UPDATE, 12:45 p.m.: Power started coming back on.
The same substation where a transformer exploded in March shorted out today, darkening stores, offices and homes across the Back Bay again and causing traffic havoc as signals went dead.
The lights went out shortly before noon, in an area that extended from Columbus Avenue to Newbury Street, down toward Symphony and into Kenmore Square - including the Prudential tower and mall and half the Northeastern campus. Berklee students had finals interrupted by the sudden loss of power. The BPL main branch in Copley Square was evacuated and shut.
The Christian Science tower, however, still had power, as did the plucky little traffic lights at Mass. Ave. and Comm. Ave.
The lights came back on around 12:20 p.m, stayed on for about 30 seconds, then went out again.
Service on the T was unaffected, although the outage stopped the esclator at Prudential - but not the elevator. Boston fire crews responded to several buildings along Comm. and Huntington avenues and Newbury Street to get people out of stuck elevators.
Boston Police reassigned officers working details overseeing repair work from the last blackout to direct traffic at key intersections. The department eventually brought in extra officers from other parts of the city to help with traffic control.