An early-morning explosion in an underground vault in downtown Long Beach sent five electrical workers to the hospital with burn injuries Tuesday, authorities said.

The contractors were doing planned maintenance for Southern California Edison in the vault adjacent to a parking structure and West Shoreline Drive when the explosion erupted at about 4:15 a.m., officials said.

The workers suffered burn injuries, with their conditions not publicly released. SCE spokesman Robert Villegas did say that three were discharged from the hospital as of Tuesday afternoon.

The utility sent a crew to investigate the cause of the blast.

“It’s a little early to tell what the cause was, and we do have a crew on-site working to make repairs,” Villegas said.

Villegas did not know exactly what maintenance the crew was performing, but he said 73 customers from were affected by a planned outage to accommodate the work. The repairs were expected to be completed by Tuesday afternoon, he said, and no additional people lost power because of the explosion.

By Tuesday afternoon, a dozen customers were still without power, Villegas said.