A lightning protection system is a system designed to protect a structure from damage due to lightning strikes by intercepting such strikes and safely passing their extremely high voltage currents to "ground". Most lightning protection systems include a network of lightning rods, metal conductors, and ground electrodes designed to provide a low resistance path to ground for potential strikes.
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Lightning protection systems are used to prevent or lessen damage to structures done by lightning strikes. Lightning protection systems mitigate the fire hazard which lightning strikes pose to structures. A lightning protection system provides a low-impedance path for the lightning current to lessen the heating effect of current flowing through flammable structural materials. If lightning travels through porous and water-saturated materials, these parts of a building may literally explode if their water content is flashed to steam by heat produced from lightning current.
Because of the high energy and current levels associated with lightning (currents can be in excess of 150,000 amps), and the very rapid rise time of a lightning strike, no lightning protection system can guarantee absolute safety from lightning. Lightning current will divide to follow every conductive path to ground, and even the divided current can cause damage. Secondary "side-flashes" can be enough to ignite a fire, blow apart brick, stone, or concrete, or injure occupants within a structure or building. However, the benefits of basic lightning protection systems have been evident for well over a century.[1]
The parts of a lightning protection system are air terminals (lightning rods or strike termination devices), bonding conductors, ground terminals (ground or "earthing" rods, plates, or mesh), and all of the connectors and supports to complete the system. The air terminals are typically arranged at or along the upper points of a roof structure, and are electrically bonded together by bonding conductors (called "down conductors" or "downleads"), which are connected by the most direct route to one or more grounding or earthing terminals.[2] Connections to the earth electrodes must not only have low resistance, but must have low self-inductance.
An example of a structure vulnerable to lightning is a wooden barn. When lightning strikes the barn, the wooden structure and its contents, may be ignited by the heat generated by lightning current conducted through parts of the structure. A basic lightning protection system would provide a conductive path between an air terminal and earth, so that most of the lightning's current will follow the path of the lightning protection system, with substantially less current traveling through flammable materials.
Originally, scientists believed that such a lightning protection system of air terminals and "downleads" directed the current of the lightning down into the earth to be "dissipated". However, high speed photography has clearly demonstrated that lightning is actually composed of both a cloud component and an oppositely charged ground component. During "cloud-to-ground" lightning, these oppositely charged components usually "meet" somewhere in the atmosphere well above the earth to equalize previously unbalanced charges. The heat generated as this electrical current flows through flammable materials is the hazard which lightning protection systems attempt to mitigate by providing a low-resistance path for the lightning circuit. No lightning protection system can be relied upon to "contain" or "control" lightning completely (nor thus far, to prevent lightning strikes), but they do seem to help immensely on most occasions of lightning strikes.
Steel framed structures can bond the structural members to earth to provide lighting protection. A metal flagpole with its foundation in the earth is its own extremely simple lightning protection system. However, the flag(s) flying from the pole during a lightning strike may be completely incinerated.
In overhead transmission lines, a ground conductor may also be the top most wire on pylons, poles, or towers. This ground conductor is intended to protect the power conductors from lightning strikes. These conductors are connected to earth either through the metal structure of a pole or tower, or by additional ground electrodes installed at regular intervals along the line. As a general rule, overhead power lines with voltages below 50 kV do not have a ground conductor, but most lines carrying more than 50 kV do. An over head transmission line may have two overhead ground conductors. The ground conductor cable may also support fibreoptic cables for data transmission ( see OPGW).
The majority of lightning protection systems in use today are of the traditional Franklin design.[2] The fundamental principle used in Franklin-type lightning protections systems is to provide a sufficiently low impedance path for the lightning to travel through to reach ground without damaging the building.[3] This is accomplished by surrounding the building in a kind of Faraday cage. A system of lightning protection conductors and lightning rods are installed on the roof of the building to intercept any lightning before it strikes the building.
Non-traditional systems aim to provide the same or similar protection with fewer components. This category can be further divided into improved lightning rods that claim an increased zone of protection (and are otherwise similar to a Franklin-type system), and systems that claim to eliminate lightning strikes altogether. The first subcategory includes early streamer emission (ESE) systems, radioactive rod systems, and laser induced systems. An example of a system that claims to eliminate lightning strikes is the charge transfer system (CTS).
Some structures are inherently more or less at risk of being struck by lightning. The risk for a structure is a function of the size (area) of a structure, the height, and the number of lightning strikes per year per mi² for the region.[4] For example, a small building will be less likely to be struck than a large one, and a building in an area with a high density of lightning strikes will be more likely to be struck than one in an area with a low density of lightning strikes. The National Fire Protection Agency provides a risk assessment worksheet in their lightning protection standard.[5]