Earthing Calculation

1. 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. 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. Schwarz’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 . He also introduced an equation for the mutual earthing resistance Rm between the buried conductor and rod bed. 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 .

where R1 earthing resistance of buried conductors in Ω R2 earthing resistance of all earthing rods in Ω Rm mutual earthing resistance between the group of buried conductors, R1 , and group of earthing rods, R2 in Ω Earthing resistance of the buried conductor

where ρ is the soil resistivity in Ω ·m Lc is the total length of all connected grid conductors in m a’ is sqrt(a. 2h) for conductors buried at depth h in m, or a’ 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 [see Figure 1(a) and (b)] Earthing resistance of the rod bed

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

The combined earthing resistance of the grid and the rod bed will be lower than the Hamed afzalipour http://electrical-engineer.blogfa.com/

earthing resistance of either component alone, but still higher than that of a parallel Combination. Schwarz compared the results of his equations to previously published theoretical work and to model tests to verify the accuracy of his equations. Since they ware published in 1954, Schwarz’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).

1.2 Earthing resistance of single rod. The earth resistance of a single rod or pipe electrode is calculated using the Fagan and Les formula.

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where L is the length of the electrode in m 2b is the diameter of the electrode in m ρ is the resistivity of the soil in Ω ·m 2.1

2.2


D: Spacing between parallel conductors d : Diameter of grid conductor h : Depth of ground grid conductor

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 direction


3. EARTHING CONDUCTOR SIZE CALCULATION General For the electrical neutral earthing, earthing conductor shall be designed so as not to exceed the allowable maximum temperature of the conductor. The calculation is made based on the criteria of cable sizing for short circuit specified .namely the conductor is sized so as not to exceed the allowable maximum temperature for the clearance time of the earth fault current by the protection device. The temperature rise of the earthing conductor during very short time fault condition is calculated by the following formula in accordance with IEC 60949 standard:

STANDARDS Code of practice for Earthing BS7430 For the earthing system calculation, IEEE std 80-2000 is referred to. Thanks Hamed afzalipour http://electrical-engineer.blogfa.com/


Earthing Calculation

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