Undergrounding refers to the replacement of overhead cables providing electrical power or telecommunications, with underground cables. This is typically performed for aesthetic purposes, and increases distribution cost.
The aerial cables that carry high-voltage electricity (e.g., 400 kV) and are supported by large pylons are generally considered the least attractive feature of the countryside but are some of the most expensive to lay underground.
Underground cables can assist the transmission of power across:
Densely populated urban areas
Areas where land is unavailable or planning consent is difficult
Rivers and other natural obstacles
Land with outstanding natural or environmental heritage
Areas of significant or prestigious infrastructural development
Land whose value must be maintained for future urban expansion and rural development
Other advantages include:
Less subject to damage from severe weather conditions (mainly lightning, wind and freezing)
Greatly reduced emission, into the surrounding area, of electromagnetic fields (EMF). All electric currents generate EMF, but the shielding provided by the earth surrounding underground cables restricts their range and power. See section below, health concerns.
Underground cables need a narrower surrounding strip of about 1–10 meters to install, whereas an overhead line requires a surrounding strip of about 20–200 meters wide to be kept permanently clear for safety, maintenance and repair.
Underground cables pose no hazard to low flying aircraft or to wildlife, and are significantly safer as they pose no shock hazard (except to the unwary digger).
Much less subject to conductor theft, illegal connections, sabotage, and damage from armed conflict.
Disadvantages include:
Undergrounding is more expensive, since the cost of burying cables at transmission voltages is several times greater than overhead power lines, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line. Above ground lines cost around $10 per foot and underground lines cost in the range of $20 to $40 per foot.
Whereas finding and repairing overhead wire breaks can be accomplished in hours, underground repairs can take days or weeks, and for this reason redundant lines are run.
Underground power cables, due to their proximity to earth, cannot be maintained live, whereas overhead power cables can be.
Operations are more difficult since the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.
The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.