LIGHTNING ARRESTERS AND RODS
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07-01-2011, 02:59 PM
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Lightning, is a form of visible discharge of electricity between rain clouds or between a rain cloud and the earth. The electric discharge is seen in the form of a brilliant arc, sometimes several kilometres long, stretching between the discharge points. How thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor.
When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes ionized along a narrow path and a lightning flash results.
Many meteorologists believe that this is how a negative charge is carried to the ground and the total negative charge of the surface of the Earth is maintained.
A lightning arrester is a device Used on Power Systems above 1000V to Protect the equipment from Lightning and Switching Surges.
A lightning arrester does not absorb or stop the lightning rather it diverts the lightning to ground and Clamps (limits) the voltage produced by the lightning thus protecting the equipment electrically in parallel with it.
At the heart of all arresters is the Metal Oxide Varistor (MOV). The MOV Disk is a Semiconductor that is sensitive to Voltage. At normal voltages the MOV disk is an insulator and will not conduct current. But at higher voltages caused by lightning it becomes a conductor. The MOV Disk is a very fast acting electronic switch.It is an open switch to standard system AC voltages and a closed switch to lightning voltages
By magnifying the MOV material 5000 times, Metal Oxide Grains and Dopants in the material can be discerned.Each MOV Disk with a 35mm diameter and a 35mm height contains about 28 Billion MOV Grains. The MOV Grains and their Junctions are the Electronic Switches that turn on and off in unison to divert the lightning around the equipment.The Switches are at the junctions between the grains.
A lightning arrester is hence essentially a collection of billions of microscopic junctions of Metal Oxide Grains that turn on and off in microseconds to form a current path from the top terminal to the ground terminal of the arrester.
Thus, in short,a Lightning Arrester is a device, used on power systems, that contains billions of electronic switches that divert lightning around sensitive equipment and saves them from damage.
A lightning rod (AUS) or lightning conductor (UK) is a metal rod or conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning.
The possibility of discharge is high on tall trees and buildings rather than to ground. If lightning strikes the building it will preferentially strike the rod, and be conducted harmlessly to ground through the wire, instead of passing through the building, where it could start a fire or cause electrocution.
Thus buildings are protected from lightning by metallic lightning rods extending to the ground from a point above the highest part of the roof. The conductor has a pointed edge on one side and the other side is connected to a long thick copper strip which runs down the building. The lower end of the strip is properly earthed. When lightning strikes it hits the rod and current flows down through the copper strip. These rods form a low-resistance path for the lightning discharge and prevent it from travelling through the structure itself.
A lightning rod is a single component in a lightning protection system. In addition to rods placed at regular intervals on the highest portions of a structure, a lightning protection system typically includes a rooftop network of conductors, multiple conductive paths from the roof to the ground, bonding connections to metallic objects within the structure and a grounding network.
The rooftop lightning rod is a metal strip or rod, usually of copper or aluminum. Lightning protection systems are installed on structures, trees, monuments, bridges or water vessels to protect from lightning damage. Individual lightning rods are sometimes called finials, air terminals or strike termination devices.
The lightning rod was invented by Benjamin Franklin in America in 1749 and, perhaps independently, by Prokop Diviš in Europe in 1754.
Lightning rods convey the current from a strike to the ground or water via a low-resistance conductor. A lightning strike is thus said to be diverted from the protected structure. However, diversion is a misnomer. Rather, the lightning rod widely used attracts and intercepts a strike that terminates near a protected structure. There is some uncertainty as to why a lightning strike is attracted to a lightning rod or similar protector, the leading assumption being that the air near the rod becomes ionized during an electrical storm, and thus highly conductive relative to the surrounding air.
Lightning rods are typically installed around the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 6.1 m or 7.6 m, depending on the height of the rod. When a flat roof has dimensions greater than 15 m by 15 m, additional air terminals will be installed in the middle of the roof at intervals of 15 m or less in a rectangular grid pattern.
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