Definition of Sag

Definition of Sag

Consider an overhead transmission line suspended between the two supports A and B as shown in the Fig. 4.2.1.

The transmission line is not fully stretched but it is allowed to sag down. The point O is the lowest point on the conductor. 

Sag : The difference in levels between point of support and the lowest point on the conductor is called sag.

The sag is denoted by S as shown in the Fig. 4.2.1.

1. Important Points Related to the Sag

It is necessary to note some points regarding the sag which are important from the sag calculation point of view. These points are,

1. When the supports are at the same level, then shape of conductor is catenary. The shape of conductor is called sag-span curve. If the sag is small compared with the span then the sag-span curve is parabola.

2. The tension at any point on the conductor acts tangentially. This is shown in the Fig. 4.2.2. At point P on the conductor, the tension T acts tangentially.

This tension T has two components, one acting horizontally while other acting vertically.

The tangent direction at point O is horizontal thus the tension at point 'O' acts horizontally as shown in the Fig. 4.2.2 and it is denoted as H.

3. The horizontal component of the tension is constant throughout the length of the wire. Thus horizontal component of tension is H at point O which is constant throughtout the length of wire. The horizontal component of tension at point P is also H.

4. The tension at the supports is approximately equal to the horizontal tension acting at any point on the conductor. The horizontal tension acting at any point is H and hence if T is tension at the support A or B then T = H.

Review Question

1. What is sag 1 What are the points to be noted for the calculation of sag ?