Operators at Diablo Canyon nuclear power plant began ramping up power in the Unit 2 reactor early Tuesday morning.

The reactor shut down Thursday when a transformer shorted out. Dust had built up on an insulator and rain caused the wet dust to ground out the transformer, said Tom Cuddy, PG&E spokesman.

“The rain was definitely a contributing factor,” he said.

Plant personnel are looking into ways to prevent a similar mishap in the future, Cuddy said. There is no estimate on when the unit will reach full power.

The plant’s other reactor remained in operation.