(UPDATE: Tesla has provided more information about the accident. See below.)

This week a Tesla Motors Model S electric vehicle caught on fire after the driver ran over a chunk of metal in the road, an incident that’s been reported all over the place now.

And this is news, why?

After all, vehicle fires are very common. One battery researcher, Jeff Dahn of Dalhousie University, pointed out to me this afternoon that there were 187,000 vehicle fires in the United States in 2011. That’s one fire for every 1,738 cars on the road. With Tesla this fire makes one out of almost 20,000. “That’s 10X less frequent,” he told me in an e-mail, typing in all caps.

Let’s leave aside the fact that even Tesla-related sneezes seem newsworthy these days. There is some real concern out there about the safety of lithium ion batteries, which is understandable because there have been well reported cases of lithium ion batteries catching fire. What’s unnerving about many of these fires is that they seem to happen spontaneously. There’s the story of someone whose cell phone caught fire while he or she was talking on it. There’s video of a laptop bursting into flame in a conference room. A house burns down because of a problem with an electric lawnmower in the garage. Days after a crash test, a Chevy Volt battery catches fire. A row of Fisker Karmas is reduced to skeletons because they got wet during hurricane Sandy, triggering a battery fire. Seemingly spontaneous lithium ion fires grounded Boeing’s 787 Dreamliner for months.

So investors and electric vehicle advocates and electric vehicle naysayers have been keeping a close eye on Tesla to see if a battery fire would ruin the company and the chances of electric vehicles to become mainstream cars.

And the fire finally happened. Here’s the good news for Tesla and electric cars.

One, the fire wasn’t spontaneous. It was apparently caused by the driver running into something. That’s far less scary. People know that from time to time, people run into things and their cars catch fire. That’s not surprising.

Two, the fire didn’t spread quickly to the whole battery pack, and as a result, no one was hurt.

The worry with lithium ion battery fires is that they have the potential to spread quickly throughout the battery, as the cells within the battery ignite their neighbors. Tesla’s CTO, JB Straubel, has said that the company has engineered the pack to prevent fires from spreading. It’s difficult to know just how successful this was, since we don’t know how much of the pack was directly damaged in the accident. But we do know at least that the fire didn’t spread throughout the whole battery, and, at least according to Tesla, it didn’t enter the passenger compartment.

But there are a couple of pieces of bad news.

First, the fire illustrated once again how difficult lithium ion battery fires are to put out. Firefighters thought they had it put out, but it reignited. There are a couple of schools of thought among battery experts about why this happens. In a battery fire, the main thing that’s burning is the liquid electrolyte, which burns best when it’s exposed to air. One school of thought is that even in the absence of air there other oxidants within the battery that can create and sustain a fire. It’s thought that the battery electrodes themselves can release oxygen, fueling the fire from within. If this is the case, all firefighters can do is to work to keep the fire from spreading and wait for the reactants to burn up.

Other research suggests that this isn’t the case. Instead, what might happen is that even once the fire is put out, the cells stay very hot and keep releasing more electrolyte in the form of vapor. Once firefighters turn off the water and oxygen can once more come into contact the vapor, it can reignite.

It seems clear that we need to do more tests and learn the best ways to put out battery fires, especially as battery-powered cars proliferate.

The second negative is that the accident raises questions about how well protected the battery is. The Tesla battery spreads out over most of the floor of the car. Contrast that to the battery on the Chevy Volt, which is tucked up inside the car, actually taking up space within the passenger compartment to keep it out of harm’s way. Was the piece of metal that the driver ran into huge, and likely to cause serious damage to any sort of car? Or was it something you wouldn’t think twice about running over in a conventional car? Is the Model S particularly vulnerable to road debris?

I’ve seen the protection on the battery, and my sense is that it would take a pretty good chunk of metal to damage it (see “How Tesla Is Driving Electric Car Innovation”). And the fact that no other Model S’s have caught fire, even in vigorous safety tests, also seems to be a good sign. Hopefully we’ll find out more about exactly what caused the accident in coming days.

UPDATE 10/4/13

Tesla has released some additional details about the accident. Here’s an excerpt:

Earlier this week, a Model S travelling at highway speed struck a large metal object, causing significant damage to the vehicle. A curved section that fell off a semi-trailer was recovered from the roadway near where the accident occurred and, according to the road crew that was on the scene, appears to be the culprit. The geometry of the object caused a powerful lever action as it went under the car, punching upward and impaling the Model S with a peak force on the order of 25 tons. Only a force of this magnitude would be strong enough to punch a 3 inch diameter hole through the quarter inch armor plate protecting the base of the vehicle …

Had a conventional gasoline car encountered the same object on the highway, the result could have been far worse. A typical gasoline car only has a thin metal sheet protecting the underbody, leaving it vulnerable to destruction of the fuel supply lines or fuel tank, which causes a pool of gasoline to form and often burn the entire car to the ground. In contrast, the combustion energy of our battery pack is only about 10% of the energy contained in a gasoline tank and is divided into 16 modules with firewalls in between. As a consequence, the effective combustion potential is only about 1% that of the fuel in a comparable gasoline sedan.