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This is the third fire in under a two-month period

The National Highway Traffic Safety Administration (NHTSA) has decided to look into the latest fire involving Tesla Motors' Model S. 

According to The Detroit News, NHTSA will review what is now the third Model S to catch fire in under a two-month period. This most recent incident took place near Smyrna, Tennessee. 

The Tennessee fire took place when a Model S driver hit a tow hitch on Interstate 24, which damaged the car’s undercarriage and caused a fire. The driver was able to exit the vehicle safely.

“NHTSA is in close communication with Tesla and local authorities gathering information about the incident to determine if additional action is necessary,” said NHTSA.

Special focus is being placed on the fact that the Model S' battery is located near the underside, making the battery an easy target when striking debris or hitting pavement -- thus increasing the risk of a fire.

This could lead to stronger methods of protecting the EV's battery pack. 

NHTSA has not opened a formal investigation on the Tesla crashes yet, likely because this is the first incident it's addressing.

Model S fire in Smyrna, Tenn. [Image Source: Associated Press]

But this isn't the first Tesla fire to occur. In early October, a Model S driver in Kent, Washington was traveling southbound on state Route 167 when he hit a piece of metal debris on the freeway. He then exited the freeway, and the car became disabled right before he smelled something burning. The car caught fire.
Tesla spokeswoman Liz Jarvis-Shean said the fire was caused by a large metallic object hitting one of the battery pack’s modules. NHTSA did not investigate the fire at the time because of the partial government shutdown, which suspended such activity. 

Another Model S fire occurred shortly after in Mexico, but that's out of NHTSA's jurisdiction. 

Tesla said all three fires were caused by crashes; not spontaneous events.

Tesla and its Model S have been in the spotlight a lot this year after the company successfully paid off its government loans nine years early, pulled a profit, unveiled new tech for its electric car and the Model S even snagged the highest safety rating from the NHTSA. But it's unclear if this hiccup will further heighten the fear surrounding lithium ion batteries for cars, and possibly even take a toll on Model S sales. 

Source: The Detroit News

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RE: Lithium batteries burn
By daboom06 on 11/11/2013 12:55:41 PM , Rating: 2
sorry, the approximation is n ln(n) - n

RE: Lithium batteries burn
By Motoman on 11/11/2013 1:03:56 PM , Rating: 2
3 isn't the sample size - 20,000 is. The rate is 3 fires per 20,000 units per month.

RE: Lithium batteries burn
By daboom06 on 11/11/2013 2:32:01 PM , Rating: 5
the sample size is small if the number of positive events is 3.

RE: Lithium batteries burn
By Solandri on 11/12/2013 5:13:48 PM , Rating: 4
Actually he's correct that the sample size is 20,000 here.

The problem with having only 3 incidents is that the uncertainty becomes larger relative to the incident rate as the incident rate becomes very small.

z = 1.96 for a 95% confidence interval
std_err = sqrt[ (p)*(1-p) / n ]
margin of error = z * std_err

e.g. If there were 10,000 events (50% observed incident rate), then the margin of error is:

1.96 * sqrt( (.5)*(1-.5)/20000 ) = 0.006929646

So the true incident rate is 50% +/- 0.69%

But if you calculate it for just 3 incidents (0.015% observed incident rate), you get:

1.96 * sqrt( (.00015)*(1-.00015)/20000 ) = 0.000169728

So the true incident rate is 0.015% +/- 0.017%. In other words, your margin of error is bigger than your incident rate. With a 95% confidence interval, all you know is that the true incident rate lies somewhere between 0 per 20,000 and 6.8 per 20,000. Which pretty much is the same thing as saying you don't know what the true incident rate is, other than it being a very small number.

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