Electric distribution circuits have been installed underground in large cities for many years to serve the central business areas. The electric load density in the central business areas is high enough to justify the expenses associated with the conventional underground system employing the use of conduits encased in concrete, manholes, vaults with submersible transformers, network protectors, and paper-insulated, lead-covered, or polyethylene-insulated, shielded, and jacketed high-voltage cables.

The conventional underground system cannot be economically justified to serve the low electric load densities found in residential areas. Direct-buried cable systems with pad-mounted transformers, submersible transformers in fiber vaults, or direct-buried transformers can be installed in residential areas at costs that are economically feasible. The converting of overhead electric distribution circuits to underground in residential areas is generally not economically feasible. Most of the conversion programs executed have been associated with renewal projects where it was necessary to remove the overhead electric facilities as a part of the demolition work.

Underground lines are relatively immune to some of the major causes of failures in overhead circuits, such as power-pole accidents; damage from lighting, wind, ice, and snowstorms; or contacts to the wires by trees or other foreign objects. Underground circuits have their own 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 be filled with water containing contaminants. Underground cables and equipment are both vulnerable to...