A major public works project to install new power cables deep beneath the Edgartown harbor to Chappaquiddick must start over, a spokesman for the electric company NStar confirmed this week.

NStar spokesman Mike Durand said three of the four conduits beneath the harbor have collapsed and must be installed again. The project, which has caused a fair amount of disruption for the people who live on Chappy, is now projected to be completed by spring.

“We’re running four new conduits and expect to be done by the end of March or the beginning of April depending on the speed,” Mr. Durand said. “That will meet our specifications. The main electric line will run through one of them, a backup will go through a second, and the third and fourth will be spares.”

The project was thought to be near completion until three weeks ago when Bay State Piping, the subcontractor hired to install the conduits, learned that structural tests showed the conduits were beginning to collapse. The only conduit working was the one that would hold the main power line to Chappaquiddick. If that cable failed, Chappaquiddick would be without power.

“The island has long gotten electricity from one cable, and that is still the case today,” Mr. Durand said. “If that cable were to fail, power would be lost until alternate arrangements would be made; that’s why we’ve taken the extra step of including a backup cable as part of the upgrade.”

The new main cable is not yet connected, and NStar will continue to provide electricity to Chappaquiddick with the old cable until the new construction is completed, Mr. Durand said.

The project dates back to September when Chappaquiddick experienced a power outage and it was discovered that the cable running beneath the harbor had come loose. NStar went to the Edgartown selectmen in October to request permission for a project to replace the cable and in the process upgrade service to Chappy. The project was expected to be finished by December.

First there were delays due to weather. Then in November Hemlock Directional Boring from Torrington, Conn., set up camp on Dock street with a 20-foot drill that was used to drill to bore tunnels 30 feet below the bottom of the harbor. The conduits that are meant to carry cables were threaded into the tunnels.

Mr. Durand said Bay State Piping will now install new conduits near the existing ones. The work is expected to begin in early March. Mr. Durand said he did not know the total cost of the project to date.

The construction project has been disruptive on both sides of the harbor between the work crews and their equipment and delays at the Chappaquiddick ferry. At one point Chappaquiddick residents had no telephone service for three days when a line was accidently damaged during construction.

“I think everybody here was hoping life was going to get more normal, more like we were somewhere else other than on Chappy,” Chappaquiddick resident Dale Carter said last week. “It is pretty tedious sometimes.”

Mr. Durand pledged better days ahead. “The new underwater line is part of a system upgrade that will improve service reliability to customers on Chappaquiddick,” he said. “In addition to the main cable and backup, we’re also working now on other upgrades to on-land technical equipment, so a combination of all that will certainly improve service reliability.”

The plan still calls for burying power lines from the Chappaquiddick ferry at the point to Litchfield Road. NStar then plans to take down the utility poles along the beginning section of Chappaquiddick Road. The unfinished work means potholes along the work area will remain filled with dirt until warmer weather comes.

“Any potholes are being addressed with temporary patches during the winter months,” Mr. Durand said. “Once the asphalt plant opens, permanent asphalt will be laid. The poles along the project route will be removed once all the new underground system is installed and connected,” he added.

“We’re hoping they’ll fix the patches as soon as the plant [at Goodale’s] opens because people drive on the wrong side and that’s okay, but when you get up to the curve just before Litchfield, then people swerve over to other side,” Chappaquiddick resident Margaret Knight said yesterday. “Maybe we’ve just been lucky.”

And despite all the problems, she praised the work by NStar. “They worked really hard and were really great,” Ms. Knight said. “We appreciated their courtesy on the road.”