Comparison Between Overhead and Underground systems

Transmission and distribution of electric power can be carried out by overhead as well as underground systems. Comparison between overhead and underground systems are given below:

  1. Public Safety: Underground system is more safer than overhead system.
  2. Initial Cost:  Underground system is more expensive
  3. Flexibility:  Overhead system is more flexible than underground system. In overhead system new conductors can be laid along the existing ones for load expansion. In case of underground system new conductors are to be laid in new channels.
  4. Working Voltage: The underground system cannot be operated above 66 KV because of insulation difficulties but overhead system can be designed for operation up to 400 KV or  higher even.
  5. Maintenance Cost: Maintenance cost of underground system is very low in comparison with that of overhead system.
  6. Frequency of Faults or Failures: As the cables are laid underground, so these are not easily accessible. The insulation is also better, so there are very few chances of power failures or fault as compared to overhead system.
  7. Frequency of Accidents: The chances of accidents in underground system are very low as compared to overhead system.

  8. Voltage Drop: In underground system because of less spacing between the conductors inductance is very low as compared to overhead lines, therefore, voltage drop is low in underground system.
  9. Appearance: Underground system of distribution or transmission is good looking because no wiring is visible. Due to its good looking, in spite of its higher cost it is adopted in modern cities.
  10. Fault Location and Repairs: Though there are very rare chances of occurring fault in underground system, but if occurs it is very difficult to locate that fault and its repair is difficult and expensive.
  11. Charging Current: On account of less spacing between the conductors the cables have much capacitance, so draw higher charging current.
  12. Jointing:  Jointing of underground cables is difficult so tapping for loads and service mains is not conveniently possible in underground system.
  13. Damage Due to Lightning and Thunderstorm: Underground system is free from interruption of service on account of thunderstorm, lighting and objects falling across the wires.
  14. Surge Effect: In underground system surge effect is smoothed down as surge energy is absorbed by the sheath.
  15. Interference to Communication Circuits: In underground system there is no interference to communication circuits.

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