Vehicle Lightning Safety - Sun Roof
Question: I'm shopping for a new car and the model I'm interested in has a glass sun roof. Until now I thought that would not matter in regards of lightning safety because the rest of the body is metal, but I found a news report where a woman’s car windshield was hit by lightning. I never thought lightning could hit a car window. Is this possible?
Just out of curiosity, there's a substantial amount of Corvettes on the road, so it might be interesting to compare those cars in regards of lightning safety to metal cars. Did anyone ever get killed by a lightning strike while driving or seeking shelter in a Corvette?
Comment from Mike Holt: (Sorry the news report link is no longer online, but you can still get the picture)
Response: Safety against electric shocks, regardless of whether the source is lightning or man-made electricity, rests on creating an equipotential zone which encompasses all the points that are likely to be touched by any part of the person's body. In case of vehicles, the metallic/conducting body of the vehicle forms an equipotential zone, thus keeping the occupants safe.
As an additional precaution, occupants are urged not to stick their arms out of the windows nor rest against the metallic sides of the vehicle through which the lightning current will be flowing.
The above protection is unaffected by whether the vehicle is insulated from the ground by means of rubber tires, or is in contact with the ground as in the case of trams and trains which have metallic wheels. Hence, the statement in the news article that "the rubber tires saved the life of the motorist" is false and should be ignored.
A lightning strike to a vehicle can terminate on any point of its upward facing surfaces or on the radio antenna, with the antenna receiving a significant percentage of all strokes. However, the probability that the stroke will terminate on any of the glass surfaces is very small. In the reported case, there does not appear to be any deployed antenna and the stroke appears to have terminated on the tip of the metallic wiper on the passenger side of the windshield. The resulting localized heating then shattered the windshield.
The risk of shattering of the glass of the car is outside the scope of the protection afforded by the equipotential rule. This applies regardless of whether the damage is caused by a stone or by the heating from a lightning stroke. However, based on typical configurations, the shattering of the windshield is not likely to compromise integrity of the equipotential zone. This is evident in this case as no electric shock was reported and the driver was able to steer the car to the side of the road.
The above applies both to windshields and sun roofs. In other words, a sun roof will have negligible impact on lightning safety. The undesirability of sun roofs rather rests on risk of rain leakage as the car ages and on weakening the frame, thus making it less safe in case the car overturns.
Abdul M. Mousa, Ph.D., P. Eng., Fellow IEEE Co-moderator
Correspondence obtained from the Lightning Protection forum on yahoogroups.com which discusses Lightning and Power Quality issues.
Visit this forum here:
http://groups.yahoo.com/group/LightningProtection
