Determination of the probability function of lightning peak currents on flat ground

To design a proper lightning protection system the electro-geometrical model (EGM) is widely used to place lightning conductors on structures. Among some other methods the standard IEC 62305 makes use of a rolling sphere model, in which it is analyzed if a sphere of a certain radius might touch a structure or its protecting lightning conductors. The radii of the spheres are connected to certain lightning peak currents, and these are associated with the distribution of lightning peak currents, published by CIGRÉ (CIGRE data). From these distributions the probabilities of exceeding a certain current level can be derived. These distributions were created mainly by real tower measurements on Monte San Salvatore (MSS) by Prof. Berger. Today a new simulation methodology, the dynamic electro-geometrical model (DEGM), uses this distribution and also the whole concept of the EGM. Because the peak current distribution was obtained from tower measurements, the distribution is falsified in comparison to a distribution on flat ground. To get undistorted results from the DEGM a converted distribution to flat ground is needed. An analytic approach is introduced to describe the problem itself, and after that the real geographical environment is used to compute the real interception areas at MSS measurement station in order to convert the peak current distribution derived from “Bergers Data” to flat ground. As an outcome, the median current peak values of a “normalized” peak current distribution are lower than those of the original distribution. Additionally, this corresponds better to distributions found by LDNs (Lightning Detection Networks).