In Southeast, Ground Rods May Not Protect Homes Against Lightning

Published: August 15th, 2002

Category: Research

GAINESVILLE, Fla. — Ever since Ben Franklin came up with the idea more than 200 years ago, lightning rods have helped save millions of homes from damaging or deadly strikes.

But this traditional and inexpensive protection may not be adequate to safeguard homes throughout much of Florida and parts of the Southeast, according to new University of Florida research.

Lightning-rod systems are designed to route electricity from an intercepted strike harmlessly into the soil via wires connected to vertically driven ground rods. But in Florida, Georgia, Alabama and other Southeastern states, where sandy soils tend to remain fairly dry beneath the surface and do not conduct electricity well, the rods may actually wind up passing most of the current into the house circuitry, according to the UF study.

“Vertical ground rods in sandy soils do not conduct as much current as they are expected to,” said Vladimir Rakov, a professor of electrical and computer engineering and an expert on lightning. “If homes don’t have surge protectors or other additional protection, there is likely to be some damage.”

The UF study, published in April in the journal Institute for Electrical and Electronics Engineers Transactions on Power Delivery, is the first to use experiments involving real lighting to reveal how grounding under sandy soil conditions influences lightning strikes to homes.

As Franklin envisioned, lightning rods are designed to intercept lightning that might strike more vulnerable parts of a house, then conduct its current safely to the ground. It is the second part of this equation that doesn’t quite work out in sandy soil, the UF research found.

The engineers built a small, simulated home with a typical lightning rod system on the grounds of UF’s lightning laboratory, the International Center for Lightning Research and Testing at Camp Blanding in Clay County. Such a system usually has a copper rod extended

above the peak of a roof. “Down leads” connect the rod to vertically driven 8-foot ground rods.

For the experiment, the researchers fired wire-trailing rockets up to 2,000 feet high toward overhead storm clouds, triggering several lightning strikes. They routed the electricity from each of the strikes directly into the home’s lightning rod. The researchers then measured the electricity entering the ground versus the home.

Rakov said the international standard for lightning protection calls for ground rods to allow no more than half of a strike’s current into a home’s electrical system. In the experiments, however, more than 80 percent of the current flowed into the electrical system.

In a real home, the amount of damage caused by such a strike would depend on its severity and whether there were other protective devices present, Rakov said.

A major strike could melt insulation of electrical wires and do other damage, plus destroy any electronics or appliances plugged in at the time. However, a surge protector installed at the electric meter could reduce or impede damage, as could individual surge protectors at electrical outlets.

In two of the three strikes described in the paper, utility-meter surge protectors successfully prevented the current from making any damage in the home, Rakov said. In the third strike, when the meter was unprotected, it was destroyed. The strike also produced an electrical arc at the meter that could have caused a fire if flammable materials had been present, he said.

The study suggests, among other things, that Florida homeowners would be wise to use surge protectors both at the meter and in the home, Rakov said. Statewide, annual losses due to lightning damage are estimated at $27 million to $41 million, he said.

The results also suggest that residents in areas with sandy soil may want to consider installing lightning-protection systems more extensive than those employing vertical ground rods. Rather than routing the electricity into vertical rods, these systems direct it into a large buried wire ring or loop that circles the house. This loop has more surface area, making it better at dissipating electricity. Ring systems typically cost 10 to 20 percent more than vertical ground systems, which average a few hundred dollars for a normal home, Rakov said.

Rakov said researchers hope to test other soil types in future experiments as well as chemicals used to make sandy soils more conductive. They also hope to learn more about the effectiveness of ground rings compared with vertical ground rods.

Credits

Writer
Aaron Hoover, ahoover@ufl.edu
Source
Vladimir Rakov, rakov@ece.ufl.edu, 352-392-4242

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