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Contents A. Purpose/Scope ...................................................................................................................................... 3

B. Earth electrode rods: ............................................................................................................................ 4

C. Earth round conductor electrode: ...................................................................................................... 10

D. Earth rod with round conductor electrodes combination earthing network: .................................... 11

E. Interconnection of MV and LV Earths ................................................................................................. 12

F. Calculation of Touch and Step Potentials ........................................................................................... 14

G. Allowable touch and step potentials .................................................................................................. 15

H. Current Density at Surface of Earth Electrode .................................................................................... 16

I. Appendices : ........................................................................................................................................ 19

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A. Purpose/Scope

The purpose of this report to obtain the MV&LV earthing calculation study for load centre A of KAIA project according to BS Code books

(BS 7430:1998), (BS 7354:1990) and the design report.

All the formulas and tables are copied from BS 7430:1998 and

BS 7354:1990

This study is to be used to aid in specifying the following:

Earth electrode rod length. Number of earth electrode rods per each loop. Length of Earth round conductor electrode. Resistance of MV earthing network. Resistance of LV earthing network. Combined earth resistance interconnection of the MV and LV

networks. Touch and Step Potentials. Current Density at Surface of Earth Electrode.

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B. Earth electrode rods:

The resistance to earth of a rod or pipe electrode R, in ohms, is given by the following equation:

Load Centre A - Structural floor level is 7m above MSL with the design water table level at 4m above MSL, this level is based on recommendations of HUTA and is subject to final confirmation by them.

So the effective length of the electrode rod through the wet soil (10 ohm.m resistivity) will be [(3*2.4)-(7-4)] =4.2 m, where the total length of electrode rod 3*2.4 = 7.2 m.

L = Effective Length of the electrode exist in the wet soil = 4.2 m

d =Diameter of the earthing rod = 0.02 m

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= wet Soil resistivity (according to the design report) = 10 ohm.m

Then, R = 2.486 ohm for one earthing rod.

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The total number of electrodes around the rectangle = 24 rods each of the length of 9.6 m.

So, n = 7 and s = 41 m

R = earthing resistivity for one earthing rod = 2.486 ohm

= wet Soil resistivity = 10 ohm.m

s = distance between adjacent rods = 41 m

= factor given in Table 3 = 7.03

Then, The combined resistance of all earth rod electrodes in parallel Rn =0.394 ohm.

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C. Earth round conductor electrode:

For the round conductor electrode the resistance R, in ohms is given by the following equation:

= wet Soil resistivity = 200 ohm.m

L = length of the conductor = 1500 m

h = depth of electrode = 2 m

W = diameter of 240 mm2 bare copper conductor = 0.02 m

P = coefficient given in Table 5 for Two lengths at 90 electrode arrangement = 4

Q = coefficient given in Table 5 for Two lengths at 90 electrode arrangement = 0.9

Then, the resistance R for the round conductor electrode = 0.206 ohm.

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D. Earth rod with round conductor electrodes combination earthing network:

The equivalent resistance of the earthing network RLV for LV system

RLV = 0.1353ohm

The equivalent resistance of the earthing network RMV for MV system

RMV = 0.1353 ohm

According to the design report to ensure the ground potential rise meets the requirement the resistance of the earth at the load center must be: -

This condition will be achieved as R = 0.1353 ohm less than 0.143 ohm

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E. Interconnection of MV and LV Earths The MV and LV earth electrodes shall be interconnected within the ground via a disconnectable test link.

So the combined earth resistance interconnection of the MV and LV systems can be determined from the following equation:

Then,

RT = 0.067 ohm

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F. Calculation of Touch and Step Potentials

- [m] Ground resistivity = 200

V - [V] Ground potential rise = 201

L - [m] Earth round electrode length = 1500

h - [m] Depth of earth conductor = 2

d - [m] Diameter of buried conductor = 0.02

D - [m] spacing between parallel conductors = 41

ki= (0.15n+0.7) = 1.75, where n = 7

Then, r = 171.13, where grid area equal to 92000

R = 0.4255 ohm

Then,

VT = 93.6 V and VS = 10.48 V

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G. Allowable touch and step potentials BS7354 defines the following equations for calculating the allowable touch and step potentials: -

Where:

Body resistance = 1,000

Footwear resistance = 4,000

Contact resistance = 3

It is taken from curve c2, Figure 5 of PD 6519-1:1988. At 1 second this can be taken as 50mA.so Allowable Touch and Step Voltages as following table:

Alternatively, Figure 3(a) in BS7354 graphs allowable touch and step potentials as a function of the duration of the fault. Taking the maximum time of 1 second and the minimum resistivity value gives VT < 240V and VS < 720V.

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H. Current Density at Surface of Earth Electrode

Section 15 of BS7430 gives the following equation for the allowable current density at the surface of an electrode:

Where =200 ohm.m in the electrode level and t= 1 sec.

Then,

J= 537 A/m2

Tabulating the above equation against the soil resistivity data from 3.6 to 12m within Table 1 gives the following:

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Considering a 16mm diameter earth electrode, its surface area is given by:

=.

Where:

SA - [m2] The surface area

l - [m] The electrode length

d - [m] The electrode diameter

and the earth fault current is 3000A, the minimum electrode length can be calculated using the following formula:

3000/ .

Where:

J - [A/m2] The maximum current density of the earth electrode


3000 [A] - Earth fault current

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I. Appendices : 1- ATK005-422-C100-FD-X-RPT-0000: Design Report Load Centres

A, B, C and AC. 2- Appendix G. Earthing Calculations. 3- 422-C240-FD-E-RPT-00010-B: MV ELECTRICAL EARTHING

GENERAL REQUIREMENTS.

100% Design Report Load Centres A, B, C and AC

Atkins Tracker Number: ATK005-422-C100-FD-X-RPT-00001 69

14. Earthing The earthing design for each Load Centre will generally follow the guiding principles as defined in Atkins Report 422-C240-FD-E-RPT-0004 which sets out the design basis for the Medium Voltage and Low Voltage earthing systems. The earthing system is being designed in accordance with the British Standard Code of Practice for Earthing BS 7430:1998 and also Section 7 Earthing of BS 7354:1990 and in particular the guidance given in this document regarding the management of Step and Touch Potentials.

14.1. Earth Electrode Design

The general ground conditions across the KAIA site are indicated to be a mix of sands, gravels and clays which in a dry state would generally exhibit relatively high values of electrical resistivity. However the ground water table is also at a generally high level and the ground water is indicated to be of a saline nature which, from an earth electrode design basis, provides good conditions for achieving an earth electrode of low ohmic value without need to install an extensive earth electrode network. The depth of the water table does vary across the site with water tables at the Load Centre locations being as follows:

Load Centre A - Structural floor level is 7m above MSL with the design water table level at 4m above MSL

Load Centre B - Structural floor level is 8.3m above MSL with the design water table level at 4.3m above MSL

Load Centre C - Structural floor level is 26.5m above MSL with the design water table level at 20.75m above MSL

Load Centre AC - Structural floor level is 25.75m above MSL with the design water table level at 22m above MSL

Note: The design water table level is based on a long-term uncontrolled design of groundwater level = existing groundwater level plus 2m. These levels are based on recommendations of HUTA and are subject to final confirmation by them.

In order to take advantage of the ground water requires a relatively deep electrode system to be installed and would need to be in some form of deep bored or driven electrode such as earth rods. In such instances advantage can be taken of other deep structures such as re-inforced concrete foundations or deep bored piles.

In respect of the Load Centres the foundation design of the majority of the buildings are constructed from raft type foundations the depths of with are such that they will not necessarily sit below the ground water table. Originally an architectural screen was proposed to be installed around a number of the Load Centre buildings, the height of which necessitated the need for deep bored piles to form the support for the foundation beam upon which the screen would sit. The piles will generally be in accordance with the parameters as given below and depending on the footprint of the particular Load Centre which would require in excess of 100 piles to be installed per Load Centre.

Table 21 - Foundation pile details Parameter Value



Depth of pile (approximate) Typically 12.5 metres - occasional piles up to 17m

Pile diameter 800 mm

Size of reinforcement H25S

Length of reinforcement Full length of pile

No. of reinforcing bars/formation

11 bars arranged in a ring

Spacing between piles 2m to 4m

The architectural screen is no longer part of the scope for the Load Centres however, it is proposed to retain a number of the pile foundation in the design to form the primary MV and LV earth electrodes.

Using an assumed worst case for Load Centre C where the ground water is typically just under 6 metres below the finished ground level would mean that for a 12.5 metre long pile the effective length of pile within the water table would be 6.5 metres. Using the formulae from BS 7430:1998 and assumed resistivities of wet soil and concrete of 10 ohm metres and 30 ohm metres respectively would give a typical effective resistance to earth for each pile of approximately 1.8. In accordance with Atkins report 422-C240-FD-E-RPT-0004 the maximum resistance of the earth electrode shall be 0.143 in order to limit the maximum Ground Potential Rise to 430 volts. Thus this value of earth electrode resistance could be achieved by connecting a minimum of 14 piles in parallel to achieve the maximum required earth electrode resistance for both the MV and LV earth electrodes. It shall be noted that the piles selected to be connected to the earth electrode system shall be at least 13 metres apart (twice the effective length of the piled foundation) to ensure that the maximum effectiveness of each pile is achieved and is not diminished by interaction with adjacent piles.

It is therefore proposed that a minimum number of 16 piles per electrode will be used to form the earth electrode using piles at each change of direction in the foundation. In other words 16 piles will be used to form the MV earth electrode and a further 16 shall be used to form the LV electrode.

In addition a horizontal earth electrode will be installed buried typically at a depth of 2 metres below finished ground level which will be used to interconnect each of the piles forming the earth electrode and in addition will be laid around the perimeter of each of the primary buildings and structures within the Load Centre. The horizontal earth electrode will be formed from either copper strip or bare stranded copper conductor having a minimum cross sectional area of 120mm2. It is envisaged that a minimum length of 1,500 metres of interconnecting conductor laid directly in the ground will be required per Load Centre. Since this will typically be laid at a position in the ground which is above the water table and assuming reasonably dry conditions at this level it is envisaged the soil resistivity seen by the horizontal electrode will be in the order of 200 ohm metres. Based on the formulae in BS 7430:1998 this length of conductor and assumed soil resistivity would have a typical effective resistance to earth of 0.8 ohms which can further contribute to an overall lowering of the earth electrode resistance.

Calculation sheets for the above are available in Appendix G.

The calculation for Ground Potential Rise is shown in Appendix G. The configuration of earth electrode as defined above gives a Ground Potential Rise of 215.6V.



Maximum Allowable Touch and Step potentials have been determined from Figure 3 (a) of BS 7354:1990 as set out below:

Table 22 - Load Centre Touch and Step Potentials Touch and Step Potentials Allowable Value

(V)

Touch Potential 240

Step Potential 750

The above figures were determined using the most conservative values possible. It is clear that as the Ground Potential Rise is less than the maximum allowable value for Touch potential hence it is unnecessary to calculate the Touch and Step potentials as clearly it is not possible for them to exceed the limits shown in Table 22.

14.2. Perimeter Fence

Perimeter fences formed of conductive materials have to be earthed in order to protect personnel both within the site and outside of the site of being exposed to potentially injurious voltage and current during system faults on the electrical network. The fence can either be connected to the main earth electrode within the site or independently earthed. Where a fence is tied to the main earth electrode it can in some instances be required to install an additional equipotential counterpoise conductor on the outside of the fence throughout the whole length of the fence. This is to ensure that under system fault conditions where the ground potential within the site becomes higher than the general body of earth that personnel outside of the site that come into contact with the fence are at the same ground potential as the ground within the site.

The alternative approach is to independently earth the fence and in such instances it has to be ensured that the main earth electrode is a minimum distance from the fence (typically a minimum of 3.0 metres). Since for the Load Centres the earth electrode will in the majority of the area be quite significant distances from the fence and the touch and step potentials are well below the acceptable limits it is proposed that the fence shall be independently earthed. This will be achieved by employing earth rods of typical depths of 5.66metres (12 feet) driven into the ground at each change of direction in the fence and also at maximum intervals of 100 metres in straight sections of fences. Where the fence is not continuous such as at gate positions earth rods shall also be located at either side of the break in continuity and an interconnecting conductor shall be installed to connect the two earth rods together and maintain electrical continuity of the fence.

14.3. MV and LV Earthing and Distribution Networks

The proposed system design is to provide separate earth electrodes for the MV and LV earths. These electrodes are to be interconnected at two points. As described above the electrodes shall be formed by interconnecting a series of foundation piles using buried copper conductor. There will then be two distribution systems around the Load Centre site (one for MV and one for LV earth) which will allow each facility to be connected to the earth electrode as required.



Separate MV and LV equipment earth bars will be installed in all equipment and plant rooms to which all equipment earths, equipment metal work and all other extraneous and exposed metalwork will be connected. These connections will be at sufficient intervals to ensure adequate equipotential bonding of all exposed metalwork is achieved. This will be facilitated by the installation of earthing distribution networks formed of bare copper strip affixed to walls at a suitable levels and locations that will afford connections to be made from the various items of equipment and plant and extraneous metalwork. These networks will be arranged such that the equipment connections, etc to the earthing distribution system do not form obstructions, trip hazards or any other impediment to the movement of people and equipment.

14.4. Bonding of Reinforcement

The majority of the Load Centre buildings and facilities will be constructed using reinforced concrete containing a significant amount of re-inforcement steel. Whilst it is accepted that the vast majority of the steel is covered in concrete and will not generally be accessible to personnel due to the fact that some of the foundation system will form part of the earth electrode there is every possibility that under system fault conditions the re-inforcement could experience a rise in potential.

It is therefore proposed that connections shall be made to the re-inforcement steel at a number of locations throughout all the various process plant buildings and facilities. It is proposed these connections will be made at column positions and will use proprietary system that is exothermically welded to the re-inforcement steel and is brought out to the face of the concrete as shown in the detail below:



Figure 3 - Re-inforcement steel earthing connection

Connections will then be made from these re-inforcement connection points to the earthing system distribution network.

14.5. Tunnel Earthing

Where tunnels exit from the Load Centres to other facilities such as the Passenger Terminal Building, SEC supply substations etc. the earthing distribution network within the Load Centres will be extended out to the tunnel interface point and will be connected to the earthing distribution system within the tunnel. This will be done in this manner to ensure that all systems such as cable tray work, supporting steel structures, tunnel re-inforcement steel are all connected to a common earthing system and operating at the same earth potential.



Appendix G. Earthing Calculations

General Soil resistivity 200 ohm metresConcrete resistivity 30 ohm metres

Electrode Arrangement CoefficientP Q

Horizontal Electrode Strip RoundLength of conductor 250 metres Single 2 -1.0 -1.3Depth of earth electrode 2 metres Two lengths @ 90 4 0.5 0.9Width/diameter of earthing conductor 0.0175 metres Three lengths @ 120 6 1.8 2.2Electrode Arrangement Single Four lengths @ 90 8 3.6 4.1Electrode Type Round Factor FCoefficient P 2 No. of lengths FCoefficient Q 1.3- 2 0.611 No. of lengths in parallel 2 3 0.443 Spacing between electrodes 80 metres 4 0.362 Factor F 0.611

Resistance of one electrode 1.76 ohmsResistance of multiple electrodes 1.07 ohms

DatePrepared byJob No.Project Title

Notes

Calculation Sheet

Calculation of Earth Electrode ResistanceHorizontal Electrode

in accordance with BS 7430 : 1998

Relevant extracts from BS 7430:1998

Project Ref C100/

NB; Foundations need to be spaced at least two times the depth of the electrode to ensure the effectiveness of each electrode is not diminished by interaction between

adjacent electrodes

Input Data

Output Data

KAIA5101762

C Prentice15/11/2012

General Soil resistivity 10 ohm metresConcrete resistivity 30 ohm metres

Structural SteelworkDiameter of reinforcing rod 0.012 metres No. of Rods GMDEffective length of reinforcing rod 6.5 metres 2 0.0346 Thickness of concrete between rods 0.15 metres 3 0.0621 Distance between rods 0.2 metres 4 0.1079 GMD of rod cluster 0.2306 metres 6 0.1687 Number of rods 8 round 8 round 0.2306 Number of foundations 16 8 square 0.2082

Structural SteelworkResistance of one foundation 1.23 ohmsResistance of multiple foundations 0.08 ohms

Project Title KAIAProject Ref C100/

Prepared by15/11/2012C Prentice

Job No. 5101762

Calculation of Earth Electrode ResistanceReinforced Concrete Foundationsin accordance with BS 7430 : 1998

NB; Foundations need to be spaced at least two times the depth of the electrode to ensure the effectiveness of each electrode is not diminished by interaction between adjacent electrodes

Notes

Calculation Sheet

Date

Output

Relevant extracts from BS 7430:1998

Input Data

GMD Look Up

Clearancet secs 100m 500m 1000m 100m 500m 1000m

0.1 2,400 2,900 3,400 7,300 9,200 11,000 0.2 1,700 2,100 2,400 5,100 6,500 8,100 0.3 1,250 1,500 1,800 3,800 4,700 6,000 0.4 880 1,050 1,250 2,700 3,300 4,200 0.5 600 700 810 1,800 2,250 2,900 0.6 410 500 590 1,250 1,600 2,000 0.7 330 390 470 1,000 1,300 1,600 0.8 280 340 410 870 1,100 1,400 0.9 260 315 380 790 1,000 1,250 1 240 300 340 750 950 1,200

Input Data1

Soil resistivity 200 ohm metresConcrete resistivity 30 ohm metresEarth fault current I 3000 ampsResistance of grid to earth R 0.07 ohms Touch Voltage Vt 240 voltsGrid potential rise V 215.6 volts Step Voltage Vs 750 voltsTotal length of buried conductor L 1500 metresDiameter of buried conductor d 0.0175 metresDepth of burial of grid h 2 metresSpacing between parallel conductors D 150 metres

Touch Voltage Vt N/A voltsStep Voltage Vs 32.7 volts

DatePrepared byJob No.Project Title KAIA

5101762C Prentice15/11/2012

Calculation Sheet

Calculation of Touch and Step Potentials

in accordance with BS 7354 : 1990

General

Output

W/out crushed rock - BS7354:1990 Figure 3

Project Ref C100/

Allowable Step Potential (V)Allowable Touch Potential (V)

Allowable touch/step voltages for soil resistivity of 100m has been conservatively used.Notes

Allowable touch and Step

Fault Clearance Time (s)

Atkins Ltd except where stated otherwise. The Atkins logo, Carbon Critical Design and the strap line Plan Design Enable are trademarks of Atkins Ltd.

Contact name: Phillip Norman Address: P O Box 5668, Manama, Kingdom of Bahrain Email: [email protected] Telephone: +973 1751 0400 / +973 3996 1420

ATK002-422-C240-FD-E-RPT-00010 1

Saudi Binladin Group Final Design Report MV Electrical Earthing General Requirements SBG/DAH Report No

MV ELECTRICAL EARTHING GENERAL REQUIREMENTS

ATK002-422-C240-FD-E-RPT-00010 2

Notice

This document and its contents have been prepared and are intended solely for Saudi Binladin Groups information and use in relation to King Abdulaziz International Airport.

[CONSULTANT] assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents.

Document History

JOB NUMBER: Package 422

TRACKER NUMBER: ATK002-422-C240-FD-E-RPT-00010

Revision Status Originated Checked Reviewed Authorised Date

A 100% submission for

DAH review Mike Hales Ingar Loftus - Mike Hales 10-05-12

B 100% submission for

DAH review Mike Hales Ingar Loftus - Mike Hales 10-10-12


ATK002-422-C240-FD-E-RPT-00010 3

Table of Contents

Chapter pages

1. Abbreviations and References 5

1.1. Abbreviations 5

1.2. References 5

1.3. Client References 5

1.4. Atkins References 6

2. Introduction 6

3. Scope of Work 7

3.1. Deliverables 7

4. Design Input Information 8

4.1. SEC 110/13.8kV Substations 8

4.2. Load Centres 8

4.3. MV / LV Transformers 8

4.4. MV/MV Transformers 8

4.5. LV Network 9

4.6. Ground Conditions 9

5. Earthing Design 10

5.1. Earth Connections from SEC Substations to Load Centres 10

5.2. Facilities with 13.8kV Connections 10

5.3. 13.8kV Cable Connections 11

5.4. Standards 12

5.5. Lightning Protection 12

5.6. Equipotential Bonding 12

5.7. Surge Arrestors 12

6. Earth Grid Designs 13

6.1. Earth Resistivity Model 13

6.2. MV Earth Current 13

6.3. Load Centre Earth Grid Requirements 14

6.4. Facility Earth Grid Requirements 14

6.5. Current density at the surface of an earth electrode 15

6.6. Single Earth Rod Resistances 16

6.7. Multiple Earth Rod Resistances 16

6.8. Concrete Encased Earth Electrodes 16

6.9. Conductor Size 17

6.10. Touch and Step Potentials 17

6.11. Concrete Rebar Earth Connection 17

7. Summary 19


ATK002-422-C240-FD-E-RPT-00010 4

Tables

Table 1 - Soil Resistivity Model ....................................................................................................................... 13Table 2 - Earth rod resistances as a function of depth .................................................................................... 16Table 3 - Earth Electrode Allowable Current Density ...................................................................................... 22Table 4 - Minimum electrode length ................................................................................................................ 23Table 5 - Detailed earth rod resistances as a function of depth ...................................................................... 24Table 6 - Resistive values of 12m earth rods in group arrangements ............................................................. 25Table 7 - Resistive values of 20m earth rods in group arrangements ............................................................. 26Table 8 - Concrete Encased Earth Electrodes ................................................................................................ 28Table 9 - Conductor Size Calculations ............................................................................................................ 29Table 10 - Allowable Touch and Step Voltages .............................................................................................. 31Table 11 - Site Resistivity Measurements ....................................................................................................... 36Table 12 - Summarised Resistivity Measurements ......................................................................................... 37

Figures

Figure 1 - Simplified Block Diagram for facility earth potential rise ................................................................. 15Figure 2 - Earth connections to rebar in concrete ........................................................................................... 18Figure 3 - Electrical Network Diagram for Facility Earth Model ....................................................................... 20

Appendix

A. Proposed Earth Electrode Designs 20

A.1. Earth Potential Rise at Facility Substations 20

A.2. Current Density at Surface of Earth Electrode 22

A.3. Single Earth Rod Resistances 24

A.4. Resistance of Groups of Earth Rods 25

A.5. Resistance of Concrete Encased Earth Electrodes 27

A.6. Conductor Sizing 29

A.7. Calculation of Touch and Step Potentials 30

A.8. Allowable touch and step potentials 31

A.9. Concrete Rebar Connections 32

A.10. Hot Zone 33

B. Site Measurements of Electrical Resistivity 34

C. Generic Earthing Schematic 38

D. CRS Responses 39


ATK002-422-C240-FD-E-RPT-00010 5

1. Abbreviations and References

The following abbreviations and references are used throughout this document.

1.1. Abbreviations

[1] HV High Voltage, above 13.8kV;

[2] KAIA King Abdulaziz International Airport;

[3] LV Low Voltage, 0.4kV and below;

[4] MV Medium Voltage, between 0.4kV and 13.8kV;

[5] NEC National Electrical Code;

[6] NER Neutral Earthing Resistor;

[7] RFI Request for Information;

[8] SEC Saudi Electricity Company;

[9] Zsc Short circuit impedance;

[10] TT Earth is independent of earth from power source

[11] TNS Earth and Neutral conductors are separate from power source

1.2. References

All system design, installation and commissioning works must comply with the manufacturers requirements, the requirements of authorities having jurisdiction and in accordance with the following reference standards and relevant publications from the following internationally recognised organisations:

[1] NFPA: National Fire Protection Association.

[2] ANSI: American National Standards Institute.

[3] IEEE: Institute of Electrical and Electronics Engineers.

[4] BS7430 - Code of practice for Earthing

[5] BS7354 - Design of High Voltage Substations, Section 7

[6] PD 6519-1:1988 - Guide to Effects of Current on Human Beings and Livestock Part 1:

General Aspects

[7] IEEE80 - Guide for Safety in AC Substation Grounding

[8] ASTM: American Society for Testing and Materials.

[9] ISO: Standard by the International Standard Organization.

[10] ICAA: International Civil Airports Association

[11] IBC 2006 - International Building Code 2006; Standards relating to Electrical Installations

and Equipment as issued by the IBC 2006

[12] The local power authority regulations (for the MV cable connections to SEC substations).

[13] The Saudi Arabian Distribution Code Issue 01 Revision 00 dated November 2008

[14] Saudi Building Code Electrical Requirements SBC 401

1.3. Client References

[1] SECTION 260526 - GROUNDING AND BONDING FOR ELECTRICAL SYSTEMS

[2] SECTION 264113 - LIGHTNING PROTECTION FOR STRUCTURES


ATK002-422-C240-FD-E-RPT-00010 6

1.4. Atkins References

[1] MV Cable Report reference ATK002-422-C240-FD-E-RPT-00008-A

2. Introduction

The King Abdulaziz International Airport (KAIA) is expanding its runway and passenger capability. To facilitate this expansion, a new Air Traffic Control tower, a new Passenger Terminal, Transport Facilities and supporting buildings and shall be constructed. The new airport shall initially support up to 30 million passengers per year, growing to 45 million in the first few years to an eventual capacity of 80 million passengers per year.

This document describes the Generic Earthing for the General MV and LV Systems to be installed in the King Abdulaziz International Airport to support the distribution of Electrical Power and provide a safe environment for operators, passengers and equipment. The specific Earthing design will be completed by the appropriate sections, this document providing an overall co-ordinated design basis. The Generic Earthing for the General MV and LV System comprises earth busbars, earth rods, cable and terminations to connect all metallic components of the System to an appropriate earth.

This document does not cover the requirements for earthing of telecommunication systems nor systems requiring special earthing requirements. This design will be coordinated with the designs for the earthing requirements for the telecommunications systems to ensure compatibility.


ATK002-422-C240-FD-E-RPT-00010 7

3. Scope of Work

The scope of work shall include, but not be limited to:

1) Provision of all generic earthing design necessary for the General MV and LV Systems. All necessary components shall meet the requirements of 2008 NFPA-70 Article 250;

2) All design services, drawing and specifications, equipment, materials, labour and services, not specifically mentioned or shown, which may be necessary to complete the generic design and installation of the Earthing for General MV and LV Systems;

3) Comply with the Contract Exhibits D1 and D2 revised in August 2011;

4) Generally comply with Exhibit D Section 260526 - Earthing and Bonding for Electrical Systems

3.1. Deliverables

The following are the deliverables for this package:

3.1.1. 70% Submission

1) Draft Design Report - Ref: ATK002-422-C240-DF-E-0004

2) Draft generic earthing requirements for buildings.

3.1.2. 100% Submission

1) Final Design Report

2) Final generic earthing requirements for buildings.

3) Earthing calculations.


ATK002-422-C240-FD-E-RPT-00010 8

4. Design Input Information

4.1. SEC 110/13.8kV Substations

The SEC transformers are proposed as: -

Primary voltage: 110kV

Secondary voltage: 13.8kV

Transformer capacity: 67MVA

Earthing on HV side: Solidly earthed

Earthing on MV side: via an NER (Neutral Earthing Resistor) of 5.3 Ohms limiting phase earth fault current to 1500A

Switchboard short circuit rating: 40kA for 3 sec on the 13.8kV supply

Star / Delta / Star

60Hz

SEC will not permit their earth to be connected to the KAIA earth system.

4.2. Load Centres

The Load Centres distribute the power from the SEC substations and have no direct affect on the earthing arrangement except for the generator connections. The generators are connected to the 13.8kV network via 1:1 isolating transformers. When the generators are operating, the SEC network is disconnected as is the SEC earthing arrangement. To ensure the network remains earthed the generator busbars have a switchable earthing arrangement to mimic the SEC earthing and this is achieved by a NER of 5.3 Ohms limiting phase earth fault current to 1500A.

4.3. MV / LV Transformers

The MV / LV transformers are proposed as: -

Primary Voltage: 13.8kV

Secondary Voltage: 400V

Transformer capacity: Various standard sizes up to 2000kVA

Earthing on MV side: Not applicable

Earthing on LV side: Direct earth connection

Delta / Star

4.4. MV/MV Transformers

The MV / MV transformers are proposed as: -

Primary Voltage: 13.8kV

Secondary Voltage: 4.16kV

Transformer capacity: Various sizes

Earthing on MV side: Not applicable


ATK002-422-C240-FD-E-RPT-00010 9

Earthing on LV side: via an NER (Neutral Earthing Resistor) limiting the short circuit current to 100A

Delta / Star

The value of the NER can be calculated by considering the phase to neutral voltage, the required earth current and then applying Ohms Law. This gives:

V = I.R

R = 4160 / ( 100 . 3 ) = 24

4.5. LV Network

The LV network is specified as 400/230V, 60Hz, 3 phase, 4 wire, solidly earthed.

4.6. Ground Conditions

The KAIA is situated 20 km north of the Jeddah City centre, between the ring road and the Madinah road. The KAIA measures around 105 square kilometres. The western boundary of the site is around 4 km from the Red Sea whereas the eastern boundary touches the mountainous region of the Arabian Shield. The surface soil at the site comprises silty / clayey / gravely sand and sandy silt. Some loose sandy conditions were observed along the southern periphery of the Airport fence.

The water table across the site varies between 24 metres below sea level at some locations and only 3 metres at others, therefore the calculated homogonous resistivity level across the site would vary depending on the height of the water table.

4.6.1. Site Subsurface Conditions

The sub-soil conditions at the site area have been formed in the recent and very recent geological past without any noteworthy geological digenesis surcharge or other densification or solidification effects. According to the investigations for the proposed site, extremely variable coral and alluvial deposits can prevail in such cases, with abraded or completely decomposed coralline detritus materials with medium dense to very dense sandy/silty to clayey marine soils of medium to good bearing capacity. The coralline soil layers were encountered at shallow depths.

The coral soils are overlain by recent deposits. These top soils are partly sandy, though in most cases silty. The soil underlying coral is alluvium comprising Wadi deposits i.e., clayey, silty sand with gravel.


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5. Earthing Design

The Earthing Design shall be compliant with BS7430 - Code of practice for Earthing. The design shall ensure the safe operation of the electrical network during normal and abnormal conditions and protection personnel and equipment. The safe operation is demonstrated by calculations.

5.1. Earth Connections from SEC Substations to Load Centres

SEC have confirmed verbally, at meeting on 21st June, they will not permit their earth within the

110/13.8kV substations to be connected to the earth within the KAIA installation. Thus the earth connection between these points is considered to be TT.

5.2. Facilities with 13.8kV Connections

All facilities with 13.8kV connections are to have a similar arrangement, which is a 13.8kV supply and dual 13.8/0.4kV transformers providing LV supplies. Hence these are considered to be common designs. This will apply to Load Centres, Passenger Terminal Building, Mosque and many other locations.

The primary requirement is to establish a MV earth and a LV earth. These are to be installed as electrically separate. To comply with the requirements of BS7430 and to enable the MV and LV earths to be interconnected, the maximum combined earth resistance allowed is 1, therefore the target value of the earthing will be: -

MV Earth < 1

LV Earth < 1

However, the maximum allowable earth value for both is 5; this value is only to be used where achieving the target value is considered to be impractical and additional calculations would be required to ensure the system is safe.

See also Section 6.4, where the value of the earth resistance is calculated to maintain the ground potential rise to below 430V. This section takes precedence over the target values.

For any earthing arrangement for the distribution network, it is important to consider that this should not clash with any additional earthing requirements for telecommunications and lightning protection.

5.2.1. MV Earth

The MV earth will be achieved by a group of earth rods installed external to the building, the number of rods depending on the ground conditions but not less than 2. These rods will be interconnected. A minimum of 4m separation will be maintained between this rod group and any LV earthing.

At any location where the MV earth does not maintain the 4m separation from the LV earth, the MV earth will be insulated to maintain the electrical isolation between the two systems.

The MV Main Earth Bar will be connected to MV Main Earth Bars in adjacent substations.

5.2.2. LV Earth

The LV earth will be achieved by a buried copper conductor surrounding the building to achieve a LV Earth Loop. Duplicate stranded copper conductor cables will connect the LV Earth Loop back to the LV Main Earth Bar. The LV Earth Loop will be supplemented with earth rods to achieve the required LV Earth Resistance. The number of earth rods will be dependent on the ground conditions but the minimum will be two earth rods.

Where practical, the LV Main Earth Bar will be connected to LV Main Earth Bars in adjacent substations.


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5.2.3. Clean Earth

The clean earth will be established via a separate group of earth rods located a minimum of 1800mm from all other earth rod groups. The clean earth shall not be connected to any other earth system. The minimum value of the earth rod group shall be 1.

Clean earths shall typically be provided for data systems, telephony, other communication systems and UPS.

5.2.4. Interconnection of MV and LV Earths

Within the substation, the MV Main Earth Bar and LV Main Earth Bar shall be electrically independent. The MV and LV earth electrodes shall be interconnected within the ground via a disconnectable test link.

Interconnection of MV and LV earths is discussed in Section 19.2 of BS7430. To permit the interconnection of the MV and LV earth, the standard requires the combined resistance of the earth electrode to be less than 1 and the rise of earth potential not to exceed 430V.

To ensure this arrangement is safe, the ground potential rise will be calculated as per Section 16 of BS7430 and confirmed as lower than the acceptable touch and step voltages within the same standard.

5.2.5. 13.8/0.4kV Transformer LV Star Point Earthing

The 13.8/0.4kV Transformer LV star point will be directly connected to the LV Main Earth Bar to provide a TNS earthing solution for the 400V network.

5.2.6. 13.8/0.4kV Transformer Foundations

The 13.8/0.4kV transformer foundations shall include a perimeter earthing conductor loop, earth rods as required, and shall be connected to the MV earth. Where transformers are mounted internal to buildings, this will be achieved by suitable connections to the reinforcement bars within the concrete, see Section 6.11.

5.2.7. MV/LV Substations

The equipment within the MV/LV substations is to be supplied as an integrated solution, incorporating MV switchgear, MV/LV transformer, LV switchgear and busduct providing a single metallic connection in terms of earthing.

The MV switchgear, MV/LV transformer and by implication the LV enclosure require connecting to the MV Earth.

It has been agreed via an RFI that the MV and LV earth systems can be connected. For this to be completed whilst maintaining system of the personnel and equipment, the specific requirements within BS7430 must be met. These are discussed in Section 5.2.4.

The outgoing LV circuits require connecting to the LV Earth mat.

To ensure safety, all exposed metal within the MV/LV substations shall be connected to the MV Main Earth Bar. This will include but not be limited to: ventilation ductwork, pipe work and structural steel. To provide an equipotential zone within the substation the concrete reinforcement bars within the substation shall also be connected to the MV Main Earth Bar.

5.3. 13.8kV Cable Connections

The 13.8kV cables include a non-magnetic bare copper drain wire screen. The revised specifications changed the screen on the MV cables to 47mm

2 per conductor that is equivalent to 141mm

2 per

three phase trefoil group. The MV Cable Design considers the requirements for the earthing of the MV cable screen.

The MV Cable Design concludes this screen is adequate to control the sheath voltage and therefore separate earth conductors are not required for control of the screen voltage.


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However, as per Section 2.6.3.2 of Specification Aprons, Taxiways, Roads, Tunnels, Bridges, Load Centres and Infrastructure, Transportation Centre and Supporting Facilities Exhibit D Part D1 requires a separate earth conductor installed in a separate duct: -

Medium Voltage: 13.8 kV and 4.16 kV, 60 Hz 3 phases, 3 wires, and ground (Ground conductor is to be installed in a separate duct).

Therefore for each 13.8kV cable connection a separate 120mm2 bare stranded copper conductor will

be installed. Where the cables are ducted, this conductor will be installed in a separate duct. The function of this conductor is to duplicate the interconnection provided by the MV cable screens and provide an interconnected MV earth network.

The MV Cable screens to the earth bar within the MV switchboards. The 120mm2 earth conductors

associated with the MV Cables will be connected to the MV Main Earth Bar.

As per Section 5.1, there is no earth connection between the SEC substations and the Load Centres. Therefore a separate earth conductors shall be installed and the screens of the MV cables earthed at the SEC substations via this separate conductor. A carefully coordinated design will be required to ensure the MV cable screens are earthed without interconnection the KAIA and SEC earth systems. Similarly, the MV cable support structures will require careful design to ensure they do not provide an earth path between the KAIA and SEC substations.

5.4. Standards

When calculating the earth electrode resistance BS7430 considers the earth rods. It does not consider the contribution from the buried electrode connecting the earth rods. Also, BS7430 does not provide a means of calculating the touch and step potentials.

Given the length of conductor required for the load centre BS7430 would under-estimate the overall electrode resistance.

Section 7 of BS7354 Design of high-voltage open-terminal stations provides the equations necessary to address these aspects. Therefore Section 7 of BS7354 will be used in the following calculations.

Informative note: BS EN 50522 Earthing of power installations exceeding 1kV AC is due for release soon and will combine the earthing requirements from BS7430 and BS7354 into one document.

5.5. Lightning Protection

The lightning protection scheme shall comply with Specification 264113 and IEC/BS EN 62305. This shall be achieved by locating earth pits to minimise the length of lightning conductors. The lightning earth pits shall have a maximum resistance of 10 and shall be interconnected to other earth systems.

5.6. Equipotential Bonding

Equipotential bonding shall be installed in compliance with BS7671. The protective conductor cross-sectional area shall not be less than 4mm

2.

5.7. Surge Arrestors

Surge Arrestors shall be connected the to earth grid using the minimum standard conductor size to provide a power frequency earth path.

Supplementary earthing shall be used between the surge arrestor and the earth grid to provide high frequency earthing. This shall utilise stranded conductors with no sharp changes of direction connected via as shorter path as practical to the earth grid. Where possible these high frequency earth connections will be made to dedicated earth rods.


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6. Earth Grid Designs

6.1. Earth Resistivity Model

Earth resistivity measurements have been taken on site at Load Centre A, Load Centre B and Load Centre C. The measurements taken are detailed in Table 11 - Site Resistivity Measurements within Appendix B.

The measurements were taken using the Wenner Array measurement system. This 4 probe measurement technique allows the electrical resistivity at increasing soil depth to be investigated by increasing the spacing of the test probes. The probe spacing effectively gives the depth at which the measurement is being taken. Therefore a resistivity model against soil depth can be built up by increasing the probe spacing. Table 12 - Summarised Resistivity Measurements shows the measurements summarised against soil depth.

Using Table 12 the following soil model against varying earth electrode depth can be determined:

Soil Depth / Probe Spacing

(m)

Maximum Resistivity

(.m)

Soil Resistivity

Model

(.m)

0.75 870 290

1.1 560 240

1.6 420 220

2.4 245 210

3.6 140 120

5.4 85 34

8.1 25 22

12 6 6

20 4 4

Table 1 - Soil Resistivity Model

The above table has been determined by considering the maximum measured values and the second maximum measured value. The second maximum measured value is used for the soil resistivity model for soil depths between 0.75 and 8.1m as the maximum value is a single value which is unrepresentative of the bulk of measurements taken. We consider this to be a worst case model and would expect the actual reading achieved on site to be no worse than the details identified.

As per Specification 260526 clause 2.2 we seek the Engineers agreement to the soil model shown in Table 1.

6.2. MV Earth Current

The MV earth fault current with two SEC transformers in parallel is 3000A. Specification 260526 clause 2.4 (C) 3 states a diversity of 0.8 is to be used in mesh systems. The interconnection between the load centres and the facilities does provide a degree of mesh design, however it is not considered this meets the requirements of the clause and therefore a diversity of 1.0 is used.


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6.3. Load Centre Earth Grid Requirements

The design of the earth grid is to comply with the requirements of BS7430. Further there is a requirement for the MV and LV earths to be interconnected. Section 19.2 of BS 7430 requires the ground potential rise to be less than 430V and the combined earth resistance to be 1 or less for interconnection of the MV and LV earths.

The maximum earth current is when two SEC transformers are connected in parallel. As per Section 4.1 each transformer will contribute 1500A giving a maximum earth current of 3000A. Thus to ensure the ground potential rise meets the requirement the resistance of the earth at the load centre must be: -

4303000 0.143 The Load Centre earth grid will achieve this value without considering the contribution from the outgoing circuits and their associated earthing.

Also, in order to meet the requirements of Specification 260526 clause 2.4 (D) 6, the above rating shall be achieved with the rod group with the lowest calculated resistance disconnected.

Both of the above points will mean the Load Centre earth resistance will, under normal conditions, be less than 0.143 as required by BS7430 and therefore the MV and LV earths can be interconnected.

6.4. Facility Earth Grid Requirements

The maximum earth fault at each facility is as per Section 6.2 that is 3000A. This fault current will flow into the local earth electrodes and back to the Load Centre via the earth conductors installed with the MV cables and the screens of the MV cables.

As per the Load Centre design, the design of the facility earth grid is to comply with the requirements of BS7430 and there is the requirement for the MV and LV earths to be interconnected. Section 19.2 of BS 7430 requires the ground potential rise to be less than 430V and the combined earth resistance to be 1 or less for interconnection of the MV and LV earths.

The facility has its own earth grid providing a local earth resistance. It is also connected to the Load Centre by a minimum of 2 MV cables each having associated screens and earth conductors. These act in series with the earth grid resistance at the Load Centre. The MV cable screens are 47mm

2 per

single phase core and therefore 141mm2 per trefoil group. The earth conductors are 120mm

2.

Thus a model is required to determine the relative flow of earth current under fault conditions. The model used is:


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Figure 1 - Simplified Block Diagram for facility earth potential rise

The voltage rise at the facility substation is calculated in Appendix A.1. This shows the maximum acceptable total earth resistance at the facility substation is 0.28.

Referring to Section 5.2 the target resistance for MV and LV earths is 1. These target for MV and LV earths must be reduced to 0.56, the combination in parallel achieving the required earth impedance.

The simplified block diagram in Figure 1 only considers 2 MV sets of MV cable screens and 2 associated earth conductors. In most installations the minimum will be 4 of each. Also, the simplified block diagram does not consider additional substations along the feeder providing additional earthing. These considerations indicate that the earth resistance stated is conservative.

6.5. Current density at the surface of an earth electrode

In general, soils have a negative temperature coefficient of resistance so that sustained current loading results in an initial decrease in electrode resistance and a consequent rise in the earth fault current for a given applied voltage. However, as soil moisture is driven away from the soil-electrode interface, the resistance increases and will ultimately become infinite if the temperature rise is sufficient. For short-duration loading this occurs in the region of 100 C and results in complete failure of the electrode.

Section 15 of BS7430 gives the relevant equation to confirm the suitable sizing of the earth electrode. Calculations are completed in Appendix A.2 for a 16mm diameter earth rod and at the various

Load Centre

3000A

IR

ILRLRR

IF

Key120mm2 Cu Earth Conductor47mm2 x 3 Cu MV Cable ScreenIF - Earth fault currentIR - Proportion of IF flowing to remote earth networkIL - Proportion of IF flowing to local earth networkVL - Voltage rise at local earth networkRL - Resistance of local earth networkRR - Resistance of remote earth networkRC - Resistance of earth conductorsRS - Resistance of MV cable screen

VL

Simplified Block Diagram

Facility


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resistivity values shown in Table 1, showing the minimum summated earth rod length required for the various resistivities under fault conditions.

6.6. Single Earth Rod Resistances

Using the resistivity values in Table 1 the resistance of different depth earth rods can be calculated. Given the layered resistivity model as per Table 1, each earth rod is calculated as a series of smaller earth rods all connected in parallel. This takes advantage of the majority of the earth rod length. Earth rod resistances are calculated in Appendix A.3 and summarised: -

Rod Length (m)

Rod Resistance

()

3.6 150.3

5.4 61.6

8.1 10.4

12 3.759

20 0.707

25 0.390

Table 2 - Earth rod resistances as a function of depth

Earth rods 9m in length or less are considered shallow and driven. Earth rods greater than 9m in length are considered deep.

To achieve the resistances required in Sections 6.3 and 6.4 and the surface current densities given in Section 6.5 and Appendix A.2, it is clear that the lower resistivities provided by longer / deeper earth rods will be required. As a practical observation, it is recommended that the minimum earth rod length considered is in excess of 12m.

The earth rod resistances calculated above should be achieved across the whole KAIA site due to the resistivity data used. The design could be optimised by taking electrical resistivity measurements at individual facilities. In the majority of locations this will provide lower resistivity values and will allow the design to be optimised.

6.7. Multiple Earth Rod Resistances

Using the single earth rod resistances in Section 6.6, the values of groups of earth rods can be calculated. Combinations of earth rods are considered in Appendix A.4. The lengths of earth rods selected are aligned to earth rod lengths in Table 2 and considered for 12m and 20m lengths.

Results are provided in Table 6 and Table 7 of Appendix A.4.

6.8. Concrete Encased Earth Electrodes

Alternative designs to the earth rod can be considered. One such alternative design is to consider concrete encased earth electrodes. The resistance of such arrangements is very much dependent of the design. Some typical calculations are provided in Appendix A.9. The results shown in Table 8 that concrete encased earth electrode can provide significant contribution to the earth grid.


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6.9. Conductor Size

The earth conductor size is checked in Appendix A.6. To comply with the requirements of 260526 the MV earth conductor is selected to have a minimum cross section of 120mm

2.

The temperature rise for a 120mm2 conductor under MV fault current is also calculated in Appendix

A.6 and is shown to be small such that it is practical for the earth conductor to be installed adjacent to the MV cables without the need for additional precautions.

6.10. Touch and Step Potentials

Formulae to calculate the touch and step potentials are included in Appendix A.7. These must be used once the earth grid design for the facility has been finalised.

The effect of a voltage applied to the body varies significantly from person to person. On a balance of probabilities, the time dependent body current used to establish the tolerable voltage is the curve c2 of Figure 5 of PD 6519-1:1988. Body resistance also varies but most standards use a value of 1,000. The contact resistance at the surface of the ground also adds resistance which limits the body current and a value of 3 times the ground resistivity per foot is taken. There is growing international acceptance that footwear resistance should be taken into account and this now is UK practice. Footwear resistance is taken as 4,000 per foot.

The allowable Touch and Step potentials are calculated in Appendix A.8 using the above information. The calculated values of touch and step potential must be less than the allowable touch and step potentials as indicated in Table 10 of Appendix A.8.

6.10.1. Hot Zone

The ground potential rise is limited by design to be less than 430V. Therefore the hot zone as designed in BS7354 is within the earth electrode. Therefore calculations as per Appendix A.10 are not necessary.

6.10.2. High Resistivity Surface Layer

The use of high resistivity surface layer can be useful for reducing the touch and step potentials. The KAIA includes significant quantities of MV equipment installed indoors on concrete floors. These areas could not implement high resistivity surface layers.

6.11. Concrete Rebar Earth Connection

Specification 260526 states the following requirements within Section 2.7 (D): -

When the reinforcing in concrete is used as a part of the earthing system the fittings used to provide a connection point at the surface of the concrete shall be exothermically welded to a reinforcing bar. This fitting shall be provided with a bolted connection for an earthing conductor. The main bars in the reinforcing shall be welded together at intervals to ensure electrical continuity throughout the reinforcing.

Summarising the requirement gives: -

Connection shall be exothermically welded to the rebar.

The fitting shall provide a bolted connection for an earth conductor.

The main bars in the reinforcement shall be welded together at intervals to ensure electrical continuity.

The effects of fault current flowing in the rebar must not be detrimental to the rebar and / or the concrete and therefore the temperature rise during fault conditions must be limited. Appendix A.9 calculates the minimum cross sectional area required to limit the temperature rise to 55

oC as

174mm2; as 16mm diameter rebar has an approximate cross sectional area of 200mm

2 this is

considered safe. The design further reduces the effect of heating on the rebar by requiring the main rebar either side of the connection to be welded together, thus distributing any fault current. This is shown diagrammatically as: -


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Figure 2 - Earth connections to rebar in concrete

Note: Whilst the calculations in Appendix A.9 consider the MV fault current, the connection to the rebar is not intended to form part of the earth fault current path. This connection is intended to ensure equipotential voltages during fault conditions to protect personnel and equipment.


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7. Summary

This report shows how the requirements for the earthing as part of the KAIA installation can be met. The report and calculations are based on the British Standard BS7430 as required by the revision to Contract Exhibits D1 and D2. British Standard BS7354 has been used for equations to calculate the touch and step potentials, as these calculations are not included within BS7430.


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A. Proposed Earth Electrode Designs

A.1. Earth Potential Rise at Facility Substations

The Simplified Block Diagram showing in Figure 1 can be represented as an electrical network diagram as below:

Figure 3 - Electrical Network Diagram for Facility Earth Model

Solving this network:

The resistance of the earth conductors and MV cable screens can be calculated by the following equation: -

=

Where:

R - [] Total resistance

- [m] Resistivity of material l - [m] Length of conductor A - [m2] Cross sectional area of conductor For copper, = 1.68 x 10-8 at 20oC With the maximum feeder length taken to be 5km long and the MV screens 141 (74 x 3)mm

2 cross

sectional area, their resistance can be calculated to be 0.596. Using the same feeder, the earth conductor resistance can be calculated to be 0.7.

RC

RC

IR

ILIR RLRR

3000A

VL

IF

Key120mm2 Cu Earth Conductor47mm2 x 3 Cu MV Cable ScreenIF - Earth fault currentIR - Proportion of IF flowing to remote earth networkIL - Proportion of IF flowing to local earth networkVL - Voltage rise at local earth networkRL - Resistance of local earth networkRR - Resistance of remote earth networkRC - Resistance of earth conductorsRS - Resistance of MV cable screenRF - Total feeder resistance

Electrical Network Diagram

RS

RS

RF


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The combined resistance of the MV cable screens and the associated earth conductors can be calculated as:

= 0.5962 . 0.720.5962 + 0.72 = 0.16 Using the model in Figure 3, the following equations can be determined using Ohms Law: -

= . = ( ) ( + ) Solving gives:

= ( + ) + + = . ( + ) + + Where:

IF - Earth fault current

IR - Proportion of IF flowing to remote earth network

IL - Proportion of IF flowing to local earth network

VL - Voltage rise at local earth network

RL - Resistance of local earth network

RR - Resistance of remote earth network

RF - Total feeder resistance

Rearranging:

= ( + )( ( + ) ) VL must be less than 430V to satisfy BS7430, IF is 3000A, RF is 0.16 and RR is 0.143 as per Section 6.3. Solving the equation shows RL is required to be less than 0.28 to maintain the VL below the 430V limit.


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A.2. Current Density at Surface of Earth Electrode

Section 15 of BS7430 gives the following equation for the allowable current density at the surface of an electrode:

J = 103 57.7

Where:


- [m] The ground resistivity

t - [sec] the duration of the fault

Tabulating the above equation against the soil resistivity data from 3.6 to 12m within Table 1 gives the following:

Soil Resistivity

(.m)

Time

(sec)

Allowable Current Density

(A/m2)

120 1 693

34 1 1303

22 1 1619

6 1 3101

4 1 3798

Table 3 - Earth Electrode Allowable Current Density

Considering a 16mm diameter earth electrode, its surface area is given by:

= . Where:

SA - [m2] The surface area

l - [m] The electrode length


and the earth fault current is 3000A, the minimum electrode length can be calculated using the following formula:

3000 .

Where:



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3000 [A] - Earth fault current

Soil Resistivity (.m)

Allowable Current Density

(A/m2)

Minimum Electrode

Length (m)

120 693 86

34 1303 46

22 1619 37

6 3101 19

4 3798 16

Table 4 - Minimum electrode length


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A.3. Single Earth Rod Resistances

Based on the resistivity values in Table 1 the resistance of earth rods of various depths can be calculated using Section 10.2 of BS7430: - R =

2L ln 8L

d 1 Section 10.2

Where:

- [m] Ground resistivity L - [m] Length of rod

d - [m] Diameter of rod = 16mm

Given the resistivity model is layered, each section of earth rod is calculated and then the total rod resistance is calculated by considering the sections in parallel (sections of earth rod with resistance greater than 100 are not included in the total rod resistance):

Rod Length (m)

Section Length

(m)

Section Resistance

()

Rod Resistance

()

1.6 0.5 345.4 Not Calculated



5.4 1.8 61.6 61.6

8.1 2.7 12.4 10.4

12 3.9 5.904 3.759

20 8 0.871 0.707

25 5 0.869 0.390

Table 5 - Detailed earth rod resistances as a function of depth

Earth rods 9m in length or less are considered shallow and driven. Earth rods greater than 9m in length are considered deep.


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A.4. Resistance of Groups of Earth Rods

Section 10.2 of BS7430 allows the resistance of a group of earth rods connected in a straight line to be calculated using the equations: -

= 1+ - Section 10.2

Where:

R - [] Resistance of single rod, as Appendix A.3

= 2Rs s - [m] Distance between rods

- Is stated in BS7430 in Tables 2 and 3 for the various configurations

n - number of earth rods

Giving: -

12m Rods

Item Equilateral

Triangle Hollow square Straight Line Units

n 3 2 3 4 5

R 3.8 3.8 3.8 3.8 3.8

s 24 24 24 24 24 m

22 22 22 22 22 m

0.038 0.038 0.038 0.038 0.038

1.66 2.71 4.51 2.15 2.54

Number of Electrodes 3 4 8 4 5

Total Length of Electrode* 11.7 15.6 31.2 15.6 19.5 m

Rn 1.35 1.05 0.56 1.03 0.83

Table 6 - Resistive values of 12m earth rods in group arrangements

* - adjusted based on resistivity and maximum current density


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20m Electrodes

Item Equilateral

Triangle Hollow square Straight Line Units

n 3 2 3 3 4

R 3.8 3.8 3.8 3.8 3.8

s 40 40 40 40 40 m

6 6 6 6 6 m

0.006 0.006 0.006 0.006 0.006

1.66 2.71 4.51 2.15 2.54

Number of Electrodes 3 4 8 3 4

Total Length of Electrode* 24 32 64 24 32 m

Rn 1.28 0.97 0.49 1.28 0.97

Table 7 - Resistive values of 20m earth rods in group arrangements

* - adjusted based on resistivity and maximum current density


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A.5. Resistance of Concrete Encased Earth Electrodes

Section 12.2 of BS7430 considers using structural steelwork encased in concrete. The resistance of a single arrangement is given by: -

= 12 ( ) 1 + + 2 Where:

- [m] Soil resistivity

c - [m] Concrete resistivity

L - [m] Length below ground

- [m] Thickness of concrete between rods and soil

z - [m] Value from Table 9 of BS7430

Considering the following arrangement:

Gives:

= 52 . . 7 8 Where:

a - [m] Radius of reinforcement bar

s - [m] Distance between adjacent rods

Using the following values:

c = 30m

= 0.15m

a = 0.006m

s = 0.2m

and the values of resistivity given in Table 1, allows the following values to be calculated:

sa


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Item Case 1 Case 2 Case 3 Units

22 6 4 m

L 12 8 5 m

R 1.44 0.77 0.93

Table 8 - Concrete Encased Earth Electrodes


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A.6. Conductor Sizing

The required conductor size can be calculated using Section 14 of BS7430: -

=

Section 14

Where:

S - [mm2] Conductor cross sectional area

I - [A] Fault current

t - [sec] Fault current duration

k - Constant from Table 10 of BS7430

The fault current is rated at earth fault current for the MV installation and LV installation. The fault duration is as per 260526 Section 2.5 (G), which also specifies the maximum final temperature of 160

oC enabling k to

be looked up in Table 10 of BS7430. These give: -

Item MV LV Units / Comments

I 3,000 40,000 A

t 1.0 0.4 Seconds

k 138 138 Limit final temperature to 160oC

S 22 183 mm2

Table 9 - Conductor Size Calculations

Specification 260526 Section 2.5 (G) states the minimum earth conductor to be 120mm2 and hence this size

is selected for the MV network.

Using further equations in Section 14 of BS7430: -

= log 2 + 1 + Where: k - [A/mm2] Current density 1 - [

oC] Initial temperature

2 - [oC] Final temperature

K and are material specific constants (for copper 226 and 254 respectively) Using the above equations it is possible to calculated the final temperature of a 120mm

2 conductor with a

3000A fault for 1 second and an initial temperature of 40oC. This gives a final temperature of 43.6

oC .


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A.7. Calculation of Touch and Step Potentials

BS7354 defines the following equations to calculate the touch and step potentials: -

= ln + 12 + 1+ + 1 0.52 - Equation [17]

= 12 + 1+ + 1 0.52 - Equation [20]

Where: VT - [V] Touch Voltage Vs - [V] Step Voltage - [m] Ground resistivity

V - [V] Ground potential rise

R - [] Earth electrode resistance

L - [m] Earth electrode length

h - [m] Depth of earth conductor

d - [m] Diameter of buried conductor

D - [m] spacing between parallel conductors

n - Number of parallel conductors / cables / lines ki = (0.15n + 0.7)


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A.8. Allowable touch and step potentials

BS7354 defines the following equations for calculating the allowable touch and step potentials: -

= + ( + )2 = { + 2( + ) }

Where:

Body resistance = 1,000

Footwear resistance = 4,000

Contact resistance = 3 It is taken from curve c2, Figure 5 of PD 6519-1:1988. At 1 second this can be taken as 50mA.

Resistivity

(m)

Allowable Touch

Voltage

(V)

Allowable Step

Voltage

(V)

500 188 600

200 165 510

100 158 480

50 154 465

Table 10 - Allowable Touch and Step Voltages

Alternatively, Figure 3(a) in BS7354 graphs allowable touch and step potentials as a function of the duration of the fault. Taking the maximum time of 1 second and the minimum resistivity value gives VT < 240V and VS < 720V.


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A.9. Concrete Rebar Connections

BS7430 includes data and calculations to consider temperature rises commencing at 150oC. The likely effect

of this temperature on rebar within concrete is considered to be detrimental to the concrete. Therefore IEEE80 Equation 10-13 is considered to be more appropriate as it allows lower temperatures to be considered: - Amm = If

TCAP x 10tc .r .r 4 .lnKo+ TmKo+ Ta [Equ 10-13]

Where: If = 3 kA This is the MV earth fault level Tm = 55 C All other values from Table 10-1 of IEEE80 Ta = 40 C r = 0.0016

r = 15.9 .cm tc = 1 sec. TCAP = 3.28 Ko = 605 Amm = 174 mm

This calculation shows that to limit the temperature rise of the rebar to 55oC will require a cross sectional

area of 174mm or greater. 16mm diameter rebar has a cross sectional area of approximately 200mm and therefore will meet this requirement.


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A.10. Hot Zone

The hot zone can be calculated using equation 23 in BS7354. This enables the extent of voltage contours outside of the earth grid to be determined. The equation is given by: -

= 1 sin2 1

Where

x - [m] is the distance from the edge of the grid to the extent of the hot zone

r - [m] is the equivalent circular plate radius

Vx - [V] is the hot zone voltage

V - [V] is the ground potential rise

Within the UK the value of Vx it usually taken as either 430V or 690V. As both the load centres and the package substations are designed to maintain the ground potential rise below 430V the hot zone is maintained within the earth electrode and the calculation of the zone is unnecessary.


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B. Site Measurements of Electrical Resistivity

The following tabulates the electrical resistivity measurements taken on site:

Load Centre

Test No Anode Depth (cm)

Probe Spacing

(m)

Resistance Reading

()

Constant K"

(2..a)

Apparent Resistivity

(.m) A 1 15 0.3 2.960 1.885 5.579

A 1 15 0.5 0.440 3.142 1.382

A 1 15 0.75 0.420 4.712 1.979

A 1 15 1.1 0.400 6.912 2.765

A 1 15 1.6 0.380 10.05 3.820

A 1 15 2.4 0.370 15.08 5.579

A 1 15 3.6 0.240 22.62 5.429

A 1 15 5.4 0.190 33.93 6.447

A 1 15 8.1 0.080 50.89 4.072

A 1 15 12 0.010 75.40 0.754

A 1A 15 0.3 4.880 1.885 9.199

A 1A 15 0.5 4.570 3.142 14.36

A 1A 15 0.75 0.180 4.712 0.848

A 1A 15 1.1 0.290 6.912 2.004

A 1A 15 1.6 0.280 10.05 2.815

A 1A 15 2.4 0.250 15.08 3.770

A 1A 15 3.6 0.160 22.62 3.619

A 1A 15 5.4 0.080 33.93 2.714

A 1A 15 8.1 0.010 50.89 0.509

A 2 15 0.3 0.560 1.885 1.056

A 2 15 0.5 0.510 3.142 1.602

A 2 15 0.75 0.370 4.712 1.744

A 2 15 1.1 0.270 6.912 1.866

A 2 15 1.6 0.400 10.05 4.021

A 2 15 2.4 0.070 15.08 1.056

A 2 15 3.6 0.050 22.62 1.131

A 2 15 5.4 0.020 33.93 0.679

A 2 15 8.1 0.000 50.89 A 2A 15 0.3 0.540 1.885 1.018

A 2A 15 0.5 0.390 3.142 1.225

A 2A 15 0.75 0.270 4.712 1.272

A 2A 15 1.1 0.200 6.912 1.382

A 2A 15 1.6 0.130 10.05 1.307

A 2A 15 2.4 0.070 15.08 1.056

A 2A 15 3.6 0.040 22.62 0.905

A 2A 15 5.4 0.010 33.93 0.339

A 3 15 0.3 4.670 1.885 8.803

A 3 15 0.5 3.180 3.142 9.990

A 3 15 0.75 3.030 4.712 14.279

A 3 15 1.1 1.670 6.912 11.542

A 3 15 1.6 0.640 10.053 6.434

A 3 15 2.4 0.090 15.080 1.357

A 3 15 3.6 0.050 22.619 1.131

A 3 15 5.4 0.010 33.929 0.339


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Load Centre


Probe Spacing

(m)

Resistance Reading

()

Constant K"

(2..a)


(.m) A 3 15 8.1 0.000 50.894

A 3 15 12 0.000 75.398 A 3A 15 0.3 5.260 1.885 9.915

A 3A 15 0.5 3.250 3.142 10.210

A 3A 15 0.75 3.180 4.712 14.985

A 3A 15 1.1 2.150 6.912 14.860

A 3A 15 1.6 1.050 10.053 10.556

A 3A 15 2.4 0.620 15.080 9.349

A 3A 15 3.6 0.190 22.619 4.298

A 3A 15 5.4 0.080 33.929 2.714

A 3A 15 8.1 0.010 50.894 0.509

A 3A 15 12 0.000 75.398 B 1 15 0.3 2.930 1.885 5.523

B 1 15 0.5 1.740 3.142 5.466

B 1 15 0.75 0.950 4.712 4.477

B 1 15 1.1 0.400 6.912 2.765

B 1 15 1.6 0.250 10.053 2.513

B 1 15 2.4 0.110 15.080 1.659

B 1 15 3.6 0.050 22.619 1.131

B 1 15 5.4 0.040 33.929 1.357

B 1 15 8.1 0.030 50.894 1.527

B 1 15 12 0.000 75.398 B 1A 15 0.3 3.790 1.885 7.144

B 1A 15 0.5 1.760 3.142 5.529

B 1A 15 0.75 0.850 4.712 4.006

B 1A 15 1.1 0.540 6.912 3.732

B 1A 15 1.6 0.260 10.053 2.614

B 1A 15 2.4 0.160 15.080 2.413

B 1A 15 3.6 0.090 22.619 2.036

B 1A 15 5.4 0.020 33.929 0.679

B 1A 15 8.1 0.010 50.894 0.509

B 1B 15 0.3 2.730 1.885 5.146

B 1B 15 0.5 1.400 3.142 4.398

B 1B 15 0.75 0.750 4.712 3.534

B 1B 15 1.1 0.470 6.912 3.248

B 1B 15 1.6 0.290 10.053 2.915

B 1B 15 2.4 0.130 15.080 1.960

B 1B 15 3.6 0.080 22.619 1.810

B 1B 15 5.4 0.080 33.929 2.714

B 1B 15 8.1 0.000 50.894 B 2 15 0.3 16.980 1.885 32.007

B 2 15 0.5 8.770 3.142 27.552

B 2 15 0.75 1.680 4.712 7.917

B 2 15 1.1 0.510 6.912 3.525

B 2 15 1.6 0.350 10.053 3.519

B 2 15 2.4 0.210 15.080 3.167

B 2 15 3.6 0.200 22.619 4.524

B 2 15 5.4 0.140 33.929 4.750

B 2 15 8.1 0.010 50.894 0.509

B 2 15 12 0.000 75.398


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Load Centre


Probe Spacing

(m)

Resistance Reading

()

Constant K"

(2..a)


(.m) B 2 15 20 0.000 125.664

B 2 15 30 0.000 188.496 B 2B 15 0.3 2.510 1.885 4.731

B 2B 15 0.5 1.220 3.142 3.833

B 2B 15 0.75 1.200 4.712 5.655

B 2B 15 1.1 0.610 6.912 4.216

B 2B 15 1.6 0.500 10.053 5.027

B 2B 15 2.4 0.460 15.080 6.937

B 2B 15 3.6 0.130 22.619 2.941

B 2B 15 5.4 0.120 33.929 4.072

B 2B 15 8.1 0.010 50.894 0.509

B 2B 15 12 0.000 75.398 C 1A 15 0.3 347.000 1.885 654.080

C 1A 15 0.5 265.000 3.142 832.522

C 1A 15 0.75 184.100 4.712 867.551

C 1A 15 1.1 80.100 6.912 553.611

C 1A 15 1.6 41.000 10.053 412.177

C 1A 15 2.4 16.000 15.080 241.274

C 1A 15 3.6 5.280 22.619 119.431

C 1A 15 5.4 1.000 33.929 33.929

C 1A 15 8.1 0.480 50.894 24.429

C 1A 300 12 0.050 75.398 3.770

C 1A 300 20 0.030 125.664 3.770

C 1A 300 30 0.020 188.496 3.770

C 1A 300 50 0.010 314.159 3.142

C 1 15 0.3 270.000 1.885 508.938

C 1 15 0.5 116.000 3.142 364.425

C 1 15 0.75 60.100 4.712 283.215

C 1 15 1.1 33.600 6.912 232.227

C 1 15 1.6 21.300 10.053 214.131

C 1 15 2.4 13.790 15.080 207.948

C 1 15 3.6 6.000 22.619 135.717

C 1 15 5.4 2.450 33.929 83.127

C 1 15 8.1 0.420 50.894 21.375

C 1 300 12 0.080 75.398 6.032

C 1 300 20 0.010 125.664 1.257

C 2 15 0.3 5.170 1.885 9.745

C 2 15 0.5 4.070 3.142 12.786

C 2 15 0.75 4.640 4.712 21.865

C 2 15 1.1 3.550 6.912 24.536

C 2 15 1.6 2.200 10.053 22.117

C 2 15 3.6 2.120 22.619 47.953

C 2 15 5.4 0.760 33.929 25.786

C 2 15 8.1 0.080 50.894 4.072

C 2 300 12 0.010 75.398 0.754

C 2 300 20 0.010 125.664 1.257

Table 11 - Site Resistivity Measurements

The apparent resistivity column is left blank where the resistance reading is 0.


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The basic measurements taken on site can be summarised as below:

Probe Spacing

(m)

Maximum Resistivity

(.m)

Minimum Resistivity

(.m)

Average Resistivity

(.m) Count

0.3 654.1 1.018 90.21 14

0.5 832.5 1.225 92.52 14

0.75 867.6 0.848 88.09 14

1.1 553.6 1.382 61.59 14

1.6 412.2 1.307 49.57 14

2.4 241.3 1.056 37.50 13

3.6 135.7 0.905 23.72 14

5.4 83.13 0.339 12.12 14

8.1 24.43 0.509 5.802 10

12 6.032 0.754 2.827 4

20 3.770 1.257 2.094 3

30 3.770 3.770 3.770 1

50 3.142 3.142 3.142 1

Table 12 - Summarised Resistivity Measurements


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C. Generic Earthing Schematic


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D. CRS Responses

Electrical comments on report ref. no. 422-C240-DF-E-RPT-00004-A Action code 3 1. General: the following shall be indicated:

a. The overall grounding system network scheme. b. ICT network scheme. c. Grounding system for lightning protection system network scheme. d. Measure the specific earth resistance of the soil for related areas / load centers. Contractors Response: A schematic diagram will be added to the report to provide an indication of the overall earthing network. Philosophy and basic proposals for ICT earthing will be included. Lightning protection is outside of the scope of this report. The measurements of electrical resistivity are being taken on site for the load centres. This information will be used to update the report.

2. Clause 4.4: Indicate the rating of NGR to limit the S.C current to 100A and indicate the calculation note validating the 100A S.C current. Contractors Response: The earthing report is not intended to design and / or size neutral earthing resistors. This information will not be included within the report.

3. Clauses 6.1 & 7: The exact ground resistivity must be measured; considering 3 values is not acceptable. Contractors Response: As above, the measurements of electrical resistivity are being taken on site for the load centres. This information will be used to update the report.

4. Clause 6.6.3: Provide reference for the calculation note where the overall earth electrode resistance is 0.12 ohm and ground potential rise of 360V were calculated. Contractors Response: The calculations are provided in Appendix A.5 as indicated in the first paragraph. The wording will be amended to clarify and Appendix A.5 updated to clarify the 360V calculation.

5. Clause 5.2.1: It is indicated that MV earth will be insulated to maintain the electrical isolation between the MV & LV system while in figure 4 both system are interconnected, Contractor is to justify. Contractors Response: The specification requires the ability to test earth electrodes. To facilitate this insulation is maintained between the MV and LV earths. Please provide an instruction if the testing is no longer a requirement.

6. Clause A1: Clarify how average ground resistivity is calculated for each earth rod. Contractors Response: The average resistivity is calculated using a weighted average which considers depth and soil resistivity at the depth. This model is very conservative. Resistivity measurements on site will supersede this model.

7. Clause A2: This item is to calculate the surface current density not to determine the minimum length of earth rod. Also, the actual current density shall be calculated as well by dividing the fault current by the grid surface area. Contractors Response: Agreed. The equation calculates the maximum current density given a specific electrical resistivity. This result is shown is table 4. Table 4 then calculates the minimum earth rod length to dissipate the fault current. An additional equation will be included to shown this calculation.

8. Clause A5: Justify calculating earth electrode resistance using BS7354 while the resistance of earth electrode and resistance of groups of electrodes were calculated in accordance to BS7430 in item A3 and A9 respectively. Contractors Response: BS7430 provides equations to calculate the resistance of individual earth rod arrangements. It does not provide equations to calculate the effective resistance of an earth network comprising earth rods and conductors in an interconnected network. BS7354 provides the equations necessary to calculate the effective resistance of an earth network. This is recognised by the standards organisation who are working on BS EN 50522 which will be a harmonised earthing standard incorporating BS7430 and section 7 of BS7354.


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9. Clause A8: a. Provide the criteria on which the MV screen C.S.A is considered to be 285 mm2. b. Indicate the reference standard where the ground potential rise formula is derived. Contractors Response: a. The screen of the MV cable has been changed based