Sag in Overhead Line

Sag in Transmission Line:

• The difference between in level between points of supports and the lowest point on the conductor is called Sag.
• The horizontal distance between two electrical supports is called the span.
• When overhead line conductor is hung between two supports, the conductor takes the natural catenary shape.

The conductor Sag should be kept to minimum in order to reduce the conductor material required and avoid extra pole height for sufficient clearance above ground level.

It is also desirable that tension in the conductor should be low to avoid the mechanical failure of conductor. In order to permit the safe tension in the conductors, they are not fully stretched but are allowed to have a Sag.

Factors Affecting the Sag in an OH Line:

• Weight of the conductor: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
• Length of the span: Sag is directly proportional to the square of the span length. Longer span gives more sag.
• Tensile strength: The sag is inversely proportional to the tension in the conductor. 
• Temperature: The sag is reduced at low temperatures and is increases at higher temperatures.
• Wind pressure: It increases sag in the inclined direction.
• Ice weight: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.