Mesh width of ground grids in shelters with small base areas for low step voltages at lightning currents

An investigation is carried out to achieve low step voltages in particular on earthing arrangements for buildings with small base areas, especially shelters. The aim is to realize earthing arrangements with mesh grids for potential control that give safe stay of persons inside the protected area in case of lightning strikes. Based on field calculations it should be determined how step-voltage-safe areas changes in case of grounding rings and rods combined with mesh grids depending on mesh width, grid size, and type of installation. The influence of mesh width as a function of base area of the buildings, the buried depth and the cross section of the mesh grid bars was considered in various practice-oriented arrangements. The resultant step-voltage-safe areas are determined and compared, with a step voltage limit value of 25 kV being set with the positive first stroke lightning current at 100 kA peak value. By means of small mesh width (0.25 m) even with small base areas, small mesh grid areas (5 m x 5 m, < 100 m(2)) and with high soil resistivity (rho(E) up to 2000 Omega.m) a sufficient safety against step voltages can be achieved. For larger areas (> 200 m(2)) larger mesh widths are sufficient, 1 m, which is the defined step width, must not be exceeded. The potential control measure with the metal grid have to be embedded in an adapted earthing system.