Puget Sound Energy says electrical arcing caused by a fallen tree created holes in natural-gas pipes, leading to an explosion at a North Seattle house. The agency defended an initial search of the neighborhood, which didn't include the area where the house was destroyed.

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After a rare electrical problem blew four holes in natural-gas pipes in Seattle’s Pinehurst neighborhood on Sunday, Puget Sound Energy says, the agency went house to house in the neighborhood to check for more leaks. Its workers stopped at nightfall, without finding more.

It wasn’t until the next day, after a huge explosion and house fire, that PSE did a much larger “leak survey” across a 5-square-mile area, working into the night. Crews found four more leaks, but say at least three are unrelated.

With customers and Seattle residents rattled Tuesday, PSE defended its initial search. Sunday’s testing area — which stopped just blocks short of the explosion site — focused on areas with similar pipe, said Martha Monfried, PSE’s communications director.

She said it would not have been safe to continue the leak survey into the night. “You can’t do residential survey work in the dark, for both worker safety and for the comfort level of homeowners,” she said.

But Mark McDonald, a natural-gas expert who speaks about catastrophic leaks, said PSE should have gone farther.

“I would go at least 10 blocks in every direction to make sure we got all the leaks,” said McDonald, president of the New England Gas Association, an umbrella group of unionized utility workers. “Night, storm, whatever, you go farther than you need to be safe. It obviously was a mistake.”

The explosion, which rattled windows across North Seattle and sent a married couple to the hospital, is under investigation by the Seattle Fire Department and state utilities regulators. On Tuesday, David Ingham, a 53-year-old Seattle City Light lineman, was released from Harborview Medical Center after being treated for burns.

His wife, Hong Ingham, 50, was in serious condition Tuesday in the hospital’s intensive-care unit with burns.

Storm blamed for “arcing”

The source of the leaks, according to the utility, originated during a windstorm Sunday.

At about 11:30 a.m., a tree came in contact with one of the three overhead electrical distribution lines on Northeast 127th Street between 12th Avenue Northeast and 10th Avenue Northeast, said Seattle City Light spokesman Scott Thomsen. The incident tripped the breakers and the circuit quickly shut off.

“Our equipment’s role is to ground out that short, and the system operated the way it’s designed to operate,” said Thomsen.

According to PSE, the electrical current, conducted through the tree, energized a wrapped steel natural-gas pipe, causing a problem known as “arcing.” The current blew a series of BB- or finger-sized holes in the pipe, according to the utility.

On Tuesday, PSE said the supply pipe and gas meter found at the blast site showed a hole just inches outside the Ingham home, at 12312 Fifth Ave. N.E., about seven blocks from where the power tripped Sunday.

The natural-gas service line to their home was pressurized at 45 pounds per square inch, according to PSE. It won’t be clear until an investigation is completed how the gas got into the home, but experts theorized that the gas could have leaked in through a foundation.

Seattle Fire Department spokesman Kyle Moore confirmed that investigators determined there was an accumulation of gas inside the house. But it wasn’t clear if the buildup was from the leak outside the house or from a second leak that might have occurred inside, he said.

Damage to the home, which was leveled, was estimated at $350,000, Moore said.

Two engineers from the state Utilities and Transportation Commission (UTC) are investigating Monday’s fire and explosion, as well as PSE’s response to Sunday’s gas leaks.

Dave Lykken, pipeline safety director for the commission, said the neighborhood’s natural-gas pipes are probably 1960s-vintage — with some new plastic pipe — and are considered safe.

UTC requires utilities to routinely check natural-gas pipelines for corrosion. PSE said it conducts neighborhood leak surveys every three years; it last checked the Pinehurst area in November 2008, said Andy Wappler, a PSE spokesman.

PSE continues investigation

PSE ended the house-to-house leak check on Sunday at dark, after canvassing from 10th Avenue Northeast to 15th Avenue Northeast, and Northeast 110th Street to Northeast 130th Street, utility spokeswoman Monfried said on Tuesday.

On Wednesday morning, she said the intended search area extended to 5th Avenue Northeast — the Ingham’s street — but crews were unable to check that far west before nightfall on Sunday.

Though it had stopped its house-to-house survey hours earlier, PSE crews also on Sunday checked the natural-gas main that serviced the home, working until 3 a.m.

Monfried said the leak survey focused on natural geographic boundaries, and on pipes similar to those affected by the “arcing” incident on Sunday. Wappler said PSE is continuing to investigate the incident itself, including why the utility initially said the explosion was unrelated to Sunday’s leaks.

“We will take what we’ve learned and make sure it doesn’t happen again,” he said.

In the more exhaustive survey ordered after the explosion, PSE found four new leaks, but said at least three were unrelated and characterized them as small enough that they would be treated as scheduled — rather than emergency — repairs in a different situation. The other leak remains under investigation. A third survey began Tuesday, and a fourth is planned.

At one location — on the 300 block of Northeast 149th Street — a clear sulfur smell of natural gas wafted as PSE subcontractors dug up a pipe buried under a driveway on Tuesday.

About 10 blocks north, another crew dug up Diane Smith’s front yard, with only a faint smell of gas. Two employees from PSE’s public-affairs office watched as reporters watched the digging.

Smith said PSE came to her door at dinner time on Monday, telling her she had a small leak in her yard and warning her to not use a cellphone or smoke in her front yard until it replaced the damaged pipe on Tuesday afternoon.

She described herself as “a little skeptical” of PSE’s handling of the incident, and, with four adults and seven kids living in the home, fearful of the gas. Why did PSE wait until Tuesday to replace the damaged pipe after detecting the leak on Monday, she asked.

“They’re digging now, but shouldn’t have they been doing this last night, or at least have shut off the gas?” she said. “Any leak is no good.”

Staff reporters Steve Miletich, Jeff Hodson, Craig Welch and Nancy Bartley contributed to this report. Jonathan Martin: 206-464-2605 or jmartin@seattletimes.com