Protected Volume of Lightning Air-Termination Systems

The IEC standard series 62305 presents basic requirements for the lightning protection design of structures. 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 surface; to verify the behavior of the designed air-termination system. The proposed original method of the protected volume section by an air termination system of lightning rods is an alternative to the graphical analysis with the rolling sphere simulation. The protected volume method proposes the mathematical calculation of the prospected profile described by the rolling sphere, and, as simpler first step, allows identifying the minimum protection height useful for a preliminary evaluation of the critical height. As second step, the method defines a shape describing the protected volume section a cautionary model that replaces the circular sector of the rolling sphere with a “vase section” shape. Given a volume to be protected, taking into account the equipment and elements to be protected, the method allows to configure the air-terminal system with rods suitable for the volume protection. In order to avoid the exposure of equipment to direct lightning, the identification of the protected volumes allows highlighting positive situations or those of overruns with respect to the volume to be protected and so taking corrective measures.