In Article " Grounding Design Calculations – Part One ", I indicated the following:
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.
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#3:
Calculation Of Tolerable Touch And Step Voltages

Terms Definition for
Step#3

Typical Shock Situations
Fig(2) and Fig(3)
show five basic situations involving a person and grounded facilities during
a fault, the basic five shock situations are as follows:

First: Calculations of Tolerable
Step Voltage
In step#3, we will calculate the maximum tolerable limits for
step voltage from its circuit equivalent as follows:
The equivalent circuit for step voltage is as in fig(4):
From Thevenin theorem:
VTh = (RB + ZTh) x IB
And, ZTh = 2Rf
Where:
RB is the
resistance of the human body in Ω
Rf is the
ground resistance of one foot (with presence of the substation grounding
system ignored) in Ω
So, the permissible total equivalent voltage becomes:
VTh = Estep = (RB + 2Rf) x IB
To get the Tolerable Step Voltage, you need to calculate
the parameters RB, Rf
&IB.

1 Calculation Of Resistance Of The Human Body
(RB)
IEEE 80 assumes the following:
Will equal 1000 Ω.
RB = 1000 Ω

2 Calculation of ground resistance of one
foot (Rf)
For the purpose of circuit analysis, the
human foot is usually represented as a conducting metallic disc and the
contact resistance of shoes, socks, etc., is neglected.
For Theoretically Uniform Soil Resistivity:
The ground resistance in ohms of a metallic
disc of radius b (m) on the surface of a homogeneous earth of resistivity ρ (Ω·m) is given by Laurent as follows:
Rf = ρ/ 4b
Traditionally, the metallic disc
representing the foot is taken as a circular plate with a radius of 0.08 m.
With only slight approximation, equations for ZTh can be obtained in numerical
form and expressed in terms of ρ as follows.
Rf = ρ/ 4b = ρ/ (4
x 0.08) = 3 ρ
And, ZTh = 2 Rf = 6 ρ
But actually,it is common to apply a thin layer (0.08m  0.15m) of high resistivity material (such as gravel, blue metal, crushed rock, etc) over the surface of the ground to help protect against dangerous touch and step voltages. This is because the surface layer material increases the contact resistance between the soil (i.e. earth) and the feet of a person standing on it, thereby lowering the current flowing through the person in the event of a fault. In this case, this thin layer affects the value of the soil resistivity ρ by a derating factor Cs which can be obtained with the analytical method (Thapar, Gerez, and Kejriwal) as follows: Where: Cs is the surface layer derating factor, ρ is the soil resistivity (Ω.m), ρs is the resistivity of the surface layer material (Ω.m), hs is the thickness of the surface layer (m). so, The effective ground resistance of one foot Rf can be calculated as follows:
Rf = 3 Cs ρs
And, ZTh = 2 Rf = 6 Cs ρs

3 Calculation of Tolerable body current IB
As stated by Dalziel and others, the nonfibrillating current of
magnitude IB at 50Hz or 60Hz and at durations ranging from 0.03–3.0 s is
related to the energy absorbed by the body as described by the following equation:
SB = (IB)^{2} x ts
Where:
IB is the rms magnitude of the current through the body in A,
ts is the duration of the current exposure in sec,
SB is the empirical constant related to the electric shock
energy tolerated by a certain percent of a given population.
Note:
From the above equation, the current through the body IB will be
calculated as follows:
IB = √SB / √ts
And given that: k = √SB
So, IB = k / √ts
Dalziel found the following Data by tests:

Tolerable Step
Voltage Equation
So, from 1, 2 &3, the Tolerable Step Voltage
equation becomes:
Estep (for 50 Kg Person) = (RB + 2Rf) x IB = (1000 + 6 Cs ρs) x 0.116 /
√ts
And,
Estep (for 70 Kg Person) = (RB + 2Rf) x IB = (1000 + 6 Cs ρs) x 0.157 /
√ts

Second: Calculations of Tolerable Touch
Voltage
In step#3, we will calculate the maximum tolerable limits for touch
voltage from its circuit equivalent as follows:
The equivalent circuit for step voltage is as in fig(5):
From Thevenin theorem:
VTh = (RB + ZTh) x IB
And, ZTh = Rf/2
Where:
RB is the
resistance of the human body in Ω
Rf is the
ground resistance of one foot (with presence of the substation grounding
system ignored) in Ω
So, the permissible total equivalent voltage becomes:
VTh = Etouch = (RB + Rf/2) x IB
To get the Tolerable Step Voltage, you need to calculate
the parameters RB, Rf
&IB.

4 Calculation Of Resistance Of The Human Body
(RB)
Same as for calculation of Tolerable Step Voltage,
so:
RB = 1000 Ω

5 Calculation of ground resistance of one
foot (Rf)
Same as for calculation of Tolerable Step Voltage. so, The effective ground resistance of one foot Rf can be
calculated as follows:
Rf = 3 Cs ρs
And, ZTh = Rf/2 = 1.5 Cs ρs

6 Calculation of Tolerable body current IB
Same as for calculation of Tolerable Step Voltage, So:
IB (for 50 Kg Person) = 0.116 / √ts
IB (for 70 Kg Person) = 0.157 / √ts

Tolerable Touch
Voltage Equation
So, from 4, 5 &6, the Tolerable Touch Voltage
equation becomes:
Etouch (50 Kg Person)= (RB + Rf/2) x IB = (1000 + 1.5 Cs ρs) x 0.116 /
√ts
And,
Etouch (70 Kg Person) = (RB + 2Rf) x IB = (1000 + 1.5 Cs ρs) x 0.157 /
√ts

Example:
Calculate the Tolerable Step and 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:
Solution:
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
Step#3: Calculate The Tolerable Step Voltage
Estep (for 70 Kg Person) = (1000 + 6 Cs ρs) x 0.157 /
√ts
= (1000 + 6 X 0.7207 X 3000) 0.157 / √0.15 = 5664.03 V

In the next Article, I will explain Other Steps from the Design
Procedures of Grounding System Design for AC Substation. Please, keep
following.
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