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Underground vs overhead distribution systems

Short answer

Overhead systems use bare conductors on poles with air insulation; low cost, easy repair, but frequent faults and poor appearance. Underground cables in trenches are expensive, safer, hidden, and far more reliable. Overhead suits rural areas; underground is preferred in dense urban centers for safety and aesthetics.

Compare the features and suitability of the underground and overhead distribution systems.

An overhead distribution system carries bare (or covered) conductors on poles, using air as insulation. An underground system uses insulated cables laid in trenches, ducts or tunnels below the ground.

Comparison of features

FeatureOverhead systemUnderground system
Initial costLowHigh (5–10 times overhead)
InsulationAir (cheap)Cable insulation (costly)
Fault frequencyHigh (lightning, trees, wind, birds)Low (protected from weather)
Fault location and repairEasy and quickDifficult and slow
Current capacityHigher (better cooling)Lower for the same size
Voltage dropHigher reactanceLower reactance, higher capacitance
Flexibility to extendEasy to tap and extendDifficult, needs joints and new trenching
Safety and public hazardRisk of contact and broken wiresVery safe
AppearancePoor (poles, wires)Hidden, good appearance
Space / right-of-wayNeeds poles and clearancesUses road edges and ducts
InterferenceCan affect communication linesNegligible
LifeAbout 25–30 years40 years or more

Suitability

  • Overhead suits rural and semi-urban areas: low load density, long distances, open space, and limited budget. Examples in Nepal are most 33 kV and 11 kV feeders and LT lines outside city cores.
  • Underground suits dense urban areas: high load density, narrow streets, high land and space value, need for safety and appearance, and important consumers needing high reliability. Examples are city centres, airports, industrial estates and heritage sites. The NEA underground distribution projects in Kathmandu Valley (Ratnapark, Maharajgunj and other areas) are examples.
  • Hilly terrain, river crossings and flood-prone ground favour overhead lines. Areas with heavy lightning or tree cover favour underground cables or covered conductors.

In practice the choice is economic: underground becomes justified when the load density is high enough that its cost per kW served, plus the value of better reliability, beats the overhead system.

✓ Answer checked by Claude Opus 5.5 (high effort)

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