Earthing Systems Design steps – Part Three

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

Earthing Systems Design Steps

A grounding system design process has (3) main steps:

  1. Data Collection,
  2. Data Analysis,
  3. Grounding Design Calculations.

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:

  1. Facility official data,
  2. Facility characteristics,
  3. Nearby area data,
  4. Electric Utility Data,
  5. Engineering data,
  6. Geographical data of the area,
  7. Geological surveys.

Today I will explain the second step of earthing system design which is Data Analysis as follows.

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Second Step: Data Analysis

Once all the soil resistivity data is collected, the data analysis begin by which we can calculate two values of soil resistivity as follows:

  1. Apparent Resistivity,
  2. Actual Resistivity.

1- Apparent Resistivity

It called apparent because it does not correspond to the actual resistivity of the soil and it can be calculated in ohm-meters by using Soil Resistivity Structure SRS Modeling in the following procedure:

Step#1: assign the used type of the Soil Resistivity Structure SRS Modeling types which are:

  1. The Uniform Model,
  2. The two-layer model,
  3. The Multi-Layer model,

Note: the multi-layer 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:
  • In case of using the uniform model, the apparent resistivity will be calculated by using the uniform model equation.
  • In case of using The two-layer model, the apparent resistivity will be calculated by using one of the following two methods:

  1. The Two-Layer Model Equations,
  2. The Two-Layer Model Curves.

Step#1: Soil Resistivity Structure SRS Modeling

There are many types of SRS models as follows:

  1. The Uniform Model,
  2. The Two-Layer Model,
  3. The Multi-Layer Model.

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.

2-The Two-Layer Model

  • During the two layer approach, the electrode will be exposed to different soil resistivity which represents a more realistic approach; also the safety assessment will address the top layer resistivity which in its turn represents a more practical situation.
  • Understanding the soil layers resistivity supports the designer in determining the type of earth grid that yields to an adequate solution, below is a couple of cases under different soil structures:

  1. Low resistance layer overlaying higher resistivity layer, mesh grid will have a better influence than driving electrodes into the high resistivity layer.
  2. High resistivity layer overlaying a lower resistivity layer, a deep electrode used to reach the lower resistivity layer will enhance the performance of the earth grid.

  • However, the two layer model is considered sufficiently accurate for modeling the behavior of grids in the majority of cases. If more than two layers are identified, the lower layers are usually combined to form a two layer equivalent model. This is done because the surface potentials are closely related to the upper layer resistivity, whilst the grid resistance, which is primarily effected by the deeper layers, is not usually adversely affected by this simplification.

3- The Multi-Layer Model

The multi-layer model is useful in providing more accurate information regarding the presence of lower resistivity layers, and hence optimizing rod driving depths.

Note: the multi-layer 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:  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


ρ= resistivity of the local soil (Ω-m)
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 40-foot spacing, the soil resistivity in ohm-meters would be 344.7 Ω-m.

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 two-layer model

The apparent resistivity will be calculated by using one of the following two methods:

Method#1: The Two-Layer Model Equations

When using two-layer 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 two-layer structure as shown in the following equations:

Method#2: The Two-Layer 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

  • With all the available data, the actual soil resistivity can be determined by using sophisticated computer programs to provide a soil model showing the soil resistivity in ohm-meters and at various layer depths.
  • Knowing at what depth the most conductive soil is located for the site allows the design engineer to model a system to meet the needs of the application. Computer based techniques are best used to identify two or more soil resistivity layers.
  • However, the Actual Resistivity calculations by using Computer programs are not in the scope of this course.

Case Study:

Multiple soil resistivity field tests are conducted at different locations, the below Table represents the field data.


  • Using the uniform model equation to determine the average soil structure for these (5) different conditions, 


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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