Line Construction, Erection and Stringing Practices
This domain tests the field sequence for building an overhead distribution line: survey and route marking, pit excavation, pole erection, cross-arm and insulator fitting, conductor pay-off, stringing, sagging, binding and earthing. It also tests the tools and safety checks used at each stage.
Core rules
- Set out the route and span before excavation. Pit depth and foundation depend on pole type, soil and loading; backfill should be compacted in layers, with concrete or boulders where specified.
- Erect poles vertically, check alignment and set stays at angles and terminal poles before full tension is applied.
- Fit cross-arms and insulators before pay-off. Use running-out blocks and a puller so the conductor is not dragged over ground, sharp edges or live circuits.
- String conductors with a dynamometer or tension control. Sag is checked by template or by calculation. Bind conductors on pin insulators; use clamps or armour rods where specified.
- Maintain the statutory clearance to ground, buildings, roads and other lines. Earth metal supports and fitting hardware as required.
Worked example
A 50 m span has a conductor weighing 0.6 kg/m. The specified sag is 0.8 m. Find the required stringing tension.
Using S = wL²/(8T), rearrange to T = wL²/(8S). T = (0.6 × 50²)/(8 × 0.8) = (0.6 × 2500)/6.4 = 1500/6.4 = 234.4 kg. If the line is tensioned to 300 kg instead, sag becomes (0.6 × 2500)/(8 × 300) = 1500/2400 = 0.625 m. That is less than specified, so the conductor is tighter than intended and support loads are higher.
Traps that cost marks
- Pulling conductor over ground or sharp edges and damaging strands.
- Setting stays after tensioning instead of before.
- Using the wrong span or weight unit in the sag formula.
- Ignoring temperature correction; a conductor sagged cold may lose clearance when hot.
- Leaving temporary connections, tools or conductor tails on the line after stringing.