Lightning and Lightning Protection: Measuring the Tolerable Risk
August 14, 2017
When considering lightning protection, architects, engineers, safety professionals and building owners typically rely on the NFPA Risk Assessment methodology to determine the risk of damage due to lightning. The risk assessment guide is found in the “Annex L” section of the NFPA 780 Standard for the Installation of Lightning Protection Systems, and provides both an at a glance, simplified assessment and a detailed calculation guide to achieve a more in-depth analysis. The NFPA risk index compares the expected direct strikes to the structure with the occupancy and contents to give an evaluation of whether lightning protection should be applied, or may be considered optional.
Federal agencies, including the Navy, VHA, GSA and USPS also use the NFPA Risk Assessment to evaluate whether or not lightning protection should be installed for both new construction and renovation projects. The NFPA’s Risk Assessment methodology takes into account lightning’s threat and the following factors:
- The building environment
- Type of construction
- Structure occupancy
- Structure contents
- Lightning stroke consequences
Once the risk has been determined, deciding on the need for protection measures is much easier. While Risk Assessment methodology is a good rule of thumb, sometimes the presence of a single risk factor is enough to render a structure a significant risk worth protecting. Historic buildings, healthcare facilities, 911 and emergency centers, server farms, industrial plants, schools and churches are often considered to be at high risk in terms of susceptibility to lightning losses. Since a single lightning strike can introduce a chain reaction of destruction, the lightning protection system (LPS) design and installation needs to follow guidelines of NFPA 780 to effectively address all aspects of this complex electrical hazard. Lightning fires can be especially destructive when a strike ignites a structural fire in any of these ways:
- Through a direct strike
- In an arc discharge between two conductive objects at different induced potentials
- By a current surge in circuitry or electrical equipment
- By an overflow of substantial electrical current, which in turn causes overheating, melting or vaporizing of metal
- By arcing of lightning current from conductors at high-resistance grounds
- Through lightning puncturing pinholes in CSST gas piping
The design of the LPS must: 1) intercept the lightning flash (provide a preferred strike receptor), 2) conduct the current to earth, 3) dissipate the current into the earth and 4) create equipotential balance to prevent hazardous potential differences between the LPS, structure and its internal systems. When considering all the factors associated with susceptibility, safety, and disruption, the cost of installing lightning protection is often considered minimal as compared to the potential for risk.
Mitigation experts stress that the path to improved community and infrastructure resilience must be “risk-informed and performance-based.” While NFPA codes and standards seek to prevent fires from igniting, “preventing” a force of nature is obviously beyond the role of any code or standard. But in terms of risk reduction and risk management, NFPA 780 is clearly a performance-based mitigation measure for addressing protection for buildings, occupants, contents and operations from lightning fires and losses. The 2017 edition of NFPA has expanded the Lightning Risk Assessment Annex L to include clarifications and revisions that parallel requirements of international lightning protection standards. Risk assessment at the very least, is critical to providing a starting point for mitigation to successfully reduce loss of life and property due to the prevalent lightning threat. If we fail to even consider or assess the threat, how can we effectively begin to lessen the impact of future lightning tragedies?