LOS ANGELES (REUTERS) - Electricity was shut off for more than 500,000 California homes and workplaces on Wednesday (Oct 9) as Pacific Gas and Electric Co (PG&E) imposed a power outage of unprecedented scale to reduce wildfire risks posed by extremely windy, dry weather.

Some of California's most devastating wildfires were sparked in recent years by damage to electrical transmission lines from recurring bouts of high winds that then spread the flames through tinder-dry vegetation into populated areas.

The planned power cut knocked out traffic signals, forced school closures and shut businesses and government offices across northern and central California, according to Mr Brian Ferguson, a spokesman for the state Office of Emergency Services.

One of the more noteworthy disruptions occurred at the University of California at Berkeley, which cancelled classes on Wednesday.

A second phase of the "public safety power shut-off", delayed until the evening, was due to extend power cuts to another 234,000 customers, the utility said, and it was considering a third phase for 42,000 more dwellings and businesses.

The action ultimately could leave nearly 800,000 homes and businesses, comprising millions of people, without electricity - the largest precautionary electricity shut-off undertaken by PG&E, California's biggest investor-owned utility.

"It's too bad that it is such a large area to be turned off," said Mr Matthew Gallagher, a resident of Vacaville, a rural town 100km north-east of San Francisco where everything from the local Walmart retail outlet to gasoline service stations were closed for lack of power.

A similar cut-off was under consideration by neighbouring utility Southern California Edison for nearly 174,000 of its customers, about 50,000 of them in Los Angeles County, should severe winds hit Southern California on Thursday as forecast, SoCal Edison spokeswoman Taelor Bakewell said.

"We are entering into a two-, three- or four-day period of extreme fire danger in California," Governor Gavin Newsom said at an event in San Diego on Wednesday.

'RED FLAG'

Gale-force wind gusts, mostly in higher elevations, were expected to intensify late on Wednesday across northern and central California before gradually migrating into Southern California overnight and Thursday, National Weather Service meteorologist Steve Anderson said. He said extremely low humidity levels added to the fire threat.

The California Department of Forestry and Fire Protection (Cal Fire) said "red-flag" warnings were posted across the state for what was shaping up to be the strongest wind event so far this season.

PG&E warned residents to prepare for outages that could last several days. But spokesman Kristi Jourdan said the utility expected to restore electricity to most customers within 24 to 48 hours after high winds abate, once power lines were inspected and any damage repaired.

Many customers live in areas where breezes were light on Wednesday, but some are served by transmission lines hit by high winds elsewhere and thus were part of a larger portion of the grid that was turned off, PG&E spokesman Jeff Smith said.

Customers were urged to stock up on flashlights, fresh batteries, first-aid supplies and cash, and to plan for healthcare needs requiring refrigerated medications or electrical devices.

The utility said it opened 28 community centres across the planned outage zone to furnish restrooms, bottled water, battery charging and air-conditioned seating during daytime hours.

PG&E has come under increased scrutiny in recent years over maintenance of transmission wires and other equipment implicated in major wildfires.

In May, state fire investigators determined that PG&E transmission lines caused the deadliest and most destructive wildfire on record in California, last year's wind-driven Camp Fire that killed 85 people in and around the town of Paradise.

Cal Fire likewise concluded that PG&E power lines had sparked a separate flurry of wildfires that swept California's wine country north of San Francisco Bay in 2017.

PG&E filed for bankruptcy in January 2019, citing potential civil liabilities in excess of US$30 billion (S$41.36 billion) from the fires.