In Article " Earthing Systems Design steps – Part One ", I indicated the following points:
Earthing Systems Design
Steps

In the above Article and Article " Earthing Systems Design steps – Part Two ", I explained the first step: Data Collection which includes the following points:
First Step: Data
Collection
Once a need for grounding system design is
established, data collection must begin. These needed data provide the basis
for all grounding design and will be obtained from:

Today
I will explain the second step of earthing system design which is Data Analysis
as follows.
You can preview the following Articles for more info:
Second Step:
Data Analysis

1 Apparent
Resistivity
Step#1: assign the used type of the Soil Resistivity Structure SRS Modeling types which are:
Note: the multilayer modeling doesn’t be used to calculate the Apparent Resistivity values, but it is used to calculate the Actual values by using computer based programs. Step#2: Calculate The Apparent Resistivity for depth (a). Step#3: Repeat step#1 for several depths to get a series of readings at different values of a, as well as in a 90 degree turned axis.
Step#4: tabulate or plot the results to get a good
idea of how the resistivity is changing with depth and will indicate the best
type of earthing electrode system to design for the subject site. The table
in below figure shows a typical record sheet for resistivity measurements.
Step#5: Calculate The
Apparent Resistivity as follows:

Step#1: Soil Resistivity Structure
SRS Modeling
There are many types of SRS models as
follows:
Note: According to IEEE 80, two
layers SRS are often a good approximation of many soil structures.
1 The Uniform Model
A uniform SRS should only be used if
the variation in the measured apparent resistivity is low, this has a rare
occurrence in practice. If a large variation occurs, the uniform soil is
unlikely to yield accurate results.
2The TwoLayer Model
3
The MultiLayer Model
The multilayer model
is useful in providing more accurate information regarding the presence of
lower resistivity layers, and hence optimizing rod driving depths.

Step#2: The Apparent Resistivity Formula
The
below formula is Using the Wenner Array method, four small electrodes
(auxiliary probes) are placed in a straight line at intervals of (A) , to a
depth of (B). A current is passed through the outer two probes, and the
potential voltage is then measured between the two inner probes. A simple Ohm’s Law equation determines the
resistance.
For
most practical circumstances, (A) is twenty times larger than (B), where we
can then make the assumption that B=0.
Then
the Resistivity (ρ) is given by: ρ= 2 ∏a R = 1.915 A R
Where:
ρ=
resistivity of the local soil (Ωm)
A=
distance between probes (m)
B
= depth of probes into the ground (m)
R
= resistance value measured by the testing device (Ω)
For
example, if an apparent soil resistance of 4.5 ohms is at a 40foot spacing,
the soil resistivity in ohmmeters would be 344.7 Ωm.

Step#5: Calculate the Apparent
Resistivity
Case#1: using
the uniform model
The apparent
resistivity will be calculated by using the uniform model equation as
follows:
Case#2: using the twolayer model
The apparent resistivity will be calculated by
using one of the following two methods:
Method#1: The TwoLayer Model Equations
When using twolayer soil structure
to determine the grid resistance, it is important to determine the reflection
factor K.
The below Equation shows the computation of the reflection K:
The apparent soil resistivity
utilizes the characteristics of the twolayer structure as shown in the
following equations:
Method#2: The TwoLayer Model Curves
The two layer soil model can be approximate
by using graphical methods described in Sunde’s chart illustrated un the
below figure.
The two layer structure consists of
the characteristics shown in the below Table.

2 Actual
resistivity

Case Study:
Multiple soil resistivity field tests are conducted at
different locations, the below Table represents the field data.
Solution:
 Using the uniform model equation to determine the average soil structure for these (5) different conditions,
So,
Field Study #1: ρ = (27.8+23.3+39.9+31.8+40.5+48.8+62.3+79.1+106)/9 = 51.05Ωm
Field Study #2: ρ = 603.88Ωm
Field Study #3: ρ = 16.57Ωm
Field Study #4: ρ = 41.64Ωm
Field Study #5: ρ = 87.58Ωm
 CDEGS software program is used to determine the two layer soil structure for these proposed case studies as in the below figure.
In the next Article, I will explain The Third Step for Earthing System Design: Grounding Design Calculations. Please, keep following.
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