Lightning Rods and Soil
Date: Summer 2013
An electrical engineer mentioned that he doesn't put much faith in lightning rods as a protection for buildings here in Central and South Florida. This is because the soil is sandy rather than "real soil" and the lines that run from the roof down to the ground would need to be buried about 20 feet to be effective and they seldom are. Is Florida's sandy soil a problem in this regard and, if so, how would the soil composition effect lightning and lightning rods?
This article from the University of Florida at: http://news.ufl.edu/2002/08/15/lightningrod/
explains the problem and provides some remedies.
Here are some excerpts:
“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 (ground) circuitry, according to the UF study…
The study suggests, among other things, that Florida homeowners would be wise to use surge protectors both at the meter and in the home…
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….”
Yes, there is a point to this. Every soil is different, and what is important is its resistance to electrical current flow. A lightning discharge follows the least resistive path to earth, and a good grounding rod provides a solid, highly conductive path into the earth. That being said, however, soils can vary greatly in their electrical conductivity even within the same soil. Dry, sandy soil is likely to be less conductive than many other types of “real” soil as you stated. If that soil is wet, however, its resistance may become fairly low, especially if there are small amounts of minerals present. To answer the question more definitively, an engineer will actually measure the electrical conductivity of the soil, perhaps under different conditions, and decide on how to best create a solid ground. Driving the grounding rod deeper is one approach. Adding salt and moisture to the soil is another, although it can cause its own problems. Creating a larger area to contact the soil is another by burying a metallic mesh in contact with the rod or by placing conductors in a radial pattern around the rod. How much effort you need to put in to the grounding system depends on the application. Large utility lines need solid grounding that can carry large currents with little resistance. If you are protecting your house, you will not need to go to such extensive measures.
Kyle Bunch, PhD, PE
Thanks for the good question. In order for lightning rods to be effective, the soil must be able to conduct electricity. To facilitate this, one can use a salt water (brine) pit in order to increase conduction. Copper should also be used. A mesh of copper could also be used to increase the surface area and thereby increase the conductivity. Before making any modifications, I would strongly recommend consulting the local building codes.
I hope this helps. Please let me know if you have more questions.
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Update: November 2011