Earthing Report

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Earthing Report

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  • Page 2 of 19

    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

  • Page 3 of 19

    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

    d - [m] The electrode diameter

    3000 [A] - Earth fault current

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  • Page 19 of 19

    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