An independent forensic engineering firm hired by NStar could not determine the underlying cause of a catastrophic power failure in March that plunged the Back Bay and other Boston neighborhoods into darkness, according to a 220-page report commissioned by the electric utility.
The report by RTI Group detailed an investigation of a massive fire at the Scotia Street substation that ultimately cut electricity to 21,790 NStar customers. To help compile the report, engineers examined tons of twisted metal and melted wire removed from the site of the blaze in 17 red dumpsters and hauled to an industrial area in South Boston.
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They concluded that direct human intervention could not have caused the outage because no one entered or left the substation near the Prudential Center in the 18 hours before the fire. Engineers confirmed NStar’s earlier findings: A connection failed on the evening of March 13 between a 115-kilovolt power line and a transformer.
The connection sparked and ignited mineral oil, used as a cooling agent and conductor in the power line. The mineral oil fueled the fire, which caused a cascade of problems that forced the shutdown of the electrical grid in much of central Boston. But the root cause of the blackout, the failure in a high-voltage connection called a pothead, remains a mystery that may never be solved.
“Due to the extent of the damage caused to the electrical substation equipment involved in the incident, the [cause of the failure in] the pothead has not been determined,” the report concluded.
NStar commissioned the report at the behest of city and state officials, who wanted a detailed explanation of the causes of a blackout that left some people without power for three days. The utility gave the document to government officials on Friday, and NStar also posted it on its website.
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The state Department of Public Utilities, which regulates NStar and other power companies, declined to discuss the report, according to a spokeswoman for the agency. The department was still reviewing the report, which included photographs and pages of maintenance and inspection reports.
The administration of Mayor Thomas M. Menino was also still digesting details Monday. James W. Hunt III, Boston’s chief of environmental and energy services, said the city appreciated that NStar had sought an independent analysis. But city officials were frustrated that the engineers could not determine what triggered the blackout.
“Any time there is a major incident we want to know the root cause,” Hunt said. “Now I think our focus is, ‘How do we prevent this type of incident from occurring again?’ ”
City officials are particularly interested in improving the fire detection and suppression systems in NStar’s facilities to stop any future blazes from triggering widespread blackouts. The utility has agreed to work with the Boston Fire Department to assess the fire systems in the six substations in central Boston.
City Council President Stephen J. Murphy said Monday that he wanted NStar officials to appear before a committee investigating the blackout and explain the findings. Murphy said he specifically wanted answers from NStar’s president, Thomas J. May.
“I expect Tom May to be part of the hearing process,” said Murphy, “not just the engineer.”
NStar spokesman Michael Durand described the report as, “a detailed, thorough analysis of the incident.” Durand said that the engineers determined that NStar’s operation and maintenance of the Scotia Street substation “met or exceeded industry standards.”
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“We know that RTI has made a number of recommendations,” Durand said. “We will be reviewing them to determine the feasibility of each.”
A problem at the same substation on Scotia Street triggered a second power outage May 8 that lasted less than an hour but amped up criticism of NStar. Later that month, the utility installed a new transformer at the substation at a cost of $2.5 to $3 million. The transformer stands 22 feet tall, weighs 380,000 pounds, and had to be hoisted into place with an enormous crane, Durand said.
Andrew Ryan can be reached at acryan@globe.com Follow him on Twitter @globeandrewryan.