Overhead vs. Underground Power Distribution Systems

A power distribution system is the electrical system between the sub-station fed by the transmission system and the consumer’s meters.

A distribution system generally is made up of feeders, distributors and the service mains. In this case, a feeder is a conductor which connects the substation (localized generating station) to the area where the power is to be distributed. Normally, no tapings are taken from the feeder; thus, the current in it remains the same throughout. The current carrying capacity is the main factor taken into consideration when designing a feeder.  A distributor is a conductor from which tappings are taken for supply to the consumers. The current through a distributor is usually not constant because tappings are taken at various places along its length. The voltage drop along the length of a distributor is the key consideration during its design. Service mains are normally the small cable which connects the distributor to the consumer’s terminals.

Related: What is the difference between a Power Transformer & Distribution Transformer?

Types of Distribution Systems

Distribution systems can be categorized based on the supply (DC distribution system and AC distribution system) or according to the construction (overhead and underground distribution systems). In this article, we will focus on the second classification and compare the two types under that category.

Overhead Distribution System

The overhead system is used for distribution. It is typically 5 to 10 times less expensive than the underground system. The spacing is usually provided between the conductors, at the supports & intermediate points. This spacing provides insulation which avoids an electric discharge between the conductors.

Underground Distribution System

This type of distribution system is employed in places where overhead construction is unfeasible, for example, it is preferred in crowded areas. In this system, line surges are suppressed by using cables, and also the utility company can save on transformers and generators from the damage due to line surges.

Overhead vs. Underground Distribution System

Table 1: Comparing Overhead system with Underground System

Overhead Distribution System	Underground Distribution System
Mainly used for distribution as it is 5 to 10 times less costly than the underground system.	Utilized in places where overhead construction is not feasible.
The voltage level used in overhead system can be as high as 400KV.	The voltage level used in underground system is below 66KV (due to difficulties in insulation).
The volume of copper needed is ¼ the volume of copper required for 2-wire DC system.	The volume of copper needed is same as that required for two wire DC system.
It is hard to locate the exact point of fault if the transmission lines are very long.	It is easier to find the exact point of fault in comparison with the overhead system.
The overhead lines can be easily repaired from the faults occurring due to short circuits, breakage of line, and so forth.	The underground lines are difficulty to repair from faults occurring due to short circuits, breakage of line, etc.
Maximum stress exists between the conductor and the supporting structure.	Maximum stress exists on the insulation between the conductors.
The transmission by overhead system is less costly.	The transmission by underground system is expensive.
High maintenance cost of the overhead system.	The maintenance cost of the underground system is less.

Related articles:

• Features and Classification of Substations
• Types of Insulators used in Power Systems
• Types of Busbar Arrangements in Grid Stations and Substations
• Key Factors to Consider In Substation Design
• Power System Protection
• Switchgear