A major power cable connecting Long Island to the upstate grid has failed, the third time it has gone out of service in two years.

PSEG Long Island said Thursday it was working with the cable’s owner, the New York Power Authority, to find the cause of the Monday failure.

The 23-mile line has an adjacent backup cable, and PSEG said the temporary loss of the cable isn’t expected to affect service.

The 660-megawatt cable, known as the Long Island Sound Transmission Cable, connects a LIPA substation in East Garden City to the upstate grid at a Con Edison substation in Sprain Brook.

The cable has failed twice in the past two years — once after it was struck by a boat anchor outside Hempstead Harbor, and last year after a switchgear problem at the East Garden City substation.

NYPA spokesman Steven Gosset, noting that those two problems were not with the cable itself, also said the line is “operated and maintained” by PSEG “on behalf of NYPA.”

“We are working with PSEG to pinpoint the exact location of the issue and determine the best course for repairs,” he said. “There are no service interruptions as a result of the outage.”

A PSEG official Thursday said the current problem appeared to be on land, not under water.

LIPA has two other major cables providing access to off-Island grids. The Cross Sound Cable connects to the New England grid and the Neptune Cable, on the South Shore, connects to a mid-Atlantic grid known as PJM via a cable under the Atlantic.

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CORRECTION: Steven Gosset’s first name was incorrect in a previous version of this story.