Grounding System Design Calculations according to type of the building
The procedures for performing the Grounding System Design Calculations can differ slightly according to the type of the building as follows:

First: Domestic, commercial and industrial premises
We mean by domestic, commercial and industrial premises, all installations up to 1,000 V ac and 1,500 V dc  between phases, with some minor exceptions.

And I explained Methods of Grounding Design Calculations of Domestic, commercial and industrial premises in the following Articles:
 Grounding Design Calculations – Part One and Grounding Design Calculations – Part Two: Equations Method and solved examples.
 Grounding Design Calculations – Part Three: Nomographs Method
 Grounding Design Calculations – Part Four: Excel Spreadsheets Method
 Grounding Design Calculations – Part Five and Grounding Design Calculations – Part Six: using Tables Method
 Grounding Design Calculations – Part Seven and Grounding Design Calculations – Part Eight: Using Online Earthing Calculators
 Grounding Design Calculations – Part Nine: Software Programs Method
You can preview the following Articles for more info:
Second: High And Medium Voltage Electricity AC Substations

I began explaining Grounding Design Calculations for second type of buildings: AC Substations in Article " Grounding Design Calculations – Part Ten " where I explained the following:
 Design Procedures for grounding system design as per IEEE 80: Guide for safety in AC substation grounding,
 Step#1: Field Data Collection,
 Step#2: Earthing Grid Conductor Sizing.
Also in " Grounding Design Calculations – Part Eleven ", I explained Step#3: Calculation Of Tolerable Touch And Step Voltages.
And in Article " Grounding Design Calculations – Part Twelve ", I explained Step#4: Preliminary Design of Grounding System for AC Substations.
And In Article " Grounding Design Calculations – Part Thirteen ", I explained Step#5: Calculation of the Preliminary Grid Resistance, Rg, Of the Grounding System in Uniform Soil
And In Article " Grounding Design Calculations – Part Fourteen ", I explained Step#6: Determination of Maximum Grid Current, IG.
Today, I will continue explaining other steps from the design procedures of grounding system for AC Substation.
Design
Procedures of Grounding System for AC Substations  Continued

Step#7: Calculation Of Maximum Grid Potential Rise And
Comparing With The Tolerable Touch Voltage From Step#3

7.1 Terms Definitions
for Step#7

7.2 Calculation of Maximum Ground Potential Rise (GPR)
Maximum Ground Potential Rise (GPR) can be
calculated by the following equation:
GPR = IG x Rg
Where:
GPR is the maximum ground potential
rise (V),
IG is the maximum grid current found earlier in Step#6 (A),

7.3 From
Step#3: Tolerable Touch Voltage Equations
Etouch (for 50 Kg Person) =
(RB + Rf/2) x IB = (1000 + 1.5 Cs ρs) x 0.116 /
√ts
And,
Etouch (for 70 Kg Person) =
(RB + 2Rf) x IB = (1000 + 1.5 Cs ρs) x 0.157 /
√ts
Where:
RB is the
resistance of the human body in Ω = 1000 Ω,
Rf is the
ground resistance of one foot (with presence of the substation grounding
system ignored) in Ω,
Cs is the surface layer derating factor found earlier in Step#3,
ρs is the resistivity of the surface layer material (Ω.m),
ts is the
duration of the current exposure in sec.

7.4 Comparing Maximum Ground Potential Rise (GPR) with the tolerable touch
voltage (Etouch)

Example#1:
For A small industrial facility with a network connection via a
transmission line and a deltawye connected transformer:
1 Calculate the erthing grid resistance
using the simplified equation For a rectangular earthing grid (see fig.2) with the following
parameters:
2 Calculate the Tolerable
Touch Voltages for 70 Kg person, if 120 mm thick layer of crushed rock
is spread on the earth's surface above ground grid in a switchyard, noting
that:
3calculate the maximum grounding
grid fault current IG, noting that:
4
Evaluate the preliminary design.
Solution:
1
Calculate
the erthing grid resistance:
Using the simplified equation,
the resistance of the earthing grid with respect to remote earth is:
2 Calculate the Tolerable Touch Voltages
for 70 Kg person:
Step#1: Calculate The Surface Layer Derating Factor Cs
Step#2: Calculate The Tolerable
Touch Voltage
Etouch (for 70 Kg Person) = (1000 + 1.5 Cs ρs) x 0.157 /
√ts
= (1000 + 1.5 X 0.7207 X 3000) 0.157 / √0.15 = 1720.04 V
3calculate the maximum grounding
grid fault current IG
Step#1: For each fault, based on its duration
time, tf, determine the value of
decrement factor Df to allow for the effects of asymmetry of the fault current wave
Since, The X/R ratio at the fault is approximately
15, the maximum fault duration 150ms and the system nominal frequency is
50Hz.
Ta is then:
The decrement factor Df is
then:
Step#2: apply the Cp Corrective projection
factor =1.2.
So, the maximum grounding grid fault
current IG is:
IG = Sf. Df. Cp. If
So, IG = 0.6 x 1.1479 x 1.2 x 3.1
KA = 2.562 KA
4
Evaluate The Preliminary Design:
This is done by Comparing Maximum Ground Potential Rise (GPR) with the tolerable touch
voltage (Etouch):
Maximum Ground Potential Rise (GPR) can be
calculated by the following equation:
GPR = IG x Rg = 2562 A X 2.2753 Ω = 5829.31 V
Since, GPR = 5829.31 V and Etouch = 1720.04 V
So, GPR > Etouch
The Preliminary grid design is
not safe, a further design
analysis is needed and we must Continue to Step#8:
Calculation of mesh and step voltages.

Step#8: Calculation
Of Mesh And Step Voltages

8.1 Terms Definitions
for Step#8

First: Mesh Voltage Calculation
The mesh voltage is the maximum touch voltage within a mesh of
an earthing grid and is calculated as follows:
Where:
IG: the maximum grid current found earlier in Step#6 (A),
LM: the effective buried length of the grid,
Km : the geometric spacing
factor,
Ki : irregularity factor, which accounts
for some of the error introduced by the assumptions made in deriving Km,
ρ: the soil
resistivity,
(IG/LM): the average current per unit of effective buried length of the
grounding system conductor.
A The geometric spacing factor K_{m} is calculated as follows:
Where:
D is the average spacing
between parallel grid conductors (m),
h is the depth of buried grid conductors (m),
d is the crosssectional diameter of a grid conductor
(m),
K_{h} is a weighting factor for depth of burial = √(1+h),
K_{ii} is a weighting factor for earth electrodes /rods on
the corner mesh,
A.1 the average spacing between parallel grid conductors D is
calculated as follows:
Where:
W_{g} and L_{g} are the width and length of the grid
respectively (m),
n_{r} and n_{c} is the number of parallel rows and
columns respectively.
A.2 The weighting factor for earth electrodes /rods on the corner mesh K_{ii} is calculated as follows:
Where:
n is a geometric factor.
A.3 The geometric factor n is calculated as follows:
n = na x nb x nc x nd
Where:
LC is the total length of horizontal grid conductors in m,
Lp is the length of grid conductors on the perimeter in m,
A is the total
area of the grid (m^{2}).
And,
nb = 1 for square grids,
nc = 1 for square and
rectangular grids,
nd = 1 for square, rectangular
and Lshaped grids.
otherwise,
Where:
L_{x} and L_{y} are
the maximum length of the grids in the x and y directions (m),
D_{m} is the maximum distance between any two points on the
grid (m).
B The irregularity factor, Ki, used in conjunction with
the above defined n is
Ki = 0.644 + 0.148 n
C the effective buried length of the grid LM is found as follows:
For grids with few or no earthing electrodes (and
none on corners or along the perimeter):
LM = L_{C }+ L_{R}_{}
Where:
L_{C} is the total length of horizontal
grid conductors (m),
L_{R} is the total length of earthing electrodes / rods (m).
For grids with earthing electrodes on the corners and
along the perimeter:
Where:
L_{C} is the total length of horizontal
grid conductors (m),
L_{R} is the total length of earthing electrodes / rods (m),
L_{r} is the length of each earthing electrode / rod (m),
L_{x} and L_{y} are
the maximum length of the grids in the x and y directions (m).

Second: Step Voltage
Calculation
The maximum allowable step voltage is calculated as follows:
Where :
ρs is the soil resistivity (Ω.m),
IG: the maximum grid current found earlier in Step#6 (A),
Ks : the geometric spacing factor,
Ki : corrective factor,
which accounts for some of the error introduced by the assumptions
made in deriving Km,
Ls : the effective buried length of the grid.
A Geometric Spacing Factor K_{s}:
The geometric spacing factor K_{s} is applicable for burial depths (h)
between 0.25m and 2.5m (0.25 m < h < 2.5 m) and is calculated
as follows:
Where:
D is the spacing between parallel
grid conductors (m),
h is the depth of buried grid conductors (m),
n is a geometric factor (as derived
above in the mesh voltage calculation).
B Effective Buried Length L_{S}:
The effective buried length L_{S} for all cases can be calculated as
follows:
L_{S
}= 0.75 L_{C }+ 0.85 L_{R}_{}
Where:
L_{C} is the total length of
horizontal grid conductors (m),
L_{R }is the total length of earthing
electrodes / rods (m).

Example#2:
Using the same data of Example#1, calculate the
Maximum Mesh and Step Voltages.
Solution:
First: Mesh Voltage Calculation
Step#1: calculate the geometric factor n
The components of the geometric
factor na, nb, nc, and nd
for the rectangular grid are:
Step#2: calculate the average spacing between parallel grid conductors D
Step#3: calculate the geometric
spacing factor K_{m}
Step#4: calculate the irregularity
factor K_{i}
Ki = 0.644 + 0.148 n = 0.644 + 0.148 x 6.4939 =
1.605
Step#5: from the above steps, the maximum mesh voltage is:
Second: Step Voltage Calculation
Step#1: calculate the geometric
spacing factor K_{s}
Step#2: calculate the effective
buried length L_{s}
L_{S }= 0.75
L_{C }+ 0.85 L_{R} = 0.75 x 890_{ }+ 0.85 x 66 =
723.6 m
Step#3: from the above
steps, calculate the
maximum allowable step voltage

In the next Article, I will explain Other Steps from the Design Procedures of Grounding System Design for AC Substation. Please, keep following.
What sort of step voltage is expected from a lightning strike 100 m away?
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