Ground electrode
The conductivity of soil at a building site affects its suitability as a ground for lightning protection. Wet clay may not be desirable from a structural perspective, yet it is highly conductive and performs well as a ground. Dry sand, gravel, and rock have more resistance and will require more extensive methods to ground. If an owner obtains a soil investigation report, it can be made available to lightning system designers and installers; contractual provisions, however, should spell out what happens if site conditions differ from those in the report.
In conductive soil, a rod that has been located at least 0.6 m (2 ft) outside the building perimeter and driven 3 m (10 ft) vertically into the earth, may provide sufficient ground. In non-conductive soils, or where soil conditions make it difficult to drive a ground rod, a shallow ground plate or ground ring will help to distribute charges over a wider area. A ground ring, also known as a counterpoise, may be more economical than installing separate ground rods at each down conductor. Ground rings are also required for tall buildings
A test well is recommended to simplify inspection of a ground. The well selected should have a cover suitable for traffic loads that may be applied.
Surge protection
Any wire that enters a building is a potential path for lightning. In addition to power lines, contemporary buildings can be connected with wires for:
- electric gates;
- telephone and cable TV services;
- wells;
- satellite dishes and antennas;
- irrigation controls;
- surveillance cameras and security systems;
- site lighting and remote power receptacles;
- fire pumps;
- building-mounted lighting and signage; and
- photovoltaic collectors and generators.
In addition to protecting against lightning, surge protectors resist transient voltage from other external sources. However, since they do not protect against surges that originate within a building, individual pieces of equipment may still require their own surge protectors.
Surge-protective devices are usually furnished as part of the lightning protection work as their selection is integral to a complete, successful, lightning protection system. However, installation is typically performed by an electrical contractor because few installers have the electrician license necessary to install surge protective devices.
Site work
Tall trees next to buildings can present a problem when stuck by lightning, either by falling on the structure, or by causing the lightning to side-flash and strike building walls that are unprotected by air terminals. NFPA 780, therefore, recommends installation of lightning protection in trees with trunks within 3 m (10 ft) of a building or overtop of a building. Consideration should also be given to protecting valuable specimen trees, and other items onsite, such as pole-mounted lights with sensitive security concerns.
Items installed in open areas, such as pieces of equipment or small temporary structures, can be protected by a mast-mounted air terminal. Large areas—such as docks and military encampments—in which a multitude of masts would not be practical can be protected by conductors draped between widely spaced poles. This overhead shielding approach is called ‘catenary lightning protection’ after the shape assumed by the cables. It has been proposed as a means to protect arenas and other large outdoor venues where it is impractical to evacuate a crowd to safety when lightning approaches.
