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- Lightning Needs Analysis (external link
provided by APL ltd)
Lightning is a capricious, random and unpredictable event. Its' physical characteristics include current levels sometimes in excess of 400 kA, temperatures to 50,000 degrees F., and speeds approaching one third the speed of light. Globally, some 2000 on-going thunderstorms cause about 100 lightning strikes to earth each second. USA insurance company information shows one homeowner's damage claim for every 57 lightning strikes. Data about commercial, government, and industrial lightning-caused losses is not available. Annually in the USA lightning causes more than 26,000 fires with damage to property (NLSI estimates) in excess of $5-6 billion.
The phenomenology of lightning strikes to earth, as presently understood, follows an approximate behavior:
1. The downward Leaders from a thundercloud pulse towards earth seeking out active electrical ground targets.
2. Ground-based objects (fences, trees, blades of grass, corners of buildings, people, lightning rods, etc., etc.) emit varying degrees of electric activity during this event. Upward Streamers are launched from some of these objects. A few tens of meters off the ground, a "collection zone" is established according to the intensified local electrical field.
3. Some Leader(s) likely will connect with some Streamer(s). Then, the "switch" is closed and the current flows. We see lightning.
Lightning effects can be direct and/or indirect. Direct effects are from resistive (ohmic) heating, arcing and burning. Indirect effects are more probable. They include capacitive, inductive and magnetic behavior. Lightning "prevention" or "protection" (in an absolute sense) is impossible. A diminution of its consequences, together with incremental safety improvements, can be obtained by the use of a holistic or systematic hazard mitigation approach, described below in generic terms.
Lightning can strike anywhere on earth - event the North and South Poles! In any U.S. geographical location, lightning storms occur as few as five times or as many as 100 times per year (see Fig. 1). -The Northeast United States has the most violent thunderstorms in the country because of the area's extremely high earth resistivity (see Fig. 2). High earth resistivity (the earth's resistance to conduct current) increases the potential of a lightning strike. If struck, structures in these areas will generally sustain more damage when there is no lightning protection system present.
Each year, thousands of homes and other properties are damaged or destroyed by lightning. It accounts for more than a quarter billion dollars in property damage annually in the United States. Lightning is responsible for more deaths and property loss than tornadoes, hurricanes and floods combined, but of these violent forces of nature, lightning is the only one we call economically afford to protect ourselves against.
Some properties have a higher risk of lightning damage. When considering installation of a lightning protection system, you may want to assess this risk. A risk assessment guide for determining lightning loss for all types of structures can be found in Appendix I of the National Fire Protection Association's Lightning Protection Code, NFPA 780. This guide takes into consideration the type of structure, type of construction, structure location, topography, occupancy, contents and lightning frequency. Information may be obtained from tile NFPA, I Batterymarch Park, Quincy, MA, 02269, (800) 344-3555.
Lightning is the visible discharge of static electricity within a cloud, between clouds, or between tile earth and a cloud. Scientists still do not fully understand what causes lightning, but most experts believe that different kinds of ice interact in a cloud. Updrafts in the clouds separate charges so that positive charges moves end up at the top of the cloud while negative flow to the bottom. When the negative charge moves down, a "pilot leader" forms. 'This leader rushes toward the earth in 150-foot discrete steps, ionizing a path in the air. 'The final breakdown generally occurs to a high object the major part of the lightning discharge current is then carried in the return stroke which flows along the ionized path.
A lighting protection system provides a means by which this discharge may enter or leave earth without passing through and damaging non-conducting parts of a structure, such as those made of wood, brick, tile of- concrete. A lightning protection system does not prevent lightning from striking; it provides a means for controlling it and preventing damage by providing a low resistance path for the discharge of lightning energy.
FIG. 3 Lightning protection system for a dwelling: 1) air terminals spaced 20 feet apart along ridges and within two feet of ridge ends; 2) down conductors; 3) minimum of two groundings at least 10-feet deep; 4) roof projections such as weather vanes connected to system; 5) air terminal located within two feet of outside corners of chimney; 6) dormers protected; 7) antenna mast connected to roof conductor:- 8) connect gutters or other grounded metals as required; 9) surge arrester installed at service panel to protect appliances; 10) transient voltage surge suppressors installed in receptacles to which computers and other electronic equipment are connected.
FIG. 4 Lightning protection system commercial/industrial installation 1) air terminals spaced 20 feet apart around the perimeter of the building; 2) down conductors; 3) ground rods at least 10-feet deep; 4) art handling units bonded to system (may be in need of air terminals mounted on unit); 5) air terminals mounted within two feet of outside corner; 6) mid-roof conductor and air terminals at maximum 50-foot spacing; 7) grounded metal bodies bonded into system; 8) surge arresters installed at main electrical panels; 9) transient voltage surge suppressors installed in receptacles to protect computers and other office equipment.
UL's Master Label Program for lightning protection involves periodic factory testing and inspection of system Components, along with field inspection components of completed installations. The program requires that all installers comply with UL’s internationally recognized Standards for lightning protection components and systems. UL,’s field representatives countercheck compliance with these Standards.
As a home or building owner, you should make sure that your installed system complies with the UL requirements. Here’s how:
Make certain that your installer is listed by UL and that a Master Label application is submitted to UL for your installation.When You request a Master Label for your system, your installer will ask you to sign the owner’s statement on the Master Label application form. The fourth (yellow) copy of the application is for your records. This should be done before the installer submits the Master Label application to UL for issuance of the Label. Make sure you receive the Master Label from the installer and place it on the protected structure as requested.
Buildings that are changed structurally or provided with additions can be re-examined under UL’s Reconditioned Lightning Protection Program. Under this program, the entire system must comply with the current UL Standards.
Install a UL Master Label Lightning Protection System that complies with
current nationally recognized codes. Lightning protection systems consist
of air terminals (lightning rods) and associated fittings connected by
heavy cables to grounding equipment, providing a path for lightning current
to travel safely to ground.
Install UL Listed surge arresters at your service and telephone equipment
to prevent surges from entering the home or other buildings oil power
or telephone lines. Surges are diverted to ground, and both wiring and
appliances are protected.
Install UL Listed transient voltage surge suppressors in receptacles
to which computers and other electronic equipment are connected in order
to limit the voltage to 11/2 times the normal (maximum for solid state
Look for the UL Mark. Remember, your lightning protection system may
be installed using UL,'s requirements, but the system is not a Master
Label system unless installed by one of UL's Listed installers and a Master
Label has been sent to you. Always "Look for the Master Label"
on your lightning protection installation.
The above information was brought to you by UL.