Underground cables are used for power transmission and distribution in areas where overhead lines are impractical or unsafe — such as densely populated cities, industrial zones, airports, and river crossings. While they cost more to install than overhead lines, they offer superior reliability, safety, and aesthetics.
In this article, you will learn the construction of underground cables (layer by layer), types of cables based on voltage, insulation materials, properties, advantages over overhead lines, and when to use them.
Table of Contents
Why Use Underground Cables?
Underground cables are used where overhead lines cannot be installed due to safety, space, or environmental constraints:
- Densely populated urban areas (safety from electrocution)
- Areas near airports (height restrictions)
- River and sea crossings (submarine cables)
- Industrial plants and substations (compact layout)
- Areas with extreme weather (no wind/ice damage)
- Aesthetic requirements (no visual pollution)
Underground cables eliminate corona discharge losses and are not affected by lightning, wind, or ice loading — making them more reliable than overhead lines in harsh environments.
Construction (Layer by Layer)
An underground cable is built in concentric layers, each serving a specific purpose. From inside to outside:
1. Conductor (Core)
The current-carrying element. Made of stranded copper or aluminium. Stranding provides flexibility for bending during installation. A three-phase cable has three conductors (cores).
2. Insulation
Each conductor is wrapped with insulation of appropriate thickness depending on the operating voltage. This is the most critical layer — it must withstand the full voltage stress between conductor and ground. Common materials: impregnated paper, XLPE, PVC.
3. Metallic Sheath
A continuous tube of lead or aluminium extruded over the insulation. Its purpose is to protect the insulation from moisture, gases, and chemicals in the soil. Even tiny amounts of moisture can drastically reduce insulation resistance.
4. Bedding
A layer of fibrous material (jute or hessian tape) applied over the metallic sheath. It protects the sheath from corrosion and mechanical damage from the armouring layer above it.
5. Armouring
One or two layers of galvanized steel wire or steel tape. Provides mechanical protection against crushing, impact, and rodent damage during and after installation. Some cables (e.g., submarine cables) have heavy armouring.
6. Serving (Outer Sheath)
The outermost layer of fibrous material (jute) or PVC. Protects the armouring from atmospheric corrosion and soil chemicals. Similar to bedding but on the outside.
Construction Summary
Insulation Materials
XLPE has largely replaced oil-impregnated paper in modern installations due to its higher operating temperature, lower maintenance, and no risk of oil leakage.
Types of Cables by Voltage
- Low tension (LT): Up to 1 kV — used for house wiring and distribution
- High tension (HT): 1 kV to 11 kV — used for primary distribution
- Super tension (ST): 11 kV to 33 kV — used for sub-transmission
- Extra high tension (EHT): 33 kV to 132 kV — used for transmission
- Extra super voltage: Above 132 kV — used for bulk power transmission (XLPE insulated)
Properties of Underground Cables
A good underground cable must have the following properties:
- High insulation resistance: To prevent leakage current and ensure safety
- High mechanical strength: To withstand rough handling during laying and soil pressure after installation
- Non-hygroscopic: Must not absorb moisture from soil — moisture drastically reduces insulation resistance and accelerates breakdown
- Non-inflammable: Should not catch fire or support combustion
- Chemical resistance: Must withstand acids, alkalis, and other chemicals present in soil
- Flexibility: Must be flexible enough to bend around corners during installation
- Low thermal resistance: Must dissipate heat generated by I²R losses — poor heat dissipation limits current capacity
Underground Cable vs Overhead Line
Methods of Laying Underground Cables
1. Direct Burial
Cable is laid directly in a trench (typically 0.75–1.5 m deep) and covered with sand, protective tiles, and backfill soil. Cheapest method but difficult to repair.
2. Draw-in System (Duct System)
Concrete or PVC ducts are laid first, then cables are pulled through them. Most common in urban areas — allows easy cable replacement without excavation.
3. Solid System (Trough System)
Cable is laid in a cast iron or concrete trough with a removable cover. Provides good mechanical protection and easy access for maintenance. Used in substations and industrial plants.
FAQs
Why is the insulation thickness of underground cables much greater than overhead lines?
Overhead lines use air as insulation (which is free and has self-restoring properties). Underground cables must provide all insulation through solid/liquid materials. Also, cables operate in close proximity to grounded sheaths and soil, requiring higher insulation stress capability.
Why do underground cables have higher capacitance than overhead lines?
Because the conductor-to-sheath distance in a cable is very small (a few mm of insulation) compared to the conductor-to-ground distance in overhead lines (several meters of air). Capacitance is inversely proportional to the distance between conductors.
What limits the current capacity of underground cables?
Heat dissipation. Unlike overhead lines that are cooled by air, underground cables are surrounded by soil which is a poor thermal conductor. The I²R heat generated must be conducted through insulation, sheath, and soil to the surface. If the cable overheats, insulation degrades and fails.
Why is XLPE preferred over paper insulation in modern cables?
XLPE has higher operating temperature (90°C vs 65°C for paper), doesn't require oil impregnation, is lighter, has lower dielectric losses, and doesn't leak oil if the sheath is damaged. It also allows higher current ratings for the same conductor size.
Can underground cables be used for very long distances?
For AC cables, the high capacitance limits practical length to about 40–80 km (charging current becomes excessive). For longer distances, HVDC cables are used — they don't have capacitive charging current issues. The longest submarine power cable (NordLink) is over 600 km using HVDC.