Question

Describe with sketches the process of laying of underground cables by the drawing in method.

Answer 1

Procedure:
 A trench of minimum 60cm deep is made along with cable route.
 Width of trench depends on number of conduits to be laid.
 Separate pipes are provided for each cable.
 Spacing between 2 cables (conduit) is between 25 cm to 75 cm.
 Diameter of pipe is 2 to 3 cm, greater than cable diameter for easy handling of
cable.
 Pipe used may be cement pipe, DWC pipe or ducts of glad stone are used.
 For Maintenance and other cable work, man-holes are provided at suitable
distance.
 Size of man-holes should be large enough to allow a person to enter into duct
without difficulty.
 Unarmored cables are used in this type.

Answer 2

The "Drawing In" Method for Laying Underground Cables

This method involves first installing a system of pipes or conduits (called ducts) underground. The cables are then pulled, or "drawn," through these ducts at a later stage.

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Step-by-Step Process with Sketches (described from the diagram)

Step 1: Trench Excavation

• Action: A trench is excavated along the predetermined cable route.
• Sketch Description (referencing the diagram):
  • The trench is dug to a specific depth, labeled as "D" in the diagram. This depth is determined by regulations and the type of ground (e.g., under a roadway vs. a footpath, as would be specified in "Table 3").
  • The width of the trench must be sufficient to accommodate the ducts plus a minimum clearance on all sides for the protective encasement.

Step 2: Laying the Duct System

This is the most critical part of preparing for the "drawing in" method.

• Action: A protective base is laid, followed by the ducts themselves, which are then encased for protection.

• Sketch Description (referencing the diagram from bottom to top):

  1. Base Layer: A layer of "14:1 Sand cement encasement" or sometimes just fine sand is laid at the bottom of the trench. The diagram specifies this base should be a minimum of 100 mm thick. This provides a smooth, stable, and protective bed for the ducts.

  2. Placing the Ducts: The "Ducts" (the circular pipes in the diagram) are laid on top of this base. They are arranged in a specific formation (the diagram shows a 2-row by 3-column formation). Spacers are used to maintain a precise separation between them, ensuring the encasement material can flow all around each duct. The diagram indicates a minimum clearance of 50 mm between ducts and from the outer ducts to the trench wall.

  3. Encasement: The "14:1 Sand cement encasement" is then poured around and over the ducts, completely surrounding them. This solidifies into a hard, protective block, shielding the ducts from ground pressure and future impacts. The diagram shows this encasement extending at least 50 mm on the sides and above the top layer of ducts.

Step 3: Protective Layers and Backfilling

Once the duct encasement is set, the trench is backfilled in layers to provide warning and protection.

• Action: Protective slabs and warning tapes are installed as the trench is filled with soil.

• Sketch Description (referencing the diagram from the ducts upwards):

  1. Protective Slab: Approximately 100 mm above the duct encasement, a "Concrete or PVC danger slab" is laid. This provides a hard physical barrier to prevent damage from future digging with shovels or light machinery.

  2. Warning Tape: Further up, about 300 mm below the final ground level, an "Orange warning tape" is unrolled along the length of the trench. This serves as a bright, visual indicator to anyone excavating in the area that buried electrical services are below.

  3. Final Backfill: The rest of the trench is filled with "Back fill" (usually the excavated soil, cleared of large rocks) and compacted in layers to prevent future settlement. The surface is then restored to its original condition, creating the "Finished ground line".

Step 4: Drawing (Pulling) the Cable

This step can be done long after the trench is backfilled and the surface is restored.

• Action: The cable is pulled through the pre-installed empty duct from one end to the other (e.g., between two manholes or jointing pits).
• Sketch Description of the pulling process:
  1. A draw-rope is fed through the chosen duct. This is often done using flexible fiberglass rods pushed through the duct or by using a "mouse" (a small piston) that is blown through the duct with compressed air.
  2. The cable is prepared at one end. A "pulling eye" is attached to the cable's conductors, and a "cable sock" (a wire mesh grip) is fitted over the cable sheath.
  3. The draw-rope is attached to the cable's pulling eye.
  4. A winch is set up at the far end of the duct. As the winch pulls the draw-rope, the cable is carefully guided into the duct. Cable-pulling lubricant is applied to the cable as it enters the duct to reduce friction and prevent damage to the cable sheath.
  5. The cable is drawn steadily through the duct until it emerges at the other end.

Advantages of the Drawing-In Method

• Flexibility: New cables can be added in spare ducts, or old ones can be replaced without the massive cost and disruption of re-excavating the entire trench.
• Protection: The cables are very well protected from mechanical damage by the duct and concrete encasement system.
• Reduced Damage: The cable itself is not exposed to potential damage from rocks or rough handling during the backfilling process.

The primary disadvantage is the higher initial installation cost due to the materials and labor for the duct system. However, this is often offset by the long-term flexibility and protection it provides.