Page 318 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Various transmission systems

1 Overhead transmission system
2 Underground transmission system
Overhead transmission system
1 Types of OH lines

i Line in horizontal
ii Line in vertical
2 Materials used in oh system
i Poles
ii Cross arm brackets
iii Wires
iv Insulators
v Stay rods and stay wires
vi Guards

Poles
The selection of a particular type depends on the number and weight of wires used type of ground and type of
work.

Type of poles used are
1 Wooden pole – This type of poles is used for temporary electric lines. The length of wooden pole is kept 6-9
meters out of which 1-2 meters buried in the ground.

2 Cement concrete pole – he length of the pole is kept7-10 meters out of which 1.5 – 2.5 meters length is
buried in the ground
3 Steel pole – These poles are made in different shapes they are
a Rail line type pole
b Tubular pole
c Compound pole
d Tower
Tower
For EHT lines lattice structured towers are used. Their height is kept more than the other type poles and they are
constructed by joining a number of angle iron pieces
3 Cross arm brackets
The wires and the insulators are balanced on both sides of cross arm brackets. They are fitted on the top side of
the pole. The 3 length of a cross arm brackets should be kept such that the wires may not touch each other due
to wind and storm. The brackets are fitted with I type clamps made of iron.
4 Wires

Aluminum, copper and steel wires are used for installation of overhead lines. The use of single conductor wire
is limited to 6 SWG and the wires thicker than this are always of stranded type. The strength of stranded cable
can be increased by 10 times by using one steel wire. This type wire is known as ACSR wire and mostly used for
EHT lines.
5 Insulators

Porcelain or glass insulators are mounted on the cross arm so as to isolate the wires from the cross arm, pole
etc. The wires are tied on the insulators. Polished porcelain insulators are preferred because of their increased
insulation strength.

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