King County deputies first responded to a report of an explosion about 4:30 a.m. at the Puget Sound Energy substation between Renton and Issaquah.
Firefighters removed the injured man and took him to Harborview Medical Center in Seattle with second- and third-degree burns over 60 percent of his body, Eastside Fire and Rescue Battalion Chief Dave McDaniel said.
The man, estimated to be about 30 years old, is in critical condition in the intensive care unit, hospital spokeswoman Susan Gregg said.
The incident knocked out power for about 2,800 utility customers in the area, but service was restored by 8:30 a.m., Puget Sound Energy spokesman Ray Lane said.
It's assumed the man broke through the perimeter fence in an attempt steal wire to resell as scrap, Lane said. Such wire thefts have been a problem for years for utilities around the country.
Puget Sound Energy has upgraded security measures and patrols in an attempt to stem the thefts, he said.
"But there is just so much you can do to stop those who are bound and determined to break in," Lane said. "They put their own safety and the safety of others on the line to steal that copper to sell."
Such wire thieves don't know what they're doing, he said.
"I think people underestimate the type of equipment in a substation that should not be messed with," Lane said. "They have no idea what the copper is attached to or how powerful in terms of electricity force."
Lane also said there was a minimal amount of copper in the wire that the intruder cut into. He described it as a copper-coated steel wire that is being used more and more at PSE substations.
"It works the same as the old-fashioned pure copper wiring. Copper weld is worth far less than copper, making the re-sell value much less attractive," Lane said.