June 14, 2023


The electrical resistance of an earth electrode is intrinsically tied to the unique resistivity characteristics exhibited by the soil in which it is meticulously placed and driven. Therefore, the precise assessment and measurement of soil resistivity assume paramount significance in the meticulous design of earthing installations.

The concept of resistivity, a fundamental property applicable to various materials, is rigorously defined by the American Society for Testing and Materials (ASTM), an organization renowned for its publication of rigorous testing and measurement standards. In the specific context of soil, resistivity serves as a reliable indicator of its inherent ability to conduct electric current effectively.

Within the soil medium, the movement of electric charge predominantly occurs through electrolytic mechanisms, intricately connected to the transport of ions present in the moisture content. An in-depth understanding of soil resistivity at specific locations, along with its intricate dependencies on factors like temperature, depth, and moisture content, equips us with indispensable knowledge.

This knowledge, in turn, allows us to adopt optimal strategies to attain and maintain the desired earth resistance values throughout the entire lifespan of the installation, ensuring minimal expenditure and mitigating potential operational challenges.

Why Testing the Soil Resistivity Important?

Testing the resistivity of soil holds immense importance in grounding systems due to various compelling reasons. The primary objective of a grounding system is to establish a unified reference potential for multiple components, including the building structure, power supply system, electrical conduits, plant steelwork, and instrumentation system. However, achieving a desirable low-resistance connection to the Earth can be challenging and depends on several factors:

Soil Resistivity: The resistivity of the soil plays a pivotal role in grounding system design. It determines the system’s overall resistance to the Earth. By testing the soil resistivity, engineers can obtain crucial information that influences the selection of suitable grounding strategies and equipment.

Stratification: The soil may exhibit different layers or stratifications, each with varying resistivity values. By conducting resistivity tests, engineers can identify and account for these variations, enabling them to design grounding systems that consider the specific soil conditions and optimize their performance.

Electrode Size and Type: The choice of electrode size and type is influenced by the soil resistivity. Testing the soil resistivity allows engineers to select appropriate electrodes that can effectively dissipate electrical currents and minimize resistance to the Earth.

Burial Depth: The depth at which the grounding electrode is buried affects its interaction with the surrounding soil. Soil resistivity testing provides insights into how the resistivity changes with depth, enabling engineers to determine the optimal burial depth for the electrode to achieve desired grounding performance.

Soil Moisture and Chemical Composition: Soil resistivity can be influenced by factors such as moisture content and chemical composition. These parameters can impact the conductivity of the soil. By testing the soil resistivity, engineers can account for these variations and make informed decisions when designing grounding systems.

The purposes of soil resistivity testing are multifold:

⦁ Obtaining a comprehensive set of measurements that can be analyzed to derive an equivalent model for the electrical behavior of the Earth, as perceived by the specific grounding system.

⦁ Assisting in geophysical surveys to explore depth to bedrock, identify core locations, and detect geological phenomena by utilizing soil resistivity values.

⦁ Assessing the level of corrosion in underground pipelines, as changes in resistivity can indicate potential corrosion issues.

Given its significant influence on grounding system design, soil resistivity testing is crucial before implementing a new grounding system. It provides valuable insights into the soil’s resistivity characteristics, enabling engineers to tailor the design to optimize performance and ensure reliable grounding.

What is Done During the Testing of Soil Resistivity?

Soil resistivity exhibits significant variability due to various factors, including the type of earth, stratification, moisture content, temperature, chemical composition, presence of metal and concrete structures, and topography. It is crucial to consider these factors during soil resistivity testing to obtain accurate and reliable results.

The following guidelines can help overcome potential challenges and ensure meaningful data gathering and testing:

Initial Research Phase: Conducting thorough background research is essential to determine the testing program and establish a basis for interpreting the results. Gathering data on nearby metallic structures, geological characteristics, geographical factors, and meteorological conditions provides valuable context. Geological data, such as information on strata types and thicknesses, aids in understanding water retention properties and anticipated resistivity variations due to moisture content.

Comparison with Rainfall Data: Comparing the obtained soil resistivity results with seasonal average, minimum, and maximum rainfall data for the location helps verify their accuracy. This assessment takes into account the potential influence of precipitation on soil resistivity.

Test Method Selection: When choosing a test type, consider factors such as maximum probe depths, cable lengths required, measuring technique efficiency, cost, and ease of data interpretation. These considerations ensure the selected method aligns with the project requirements.

It is important to note that temperature and moisture content become more stable with increasing distance below the earth’s surface. Therefore, to ensure year-round effectiveness, grounding systems should be installed at deeper depths whenever possible.

Test Methods

Various methods are available for acquiring soil resistivity measurements. The most commonly used methods include the Wenner method and Schlumberger method, also known as the four-pin methods. Regardless of the specific method employed, the fundamental concept involves injecting a known current into the soil and measuring the resulting voltage. This current flow generates equipotential lines, which can be visualized to analyze the soil’s resistive properties across different layers.

By selecting appropriate test methods and following these guidelines, accurate soil resistivity data can be obtained, enabling informed decision-making during grounding system design and installation.

Wenner Method

The Wenner method, renowned as the Wenner alpha four-pin technique, holds a preeminent position in the realm of soil resistivity measurements due to its widespread adoption. This method is favored for its efficacy and dependability. It entails strategically positioning four pins at equidistant intervals within the soil.

A controlled current is then introduced through the outermost electrodes, while the resulting voltage between the inner electrodes is meticulously recorded. By employing this measurement approach, accurate and comprehensive assessments of soil resistivity at varying depths can be obtained.

In instances where the probe depth (b) is significantly smaller than the distance between the outer electrodes (a), an approximation of the apparent resistivity can be derived using the provided formula.

This approximation holds true when the electrodes penetrate the ground to a limited extent. The Wenner method proves especially valuable when characterizing the soil in direct contact with grounding systems, as it allows for measurements with smaller electrode spacing.

Moreover, by increasing the electrode spacing, it becomes feasible to conduct deeper measurements, accommodating the dimensions of the specific grounding system under evaluation. This flexibility is crucial for accurately capturing resistivity variations that occur at different depths.

Schlumberger Method

In the Schlumberger method, the distance between the voltage electrodes “a” and the distances from a voltage electrode and a current electrode “c” are different (see figure).

If b << a and b << c (as usually happens), the apparent resistivity can be calculated as follows:

The Schlumberger method encompasses two configurations: a > c, referred to as the “Schlumberger-Palmer method,” and a < c, known as the “Schlumberger method.” Notably, the Schlumberger method offers advantages over the Wenner method, such as reduced labor requirements as there is no need to reinstall the interior voltage electrodes for each measurement.

The Schlumberger method boasts several merits, including the utilization of shorter measurement cables, reduced space requirements, and faster testing duration, while yielding resistivity measurements that are equivalent to those obtained through the Wenner method.

It is important to consider that the choice of configuration (c > a or c < a) in the Schlumberger method may dictate the need for more sensitive instruments to handle lower measured resistances or, conversely, facilitate easier measurements with higher measured resistances.

Steps usually performed during testing

Traverse Locations
Recognizing the significant variability of soil resistivity across different locations and depths within a site, relying on a single resistivity measurement is often inadequate. To obtain a comprehensive understanding of soil resistivity differences, it is advisable to conduct a thorough survey. Utilizing techniques such as Line Traverse surveys can effectively identify variations in soil resistivity at specific locations, leading to improved cost efficiency by optimizing labor and material usage to achieve the desired resistance value.

Spacing Range
Determining the appropriate spacing range is crucial for precise measurements. Close probe spacings, typically below 1 meter, are necessary to evaluate the resistivity of upper soil layers, particularly when quantifying touch and step voltages. On the other hand, larger spacings are utilized to assess grid impedance and remote voltage gradients.

It is important to note that calculations involving larger spacings can present significant challenges, particularly if the bottom layer exhibits higher resistivity (ρ2 > ρ1). In such cases, it is imperative to measure a realistic value of ρ2 to mitigate errors resulting from insufficient spacing.

Our Recommendations:

⦁ Minimizing mutual coupling or interference caused by leads running parallel to power lines, which can distort measurements.

⦁ Ensuring the adequacy and calibration of the instrumentation and setup used for testing.

⦁ Conducting operational checks to verify the accuracy and functionality of the equipment throughout the testing process.

⦁ Reducing contact resistance to achieve reliable measurements by ensuring proper contact between the electrodes and the soil.

⦁ Instructing testing personnel to utilize finer test spacing in areas where sharp changes in resistivity are observed, facilitating more detailed and precise measurements.

⦁ Plotting test results in real-time during the testing process to promptly identify any problematic areas or anomalies that may require further investigation.

How is Soil Resistivity Testing Performed?

Conducting a soil resistivity test involves a set of precise procedures to ensure accurate measurements and provide valuable insights for designing an effective grounding system. Let’s review them:

Placement of Equally Spaced Probes: Four auxiliary probes are strategically positioned in the ground, maintaining equal intervals along a straight line. Care is taken to insert them to an appropriate depth, ensuring reliable results.

Connection to Earth Resistance Tester: The four probes are connected to an earth resistance tester, a specialized device used for measuring electrical resistance in the ground.

Activation of DC Test Mode: The DC test mode is selected on the resistance tester, initiating the test sequence. This mode allows for the measurement of resistance under direct current conditions.

Recording of Resistance Value: The resistance figure (R) obtained from the test is accurately recorded. This value represents the electrical resistance encountered by the current flowing through the soil.

Calculation of Soil Resistivity: The soil resistivity (r) in ohms/cm is calculated using the formula:
r = 2 ρaR

In this formula, R represents the measured resistance value in ohms, and a denotes the separation distance between the probes in meters. The resistivity value provides insights into the soil’s ability to conduct electrical current.

By following these meticulous steps, a comprehensive assessment of soil resistivity can be achieved, facilitating the design and implementation of an optimal grounding system tailored to meet the specific requirements of the site.

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