In case of mechanical design of over head line, main components of overhead lines are (i) conductors, (ii) supports, (iii) insulators. Beside these (a) bracket and cross arm, (b) stray and strat, (c) lighting arrestor, (d) damper, (e) earthing, (f) guard, (g) danger board etc. are also vital component in over head line.

Conductor: A conductor is one, trough which electricity conducts. It plays an important role from economic point of view. So, we should be careful for selecting proper conductor material.
Properties of good conducting materials:
(i) High electrical conductivity.
(ii) High tensile strength.
(iii) Low specific gravity.
(iv) Low cost.
(v) Free from atmospheric hazard.

Solid and Stranded conductor:
Line conductors are two types, solid and stranded. Solid conductors are made by single conductor whereas stranded conductors are made by group of conductor. Solid wires are used only when x-section is small, otherwise in case of larger cross section and longer span, continuous vibration, and swinging may produce fracture at support. In case of stranded conductor, one conductor is main and group of conductors surrounding it make the whole conductor flexible. Every conductor inside a stranded conductor is known as strand and total no of strand is 3, 7, 19, 37, or 61.

If there are n layers, total number of individual wires are 3n(n+1)+1.
And if the diameter of one strand is d, then total radius of stranded conductor is (2n+1)d.
Conductor Materials: Depending upon electrical, mechanical and economical condition, common conductor materials are,
1. Copper: It has high electrical conductivity as well as tensile strength. Specific resistance of hard drawn conductor is 1.72 µΩ/cm3. It has high current density too, but the cost of copper is too high and also it is not easily available. So the use of copper is minimum in now a day.

2. Aluminium: The specific resistance of hard drawn aluminium is 2.83 µΩ/cm3. Nthe weight is much lighter than copper and cost also much lower. But conductivity and tensile strength also much lower than copper. Conductivity of aluminium is 60% that of copper and tensile strength is 50%.

Comparisons between copper and aluminium for choosing conductor materials:

The aluminium conductor mainly uses are two types
(a) Aluminium conductor steel reinforced (A.C.S.R): To increase the tensile strength of aluminium conductor, it is reinforced with a core of galvanized steel wires. The composite conductor with high tensile strength and lower sag is known as steel cored aluminium or aluminium conductor steel reinforced abbreviated as A.C.S.R. the x-section of steel (central core) and aluminium(outer layer) are same.

In transmission system normally used A.C.S.R conductors are

(b) Aluminium stranded conductor (A.S.C) or all aluminium conductors (A.A.C): This type of stranded conductors is made by only aluminium wire. The numbers of strands are 3,7,19, or37. As only aluminium is uses, the tensile strength is much lower. It is uses in distribution system with less span between poles.

3. All aluminium alloy conductor (A.A.A.C): aluminium, manganese and silicon makes an alloy which has high tensile strength. No steel conductor is required as it has high tensile strength. The weight of A.A.A.C conductor is lower than A.C.S.R conductor, so sag also less. The resistance is lower but conductivity is higher which makes low loss. Weather effect on this conductor is less and melting point is high, so theft is critical. Now a day it is uses.

4. Cadmium copper: With alloying 1 to 2% cadmium with copper makes higher tensile strength with good conductivity ( 15% lower than hard drawn copper). So in very long span this type of conductor is uses. Only disadvantage is it has high cost.

5. Steel conductor: As steel has high tensile strength, galvanized steel conductor may use in long span. The major drawback is its lower conductivity (only 10% than copper). It uses in rural areas with bad climate where economy is main consideration. Now a days these type of conductor are never used.

6. Bronze conductor: Harmful gases like ammonia may damage conductor material. Phosphor bronze or silicon bronze conductor are eligible to protect from ammonia. So, where ammonia is present in the air, this type of conductor is uses.

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