Electrical Transmission and Distribution--Earthing (Grounding) and Bonding

1 INTRODUCTION

The purpose of a substation earthing system is to ensure safe conditions for personnel and plant in and around the site during normal and earth fault conditions. To achieve this, it needs to perform the following functions:

_ Provide equipotential grading by electrodes or similar, to control touch and step potentials.

_ Enable connection (bonding) of necessary exposed extraneous conductive parts to earth.

_ Provide a route for the passage of fault current which does not result in any thermal or mechanical damage to connected plant and allows protective equipment to operate.

_ Provide an earth connection for transformer neutrals, sometimes via an impedance to restrict the fault current magnitude.

_ Minimize electromagnetic interference between power and other systems such as control or communication cables or pipelines.

This section is an over-view of the subject, dealing mainly with the design limits and the general procedure followed to design an electrode system, including a computer analysis. Additionally, a brief overview of protective multiple earthing (PME) is given.

Small power and lighting installation earthing is covered in Section 7; system earthing (grounding) is reviewed in Section 10, Section 10.2.

2 DESIGN CRITERIA

2.1 Touch and Step Voltages

Substation 'earthed' (grounded) metal work (consisting of switchgear enclosures, supports, fencing, etc.) and overhead line steel supports all have an impedance to true earth. When fault current flows through them to earth, a voltage rise will occur. This 'earth potential rise' (EPR) is the maximum voltage that the earthing system of an installation may attain relative to a remote point assumed to be at true (zero) earth potential. The EPR is the product of the current that returns to its remote sources via the soil and the earthing system impedance. The overall (gross) fault current calculated will normally include amounts that may return via metallic routes such as the other (un-faulted) phase conductors, the metal screens/sheaths of buried cables and the earth wires (if fitted) of an overhead line. These current flows are subtracted from gross value to leave the amount that will return via the soil. This current will flow through the local electrode system and any other electrodes connected in parallel to it. These include sheet steel foundations, the steel legs of trans mission towers, large pipes and the lead sheath of cables that have a conductive outer covering (such as Hessian).

Whilst the EPR exists, voltages will occur in and around the installation.

A number of voltage definitions are used to characterize the situation at any point. They include:

_ The 'touch voltage', which is the potential difference between the EPR on a structure and the surface potential at a point where a person is standing (normally 1 m away), whilst at the same time having one or both hands in contact with the structure.

_ The 'step voltage', which is the difference in surface potential experienced by a person bridging a distance of 1 m with their feet.

_ The 'transfer potential' is that between steelwork (physically distant from the installation, but bonded to it) and the remote local earth. This could occur at any point along a cable, pipe or steel fence. The design normally seeks to prevent dangerous transfer potentials occurring, by limiting the EPR or removing the electrical connection between the steelwork and installation earth.

_ The 'mesh voltage' is a quantity used in the American Standard, IEEE 80, and is the touch voltage seen at the centre of a mesh of the substation buried earthing grid.

Calculations seek to find the worst case value of touch and step voltage for the design and compare it against tolerable voltage limits. These different scenarios are illustrated in FIG. 1, extracted from the North American Standard IEEE 80.

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FIG. 1 Illustration of the voltages that occur during an earth fault (from IEEE 80) ES =step voltage; Et =touch voltage; Em 5mesh voltage; Etrrd =transfer potential.

Surface potential profile Etrrd ˜ GPR Transferred voltage Mesh voltage Touch voltage Step voltage Remote earth Em Et Es

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2.2 Touch and Step Voltage Limits

Electro-pathological effects on the human body are produced when current passes through it, and to avoid death we are concerned about the proportion which flows in the region of the heart. IEC 60479-1 (2004) 'The effects of current on Human Beings and Livestock' provides the internationally accepted best available guidance on the subject, and is used as the basis of touch and step voltage limits in many standards _ particularly those in Europe. The first edition (called IEC 479) was published in 1974 and was influenced by the early research work of Dalziel. This data was subsequently improved in the 1984 and 1994 editions, which are still used in many standards. A new and extended version of IEC 60479-1 was completed in 2007.

It extended the documented work to include different 'touch contact' areas and to take account of whether the ground surface is wet. Original work in the area included that by Biegelmeier, who confirmed some observations by subjecting himself to electric shocks! The most often referenced part of IEC 60479-1 is illustrated in FIG. 2. This uses current flowing in the region of the heart against its duration to establish the boundaries between different threshold levels.

The document shows that humans are particularly vulnerable to alternating currents of between 50 and 60 Hz with a threshold of perception of about 1 mA. With increasing current, the effects move from muscular contraction, unconsciousness, fibrillation of the heart, respiratory nerve blockage to burning. Humans can generally withstand higher currents at DC and higher frequencies. The current limits are time dependant, because the threshold of fibrillation rapidly decreases if the alternating current flow persists for more than one cardiac cycle. For shock durations below one cardiac cycle, the ventricular fibrillation threshold current is nearly constant down to very short times. The risk of ventricular fibrillation is much lower at short intervals because there is less chance of current flow during the vulnerable period in the 'T' phase of the cardiac cycle. This is illustrated in FIG. 3 and only occupies approximately 10% to 20% of the total cardiac cycle. For this reason, there is a pronounced 'kink' in threshold values in the standards.

Earthing standards convert the tolerable currents into touch and step potential limits because it’s easier to make voltage calculations and measurements at electrical installations. A typical set of touch and step potential limits are shown in FIG. 5.

FIG. 2 Time/current zones of effects of AC currents (15_100 Hz) on persons (IEC 60479-1).

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Zones | Physiological effects

1. Usually no reaction effects.

2. Usually no harmful physiological effects.

3.

4.

Usually no organic damage to be expected. Likelihood of muscular contractions and difficulty in breathing, reversible disturbances of formation, and conduction of impulses in the heart, including arterial fibrillation and transient cardiac arrest without ventricular fibrillation increasing with current magnitude and time.

In addition to the effects of zone 3, probability of ventricular fibrillation increasing up to about 5% (curve C2), up to about 50% (curve C3), and above 50% beyond curve C3. Increasing with magnitude and time, patho-physiological effects such as cardiac arrest, breathing arrest, and heavy burns may occur.

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FIG. 3 Typical human heart cycle.

S Q T P Auricles Ventricles Spread of excitation Recovery from excitation Vulnerable period of the ventricles during the 'T' phase

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As can be seen, the tolerable voltages are surprisingly high at short clearance times, although it should be noted that a person would still experience a considerable shock. The aim is to avoid the shock being sufficient to cause ventricular fibrillation.

The earthing coverage in the standards is very varied and even taken together, the guidance offered to designers until quite recently was sparse and often conflicting. The most recent versions of the standards now include reasonably detailed design guidance. Of further concern is the fact that National and International Standards still quote different touch and step potential limits and different ways of calculating them. For example, within Europe they are based on the IEC 60469 curves, whilst in America they are based upon equations such as those illustrated in Section 3.4.

Partly to address the above problems, a section in the new IEC standard (IEC 61936-1) was developed. This has now been extended into a European Standard, EN 50522, published in 2010. The text sets out the methodology to be used to establish the touch and step voltage limits and provides a design flow chart, together with supporting guidance to show how the earthing system needs to be designed. European National Committees will use this to update their National Standards.

Most standards (including IEEE 80 and BS 7354) take into account the resistivity of the substation surface material and whether a small thickness high resistivity surface layer such as crushed rock is used. The results pro vide slightly higher admissible touch and step potential voltages whilst stressing the advantages of rapid fault clearance times.

3 SUBSTATION EARTHING CALCULATIONS

3.1 Environmental Conditions

3.1.1 Introduction

In order to calculate the required earthing parameters at a new site, a measurement of the soil resistivity (ohm meters), site dimensions and site specific earth fault levels (kA) are necessary. An initial grid layout is designed, based upon the site dimensions and influenced by the soil structure. Its resistance, EPR, and touch and step voltages are then calculated. These are com pared to the tolerable voltage limits and the grid adjusted, as necessary, to enable these requirements to be met. In some cases the calculated resistance is needed to revise the fault current and the voltages it creates. The process continues until the calculated safety voltages are all lower than the limits or specific remedial measures imposed.

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TBL. 1 K factors used for calculating minimum earth conductor sizes

1. Bare copper conductors Initial temperature of conductor 530_ C Maximum temperature of conductor 5150_ C or 200_ C (fire risk conditions) K factor (BS 7671) 5138 (normal conditions) or 159 (fire risk) (normal)

2. XLPE insulated copper/aluminum cable Single core Multicore Initial temperature of conductor 530_ C90_ C Maximum temperature of conductor 5250_ C 250_ C K factor (BS 7671) copper 5176 143 K factor (BS 7671) aluminum 5116 94

3. Bare steel electrode Initial temperature of conductor 530_ C (normal conditions) Maximum temperature of conductor 5200_ C K factor (BS 7671) 558

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3.1.2 Relevant Fault Conditions

The fault conditions are calculated using short circuit analysis techniques as described in Sections 1 and 26. The substation may be a switching centre and/or voltage transformation point. The substation earthing is common to the different voltage levels, so the calculations need to cater for a fault within the site at the highest voltage level and external to the site at the lower voltage levels. Only in exceptional cases will faults within the substation at the lower voltage level create a significant EPR, because the majority of the fault current in this case should normally circulate within the earth grid rather than flow into the soil.

Depending upon the voltage level and applicable standard, a fault duration of 1 to 3 seconds is used for the conductor sizing calculations. The conductor cross section required can be calculated using the following formula:

where

S=required cross sectional area of conductor in mm^2

I=value (AC rms) of fault current in amperes

t=standard substation design time or operating time of backup disconnecting device, in seconds

K=factor dependent on the material of the conductor, the insulation and other parts and the initial and final temperatures. Some K factors that are mainly used for building services for different conductors are listed in TBL. 1 (selected from BS 7671, but the same figures may be found in IEC 60364). Different values are used within substations for the buried grid and structure connections _ based For example on higher final temperatures.

For bare copper:

For example, a 13-kA, 1 second design fault level will require a bare cop per conductor size (based upon a K factor of 159) of 82 mm2 or its nearest equivalent standard size.

The conductor size needs to include an amount for corrosion over the lifetime of the site and can take into account the effect of duplicate paths to reduce the conductor cross-sectional area.

The exposure time for touch and step voltages depends upon the fault clearance time and this is provided from the fault level study mentioned earlier. As a general guide, 132 kV and above, circuits have a clearance time of less than 0.2 seconds. At lower voltages, clearance times in the range 0.3 to 1 second are the most common.

3.1.3 Earth Resistivity

Measurements are normally carried out using the Wenner method and the data is used to arrive at a representative soil model for the site. Whilst the measurements would best be carried out in representative weather conditions, this is clearly not always possible, so allowance for seasonal effects may need to be made in the model. This would normally be done by modifying the resistivity and/or depth of the surface layer. Some typical soil resistivity values are shown in TBL. 2.

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TBL. 2 Typical ground resistivity values

Ground type Resistivity (ohm meters)

Loams, garden soils, etc. 5_50 Clays 10_100 Chalk 30_100 Clay, sand and gravel mixture 40_250 Marsh, peat 150_300 Sand 250_500 Slates and shales 300_3000 Rock 1000_10000

=== Measurements are taken for a range of probe separations, each of which is a general indicator of the depth to which the individual measured value applies. Measurements in a number of directions would be taken and aver aged values (excluding obvious errors) for each separation distance would be used to derive the initial soil model.

A number of computer programs are commercially available and used to translate the data into a representative soil model. It’s useful to have both the average model and the data spread, so that the error band is known, as this will influence the subsequent calculations or suggest that the derived soil model be modified to improve its accuracy.

It’s possible to use formulae or graphical methods to derive a two layer model. The formula below compares the resistivity, s1, of the upper layer of depth h1 with the lower layer of resistivity, s2.

The value ss is the resistivity measured at a depth a. IEEE 80 includes a number of graphs to achieve the same result, based on the work of Sunde. It’s unusual to use formulae now, because the interactive computer pro grams available can quickly provide a model which may have a number of vertical or horizontal interfaces. Often a three-layer model is necessary as a minimum to provide sufficient accuracy.

The soil model values are used in formula or a computer program to calculate the earth resistance and hazard voltages (see Section 2).

3.2 Earthing Materials

3.2.1 Conductors

The conductors used must be capable of carrying the anticipated fault current and cope with corrosion over the lifetime of the installation. Bare copper is normally used for a substation earthing grid, being buried at depths between 0.6 and 1.0 m in rectangles of between 3 and 7 m side length. Equipment connections are generally laid at a shallower depth of about 0.2 m. Because of mechanical and thermal criteria, it’s unusual for copper of less than 70 mm^2 to be used.

Aluminum is often used for above ground connections and could be used below ground if it’s certain that the soil won’t cause corrosion problems, but most standards prohibit this. Some protection, such as painting with bitu mastic paint, is recommended in the area where the conductor emerges from the ground, as corrosion may occur here and just below. Where not connected directly to the electrode, all metallic substation plant is bonded via above ground conductors.

3.2.2 Connections

The connection methods used for below ground application are welded, brazed, compression, or exothermic. For above ground use, bolted connections are used in addition to these.

Soldering is not permitted, because the heat generated during fault conditions could cause failure.

Bolted joints should normally have their contact faces tinned or treated with an electrical grease, and particular care is necessary for connections between dissimilar metals, such as copper and aluminum. The standards (such as IEEE 837) and national codes of practice offer much advice on connection methods.

3.2.3 Earth rods

The earth grid's horizontal electrodes may be supplemented by vertical rods to assist the dissipation of earth fault current, further reduce the overall sub station earthing resistance, and provide some stability against seasonal changes. This is especially useful for small area substation sites (such as GIS substations) or where the rod is of sufficient length to enter lower resistivity soil or the water table. Rods may be of solid copper or copper clad steel and are usually of 1.2 m length with screw threads and joints for connecting together in order to obtain the required length for installation in the soil.

The formula for the effective resistance, RROD ohms of a single earth rod is given by:

RROD = Bare vertical earthing rod effective resistance (ohms)

S=Resistivity of soil (ohm meters)

L=Length of earthing rod (meters)

D=Diameter of earthing rod (meters)

Long rods are often fitted with test facilities so that their resistance can be measured during regular maintenance checks. Individual rods are also specified for line traps, current and voltage transformers (CVTs) and surge arresters. Care must be taken with CT and VT earthing to ensure that current loops don’t cause protection maloperation.

Some examples of standard manufacturers earthing fittings and connections are shown in FIG. 4.

3.2.4 Substation Fence Earthing

Two substation fence earthing practices are generally used, namely:

_ Extending the substation earth grid 0.5_1.5 m beyond the fence perimeter and bonding it to the grid at regular intervals.

_ Placing the fence beyond the perimeter of the switchyard earthing grid and providing it with its own earth rod system which is independent of the main substation earth grid.

Including the fence in the earth grid system can reduce the substation earth resistance and allows the earthing system design to be simplified, but requires an increased land area, puts electrode into publicly accessible areas, requires protection against theft or damage and can produce higher touch and step voltages external to the site than otherwise necessary.

Isolation of the fence from the main grid system is a safer option, but does involve an ongoing maintenance responsibility to ensure that isolation is maintained. Inadvertent connections could give rise to dangerous potentials under fault conditions.

Special fence earthing arrangements are necessary near single phase reactors or other substation plant generating high electromagnetic fields. It’s necessary to electrically separate the fence into short, individually earthed sections to avoid large induced circulating currents. Experience shows that this must not be overlooked, to avoid cases such as substation gates being literally welded together due to induced circulating current associated with large static VAr compensation equipment.

FIG. 4 Examples of manufacturers' earthing fittings.

3.3 Earth Resistance and Earth Potential Rise --- LK005 LK007 LK043 LK045 Rod Driving stud Coupling

A simplified formula for approximating the resistance, R ohms, of a substation earth grid of horizontal electrode is:

This needs more terms to be added if rods are present, to account for their combined effect. An example from BS 7354 is:

s=soil resistivity (ohm meters)

r=equivalent circular plate radius (m)

h=depth of buried grid (m)

L=total length of buried conductors (m)

KR = constant concerned with the number of vertical earthing rods used in the overall substation earthing grid design, which is normally supplied as a table or graph in the relevant standard.

Equations from IEEE 80 and BS 7354 are normally introduced into mathematical packages or a spreadsheet to enable the resistance values to be calculated quite easily. It’s important to be able to compare calculated and measured values to see how accurate the formulae are in different situations.

TBL. 3 (from IEEE 80) compares calculated and measured earth grid resistances for five different substation sites and configurations, the calculations being based upon the simplified equation.

Because of the approximations introduced by the formulae and especially the fact that the soil more often needs to be represented as a three or more layer model, it’s more common now for designers to use purpose-designed computer programs to carry out the necessary calculations. These account for the actual electrode/grid shape and the soil structure and some can also account for the longitudinal impedance of the earth conductors at a range of different frequencies. Use of these programs enables a closer match between calculated (predicted) and measured values than suggested in TBL. 3.

Most designers will continually examine differences between these values and amend procedures to improve the outcome of their design studies.

The input data and examples of the output parameters provided when an earthing design study is undertaken using formulae or a computer program are listed in TBL. 4.

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TBL. 3 Typical substation earthing grid resistances (IEEE 80) Parameter

Sub 1 Sub 2 Sub 3 Sub 4 Sub 5 Sand and Gravel Sandy loam Sand and clay Sand and gravel Soil and clay Resistivity (O m) 2000 800 200 1300 28 Grid area (m2 ) 1408.3 5661.4 1751.1 1464 5711.6 Buried length (m) 951 2895.6 541 1164.3 914.4 Calculated resistance (O) 25.7 4.97 2.55 16.15 0.19 Measured resistance (O) 39.0 4.10 3.65 18.2 0.21

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TBL. 4 Data needed and outputs from substation earthing calculations

Input parameters Site length (m) Site width (m) Fault current (A) Fault maximum duration(s) (typically 1 s) Normal fault duration(s) (typically 0.2 s) Soil resistivity (ohm meters) Crushed rock resistivity (ohm meters) _ (typically 3,000ohm meters) Minimum depth of crushed rock (m) Grid depth (m) Grid spacing (m) Required resistance to earth (ohms), if known Calculated parameters Conductor diameter (m) Number of parallel conductors Number of cross conductors Reduction factor, C Coefficient, Kim Coefficient, Km Coefficient, Kis Coefficient, Ks Theoretical conductor length (m) Actual grid conductor length (m) Grid corner conductors (m) Total rod length (m) Overall resistance to earth (ohms) Tolerable step and touch voltage on crushed rock (V) Tolerable step and touch voltage on natural soil (V) Generated maximum step voltage (V) Generated maximum mesh voltage (V) Data used for budget and material ordering Total length of tape or stranded conductor required Total length of earth rods

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For small substation sites, where access may not be available for the installation of an earth grid of sufficient size, a satisfactory earthing arrangement may often be achieved by installing copper tape in the ground around the periphery of the substation buildings. This may have additional earth rods connected and the building floor reinforcement may also be used to supplement the design. When necessary to account for the effect of the steel reinforcements, a typical value for the resistivity of damp concrete is 90 ohm meters. More often a sufficiently accurate result is obtained by assuming the same resistivity as the surrounding soil.

The substation EPR is then the product of the total substation earth impedance, Z, and the amount of fault current that flows through it into the soil. Use of reduction factors or separate calculations enables the current flowing into the earth grid to be determined. Simplified equations from the standards should be used with caution _ especially when calculating the split between the ground and sheath return currents in cable systems. The value of soil resistivity used in them must be selected carefully and large errors can result if the soil has several layers of markedly different resistivity, because this will react in a completely different way to a uniform soil. The substation EPR, UE, is then used to calculate the touch and step potentials that are compared against the applicable limits.

3.4 Hazard Voltage Tolerable Limits

3.4.1 General

Examples of some tolerable touch and step potential limits, as derived from IEC 60479-1 and IEEE 80 current/maximum disconnection time curves, are shown in FIG. 5. From this figure the large difference between the limit values calculated using different standards is evident.

The touch and step voltage limits may be individually calculated for the specific substation soil conditions if required, using formulae from standards.

For example, the IEEE 80 formulae are:

Ut (tol)=tolerable touch voltage (volts)

Us (tol)=tolerable step voltage (volts)

ss = resistivity of earth surface layer (say 3,000 Ohm/m for crushed rock)

tE = maximum exposure time to be taken into account for touch and step voltages (say 0.5 seconds)

kw = body weight factor (116 and 157 for 50 kg and 70 kg body weight, respectively)

For a 50 kg person, 3,000 Ohm/m crushed rock surface layer and 0.5 second maximum exposure time, values for allowable touch and step voltages using the IEEE 80 formula are 885 V and 3,134 V, respectively. Note that this differs from the value obtained using IEC 60479-1.

3.4.2 Hot Sites

A special case of transferred potential (Section .2.1 and FIG. 1) arises in respect of telecommunication circuits connected to substation sites. The rise of earth potential (ROEP), also referred to as ground potential rise (GPR) or earth potential rise (EPR), in the substation under earth fault conditions can adversely affect telecommunications equipment and present a shock hazard to telecommunication workers. To protect staff, equipment and users, the International Telecommunications Union has set recommended limits for GPR in MV and HV networks. With high-speed, high reliability, protection (clearance time ,200 ms), a site is classed as 'hot' if the most onerous fault causes a GPR at the substation boundary of 650 V or more. With slower protection, a site is classed as 'hot' if the GPR is 430 V. If these limits are exceeded at the boundary, the 650- or 430-V contour at the surrounding surface must be determined and the extent of the 'hot zone' notified to the telecommunication authorities.

Special protection measures are also then required for telecommunications equipment and in respect of the substation earthing in such cases, it’s recommended that the HV and LV earths are segregated so that any potential rise on the HV earth during a fault is not transferred to the LV system.

4 COMPUTER SIMULATION

Most major engineering project companies and designers now have in-house computer programs to evaluate different substation earthing arrangements. A summary of some of the more important stages in the design process is shown in FIG. 6.

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Field Data, Substation and equipment layout, Soil resistivity survey data (if required), Details of pile and building reinforcing bars, if to be included in earth grid, Maximum earth fault current magnitude and ground return component Fault current durations for both conductor sizing & hazard voltage calculation, Conductor

Conductor material Jointing methods Conductor size Hazard Voltages Tolerable Limit Values Standard Applicable (e.g. IEEE 80, BS 7430) Calculation or look-up values from tables Initial Grid Design Surround area available Run cross conductors near structure connection points Basic grid dimensions Number of parallel conductors in x and y directions Total length of grid conductors Supplementary earth rods, fencing, cable sheaths, substation control building floor reinforcements, etc.

Calculate earth impedance, EPR, and hazard voltages Modify Design Change grid spacing or add electrodes Change grid conductor length Add supplementary earth rods Change area occupied by grid Account for soil structure Check Voltages Against Tolerable or Other Limit Values Transferred potential Touch voltage Step voltage Mesh voltage External voltage contours Definitive Design Earthing layout drawing updated Grid and earth rod connection details Finalize calculation notes, QA procedures/checks Materials take-off Order materials

FIG. 6 Some calculation stages and data used for substation earth grid design.

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A simplified layout for a 132/11 kV open terminal, two transformer substation is shown in FIG. 7. The substation has a separately earthed fence. The two incoming circuits are carried in on a steel tower line that has a terminal tower within the substation. Overhead connections are provided from this to switchgear and each transformer. The busbar supports, surge arresters, and other equipment have been excluded for clarity. The earthing design is quite straightforward, with an electrode loop (or perimeter) about 2 m inside the fence, electrode rectangles surrounding the equipment and an electrode quite close to the plant items to enable short connections to them. The steel mesh rebar of the switchroom would also be connected to the electrode system.

The procedure followed in modeling the electrode is generally:

1. Site soil survey measurements are first taken and analyzed.

2. The resistance of the earth grid (grounding resistance of the primary electrode) is calculated based on the initial design and soil model.

3. The total impedance of the earthing installation is calculated. In this case there is a parallel earth contribution via the tower line and its overhead earth wire to include.

4. The fault current data is analyzed and the amount flowing into the grid and tower line calculated. The former is multiplied by the total earth impedance to provide the EPR.

5. Contour plots of the required potentials are produced by the computer soft ware. For example, in FIG. 8, the touch voltages across the site are shown, whilst the external potential contours are shown in FIG. The plots can be provided to show actual potentials, but in the figures they are shown as percentages of the EPR. If this was calculated as 2,000 V, then the 30% touch voltage contours in FIG. 8 correspond to a voltage of 600 V, whilst in FIG. 9 the 'level 2' contour is 40% of the EPR, or a value of 800 V.

6. Once the final studies have been completed, the total quantities of earthing materials necessary to achieve the design can be produced from the model and used for budgeting purposes.

The studies were carried out using the CDEGS computer package. Note that this package actually provides additional clarification of the output diagrams with the use of colour.

FIG. 7 Basic electrode/grid design for a 132/11 kV substation (Strategy and Solutions)

FIG. 8 Touch voltages across a 132/11 kV substation ( Strategy and Solutions).

FIG. 9 Surface potentials external to a 132 kV/11 kV substation (Strategy and Solutions).

5 PROTECTIVE MULTIPLE EARTHING

The characterization of LV distribution earthing systems and the European system of identifying them is described in Section 7, Section 7.3. Where the system formally identified as TN-C-S is used by the Utility, it’s known as Protective Multiple Earthing (PME). The same conductor is used to carry earth and neutral from the distribution substation to the consumer's premises, and then the single conductor is separated into neutral and earth conductors at the point of supply, so as to provide the consumer with a separate earth terminal.

It’s essential that the ROEP of such a system does not exceed 430 V (see Section 8.3.4.2) and to achieve this, the combined neutral and earth (CNE) conductor is earthed at the substation, at the consumer's premises, and at other places in the network _ hence, the PME classification. The choice of these other earthing locations is determined not only by the system earth impedance, but also by the need to minimize the risk of high voltage on the consumer's earth terminal in the event of an open-circuit fault on the mains.

For example, the neutral conductor of every leg of the PME low voltage sys tem should be bonded to earth at its remote end. In an overhead supply sys tem it will be necessary to ensure that the HV and LV earths at the substation are isolated to ensure the ROEP limit is not exceeded.

Many supply authorities[9,10] and national authorities[11] have their own stringent rules about the steps necessary to achieve safety on PME systems, and have specific rules about the treatment of special sites _ e.g. construction sites, caravan sites, mines, telecommunications equipment.[11,12] Such rules include steps to ensure the integrity of the supply neutral/earth conductor and qualifications on its area of cross section.

Often, specially designed CNE cables are used in underground PME systems, and on overhead distribution, an aerial bundled conductor (ABC) with the CNE concentrically arranged over the three phase conductors.

It may be noted that the distribution systems common in US practice, which bond the HV and LV neutral together and to ground at each local sup ply transformer and have a ground busbar also connected to ground at each consumer's premises, comprise a multiple earthed system.

REFERENCES:

1. IEC 60479-1. The effects of current on human beings and livestock; 2004.

2. IEEE Standard 80. Guide for safety in AC substation grounding; 2000.

3. Tagg GF. Resistances. London: George Newnes; 1964.

4. Bayliss CR, Turner H. Shock voltage design criteria. Electrical Rev 1990;18_33 May. 23_25.

5. IEC 61936-1. System engineering and erection of power installations in systems with nominal voltages above 1 kV AC and 1.5 kV DC, particularly concerning safety aspects.

6. BS 7430: 1998 (formerly CP. 1013:1965). Code of practice for earthing.

7. IEEE Std 837-2002 (revision of ANSI/IEEE Std 837-1984). IEEE standard for qualifying permanent connections used in substation grounding.

8. ANSI/IEEE Std 81-1983 (revision of IEEE Std 81-1962). IEEE guide for measuring earth resistivity, ground impedance, and earth surface potentials of a ground system.

9. Earthing and bonding. Distribution construction manual. UK: Power Networks; 2010.

10. East Midlands Electricity. Earthing manual. UK: EON; 2002.

11. Engineering Recommendation G12/3. National code of practice on the application of protective multiple earthing to low voltage networks. [Note: A new version (G12/4) is presently prepared, to include guidance on locating earth electrodes and ensuring compliance with the latest UK electricity supply, quality etc. regulations.] 12. EN50522-2010. Earthing of power installations exceeding 1 kV AC.