Overhead Distribution Lines: Conductors, Supports, Insulators and Hardware
This domain tests the physical overhead line: conductor types, pole and support types, insulators, cross-arms, binders, stays and hardware. You should be able to match a component to its function and recognise why a particular insulator or conductor is selected for 11 kV or 33 kV work.
Core rules
- Common overhead conductors are ACSR (aluminium conductors steel reinforced), AAAC (all aluminium alloy conductor) and copper. ACSR uses a steel core for strength; AAAC uses aluminium alloy for strength and conductivity.
- Supports include RCC/PCC poles, steel tubular poles and lattice structures. Stays and stay wires resist unbalanced pull at angles, terminations and taps.
- Pin insulators are common on 11 kV lines; suspension disc strings are used at higher voltages such as 33 kV and above; shackle insulators are used on LT lines. Materials include porcelain, glass and polymer.
- Hardware includes cross-arms, insulator pins, clamps, binders, armour rods, stay sets, earth wire and danger plates. Hardware must be galvanised and correctly rated.
- Sag depends on conductor weight, span and tension. For a level span, approximate sag S = wL²/(8T), where w is weight per unit length, L is span and T is tension.
Worked example
A 60 m span uses a conductor weighing 0.8 kg/m. The stringing tension is 300 kg. Estimate the sag.
S = wL²/(8T) = (0.8 × 60²)/(8 × 300) = (0.8 × 3600)/2400 = 2880/2400 = 1.2 m. If the same conductor is tensioned to only 240 kg, sag becomes (0.8 × 3600)/(8 × 240) = 2880/1920 = 1.5 m. Lower tension means more sag and lower ground clearance. Higher tension reduces sag but increases conductor and support stress.
Traps that cost marks
- Treating sag as independent of tension; sag rises sharply as tension falls.
- Mixing conductor weight per metre with total span weight.
- Using a pin insulator where a suspension string is required for the voltage level.
- Ignoring temperature: conductors expand in heat, so sag increases on a hot afternoon.
- Forgetting that stays resist tension at angles, not just vertical weight.