On Friday (the 18th April) night I had first hand experience on lightning as my home was struck. The house is made of steel reinforced concrete with an asbestos roof supported by a wooden structure. The building is properly grounded through steel reinforcement at the foundation, however due to the busy schedule, I could not implement the air-termination system that I designed few years ago.
On Friday there was a severe thunderstorm started around 10 pm. Around 10.15 pm the power was interrupted probably due to the tripping of the substation. The house was in darkness and around 10.20 pm we heard a tremendous sound followed by some cracking noises. My wife and I were standing almost middle of the living room while my elder son was in the kitchen. For a moment the house became very bright as all the lamps were illuminated for a brief second and some sparking occurred at the plug points. My son said that he saw a large spark at a plug point in the kitchen which was almost in total darkness. Neither of us experienced any step potential and my younger son, who was fast a sleep, had no idea of what has happened until the morning. I plugged out all the equipment at the very first hearing of the thunder except the antenna cable of one of the TV sets (I forgot it). Thus, none of the equipment other than this TV was affected. The TV (of which the antenna was right below the point of strike) was not working as we tried to operate it in the morning. I could not open it yet to have a look at the damage (No burning marks from exterior). There were no signs of damage in the antenna which was just 1-2 m below the struck point.
In the morning we found that a part of our upper roof is badly damaged. The roof and the supporting wooden structure have been ripped off. Fortunately there are two steel bars of the concrete reinforcement that have been extended above the column surface to support the wooden bars from falling due to wind. It seems that the current has entered these bars and grounded through the steel reinforcement structure. There was no damage to the other parts of the house, except for some cracks appeared in the concrete column just underneath the point of strike.
Interestingly there are 3 tall metal communication towers situated at two sides of the house within about 100 m from the building
The following statements are based on my observations in the morning.
1. This is another example that corners of the buildings are more vulnerable than the highest elevations
2. It is re-confirmed that a metal structure underneath an insulation material can not prevent point-of-strike damage, which can be very severe (fortunately the roof has not caught fire, partly due to the very high fire resistance property of asbestos and the heavy rain)
3. Steel reinforcement of the structure provides very good path for the lightning current to flow towards ground.
4. Even the distributed current flow in the steel reinforcement structure can not prevent line wires being raised to very high potentials (perhaps a part of the lightning current has entered the wiring system through the lamp which is right underneath the point of strike).
5. You can not rely on the protection given by steel towers in the neighborhood.
Some photos of the damage and point of strike are attached for your observation. Please let me know if anyone need further information or other photos of the incident.
Chandima
Dr. Chandima Gomes
Department of Physics
University of Colombo
