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The San Gabriel Complex Fire that continues to smolder more than a week after erupting temporarily took out two parts of the electrical grid last week but did not severely interrupt the power supply to the L.A. Basin.
Power disruptions from fires are often easily minimized by workarounds on the region's massive electrical network.
But utility officials caution that this summer's anticipated shortages of natural gas — the primary source of fuel for local power companies — add a new uncertainty to the region's power reliability.
The San Gabriel Complex Fire began as two separate blazes, the Reservoir Fire, which burned a string of power poles providing electricity to three flood control dams in the San Gabriel Mountains. And the Fish Fire, which caused a high powered transmission line above Duarte to fail.
Here are some basics on why the region's lights usually stay on when the local mountains burn and why this summer could be different .
Utility experts say Southern California's network of power transmission and distribution lines has enough built-in redundancy so that wildfires generally do not interrupt power to a wide area.
If you think of the power grid like a network of streets, the big transmission lines are like freeways and the smaller distribution lines are surface streets. When one portion of the system fails because of fire or a power line is de-energized to protect firefighters working nearby, power can be re-routed to users through other lines.
Fires can burn the wooden utility poles that carry power to homes and businesses. Burned poles can drop live power lines onto metal fences, carrying a risky current hundreds of feet away where it can shock firefighters or others. So it's not unusual for firefighters to request power utilities to turn off the power to areas where fires are raging.
Fires can also interfere with the flow of power along major transmission lines. During a fire, smoke can cause the electrical current to arc away from power lines onto a metal tower and down into the ground. That can cause whole power lines to short out. Likewise, fire retardant inadvertently dropped on power lines can cause similar arcs and shorts.
There were two main incidents, each illustrating a different aspect of how power can go down in a wildfire.
Smoke from the Fish Fire or an aircraft drop of fire retardant caused one of Southern California Edison's 220-thousand volt power lines above Duarte to lose power, said Troy Whitman, Edison's top fire management officer. Smoke and fire retardant can conduct electricity, and cause the current in the line to arc onto the tower or down to the ground, and the line can lose power. In a case like that, Edison would re-route the power through other lines coming into the L.A. Basin to minimize power disruptions.
In a separate incident, the Reservoir Fire burned a dozen power poles that are part of the distribution system feeding electricity to three Los Angeles County Flood Control District dams along Highway 39. Backup generators have been providing power to the San Gabriel, Morris and Cogswell dams, but it could take a few weeks to replace the power poles and electrical wires that burned. Some are far up the side of mountain slopes and would need to be set in place by helicopters.
Utility officials say the continued shutdown of Southern California Gas Company's underground gas storage field at Aliso Canyon near Porter Ranch increases the potential for power outages this summer during fires.
Under normal circumstances, the utilities that rely on natural gas to fuel their electrical generating plants can draw enough gas from the distribution pipelines of SoCal Gas. But on the hottest days, when the region demands more power than usual, the extra gas to make the electricity comes from the underground storage field at Aliso Canyon, near Porter Ranch.
This year is different, because the storage field is not available to provide that extra measure of gas.
The rupture of a natural gas well connected to the reservoir caused state officials to temporarily shut down the storage field. That lack of storage capacity has prompted warnings from the state Energy Commission that the gas supplies for utilities could be cut off if there is a shortage.
The worst-case scenario is that a wildfire could limit the flow of electricity into the L.A. Basin from one of about 20 big transmission lines, at the same time a heat wave is stressing the power grid and gas supplies to power plants are low.
The result could be widespread power outages.