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Lightning protection system
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.
Description
Lightning protection systems are used to lessen damage to structures by lightning strikes.
Lightning protection systems mitigate the fire hazard which lightning poses 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. Porous, water-saturated
materials may literally explode if their water content is flashed to steam by heat produced from
lightning current.
To appreciate the limitations of lightning protection systems, it is important to understand the
magnitude of lightning's energy. Because of the high electrical potential of lightning (often
exceeding 100 million volts and 40,000 amperes), and the very rapid rise time of a lighting
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.

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.
[3]

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.
[4]
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.
Although some are sold and installed, no such system has demonstrated this ability. 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).
Traditional System
The traditional Franklin type lightning protection system has 3 main parts.

The roof circuit

Interconnection to grounding electrodes

Grounding electrodes
[edit] Roof circuit

If one imagines the center of the sphere moving at a constant rate as it is rolled over the building,
the parts of the building that are in contact with the sphere the longest are those that are most
likely to be struck by lightning.
[4]
A typical example of a part of a building that would be a
higher risk would be the edge of the roof as the sphere rests on this longer than any flat portion
of the roof. This is a simple explanation of why more air lightning rods are required on peaks or
the edges of flat roofs than are required on the middle of flat roofs.
Additional precautions must be taken to prevent side-flashes between the lightning protection
system and conductive objects on or in a structure. The surge of current through a lightning
protection conductor creates a difference in electric potential or "voltage" between it and any
conductive objects near it. This difference can be large enough to allow current to jump to and
flow through such objects, possibly causing significant damage, especially to structures housing
flammable or explosive materials. The most effective way to prevent such damage is to establish
and maintain low resistance electrical continuity between the lightning protection system and any
objects susceptible to a side-flash, effectively reducing the difference in voltage to near zero.
Such "bonding" allows the voltage on a lightning protection conductor and any nearby
conductive objects to rise and fall in tandem, thereby eliminating any risk of a side-flash.
[9]

[edit] Interconnection to grounding electrodes
The roof circuit must be electrically continuous with some method of grounding. This is usually
accomplished in one of two ways. The first way is to use copper or aluminum cables to connect
the roof circuit to the grounding electrodes. Virtually all structures require two or more
connections from the roof to the ground.
[10]
If a structure's perimeter is greater than 76 m, the
structure will require a connection for every 20m of perimeter or fraction thereof.
[11]
For

Early streamer emission (ESE) lightning conductors were invented in the 1980s. They aim to
provide a much wider lightning protection area than traditional Franklin lightning rods. In the
practice that ESE devices of all types fail to meet their technical and performance claims and are
not better than Franklin lightning rod systems.
[edit] Radioactive rod systems
Radioactive rod systems' main differentiating feature from traditional Franklin type systems is
the use of radioactive materials in the lightning rods. The theory behind this is that the
radioactive properties of the rods can ionize the air around the rod sufficiently to increase the
likelihood of that rod being struck, rather than the building itself. This, in effect, would increase
the zone of protection provided by the lightning rod. Unfortunately, the efficacy of radioactive
lightning has been shown to be less than that of regular lightning rods, as the radioactive
materials are only able to ionize air around the lightning rod for a short distance that doesn't
positively affect the chance of being struck.
[15]

[edit] Laser induced systems
Laser induction of lightning strikes is currently being researched by scientists.
[16]
The main
principle of these systems is that a sufficiently powerful and properly tuned laser can ionize air
from the clouds to the ground level. This ionization reduces the breakdown voltage of the air and
provides a lower resistance path for the lightning to travel on. This acts like a lightning rod tall
enough to reach the clouds. So far, scientists have only been able to trigger lightning activity in
the clouds and have not been able to induce a cloud-to-ground strike.
[17]

[edit] Charge transfer systems
Charge transfer systems claim to eliminate the charge buildup on a structure by transferring it to
the surrounding area, thereby eliminating the potential for a lightning strike. This is the same
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