Part of Unalaska went dark this morning when power crews accidentally tripped a breaker during a routine maintenance project.

Public utilities director Dan Winters says a crew was working on one of the diesel engines at the powerhouse, which caused the overall power load to fluctuate. The other engines failed to keep up and tripped a breaker, shutting down the town circuit -- which includes City Hall and the public library.

The outage lasted about 15 minutes. While he says he’s sorry for the inconvenience to residents, Winter says he can’t guarantee there won’t be more outages like this in the future.

Right now, the engines are being tested two to three times a week. The tests aren’t supposed to affect the grid at all. But Winters says there’s always a chance that crews could encounter a bug in the computer programs or electrical equipment that run the engines, which can prompt an outage. New equipment, like the CAT engine that was installed in 2012, can be especially glitchy.

There have been more than a dozen outages so far this year, but Winters says none have been citywide. So far, they’ve been confined to individual neighborhoods.

Unalaska’s power grid is designed so that the town circuit will shut down first. Depending on the severity of the outage, Captain’s Bay shuts down next, followed by Unalaska’s school, the Valley neighborhood, and then, the East Point Road neighborhood.

Winters says the public utilities crew is aiming to complete all of its routine maintenance work by the time winter starts.