Lightning Protection for the Farm
U.S. Department of Agriculture, 1959
by Harry L. Garver, agricultural engineer, Agricultural Engineering Research Division, Agricultural Research Service
Lightning is a particularly dangerous threat on the farm.
Every year over 400 persons are killed and over 1,000 are injured by lightning in the United States. Nearly all of these fatalities and injuries occur in rural districts.
Lightning is a major cause of farm fires. In Iowa from 1930 to 1947, fires started by lightning caused an average annual property loss of over $160,000.
Livestock and trees are also major victims of lightning damage.
Lightning protection can save the farmer money in two ways: By preventing loss of life and property, and by reducing the cost of his fire insurance.
THE NATURE OF LIGHTNING
Lightning is electricity that has both high amperage (rate of flow) and high voltage (pressure).
High voltage enables lightning to travel great distances through the air. High amperage is the main reason for lightning’s destructive power.
Experiments have proved that a charge of electricity must have a thousand times the voltage of household current to travel, or jump, just 1 foot through the air. Lightning, therefore, which usually travels over 2,000 feet between cloud and earth, must have extremely high voltage. But high voltage without large amperage is relatively harmless. The amperage of lightning discharges between clouds and the earth sometimes reaches 200,000 amperes or more.
Lightning follows the line of least resistance. The air through which lightning must pass between clouds and the earth is an insulating material of high resistance. Materials used in building construction have less electrical resistance than air. When such materials lie between the clouds and the earth, lightning naturally goes along the line of low resistance that they provide.
Lightning-protection systems for buildings give lightning ready-made lines of low resistance. They do this by providing unbroken bodies of material that have lower resistance than any other in the immediate neighborhood. A protection system routes lightning along a known, controlled course between the air and the moist earth. Well-installed and maintained, a lightning-protection system will route lightning with over 90-percent effectiveness.
PROTECTION FOR BUILDINGS
Lightning-protection systems for buildings consist of three parts—air terminals (“rods” or “points”), conductors, and ground connections (fig. 1).
All materials used in lightning-protection systems should comply with the specifications of the Code for Protection Against Lightning published by the National Bureau of Standards.
Air terminals are rods or tubes of metal (fig. 2) that are installed at every projecting high point of a building, such as roof peaks, chimneys, dormers, ventilators, gables, flagpoles, towers, and water tanks.
Terminals are manufactured in various lengths, but usually measure between 10 and 24 inches from tip to base. When terminals are made of copper tubing, they should be at least 5/8 inch in diameter and have a wall thickness over 0.032 inch.
Space the terminals along roof ridges, railings, and parapets according to the length of the terminals. (Recommended spacing also applies to metal-covered roofs.) If they are less than 24 inches long, space them less than 20 feet apart. If they are between 24 and 60 inches long, space them no more than 25 feet apart. There should be a terminal within 2 feet of each gable end of any roof.
Short terminals — usually 10 inches long — are satisfactory for projecting parts of a building. They are usually clamped directly to the conductor cable. Longer terminals have bases that are attached to the building, and the conductors are clamped to the bases. Air terminals over 18 inches long are usually supported by metal tripods (fig. 3). Wind, snow, and ice seldom damage well-erected air terminals. Do not mount ornaments and weather vanes on air terminals. They may weaken terminal mountings.
Air terminals on chimneys must be coated with lead to prevent corrosion from smoke fumes. They should project 10 to 24 inches above the top of the chimney (fig. 4).
Silos and towers with peaked tops need 1 or more air terminals; those with flat tops need 2 or more.
Conductors are the parts of lightning-protection systems that connect air terminals with grounds. They are made of any good electricity-conducting material that will stand exposure to weather. Aluminum or copper is now used in most installations, instead of the once-common galvanized steel. Aluminum and galvanized steel corrode and lose strength in salt air. Do not use these materials for conductors where salt air is common.
The conducting capacity of a conductor depends on its weight. The minimum acceptable weight, per thousand feet, is: Copper, 187.5 pounds; aluminum, 95 pounds; and galvanized steel, 320 pounds.