Why Not Underground Lines?
Members often ask why Northern Lights Inc. doe not install all power lines underground. The answer is not simple. It comes down to balancing the ability to provide quality electric service and offering it at an affordable price.
To understand the pros and cons of underground and overhead power lines, let’s look at how each is built.
Take, for example, a mile-long route for a new distribution power line extension over rocky, rugged terrain, with road and creek crossings.
Before construction starts, underground locates via 811 are done. If needed, the path is cleared of trees. This is done for both overhead and underground power lines. Underground line rights-of-way do not need regular tree trimming after installation.
A mile-long overhead power line requires 13 to 15 wood poles. The ground disturbance of the poles is relatively limited. Rocky areas often can be avoided by moving a pole ahead or back. Special equipment is used to dig holes in rocky areas.
Existing underground utilities are easy to avoid. We pass over road and creek crossings by spanning them with poles on both sides. Uninsulated wires are strung between the poles.
If this same mile of line were installed underground, we must dig a continuous 3-foot-deep trench using a cable plow and lay the insulated cable in the trench. A number of junction boxes are installed to connect sections of cable and services off the underground line.
Using the underground locate markers, crews must carefully avoid other underground utilities. Crews must hand dig or use a vacuum excavator to avoid damaging other underground utilities.
Trenching through rock is hard work. Large boulders are dug up with a backhoe or mini-excavator before workers plow in the cable. In steep areas, we must dig with a backhoe or mini excavator. To get under a road or creek, a contract crew bores under both.
Installing this section of line underground takes more time and money than overhead line. Typically, underground lines are four to eight times more expensive than overhead lines. However, there are good reasons to bury power lines in some areas, even with the added costs.
Throughout the life of a line, the cost of potential outages and damage must be weighed. Areas susceptible to ice, heavy snow loading, or extreme winds may be better served by an underground line. An underground line is impacted less by vegetation outages.
Wildfire prevention is another consideration.
Damage to overhead lines is typically much faster to find and repair compared to underground cable.
NLI has approximately 1,240 miles of underground power lines and 1,570 miles of overhead power lines. The mix of underground and overhead construction used by NLI provides you with the highest possible quality of service at the lowest possible cost.
Northern Lights Engineering & Operations Manager Kristin Mettke is an electrical engineer and has worked in the electric utility industry most of her career.