‘Tis the season for festive lights, family gatherings—and residential building fires. Usually wintertime home fires start because of factors like heating, cooking, and dry trees strung with lights close to fireplaces and candles, along with the simple fact that more of our activities take place indoors. But last month, the owner of a Tesla Model S electric sedan in Irvine, California awoke just before 3 a.m. to a fire in her garage, where the car was plugged into a 240-volt wall socket.
As the fourth report since October of a fire involving the best-selling electric car, the incident has garnered a tad more attention than your typical electrical fire. Citing a copy of a report by the local fire authority, Reuters has reported that investigators could not pin down the definitive cause of the fire. “The most probable cause of this fire is a high resistance connection at the wall socket or the Universal Mobile Connector from the Tesla charging system,” the report says, according to Reuters. Tesla, meanwhile, has conducted its own inspection of the car, its charging cable, and the vehicle’s data log, and concluded that the battery was charging normally. The company said in a statement, “Based on our inspection of the site, the car and the logs, we know that this was absolutely not the car, the battery or the charge electronics. There was a fire at the wall socket where the Model S was plugged in, but the car itself was not part of the fire. The cable was fine on the vehicle side; the damage was on the wall side.”
The most likely explanation, according to fire protection engineer Peter Sunderland, an associate professor at the University of Maryland, is poor wiring. “Imagine a wire barely touching onto a screw,” he said. That bad contact can create resistance heating. “Otherwise, if that’s done properly, two things need to be a problem: the car needs to be drawing too much current, and the circuit breaker in the house didn’t activate. Both those things can happen, but it’s unlikely both those things happen at the same time—it’s a two-point failure.” Sunderland, whose current work includes fire-testing lithium battery cells for Ford, also offered a third explanation. “It’s possible the electrical outlet was faulty, just made badly in the factory.”
Electrical fires make up a small portion of all home structure fires—only about 13 percent between 2007 and 2011, according to the National Fire Protection Association. But they exact a high toll, resulting in more deaths and higher dollar losses on average than other types of residential fires. Electrical issues leading to fire can include short circuits from worn or defective insulation, faulty contacts, broken conductors, and mechanical failures.
Especially in older homes, overloading circuits is a risk. Fires can result from “outdated wiring that is deteriorating, inappropriately amended, or insufficient for the electrical loads of a typical household in the 21st Century,” the U.S. Fire Administration warns. “If an outlet is added to an existing circuit, then the load easily can be more than the wiring originally was designed to conduct.”
Of course, 20th-century technologies present their own risks. “Gasoline is very flammable,” Sunderland said. “And that can catch fire in your garage, too.”