Question

Draw and explain the construction of underground cables.

Answer 1

### Construction of Underground Cables

Underground cables are used to transmit electrical power or signals below the ground, offering protection from weather, accidental damage, and other environmental factors. These cables are typically more expensive than overhead cables but provide greater reliability and safety. Let’s break down the construction and layers involved in a typical underground cable:

Basic Construction of an Underground Cable

1. Conductor (Core)
   

   - Material: Usually made from copper or aluminum.
   - Function: The conductor is the main part of the cable, responsible for carrying the electrical current.
   - Shape: It is often circular, but it can also be compact or shaped in other ways for better packing and handling.

1. Insulation
   

   - Material: Typically made from materials like PVC (Polyvinyl Chloride), XLPE (Cross-linked Polyethylene), or rubber.
   - Function: The insulation prevents the electrical current from escaping the conductor and ensures that the current only flows through the conductor, avoiding short circuits or leakage to other parts of the cable.

1. Conductor Screen (Semiconducting Layer)
   

   - Material: A layer of semiconducting material like carbon-loaded polyethylene.
   - Function: This layer smooths out the electric field at the surface of the conductor and reduces electrical stress, ensuring that the insulation lasts longer.

1. Insulation Screen
   

   - Material: A layer of semiconducting material (or another conductive material) that works with the insulation to distribute the electric field evenly across the cable.
   - Function: It improves the performance of the insulation and reduces the chances of electrical breakdowns due to external stress.

1. Armour (Metallic Layer)
   

   - Material: Usually made of steel wires or galvanized steel strips.
   - Function: Provides mechanical protection to the cable, protecting it from physical damage during installation or due to external pressure, like from digging or rodents. It also provides grounding for the cable, preventing it from carrying any unwanted voltage.

1. Outer Sheath (Final Protective Layer)
   

   - Material: A tough outer layer, typically made from PVC, PE (Polyethylene), or other durable materials.
   - Function: The outer sheath provides protection against moisture, chemicals, and physical damage. It is also designed to resist environmental factors such as UV rays, extreme temperatures, and physical wear from installation and operation.

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Diagram of Underground Cable Construction

`plaintext
  ________________________________
 /                                \
|   Outer Sheath (PVC/PE)          |  <- Protective Layer
|__________________________________|
|   Armour (Steel Wires/Strips)    |  <- Mechanical Protection
|__________________________________|
|   Insulation Screen (Semiconductor) |  <- Reduces Electric Stress
|__________________________________|
|   Insulation (PVC, XLPE, Rubber) |  <- Prevents Current Leakage
|__________________________________|
|   Conductor (Copper/Aluminum)    |  <- Carries Electrical Current
|__________________________________|
`

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Explanation of Layers

1. Conductor: This is the central part of the cable. The electrical current flows through the conductor. Copper and aluminum are preferred because they are excellent conductors of electricity, with copper having a slightly better conductivity.
   

1. Insulation: The insulation ensures that the electrical current stays inside the conductor and does not leak out, protecting the surrounding environment and preventing short circuits. For underground cables, the insulation needs to be water-resistant, durable, and able to handle high voltages.
   

1. Conductor Screen: This layer helps to make sure the electric field around the conductor is uniform, avoiding stress points that could lead to insulation failure. It’s typically a semiconducting layer that’s added to smooth out potential spikes in electrical stress.
   

1. Armour: The steel or metal wires around the cable act as a shield against physical damage. When the cable is buried underground, this armour helps it resist forces such as digging, pressure, or accidental damage. It also helps in grounding the cable to prevent electrical shocks.
   

1. Outer Sheath: The outer layer (sheath) protects the entire cable from environmental damage, including moisture, chemicals, and mechanical wear. It ensures that no foreign elements can reach the internal parts of the cable.
   

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Advantages of Underground Cables

1. Safety: Less prone to accidents like falling branches or storms.
   

1. Aesthetics: More visually appealing since no overhead lines are visible.
   

1. Reliability: Less interference from environmental factors, offering more consistent service.
   

1. Protection: Protected from accidental damage, weather, and animals.
   

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Disadvantages of Underground Cables

1. Cost: Underground cables are more expensive to install due to the need for trenching and specialized materials.
   

1. Repair Challenges: If a fault occurs, it's more difficult and time-consuming to locate and repair compared to overhead cables.
   

1. Heat Dissipation: Underground cables generally have higher operating temperatures because of limited airflow for cooling.
   

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This covers the basic construction and explanation of underground cables. They are critical in modern infrastructure, especially for urban areas or places where aesthetic and safety concerns outweigh the cost considerations.