Evaluation of mesh and touch voltage at substations

Site tests, to measure the voltages on the surface of the ground above a grid for comparison with calculated values, are only of general interest if the ground has uniform resistivity at and below the grid. However, such conditions rarely if ever exist so the only practical alternative way of obtaining general data is to make tests on model grids in a tank where the electrolyte can be maintained at an homogeneous resistivity. A comprehensive series of such tests has been made in which voltages calculated using empirical formulas given in four of the most commonly used standards are compared with model test results and, as the correspondence is unsatisfactory, new empirical formulas have been developed. That identified the need to account for the shape of the grid and the meshes, and the distortion of the voltage contours close to the grid due to the heterogeneous nature of the local ground. New equations more closely agree with test results. A method of taking into account that the grid is not at the ground surface in the calculation of the HOT distance is given