With law enforcement, private security and even consumers using Taser gear, a ruling this week in San Francisco sets standards of use and level of claims. Police need reasons to believe a suspect is dangerous before firing a Taser and cannot use their stun gun simply because the person is disobeying orders or acting erratically, a federal appeals court ruled Monday. The decision by the Ninth U.S. Circuit Court of Appeals sets judicial standards for police and for people who claim they were victims of excessive force after police hit them with a Taser dart. Though stun guns may offer a valuable, nonlethal alternative to deadly force in defusing dangerous situations, the judge said, they inflict a “painful and frightening blow” and must be used only when substantial force is necessary and other options are unavailable.