L : Total length of ground rods r L : The length of each ground rods C L : Total length of the conductor in the horizontal grid p L : Peripheral length of grid A Lx Ly L x : The maximum length of the grid in the x direction L y : The maximum length of the grid in the y directionДЈ.

m 2.1Д: Spacing between parallel conductors d : Diameter of grid conductor h : Depth of ground grid conductor.

Where L is the length of the electrode in m 2b is the diameter of the electrode in m ПЃ is the resistivity of the soil in в„¦ The earth resistance of a single rod or pipe electrode is calculated using the Fagan and Les formula. Since they ware published in 1954, SchwarzвЂ™s equations have been modified by Kercel to provide equations for constants k1 and k2 and further expanded to include the use of equations in two-layer soil (Naham and Salamon).ДЎ.2 Earthing resistance of single rod. Schwarz compared the results of his equations to previously published theoretical work and to model tests to verify the accuracy of his equations. The combined earthing resistance of the grid and the rod bed will be lower than the Hamed afzalipour Д®arthing resistance of either component alone, but still higher than that of a parallel Combination. Where Lr is the length of each rod in m 2b is the diameter of rod in m nR number of rods placed in area A Mutual earthing resistance between the buried conductor and the rod bed 2h) for conductors buried at depth h in m, or aвЂ™ is a for conductor on earth surface in m 2a is the diameter of conductor in m A is the area covered by conductors in m2 k1, k2 are the coefficients Earthing resistance of the rod bed

m Lc is the total length of all connected grid conductors in m aвЂ™ is sqrt(a.

Where R1 earthing resistance of buried conductors in в„¦ R2 earthing resistance of all earthing rods in в„¦ Rm mutual earthing resistance between the group of buried conductors, R1, and group of earthing rods, R2 in в„¦ Earthing resistance of the buried conductor Schwarz used the following equation to combine the resistance of the buried conductors, rods, and mutual earthing resistance to calculate the total system resistance, Rg. He also introduced an equation for the mutual earthing resistance Rm between the buried conductor and rod bed. SchwarzвЂ™s equations extended accepted equations for a straight horizontal wire to represent the earthing resistance, R1, of a grid consisting of crisscrossing conductors, and a sphere embedded in the earth to represent earthing rods, R2. Schwarz developed the following set of equations to determine the total resistance of an earthing system in a homogeneous soil consisting of horizontal (buried conductors) and vertical (rods) electrodes. METHODS OF CALCULATING EARTH ROD RESISTANCE CALCULATION 1.1 General The buried conductor size, total earthing loop resistance, touch voltage and step voltage are studied based on the calculation by Schwarz equations, which are defined by IEEE std 80-2000.

= A (Rated Fault current of LV switchgear to be considered)T = Duration of fault current in Seconds ( example - 0.3 s for EMAL) S = Cross section of copper conductor in mm2 Main Earth Grid SizingThe minimum conductor size of main earthing grid is given by the formula given in Clause no. Substitute the values and calculate the value of equivalent resistance Rn (it should be less than 1 ohm)Rn=? R = Resistance of one rod in isolation = Rn = Resistance of n rods in parallel, n = X ( as calculated above- minimum to be considered)s = Distance between adjacent rods in meter (20 m average value considered)= Soil resistivity in Ohm-meters = 30 The equivalent resistance of n rod electrodes in parallel can be obtained from the formula given inBS 7430, Clause No.

Use 1/ R value to get number of electrodes required ( let us assume X) Calculate how many rods are required to achieve a value of 1 or less? L= Length of the earth electrode in meters = 3.6Д= Diameter of the earth rod in meters = 0.02 (20 mm) Approx.ДЇind R of one Rod. = Resistivity of the Soil in Ohm-meters =30 Resistance to earth of an electrode (rod) is given by the formula (Refer BS 7430, Clause no. Consider Soil resistivity of 30 Ohm-meter All earth electrodes shall be interconnected through the earthing grid.Д¦.

Consider Earth electrode of 19mm diameter and 3.6 m length.ДҐ.

The earthing cable/grid shall comprise of stranded copper conductor with green/yellow PVC Sheathing.ДЈ. The overall earthing network resistance shall be maintained less than 1.Дў. IEEE142 Recommended Practice for grounding of Industrial & Commercial Power SystemsДЎ. Earthing Calculation for Substation buildingScopeDefine the Size of Main Earthing Conductor and Minimum number of earth electrodes required to achieve an overall earth resistivity of 1

Earthing calculation formula pdf