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Updated: July 19, 2013 19:57 IST

When lightning strikes

C. SATISH
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Tall buildings tend to attract lightning more. Photo: S. James
The Hindu
Tall buildings tend to attract lightning more. Photo: S. James

...how well is your building protected?

This is the season for lightning. Lightning is spectacular to look at but could be dangerously damaging. Benjamin Franklin in 1752 showed that lightning was electricity using his famous kite experiment. A lightning flash can average 500 KV and currents of about 50 kilo amperes. It lasts only about 10 to 50 micro seconds. The estimated power per stroke is 10 lakh mega watts though it lasts only a split second.

Most lightning occurs between the clouds and only about 15 per cent of it reaches the ground. Tall buildings tend to attract lightning more. Empire State Building in New York is struck nearly 100 times a year. Lightning can cause deaths and injuries to human beings and severe damage to buildings and structures.

The threat of damage to property and equipment is growing as IT systems, communication networks and sensitive electronic devices are now being used without proper protection. The very high current in lightning strikes produces a large amount of heat. Air is not a good conductor and so during lightning, air becomes super-heated to temperatures hotter than the surface of the sun. This burst of heat produces an intense arc of light and waves of thundering sound. When lightning strikes a structure, the very high current searches for the easiest path like metal pipes, power and communication wiring, metal railings — all of which are not designed to carry high currents. This generates heating, melting, fire and smoke.

The safety of a structure and its occupants exposed to lightning is dependent on a correctly designed and installed lightning protection system. In spite of national and international standards, damage to property and human beings by lightning still occurs due to lack of understanding or disregard of the principles of lightning protection. The four basic parts of lightning protection system are

1. Air terminals

2. Down conductors

3. Earth connection and

4. Surge protection.

The function of a lightning protection system is to convey the very high lightning discharge currents safely to earth through a low resistance path. Air terminals are also called lightning rods. These are made of copper and fixed on top of the structure. Typically, these are spaced six meters apart on the edge of the building and 15 meters on the interior of the roof. Cross connection of air terminals is also done. These air terminals attract lightning, drawing high voltage currents into the protection system and away from the structure and its contents. The air terminals are connected to down conductors of sufficient thickness and low resistance. There should be one down conductor for every 20 meters of the building perimeter. If the building is above 20 meters high, the spacing should be reduced to 10 meters. The function of a down conductor is to conduct the lightning strike to earth. These down conductors need to be direct and vertical with minimum bends and joints. Each down conductor must have a separate earth connection. The connection is obtained by a copper-clad steel rod of about 10 ft length and 1/2 inch diameter driven deep into earth pits. The resistance to earth should be low enough to discharge a large lightning current safely into the earth.

The failure to provide low resistance earthing will cause the lightning protection system to be ineffective and result in property damage and risk to human life. All components in a lightning protection system need to be corrosion-proof, large-sized and rigidly installed.

Lightning can cause surge voltages in power and phone lines, computers, TVs etc., despite a lightning protection system. Surge protectors should be used to protect them. Surge protectors are connected to electrically live parts and to the earth and have non-linear resistances that offer high resistance when there is no surge and low resistance in case of surge.

The writer is an electrical engineer and can be mailed at c.satish@ieee.org

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