SAN DIEGO, Oct. 24 (UPI) -- A transmission emergency on California’s power grid stretched into a third day Wednesday, as smoke and ash forced the shutdown of major power lines.

The California Independent System Operator (Cal ISO), the agency that oversees the grid, called for the conservation of electricity, as the loss of transmission lines crimped the ability to move power around the region and import electricity from outside the state.

"The San Diego area continues to be hardest hit, where several important transmission lines remain out of service," Cal ISO said in a statement. "Some lines have experienced significant damage and there are hundreds of transmission and distribution structures destroyed."

San Diego Gas-and-Electric Co. said the 500-kilovolt Southwest Powerlink remained out of service. The transmission line is the major import route for electricity produced in Arizona.

Power lines do not have to be physically damaged to be knocked out of service. A build-up of smoke and ash can cause power lines to trip out of service, and must be cleaned before lines can be re-energized.