Ruling span and surveying requirements

Ruling span

In a line section between two tension (anchor) towers, the suspension strings let the tension equalise across spans of unequal length. The ruling (equivalent) span is the single hypothetical span whose tension changes with temperature and loading in the same way as the whole section. It is used to prepare the stringing (sag–tension) charts:

$$L_r = \sqrt{\frac{L_1^3 + L_2^3 + \dots + L_n^3}{L_1 + L_2 + \dots + L_n}}$$

A simpler approximation is $L_r \approx L_{avg} + \frac{2}{3}(L_{max} - L_{avg})$.

Surveying requirements for T&D line design

1. Reconnaissance survey: study topographic maps or satellite images and visit the site. Identify alternative routes, avoiding forests, settlements, protected areas, landslide zones and airports.
2. Preliminary (walk-over) survey: choose the best route for length, access, crossings and right-of-way cost. Fix the angle points.
3. Detailed survey:
   • Traverse along the route with total station or GPS. Record angle points and deviation angles.
   • Take the longitudinal profile along the centre line and cross-sections on slopes.
   • Plot the profile and use a sag template to place towers.
4. Tower spotting: place towers on the profile using the hot-curve (maximum sag) and cold-curve (uplift) templates. Ground clearance must be kept everywhere. This also gives the ruling span of each section.
5. Check survey: stake out the tower locations and confirm the clearances to roads, power and telecom lines, rivers and buildings.
6. Soil investigation: test the soil at the tower locations to classify it for foundation design.
7. Right-of-way and land records: identify land owners and the trees to cut, for compensation and permits.
8. Distribution lines: survey the road-side routes, the load locations for transformer siting, and the pole spans (about 50–80 m).