Raising a crossarm

If you modify an existing span by raising or lowering the crossarm on one end of the span, what changes occur to the tensions?

Power lines are modelled as catenary curves.

“Although commercially available wires and cables are not truly flexible, they will, in very short spans, conform closer to a catenary than to any other curve. In longer spans, the conductors may be considered as truly flexible since they will sag in the shape of a catenary curve.”1

AS/NZS 7000:2016 gives equations for the catenary curve in Appendix R.

Consider a level span with these properties:

  • Krypton conductor, weight 0.433 kg/m
  • Conductor breaking load 37.4 kN
  • 60m span
  • strung at 10% CBL at 15°C, standard temperature 15°C

Stringing tension at 10% CBL is 3.74 kN = 3740 N

W (mass/meter) = 0.433 × 9.81 = 4.248 N/m

Catenary constant \(C=\frac{H}{W}\tag{R8}\)

C = 880.47 m

Conductor catenary length \(S=\sqrt{\left ( 2C sinh\frac{L}{2C} \right )^{2}+h^{2}}\tag{R21}\)
S = 60.012 m ie only 12mm longer than the span length.

With the length of conductor being only slightly longer than the span length, raising one crossarm creates a problem. Suppose you want to raise the crossarm 1.5m. A triangle with sides of 60 and 1.5 gives the hypotenuse of 60.019. So a straight line between the new attachment positions is longer than the available length of conductor.

Depending on the configuration you may not be able to raise one crossarm with the conductor intact—it is more likely you will need to sleeve in an additional length of conductor and therefore you can retension the span as desired.

  1. H Farr, Transmission Line Design Manual, Denver, US Dept. of the Interior, 1980, p. 15