Correction: An earlier version of this story mistakenly said the PG&E work is part of an electrical liability campaign. It's actually part of an electrical reliability campaign, according to PG&E spokesman Kory Raftery.

PG&E is wrapping up a week of aerial maintenance of high-voltage transmission lines leading away from Diablo Canyon nuclear power plant.

The project was unusual because everything — workers, material and equipment — was ferried to and from the transmission towers by helicopter.

“We didn’t touch ground anywhere around the structures,” said Pete Dominquez, electric transmission supervisor in charge of the project.

In all, three crews replaced insulators and did other work on 18 power line towers along a 27-mile stretch of 500-kilovolt transmission lines between Diablo Canyon and Templeton.

Although it may sound expensive to use helicopters, it is actually cheaper, Dominquez said. It is also better for the environment because utility trucks don’t have to drive through ecologically sensitive areas, he said.

Helicopters are cheaper because they are faster, particularly in the rugged terrain of the Irish Hills behind the power plant. It can take two days to perform the maintenance on one transmission tower in a remote area if the crews drive to it. Using helicopters, three to five towers can be done in a day, Dominquez said.

An important component of the work is replacing insulators. These are strings of ceramic disks that prevent the electricity flowing through the lines from being lost by jumping to the tower.

The insulators can last 30 or 40 years but are replaced sooner if they are damaged, Dominquez said. Lightning strikes and stray hunters’ bullets are several ways the insulators can be damaged.

The tower maintenance work is part of a company-wide electrical reliability campaign, said Kory Raftery, a PG&E spokesman.

Diablo Canyon provides about 20 percent of the utility’s power.

Three sets of high-voltage transmission lines carry electricity from the plant to a distribution station near Fresno, where it is fed into the state’s electrical grid. Maintenance work is also being done on the utility’s 70-kilovolt distribution lines throughout the area.

The maintenance is scheduled to end today.