Lightning protection system design
Various materials are used for and to make up lightning protection system design, so it is of grave importance to carefully consider where the air terminal will provide the greatest protection. The historical understanding of lightning, from statements made by Ben Franklin, assumed that each lightning pole protected an area of 45 degrees. This has been proved to be insufficient for protecting structures which extends to higher heights, as it is also possible for lightning to strike the building from the side.
A lightning protection system design simulation system based on a better knowledge of termination targeting of lightning, this system is called the Rolling Sphere Method and was developed by Dr Tibor Horváth. It has become the standard method by which traditional Franklin Rod systems are installed. To better understand this, it requires knowledge of how lightning ‘moves’. As the step leader of a lightning bolt jumps toward the ground, it steps toward grounded objects in the closest proximity to its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electric current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere’s radius as 46 m near the ground.
An object outside the critical distance is unlikely to be struck by the leader if there is a solidly grounded object within the critical distance. Locations that are considered safe can be determined by imagining a leader’s potential paths as a sphere that travels from the cloud to the ground. For lightning protection, it suffices to consider all possible spheres as they touch potential strike points. To determine strike points, consider a sphere rolling over the terrain. At each point, a potential leader position is simulated. It is most likely to strike where the sphere touches the ground. Points that the sphere cannot roll across and touch are safest from lightning. Lightning protection systems should be placed where they will prevent the sphere from touching a structure. A weak point in most lightning diversion systems is in transporting the captured discharge from the lightning rod to the ground, though. A lightning protection pole are typically installed around the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 6.1 m or 7.6 m, depending on the height of the rod. When a flat roof has dimensions greater than 15 m by 15 m, additional air terminals will be installed in the middle of the roof at intervals of 15 m or less in a rectangular grid pattern.