In a case that could set the first broad judicial standards for the use of Tasers, a federal appeals court in California has ruled that the police can be held liable for using one of the devices against an unarmed person during a traffic stop.

The United States Court of Appeals for the Ninth Circuit, based in San Francisco, said the electrically disabling device constituted excessive force when used against an unarmed man who did not pose a threat, and it refused to allow a police officer immunity for its use.

In a vividly worded opinion issued by the court this week, Judge Kim McLane Wardlaw described a “bad morning” for Carl Bryan, a 21-year-old Californian who drove over large stretches of Southern California to retrieve car keys mistakenly taken by a friend and ended up being Tasered by a Coronado, Calif., policeman and breaking four teeth when he fell to the ground.

Mr. Bryan was stopped twice on his driving odyssey, once for speeding and once for not wearing his seat belt. After the second stop, he was “agitated, standing outside his car, yelling gibberish and hitting his thighs, clad only in his boxer shorts and tennis shoes,” the court said.