Q: What is the difference in technologies behind overhead transmission lines and underground lines?
A. Overhead and underground lines make use of very different technologies. There is much more to putting transmission lines underground than just burying them.
Underground lines can more easily be routed through highly developed urban areas, ideally within tunnels beneath city streets. And the fact that the lines are out of sight once they are constructed appeals to most people – especially those who live along the route. But whereas overhead lines can span wetlands and other waterways, constructing underground lines requires disturbing every foot of the route, increasing construction impacts on roadways, traffic, wetlands, and wildlife habitats.
Overhead lines are typically bare wire that is “insulated” by the air and spacing maintained between the conductors and with the ground.
Underground cables, because they are placed close together and are surrounded by ground, require heavy insulation surrounding the conductive core of the cable. The thickness of the resulting cable means that only relatively shorter lengths can fit onto reels to be transported along roadways to the construction site, and these relatively shorter segments of cable must be spliced together in concrete “vaults” the size of a large room, to avoid a short on the cable and to allow for worker safety. All these factors make an underground line significantly more expensive than a comparable length of overhead line.
Finally, high-voltage underground lines also have different electrical characteristics compared with overhead lines, which adds to the complexity of operating the power grid.
Learn About Rights-of-Way
Read more about CL&P, WMECo and PSNH's policies related to rights-of-way use -- including permitted and non-permitted uses -- and maintenance practices. Rights-of-way acquisition and easements are also explained.