You can't stop lightning from striking a building, but many facilities employ lightning protection systems meant to conduct the electrical discharge from a strike through the building to the ground along specific conductive pathways, minimizing damage to the building's structural and electrical systems.
Although safety systems in modern nuclear weapons employ a series of electrical and mechanical switches that would prevent a stray electrical discharge from detonating a weapon or its high explosives, researchers wanted hard data about the electromagnetic field that might pass through a concrete structure struck by lightning, as well as about how electric current travels through a structure's structural elements. Such data, it was hoped, would lead to improvements in the way such facilities are constructed in the future.
"Little data were available regarding the electromagnetic effects of a direct lightning strike to a concrete structure," says Marvin Morris, Manager of Electromagnetic Test and Analysis Dept. 9753. "We knew weapons magazines had been struck before, and we were confident in the safety of the weapons, but we wanted a set of verifiable measurements based on real lightning strikes to show us how to make improvements."
Normally air serves as an insulator, preventing an electrical discharge between the ground and an electrified cloud until the charge differential between the two becomes so great that a conductive "bridge" of ionized air particles is formed, initiating a strike.
In a triggered lightning test, a small rocket is launched toward a thundercloud above the object being tested. The rocket unspools a fine trailing wire attached at one end to a grounded structure, providing a ready conductor. The process not only triggers a flash in most cases, it also aims the strike to a point where its effects can be evaluated. It's really just a modification of Ben Franklin's historic kite and key test, Marvin says.
Sandia has conducted rocket-triggered lightning tests as part of its nuclear weapons safety mission since the early 1980s.
Sensors at various locations on the tested structures helped researchers measure currents in concrete rebar elements and in the lightning protection systems, voltage differences among structural elements, and inductance levels inside the structures.
"What we found out was that the lightning protection system played a limited role in directing current from a lightning strike," he says. "Current traveled through the rebar, through concrete, through pipes, through cables, through vent stacks, and through the electrical systems, but in few cases did the lightning protection systems significantly affect the electromagnetic field inside the building."
The tests did show, however, that the degree to which the steel rebar rods in the concrete structure were connected played a far greater role in controlling electromagnetic effects inside the structure than did the lightning protection systems.
"When rebar and other conductors in a building form what is essentially a Faraday cage around the building, current from a strike tends to flow around the structure's shell and into the ground through pipes and other large metal conductors," he says. (A Faraday cage is a shell made of conductive mesh or parallel wires that routes an electrical current around the outside of the shell.)
As a result of the tests, Sandia and the Army developed a system of rack-mounted measurement equipment and associated testing techniques. That equipment and a portable suitcase-sized version now under joint development will help the Defense Special Weapons Agency and the Army evaluate rebar integrity at thousands of weapons storage magazines worldwide and possibly abandon intensive, expensive maintenance procedures required to keep their lightning protection systems operational.
The tests also led to safety improvements for nuclear weapons assembly and disassembly cells and bays at DOE's Pantex plant near Amarillo, Texas. Subsequent Sandia tests helped verify the rebar integrity of Pantex's critical facilities, and improved surge protectors were installed in the buildings' electrical systems to minimize voltage spikes.
As a further outcome of the Sandia research, DOE, the DoD, and the Defense Special Weapons Agency are looking into updating their construction standards for new weapon storage magazines to include guidance on rebar construction.
Marvin suspects that the standards for other types of facilities - including those that house vital information or electronic systems (such as financial institutions and supercomputer facilities) and those at which a lightning strike might cause a serious accident or jeopardize people's safety (such as petrochemical plants and oil refineries) - might also be updated sometime in the future.
Because of a lack of technical information supporting the effectiveness of lightning protection systems, the NFPA subcommittee that writes national construction standards for lightning protection recently downgraded its lightning-related standards, making lightning protection systems optional rather than required. Marvin expects that the NFPA will write new codes incorporating the Sandia findings.
Sandia-led triggered lightning research continues. The University of Florida is conducting another series of lightning studies under contract with Sandia at its lightning research center in Gainesville, primarily to better understand how different grounding techniques can control the ground arcing that results from a lightning strike.
"You can't stop ground arcing, but we're showing that you can get arcs to occur where you want them to," he says.
The ongoing tests, it is hoped, will lead to further lightning safety improvements inside and outside the federal government.
"We sort of knew traditional lightning protection systems couldn't work based on back-of-the-envelope calculations," adds Marvin. "But there's nothing more convincing than measured data."
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Last Modified: April 28, 1997