--- title: "Underground vs overhead distribution systems" subject: "Transmission and Distribution System Design" chapter: 5 key: "semester_8/transmission_and_distribution_system_design:underground-vs-overhead-distribution" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/underground-vs-overhead-distribution site: BCT Engineering --- # Underground vs overhead distribution systems Transmission and Distribution System Design · Chapter 5: Electric Power Distribution - Asked in: 2080 Chaitra, 6 marks; 2079 Chaitra, 3 marks; 2078 Kartik, 6 marks; 2074 Magh, 4 marks (asked 4 times) - Video: [Difference between Overhead and Underground distribution systems || DISTRIBUTION SYSTEMS || PS 1](https://www.youtube.com/watch?v=aWmga5qlz8s) — E4J Lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/underground-vs-overhead-distribution - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-5 ## Question Compare the features and suitability of the underground and 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. ## Answer 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 | Feature | Overhead system | Underground system | |---|---|---| | Initial cost | Low | High (5–10 times overhead) | | Insulation | Air (cheap) | Cable insulation (costly) | | Fault frequency | High (lightning, trees, wind, birds) | Low (protected from weather) | | Fault location and repair | Easy and quick | Difficult and slow | | Current capacity | Higher (better cooling) | Lower for the same size | | Voltage drop | Higher reactance | Lower reactance, higher capacitance | | Flexibility to extend | Easy to tap and extend | Difficult, needs joints and new trenching | | Safety and public hazard | Risk of contact and broken wires | Very safe | | Appearance | Poor (poles, wires) | Hidden, good appearance | | Space / right-of-way | Needs poles and clearances | Uses road edges and ducts | | Interference | Can affect communication lines | Negligible | | Life | About 25–30 years | 40 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does initial cost compare? A. Underground is cheaper B. They are equal C. Overhead is 5-10 times costlier D. Underground is much higher (5-10 times overhead) Answer: D. Cable insulation and trenching make underground costly. (Key point: Initial cost) 2. Which statement is correct? A. Overhead has more faults but is easy to locate and repair B. Underground has more faults C. Underground faults are repaired quickly D. Overhead lines have no faults Answer: A. Overhead is exposed to weather, but faults are easy to find; underground has few but hard-to-locate faults. (Key point: Faults and repair) 3. Where does overhead suit best? A. City centres and airports B. Heritage sites C. Rural and semi-urban areas with low density and long distances D. Dense urban cores Answer: C. Low load density, long distances and limited budget favour overhead. (Key point: Overhead suitability) 4. Why is underground used in dense urban areas? A. It is cheapest there B. High load density, little space, safety, appearance and reliability needs C. It has higher current capacity D. It is easy to extend Answer: B. These factors justify the high cost in cities. 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