Issue link: https://geospatial.trimble.com/en/resources/i/1415432
24 Spring 2019 Flammable or Combustible Liquids'. The NFPA standard is NFPA 780, 'Standard for the Installation of Lightning Protection Systems'. Since this article centres on petroleum storage tanks and the API documents specifically address these, the API documents should be considered primary. However, they offer little guidance regarding location and spacing of lightning protection system components. While NFPA 780 does provide such guidance, the designs contained in the document were originally intended to provide protection for wood barns and houses, not industrial facilities. As such, NFPA 780 is somewhat lacking when applied to tanks. Lightning attachment The attachment issue concerns the acceptability of substituting the dome structure for air terminals (lightning rods) or other strike termination devices. Reviewing applicable standards and recommended practices, NFPA 780 is the lightning protection standard for the US. NFPA 780, 4.6.1.4 states that metal parts of a structure that are exposed to direct lightning flashes and that have a metal thickness of 3/16 in. (4.8 mm) or greater only require connection to the lightning protection (conductor) system. That thickness requirement ensures that the structure is adequate to accept lightning attachment without burn-through or other damage from the heat or impact of a direct lightning attachment. The structural parts of the dome, including the nodes and beams, can usually meet the thickness requirements to serve as strike termination devices. However, the panels and node covers usually do not. One of the principles of conventional lightning protection is that any portion of a structure not meeting the requirements of a strike termination device must fall within the zone of protection of a portion of the structure that does meet those requirements. The zone of protection is defined as the space adjacent to a lightning protection system that is substantially immune to direct lightning flashes. For example, a lightning rod's zone of protection is the space around it wherein lightning will attach to that lightning rod in preference to anything else around it. As long as the entire protected structure is located within the zone of protection, lightning will always attach to the lightning rod and be harmlessly conveyed around the protected structure to ground, thereby sparing attachment to the parts of the structure that are subject to burn-through or other damage. There are three methods that determine the zone of protection of a lightning rod: layout, angle and rolling sphere. The requirements of these methods are not met by the structure of a geodesic dome. Therefore, there is nothing on the dome that will protect the panels, leaving them subject to direct attachment and burn-through. However, the requirements may be met by installing lightning rods on the dome in such a manner that all areas of the dome are within the zone of protection of one or more lightning rods. The easiest way to accomplish this is to install lightning rods on the nodes, spaced so that all panels are protected. Conduction of current NFPA 780, 4.9 describes the conductor system intended to convey lightning strike energy from the air terminal to ground. Section 4.9.3 allows certain metallic structural members to be substituted for a wire or cable conductor system. If structural steel is electrically continuous and greater than 3/16 in. thick, it may be used as the conductor system. However, conductors are not subject to the burn-through caused by a direct lightning attachment. They only conduct lightning current, so the thickness requirement is probably a holdover from that for a strike termination device. A thickness requirement of 0.064 in. is specified in NFPA 780, 4.9.3.2 for metal handrails and ladders and in 4.6.5.3 for metal masts, so it follows that the minimum thickness of any substituted metal need only be 0.064 in. NFPA 780, 4.19 addresses structural metallic systems. This section allows the metal framework of a structure to be utilised as the main conductor in a lightning protection system. The framework of a geodesic dome is analogous to the structural steel in an ordinary structure. This section also contains the 3/16 in. thickness requirement, but again, the argument can be supported for allowing the substitution of 0.064 in. thickness. Figure 2. Aftermath of a direct lightning strike. Figure 1. Geodesic dome storage tanks.