7.2.1 Simplified calculation of the separation distance

A direct lightning strike on the external lightning protection system of a building causes strong, temporally variable magnetic fields due to the currents flowing through the air-termination systems and down conductors. These fields induce high voltages in the resulting loops between the air-termination system and the down conductor on the one hand and metallic components, installations and internal systems on the other, which, in extreme cases, can reach values in the megavolt range.
To prevent uncontrolled flashovers, the external lightning protection system must be adequately insulated from metallic components and internal systems. The decisive factor here is what is known as the separation distance, which is derived from the required impulse withstand voltage at the point of proximity.
The separation distance is not uniformly specified for every building, but depends on the location of the proximity, the most unfavourable point of strike and, in particular, the dielectric strength at the location in question. This dielectric strength is in turn determined by the maximum value and the temporal progression of the induced voltage.
Both theoretical considerations and experimental results show that the required separation distance is additionally influenced by material properties, the current steepness and the current distribution between the lightning-current-carrying conductors. In IEC 62305-3, these influences are taken into account by the coefficients k_i (current steepness) k_m (material) and k_c (current distribution). The required separation distance s (in metres) is calculated using equation 7.2.1.1: