Earthing Systems Design steps – Part Two


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.


And 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.



Also, in that Article, I explained all the above points except How to perform Soil Resistivity Survey.







Resistivity Definition


  • Electrical Resistivity is the measurement of the specific resistance of a given material. It is expressed in ohm-meters and represents the resistance measured between two plates covering opposite sides of a 1 m cube.
  • Soil Resistivity is the key factor that determines the resistance or performance of an electrical grounding system. It is the starting point of any electrical grounding design.






Factors Affecting Soil Resistivity

There are many factors that affect on the value of the soil resistivity and the earth resistivity depends significantly on these factors which are:

  1. Physical Composition,
  2. Moisture,
  3. Chemical Composition,
  4. Temperature.


These Factors were explained in detail in Article " Electrical Properties of the Earthing System ". Please, review their effects on the soil resistivity in the referred article.






Methods for Getting Soil Resistivity Value

In Earthing System Design, the Soil resistivity Value must be known by one of the two following methods:
  1. Estimation from soil resistivity tables according to the soil nature.
  2. Calculation from soil resistivity Testing.

In case that soil resistivity will be estimated from tables there is no need to perform the Geological Surveys.







First Method: Estimation From Soil Resistivity Tables According To The Soil Nature


Table 1
Effect of soil type on resistivity
Soil type
Typical resistivity ohm-m
Marshy Ground
2 - 2.7
Loam and clay
4 - 150
Chalk
600 - 400
Sand
90 - 8000
Peat
200 upwards
Sandy Gravel
300 - 500
Rock
1000 upwards


Note:

Where no information is available about the value of ρ it is usually assumed ρ = 100 Ωm.







Second Method: Calculation From Soil Resistivity Testing
                         




What Is The Soil Resistivity Testing?


  • Soil resistivity testing is the process of measuring a volume of soil to determine the conductivity of the soil. The resulting soil resistivity is expressed in ohm-meter or ohm-centimeter.
  • Soil resistivity testing is the single most critical factor in electrical grounding design. This is true when discussing simple electrical design, to dedicated low-resistance grounding systems, or to the far more complex issues involved in Ground Potential Rise Studies (GPR). Good soil models are the basis of all grounding designs and they are developed from accurate soil resistivity testing.
  • This soil resistivity test is commonly performed at raw land sites, during the design and planning of grounding systems specific to the tested site.


 Note:

 it is not acceptable to make soil resistivity test for disturbed (backfilled) land sites.





Methods Of Soil Resistivity Testing

There are mony methods used for soil resistivity testing which are:
  1. The Wenner 4-Point Method,
  2. General Method,
  3. Schlumberger Method.



  • The Wenner 4-point Method is by far the most used test method to measure the resistivity of soil. Other methods such as the General and Schlumberger methods which are infrequently used for grounding design applications and vary only slightly in how the probes are spaced when compared to the Wenner Method.
  • Proper soil resistivity testing using the Wenner 4-point method is recommended because of its accuracy.






Criteria For Soil Resistivity Test By Using Four (4) Probes Method





  • The soil resistivity test spaces four (4) probes out at equal distances to approximate the depth of the soil to be tested. This means that if the probes spaced at 5’ distance across the earth, they will read 5’ in depth. The same is true if you space the probes 40’ across the earth, you get a weighted average soil resistance from 0’ down to 40’ in depth, and all points in between.
  • Typical spacings will be 1’, 1.5’, 2’, 3’, 4.5’, 7’, 10’, etc., with each spacing increasing from the preceding one by a factor of approximately 1.5 up to the maximum spacing vale.
  • The maximum spacing Value equal to (1 to 3) times the maximum diagonal dimension of the grounding system being designed, resulting in a maximum distance between the outer current electrodes of (3 to 9) times the maximum diagonal dimension of the future grounding system.
  • 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.
  • This set of measurements is typically repeated, albeit with shorter maximum spacings, several times around the location at right angles and diagonally to each other to ensure accurate readings.
  • These readings are usually processed with computer software to determine the actual resistivity of the soil as a function of depth.






Conducting a Wenner 4-point (or four-pin) Soil Resistivity Test

The following describes how to take one “traverse” or set of measurements. As the “4-point” indicates, the test consists of 4 pins that must be inserted into the earth.





  1. The outer two pins are called the Current probes, C1 and C2. These are the probes that inject current into the earth. The inner two probes are the Potential probes, P1 and P2. These are the probes that take the actual soil resistance measurement.
  2. In the following Wenner 4-Point Test Setup diagram, a probe C1 is driven into the earth at the corner of the area to be measured. Probes P1, P2, & C2 are driven at 5’, 10’ & 15’ respectively from rod C1 in a straight line to measure the soil resistivity from 0’ to 5’ in depth. C1 & C2 are the outer probes and P1 & P2 are the inner probes. At this point, a known current is applied across probes C1 & C2, while the resulting voltage is measured across P1 & P2. Ohm’s law can then be applied to calculate the measured apparent resistance.
  3. Probes C2, P1 & P2 can then be moved out to 10’, 20’ & 30’ spacing to measure the resistance of the earth from 0’ to 10’ in depth.
  4. Continue moving the three probes (C2, P1 & P2) away from C1 at equal intervals to approximate the depth of the soil to be measured.


 Note:

 the performance of the electrode can be influenced by soil resistivities at depths that are considerably deeper than the depth of the electrode, particularly for extensive horizontal electrodes, such as water pipes, building foundations or grounding grids.






Soil Resistance Meters





There are basically two types of soil resistance meters:

  1. Low-Frequency models,
  2. High-Frequency models.


Both meter types can be used for 4-point & 3-point testing, and can even be used as standard (2-point) volt meter for measuring common soil resistivity.


Criteria for Selecting the Best Soil Resistance Meters

Care should always be given when selecting a soil resistance meter, as the electronics involved in signal filtering are highly specialized. Electrically speaking, the earth can be a noisy place. Overhead power lines, electric substations, railroad tracks, various signal transmitters and many other sources contribute to signal noise found in any given location. Harmonics, 60 Hz background noise, and magnetic field coupling can distort the measurement signal, resulting in apparent soil resistivity readings that are larger by an order of magnitude, particularly with large spacings. Selecting equipment with electronic packages capable of discriminating between these signals is critical.


1- High-Frequency Meters

  • High-Frequency soil resistance meters typically use a pulses operating at 128 pulses per second, or other pulse rates except 60. These High-Frequency meters typically suffer from the inability to generate sufficient voltage to handle long traverses and generally should not be used for probe spacings greater than 100 feet.
  • Furthermore, the High-Frequency signal flowing in the current lead induces a noise voltage in the potential leads, which cannot be completely filtered out: this noise becomes greater than the measured signal as the soil resistivity decreases and the pin spacing increases. High-Frequency meters are less expensive than their Low-Frequency counterparts, and are by far the most common meter used in soil resistivity testing.


2- Low-Frequency Meters

  • Low-Frequency meters actually generate low frequency pulses (on the order of 0.5 to 2.0 seconds per pulse), are the preferred equipment for soil resistivity testing, as they do away with the induction problem from which the High-Frequency meters suffer. However they can be very expensive to purchase.
  • Depending upon the equipment’s maximum voltage, Low-Frequency meters can take readings with extremely large probe spacings and often many thousands of feet in distance.
  • Typically, the electronics filtering packages offered in Low-Frequency meters are superior to those found in High-Frequency meters.





Proper Measurement of Soil Resistivity

For taking more accurate readings in the soil resistivity testing, the following points must be considered:

  • Take the seasonal variation in soil and its electrical characteristics and therefore in ground resistance into consideration.
  • Define the climatic condition (temperature, humidity, etc.) and soil condition (moisture, temperature and chemical content) during the test.
  • Refer to long term studies that have been done on soil resistivity variations in the area of concern for adjustment of the measurement results.
  • The Variations of measured resistivity when plotted versus probe spacing indicate that the earth is non-uniform, and a two-layer soil model must be used. Using a single-layer model in such a situation cause significant errors in resistivity.
  • Calculations based on two-layer soil model usually give correct values for the ground resistance and for the step and touch potentials on the surface of the ground. However, when the soil has a multilayer structure and certain combination of different layers resistivity exist the two-layer model of the soil may give unreasonable results.


The two layers required for the soil modeling are called:
  1. Shallow Depth Readings,
  2. Deep Readings.



1- Shallow Depth Readings






  • Shallow depth readings, as little as 6” in depth, are exceedingly important for most, if not all, grounding designs. As described above, the deeper soil resistivity readings are actually weighted averages of the soil resistivity from the earth surface down to depth, and include all the shallow resistance readings above it. The trick in developing the final soil model is to pull out the actual resistance of the soil at depth, and that requires “subtracting” the top layers from the deep readings. The following figure demonstrates how the shallowest readings impact deeper ones below it.
  • As you can see in the following diagram, if you have a 5’ reading of 50 ohm-meters and a 10’ reading of 75-ohmmeter soil, the actual soil resistivity from 5’ to 10’ might be 100 ohm-meters (the point here is to illustrate a concept: pre-computed curves or computer software are needed to properly interpret the data). The same follows true for larger pin spacings.
  •  The shallowest readings are used over and over again in determining the actual resistivity at depth.
  • Shallow depth readings of 6-inches, 1-foot, 1.5-feet, 2-feet and 2.5-feet are important for grounding design, because grounding conductors are typically buried at 1.5 to 2.5-feet below the surface of the earth. To accurately calculate how those conductors will perform at these depths shallow soil readings must be taken.
  • These shallow readings become even more important when engineers calculate Ground Potential Rise, Touch Voltages and Step Voltages.
  • It is critical that the measurement probes and current probes be inserted into the earth to the proper depth for shallow soil resistivity readings. If the probes are driven too deep, then it can be difficult to resolve the resistivity of the shallow soil.
  • A rule of thumb is that the penetration depth of the potential probes should be no more than 10% of the pin spacing, whereas the current probes must not be driven more than 30% of the pin spacing.


2- Deep Readings

  • Often, the type of meter used determines the maximum depth or spacing that can be read. A general guideline is that High-Frequency soil resistivity meters are good for no more than 100-feet pin spacings, particularly in low resistivity soils. 
  • For greater pin spacings, Low-Frequency soil resistivity meters are required. They can generate the required voltage needed to push the signal through the soil at deep distances and detect a weak signal, free of induced voltage from the current injection leads.




In the next Article, I will explain The Second Step for Earthing System Design: Data Analysis. Please, keep following.




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