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Lightning protection systems. (2014) Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice. Retrieved from http://articles.extension.org/pages/71216/lightning-protection-systems.
Lightning protection systems are recommended for all barns to reduce the risk of damage sustained from a lightning strike. Thunderstorms involving lightning occur across the United States but are most prevalent in central and eastern states. Lightning is a stream of pure energy, approximately 1/2- to 3/4-inch wide and surrounded by 4 inches of extremely hot air, that is looking for the path of least resistance between the clouds and the ground. The amperage from a lightning flash can be approximately 2,000 times greater than the current in a typical home.
Lightning and Potential Damage
The powerful force of lightning can ignite fires in buildings, damage electrical equipment, and electrocute humans and livestock. Typically, lightning enters a building by striking a metal object on the roof, directly striking the building, hitting a tree or structure (for example, a silo) that causes the strike to jump to a nearby building, or striking a power line or wire fence that provides a path into the structure. You can protect your farm or ranch structures by installing a lightning protection system, which will direct a strike away from your buildings and dissipate the strike in a safe manner.
Lightning Protection System Components
(Source: Penn State Ag Safety & Health)
A lightning protection system consists of the following five parts: air terminals (lightning rods), conductors, ground connections (electrodes), bonding, and lightning arrestors.
Air terminals. Air terminals, or lightning rods, are metal rods or tubes installed at every projecting high point of a building—such as the peak, a dormer, a flagpole, or a water tank—to intercept a lightning bolt. Solid copper rods should be a minimum of 3/8-inch in diameter, and solid aluminum rods should be a minimum of 1/2-inch in diameter. Rods should extend between 10 and 36 inches above the projecting object. Typically, rods are 10 to 24 inches long; extra support or a brace is needed for a rod that is more than 24 inches long. The most effective spacing is 20 feet apart for rods that are less than 24 inches long or 25 feet apart for rods that are between 24 and 36 inches long. Additionally, a rod should be located within 24 inches of the end of any building ridge or projecting object. Strategic placement of rods on a structure ensures that lightning will strike the rods rather than another part of the building.
Conductors. Conductors, which are copper or aluminum cables, provide the connection between the air terminals and the earth to direct the lightning strike deep into the earth where it can safely dissipate. Choose copper or aluminum rather than a combination of the two because galvanic or chemically corrosive action can occur between the two elements. Main conductors connect all of the lightning rods with the down conductors and then connect to the ground connections.
Ground connections. Ground connections, or electrodes, provide contact with the ground to safely dissipate the lightning charge. A minimum of two ground connections should be used for most buildings; additional ones may be needed for larger structures. The type of ground connection may depend on the conductivity of the soil in your area. Ground electrodes should be 1/2-inch diameter, 10-foot long copper-clad steel or solid copper rods driven at least 8 feet into the ground.
Bonding. Bonding involves branch conductors that protect against sideflashes by connecting metal objects (such as ventilation fans, water pipes, and so on) with the grounding system. Common grounding can eliminate lightning sideflashes. Grounding is achieved when all electrical systems, telephone systems, and underground metal piping are connected to the lightning protection system.
Lightning arrestors. Lightning arrestors provide protection against a strike entering your building through the electrical wiring system and thereby causing potential power surges that may result in severe damage to electrical devices. To provide the best possible protection, lightning arrestors should be installed on the building’s exterior where the electrical service enters the building or at the interior service entrance.
Protection of Livestock and Trees
Examine your farm or ranch with a certified installer to determine whether lightning protection should be extended to protect valuable trees; trees located within 10 feet of a structure, such as a silo; or trees used for shade by livestock. If livestock stand under a tree, they can be killed by a direct lightning strike to the tree or from contact with resultant charged soil. To avoid this scenario, consider removing trees favored by livestock, fencing livestock away from trees, or providing protection with a conductor system.
Lightning protection for a tree involves placing air terminals at the tips of the main trunk and attaching a full-size grounding cable to a ground rod. The ground rod should be located away from the tree’s root system. Air terminals with smaller cables can be attached to main branches. If the tree is 3 feet in diameter or larger, use two ground rods attached to the main conductor system.
Protection of Fencing
Lightning can travel up to 2 miles along an ungrounded wire fence, posing a threat to humans and livestock. Fences may be attached to wooden posts, steel posts set in concrete or to buildings, and even trees (not recommended). In all circumstances, the fence should be grounded to safely route the lightning’s voltage into the earth. To ground a fence, drive 1/2-inch steel rods or 3/4-inch pipe 5 to 10 feet into the ground next to wooden fence posts at intervals of 150 feet. Allow a few inches of the ground rod or pipe to extend past the top of the adjacent fence post. Attach the rod or pipe to the fence post with pipe straps to ensure a tight connection.
System Installation and Maintenance
A certified installer should install your lightning protection system to reduce the risk of a system failure and to ensure that your system meets necessary codes and standards. The Lightning Protection Institute certifies systems meeting all its requirements. To maintain a system’s certification, regular maintenance and annual inspection must be completed. Damage due to high winds, building additions, and roof repairs or upgrades can alter a system’s performance. To locate a certified installer in your area, click one of the resource links below:
Lightning Protection Institute
Resources
Click here for more information about structural lightning safety from the National Lightning Safety Institute.
Click the link below for more detailed information about the related topic.
Use the following format to cite this article:
Lightning protection systems. (2014) Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice. Retrieved from http://articles.extension.org/pages/71216/lightning-protection-systems.
Sources
Chamberlain, D. and Hallman, E. (1995) Lightning protection for farms. Cornell Cooperative Extension. Retrieved from http://ecommons.library.cornell.edu/bitstream/1813/5168/2/LIGHTNING%20PROTECTION%20FOR%20FARMS.pdf.
Linn, R. (1993) Lightning protection for the farm. Montguide. Montana State University. No longer available online.
Murphy, D. (1988) Lightning protection for the farm. The Pennsylvania State University. Retrieved from http://nasdonline.org/1168/d001010/lightning-protection-for-the-farm.html.
Specifications for lightning protection – ASAE engineering practice. (1998) The Disaster Handbook 1998 National Edition. University of Florida Institute of Food and Agricultural Services. No longer available online.