Page 327 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 12 Fig 13 Fig 14

Overhead poles and method of joining aluminium
conductors

Objectives: At the end of this lesson you shall be able to
• state transmission and distribution by O.H lines
• list out the main components and explain each of them
• state the types of power lines with respect to the classification of voltage
• state sag in O.H lines.

Overhead lines: Electric power, which is generated from generating plant / station to the consumer end is
transmitted and distributed either by means of overhead lines (O.H) or by under ground cables (U.G. cables).
Main components used in O.H lines
An overhead line may be used to transmit or distribute electric power.

i Conductors which carry electric power from the sending end station to the receiving end station.
ii Supports which may be poles or towers and keep the conductors at a suitable level above the ground.
iii Insulators which are attached to supports and insulate the conductors from the ground.
iv Cross arms which provide support to the insulators.
v Miscellaneous items such as phase plates, danger plates, lightning arrestors, anti-climbing wires etc.
Commonly used conductor materials

The most commonly used conductor material for overhead lines are copper, aluminium, steel reinforced aluminium,
galvanized steel and cadmium copper.
All conductors used for overhead lines are preferably stranded in order to increase the flexibility. In stranded
conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18,
24...
Line Supports: The supporting structures for overhead line conductors are various types of poles and towers
called line supports. In general, the line supports should have the following properties:
i High mechanical strength to withstand the weight of conductors and wind loads etc.

ii Light in weight without the loss of mechanical strength
iii Cheap in cost and economical to maintain.
iv Longer life
v Easy accessibility of conductors for maintenance
The line supports used for transmission and distribution of electric power are of various types including wooden,
poles, steel poles, R.C.C poles and lattice steel towers.
Wooden poles (Fig 1): These are made of seasoned wood (sal or ehir) and are suitable for lines of moderate
cross sectional area and of, relatively shorter spans, say up to 50 metres. Such supports are cheap, easily

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