In a preliminary finding that will undergo more checking, operators at Diablo Canyon nuclear power plant think the Unit 2 reactor shut down Thursday due to a transformer that grounded out.

Blair Jones, a spokesman for plant operator PG&E, said Friday that evidence showed dust had built up on an insulator that was on a transformer outside the reactor building.

When rain showers fell and moistened the dust, conditions were just right for an electrical arc on the insulator, he said.

“Our equipment detected that, and Unit 2 shut down as designed,” Jones said.

Operators will run more tests to make sure they have correctly diagnosed the cause of the shutdown, and there is no estimated time for restarting Unit 2, Jones said.

The Unit 1 reactor at Diablo Canyon was unaffected and remains in operation.