WASHINGTON — Investigators said Saturday that the blackout of much of the Northeast and part of Canada was likely triggered by three transmission line failures in northern Ohio, but they remained at a loss as to why the problem caused such a massive shutdown of the power grid.
Officials said it could be months before a thorough examination of the blackout is completed and recommendations can be made for preventing a recurrence. The line failures near Cleveland will be a key focus of the investigation.
"We now are fairly certain this disturbance started in Ohio," said Michehl R. Gent, president and CEO of the North American Electric Reliability Council, a private standards-setting organization that monitors the power grid.
In Ohio, the transmission system is operated by FirstEnergy Corp., a public utility with headquarters in Akron. Its seven utility companies make up the nation's fourth-largest investor- owned electric system.
"We are now trying to determine why this situation was not brought under control after the first three transmission lines relayed out of service. We will get to the bottom of this," Gent said.
He also announced that as of 10 a.m. Saturday, the power systems in the path of the blackout, which began Thursday afternoon, "had returned virtually all customers to normal electric service."
Pockets of inconvenience continued nonetheless.
Rolling blackouts were predicted for Detroit and Canada's Ontario province. In the Cleveland area, 1.5 million people lost water service, which is provided with the assistance of electrical pumps that move water uphill from Lake Erie. Although power was restored Friday, customers still had low water pressure, and what trickled out of the faucets needed to be boiled for safety.
Detroit officials said 50,000 people were still without power on Saturday, and authorities said it might be the end of the weekend before service to all 2.3 million residents is fully restored. The Detroit Metropolitan Airport was running on a limited basis, and Michigan Gov. Jennifer Granholm declared a state of emergency.
For the most part, the affected areas were returning to normal, though the nine nuclear power plants that had shut down automatically were not yet back on-line. Nuclear power plants generally take longer to return to service than generators fueled by coal, natural gas and oil.
Power to New York City was declared completely restored, and the city's 24 main subway lines and commuter rail services were operating Saturday -- a vastly different picture from Thursday evening, when hundreds of thousands of stranded New Yorkers trudged home on foot in record heat.
Gent said his investigators are keen on finding out why the power grid did not automatically isolate the problems in Ohio, as it was supposed to do, and shield the rest of the network from collapsing.
"The system has been designed and rules have been created to prevent this escalation and cascading," Gent said. "It should have stopped, we think, after the first three" line failures.
To get to those answers, he said, will take some time, noting that examiners will be reviewing more than 10,000 pages of data, including automatically generated logs on power flows over transmission lines.
Complicating the investigation, he said, is that when the lights went out, "events were coming in so fast and furious that [some] weren't even being logged in a timely way."
In a statement released Saturday, FirstEnergy said that on early Thursday afternoon, Unit Five of the Eastlake Plant in Eastlake, Ohio, tripped off-line. Later in the afternoon, the statement said, three FirstEnergy transmission lines and one jointly owned by American Electric Power and FirstEnergy tripped out of service, though one of the lines did reconnect.
According to FirstEnergy, the Midwest Independent System Operator, or MISO, which oversees 111,000 miles of high-voltage transmission lines across 15 states, including Ohio, indicated that there were also a number of other transmission-line trips in the region outside of FirstEnergy's system.
FirstEnergy's statement said that its system appeared to be stable and that no customers were reporting problems, thus "no isolation of FirstEnergy's system was called for."
However, the company noted that while its computerized system for monitoring and controlling its transmission and generation system was operating, the alarm screen function was not.
"These are very complex issues that will take time to analyze and work through," FirstEnergy Chairman and CEO H. Peter Burg said in the statement.
Perhaps the first to see the system unravel were technicians in a nondescript office building in Carmel, Ind., the home of MISO.
The outage played across double-decker rows of monitors in MISO's computer control center as the transmission lines tripped off-line. A normal crew of about eight system operators worked frantically to contain the damage.