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KNOB & TUBE WIRING
LIGHTING, EXTERIOR GUIDE
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LIGHTNING PROTECTION SYSTEMS
LOW VOLTAGE BUILDING WIRING
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This article series describes common lightning protection systems, certification, installation, and lightning protection system inspection. We provide information about lightning strikes, lightning hazards, related equipment, sources of lightning protection system installers, and lightning strike risk assessment
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
A lightning protection system provides a safe pathway for energy from a lightning strike to find its way to earth, encouraging the bolt to bypass building components that might otherwise be damaged. So it's job is both to provide a path for the lightning strike to reach ground (instead of reaching the ground through the building or its components), and to avoid damaging the building as the electrical current of the lightning bolt flows through the protection system components. In other words, a lightning rod and its components do not prevent a lightning strike, rather they give it a safer pathway to earth than would occur if the energy passed through building components on the way down.
The basic components of a lightning protection system include the following
As we mentioned above, 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. This means that the energy from a lightning strike is directed along a heavy metal cable from an air terminal or other protection device down to a rod driven into the earth.
If an easier path is not provided for the energy (or current) to flow to earth, the energy from the lightning strike will find its own, often remarkably odd, pathway to earth anyway, perhaps moving on building water piping, electrical wiring, or even through damp timbers.
When the tremendous energy of a lightning strike moves through these building components instead of through a desired and safest cable designed for that purpose, damage to the building is likely, including explosion, fire, or electrical damage.
But lightning protection systems have a confusing history. The earliest lightning rod design, by Ben Franklin in 1752, ended in a sharp pointed trident which may have actually created an electrical field around the tip which invited strikes.
You'll notice that modern lightning rods have a short blunt tip and that they are connected to earth by heavy metal cables, often using braided copper. Having a properly-designed lightning protection system is very important if you're going to have one at all, since an improperly designed or installed system might actually increase the chances of a building being damaged by a lightning hit.
Lightning strikes at buildings and other sudden electrical currents (such as a tree touching a high tension power line) produce very high voltages which can take surprising routes at a property such as following underground tree roots, metal porch railings, and copper or steel building water pipes.
Lightning protection may be needed for tall trees on a property, either to protect the trees themselves from damage, or to avoid subsequent tree damage to nearby buildings if the tree is stuck. As a child on the Rappahannock river in Virginia I watched storms come up the river as a solid front of rain and lightning. Our house, on a hill overlooking the river, had a metal roof and had been protected with a lightning rod system. But we regularly saw nearby trees take a strike. The energy from the lightning strike appeared to vaporize the sap within the tree, causing it to literally explode outwards.
We've also inspected a home damaged by an indirect electrical strike which originated below an electrical power transmission line at a nearby tree, traveled down the tree under the garage concrete slab, exploding the slab (there was perhaps water below the slab), blowing the car up through the garage roof. The electrical energy from this strike continued, following the roots of the tree across the property, up a metal entry stair railing, and into metal siding on the home and into the home's metal water supply piping system. Aluminum rivets holding corner trim onto the aluminum-sided home melted and the siding corner trim fell to the ground. The water main melted, leaving a burst and leaking water entry line which then flooded the home's basement.
See Lightning Protection for Trees for more details of protecting trees from lightning strikes.
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