Now that power has been restored to 1.4 million San Diego Gas & Electric customers, state and federal regulators are focused on figuring out who’s responsible for San Diego’s massive blackout and what measures, if any, could have prevented it.

SDG&E says a cascading series of failures caused the collapse of the region’s power grid Thursday afternoon. The utility says it was started by a single repair job gone wrong more than a hundred miles away in Arizona. Here’s a rundown of four major questions that haven’t been fully answered:

What exactly went wrong?

SDG&E, the region’s major electricity provider, blames an outage near Yuma, Ariz., for starting the unprecedented collapse of its system. It says that outage stopped power from coming into the county, knocked out the San Onofre nuclear power plant and shut down the entire system.

Arizona Public Service, the utility that operates the power station where the outage happened, has released few details so far. It said an employee was trying to make repairs, a short circuit occurred and the safeguards that typically prevent an outage from spreading didn’t work.

“It should not have impacted a single customer and we want to know why that didn’t happen,” APS spokesman Damon Gross said.

The uncontained surge from the outage cut all power from a high-voltage transmission line connecting Arizona and California, and initiated a chain of events that also cut off the San Onofre plant, which was serving as San Diego’s other main source Thursday.

All together, that turned off the lights for millions of people for 12 hours.

Why did the San Onofre plant shut down?

On Thursday, San Onofre was San Diego’s main source of electricity. Before the blackout, SDG&E says its customers were using about 4,300 megawatts of power. About 2,200 megawatts came from San Onofre, more than 1,500 megawatts came from the Arizona power line and the rest came from local plants. (One megawatt can power several hundred homes.)

Losing the power line connecting us to Arizona placed more pressure on San Diego in two ways. First, the grid instantly lost about a third of its power supply. Second, the customers normally served by the Arizona power line were transferred to nearby power grids, including San Diego’s, which are all interconnected.

It’s unclear how the shift happened. Jim Avery, SDG&E’s senior vice president of power supply, said it occurred automatically when the Arizona power line went out. The California Independent System Operator, the agency ultimately responsible for operating the power grid, did not return messages seeking an explanation.

Utility watchdog Michael Shames doubts the shift occurred automatically. Shames, executive director of the Utility Consumers’ Action Network, said San Diego’s grid is operated in real-time by the Independent System Operator and SDG&E. A shift of that magnitude would not have happened without both agencies being involved, he said. Somebody made the decision to accept more customers, he said, but it’s still unknown who.

Whether the shift happened automatically or by choice, Avery said it was crucial step in the downfall of San Diego’s systems. Adding more customers immediately boosted the amount of power being demanded from San Onofre and overwhelmed the power plant’s capabilities.

San Onofre then turned off for the same reason the breaker in your home turns off power when you plug too many appliances into an outlet, Avery said. Before a surge in power damaged San Onofre’s equipment, computer sensors turned off the plant, he said. (And turning the power back on takes a lot longer.)

However, the company that operates San Onofre hasn’t gone that far. Steve Conroy, spokesman for Southern California Edison, said the plant was operating at full capacity before the Arizona power line was cut and the company is still investigating why the plant turned off.

How could San Diego’s entire power grid have collapsed from one short circuit?

Shames said the amount of power available from local power plants is designed to support San Diego’s grid, even during peak hours in sweltering heat (like Thursday’s). And when the region has lost power from San Onofre in the past, SDG&E said it has still managed to keep most of the lights on or brownout only some areas.

But Avery said SDG&E never anticipated what happened Thursday. The company planned how to maintain power without San Onofre and the Arizona power line, but never for maintaining power in those conditions while also handling more customers from Arizona. (It isn’t clear how many additional customers were transferred. SDG&E referred follow-up questions to the Independent System Operator, which didn’t return messages.)

“We planned for what we deem to be a credible threat. We do not plan for every contingency,” Avery said. “What occurred is greater than we planned for.”

Because SDG&E never planned for it, Avery said, the safeguards to prevent a system-wide collapse didn’t work. San Diego has three substations where grid managers and utility officials can stop a surge from turning off local power plants, too. They closed none before the blackout. If they had, the whole system wouldn’t have failed.

Avery said the new conditions arrived too fast for humans to act and the computer sensors that could have automatically closed the gateways didn’t because the scenario had never been planned for.

“The system did exactly as it was designed to do,” Avery repeated several times.

Murray Jennex, an associate professor at San Diego State who’s worked with utilities for 20 years, said computer sensors should have stopped the outage in Arizona from spreading to San Diego’s system.

Shames said if the Independent System Operator transferred the Arizona customers to San Diego without a plan approved by state oversight officials, it would have been illegal.

“Heads are going to roll at [the Independent System Operator] if that’s true,” Shames said. “What SDG&E is telling you doesn’t add up.”

So who’s ultimately responsible for the blackout?

That’s the question several state and federal regulators have set out to answer in the coming weeks. The California Independent System Operator ultimately operates the state’s power grid, including San Diego, and has promised to participate with those investigations and release more information about why the blackout happened when it becomes available.

The Federal Energy Regulatory Commission and North American Electric Reliability Corp. have begun a joint inquiry. So has the Western Electric Coordinating Council, the regional group that manages the western power grid.

If you have any insight to share about who’s responsible, please contact me by email or leave a comment below.

Keegan Kyle is a news reporter for He writes about public safety and handles the Fact Check Blog. What should he write about next?

Please contact him directly at or 619.550.5668. You can also find him on Twitter (@keegankyle) and Facebook.

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