A large portion of northwest Corvallis was without power starting about 11 a.m. Monday, when four transformers went out at Pacific Power's Grant substation. In all, some 10,681 customers, both businesses and residences, lost power when a squirrel entered the substation and made contact with a transformer, according to Pacific Power spokeswoman Bekki Witt.

Through the afternoon, repair workers restored one transformer at a time. All customers had power again by 3:36 p.m., according to Witt, although some some customers were already back online by noon.

Witt said the squirrel caused a flashover, or spark-like flash, that spread from one transformer to another which was the reason for the large size of the outage. Transformers take the high voltage generated by power stations and transform it to lower voltage for consumer use.

When something - like a squirrel - gets into the circuit at a fuse or transformer, a breaker is tripped, shutting down the flow of electricity to prevent further damage. If there is any significant damage from sparks or large jolts of electricity, it can take awhile to restore the flow of power.

Why are substations are so vulnerable to small wildlife?

"Our biggest concern is keeping people out," Witt said, "because it's dangerous."

Pacific Power tries to keep birds and squirrels out using fences, but it would be difficult and expensive to completely encase every transformer. Some transformers are mounted on power poles and squirrels or birds can just walk along power lines to get to them. Witt said the company had made extra efforts to protect some endangered species, such as an endangered squirrel in Washington state and had used underground fences to protect tortoises in Utah.

"We're not going to prevent all of them from getting in," she said.