Underground Cables: Answers for Beginners

Underground cables play an important part in electrical installation. In this article, I will explain what you should know about underground cables. Let’s get started.

underground cables construction

The construction of underground cables involves several key steps to ensure a reliable and durable installation.

The process may vary based on factors such as the type of cable, local regulations, soil conditions, and the specific requirements of the project. Here is a general overview of the construction process for underground cables:

  1. Site Survey and Planning:
    • Conduct a thorough site survey to assess the terrain, soil conditions, and potential obstacles. Develop a detailed plan that includes the route of the cable, burial depth, and any necessary permits or approvals.
  2. Trenching:
    • Excavate a trench along the planned route for the underground cable. The dimensions of the trench depend on factors such as the size and type of cable, as well as local regulations.
  3. Bedding Installation:
    • Place a layer of bedding material at the bottom of the trench. The bedding material, often composed of sand or a sand/gravel mixture, provides support for the cable and helps distribute the load.
  4. Cable Installation:
    • Lay the cables into the trench on top of the bedding material. Carefully position the cables according to the project specifications. Larger projects may involve the installation of multiple cables, each serving a different purpose.
  5. Backfilling:
    • Surround the cables with additional layers of sand or other suitable backfill material. This helps to further protect the cables and provides stability to the trench.
  6. Sealing and Protection:
    • In some cases, especially when using lead sheaths, additional sealing and protection measures may be applied. This can include lead sheaths for moisture resistance and protection against external factors.
  7. Termination and Jointing:
    • Install cable terminations and joints where necessary. Proper termination and jointing are critical for maintaining the integrity of the electrical connection and ensuring the reliability of the cable system.
  8. Testing:
    • Perform tests to ensure the integrity and performance of the installed cables. This may include insulation resistance tests, continuity checks, and other electrical tests to verify that the cable meets specified standards.
  9. Surface Restoration:
    • Restore the surface above the trench. This involves backfilling the trench with soil and restoring any pavement or landscaping that was disrupted during the installation.
  10. Documentation:
    • Maintain detailed documentation of the cable installation, including “as-built” drawings and records of test results. This documentation is essential for future maintenance, repairs, or expansions of the cable system.

Throughout the construction process, adherence to safety standards and environmental regulations is crucial. It’s also important to consider factors such as cable type, voltage level, and the specific requirements of the project.

The construction of underground cables requires coordination among various stakeholders, including engineers, contractors, and regulatory authorities, to ensure a successful and compliant installation.

When are underground cables better than OHTL?

The choice between underground cables and overhead transmission lines (OHTL) depends on various factors, and each option has its advantages and disadvantages. Here are some key considerations for both:

  1. Cost:
    • Underground Cables: Generally, the installation of underground cables is more expensive than overhead lines. Excavation and burial costs, as well as the cost of the cables themselves, contribute to the higher overall expense.
    • OHTL: Overhead transmission lines are typically less expensive to install than underground cables. The cost of materials and installation for poles and conductors is generally lower.
  2. Aesthetics:
    • Underground Cables: Underground cables are hidden from view, providing a more aesthetically pleasing landscape. This can be an important factor in urban or scenic areas.
    • OHTL: Overhead lines can be more visually intrusive, especially in residential or environmental areas, and may face opposition from communities concerned about visual impact.
  3. Reliability:
    • Underground Cables: Underground cables are less susceptible to weather-related disturbances, such as lightning and high winds, leading to higher reliability. They are also less prone to physical damage from external factors like falling trees.
    • OHTL: Overhead lines may be more vulnerable to weather conditions, especially in areas prone to storms, high winds, or ice. However, they are generally easier to access for maintenance and repairs.
  4. Maintenance:
    • Underground Cables: Maintenance of underground cables can be more challenging and costly. Locating and repairing faults may require specialized equipment and expertise.
    • OHTL: Overhead lines are generally easier to access for routine maintenance and repairs. Line inspections and repairs can be carried out more quickly compared to underground systems.
  5. Environmental Impact:
    • Underground Cables: The installation of underground cables may involve some environmental disruption during the excavation process. However, once installed, they have minimal impact on the visual environment.
    • OHTL: Overhead lines can have a more noticeable impact on the landscape, and there may be concerns about the impact on wildlife and vegetation.
  6. Capacity and Voltage:
    • Underground Cables: Underground cables may have limitations on capacity and voltage compared to overhead lines. This can be a consideration for large-scale transmission projects.
    • OHTL: Overhead lines can typically accommodate higher capacities and voltages, making them suitable for long-distance and high-capacity transmission.

The choice between underground cables and OHTL is often made based on a combination of these factors, and the specific requirements of a project or the preferences of stakeholders involved. It’s not a one-size-fits-all decision and depends on the circumstances of each individual case.

Why underground cables are laid at a certain depth?

Underground cables are laid at a certain depth for several reasons, and the specific depth can vary depending on factors such as local regulations, soil conditions, and the type of cable being used. Here are some reasons for burying underground cables at a certain depth:

  1. Protection from External Factors:
    • Mechanical Protection: Burying cables at a certain depth provides mechanical protection, shielding them from physical damage due to excavation activities, construction work, or accidental impacts. This helps prevent disruptions to the power supply and minimizes the risk of cable damage.
  2. Temperature Stability:
    • Thermal Stability: Underground cables are subjected to relatively stable temperatures compared to aboveground cables. Burying them at a certain depth helps maintain a more constant temperature, reducing the risk of overheating or cooling, which can affect the cable’s performance.
  3. Protection from Environmental Conditions:
    • Protection from Weather: Underground placement protects cables from weather-related conditions such as storms, ice, and wind, which can impact overhead lines. This enhances the reliability of the power distribution system.
  4. Safety:
    • Public Safety: Burying cables at a sufficient depth enhances public safety by minimizing the risk of accidental contact with live cables. This is especially important in urban areas where people may be digging for various reasons.
  5. Ease of Installation and Maintenance:
    • Access for Installation and Repair: Cables buried at an appropriate depth are still accessible for installation and maintenance purposes. While not as easily accessible as overhead lines, underground cables can be reached using specialized equipment for repairs and upgrades.
  6. Compliance with Regulations:
    • Regulatory Requirements: Local regulations and standards often dictate the minimum burial depth for underground cables. Compliance with these regulations ensures that the installation meets safety and environmental standards.
  7. Soil Characteristics:
    • Soil Stability: The type of soil in a particular area can influence the depth at which cables are buried. In some cases, soils with certain characteristics may require deeper burial to ensure the stability of the cables and protect them from external forces.
  8. Voltage Level and Cable Type:
    • Voltage Considerations: The voltage level of the cables and the specific type of cable being used can also impact the recommended burial depth. Higher voltage cables may require greater depth for safety and performance reasons.

It’s important to note that the depth at which underground cables are buried is determined through engineering analysis, considering a combination of these factors.

The goal is to balance the need for protection, safety, and accessibility while ensuring the reliable and efficient transmission of electrical power.

Bedding, Bricks, and Sand on the Underground Cables

It sounds like you’re referring to the construction of underground cable systems and the materials used in the process.

When installing underground cables, especially in urban areas or places where aesthetics are important, a combination of materials is often used to provide support, insulation, and protection to the cables. Here’s a breakdown of each component:

  1. Bedding:
    • The bedding refers to the layer of material directly beneath the underground cables. This layer is typically composed of a material that provides support and helps distribute the load. Common materials used for bedding include sand or a sand and gravel mixture. The bedding material is chosen to be stable and to prevent damage to the cable during installation and over time.
  2. Bricks or Concrete Encasement:
    • In some cases, especially when additional protection is required, bricks or concrete encasement may be used around the cables. This provides extra physical protection and helps to distribute the load even further. The encasement can also offer some resistance to external forces and prevent damage from excavation or accidental impacts.
  3. Sand:
    • Sand is often used as a filler material around the cables and within the trench. It provides additional insulation and helps to disperse any heat generated by the cables. Sand is chosen for its thermal properties and its ability to resist moisture, which is crucial for protecting the cables from environmental factors.

The typical sequence of layers from the bottom up might look like this:

  • Bedding (Sand or Sand/Gravel mixture): Provides support and load distribution.
  • Cables: The actual electrical cables are laid on top of the bedding.
  • Sand: Surrounds the cables, providing thermal insulation and moisture resistance.
  • Bricks or Concrete Encasement: Offers additional physical protection and load distribution.
  • Backfill Material: The trench is then backfilled with soil or other suitable material.

The specific materials and methods used can vary depending on factors such as soil conditions, local regulations, and the type of cables being installed.

The goal is to create a stable and protective environment for the cables, ensuring their longevity and the reliability of the electrical system.

Additionally, safety standards and environmental considerations play a crucial role in the selection of materials and construction practices.

What happens if an underground cable gets wet?

The impact of water on underground cables depends on various factors, including the type of cable, the insulation material, and the duration and extent of exposure. Here are some general considerations:

  1. Insulation Resistance:
    • Water can compromise the insulation resistance of cables. If water enters the cable and comes into contact with the conductors, it can reduce the insulation resistance, potentially leading to electrical faults, short circuits, or disruptions in power transmission.
  2. Corrosion:
    • Water exposure may accelerate the corrosion of metallic components within the cable, such as conductors or metallic shielding. Corrosion can degrade the structural integrity of the cable and affect its electrical performance.
  3. Cable Performance:
    • Water ingress can affect the electrical properties of the cable, including capacitance and inductance. This can lead to changes in signal transmission characteristics and impact the overall performance of the cable.
  4. Safety Hazards:
    • Water and electricity are a dangerous combination. If water enters the cable and creates a conductive path, there is an increased risk of electrical shock or damage to equipment connected to the cable.
  5. Deterioration of Insulation Material:
    • Prolonged exposure to water can lead to the deterioration of the insulation material. Different cable types use various insulation materials, and some are more resistant to water than others. However, over time, even water-resistant insulation can degrade, especially if the cable is not designed for direct burial in wet conditions.
  6. Moisture Ingress Testing:
    • High-quality underground cables are designed to resist moisture ingress. They undergo testing to ensure their ability to withstand environmental conditions. Moisture ingress testing involves subjecting cables to simulated wet conditions to verify their resistance to water penetration.
  7. Mitigation Measures:
    • Various measures are employed to mitigate the impact of water on underground cables, such as using moisture-resistant insulation materials, providing proper cable sealing and jointing techniques, and maintaining a suitable burial depth to minimize the risk of water ingress.

It’s crucial to follow industry standards and guidelines for the installation and maintenance of underground cables to minimize the impact of water exposure.

Regular inspections, testing, and preventive maintenance can help identify and address issues before they lead to significant failures.

If an underground cable is suspected to be compromised by water, it may need to be inspected and repaired or replaced as necessary.

Why are Underground Cables Stranded Conductors?

Underground cables often use stranded conductors for several practical reasons related to flexibility, durability, and ease of installation.

Here are some of the key reasons why stranded conductors are commonly used in underground cables:

  1. Flexibility:
    • Stranded conductors are more flexible than solid conductors. This flexibility is crucial during the installation process, especially when dealing with the bends and twists that are often encountered in underground cable routes. The ability of stranded conductors to bend and flex makes them easier to handle during installation in trenches or conduits.
  2. Resistance to Fatigue:
    • The repeated bending and flexing that may occur during the installation of cables, as well as movements caused by factors like ground settlement, temperature variations, or other environmental stresses, can lead to metal fatigue. Stranded conductors are better able to resist fatigue, as the individual strands can move independently, reducing the risk of breakage.
  3. Improved Conduction:
    • Stranded conductors provide a larger surface area for current flow compared to a solid conductor of the same cross-sectional area. This increased surface area helps reduce the resistance of the conductor, improving the overall efficiency of power transmission.
  4. Thermal Stability:
    • Stranded conductors offer better thermal stability compared to solid conductors. This is important in managing the heat generated during the transmission of electrical power. The ability of the strands to dissipate heat more effectively helps maintain the cable’s performance under varying load conditions.
  5. Reduction of Skin Effect:
    • At high frequencies, alternating current tends to concentrate near the surface of a conductor, a phenomenon known as the skin effect. Stranded conductors, by virtue of their multiple smaller strands, can mitigate the skin effect, improving the uniform distribution of current across the cross-sectional area.
  6. Ease of Termination:
    • Stranded conductors are generally easier to terminate than solid conductors. They provide more points of contact, making it simpler to connect the conductor to connectors, terminations, or other components within the cable system.
  7. Resistance to Breakage:
    • Stranded conductors are less prone to breakage caused by external factors like bending or mechanical stress. The flexibility and redundancy of strands in a stranded conductor contribute to its durability and resistance to damage.

While stranded conductors offer these advantages, it’s essential to consider the specific requirements of the application.

The choice of conductor type (stranded or solid) depends on factors such as the application, voltage level, current capacity, and environmental conditions.

For underground cables, where flexibility, durability, and ease of installation are critical, stranded conductors are a common and practical choice.

Why lead sheath is used in underground cables?

Lead sheathing is used in underground cables for several reasons, primarily related to its properties that enhance the durability, protection, and longevity of the cable. Here are some key reasons why lead sheaths are employed in underground cables:

  1. Waterproofing and Moisture Resistance:
    • One of the primary functions of lead sheathing is to provide excellent waterproofing and moisture resistance. The lead sheath forms a tight, continuous barrier around the cable, preventing water and moisture from penetrating into the cable core. This is crucial for maintaining the electrical performance of the cable and preventing insulation degradation.
  2. Chemical Resistance:
    • Lead is highly resistant to many chemicals and corrosive substances. The lead sheath acts as a protective barrier, shielding the cable from exposure to potentially corrosive elements in the surrounding environment, such as soil chemicals or groundwater contaminants.
  3. Mechanical Protection:
    • Lead sheathing adds a robust and durable layer that protects the cable from mechanical damage. It helps resist external forces, such as pressure from surrounding soil or rocks and provides a level of physical protection during installation, maintenance, or accidental impacts.
  4. Flexibility:
    • Lead sheaths are relatively flexible, which is advantageous in applications where the cable may experience bending or movement. The flexibility of the lead sheath contributes to the overall flexibility of the cable, making it easier to handle during installation and reducing the risk of damage.
  5. Longevity and Stability:
    • Lead has a high resistance to corrosion and environmental degradation over time. The use of lead sheathing enhances the longevity and stability of the cable, ensuring a longer service life in underground applications.
  6. Electromagnetic Shielding:
    • Lead sheaths provide electromagnetic shielding, helping to contain the electric and magnetic fields generated by the conductors within the cable. This can be important in applications where electromagnetic interference needs to be minimized or controlled.
  7. Fire Resistance:
    • Lead has fire-resistant properties, and the lead sheath contributes to the overall fire resistance of the cable. This can be an important safety feature, especially in densely populated areas or critical infrastructure where fire resistance is a priority.

It’s important to note that while lead sheathing offers these advantages, there are environmental and health considerations associated with the use of lead.

In some regions, there are regulations and standards in place to address the safe use and disposal of lead-containing materials.

As a result, alternative materials with similar protective properties, such as corrugated aluminum sheaths or polymer materials, are sometimes used as substitutes for lead sheathing in modern cable designs.

Overhead vs. Underground Cables

 Overhead CablesUnderground Cables
ConstructionL complicated in manufacturingMore complicated.
InstallationThe cables are mounted on high steel and aluminum towers.Cables are laid underground.
LocationMainly used to transmit power outside cities and residential areas.Transmits power inside cities and residential areas.
The insulators are used to connect the conductor with the towers.No concept of towers is there in this method.
InsulationConductors without insulation are used.A very sophisticated kind of insulation is used.
Conductor sizeThe size (cross-sectional area) of the wire is way smaller than the underground cables.The cross-sectional wire is higher than that of overhead cables.
Environmental effectThe conductor may get damaged due to environmental factors.The conductor is not damaged by environmental factors.
This circuitry has insulator-dominant characteristics.The circuitry has capacitive characteristics. (Circuitry means the transmission circuit from the source to the end)
High voltage is used for the Power transmission.A high current is used for the Power transmission.
Proximity effectProximity Effect is negligible i.e., 0.The proximity effect is much higher than OHTC.
BranchingExtension or getting a branch from the cable is easy.Extension or getting a branch from the cable is a headache.
Corona lossesCorona discharge is present. A reasonable corona discharge is present in windy areas.No concept of corona discharge. The underground cables are filled with materials like sand and cement. So, air has no reach to the conductor.
Falling riskRisk of fallingNo falling risk.
MaintenanceEasy to repair.Difficult to repair.
Fault locationEasy to locate the fault.Locating of fault is difficult.
Current capacityHigher current capacity due to good cooling.Lower current capacity.
Manufacturing CostLines of overhead cables are cheaper.Lines of underground cables are expensive as many layers of insulators are present over the core conductor.
Voltage levelsHigh and ultra high voltages power transmission up to 400 KVNot suitable for ultra-high voltages. Up to 66kv
Life TimeThe life of cables is lesser than underground cables as the environmental factors lead to metallic loss of the conductor.The life of the cable is even up to 40-50 years as the metallic loss is 0 unless manually done.
InstallationEasier and straightforwardNeed digging activity
CoolingEasier by natural airDepends on soil heat conductivity.
OHTL vs underground cables