohm p =

Where:

?: Soil resistivity in Ohm-m S: Distance shown in FIG. 2 in meters R: Resistance measured in Ohms Since the soil is usually not very homogeneous especially near the surface, the depth to which the pins are driven and the separation between the pins will cause resistivity FIG.s to vary and can indicate the type of soil at different depths. The calculated value of resistivity can be taken to represent the value at the depth of 0.8S where S is the electrode spacing. The test is repeated at different values of S viz. 1, 2, 3, 5, 10 and 15 m and tabulated in the format shown in Table A.6 in Section A. They can also be plotted in the form of a graph.

A study of the values will give some indication of the type of soil involved. A rapid increase of resistivity at increasing D values shows layers of soil with higher resistivity. A very rapid increase may indicate the presence of rock and will possibly prevent use of vertical electrode. On the other hand, decrease of soil resistivity as D increases will indicate lower-resistivity soils in deeper layers where vertical electrodes can be installed with advantage.

In the case of any abnormality in the values, the test can be repeated after driving the pins along a different direction.

Errors can be caused by various factors in this measurement. They are as follows:

1 Errors due to stray currents

Stray currents in the soil may be the result of one or more of the following reasons:

• Differential salinity

• Differential aeration of the soil

• Bacteriological action

• Galvanic action (more on this later in the section)

• Ground return currents due to electric traction systems nearby

• Currents from multiple grounding of distribution system neutrals.

These stray currents appear as potential drop across the voltage electrodes without a corresponding current from the instrument's current source. Thus, they result in exaggerated resistivity measurements. This can be avoided by selecting an instrument source frequency, which is different from the stray currents, and providing filters that reject other frequencies.

2 Coupling between test leads

Improper insulation may give rise to leakage currents between the leads, which will result in errors. Ensuring good insulation and running the current and potential leads with a gap of at least 100 mm will prevent errors due to leakage.

3 Buried metallic objects

Buried metallic objects such as pipelines, fences, etc. may cause problems with readings.

If presence of such objects is known, it will be advisable to orient the leads perpendicular to the buried object.

Resistance of a single rod electrode

The resistance of a ground electrode can be calculated once the soil resistivity is known.

For a rod driven vertically into ground, the electrode resistance is given by the following formula

. / 2{log(8/)1} RLL d ohm p =-

Where R: is the resistance of the electrode in ohm

?: Soil resistivity in ohm meter L: Length of electrode buried in soil D: Outer dia of earth rod in m A simplified formula for an electrode of 5/8 in. (16 mm) diameter driven 10 ft (3 m) into the ground is:

/ 3.35 R ohm =

Where R: the resistance of the electrode in ohms

?: the soil resistivity in ohm meter

Knowledge of the soil resistivity alone is thus adequate to assess the electrode resistance to a reasonable degree of accuracy. The IEEE 142 gives the following table ( FIG. 5) for ready reference and can be used to arrive at the resistance value of the standard ground rod for different types of soil.

Soil Type Average Resistivity (Ohm m)

Resistance of Rod Dia. 5/8 in. Length 10 ft in ohms Well-graded gravel 600-1000 180-300 Poorly graded gravel 1000-2500 300-750 Clayey gravel 200-400 60-120 Silty sand 100-800 30-150 Clayey sands 50-200 15-60 Silty or clayey sand with slight plasticity 30-80 9-24 Fine sandy soil 80-300 24-90 Gravelly clays 20-60 17-18 Inorganic clays of high plasticity 10-55 3-16

FIG. 5 Soil resistivity for different soil types

Resistance distribution in soil surrounding a single electrode

The resistance of the soil layers immediately in the vicinity of the soil is significant in deciding the electrode resistance. To illustrate this let us see FIG. 6.

A current that flows into the ground from a buried electrode flows radially outward from the electrode. It’s therefore reasonable to assume for the purpose of calculating the soil resistance that the soil is arranged as concentric shells of identical thickness with the electrode at the center. The total resistance can thus be taken as the sum of the resistance of each shell taken in tandem.

FIG. 6 Soil resistance distribution around a vertically driven electrode

The resistance of each shell is given by the formula:

/ R LA ohm =

Where

R: Resistance of the shell L: Resistance of the shell A: Surface are (inner) of the shell

The area of the shells keeps increasing as we move away from the electrode. Thus, the resistance of the shells keeps reducing in value. The IEEE 142 has tabulated this variation (see FIG. 7).

It can be seen from FIG. 7 that the first 0.1 ft accounts for 25% of the resistance value and the first 1 ft for 68%. At 10 ft (equal to the rod length), 94% of the resistance value has been achieved. For this reason, lowering of soil resistivity in the immediate vicinity of the electrode is the key to lowering the electrode resistance.

Also, placing more ground electrodes in the vicinity will only interfere with the conduction of current since the current from one electrode will increase the ground potential, which will have the effect of decreasing the current flow from the other nearby electrode (and vice versa).

FIG. 7 Radial variation of soil resistance around a rod electrode

Distance from Electrode (in feet)

App. % of Total Resistance

Current-carrying capacity of an electrode

When current flow through a ground electrode into ground is low, the heat generated in the ground layers gets dissipated fairly fast and does not lead to any appreciable temperature rise. On the other hand , for a high current flow as happens during faults in solidly grounded systems, the effect would be quite different. As we saw earlier, the bulk of the resistance is concentrated in the immediate vicinity of the electrode.

Without adequate time for the heat generated to be conducted away, the temperature of the ground layers surrounding the ground electrode rises sharply and causes evaporation of soil moisture around the electrode. If this persists, the soil around can become dry losing all the moisture present in it resulting in arcing in the ground around the electrode. Thus, a smoking or steaming electrode results in an electrode that is ineffective. To prevent this from happening, it’s essential to limit the flow of current flowing into the ground through an electrode as indicated by the following formula:

Where I: Maximum permissible current in amp d: Diameter of the rod (m)

L: Buried length of rod (meters)

?: Soil resistivity ohm-m t: Time of fault flow in second

Use of multiple ground rods in parallel

When it’s not possible to obtain the minimum resistance stipulations or the ground fault current cannot be dissipated to the soil with a single electrode, use of multiple ground rods in parallel configuration can be resorted to. The rods are generally arranged in a straight line or in the form of a hollow rectangle or circle with the separation between the rods not lower than the length of one rod. As we have seen earlier in this section, the soil layers immediately surrounding the electrode contribute substantially to the electrode resistance. More than 98% of the resistance is due to a soil cylinder hemisphere of 1.1 times the electrode length. This is called the 'critical cylinder'. Placing electrodes close to each other thus interferes with the conduction of current from each electrode and lowers the effectiveness.

It’s also of interest to note that the combined ground resistance of multiple rods does not bear a direct relationship to the number of rods. Instead, it’s determined by the formula:

R = R/N × F

R: Combined ground resistance of the electrode system having N electrode (Ohms)

R: Resistance of a single typical electrode (Ohms)

F: Factor F in the table shown in FIG. 6 for Number of Roads = N The table in FIG. 8 shows the value of the factor F used above.

No. of Rods F

2 1.16

3 1.29

4 1.36

8 1.68 12 1.80 16 1.92 20 2.00 24 2.16 FIG. 8

Factor F for multiple ground rods

Measurement of ground resistance of an electrode

The resistance of a single ground electrode (as well as small grounding systems using multiple rods) can be measured using the 3-point (or 3-pin) method. The apparatus for this purpose is the same that is used for soil resistivity, viz. the ground resistance tester (see FIG. 9). This method, however, may not yield correct results when applied to large grounding systems of very low resistance.

The measurement of electrode resistance is done in order to:

• Check on correctness of calculations and assumptions made

• Verify the adequacy after installation and

• Detect changes in an existing installation.

In this case, the ground electrode itself serves both as a current and potential electrode.

The other electrode farther from the electrode is the other current electrode and the nearer one is the second potential electrode. The resistance can directly be read off the instrument.

To get correct results, the current electrode must be placed at a distance of at least 10 times the length of the electrode being measured and the potential electrode at half the distance. A very similar method can be adopted for the measurement of ground grids, which are used commonly in HV substations (usually outdoor switchyards) (refer FIG. 10).

FIG. 9 Measurement of electrode resistance by 3-point method

FIG. 10 Measurement of resistance of a ground grid

The problems mentioned in the section on measurement of soil resistivity are applicable in this case too and appropriate precautions must be taken to ensure accuracy. A more detailed approach given in the South African standard SCSASAAL9 is described in Section A and can be used for better results.

Concrete-encased electrodes

Concrete foundations below ground level provide an excellent means of obtaining a low resistance electrode system. Since concrete has a resistivity of about 30 ohm m at 20 °C, a rod embedded within a concrete encasement gives a very low electrode resistance compared to most rods buried in ground directly. Since buildings are usually constructed using steel-reinforced concrete, it’s possible to use the reinforcement rod as the conductor of the electrode by ensuring that an electrical connection can be established with the main rebar of each foundation. The size of the rebar as well as the bonding between the bars of different concrete members must be done so as to ensure that ground fault currents can be handled without excessive heating. Such heating may cause weakening and eventual failure of the concrete member itself. Alternatively, copper rods embedded within concrete can also be used.

Concrete electrodes are often referred to as 'Ufer' electrodes in honor of Mr Ufer, who performed a large amount of research into concrete-encased electrodes. The rebars used are required to be either bare or zinc coated. Normally, the following applies to a rebar used as an earthing electrode:

• Minimum length of 6 m

• Minimum diameter 13 mm.

...and installed:

• In a minimum of 50 mm of concrete

• Concrete is in direct contact with earth

• Located within and near the bottom of a concrete foundation or footing

• Permitted to be bonded together by the use of steel tie wire.

With respect to the last point, steel tie wire is not the best means of ensuring that the rebars make good continuity. Excellent joining products are available in the market, which are especially designed for joining construction rebars throughout the construction.

By proper joining of the rebars in multi-level buildings, exceptionally good performance can be achieved. An extremely low-resistance path to earth for lightning and earth fault currents is ensured as the mass of the building keeps the foundation in good contact with the soil. Some examples of splicing products available in the market for jointing of rebars are shown in FIG. 11a-c.

A recent advancement for solving difficult earthing problems is the use of conductive concrete to form a good earth. Normally, this form of concrete is a special blend of carbon and cement that is spread around electrodes of copper. These are normally installed in a horizontal configuration by digging a trench of approximately a half a meter wide and 600 mm deep. The flat copper or rods are then installed in the center of the trench. The conductive concrete is then applied dry to the copper and spread to approximately 4 cm thick over the copper to the edges of the trench.

The trench is then backfilled and the conductive concrete then absorbs moisture from the soil and sets to about 15 Mpa.

It’s also possible to install these electrodes vertically. However, in this case, the conductive concrete has to be made up as a slurry and pumped to the bottom of the hole to displace water or mud.

FIG. 11a Threaded splice joint using a coupler

FIG. 11b A comparison between lap splice using tie wire and threaded (mechanical) splice

FIG. 11c A welded splice joint with sleeve

Corrosion problems in electrical grounding systems

Buried electrode systems bonded to other facilities embedded in ground such as piping/conduits can form galvanic cells when they involve dissimilar metals having differing galvanic potential. These cells, which are formed from the dissimilar metals as electrodes and the ground as the electrolyte, set up galvanic current through the bonding connections (refer FIG. 12). For example, copper electrodes and steel pipes used as a part of grounding system can cause cells of 0.38-V potential difference with copper as the positive electrode. This circulates a current, which causes corrosion of the metal in the electrode from which current flows into the ground. A galvanic current of 1-A DC flowing for a 1-year period can corrode away about 10 kg of steel.

This can be avoided by the use of materials with the same galvanic potential in the construction of ground electrode systems. Other methods such as use of sacrificial materials as anodes and injection of DC currents help to control this type of corrosion.

FIG. 12 Galvanic action of a ground electrode system

Maintenance of grounding system

A properly scheduled and executed maintenance plan is necessary to maintain a grounding system in proper order. This is essential because the efficacy of the system can be affected over a period of time due to corrosion of metallic electrodes and connections. Periodic measurement of the ground electrode resistance and recording them for comparison and analysis later is a must. In the case of any problems, repairs or soil treatment must be taken up to bring the ground electrode system resistance back to permissible values.

Chemical electrodes

We have seen earlier in this section that the resistance of the ground electrode is influenced by the soil immediately surrounding the electrode. It’s also influenced by the ambient conditions of the soil such as moisture and temperature. Thus, it’s difficult to obtain acceptable values of grounding resistance in areas where:

• Natural soil is of very high resistivity such as rocky material, sand without vegetation, etc.

• During part of the year, the resistance may become excessive because of the absence of moisture.

• Soil temperature remains extremely low as in the case of polar regions or those close to the polar circle (called as permafrost condition, where the ground is below freezing temperature).

It thus follows that the performance of an electrode can be improved by using chemically treated soil to lower the soil resistivity and to control the ambient factors.

While the soil temperature cannot be controlled, it’s possible to ensure presence of moisture around the electrode. Soil treatment by addition of hygroscopic materials and by mechanisms to add water to the soil around the electrode are common methods of achieving this objective. Also, the resistivity behavior in permafrost conditions can be improved by soil conditioning, thus improving the electrode resistance dramatically.

Tests performed by the US Corps of Engineers in Alaska have proved that the resistance of a simple conventional electrode can be lowered by a factor of over twenty (i.e. 1/20). The treatment involved simply replaces some of the soil in close proximity with the electrode by conditioned backfill material. Refer FIG. 13 for the result of tests conducted at Point Barrow, Alaska, which illustrates that the electrode resistance has dropped from a high of 20 000 ohm to a maximum of 1000 ohm by soil treatment.

FIG. 13 Result of soil treatment on electrode resistance

The principle of improving the soil conductivity has been applied for a long time in ground electrode construction. The example shown earlier in FIG. 1.5 belongs to this category. In this example, the hollow earthing tube contains sodium chloride, which absorbs moisture from surrounding air and leaches out to the soil to lower its resistivity.

The backfill is soil mixed with charcoal and also sodium chloride. Since moisture in air is essential for this construction, means are provided to externally add water during dry weather.

These basic principles are used by several vendors who manufacture electrodes for applications involving problem areas. In these cases, both the electrode fill material and the augmented backfill are decided based on the soil properties so that moisture can be absorbed from surrounding soil itself and preserved in the portion immediately surrounding the electrode. In some systems, automated moisture addition devices are provided to augment this effect. A typical system by a vendor incorporating a solar powered moisture control mechanism is shown in FIG. 14a and b.

FIG. 14a Arrangement of chemical electrode with moisturizing mechanism

FIG. 14b Control system for moisture addition

Summary

In this section, we discussed the grounding electrodes that are commonly used in electrical systems and about their resistance. The details of soil resistivity and its measurement were examined. The methods to estimate the grounding resistance of single and multiple electrodes were reviewed. The measurement of ground electrode resistance was also covered. The need for limiting the ground current flow to prevent smoking electrode phenomenon was reviewed. The improvement of soil to obtain better conductivity by use of conducting salts and bentonite clay was covered. Corrosion of ground electrode systems and the ways of preventing the same were covered briefly. Various tables, which will be of assistance in designing grounding systems, have been given in this section. These along with any applicable local codes may be deployed to ensure that the grounding system will function correctly and fulfill its primary purpose of achieving human safety. Section A at the end of this guide gives an example of standardized ground electrode systems recommended by some national standards and procedure for measurement of electrode resistance.