A 132 kV transmission line has the following data: Weight of conductor = 700 kg/km; Length of span = 250 m; Ultimate strength = 4375 kg; Safety factor = 2. Calculate the height above ground at which the conductor should be supported. (Ground clearance required is 10 m.) 

The correct answer is option 3):(12.5 m )

Concept:

Sag in a Transmission Line:

• Sag (s) is defined as the vertical difference in level between points of support (most commonly transmission towers) and the lowest point of the conductor. When the same levelled two supports hold the conductor, a bent shape arises in the conductor.
• Sag is very small with respect to the span of the conductor. The Sag span curve is parabolic. The tension in each point of the conductor acts always tangentially

Calculation:

Given,

Weight of the conductor, W = 700/1000 = 0.70 kg

Working tension , T = ultimate strength / safety factor = 4375 / 2 = 2187.5 kg

Span length , l = 250 m

\(Sag\,,s=\dfrac{Wl^2}{8T}=\dfrac{0.70\times250^2 }{8\times2187.5}\)

= 2.5

Ground clearance = 10 m

The conductor should be supported at a height = 10 +2.5

=12.5  m