Page 320 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Overhead versus underground system

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.

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.
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.
Corona effect
When the applied voltage exceeds a certain value (critical disruptive voltage) the conductors are surrounded by a
faint violet glow called corona. This is occurred by the ionization of air surrounded by a hissing sound. When the
surface is even the effect of corona is less.
Advantages of corona

1 Acting as valve for the surges
2 Effect of transients produced by surges is reduced
3 Electrostatic stress is reduced
Disadvantages of corona
1 Power loss and radio interference

2 Produce ozone layer, hissing noise.
3 Insulation damage
Methods of reducing corona
1 By increasing conductor size
2 By increasing conductor spacing

3 By using bundled conductor
Skin effect
Skin effect is a tendency for alternating current to flow mostly near the outer surface of a conductor. This effect
become more and more apparent as the frequency increases. This effect is most pronounced in radio frequency
system and transmission lines.
Factors affecting
The skin effect in an ac system depends on a number of factors

1 Shape of conductor
2 Type of material
3 Diameter of the conductor
4 Operational frequency

Ferranti effect
In all electrical systems current flows from the region of higher potential to the region of lower potential to
compensate for the electrical potential difference that exist in the system. In medium distance transmission line or

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116