Cornell Agricultural Safety and Health Program
Lightning, one of nature's most
powerful forces, can cause a great deal
of damage, particularly in a farm
environment. A lightning strike can start
fires in buildings, damage electrical
equipment, and electrocute humans and
livestock. Losses from lightning can be
very costly. Replacing buildings,
equipment, or livestock disrupts farm
operations and incurs considerable
expense, and of course a human life
cannot be replaced. Fortunately, most
losses caused by lightning strikes can be
prevented by installing proper lightning
Characteristics of Lightning
Positive and negative electrical charges exist throughout nature. Under normal conditions, these charges are paired up and neutral, exhibiting no net electrical charge. The charges, however, have the ability to move about and separate. Strong air currents, moist air, and extreme temperature differences can all disrupt the natural balance of these charges.
Lightning occurs when the imbalance between charges becomes too great. Certain weather conditions can cause an abundance of negative charges to gather on the bottom of clouds, while positive charges accumulate on buildings, trees, or any objects that project above the ground. When the negative and positive charges build to high enough levels, a streamer of negative charges moves erratically toward the earth. At the same time, a short leader of positive charges may move a short distance up into the air. When the two charges meet, the downward moving streamer completes the grounding path as the positive charges instantaneously move back up the path to the clouds (see fig. 1). The resulting flash is lightning. A lightning "stroke" happens very quickly and contains a great deal of electrical energy.
Lone trees and isolated buildings, which are closer to the clouds than their surroundings, tend to concentrate positive charges. Consequently, they are frequently the objects of lightning strikes. High objects actually intercept lightning strikes from other nearby objects. That is why it Is particularly important to take precautionary measures to protect farm buildings from the damage lightning strikes can produce.
Principles of Building Protection
Lightning can enter a building in one of four ways:
- It can strike a metal object on the roof.
- It can strike a building directly (called a direct strike).
- It can strike a tree or silo near the building and jump to the building. This occurs when the building provides an easier path to ground.
- It can strike a power line or a wire fence and follow the line or fence to the building.
Depending on their location, some silos should be tied into a building's lightning protection system. It is also possible to extend protection to trees situated near farm structures or that offer cover for livestock. Detailed specifications for these systems can be found in the codes and standards cited later in this fact sheet.
Installation Codes and Standards
Installing lightning protection systems is not a do-it-yourself job. To ensure that a lightning protection system is safe and effective, it should be designed and installed by trained professionals.
Certain codes and standards must be followed when lightning protection systems are installed. Standards and sources are listed below:
LPI-175: The lightning protection code, published by the Lightning Protection Institute.
NFPA 78: National Fire Protection Association Lightning Protection Code.
ASAE EP381: American Society of Agricultural Engineers, Engineering Practice.
96AUL: Requirements for Master Label for Lightning Protection, developed by Underwriters' Laboratories.
The Lightning Protection Institute will certify a lightning protection system that meets all its requirements. To retain certification, the system must undergo regular maintenance and be inspected annually. Maintenance of any lightning protection system is vital to make sure the system will work when it is needed. Weather conditions, such as high winds, can damage components of a lightning protection system. Building additions and re-roofing can also affect a system's performance.
Components of the System
The major components of a building's lightning protection system are air terminals, conductors, and ground electrodes (see fig. 2).
Air terminals are more commonly known as lightning rods. They are placed at intervals on the roof and on any high points projecting from the roof. Lightning rods are made of solid copper or aluminum and are drawn to a point. Their design and placement ensure that lightning will strike them and not another part of a building. Specifications for lightning rods vary depending on a roof's type and size. Recommended height, anchoring methods, and spacing intervals can be found in the codes and standards previously listed.
Conductors are specially designed cables made from copper or aluminum that provide a low-resistance path to the ground for lightning's electrical charges. Conductors can be classified into three categories:
- Main conductors interconnect all the lightning rods and down conductors.
- Down conductors connect the main conductors to the ground. Each building needs at least two down conductors located at opposite corners of the building. Codes should be consulted to determine the number and location of down conductors for different building types.
- Branch conductors connect metal objects such as vent fans, gutters, and water pipes to the grounding system to protect against possible sideflashes.
The best method for eliminating lightning induced sideflashes between metal bodies is common grounding. This means that the grounds for all the electrical systems, the telephone service, and underground metal piping are connected to the lightning protection system. Plastic piping, which has become common in recent years, does not conduct lightning's electrical charges and requires special grounding.
When lightning strikes a power line, it can travel along the line and enter a building's wiring system, causing a power surge that can damage wiring and electrical equipment. To prevent this from happening, lightning arresters should be installed outside, where the electric service enters a building, or at the inside service entrance. The arrester supplies a ground so that a power surge will not enter the building. If a farm has several buildings with separate electric service entrances, a grounded lightning arrester should be installed in each building.
Wire Fence Grounding
Ungrounded wire fences can be very hazardous to livestock and humans who are in the vicinity of the fence when lightning strikes it. Lightning strikes can travel almost two miles along an ungrounded fence. Wire fences supported by wooden or steel posts set in concrete are not grounded. The best way to ground these fences is to drive 1/2- or 3/4- inch steel rods or pipes next to the fence posts at least 5 feet into the ground, at intervals of no more than 150 feet along the fence (see fig. 3). The grounding rod should be securely fastened so that all the fence wires are in contact with the rod. Substituting galvanized steel fence posts for wooden posts at intervals of not more than 150 feet is also effective.
Electric fences should not be grounded in the manner described above because they already include a path to ground in their circuitry.
Several precautions should be taken for protection from a lightning strike during a storm:
- Stay away from indoor water faucets, telephones, appliances, and lamps. These objects are all connected to outdoor conductors.
- Stay clear of chimneys, fireplaces, and stovepipes. Lightning will often strike chimneys, which then become a lightning stroke path.
- Don't get out of a closed vehicle until the storm passes.
- When no shelter is available, seek a low spot away from lone trees or fences and lie down.
Lightning can be a very destructive force, but steps can be taken to protect livestock, property, and human lives. Hire professionally trained personnel to design and install effective lightning protection systems on vulnerable buildings. Install lightning arresters at all electric service entrances to buildings to protect interior wiring and electrical equipment from a power surge caused by lightning. Ground wire fences to prevent hazards to livestock and humans. A small investment now can protect family members, farm workers, property, and equipment from lightning devastation.
OTHER RURAL HEALTH AND SAFETY FACT SHEETS INCLUDE:
Power Take-Off Safety
Electrical Safety on the Farm
Slow Moving Vehicle Emblems
Safer Farm Environments for Children
Safe Animal Handling
This publication is issued to further Cooperative Extension work mandated by acts of Congress of May 8 and June 30, 1914. It was produced with the cooperation of the U.S. Department of Agriculture; Cornell Cooperative Extension; the New York State College of Agriculture and Life Sciences, New York State College of Human Ecology, and New York State College of Veterinary Medicine, at Cornell University.
Designer: Dennis E Kulis
Editor: David A. Poland
Illustrations by Jim Houghton
For additional information: call 1-877-257-9777
Cornell Agricultural Health & Safety Program
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
Reviewed for NASD: 07/2005