I was involved recently in a discussion on lightning protection for various types of buildings and what standards applied to those installations or if the installations required compliance with NFPA 780, Standard for the Installation of Lightning Protection Systems; UL 96A, Standard for Installation Requirements for Lightning Protection Systems; or IEEE-142, Standard for Static and Lightning Protection Grounding. The answer was yes; all three are often necessary for any installation.
IEEE-142 2007 provides background information on lightning protection and references NFPA 780 for the application and installation requirements as applied to buildings and other structures, such as tank farms, smokestacks, steeples, television antennas, FM radio antennas, cell towers, and other tall structures. UL 96A and NFPA 780 cover installation requirements for lightning protection systems. The National Electrical Code (NEC) covers bonding and grounding issues of the electrical system as they relate to the lightning protection installation. A basic understanding of lightning is provided by IEEE-142, as well as other sources, and is necessary before accessing NFPA 780, UL 96A, or the NEC relating to a lightning protection system.
According to Wikipedia, “Lightning is a massive electrostatic discharge between electrically charged regions within clouds, or between a cloud and the Earth’s surface. The charged regions within the atmosphere temporarily equalize themselves through a lightning flash, commonly referred to as a ‘strike’ if it hits an object on the ground. There are three primary types; from a cloud to itself (intra-cloud or IC); from one cloud to another cloud (CC) and finally between a cloud and the ground (CG).”
The discharge that most concerns us is the CG flash. Charged cells in clouds attract charges of opposite polarity from the earth’s surface or from objects located directly below the clouds.
When the opposite polarity between the cloud and the earth or object reaches a critical level, the natural insulation of the air “breaks down” in an ionized path that results in a high current discharge, temporarily neutralizing the difference of potential between the cloud and the earth or object at ground level.
According to “The Encyclopedia of World Climatology,” Lightning occurs approximately 40 to 50 times per second worldwide, resulting in nearly 1.4 billion flashes per year. Many factors affect the frequency, distribution, strength and physical properties of a “typical” lightning flash. These factors include ground elevation, latitude, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, and more.
According to NASA, about 70 percent of lightning occurs over land in the tropics where atmospheric convection is the greatest. This occurs from both the mixture of warmer and colder air masses, as well as differences in moisture concentrations, and it generally happens at the boundaries between them. The ratio between IC, CC and CG lightning may also vary by season in middle latitudes. Warm ocean currents flowing past drier land masses, such as the Gulf Stream, partially explain why the Southeast United States, although located outside of the tropics, experiences a higher-than-expected lightning frequency.
Fun facts: the science of lightning is called “fulminology,” and the fear of lightning is called “astraphobia.”
As stated in IEEE-142, “The discharge current increases from zero to a maximum current in 1 microseconds to 10 microseconds and then declines to half the peak current in 20 to 1,000 microseconds. Lightning flashes usually consist of a sequence of individual return strokes that transfer significant electrical charge usually from cloud to earth. The original, rather slowly developing stroke is followed, on the average, by two to three subsequent strokes, but may contain as few as one to as many as 20 strokes. Each stroke may have peak currents from 2,000 amps to 300,000A and has a nominal duration of 20 microseconds to 50 microseconds. The number of strokes in a flash is typically referred to as the multiplicity of strokes. Subsequent strokes, after the initial stroke, develop much more rapidly than the original stroke, with rates of rise from two to 10 times that of the original stroke and are not linear, but exponential with the rate of rise increasing rapidly in the last few tenths of a microsecond of rise time.” IEEE-142 contains much more background information on the phenomena of lightning and should be consulted as one of the authoritative manuscripts on lightning. NFPA 780 and UL 96A contain the installation requirements for protection systems.
The general attitude behind lightning protection is that lightning cannot be prevented; however, it can be intercepted or diverted to minimize or avoid damage to buildings, structures and electrical systems by properly designed and constructed systems. Close examination of the lightning protection standards referenced in the previous paragraphs will help ensure that lightning protection systems can be effectively installed.