The latest 2023 edition of NFPA 780 covers traditional lightning protection techniques for a wide range of structures such as ordinary buildings, structures of wind turbines, watercraft, and those with flammable and explosive materials. The purpose of the standard is to safeguard persons and property from hazards arising from exposure to lightning strikes.
Guidelines for a Basic Lightning Protection System:
A lightning protection system is a complete system that comprises conductors, including strike termination devices, interconnecting conductors, surge protective devices, grounding electrodes, bonding conductors, other connectors etc. We shall understand the important parameters mentioned in the standard for an ideal lightning protection system.
Material used: The material for the lightning protection system must be selected from the Class I or Class II materials list mentioned in the standard, according to the environmental factors and structural requirements. High-grade copper or copper alloys with the same corrosion resistance are preferred for lightning protection systems. Conductors which are electrical grade aluminium and with a minimum chemical composition of 99% aluminium can be used. But they should not be in contact with the copper roofing material or other copper surfaces etc. Bimetallic connectors or fittings shall be used for bonding dissimilar metals.
Strike Termination Devices: A strike termination device is a conductive metal component of the lightning protection system that is capable of receiving the lightning strike and conducting it to the ground. This includes air terminals, metal masts, metal parts of the structure used as part of the lightning protection system, overhead grounding conductors etc. The tip of the air terminal shall not be less than 10inch above the structure or area it is to protect. The overhead ground wire shall be made of aluminium, copper, stainless steel, galvanized steel etc. It also gives information about the type, size and placement of strike termination devices for different roof types.
Zones of Protection: The geometry of the structure decides the zones of protection. The different methods used for determining the overall zones of the structure are; air terminal placements, the angle method, and the rolling sphere method. We have detailed these methods in our previous blogs.
Conductors: The main conductor should interconnect all the strike termination devices in the structure. It should have two or more paths from the strike termination device connecting to the ground. Permanent metal handrails and ladders that are exposed to direct lightning strikes and are electrically continuous shall be used as main conductors. They need to have a minimum thickness of 1.63mm. The main conductor should not have a bend with an included angle of less than 90 degrees.
At least two down conductors must be present for a structure and shall be as separated as possible. The location of the down conductors depends on the position of the strike termination device, earth conditions, the location of the underground metallic piping system and so on.
Conductor Fasteners and Connectors: Conductor connections shall be of bolted, exothermically welded, high compression or crimp type. Connectors must be used to withstand a pull test of 890N. The fasteners used should be of the same material as that of the conductor or equally resistant to corrosion as the conductor.
Grounding Electrodes: The down conductors must be connected to one or more grounding electrodes dedicated to the lightning protection system according to the requirement of the structure. The ground rods must be solid copper, copper-clad steel or stainless steel (depending on the soil type and corrosion). There are different types of ground electrodes explained in the standard which include; ground rods, plate electrodes, radials, ring electrodes, etc.
Common Bonding of Grounded System: All the grounded metallic conductors, including the metallic piping system that can become a path for the lightning currents, shall be interconnected to the lightning protection system to provide a common ground potential. This can include the metallic pipings of water service lines, gas pipings, underground conduits etc.
Potential Equalisation: Potential equalisation is required to bring all the metal conducting parts to an equal potential level by bonding them. It is done based on three levels ground level, roof level and intermediate level potential equalization. The material, size and other requirements for the equipotential bonding are mentioned in the standard.
Structural Metallic Systems: The metallic framework of the structure can be utilised as the main conductor of the lightning protection system, if it is made in such a manner that it offers a continuous path for current flow by the methods specified in the Standards. The strike termination devices must be connected to the metal framework using exterior conductors. The connections can be made by bonding, welding, brazing, drilling and tapping.
Surge Protection: Surge protection includes the Type I and Type II surge protection devices, surge arresters and surge protectors installed within the structure. Surge protection devices(SPDs) are to be installed at the load side of an incoming or branching circuit line or any device. It mentions the requirements and conditions for surge protection devices, surge arresters and surge protectors separately.
Lightning Protection for Complex Facilities
Above are the basic protection techniques and parameters in a lightning protection system for a normal structure or a building. As the complexity and functionality of the facility increase, the requirements for lightning protection also increase. The lightning protection for miscellaneous structures and special occupancies gives the requirements for high masts, flagpoles, air-inflated structures, metal towers and tanks, rooftop helipads and so on. The standard details about the lightning protection requirements for the following structures:
1. Protection for structures with flammable vapours, gases and liquids that can give off flammable vapours.
2. Protection of structures housing explosive materials.
3. Protection for wind turbines.
4. Protection for watercraft.
5. Protection for airfield lighting circuits.
6. Protection for solar arrays.
Conclusion
The standard also explains the various lightning protection components and the maintenance, testing and inspection routines to be followed by the authority having jurisdiction. The protection methods for other temporary structures, lightning risk assessment and personal safety from lightning are also explained in NFPA 780.
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