Overhead Versus Underground System:
The distribution system can be overhead or underground. Overhead
lines are generally mounted on wooden, concrete or steel poles which are
arranged to carry distribution transformers in addition to the conductors. The
underground system uses conduits, cables and manholes under the surface of
streets and sidewalks. The choice between overhead and underground system
depends upon a number of widely differing factors. Therefore, it is desirable
to make a comparison between the two.
(i)Public safety: The underground system is more safe than overhead system
because all distribution wiring is placed underground and there are little
chances of any hazard.
(ii) Initial cost: The underground system is more expensive due to the high
cost of trenching, conduits, cables, manholes and other special equipment.
The initial cost of an underground system may be five to ten times than that
of an overhead system.
(iii) Flexibility: The overhead system is much more flexible than the
underground system. In the latter case, manholes, duct lines etc., are
permanently placed once installed and the load expansion can only be met by
laying new lines. However, on an overhead system, poles, wires, transformers
etc., can be easily shifted to meet the changes in load conditions.
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(iv) Faults: The chances of faults in underground system are very rare as the
cables are laid underground and are generally provided with better insulation.
(v) Appearance: The general appearance of an underground system is
better as all the distribution lines are invisible. This factor is exerting
considerable public pressure on electric supply companies to switch over to
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underground system.
(vi) Fault location and repairs: In general, there are little chances of faults
in an underground system. However, if a fault does occur, it is difficult to
locate and repair on this system. On an overhead system, the conductors are
visible and easily accessible so that fault locations and repairs can be easily
made.
(vii) Current carrying capacity and voltage drop: An overhead
distribution conductor has a considerably higher current carrying capacity
than an underground cable conductor of the same material and cross-section.
On the other hand, underground cable conductor has much lower inductive
reactance than that of an overhead conductor because of closer spacing of
conductors.
(viii) Useful life: The useful life of underground system is much longer than
that of an overhead system. An overhead system may have a useful lifeof 25 years, whereas an underground system may have a useful life of more
than 50 years.
(ix) Maintenance cost: The maintenance cost of underground system is
very low as compared with that of overhead system because of less chances
of faults and service interruptions from wind, ice, lightning as well as from
traffic hazards.
(x) Interference with communication circuits: An overhead system
causes electromagnetic interference with the telephone lines. The power line
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currents are superimposed on speech currents, resulting in the potential of the
communication channel being raised to an undesirable level. However, there
is no such interference with the underground system.
It is clear from the above comparison that each system has its own
advantages and disadvantages. However, comparative economics ( i.e.,
annual cost of operation) is the most powerful factor influencing the choice
between underground and overhead system. The greater capital cost of
underground system prohibits its use for distribution. But sometimes noneconomic factors ( e.g., general appearance, public safety etc.) exert
considerable influence on choosing underground system. In general,
overhead system is adopted for distribution and the use of underground
system is made only where overhead construction is impracticable or
prohibited by local laws.