There are more than 3 million miles of overhead electrical lines crisscrossing every town and state in the country and approximately 1% of all lines in the US are underground (EEI estimate from 2009). Underground lines are typically built when there are no feasible overhead alternatives. For example, under grounding may be a better option in dense, urban areas. Under grounding may also be required by the Federal Aviation Administration (FAA) next to airports, and can provide greater reliability in areas where more extreme weather such as fires, heavy snow and ice, or hurricanes occur frequently.
Because extended power outages pose risks to health, safety, security and business, more communities and utilities are considering converting at least some portion of their overhead distribution lines to underground. Making the switch to underground cable is not a simple or easy decision. Constructing an overhead line typically costs many times less than installing underground cable. And, while it’s tempting to think that underground lines are immune to service disruptions, they do experience outages and are more expensive to maintain, replace and upgrade.
A number of factors must, therefore, be weighed when considering whether overhead or underground lines are the best option:
Although less susceptible to weather related outages, failures do occur on underground lines. Locating the cause of these outages can be considerably more difficult since the lines are not visible. It requires us to use special equipment to test and locate the issue, excavate the area to access the cable, and, if a cable is damaged, cut out and add a new section. Storm-related flooding can not only cause prolonged outages in underground systems, but also shorten the life of underground cables.
Repairs take longer for an underground system, have to be done more frequently and are generally more costly over time. Similarly, inspection and maintenance of underground lines typically takes longer and restoration can take several weeks when there is a line failure. Issues with overhead lines are more easily and quickly identified and fixed since the lines are visible and easily accessible. However, overhead lines require tree trimming as part of their maintenance.
Underground cables need to be heavily insulated so that they can withstand the pressure of being buried, but must also allow heat, which can damage the wires, to escape safely. Underground power lines do not last as long as overhead cables, mostly due to the breakdown of the insulation. Overhead cables, on the other hand, do not need to be as heavily insulated because they are not buried and air flowing around the cables keeps them cool.
Underground systems are much more expensive to build than overhead ones, a cost that is typically borne by customers. As a general rule, underground transmission and distribution lines are 5 to 10 times more expensive to install than above ground lines.
The best time to install underground cables is when an area is being developed. This is considerably less expensive than converting overhead to underground. Newer communities often request underground distribution lines for improved aesthetics. Except for above ground terminals, there is little visible evidence of the grid when underground cables are installed.
No one single choice can be a solution to all utility issues. While there are advantages in using an underground power line, the process for restoring power in the case of an outage is particularly more cumbersome than with an overhead power line. How complex the recovery process will be depends on what caused the outage.
We encourage you to do your own research on under grounding. Below are a couple of examples for further information: