Dennis
J. Murphy
Pennsylvania State University Cooperative Extension
Lightning
is the leading cause of farm fires. Plan 6368 illustrates
some appropriate arrangements for lightning protection for
farm buildings. A well-installed and maintained lightning
protection system (see Figure 1) routes lightning along a
known, controlled course between the air and moist earth with
over 90 percent effectiveness in preventing damage. Such systems
can prevent damage to a building or any loss of income related
to this damage.
Lightning
protection systems consist of five parts: 1) air terminals,
2) conductors, 3) ground connections, 4) bonding, and 5) arrestors.
- Air
terminals are rods or tubes of metal that are installed
at every projecting high point of a building, such as root
peaks, chimneys, dormers, ventilators, gables, flagpoles,
towers and water tanks. To be effective they must not be
spaced too widely apart.
- Conductors
connect air terminals with grounds. Conductors are copper
or aluminum cables. Galvanic action will occur between aluminum
and copper; therefore, only one metal should be used for
the system or direct contact between the two should be avoided.
- Grounds
and ground connections provide contact with the earth for
dissipation of the lightning charge (see Figure 2). Usually,
at least two ground connections are needed for any building
-- more with large or complex structures. They should be
apart from building foundations and extend deeply enough
to reach moist subsurface earth no matter how dry the weather.
- Bonding
is the interconnecting of metal parts to prevent sideflash.
- Lightning
arrestors guard against damage that might occur by way of
the electric power lines. Properly designed lightning arrestors
should be placed between the power circuit and ground where
the circuit enters the building. Large trees need protection
from lightning. In addition, trees that are taller than
or within 10 feet of a building need protection to prevent
flashover. Also, lightning may cause a tree to fall on a
building.
Livestock often are killed when they are near a fence or tree
that receives a lightning discharge. Wire fences need to be
grounded. Use galvanized steel posts at 150-foot intervals
along the fence. It also is recommended that long runs of
wire fence be interrupted. Lone trees should be either fenced
off to keep livestock away from them or be protected by a
lightning protection system (see Figure 3).
In an
approved lightning protection system the house, barns, sheds,
silo and all other buildings are protected. All metal tracks,
guys, lines and other metal bodies are bonded to the system
as required. Arrestors are installed where needed. Lone trees
are protected. Metal fences are properly grounded. Electrical
entrance services have Underwriters Laboratory (UL) approved
arrestors.
Local
fire officials should be contacted for conformance with local
codes as may apply. Only reputable contractors should be used.
The recommendation on the plan incorporates, with permission,
the 1983 recommendations of the lightning Protection Institute
(LPI), 46 North Ayer, Harvard, IL 60033. LPI has a lightning
system certification program and certifies journeymen installers,
master installers and professional designer inspectors.
For
more copies of this plan, contact your county Extension agent
or agricultural engineer at your landgrant university. If
you do not know the location of your State university, send
your request to Agricultural Engineer, Extension Service,
U.S. Department of Agriculture, Washington, D.C. 20250. Your
request will be forwarded to the correct university.
Disclaimer
and Reproduction Information: Information in NASD does not
represent NIOSH policy. Information included in NASD appears
by permission of the author and/or copyright holder. More
NASD Review: 04/2002
This
document was published in 1988 as Pennsylvania State University
Fact Sheet Safety
29
,
Pennsylvania Cooperative Extension Service. For more information,
contact Pennsylvania State University College of Agricultural
Sciences, Agricultural Engineering Department, 246 Agricultural
Engineering Building, University Park, PA 16802.
Dennis
J. Murphy, professor, Agricultural Engineering Department,
Cooperative Extension Service, Pennsylvania State University,
University Park, PA 16802.
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