An unknown malfunction in an underground piece of electrical equipment knocked out power to 940 customers in Boston’s South End, NSTAR said.

The outage occured at around 9:45 a.m. in the West Newman Street and Rutland Street area, said NSTAR spokesman Michael Durand.

About 340 customers had power restored within an hour, and NSTAR was working to have all power restored by mid-afternoon, Durand said. He could not say what caused the malfunction.

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