The purpose of the protection measures against lightning is to reach a balance between the cost of protecting lines and plant from lightning damage and the cost of repair and the tolerable number of faults, in a given area. These costs will depend on the number of thunderstorm days per year (keraunic level), soil resistivity and the degree of shielding provided by buildings, water pipes, and metallic structures in general. A classification of zones according to the risk of lightning strikes appears to be useful. It is not possible, however, to make a universally valid classification solely in terms of the number of thunderstorm days per year, because the effect of lightning on telecommunication cables (which is the real criterion) differs according to the other factors involved, including the local earth resistivity and the topographical situation. A relatively low keraunic level may therefore give rise to a low fault rate in one region but to a high fault rate in another. Each country must make its own local observations to determine the relation between the keraunic level in their areas and the fault liability, and to classify their zones accordingly.