Underground Electric Transmission Installations Gaining Traction

By Jeff Griffin, Senior Editor | June 2010 Vol. 65 No. 6
Two jack and bore underground duct installations for Progress Energy in St Petersburg, Fla. Photo: Brierley Associates

Electrical cable for power distribution systems has been placed underground for decades. In fact, it would be unusual today to even consider installing aerial power or communications cable in a new residential development, office park, government or institutional campus.

However, most power transmission lines remain overhead, suspended from steel towers. That’s because high-voltage power lines generate heat; being in open air allows that heat to dissipate. Being confined in underground spaces is not a good environment for high-voltage power cable. In addition, underground construction costs can be much greater than constructing aerial transmission lines.

Even so, there are areas, primarily in urban centers, where short segments of power transmission networks must be put underground. Today, advances in technology make it more feasible to bury high-voltage power cable.

A primary reason is the development of cable using plastic for insulation which has higher thermal tolerances than paper, oil-impregnated paper and fluid-filled pipes for insulating electric cable. With cable that can effectively and dependably carry high voltages underground, a narrowing gap between underground costs compared to overhead construction, and a growing list of successful underground transmission installations, utility providers, project planners, engineers, developers and communities are more open to considering underground segments of transmission projects.

Many factors
Ultimately, many factors influence consideration of installing transmission lines underground, says Brian Dorwart, P.E. PG, Brierley Associates, Bedford, NH.

“The reliability of underground infrastructure, along with regulatory issues and public demand, are factors,” Dorwart explains. “Cost ultimately may be the deciding factor, so it is important to note that the gap between underground and aerial construction costs has narrowed. The common rule of thumb once was that underground was 20 times more expensive than aerial. Today the difference generally used is that underground costs 10 times more.”

Aside from cost, it is generally accepted that underground cable is more secure than overhead cable.