Rolling Sphere Method for Lightning Protection
Designers use the rolling sphere method to help determine where air terminals or strike termination devices are required on a building. It assumes that the electrically-charged field that produces a lightning strike has a 150 ft. radius. The designer visualizes a sphere with that radius rolling over the surface of the building. Any place the sphere touches the building is a location (shown in red) where lightning can strike the building. By installing air terminals (shown in green), the sphere cannot touch the building because electrical charges flow through the lightning protection system into the ground.
The rolling sphere method is included in the National Fire Protection Association document NFPA 780 — Standard for the Installation of Lightning Protection Systems. The standard is based upon rigorous scientific research, testing with real lightning, and more than two hundred years of proven performance.
Some companies claim their “early streamer emission” air terminals are effective beyond the 150 ft. radius some even claim they can protect an entire building with a single air terminal regardless of the configuration of the structure. Others claim their “dissipation array systems” can prevent lightning altogether. Be aware — their claims of exaggerated performance are not substantiated. In fact, courts, governmental agencies, and scientific committees have rejected their claims.