The difference in level between points of supports and the lowest point on the conductor is called sag in overhead transmission lines.
While erecting an overhead line, it is very important that conductors are under safe tension. If the conductors are too much stretched 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.
Figure 1 shows a conductor suspended between two equal levels supports A and B. The conductor is not fully stretched but is allowed to have a dip. The lowest point on the conductor is O and the sag is S. The following points may be noted :
- When the conductor is suspended between two supports at the same level, it takes the shape of catenary. However, if the sag is very small compared with the span, then the sag-span curve is like a parabola.
- The tension at any point on the conductor acts tangentially. Thus tension To at the lowest point O acts horizontally as shown in figure 2.
- The horizontal component of tension is constant throughout the length of the wire.
- The tension at supports is approximately equal to the horizontal tension acting at any point on the wire. Thus if T is the tension at the support B, then T = To.
Conductor Sag and Tension
Conductor sag and tension are an important consideration in the mechanical design of overhead lines.
The conductor sag should be kept to a minimum in order to reduce the conductor material required and to 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 the conductor and to permit the use of less strong supports.
However, low conductor tension and minimum sag are not possible. It is because low sag means a tight wire and high tension, whereas a low tension means a loose wire and increased sag. Therefore, in actual practice, a compromise in made between the two.
Steel core strands have been used to increase the tensile strength and reduce thermal sag of bare overhead conductors to accommodate longer spans between fewer or shorter structures.