In a statement was related to a 'procedure' at the North Gila substation northeast of Yuma. The error resulted in the 500 kV transmission line near Yuma tripping offline. Existing measures should have isolated the resulting outage to the Yuma area, the statement said.

"The reason that did not occur in this case will be the focal point of the investigation into the event," APS said. That review is already under way.

In a cascading blackout, problems in one section of a power gird ripple out over the entire gird. Similar, larger blackouts have happened elsewhere.

In 2008 for instance, a fire in a substation near Miami triggered a cascading blackout across a large swath of Florida, leaving three million people without power for hours. In 2003, a similar affected close to 15 million people in New York, Connecticut and even parts of Canada and the Midwest.

Investigators the problem started when an engineer with Midwest Independent Transmission System Operator disabled a software function that allowed the utility to determine the real-time status of the power grid in its region. That problem was later exacerbated by a software failure at FirstEnergy Corp., which contributed significantly to the problem.