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,
undergrounding may be a better option in dense, urban areas. Undergrounding 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.
It is important to know where these underground lines may be
located before you start digging. We ask that you call 8-1-1 before digging.