SEATTLE, Washington - When debris on a Seattle highway pierced the battery of a R700 000-plus Tesla Model S and touched off a raging fire it raised new safety concerns for electric-vehicle owners.It also caused rare jitters among investors, who of late have viewed Tesla as nearly invincible.Battery cars have scored well in government tests of front and side crashes - the Tesla S earned the highest score possible - but Tuesday's (Oct 1 2013) incident demonstrated that real-world driving could reveal some vulnerabilities that don't show up in laboratory testing.NEW CHALLENGESKelley Blue Book senior analyst Karl Brauer said: "The safety challenges related to electric cars are still in the early stages of being tested and addressed."Tesla said the Seattle-area driver hit a large metal object in the road, which damaged a battery cell and caused a fire. The company said the car acted as designed by containing the blaze in the front of the car. Still, experts said Thursday (Oct 3) that while incidents like this would happen again, they were rare. And electric cars were still safer than a tank full of petrol. The Tesla fire also showed that automakers needed to strengthen battery shields and that fire crews needed more training to deal with battery-car blazes.Of the estimated 194 000 vehicle fires in the US each year, most are in cars and trucks with liquid-fuelled engines. Electric vehicles made up less than 1% of the cars sold in the US.Tesla says this was the only fire ever to have happened in one of its batteries. A Chevrolet Volt made caught fire two years earlier after a government crash test but neither GM nor Nissan, which make the top-selling electric cars in the nation, knew of any real-world fires in their vehicles. SHARE FALLOhio State University’s Centre for Automotive Research director Giorgio Rizzoni said: "If you think about what you'd rather be close to, 40 litres of petrol or a battery pack, I'd pick the battery pack."Still, an online video of the Tesla fire spooked investors and caused a sell-off Wednesday and Thursday. Tesla shares fell 6% Wednesday and they closed down to the equivalent of R76.47, or 4.2%, at about R1379. At that price, Tesla's market value has dropped about R24-billion in the past few days.Still, if an investor purchased a Tesla share at the equivalent of R350 on January 2 2013 he would be sitting on a gain of nearly 400%. Tesla had dazzled Wall Street by selling more vehicles than expected and posting its first quarterly net profit earlier in 2013. Tesla said that on Tuesday that the Model S warned the driver of problems from the collision. He pulled off the road, smelled smoke and saw flames. A company spokeswoman said the fire originated in a battery cell damaged in the collision but the car's design prevented the fire from spreading to the rest of the battery and contained it in the front.FIRE CAUSESThomas Habetler, an electrical engineering professor at Georgia Tech, theorised that the highway debris punctured a shield and a battery cell, causing a short-circuit, bypassing fuses and electrically linking one battery terminal to another. "You're going to have arcing and sparking in that case, which can cause whatever it is to light on fire," he said.Leaking battery coolant also could have caused a short-circuit, he said.Habetler and Rizzoni said electric cars were designed to withstand a blow from highway debris. Fires were so rare that this one shouldn't give anyone pause about buying. Rizzoni said: "My feeling is this was a case of prodigious bad luck."Tesla said it had already inspected the Model S. Galves wrote that the company's ability to monitor cars remotely should result in a detailed report on the cause.Fire department Captain Kyle Ohashi said crews learned lessons from the Tesla fire. For one, the dry chemical extinguisher seemed to work better than water to combat the blaze. And he said the department was now aware that accessing the battery pack in a Tesla is quite difficult.