ATLANTA — The lights were back on at the world’s busiest airport on Monday, but ticket counters were jammed with hundreds of passengers looking for a way out, after an intense electrical fire blacked the airport out for much of Sunday. Though the fire caused no injuries, it exposed flaws in a system whose temporary failure was felt by travelers around the globe.

The fire at Hartsfield-Jackson Atlanta International Airport had not only knocked out the airport’s primary power source, it also prevented the backup system from kicking in. “The fire was so intense, so hot, that it caused a switching system, which switches to the backup, to malfunction,” the mayor of Atlanta, Kasim Reed, said in a news conference.

The fact that a key element of the backup system could be vulnerable to a fire that hobbled the main one was a problem that Mayor Reed said would probably be addressed. He said Georgia Power, the regional utility that runs the underground facility where the fire occurred, was considering a series of concrete casings that would protect the switches in case of another blaze.

But critics had already spent a day expressing their astonishment and anger over the failure. Anthony R. Foxx, who was transportation secretary during the Obama administration, was among the many passengers who spent several hours stuck aboard aircraft stranded on the Atlanta tarmac on Sunday, unable to unload at the terminal. In a series of posts on Twitter, Mr. Foxx wrote that there was “no excuse for lack of workable redundant power source. None!”