An electrical current surging from a severed Seattle City Light power line punched a hole in a natural gas line causing a leak that led to an explosion of a North Seattle home in 2011, said Washington state pipeline investigators last week.

Following a comprehensive investigation into the Sept. 26, 2011 explosion of a home located in Seattle’s Pinehurst neighborhood, pipeline engineers and safety experts with the Washington Utilities and Transportation Commission (UTC) found the most likely cause of the explosion was natural gas leaking from Puget Sound Energy’s (PSE) service pipe located outside the Pinehurst home.

In an investigative report on the incident issued last week, the UTC pipeline safety staff said the natural gas leaking from the pipe likely migrated into the crawlspace, eventually igniting and causing the explosion and fire that destroyed a house and resulted in injuries to two homeowners. The explosion also caused property damage to surrounding homes in the Northgate area.

A downed power line that fell the day before the explosion had energized a metal fence post, an abandoned buried metal-water pipe and finally, PSE’s gas-distribution piping. The leak that led to the explosion was attributed to a problem known as “electrical arcing,” in which a powerful jolt of stray electricity energized the metal natural gas piping.

“The hole in the PSE service pipe serving the exploded home was caused by an electricity arcing process that was initiated by a downed Seattle City Light power line,” said pipeline safety experts in their investigative report on the incident. The electricity in the gas distribution piping in the Pinehurst neighborhood caused leaks in four locations, creating finger-sized holes in the gas pipe at each site.

On the Sunday before the explosion, PSE gas workers found leaks in three pipes, but did not find the leak at the exploded house until late Monday. The house exploded around 6 a.m. Monday and the two occupants were hospitalized. Neither the couple nor any of their neighbors reported a possible gas leak before the explosion. Gas workers continued the next day to check neighborhoods in a two-mile radius of the house that exploded. After the hole was found in the pipe, the gas company expanded the survey to a five-mile radius Tuesday.

UTC pipeline safety engineers are recommending PSE incur penalties for violating gas-safety rules. While the company promptly responded to odor reports and conducted a leak survey in the area, it failed to leak survey all the gas lines located in the area, including the service line to the home that exploded and the gas main located in and under the adjoining cul-de-sac.

The UTC monitors natural gas pipeline safety regulations on Puget Sound Energy’s 12,000 miles of natural-gas distribution system in Washington. The commission has the authority to levy fines against pipeline operators found out of compliance. The UTC’s pipeline safety program performs inspections regularly on the state’s 31 operators.