To help determine where air terminals or strike termination devices are required on a building, our designers use the rolling sphere method. Included in National Fire Protection Association document NFPA 780 —Standard for the Installation of Lightning Protection Systems, the method is based upon rigorous scientific research, testing with real lightning, and more than two hundred years of proven performance. Also see NFPA 780.
The rolling sphere lightning protection method assumes 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. In the video below, any place the red sphere touches the building is a location 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.