Two electricity company workers were hurt Wednesday afternoon when their truck tumbled nearly four stories off the Ravenel Bridge in downtown Charleston and crashed onto the grass below.

The Sumter Utilities truck was leaving the bridge on southbound U.S. Highway 17 when its boom snagged a sign marking the Meeting Street exit and pulled the rig over a wall.

With the truck on its side 36 feet below the bridge, rescuers cut the two men from the cab. Their injuries were said to be serious but not life-threatening.

It was the second accident for a utility company that slowed traffic on the bridge Wednesday.

About 11 a.m., a crew from S.C. Electric & Gas was using a machine to pull a rope across the bridge as part of a line project above the bridge when the device’s brake mechanism failed, company spokeswoman Kim Asbill said.

The rope snapped, causing three light poles to break.

Authorities shut down the bridge for a half-hour as crews cleared the debris. Motorists left their vehicles and stood on the pavement during the delay.

The afternoon crash wasn’t related to the earlier mishap, Asbill said, though the Sumter Utilities crew was acting as an SCE&G contractor at the time.

Investigators were still looking into the circumstances leading to the 3 p.m. wreck. Other Sumter workers at the site said they saw what happened, but they declined to talk about it. Company spokesman Steve Barwick said he thought the crew had been doing ordinary maintenance, but he declined to comment further.

The vehicle’s boom was somehow not secure, Charleston Police Department spokesman Charles Francis said, causing it to jut out to the side and catch on the yellow and green sign as the truck passed. The sign was affixed to a concrete protrusion outside the barrier wall next to the exit lane. The boom hit a canister-like support for the sign, severing it from its base and sending it into a free fall, according to regional S.C. Department of Transportation spokesman James Law. Two light posts, also on the far side of the wall, were felled.

The DOT worked late Wednesday to devise a temporary sign to indicate the exit-only lane toward Meeting Street.

The lack of a sign “may be a problem” for motorists unfamiliar with the area, Law said, but engineers were already devising a replacement.

Law said he did not know how much that would cost.