Heavy damage to a transmission substation in San Jose, Calif., resulting from gunfire during an apparent vandalism on early Tuesday morning prompted the California Independent System Operator (CAISO) to urge residents and businesses in the Santa Clara and Silicon Valley region to conserve electricity in an effort to maintain reliability.

Pacific Gas and Electric (PG&E) reported "heavy damage" to an undetermined number of transformers at its Metcalf transmission substation in San Jose, a critical hub in the Bay Area transmission system. Gunshots were heard during the apparent vandalism event shortly before 2 a.m. on Tuesday, CAISO said.

PG&E said the gunshots apparently caused cooling oil to leak from a transformer bank. The leak has since been contained, but Santa Clara law enforcement continues to work closely with PG&E to investigate the incident. Workers and fire crews were cleaning up hazardous materials on Wednesday. Cleanup could take several weeks, CAISO noted.

"As damage assessments continue, additional equipment at the substation may be taken out-of-service. This will limit transmission capability in this area of the high-voltage grid, which is why conservation is required," CAISO said in a Flex Alert issued on Tuesday.

Sources: POWERnews, CAISO, PG&E