Michael Ray Russell
P.E.
Memphis Light, Gas and Water
There`s a familiar quote from Shakespeare`s Hamlet, "To be or not to be: that is the question." However, when it comes to electric utilities, the saying becomes, "Overhead or underground: that is the question." Without a doubt, modern society has grown to depend on the convenience of electricity, and electric service is now seen as a necessity, not merely a "want." However, the question of how to provide this service remains.
No one is more aware of this dilemma than Memphis Light, Gas and Water (MLGW). Located in Memphis, Tenn., MLGW is the nation`s largest three-service public utility. In its 1999 Annual Directory and Statistical Report, the American Public Power Association ranked MLGW as the sixth largest public power utility by the number of electric customers served and the eighth largest by electric revenues. MLGW buys electric power from the Tennessee Valley Authority (TVA) and distributes it to nearly 400,000 residential, commercial and industrial customers in Shelby County.
Since underground power line construction nearly always costs more than equivalent overhead power line construction, underground construction is used only where cost is not the principle consideration. Besides higher cost, underground lines have other disadvantages, including the difficulty of locating faults and longer time required for repairs. There are also other problems such as entrance of moisture, corrosion, cable dig-ins, insulation failures as a result of switching surges or corona, and damage during installation. Underground equipment must be designed for long life in below-ground enclosures that may become filled with water or other contaminates. Underground cables and equipment are vulnerable to the entrance of moisture as a result of flaws in splices, terminations, gaskets and connecting devices. Chemical corrosion or electrolysis can damage any of the exposed metal installed as a part of the underground distribution system.
These disadvantages are somewhat offset by the fact that underground lines are subject to fewer interruptions because they are protected from storms, automobiles, fires and malicious mischief. Also, interruptions do not occur all at once, as they would in overhead lines during a severe storm. Underground lines are relatively immune to some of the major causes of failures in overhead circuits, such as power pole accidents; damage from lightning, wind, ice and snowstorms; or contacts to the wires by trees or other foreign objects.
One of the major disadvantages of underground lines-costs-is somewhat lessened for MLGW by the fact that the utility also provides gas services. Part of the major cost of installing underground lines is the labor involved in opening and closing the ditch. Since MLGW often has to provide a ditch for the installation of the gas utilities, there is a smaller amount of additional labor to install the electric utilities in that same ditch. This is known as "joint trench" construction.
Whether or not underground construction is adopted depends upon one or more of the following considerations:
- load density in the area to be served;
- cost and availability of overhead rights-of-way;
- presence or absence of obstructions that cannot be bypassed by overhead construction;
- advisability of protecting the line from mechanical damage;
- importance of the appearance of the environment;
- laws, rules, or ordinances of local municipalities;
- policy guidelines of local utility providers;
- consideration of overhead clearances;
- proximity to airports; and
- elimination of crossings of other lines.
Nationally, the unsightly appearance of power lines and poles and the dangerous electrical hazards posed by falling power lines have caused a trend toward underground construction. Most new, urban residential developments use underground rather than overhead services, and many also use underground rather than overhead primary lines. When the primary lines are installed underground, a pad-mounted transformer (electrically the same as the pole-mounted transformer in an overhead system) is used to step the primary voltage down to the 120/240 volts necessary for houses. Underground power lines serve the same purpose as overhead distribution lines: to bring power to the customer`s premises. However, underground lines can be considered a lot safer for the public.
Figures 1 and 2 show the trend toward underground lines at MLGW over the past 10 years. The percentage of lots served by underground primary lines has risen from slightly more than 40 percent to 80 percent.
Two of the major driving forces for this trend at MLGW are the customer policies/guidelines of our company and the ordinances of local municipalities. MLGW has implemented policies to encourage developers/customers to install electric utilities underground. MLGW`s policy requires that most new subdivisions have electric utilities installed on the front property line. Even though the developers/customers may choose to have these electric utilities installed overhead or underground, the majority does not want overhead facilities on the front property line, primarily due to aesthetics. The larger the lot size, the greater chance the developer will request overhead facilities on the front property line. For example, most large lot developments (four acres or more per lot) request overhead on the front property line. However, the majority of subdivision developments have smaller lot sizes.
In summary, over the past 10 years at MLGW, the trend has obviously shown that developers/customers are requesting underground utilities more often than overhead. So, in Memphis and Shelby County at least, the answer to the question of "overhead or underground" seems to be loud and clear-underground.
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