The group that oversees the power grid serving the Western United States has requested potentially sensitive market information as it investigates the Sept. 8 power outage that left 7 million people in the dark in California and Arizona, the California grid operator said on Wednesday.

The California Independent System Operator (ISO) warned market participants the Western Electricity Coordinating Council (WECC) has asked for operator logs and recordings of control room conversations from 12:30 p.m. PDT (1930 GMT) Sept. 8 through 9 a.m. Sept. 9.

The blackout knocked about 4,300 megawatts of generation offline, including the two-reactor San Onofre nuclear power station in California.

It was triggered when an employee of Pinnacle West Capital Corp's (PNW.N) Arizona utility working on faulty equipment at a Yuma substation caused the failure of a high-voltage transmission line that supplied electricity to Southern California. [ID:nN1E7880CH].

Power was knocked out to customers of Sempra Energy (SRE.N) in San Diego and to residents in parts of western Arizona and northern Baja Mexico for about seven hours.

Early estimates of losses from the blackout were $97 million to $118 million.

The California ISO was forced to suspend wholesale market activity during the blackout.

To the extent these recordings and logs contain confidential or commercially sensitive information under Cal ISO rules, the agency will seek confidential treatment, it said in a market notice.

To protect sensitive information, however, market participants will have to produce an order from the Federal Energy Regulatory Commission (FERC) prohibiting disclosure by Oct. 5, the ISO said.

WECC, based in Salt Lake City, Utah, is coordinating a larger investigation into the cause of blackout that includes FERC, the U.S. Energy Department, the North American Electric Reliability Corp (NERC), along with state regulators.

WECC is one of eight regional entities that works to enforce grid operating standards developed by NERC.

Violation of NERC standards can result in enforcement actions that include financial penalties and other utility action to prevent problems from reoccurring.