Under the phase, the company is installing new wires, repairing steelwork on the 345 pylons which make up the power line.
The transmission line runs south from a substation at Harker, near Carlisle to a substation at Old Hutton near Kendal and then on to a point on the national grid system at Quernmore in Lancashire.
After completion of the project, more than 300 tons of steelwork will be replaced and over 1,000km of new wires will be suspended on the pylons.
National Grid said work to replace the wires on one side of the pylons is complete and work on the other side wires has been started.
The work on the line is anticipated to be completed by November 2013 and temporary offices and working areas around pylons will be removed by April 2014.
National Grid project manager William Fenton said, "By using our teams of engineers and their ground support teams more efficiently, we’re on target to complete this job in just a year – quite an undertaking when you consider the length of the line and the terrain it crosses."
