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NStar faulted for second Back Bay blackout

State regulators called this week’s power outage in the Back Bay unacceptable and demanded Friday that electric company NStar provide a full account of what is being done to prevent repeat blackouts.

The regulators, from the Department of Public Utilities, told NStar officials they must maintain the “highest standards’’ as they repair an electrical substation that caught fire in March, cutting power for two days to a broad swath of the city.

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NStar officials assured department officials that the cause of this week’s outage, which affected about 12,500 customers and lasted less than an hour, has been remedied and “will not happen again,’’ David Cash, public utilities commissioner, said in a prepared statement.

Cash said NStar must submit a full report by June 1 that details what caused the outages and what steps crews have taken to bolster the system. “The DPU intends to closely monitor NStar’s restoration efforts, assess the June reports, and take all appropriate further action,’’ he said.

The meeting was closed to the public. An Nstar spokeswoman said crews are working to get return operations at the substation to normal and pledged to submit its consultant’s report by June 1.

Earlier this week, an NStar spokesman said Tuesday’s outage was caused by failure of a power supply, causing a “protective system’’ to shut down a transformer and a 115,000-volt line. It was not clear why the power supply unit failed, but it was taken out of service and replaced.

Crews have been working to install a second transformer at the substation, as well as additional backup systems to guard against outages. The work should be completed by mid-June.

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NStar said its forthcoming report will provide more details, including a consultant’s analysis of the fire’s cause.

Earlier this week, Mayor Thomas M. Menino of Boston called on state regulators to demand answers from NStar.

This week’s outage, he wrote, “is just the latest reminder that NStar still has not provided a full explanation about what caused the fire and major outage in March.’’

“Boston deserves a thorough explanation of the Back Bay power outage, now outages, urgently,’’ Menino wrote.

“NStar must take whatever steps necessary to prevent outages like the one that occurred March 13 and their lingering after-effects,’’ the mayor said. “I ask that you use the full force of your authority to make sure they do.’’

The March outage affected some 21,000 customers, costing businesses millions of dollars. Menino called on NStar to reimburse businesses for lost income and wages, but the company refused to do so.

Peter Schworm can be reached at schworm@globe.com. Follow him on Twitter @globepete.

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