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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
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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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Page 4 of 19
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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Page 5 of 19
= wet Soil resistivity (according to the design report) = 10
ohm.m
Then, R = 2.486 ohm for one earthing rod.
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Page 6 of 19
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Page 7 of 19
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Page 8 of 19
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Page 9 of 19
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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Page 10 of 19
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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Page 11 of 19
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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Page 12 of 19
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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Page 13 of 19
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Page 14 of 19
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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Page 15 of 19
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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Page 16 of 19
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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Page 17 of 19
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 18 of 19
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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 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
-
100% Design Report Load Centres A, B, C and AC
Atkins Tracker Number: ATK005-422-C100-FD-X-RPT-00001 70
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.
-
100% Design Report Load Centres A, B, C and AC
Atkins Tracker Number: ATK005-422-C100-FD-X-RPT-00001 71
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.
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100% Design Report Load Centres A, B, C and AC
Atkins Tracker Number: ATK005-422-C100-FD-X-RPT-00001 72
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:
-
100% Design Report Load Centres A, B, C and AC
Atkins Tracker Number: ATK005-422-C100-FD-X-RPT-00001 73
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.
-
100% Design Report Load Centres A, B, C and AC
Atkins Tracker Number: ATK005-422-C100-FD-X-RPT-00001 88
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
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MV ELECTRICAL EARTHING GENERAL REQUIREMENTS
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
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MV ELECTRICAL EARTHING GENERAL REQUIREMENTS
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
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MV ELECTRICAL EARTHING GENERAL REQUIREMENTS
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
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MV ELECTRICAL EARTHING GENERAL REQUIREMENTS
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.
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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.
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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
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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:
J - [A/m2] The maximum current density of the earth
electrode
- [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
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
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d - [m] The electrode diameter
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
2.4 0.8 218.5 Not Calculated
3.6 1.2 150.3 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
Test No Anode Depth (cm)
Probe Spacing
(m)
Resistance Reading
()
Constant K"
(2..a)
Apparent Resistivity
(.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
Test No Anode Depth (cm)
Probe Spacing
(m)
Resistance Reading
()
Constant K"
(2..a)
Apparent Resistivity
(.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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MV ELECTRICAL EARTHING GENERAL REQUIREMENTS
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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