Transmission and Distribution Lines
Domain 3 - Transmission and Distribution Lines. Questions cover supports, spans, sag and the statutory clearances that decide whether an erected line is acceptable.
Line basics
- 11 kV HT lines use PSC or steel tubular poles with pin insulators; 33 kV and above use lattice towers or RS joists carrying disc strings.
- Typical spans run 50–65 m on LT and 60–100 m on 11 kV, longer on transmission lines.
- For a level span, sag S = wL²/(8T), where w is weight per unit length, L the span and T the tension. Sag increases with temperature, ice and wind, and falls as tension rises.
- Clearance above ground, roads, buildings and other lines is fixed by IE Rule 77 and must be verified after stringing. Extra clearance comes from taller poles, not from over-tensioning a conductor.
- Guard wires, barbed-wire cradles and Danger plates are required at road crossings and wherever the public can reach the line.
Worked example
A 60 m span of conductor weighing 1.0 kg/m is strung at 600 kgf tension: S = (1.0 × 60²)/(8 × 600) = 3600/4800 = 0.75 m. A 9 m pole buried 1.8 m leaves 7.2 m; subtract a 0.5 m cross-arm-and-insulator stack and 0.75 m sag, giving 5.95 m at mid-span. That clears a 5.2 m minimum over open ground, but a road crossing needing more height would fail and require a taller support.
Traps
- Reversing the sag–tension relationship: higher tension gives lower sag.
- Mixing w in kg/m with T in newtons without converting.
- Checking clearance at the pole instead of at mid-span, where sag is greatest.
- Ignoring temperature: sag set on a cold morning can become excessive by afternoon.