Earthing Grid resistance

The approximate grid resistance epo without the earthing rods can be found from Figure H.la, which was derived from Figure B.l of IEEE80 but applicable to small sizes of the mesh ) epo> is approximately -... 

The connection to earth can be made utilizing earth rods, earth plates, earth lattices or grids, or buried strip conductors. Selection depends on the locality and type of ground conditions at the site. The effectiveness of the earth connection is, in turn, dependent on the resistivity of the soil at the site, the type of soil and climatic conditions have a direct effect upon the resistance of the connection made. Typical values of resistivity — m) for different types of soil are ... 

It will be assumed that the source impedance Z feeding the transformers is small enough to be neglected. To illustrate the difficulty in finding a suitably low resistance to earth it will be assumed that the secondary winding of the transformer is solidly earthed and hence the NER resistance Rn is zero, a non-zero value will be recommended at the conclusion of the calculations. However, it will be assumed that the resistance to earth / en at the somce transformer is 1.5 ohms. The resistance to earth at the far end pole is R, which needs to be determined. This requires a suitable grid and rod system to be chosen. The calculation process will be carried out in a series of steps. 

Table H.lc. Data for earth resistance, touch voltage, ground potential rise and corner mesh voltage for different grid and rod designs...

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