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| Sunday, 21 March 2004 |
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Features |  |
| News Business Features Editorial
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Are lightning hazards common only in April? K. R. Abhayasingha, Deputy Director, Department of Meteorology, The calamity caused by rain, winds, floods and earthslips during the last Vesak week is an unforgettable example for one to realize how violent a natural hazard can be. Scanning the historic events will disclose that repetitions of such calamity are normal. Cyclones that crossed Sri Lanka in 1978, 1992 and 2000 and the hazardous droughts in 1983 and 2002 caused considerable loss to the economy of our country. Hundreds of people were killed during the passage of the 1978 cyclone across our region. Fortunately, thanks to modern technology, 2000 cyclone could account only 9 lives. But the property damage it did from Trincomalee to Puttalam was accounted in billions of rupees. Climate parameters and systems that lead to natural hazards in the tropics are mainly the rain, temperature, winds, tropical cyclones, thunderstorms and tornadoes. Since direct and indirect effects of the climate parameters result in natural hazards, weather forecasting plays an important and necessary role in the prevention and mitigation of natural hazards. The impact of natural hazards normally depends on our understanding of characteristics of climate systems, forecasting capabilities and also the validity of the precautionary steps we launch. However quantitative analysis of natural hazards, particularly of death toll, resulted by weather systems reveals that lightning accidents come to a leading place. Lightning is hazardous. Lightning incidents in Sri Lanka lead to 50 deaths of human beings and damage to property of billions of rupees worth every year. This is a serious matter. Lightning activity over Sri Lanka shows peaks during two inter-monsoon seasons, March-May and October-November. During these periods convective clouds (Cumulonimbus) develop over most parts of the country particularly during the afternoon or evening. From the list of the months of significance, April is on top. For this reason you may have questioned yourself 'Are Lightning hazards common only in April'? The figure 1 depicts results of a study done in Sri Lanka. Frequency of stations reported thunderstorms in each month are shown in the figure. It is clearly seen that there is a probability of occurring thunderstorms and hence lightning hazards during the whole year with peaks during inter monsoons.Since thunderstorms develop under any atmospheric conditions that are favourable in developing convective clouds, we should be alert in launching precautionary steps to reduce lightning hazards during all seasons. Lightning is the discharge of electricity from a thundercloud to another cloud, to the air surrounding the cloud or to earth. The magnitude of the current associated with a lightning strike could range from 3,000 - 200,000 amperes. It is this sudden discharge of an extremely large current and the voltages it induces that cause the damage. On average, the electric current of a ground lightning flash is about 25,000 amperes (25,000 A) and the potential difference between a charged cloud and the earth is about 100 million volts. Therefore the energy of a lightning flash bringing 5 coulombs to ground is about 500 million Joules. Lightning may be categorized into three types - * Cloud Flash (intra cloud flash) - Discharge within the cloud * Air Flash - Flash between clouds and the atmosphere * Ground Flash - Discharge between the cloud and the earth The Ground flash is the most important as a hazardous event and is the electrical discharge usually between the negative charge of the cloud and the induced positive charge on the ground or structures on it (figure 1)
* Direct Strikes When a person or object is struck directly it is called a direct strike. This is the source of the most serious damage by lightning. * Contact or Contact Potential When a person stays in physical contact with an object struck by lightning, for example, leaning against a tree, the lightning current is diverted into the person at the contact point. This is because the resistance to the current of the tree or such object to earth is generally higher than the path through a person. * Side Flash This happens when a person or object is close to an object struck by lightning. The lightning current travels down the object and may jump across the gap to the person. A side flash may occur even from a lightning conductor (the copper strip conducting the current to earth) when the current jumps across to a person or object close to the conductor. * Step Voltage This occurs when a person contacts the ground with several parts of his body. When the lightning strikes the ground some distance from him, the current disperses on the ground surface, part of it enters him through the body part near the struck point, and leaves again from the part of the body distant from the struck point. For example, for a person standing with his feet apart, a current path can be established up one leg and down the other. This is often how animals are killed or injured by lightning. * Surge Propagation When lightning strikes a power or communication networks either directly or close by to it, it causes a voltage surge to be created in the network. This surge travels down the line causing a voltage surge to strike persons staying close to or in touch with electrical apparatus or appliances. Equipment and apparatus (not properly protected) are also frequently damaged by surges caused by lightning. This also may be the cause of fires. A lightning flash, after travelling a distance about half a kilometre from clouds selects the shortest conducting path to go to earth before getting neutralized. Therefore the tallest objects of a location are the best supporters for a lightning flash in its way to ground. In a natural environment, tall trees are the lightning-attracters. Otherwise conducting (metal) structures or objects (like communication towers, telephone and power supply lines and TV antenna) will attract the close by lightning flashes. In the absence of such objects, the buildings in the location are vulnerable to direct hit of thunderbolts. Lightning is a natural phenomenon and therefore it is a difficult thing to stop it. But it has been found that the lightning caused hazards could be reduced with a number of precautionary measures, which are introduced in order to pass the high current surge of a direct or indirect lightning strike to the ground safely. Some of them are, 1) install good earth for the electric circuit of buildings. A lightning flash conducted through a power supply circuit may be equally hazardous as a direct lightning strike. In buildings powered by the main power supply lines without the earth wire, this situation may be very serious. The galvanized iron rods about 4.5 feet long which are usually used in installation of earth rods should be vertically pressed into the soil so that the good contact between rod and the soil is maintained. Application of any mixture with salt is not advisable. But installing the rod in a place with wet soil will lead to reduction of the resistance. Also it is an easy thing to keep the soil wet/dam by adding water to the rod hole on a regular basis particularly in places with sandy or rocky soil. There are a number of ways of installing earth rods to have its resistance low. Installing a set of parallel rods and installing several number of earth rods around the building and connecting each to the electric circuit of the building at several points. 2) Use power surge diverters Supply lines of power and communication can be interrupted by a gadget called power diverter which is capable in directing high currents intruding a building along supply cables as a result of a lightning flash to the ground without allowing it to come into the interior parts of the building circuit. 3) Installing lightning conductors/arresters Direct hit of a lightning flash to a building is the most dangerous event when the lightning hazards are concerned. Lonely and tall buildings are the most vulnerable to lightning. Protection of buildings from direct hits of lightning has been discussed even in the time of the leading scientist, Benjamin Franklin in this field. The lightning rod (LR) (or lightning arrester or conductor), which was first introduced by Franklin, is the widely used device for protection of buildings even these days. continuous research in this field has been useful to apply modifications to the lightning rod to make it efficient in conducting lightning currents to the ground protecting the building and the property inside (figure 2).
The case has to be taken to have the ground resistance as low as possible and to avoid margins for sparks of the surge to close by parts of the building. Precautionary steps like installing lightning rods and power diverters should be planned and performed in time without waiting till a lightning flash strikes your house. For information on lightning and precautionary measures browse the web, www.meteo.sltlk. |
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News | Business | Features
| Editorial | Security Produced by Lake House |
During bad weather conditions with thunderstorms, lightning surges may
reach the inside of a building in one of the following ways.
The LR usually consists of three parts namely, air terminal, down
conductor and the ground terminal (figure 2). The installation should be
performed properly, according to the standards, to reach the necessary
requirement of the characteristics of an efficient arrester.
