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Mechanical Design of Overhead 


The main components of an overhead line are the support ( Poles or tower ), cross arms and clamps, insulators, Conductors, Guys and stays, lighting arrestors, fuses and isolating switches, Continuous Earth wire, vee guards, guard wire, phase plates, bird guards, barbed wire, danger plates, beads for jumpers, vibration dampers etc. 

Line Supports :- 

The support used for transmission and Distribution of Electrical power must have the characteristics high mechanical strength, light in weight, low initial as well as maintenance cost, longer life, good looking and easily accessible for painting and erection of line Conductors. The line Supports are of various types including wood, steel, reinforced concrete poles and lattice steel towers. 

Wooden Poles :- 

Wooden pole
Wooden pole

These are cheapest, easily available, providing insulating properties and there fore, are extensively used for distribution purposes, specially in rural areas, keeping the cost low. Their used is usually ( up to 60 meter ). 

The Wooden poles, well impregnated with creosite Oil or any Preservative compound, have life from 25 to 30 years. The Disadvantages of such as support is that these need Periodical inspection because they tend to rot and their life is short.

Steel Poles :- Steel Poles are of three types 

A) Rail poles

B) Tubular Poles and

C) Rolled Steel joists

Theses poles posses geater mechanical strength and so permit use of longer spans
( 50 - 80 m ) but cost is Higher. 

The average life of steel Poles is more than 40 years.

RCC Poles :- These give good Outlook, need no maintenance, have got insulating properties and resistance against chemical acting, Very strong and can be used for longer spans ( 80 - 200m ) and have Very long life.

Since these poles are Very heavy, transportation cost is heavy and require care in handling and erection 

Lattice Steel Towers :- These are mechanically stronger and have got longer life. 
Due to robust Construction long spans 
( 300m and above ) can be used and are much use full for crossing field, valleys, railway lines, revers etc.

Even though these are two to four times costlier than wooden poles, yet for all supports and longer spans these prove economical.

These towers need Periodical painting or galvanising for protection against corrosion.

Narrow - base lattice steel towers are used for transmission at 33 kv and board - base lattice steel towers are used for transmission at 66 kv and above.

Conductor Materials 

The conductor Materials used for Transmission and Distribution of Electrical power should have the characteristics - high Conductivity, high tensile strength in order to withstand mechanical stresses, low specific gravity in order to give low weight per unit volume, low cost and no brittleness.

Aluminum Conductor
Aluminum Conductor

Copper, aluminum, steel and steel - cored aluminum, galvanized steel Conductors are
Generally used for this purpose. 

All Conductors used for overhead lines are preferably stranded in order to increase the flexibility.

Solid wires, except of smaller sizes, are difficult to handle and when employed for long spans tend to Crystallise at the points of Support because of swinging in winds.

Hard drawn copper Conductor is the best Conductor owing to its high Electrical Conductivity and great tensile strength and is ideally suitable for Transmission and Distribution, but owing to the difficulty of importing it due to lack of foreign exchange in our country, the trend nowadays is to use aluminum in place of copper.

Aluminum is cheaper in cost and lighter in weight but poor in Conductivity and tensile strength.

Usually aluminum Conductors are used for Transmission of heavy current, where Conductors size is large and it's cost form a large Proportion of the total cost of the complete installation.

Steel core aluminum (ACSR) Conductor consists of a core of galvanized steel strand surrounded by a number of aluminum strands.

The steel core takes a larger percentage of mechanical stresses while the aluminum carries the bulk of the current.

ACSR Conductor being of high tensile strength and lighter in weight Produces small sag and therefore, long spans can be used. 
Copper materials
Copper materials

Galvanized steel Conductors have been used for extremely long spans. There are found most suitable in rural areas, where cheapness is the main Consideration.

The Conductors is not suitable for EHT lines for the purpose of transmitting large power over a long distance because of poor Conductivity and high internal Reactance.
Galvanized steel wire is generally used as stay wire, Earth wire and structural components.

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