This time delayed-exposure shows streaks of lights from traffic on Interstate 5 with a dark San Diego skyline behind it. (Sean M. Haffey/AP)

APS put out a statement saying that the switch clicked off when a power line was tripped during maintenance work in North Gila, Ariz. The outage should have been contained to the surrounding area, and the power company said it would be investigating why backup protection did not work.

No injuries have been reported, though there were two sewage spills that contamenated the Los Peñasquitos Lagoon and Sweetwater River.

Here’s one San Diegan’s take on the day: