Despite the fact underground lines tend to be more reliable and are less likely to fail, they generally have higher installation and maintenance costs than the overhead alternative.
Another notable disadvantage for underground lines is the ability to find the source of an extended outage when power is lost from beneath the surface. That’s because the problem is most likely buried about three feet underground.
For Shenandoah Valley Electric Cooperative (SVEC) line crews, it can be a tedious process of elimination to identify what needs repair.
“Everything that leads up to [restoration] takes so much time,” Manager of Distribution Systems Ben Cash said.
The most obvious difference in restoring power to underground lines versus overhead is the visual component: You can’t fix what you can’t see.
For underground troubles, it’s instead a matter of fixing what you hear or feel first. Crews use a “thumping” device to connect to failed underground line sections that will send signals over the line to pinpoint the exact location of the problem, Line Superintendent Scott Austin said.
“At this time you could start your repairs,” he said.
Even as fault indicator technology and sectionalizing equipment improves to decrease the time it takes to isolate a problem, Austin said line crews could still have about a three-hour delay in digging since they might have to wait for Miss Utility. The restoration could then require digging down several feet to create space large enough for a lineman to handle the underground cable.
That’s a pretty complicated solution compared to the relatively simple eye test used for overhead issues that can lead to an almost immediate fix. It’s easy to spot a downed wire, a bad insulator or transformer, or a tree on the line caused by weather or human error of someone cutting a limb onto the line.
“They will be able to find these issues with a visual inspection of the lines, and once the problems are found, they can start the repairs,” Austin said.
Cash said, “At least when bad weather hits, with ice and trees hanging on lines, that weather does not affect the underground cable.”
It’s important to understand that underground lines aren’t fool proof. They are more vulnerable to soil corrosion, the movement of moisture into the cable and animals that may find their way in to wreak havoc. Age is also a factor in causing outages. Underground lines don’t last as long as overhead wires, though technological advancements have been made over the years.
Flooding can also be a major issue for buried wire, and lightning still has an impact, too, the U.S. Energy Information Administration notes on its website.
“Underground distribution circuits are typically still linked to aboveground facilities, such as substations and transmission lines, so homes and buildings served by underground infrastructure will not necessarily be spared in the event of system-wide outages from a major storm,” the agency states.
At SVEC, underground-related outages are fairly uncommon since these facilities are protected from most dangers. Since December 2012, only about 2 percent of the overall time that SVEC member-owners have been out of power has been linked to underground components, Manager of District Operations Greg Rogers said.
However, it’s still a situation the Cooperative monitors closely.
“While the number is low, it is increasing,” Rogers said. “When it does affect consumers, it is significant in their minds due to the property damage of digging a new ditch in established subdivisions and the length of the outage they experience.”
The general lesson to take away is that underground lines provide a tradeoff when outages occur. Ice and wind aren’t factors as they are for overhead cable, but rectifying any problem can be time consuming and may require a considerable digging expense for the Cooperative.