Federal regulators closed an investigation of Tesla Motors' Model S sedans earlier this month, after concluding that three cars destroyed by fire after running over road debris didn't represent a "defect trend."

That conclusion, released March 28, does not mean the US National Highway Traffic Safety Administration (NHTSA) believes there is no safety-related defect in the design of the all-electric sports sedans.

An NHTSA report concluded that sudden fires starting in the bank of 7,000 batteries that power Tesla Model S sports sedans were the result of road debris that punctured the aluminum shield protecting the cars' 1,300-pound battery packs, not a design or manufacturing defect.

Safety officials in Germany issued a report in December clearing Tesla of blame, saying "no manufacturer-related defects could be found."

NHTSA accepted Tesla's explanation that, under the right conditions, it is possible for objects passing under the car to get snagged on the leading edge of the plate protecting the batteries, then spike sharply upward if the opposite end digs into the pavement.

As evidence, Tesla showed crash-test video it shot using a three-ball trailer hitch -- the same type that evidently fell from another vehicle before puncturing the battery shield of a Model S November 13 in Tennessee, which passed over the debris and caught fire after the trailer hitch smashed through the battery shield and set off a "thermal runaway" reaction in the batteries, which overheated, caught fire, and destroyed the car.

By: DocMemory
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