An investigation meant to shine light on a September blackout found that electric providers' lack of planning, communication, and awareness of their grid neighbors' operations were to blame for leaving nearly three million customers in the dark.

“There is no limit to how much improvement in communication and coordination should be made,” said Dave Nevius, senior vice president of the North American Electric Reliability Corporation, according to the Los Angeles Times.

The cascading blackout was started September 8, 2011, by an Arizona Public Service employee who made an error that took a down a single transmission line. After that, a series of missteps and equipment overloads converged to cause the widespread outage that affected customers in Arizona, California, and Mexico, according to the investigation report. Many customers didn’t have power restored until the following day. Notably, the overloads took down the San Onofre Nuclear Generating Station, which provides electricity to approximately 1.4 million customers in the San Diego area.

If agencies had been in communication regarding the real-time status of their systems, the spread of the outages could have been limited, the report stated. As it was, operators saw changes in the flow to their own systems, but didn’t realize their significance and take corrective action.

“This event underscores how complex and interdependent our North American electric systems are and the critical importance of information sharing, communications, system studies and coordinated operations among all interconnected systems,” NERC President Gerry Cauley said in a press release Monday. “Transmission operators, balancing authorities and reliability coordinators must work together to effectively manage this complex system.”

The investigation, conducted by NERC and the Federal Energy Regulatory Commission, also suggested that utilities may have set safety systems that automatically take facilities offline in response to overload to be too sensitive. While the systems are meant to regulate problems and protect equipment, in this case they worked to facilitate the spread of the blackout.

For an examination of the impacts of the blackout on lifeline infrastructure in the San Diego area, see Natural Hazards Center Quick Response Research Report #228 by Scott Miles, Hannah Gallagher, and Charles Huxford, currently in prepress at this link: QR228: Quick Response Research on the September 8, 2011, San Diego Blackout.