Here is an interesting report regarding a recent lightning strike to a home:
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.
Dr. Chandima Gomes Department of Physics University of Colombo
According to Standard NFPA 780 (USA), the design of the shielding system is usually based on a striking distance (radius of the Rolling Sphere) of 150 ft (about 46 m). As such, the communication tower, regardless of its height, cannot be relied upon to protect a house which is located 100 m away from it. Abdul M. Mousa, Ph.D., P. Eng. Fellow IEEE Lightning Protection Consultant Vancouver, Canada abdul_mousa@ hotmail.com
Bryan P. Holland, ECO. Secretary - IAEI Florida Chapter
Interesting story! There's a lot you can do to reduce the liklihood of a lightning strike, but nothing you can really do to prevent it, short of building a giant cage around your house. (Which is, incidently, how they protect ammunition depots and things of that nature which you really really don't want struck by lightning.)
I wonder if the steel rebar in his house was actually bonded and grounded, or if it was merely the most convenient path towards the ground?
I am not sure what the building code is in Colombo but in Florida the steel in a house is a fairly good Faraday cage. The footer steel is tied to the steel in the grouted cells and that is tied to the tie beam steel. The straps over the trusses is hooked under the tie beam steel. It sounds like that is the path he is talking about. The lightning blew a hole through the roof sheathing to get to the truss straps.
It seems Dr. Gomes is making what seems to be a resonable assumption the lightning somehow coupled onto the AC line (the hot), from which it arced to the ground (including grounded neutral) at the outlets. I believe this is somewhat incorrect. From his description of the lightning strike hitting the grounded building structure, I would expect the well-grounded structure, the earth around it, and the ground rods and ground/neutral wiring attached at the service panel, bounced quite high relative to the AC line hot conductor which would not have been carrying the same level of lightning currents. As a result, there was a flashover at the outlets due to the ground rising as opposed to the AC line rising. Ground potential rise is quite large with even "small" 2000 amp transients. I would expect an enormous ground potential rise with a direct strike, even to such a well grounded structure. The line conductors are referenced to the ground(neutral) window present at the utility transformer ground rod(if present) and neutral wire and not the ground window of the building structure/AC panel/local ground rods.
Lightning doesn't always have a single point of impact. It can bounce around (or maybe fork) and hit a couple different places at the same time. He may have really had a hit on the power line and the house at the same time.
Could be that it hit the AC line too, and that if it did, the ground and AC line could still be at vastly different potentials (enough to arc across the insulation from hot to neutral at the outlet) even though lightning struck both simultaneously. But, I think we can all agree the earthed house did not "ground out" the lightning bolt. If it did just strike the structure, I would expect the effect I described. Draw out a grounded house with the panel connections including local ground rod, the connections going to the pole or remote poco transformer, and the grounding of the neutral at the transformer, and I think you will see the same thing. The 120 volt power is essentially at ground potential - its only 120 volts away from what is considered 0 volts at the pole and everywhere except from where the strike hit. The 120 volt lines will rise or fall with the voltage of the POCO neutral they are tied to, rather than the ground potential at the customers end.
The last few times I've heard about arcing observed at the outlets, it's always been GFCI outlets in the kitchen or bathroom. Is this just coincidence, or is there something about a GFCI receptacle that causes it to spark?
I believe the steel in the house was very effective in grounding out the lightning but it didn't happen until it had blown a hole in the roof sheathing to get to it. By then it had sought alternative paths, probably through the power line. I have said this before. I took a direct hit on my weather station mast last summer with minimal damage but I did have an air terminal on top of it with a #2 going as straight to ground as possible.