High-voltage underground cables have been installed in response to adverse public response to the visually offensive high-rise transmission towers in or close to populated communities. Underground cables rated for voltages as high as 500 kV have been developed.
They were first placed in service in the United States in 1976. Traditionally, underground cable systems have been installed in cities and other heavily populated areas, where open high-voltage lines present a safety hazard.
They have also been installed where overhead lines were not practical, in locations such as air port approaches because of aircraft safety issues, or water crossings where overhead lines are not feasible because of interference with water traffic.
For crossing large bodies of water, trenches are dug or dredged to depths related directly to the voltage being carried by the cable, and the crossings are marked near the shore lines. Extruded dielectric cables have become the U.S. standard for voltages to 161 kV.
Low-pressure cables have hollow cores for the circulation of oil under low pressure. The oil provides temporary protection of the enclosed wires from water damage should the cable sheath develop a leak.
High-pressure oil-filled pipe-type cables are commonly installed for 230- and 345-kV applications in the United States. Oil is circulated in the pipe under high pressure (14 kg/cm2 or 200 psi).
Most new cable installations make use of extruded dielectric, but pipe-type cables account for 75 percent of the approximately 2400 circuit miles now in service.
From 15 to 20 percent of the cable is extruded dielectric, and most of the remainder is self-contained fluid filled cable.