Lightning Protection System

The Full Picture

The aim of the system is not to prevent a strike but to control it. A National Weather Service facilities directive puts it this way: the system's intention is to provide preferred paths for lightning discharges to enter or leave the earth without damaging the facility or injuring people. Air terminals, often called lightning rods, intercept the strike. Conductors carry the current along the roof and down the structure, and a grounding system disperses it into the earth.

Installation practice follows NFPA 780, the standard for lightning protection system installation, together with product listings. NFPA 780 is the standard most specs cite, and the edition and any local requirements come from the project documents. The design may be shown on the electrical drawings or delivered as a separate specialty drawing from the system supplier. Many projects carry it as a specialty subcontract, so scope is often defined by the spec section for lightning protection, not by the electrical plans.

On the documents, look for a roof plan showing air terminal locations and the roof conductor, a grounding plan showing electrodes and any ground ring, building elevations or details with down conductor routes, and a detail sheet for connections to steel, roof equipment and the ground system. The takeoff units mix length and count. Conductor is taken off in LF, including roof loops, down conductors and ground rings. Air terminals, bonds to roof equipment and steel, splice and connection hardware, and ground rods or plates are counted EA.

Common mistakes are measuring only the roof and omitting the down conductors and ground ring, missing bonds to metal roof equipment and rooftop units that the detail calls for, and ignoring roof conditions. Penetrations and attachments to the roofing system should be coordinated with the roofer, so a roof warranty is not affected, and ground-level work needs excavation and backfill from the site scope.