LOS ANGELES - A series of Department of Water and Power problems have raised questions about the reliability of electrical service in the nation's second-largest city.

A power outage blacked out more than 71,000 utility customers including City Hall, police headquarters and other government buildings in the civic center area for up to 15 minutes Monday.

It was the latest in series of problems for the city Department of Water and Power, which has 1.4 million customers.

The electrical outage also extended outside the civic center and affected some residential and commercial buildings, said DWP spokeswoman Gale Harris.

The incident triggered numerous calls reporting malfunctioning traffic lights and stuck elevators, said Fire Department spokesman Brian Ballton.

Courts, DWP headquarters and the Police Department's Parker Center were among major buildings affected.

The failure involved a transformer bank at a North Main Street electrical receiving station but the exact cause remained under investigation, Harris said.

Power went out at 11:41 a.m. Some of the 71,200 affected customers were back up in seven minutes, and power was restored to the remainder at 11:56 a.m., Harris said.

A series of DWP problems have raised questions about the reliability of electrical service in the nation's second-largest city.

A 20-minute outage hit about 4,000 civic center customers on Aug. 2, and during July's record heat wave 860 transformers failed, leaving thousands of customers without power — for days in some cases.

The latter incident revealed that city's aging power infrastructure may not be capable of keeping up with the amount of electricity that customers can demand.

The DWP had planned to spend $2.4 billion on improvements over the next five years, but officials say the utility may need to move faster, adding another $100 million or more to costs.