What is the Sag in Transmission Lines and How to Calculate it ( Application of Sag Template)

The Sag in Transmission Lines is applied to the ground profile by moving the same horizontally while always ensuring that the vertical axis or centerline is held vertical.  The sag of high voltage transmission line can simply define as the difference between one level point of and the lowest point of the conductor. The Structure Positions are marked where the tower footing curve cuts the ground profile, while the ground clearance curve is just clear and above the ground profile. The ground clearance curve shall not only clear the route centerline ground profile but also the ground profile to the left or right of the centerline up to a distance equal to the maximum swing of conductor including cross-arm spread on either side.

Besides normal ground clearance, the clearance between power conductor and objects like, other power or telecommunication lines, houses, trolley wires, roads, railway tracks, ‘anal embankments etc., shall be checked. Extra clearance of power line can be got either by reducing the span or by providing lower body extension which is depending on which alternative is most economical.

The weight span on either side of the tower for practical purposes is obtained by marking the low points of the sag in the two adjacent spans and sealing the same off.

On steep sloping spans, the low points may fall beyond the lower support; this indicated that the conductor in the uphill span exerts a negative or Upward pull on the lower tower.

The amount of this upward pull is equal to the weight of the conductor from the lower tower to the low point in the Sag in Transmission Lines , Should the upward pull of the uphill span be greater than the downward load of the next adjacent span, actual uplift would be caused and the conductor Would tend to swing clear of the tower upward.

For any easy check whether a tower is under uplift or not, the following method may be adopted. The template is applied horizontally to the tops of alternate supports coincide with the cold template. If the curve is above the intermediate support, the support is under uplift and has to be extended Until it touches the cold template and so eliminates uplift.

If request standard body extensions are not available for extending the tower to desired height, a tower designed for uplift shall have to be provided.

The suspension tower shall be checked for uplift under normal working condition only, that is both adjacent spans intact. The section and angle tower shall be checked for uplift under both normal and broken wire condition.


Weight Span Calculation of Sag in Transmission Line

Sag in Transmission Lines

The analytical method for calculating weight span is given below:

The distance of ‘Null point’ or ‘Low point’ of the conductor from a center of the span is given by the formula.

x = (Th) / (w l)


x = distance of a low point from the center of the span

T = conductor tension in kg

h = difference between conductor support level in “m”

l = span length in “m”

So Weight span:

For towers A, right-hand side only: a = l/2 – x

For tower B, left hand side only : b= l/2 + x

Similarly, weight span for the other side of towers can calculate. If the sum of ‘a’ and ‘b’ calculated for a particular tower is negative, the tower is under ‘uplift’.

t is also evident that maximum weight spans are obtained by the worst condition of wind loading when T is maximum, which means the vertical component of worst load Sag in Transmission Lines should be taken for ‘cold curve’ of sag template in order to assess uplift on towers.


One thought on “What is the Sag in Transmission Lines and How to Calculate it ( Application of Sag Template)

  • June 29, 2017 at 3:06 pm

    sir what is formula to calculate electric clearance between two overhead line crossing


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