The operator of the Palo Verde Nuclear Generating Station west of Phoenix says a backup generator being tested at the plant failed but the plant is operating normally.
Arizona Public Service Co. says Thursday morning's incident triggered a brief and minor emergency alert but did not involve the plant's reactors.
An APS statement says the incident happened during routine monthly testing of one of two backup generators at the plant's No. 3 reactor. The diesel generator had a major failure during startup and a portable generator is being moved in to take its place.
The plant 50 miles west of Phoenix has three reactors, and each has two diesel backup generators to run reactor systems in case of a power failure. All three reactors are running at 100 percent power.