The Rolling Sphere Method
Lightning protection system designers use the “rolling sphere method for lightning protection” to help determine where air terminals or strike termination devices are required on the roof or other parts of a building. The method assumes that the electrically-charged field ahead of a bolt of lightning that is capable of causing damage is a distance of approximately 150 ft. To simulate how lightning will interact with a structure, the designer visualizes a sphere with a 150 ft. radius rolling over the surface of the building. Any place the sphere touches the building is a location where lightning can strike. By placing air terminals at regular intervals in these locations, the lightning strike is intercepted by the lightning protection system so that the electrical charges can flow safely to the ground.
Animation of the use of the Rolling Sphere Method to place air terminals on top of a building.
Air terminals (shown in green) or other strike termination devices must be located to prevent a sphere with a 150-foot radius (red) from contacting the building. (Animation Courtesy Scientific Lightning Solutions, LLC)
Beware of Unproven Design Methods
The rolling sphere method is included in the National Fire Protection Association document NFPA 780 — Standard for the Installation of Lightning Protection Systems. It is based on extensive, peer-reviewed science and decades of field observations. Some companies, however, claim their “early streamer emission” air terminals are effective well beyond the distances embodied in Standard NFPA 780. Some even claim they can protect an entire building with a single air terminal regardless of the configuration of the structure. Other companies claim their “charge dissipation array” systems can prevent a lightning event altogether. Be wary — these claims of exaggerated performance are not substantiated. In fact, courts, governmental agencies, and scientific committees have rejected the claims made by these fringe companies.
For an article explaining the shortcomings of non-standard system claims, click on the link below.
Not All Lightning Protection is Created Equal
Drawing depicting the application of the rolling sphere method to determine zones of protection on a structure.
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.
Application of the Rolling Sphere Method can identify architectural elements that might be able to create a zone of protection and eliminate some air terminal locations.
A rolling sphere analysis determined that air terminals are not required on this sculptural grouping or pediment at the Smithsonian Arts and Industries Building. They are in the “zone of protection” created by the air terminals on the flanking towers. (Photo Lydia Liu, creativecommons.org/licenses/by/2.0/)