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The Standard IEC 62305 provides advanced information for the design of lightning protection of structures (LPS). In determining the position of the air-termination system, two methods are generally suggested: the protective angle method and the rolling sphere method. Adopting a 3-D graphical simulation, the “rolling sphere” method allows one: to analyze the protected volume at the building roof inside of the rolling ply surface; to verify the behavior of the designed air-termination system; to evaluate the contribution of the surrounding structures in limiting the lightning current flowing in the down-conductors. This current affects the probability of dangerous sparking and the separation distance for the protection of internal installations. This paper presents this approach by a case study of a building in a complex of structures: it highlights particularly the worst cases of the lightning impact related to the designed air-termination system.