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Lightning protection
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Chennai faces about 170 of thunder storm days per year and this will cause 15 lightning flashes per sq km per year, writes A.R.Santha Kumar
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Photo: Ch. Vijaya Bhaskar
Vigil a dire need: Lightning is always a potential hazard in coastal areas and needs preventive measures.
Lightning can be described as a visible discharge of static electric charges occurring within a cloud, between clouds, or between a cloud and earth. Such charges always develop in pairs, one negative and one positive. The potential power depends on the size of the charge that builds up between these opposite charges which are separated by an insulating air gap. When the electrical potential between the positive and negative charges becomes great enough to jump the air gap
insulator, the charges rush to meet.
This action produces a sudden release of energy, heating the air to incandescence to form the intense white spark we call lightning. Lightning is more apt to strike on projecting objects.
Frequency of hazard
Consider the frequency and severity of thunderstorms in the area, the value and nature of each building and its contents, the relative building exposure (whether located on a hill or in a valley), and the hazards to people and/or livestock. Frequency of the lightning hazard depends on the number of thunder storm days at a location. For instance, for Chennai the average number of thunder storm days per year is about 170.This is likely to cause about 15 lightning flashes per sq km per year. This shows that one should worry about this hazard in Chennai and its adjoining east coastal areas.
Need for protection
The need for protection is evident in the following cases:
Where essential public services are concentrated like Railway Stations
Where large number of people congregate like cinema halls and places of worship
Where there are very tall and isolated structures
Where there are structures of historic and public importance like secretariat, municipal administrative head quarters etc.
Where the area is prone to lightning strokes
Protection Method
The fundamental principle of lightning protection is to provide a safe conductor pathway for lightning to follow without destroying a structure or other object. Adequate protection systems for buildings must include:
1) Properly sized, placed, and installed air terminals (lightning rods) to receive the lightning strike;
2) Conductors designed to carry the discharge from the air terminal to the ground; and
3) Ground rods properly sized and installed to carry the charge to the earth.
Conductor materials used in a protection system must be resistant to corrosion. Copper, copper alloys, copper-clad steel, and aluminium are all approved materials, with copper and aluminium the most common. Because aluminium conductors corrode when in contact with earth, aluminium should terminate at least one foot above ground level and connect with corrosion-resistant copper conductors and ground rods. Always use a special bi-metal connector to avoid corrosion when bonding aluminium and copper conductors.
The installation of a lightning protection system is as important as the materials used. A sufficient number of air terminals or points must be spaced properly. The spacing of air terminals depends on the roof shape and slope, but generally is 7-9 m along the ridge and on any tall projections such as chimneys or vent pipes.
Conductors must be properly sized and installed to avoid sharp turns. The momentum of the huge surge of electrons in a lightning strike can straighten sharp conductor turns and jump off the conductor onto other building parts.
Metal conduits, gutters, drain pipes, pipe vents, metal water pipe, radio and television antennas, metal roofing, fences and other metal objects should be bonded to main down conductors and ground rods to prevent side flashes.
Lightning system ground rods (electrodes) must be driven to a minimum depth of ten feet. Use no less than two rods at opposite corners of major buildings.
Education
Lightning safety should be practiced by all people during thunderstorms. Preparedness includes: get indoors or in a car; avoid water and all metal objects; get off the high ground; avoid solitary trees; stay off the telephone. If caught outdoors during nearby lightning, adopt the Lightning Safety Position (LSP). LSP means staying away from other people, taking off all metal objects, crouching with feet together, head bowed, and placing hands on ears to reduce acoustic shock.
Do not resume outdoor activities until 20 minutes has past from the last observable thunder or lightning.
Codes & standards
Bureau of Indian Standard code of practice IS 2309 details precautions to be taken for different types of buildings. It is prudent to follow this code especially if the building is more than 4 storeys high.
Be aware of the lightning hazard in your area. Be aware of how you can protect yourself and your property.
The author is emeritus Professor, Department of Civil Engineering, IIT,Madras and former Dean, Anna University.
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Property Plus
Bangalore
Chennai
Hyderabad
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Thiruvananthapuram
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