Although the original distribution system pioneered by Thomas
Edison was a direct current low-voltage system installed underground,
the widespread expansion of electric systems was based principally on
the adoption of alternating current (through the application of transformers)
and the very economic overhead type of construction.
While the chief limitation to the adoption of underground systems
is economic, there are other reasons that argue against its selection. The
necessity for ducts, for manholes, and for cables that require expensive
insulation and lead sheaths, short pulls, and a relatively large number
of splices, and the special requirements to make equipment waterproof
and safe for installation underground all tend to make investment costs
several times as great as for overhead systems of comparable characteristics.
Where loads become so great, however, that the number of pole
lines and the congestion of conductors on such lines become impractical
from safety, operational, and appearance viewpoints, there is no alternative
but to place the lines underground. In such areas, traffic conditions
are usually so severe that difficulty is experienced in building and
maintaining overhead systems; moreover, the heavy traffic itself presents
additional hazards from vehicles striking the poles.
While an underground system is not exposed to damage and interruptions
from storms, traffic, etc., on the other hand, when trouble does
occur, it is very much more difficult and time-consuming to locate and
repair than in the overhead system. For this reason, additional provisions
and expenditures are made for maintaining service reliability;
these include duplicate facilities, throwover schemes, networks, etc.
Also, the lesser ability for heat radiation in an underground system does
not permit the loading and overloading of conductors and equipment
possible with overhead systems.
With plastics taking over the functions of insulation and sheathing
in underground cables, and the ability of these materials to be buried
directly in the ground, the economic advantage of overhead systems,
though still favorable, is markedly reduced.
The recent greater emphasis on environment (appearance) also has contributed to a greater pressure
for underground installations. Overhead systems will, however, prevail
to a very great extent for some time, and will be in almost exclusive use
in rural areas.
