Following the third report of a Model S in flames, a US safety agency has launched an investigation into Tesla's electric car fires. How does Tesla plan to reduce the risk of fires? A software update...DETROIT, US - The US government's vehicle safety watchdog has opened an investigation into battery fires in Tesla Model S electric cars.The National Highway Traffic Safety Administration (NHTSA) says fires broke out in two of the cars in the US after the undercarriage hit metal road debris. The debris pierced the batteries and caused a thermal reaction. In each case, the car warned the driver of the damage, and both escaped without harm.The probe affects more than 13 000 cars from the 2013 model year sold in the US. Tesla has sold about 19 000 cars worldwide. Video: Tesla Model S on FireRECALL THREATTesla's batteries are located beneath the passenger compartment and protected by a metal shield. Experts say that damaged batteries can cause arcing and sparks which could result in a fire.NHTSA said it opened the preliminary evaluation "to examine the potential risks associated with undercarriage strikes" on the Tesla cars. The investigation could lead to a recall, but a decision likely is months away.Tesla CEO Elon Musk said in a blog posting that the company asked the government to investigate, even though its cars catch fire at a far lower rate than gas-powered vehicles.Musk said: "While we think it is highly unlikely, if something is discovered that would result in a material improvement in occupant fire safety, we will immediately apply that change to new cars and offer it as a free retrofit to all existing cars."He also wrote that Tesla has released a software update to give the car more ground clearance at highway speeds. Musk wrote that the software changes were made to reduces the chances of underbody damage. He said: "The theoretical probability of a fire injury is already vanishingly small, and the actual number to date is zero."Tesla plans another software update in January 2013 to give the driver more control of the air suspension ride height.194 000 VEHICLE FIRESThe low-slung Model S has a 15cm clearance between the ground and the undercarriage but the Tesla automatically lowers itself at highway speeds, the company's website said.According to the US Fire Administration, there are around 194 000 vehicle fires on US roads each year. The vast majority (61%) start in engines while 15% occur in cabins. Approximately 300 people die and 1250 are injured in US vehicle fires each year. Electric vehicles make up less than 1% of vehicles in the US.General Motors and Nissan make the top-selling battery-powered cars in the nation, the Volt and Leaf. Neither reports on any blazes in those vehicles. A Chevrolet Volt caught fire in 2011 after a government crash test but the US government closed an investigation into the incident after GM agreed to a safety campaign to bolster shielding around the battery.'POTENTIALLY DISASTROUS'Musk wrote that at first, a NHTSA investigation didn't seem like a good use of the agency's time given the higher frequency of gasoline-powered car fires. Musk said: "If a false perception about the safety of electric cars is allowed to linger, it will delay the advent of sustainable transport and increase the risk of global climate change, with potentially disastrous consequences worldwide. That can't be allowed."He has said previously that the car won't be recalled.Tesla's stock rose more than 400% earlier in 2013 but it has fallen 37% since news of the first fire was reported in October 2013.The first US fire occurred along a freeway near Seattle when a Model S struck a curved metal object which pierced the shield and battery. In the second case, a Model S caught fire on November 6 near Tennessee after the driver struck a trailer hitch in the road. Another fire was reported on October 17 in Mexico when a Model S burned after a high-speed crash.