Sag in overhead lines

While erecting an overhead line,it is very important that conductors are under safe tension.If the conductors are too much streched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension.In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag.



The difference in level between points of supports and the lowest point on the conductor is called SAG.
The figure above shows a conductor suspended between two equilevel supports A and C.The conductor is not fully stretched but is allowed to have a dip.


The generally sag is calculated as

s=w(l)^2/8T

Where,

S is the sag

l is the length of the conductor

T is the tension in the conductor

Effect of wind and ice loading: The above

 formulae for sag are true only in still air and at

normal temperature when the conductor is acted

by its weight only.However, in actual practice, a

conductor may have ice coating and

simultaneaously subjected to wind pressure.The

weight of ice acts vertically downwards i.e.,in the

same direction as the weight of the

conductor.The force due to the wind is assumed

to act horizontally i.e., at right angles to the

projected surface of the conductor.