Date: 28 December 2023
NEW TERMINAL BUILDING AT SHARM-EL-SHEIKH, EGYPT
Earthing Resistance Value Calculation
As Spec. Calls for :
Rt ≤5 Ω for Mains Power Earthing Network .
Rt ≤1 Ω for communications, light Current System (clean earthing network)…etc.
Rt ≤10 Ω for lightning protection earthing Network.
Description
The Calculation shall consider the following elements for calculations.
I- Artificial vertical earth electrodes, (2.4 m length and 5/8“ (16 mm) diameter).
II- Horizontal earth electrodes formed by linking Bare Copper Conductors.
III - Natural earthing electrodes due to volume of base foundations.
Equations:-
1. Calculation of Vertical Electrodes
2. Calculation of Horizontal Electrodes
3. Sizing of Main Earthing conductors
Eq. (3)- IEE 543-2
4. Soil description and properties
1
Soil: Homogeneous fine Salty sand with Salt soil moisture contents average 15%.
(Soil Resistivity) = 1800 Ω.cm (Schedule 7.4-IEEE Std. 142-1991)
(Actual on site) = 3500 Ω.cm (larger 2 times).
(As per Otak Trading & Contracting Co., Former site survey in Sharm El Sheikh
Airport Mall).
Horizontal conductor laying depth = 60 cm
Legend And Abbr.
L: Electrode length in cm. Vertical Electrodes Eq.(1)
a: Radius in cm.
2L: buried Horizontal Wire Length in cm. Horizontal Electrodes Eq.(2)
s/2: Laying Depth in cm.
t : Operating time of disconnecting device in Seconds.
= 5 sec. for low voltage as per spec.
= 3 sec for Medium voltage as per spec. Eq.(3)
S = C.S.A in mm²
I = R.M.S A.C fault current.
K = Factor depending on material of protective conductor
, insulation and initial and final temperature.
( IEE Std. 543-2, tables 54B, 54C, 54D And 54E
(We shall proceed with calculations as = = ).
F: Multiplying Factor for Multiple Rods As per Table 14- IEEE Std. 142-1991
Table 14—Multiplying Factors for Multiple Rods
IEEE Std. 142-1991
2
Number of Rods F
2 1.16
3 1.29
4 1.36
8 1.68
12 1.80
16 1.92
20 2.00
24 2.16
Calculation's Concept And Methodology .
To achieve and verify specification's requirements for total grounding resistance, the
following equality has to be verified:-
Rt ≤5 Ω for Mains Power Earthing Network.
Rt ≤1 Ω for communications, light Current System (clean earthing network)…etc.
Rt ≤10 Ω for lightning protection earthing Network.
Eq.(4)
(F)- will be the highest multiplier factor.
Eq.(5)
From Eq.(1)
From Eq.(2)
2L = 5000 cm L = 2500 cm
3
S/2 =60 cm S = 120 cm , a = 0.775 cm
(Bare Copper conductor 1x150mm2)
From Eq.(5)
Let F=2.16 highest multiplier factor.
Taking into consideration that (N) is the No. of electrodes for both vertical and horizontal ones.
From the above results, we find that:-
1. It is sufficient to use Two vertical electrodes and Two linking horizontal ones for Mains
Earthing Network.
2. Also, Six vertical electrodes and Six linking horizontal ones to more satisfy the required
earth resistance for Clean Earthing Network.
3. Also, Two vertical electrodes and Two linking horizontal ones to satisfy required earth
resistance.
Regarding the single line diagram for total earthing grid Dwg. E-32 and as per
specifications.
Include at least Two earthing electrodes for each main earth bar for M.D.B’s, R.M.U, M.V
SWITCHGEARS, TRANSFORMERS AND GENERATOR ROOM, this will lead to select:
14 Nos. vertical electrode for mains power earthing network including power plant.
6 Nos. vertical electrodes for clean earthing for terminal building and power plant.
17 Nos. for lightning protection network.
All the above arrangements will be shown on the related drawings.
CONCLUSION
The total number of vertical electrodes for Mains Power, Clean Earthing and Lightning
protection will be up to 37 Nos,.Vertical and 37 Nos., horizontal ones, which result in a
total earthing grid resistance of ( from eq. (5):
4
This value is so below required value for both Mains Power and Clean Earth Resistance.
Also, we have neglected the contribution of natural foundation value, which if added will improve
better even the Soil Resistivity is higher than assumed.
Sizing of main Earth Conductor
Earth Conductor C.S.A has to the following calculated value from the equation.
S = √I²t mm² (IEE 543-2)
K
5
Generally speaking the symmetrical R.M.S, A.C bolted three phase Isc (KA) is equal to the line to
ground fault, acct. the attached dwg. (D1), the ground fault will be splitted to the value of ( Ig / 2).
From table: 3-phase short-circuit currents of typical distribution transformers, IEC-standard 20 / 0.4 kV
transformers and regarding the worst case for oil transformer 2000 KVA in power plant.
transformer rated power (kVA) 10 12 16 20 2
00 50 00 00 5
0
0
transformer current Ir (A) 13 17 21 27 3
75 18 99 49 4
3
7
oil-immersed transformer Isc (kA)
Psc = 250 MVA 21. 26. 33. 40. 4
5 4 1 4 9
.
1
Psc = 500 MVA 22. 27. 34. 43. 5
2 5 8 0 2.
9
cast-resin transformer Isc (kA)
Psc = 250 MVA 21. 26. 33. 40. 4
5 4 1 4 9
.
1
Psc = 500 MVA 22. 27. 34. 43. 5
2 5 8 0 2
.
9
3-phase short-circuit currents of typical distribution transformers.
IEC-standard 20 / 0.4 kV transformers
Isc = 43.0 KA.
Acct. BS 7430-1991:-
Ig = ( R.M.S AC), For low tension, at the grid.
t = 1 sec. for clearing fault.
For medium tension network ruptures capacity is 500 MVA
500000 = √3 x 22 x I
Isc = 13.12 KA ( R.M.S AC).
6
1. FOR MEDIUM TENSION
Network ruptures capacity 500 MVA
K= 143 For Indoor insulated copper conductors for maximum temp.160ºC
Table 54 B IEE
K= 228 For bare buried outside conductors with maximum temperature 500ºC
Table 54 E – IEE
For inner insulated conductors.
For outer bare main grid conductors.
Select : 2(1 x 150) mm2 , Copper PVC insulated (As per Design)
Selected: 1 x 150 mm2, bare buried Copper Conductor, for out-door main earthing.
2. LOW TENSION
Ig = 28.7 KA
For inner insulated conductors
For outer bare main grid conductors.
Selected: : 2(1 x 150) mm2 for Indoor Connection (As Per Design).
Selected: 1 x 150 mm2, bare buried Copper Conductor, for out-door main earthing.
