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When lightning strikes:

Factors to consider for metal construction and lightning protection

Kim Loehr, Posted 09/01/2009

The probabilities of lightning striking a metal-roofed structure are no more or less than lightning striking any other kind of structure. Yet when a metal roof is called for, questions often arise regarding risks associated with lightning. A metal roof does not attract lightning, nor does it protect a structure against lightning. Assessing the need for lightning protection typically involves comparing the expected lightning frequency with the tolerable lightning frequency risk-which means weighing the lightning probability against consequence to life, environment and momentary loss.

The issue of risk assessment in terms of lightning protection was addressed by the General Accountability Office in May 2005, when it released the results of a five-month evaluation of the federal government's approach to protecting facilities from lightning strikes. The GAO found that when government agencies call for lightning protection, they require installation to conform to the National Fire Protection Association Standard 780 and Underwriters Laboratories Standard UL96.

The NFPA 780 includes a Risk Assessment Guide, which is typically used by federal agencies, such as the Navy, the Verterans Health Administration, the General Services Administration and the U.S. Postal Service, to evaluate whether or not lightning protection should be installed for new construction and/or renovation projects.

 

What is the Risk?

Lightning is one of the most underrated severe weather hazards, yet the statistics for lightning damage are staggering. According to UL, lightning accounts for more than one billion dollars annually in structural damage to buildings in the United States. What's not reported is the loss of business, downtime and liability when business or commercial tenants are forced to shut down to repair lightning damage. Facility owner's who have witnessed lightning's destructive power understand the need to protect their personnel, structure and contents from lightning. Unlike threats posed by other forces of nature, such as tornadoes, hurricanes or floods, lightning's incredible power can be controlled on a specified path.

According to the National Weather Service, there are three main ways that lightning enters buildings: a direct strike, through wires or pipes that extend outside the structure, and through the ground. Regardless of the method of entrance, once inside the structure, the lightning can travel through the electrical, communication, or data wiring, along with plumbing, gas or process piping systems. Lightning can also travel through structural steel framing and reinforcing rods in concrete walls or flooring. On the outside of the structure, lightning can travel along the outer shell and may follow conductive metal vents, roof drainage elements and external supports as it seeks a path to ground. Packing up to 100 million volts of electricity, a lightning strike to an unprotected structure can be disastrous. The good news, again, is that lightning losses can be prevented.

How the System Protects the Structure

Lightning is electricity. When electricity is confined to a properly designed conductive path, damage can be minimized. Destruction results when electricity encounters resistance, similar to the resistance used in arc welding. When electrical current runs through an arc welder, the resistance it encounters when arcing through air generates the heat necessary to melt steel. Metallic roofing and siding systems are designed to fulfill the mechanical function of weather exclusion from the building but are not designed for electrical continuity. The anchoring sections of roofing and siding systems are not constructed to carry current because, in most situations, the thickness of the metal used is insufficient to provide a guaranteed path for lightning. The highly conductive copper and aluminum materials used in a lightning protection system provide a low resistance path for lightning to travel without resistance. When the lightning protection network is in place, a lightning strike is intercepted and directed to ground without impact to a structure or its contents.

 

Without the presence of the low resistance path provided by a lightning protection system (network), the lightning will fight its way through non-conductive building materials like wood, brick, rubber membranes, glass, plastic, etc., on its way to earth ground. The resistance the lightning encounters will produce heat, fires and even explosions. It is also common for lightning to travel via conductive matter it finds along the way, including fl ashing, structural members, plumbing and/or wiring for power, communication or data. None of these materials are designed to provide a safe path to ground for lightning. Providing this safe path to ground is the objective of a lightning protection system design.

Roof and Ground Protection Network

While the concept behind lightning protection is relatively simple, the requirements for installation are specific and often complex. The single best way to ensure proper system design and installation is to specify compliance with nationally recognized safety standards for lightning protection. The LPI-175 Standard of Practice (based on NFPA 780 and UL 96 requirements) is published by the Lightning Protection Institute, Maryville, Mo., as a resource to help ensure quality control for lightning protection installation and materials. A complete lightning protection system includes the following elements:

• A network of prominent strike termination devices.
• A network of ground terminations (ground electrodes).
• A network of conductors or qualifi ed structural steel members interconnecting the strike and ground terminations.
• Interconnections with other metallic grounded building systems.
• Surge protection devices on all incoming power, data and communication lines.

A properly installed lightning protection system will intercept, conduct and dissipate the lightning discharge. Bonding and interconnections with metallic grounded building systems will address the secondary effects of a strike by limiting the dangers of the harmful current caused by side flashing. Surge protection provided with the lightning protection system will protect connected equipment and wiring from damaging currents and surges that can travel on utility lines. The lightning protection safety standards also permit the use of structural steel framework as down conductors for the lightning protection system, provided the framework is electrically continuous. Provisions for connecting the roof network and grounding system to the structural steel are outlined in the safety standards. Failure to make proper provisions for any of the elements involved in installation can result in inadequate protection.

 

Maintenance of the lightning protection system in accordance with industry recommendations is an important aspect of quality control. In the event a facility is changed structurally (additions, reroofing, etc.) or if building modifications are made, the lightning protection system must be repaired or updated to ensure continued compliance with installation safety standards. The NFPA Safety Standard provides guidelines for inspection and maintenance procedures, which should be established for the lightning protection system and incorporated as part of the overall maintenance program for the facility it protects. While maintenance is typically uncomplicated and inexpensive, it is an important service to ensure that mechanical damage, modifications to the structure or age, do not degrade a system. System defects or disconnections can pose safety problems that should always be addressed.

Installation requirements are specific and often complex-even for ordinary structures and roof types. Requirements for special occupancies, heavy-duty stacks and structures containing flammable vapors, gases or liquids can be especially stringent, as special considerations are dictated by NFPA 780. In addition, the safety standards mandate that metal rooftop equipment, such as ventilators, skylight frames, air-conditioning units and railings be incorporated into the lightning protection system. Connections for these objects depend on their construction, location and skin thickness. A bonding connection might be sufficient to ensure lightning conductivity, or the object may require conductors and strike termination devices, as well as the bonding connection. The experienced and certified lightning protection specialist will know how to interpret the safety standards to meet all requirements with the completed installation, regardless of roof type or composition.

Written and contributed by Kim Loehr on behalf of the Lightning Protection Institute, Maryville, Mo. LPI is a nationwide non-profit organization founded in 1955 to promote lightning protection education, awareness and safety. LPI conducts certification and educational programs and is a leading resource for lightning protection information and system requirements. Visit www.lightning.org.

www.lightning.org

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