Rolling Sphere Model
This somewhat cryptic term is used in IEC 62305 to describe how to assess the risk of a direct lightning strike to a building.
The principle is based on the idea that lightning seeks the most efficient path to ground. If the tip of a lightning bolt is represented as the center of a fictional sphere, the sphere indicates possible strike paths in all directions.
When the sphere touches the ground, it represents a potential path for the lightning.
By rolling the sphere toward and over a building, one can identify where unwanted strike paths may occur.
The model can also be used to determine strategic placements for air termination systems on the building.
The protection classes (LPS I, II, III, and IV) define different diameters of the sphere, ranging from a radius of 20 meters to a radius of 60 meters. The smallest radius represents the highest level of protection, which requires a larger air termination system than the lower classes.