California’s largest utilities have staged power shutoffs this year to prevent their equipment from sparking wildfires during the worst fire season on record. Here’s a look at the temporary blackouts:

What are the shutoffs?

A public safety power shutoff, or PSPS, involves a utility preemptively de-energizing power lines as a precaution against weather conditions that might damage equipment and spark a fire. That includes high winds that can blow down power lines, hurl branches or other debris into them or cause two lines to hit each other, causing sparks that could ignite tinder-dry grass. Power lines have been blamed for roughly half of the most destructive wildfires in California history, according to California’s Public Utility Commission. The agency ruled in 2012 that utilities had the right to shut off electricity to protect public safety, and the most recent guidelines were adopted in June. About 40 safety shutoffs have occurred since 2017.

How do they work?

Utilities monitor the weather. If high winds, scorching temperatures, low humidity or other risky fire conditions are forecast for a certain area, a utility will begin analyzing the area, including looking at historical data to determine the likelihood of wildfires. The PUC has categorized regions into three tiers of wildfire risk. The two highest tiers have a greater chance of being hit with a shutoff. However, even communities that don’t face a fire threat could lose power if they rely on lines that are de-energized in risky areas. Each utility makes its own decisions about when to call a PSPS and how to implement it. When a PSPS is planned, PUC guidelines call for utilities to make efforts to warn customers, usually two days in advance. As the weather event nears, information about estimated shutoff times, duration and size of outages are updated. Notices go out publicly through media and websites, through social media and through emails, texts and phone calls to customers. Local governments, emergency response agencies and critical infrastructure such as hospitals are also notified. Power isn’t restored until crews can inspect power equipment in the risky areas to make any repairs and ensure the lines are safe to be re-energized.

How widespread are they?

A PSPS can affect a relative handful of customers up to millions. What’s believed to be the largest was in October 2019, when PG&E cut power to more than 2 million people from the San Francisco Bay Area to the Sierra Nevada. The resulting chaos — phones and gas pumps, elevators, traffic lights and even water pumps stopped working — created furious criticism. The utility, however, said it has been working to make the blackouts smaller, shorter and more targeted, as well as providing technical changes allowing more people to keep their power on. It calls such outages a last resort. PG&E has done five shutoffs this year and the latest this week was by far the largest, affecting about 1 million people in 34 counties. Southern California Edison shut power to a small fraction of its 5 million customers this week.

What is the risk?

Catastrophic. PG&E recently emerged from bankruptcy stemming from financial fallout from several devastating wildfires caused by its utility equipment that killed more than 100 people and destroyed more than 27,000 homes and other buildings in 2017 and 2018. That included a blaze two years ago that destroyed much of the town of Paradise and killed 85 people. The utility pleaded guilty in June to 84 felony counts of involuntary manslaughter and paid $25.5 billion in settlements to cover the losses from that and other recent power line-sparked catastrophes. And authorities say the fire risk is increasing. Scientists have said climate change has made California much drier, meaning trees and other plants are more flammable. October and November are traditionally the worst months for fires, but already this year 8,600 wildfires in the state have burned an area larger than Connecticut, killed 31 people and destroyed thousands of homes.

Do they prevent wildfires?

PG&E said during its latest blackout, crews discovered at least 130 incidents of damage or hazard to its equipment. The utility says any one of them conceivably could have sparked a fire. It’s much easier to show that equipment has caused or is suspected of causing devastating blazes. PG&E avoided major wildfires during its outage this week, while Southern California Edison is trying to determine if one of its power lines started a massive fire that drove nearly 100,000 people from their homes in Orange County. The power shutoff strategy does have its critics. Jonathan Lesser, an adjunct fellow at the conservative Manhattan Institute think tank, coauthored a report that found the benefits don’t outweigh the costs to customers. He said preemptive blackouts are like a cheap insurance policy for utilities that don’t have to account for the costs to customers. When the lights go out, many businesses are forced to close or scale back, homeowners have to toss spoiled food and vital medical equipment can’t be used.