Page 323 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

3 Advantages and disadvantages (Fig 8)

Advantages
1 Less subject to damage from severe weather conditions (mainly lightning, wind and freezing).
2 Underground cables pose no hazard to low flying aircraft or to wildlife.
3 Much less subject to conductor theft, illegal connections, sabotage, and damage from armed conflict.

Disadvantages
1 Underground cable locations are not always obvious, which can lead to unwary diggers damaging cables or
being electrocuted.

2 Operations are more difficult since the high reactive power of underground cables produces large charging
currents and so makes voltage control more difficult.
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
Underground cables offer an affordable and justifiable solution for critical parts, and in some cases the entire
length, of overhead high voltage power lines. With appropriate technology used in appropriate places, the
environmental impact of underground cables can be minimised.

Fig 8

Line insulators

Objectives: At the end of this lesson you shall be able to
• explain the types of insulators and their uses.

Line insulators
The aim of using a line insulator in an overhead line is to hold the live conductor to prevent leakage of current from
the conductor to the pole. These are made of porcelain clay and are thoroughly glazed to avoid the absorption
of moisture from the atmosphere.
Properties of insulators
i High mechanical strength in order to withstand conductor load, wind load etc.
ii High electrical resistance of insulator material in order to avoid leakage currents to earth.

iii High relative permittivity of insulator material in order that dielectric strength is high.
iv The insulator material should be non - porous, free from impurities and cracks otherwise the permittivity will be
lowered.

v High ratio of puncture strength to flash over.
The most commonly used material for insulators of overhead line is porcelain but glass, steatite and special
composition materials are also used to a limited extent.

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