Soil resistivity. This topic explains measurement of soil resistivity, Wenner's four-point method, factors affecting soil resistivity. The earth acts as a conductor for carrying the current during fault conditions. Like any other metallic conductor, mild steel, MS, copper, aluminum, etc, earth also has resistivity. The process of measuring resistivity of soil is called as soil resistivity measurement. The resistivity of the soil varies from soil to soil. Soil which has ability to retain moisture has very low resistivity in the order of a few ohm meters, and dry rocky strata has the highest soil resistivity in thousands of ohm meters. Methods for measurement of soil resistivity. IEEE-80 and 81 list several methods for measuring soil resistivity. Methods include Wenner's four-point method, fall of potential method, three-point method, two-point method, etc. IEEE-81 elaborately discusses all these methods, factors affecting the accuracy of measurements and recommendations for obtaining reasonably accurate values. Wenner's four-point method is widely adopted. Soil has electrical resistivity and is very much different from soil thermal resistivity used in the cable sizing calculations. Soil thermal resistivity or the thermal conductivity of soil is used to analyze or to understand how quickly the heat generated in cables is dissipated. The resistance of one meter cube of the soil, one meter into one meter into one meter, as measured between any two opposite surfaces, is the resistivity of the soil similar to the specific resistance of conductors. The formula for soil resistivity is, where R, resistance of the conductors, Rho, soil resistivity, L is the length, and A is the area of cross-section of conductor. You will note that the unit of soil resistivity can be derived from the above expression as ohm into square meter per meter, that is ohm meters. Wenner's four-point method is very simple. It requires four mild steel electrodes, some cables, and earth resistance tester, which is nothing but Earth Megger. Megger is a registered brand name, and it is synonymous with resistance tester. We will use generic name, earth resistance tester. Electrodes or spikes supplied as part of Earth tester shall be preferably used, as the material and resistance of the electrodes are optimized for the instrument's sensitivity. Length of all four electrodes are identical. Earth resistance tester has four terminals, C1, C2, and P1, P2. A test current is injected through C1, C2, that is, a current flows in the soil between Electrodes 1 and 4. Earth tester reads potential difference between P1, P2, which is due to the product of injected current and the resistance of soil between Electrodes 2 and 3. So the soil resistance measured can be obtained from the following formula. Soil resistance is equal to measured voltage by injected current. The current injection and voltage measurement are done through four spikes or electrodes driven into the soil, as shown in the picture, placed equidistance a from each other. Several measurements are taken at various spacing, a, starting from one meter to 100 meters. Measurements are taken at a plot identified for substation in all directions, especially at diagonals. Measurements shall be taken where the earth grid is likely to be located. Measured values are tabulated. Simplified formula for calculating soil resistivity is given by; Rho is equal to 2 into Pi, into a, into R or meters, where a is equal to distance between spikes in meters and R is equal to resistance value, measured using earth tester as V by I. Measured values of R versus of a are tabulated. The value of Rho measured like this corresponds to resistivity of soil at a depth of a meters. Average of all Rho thus measured for various spacing, is considered for earthing design as soil resistivity. Note, the simplified formula is valid if a is significantly greater than D. Hence, it is sufficient to drive the electrode to a depth of about 200 millimeters. However, it is important to ensure that the electrode is in good contact with soil without any whites. So best way to ensure good contact with the soil is to drive the electrode directly into the undisturbed virgin soil. Soil resistivity depends on type of soil and varies based on following aspects: seasonal variation, temperature, moisture, dissolved salts in water. The earthing design shall take into effects of these aspects and ensure safe earthing based on onerous conditions. Accordingly, it is desirable to measure soil resistivity when the ambient temperature is high and the soil is generally dry. Measurements taken around noon or after noon during summer will result in higher soil resistivity values, which shall preferably be used in the earthing design. Next, let us move on to effect of moisture, temperature, and salt on soil resistivity. For good soil conductivity, presence of moisture is essential. It varies with season, and hence it is always advisable to penetrate the earth electrodes, not test electrode till the depth at which moisture remains throughout the year. Soil resistivity decreases with decrease in temperature but at zero degrees Celsius, its effect is predominant, at which resistivity increases sharply. Soil resistivity decreases with increase in soil salt content. However, resistivity increases again once concentration of salt content gets diluted. Given the opportunity, which is the ideal period for measuring the earth resistivity in India for grounding system design and why? The appropriate period for measurement of earth resistivity is summer. The fundamental reason is that the moisture content in the soil will be lower compared to the rainy season. Here is a quick recap of what you've learned this far. Like any other metallic conductor, earth also has resistivity. The soil resistivity varies for different types of soils and is different from thermal resistivity. The resistance of one meter cube of the soil as measured between any two opposite surfaces, is the resistivity of the soil. Wenner's four-point method is commonly used for measurement of soil resistivity. Simplified formula for calculating soil resistivity is given by Rho is equal to 2 into Pi, into a into R. Where a is equal to distance between spikes in meters and R is equal to resistance value measured using earth tester as V by I. Each value of a specific resistance Rho measured is with reference to value of a, that is spacing between the electrodes. Measured values of R versus spacing a are tabulated. Average of all Rho thus measured for various spacing is considered for earthing design as soil resistivity. Soil resistivity decreases with decrease in temperature, but at zero degrees Celsius, its effect is predominant at which resistivity increases sharply.
