Power Cable Laying

Methods, key laying requirements and common defects

Part of the training series “Distribution Line Engineering” 

A power cable route is only as reliable as the way the cable is laid. This article reviews the six laying methods used in distribution networks — direct burial, cable trench, cable tunnel, duct bank, support (overhead) laying and submarine laying — together with the points that matter most on site: bending radius, pulling force, protective bedding, clearances and earthing. It closes with the defects most often found during inspection, each with a reference standard and a remedy.

Laying methods at a glance

The choice of method depends on the route, the soil, the number of circuits and the future access requirements. The six methods below cover almost every distribution cable route in service.

Figure 1. The six cable laying methods covered in this article.

1. Direct burial laying

1.1 Method and characteristics

The cable is laid directly in an excavated trench and backfilled. It is the cheapest method and needs no structures, but access for inspection and replacement is poor, so the route must be chosen carefully and the bedding and protection must be right.

Figure 2. Typical direct-burial cross-section. The cable is bedded in sand or soft soil, covered by a concrete protection plate, and buried at least 700 mm below ground level (dimensions in millimetres).

1.2 Key laying requirements

(1) Cables laid directly in the ground should generally be steel-wire armoured cables. The soil along the route must not contain substances that corrode the cable sheath or metal covering.

(2) Before laying, check the cable for mechanical damage and confirm that the sealing ends are intact; verify that the cable type, specification and length meet the design requirements; and measure the insulation resistance to confirm it is acceptable.

(3) The cable must be paid off from the top of the drum. It must not be dragged over supports or across the ground.

Figure 3. Pay the cable off from the top of the drum (left). Dragging the cable over the ground or over supports damages the sheath (right).

(4) For mechanical pulling, use a dedicated wire-mesh pulling grip or a pulling head at the pulling end, and fit a swivel there so that the cable does not twist. The pulling force must not exceed the value permitted by the standards, and the pulling speed must be uniform.

(5) Place rollers in the trench every 3–5 m, on the principle that the cable must never sag and touch the ground while it is being pulled.

Figure 4. Mechanical pulling with a wire-mesh grip and swivel (left); rollers keep the cable off the trench bottom (right).

(6) Where the cable passes through terrain with elevation changes, excessive tension must be avoided: the cable should be laid slack — generally 1.2% to 2% longer than the trench length — and laid in a wave pattern. At bends, the bending radius must not be less than the minimum permissible bending radius: ≥ 20d for single-core oil-impregnated paper insulated cable, single-core XLPE cable and rubber-insulated cable with lead sheath and steel-tape armour; ≥ 15d for multi-core armoured lead-sheathed oil-impregnated paper cable, multi-core XLPE cable and bare lead-sheathed rubber-insulated cable; ≥ 10d for all other types, where d is the cable outside diameter.

(7) Where the cable crosses a city street, highway, railway or drain, it must be carried in a steel pipe. The internal diameter of the pipe must be at least 1.5 times the cable outside diameter and not less than 100 mm.

(8) Burial depth, the clear spacing between cables, and the minimum distances to other pipelines and to buildings — for parallel and crossing layouts — must comply with Table 1.

(9) Cables laid in parallel underground must have their joints staggered. Where a cable crosses a heat pipeline, a thermal insulation layer or an asbestos-cement pipe must be provided. Cables must never be laid parallel directly above or below a pipeline or another cable.

(10) For direct burial, a 100 mm layer of soft soil or sand is placed first. After the cable is laid, a further 100 mm of soft soil or sand is placed on top, and concrete protection plates are then laid along the whole length, extending at least 50 mm beyond both sides of the cable. In special cases bricks may be used instead of concrete protection plates.

Figure 5. Measuring the 100 mm sand or soft soil cushion around the cable, before the protection plate is laid.

(11) For cable routes in suburban areas and open ground, cable position markers must be erected.

Table 1. Minimum clearances (m) between cables, and between cables and pipes, roads and buildings, in parallel and crossing layouts.

2. Cable trench laying

2.1 Method and characteristics

Cables are carried on brackets inside a concrete or brick trench closed by cover slabs. Compared with direct burial the trench gives a defined route, easier inspection and room for several circuits — at the cost of civil works, drainage and ventilation to maintain.

Figure 6. Cables carried on brackets inside a cable trench.

Figure 7. Typical cable trench cross-section: cover slab, bracket tiers at 150 mm spacing, earthing busbar and waterproof render (dimensions in millimetres).

2.2 Key laying requirements

• Use cables with bare armour or a non-flammable outer sheath.
• Maintain the specified minimum permissible spacing between cables inside the trench.
• Install a continuous earthing conductor along the whole length of the trench, with both ends connected to the earthing electrodes.
• Galvanize all metal structures inside the trench and outdoors, or protect them with anti-rust paint.
• Provide adequate drainage for trenches and tunnels, and adequate ventilation for tunnels.
• Fit every trench with cover slabs.

3. Cable tunnel laying

3.1 Method and characteristics

In densely loaded urban routes the cables run in a walk-in tunnel on both walls, which allows continuous inspection, forced ventilation and fire separation between circuits. It is the most capital-intensive method and is used where duct banks and trenches run out of capacity.

Figure 8. Cables on wall brackets inside a cable tunnel.

Figure 9. Segmental-lined cable tunnel with cables on both walls.

3.2 Key laying requirements

• Provide an adequate number of exits: one exit where the tunnel is shorter than 7 m; two exits up to 100 m; above 100 m, the distance between any two manholes must not exceed 75 m, and the manhole diameter must be at least 700 mm.
• Install fire alarm and fire-fighting equipment and fire walls at suitable locations, and fit lockable fire doors where the tunnel passes the plant boundary wall.
• Provide a damp-proof layer on the tunnel walls and a drainage ditch with a slope of 0.5%–1% at the tunnel floor.
• Install 36 V lighting inside the tunnel.
• Earth the cable support brackets reliably.

4. Duct bank laying

4.1 Method and characteristics

Cables are pulled through a block of buried ducts cast in concrete. The duct bank protects the cable against mechanical damage and allows several circuits to share a narrow corridor under roads, but cable replacement is difficult and heat dissipation is poorer than in a trench.

Figure 10. Duct bank cross-section: 4 × 4 duct arrangement in C25 concrete, with timber formwork and angle-steel bracing (dimensions in millimetres).

Figure 11. Duct bank under construction, with ducts supported on formwork before concreting.

4.2 Key laying requirements

(1) Cables installed in ducts should have a thickened bare-lead sheath or a plastic sheath.

(2) Excavate the duct trench to a depth of at least 1.7 m, compact the original soil at the bottom and cast a concrete bedding layer of at least 60 mm.

(3) Before pulling, clear and check the ducts: no standing water, no debris blocking the bore, smooth duct mouths, and enough space for the specified bending radius. Record the results.

(4) The duct bore must be at least 1.5 times the cable outside diameter. Where the ducts cross a town street, highway or railway, the protection pipe diameter must be at least 100 mm.

(5) The top of the duct bank should be at least 0.7 m below ground level, and at least 0.5 m beneath a pavement. Where the uniform live load on the ground exceeds 10 t/m², the duct walls may be strengthened with a cast concrete layer.

(6) During pulling, place rollers at the entry manhole, the exit manhole, the protection-pipe mouths and between the pulling rope and the ground.

(7) Pull the cables in a fixed sequence: lower tiers first, then upper tiers; inner ducts first, then outer ducts.

(8) Provide manholes at corners, at branches and wherever the laying method changes. On straight runs, provide a sufficient number of manholes to make pulling practical. Manhole positions and spacing depend on the permitted pulling force, the manufactured cable length and the site conditions, and should generally not exceed 200 m. The number of bends in one duct pull should be limited — generally ten or fewer.

Figure 12. Duct mouths at a manhole (left) and a cable being pulled into a duct (right).

5. Support (overhead) laying

5.1 Method and characteristics

Cables are carried on steel brackets or cable trays, indoors or on structures. It is the normal method inside plants, substations and cable shafts, where cables run above ground and remain accessible.

Figure 13. Cables on steel supports (left) and on a cable bridge / tray system (right).

5.2 Key laying requirements

• Cable supports include angle-steel brackets, assembled (prefabricated) supports and cable trays.
• Fit a pad of elastic material between the cable and the support.
• Where cables are run on supports they should be carried in steel pipe or in pipe made of fire-resistant material.
• Earth the cable fixing supports reliably.
• Maintain the specified clearance between cables and heat pipelines or heating equipment.
• Protect cables laid in the open from direct sunlight as far as possible.

6. Submarine (underwater) laying

6.1 Method and characteristics

River and sea crossings use a dedicated laying vessel: the cable is paid out under controlled tension, sunk along a surveyed route and, where the bed allows, buried below the riverbed or seabed.

Figure 14. Submarine cable laying from a laying vessel.

Figure 15. Cable coiled in the vessel hold, ready to be paid out over the gantry.

6.2 Key laying requirements

(1) A submarine cable should be in one continuous length. Where the length exceeds the manufacturer’s capability, a flexible joint may be used.

(2) Lay river crossings where the bed is stable and the banks are rarely subject to scour. At wharves, anchorages, harbours, ferry crossings and berths, reliable protective measures are mandatory; where conditions allow, bury the cable deep.

(3) The cable must lie flat on the bed and must not be suspended. Where conditions allow, bury it at least 0.5 m below the riverbed (seabed).

(4) Where submarine cables are laid in parallel, the spacing should be at least twice the water depth at the highest water level. Where the cables are buried below the bed, the spacing is set by the laying method and the capability of the burial equipment.

(5) Protect the shore end with a pipe or protection cover plates. Protection must extend down to below the lowest water level, beyond the reach of grounding vessels and boat hooks, and up to above the highest flood level. Fix the cable at the lower end of the protected section.

(6) Where the route crosses a small river or stream, use a pipe or bury the cable deep enough below the bed.

(7) Fit an anchoring device at the joint between the shore-end submarine cable and the land cable.

(8) Determine the laying method, the choice of laying vessel and the construction organization from the cable length, diameter and weight, the water depth, the flow velocity and the bed topography.

(9) When the whole route is drum-packed, the drum may be placed ashore or on the vessel depending on the water conditions. Floats may be used during laying, but the cable must never be dragged along the bed.

(10) Where the cable cannot be drum-packed, use the loose-coil method: coil the cable in the vessel hold, lead it over the gantry and through the pulleys and brake into the discharge chute, then lay it with a tug or by self-propulsion under steel-wire pulling.

(11) The laying vessel must be selected to meet the following conditions: 1) the hold capacity, deck area and stability must suit the cable length, weight, bending radius and working space; 2) the vessel must be equipped with a brake, tension gauge, length meter, entry-angle indicator, echo sounder, navigation and positioning equipment, and communication equipment.

(12) Carry out laying at neap tide, at slack water or in the dry season, with clear visibility and a wind force below Beaufort 5.

(13) Keep the cable rack at an appropriate back-twist height. Control the laying tension according to the water depth so that the entry angle is 30°–60°. With a tractor pushing, the speed should be 20–30 m/min; with a tug or self-propelled laying, 90–150 m/min.

(14) Set guide marks on both banks as designed. Measure the position during laying, correct the course in time and check the length laid.

(15) When bringing the cable ashore, first float the slack on the water surface and then pull it ashore. Sink the floated cable along the design route.

(16) After laying, carry out a diving inspection: the cable must lie flat and must not be suspended where the bed undulates. Measure the actual position and install marker signs on both banks as designed.

7. Common defects in cable laying

The defects below are the ones most often recorded during inspection of distribution cable works. Each is given with the reference standard and, where relevant, the remedy.

7.1 Cable structures

(1) Cable manhole cover installed incorrectly

Defect: The brick ring beam at the manhole opening is not thick enough, and a heavy-duty cover was not used.

Reference standard: When installing a manhole frame in a cement pavement, cast a 20 cm thick C25 concrete ring beam on the brickwork at the opening and trowel it flat, so that the frame is fully supported, level and firm. Cast a 40 cm wide concrete protection ring around the cover and cure it for at least 10 days; heavy-duty covers must be used. Provide fall-arrest protection at the opening — covers should be theft-proof, non-slip, resistant to displacement and fall-proof.

Figure 16. Cable manhole cover installed incorrectly.

(2) Cable work pit not waterproofed

Defect: No waterproofing was applied to the work pit, and the sump is incorrectly built.

Reference standard: Provide a sump at the bottom of the work pit with a drainage slope of at least 0.3%, and two access openings for lighting, ventilation and personnel. The reserved dimensions and method for the manhole base may be adjusted to local operating practice.

Remedy: Carry out the waterproofing as required and build the sump.

Figure 17. Cable work pit not waterproofed.

(3) Debris left in the cable work pit

Defect: Debris in the work pit was not cleared.

Reference standard: Before the duct bank is installed the pit must be bedded solid, with uniform joint widths. No mud, bricks, mortar, timber or other debris may remain in the ducts.

Remedy: Clear the debris from the work pit.

Figure 18. Debris left in the cable work pit.

(4) Cable trench built to a non-compliant standard

Defect: The drainage is inadequate and water stands in the trench; the number of supports is insufficient.

Reference standard: The trench must be free of debris and standing water, and the supports must provide the required load capacity.

Remedy: Construct strictly to the specification.

Figure 19. Cable trench built to a non-compliant standard.

Key requirements for cable trench construction

(1) No debris and no standing water in the trench. Backfill preferably by hand with lime powder or coarse sand, compacted in layers not exceeding 300 mm. Cure ordinary concrete for at least 7 days. Do not render at an ambient temperature below 5 °C; below 5 °C take effective measures to guarantee quality.

(2) Walls vertical and dense, top surface level, dimensions in accordance with the design and operating requirements, and adequate load capacity.

(3) Supports preferably one set every 0.8 m. Do not install supports within 2 m of the transition into a duct. Clear distance from the lowest support to the trench bottom 50–100 mm. Earth the supports reliably.

(4) The trench must prevent the ingress and seepage of water, with a longitudinal drainage slope of at least 0.5% and a sump at the lowest point. Do not stack materials within 1500 mm of the trench edge. Provide expansion joints in long trenches.

(5) Cable duct bank built to a non-compliant standard

Defect: Duct mouths are not smooth and have sharp projections; duct mouths are not sealed; no spare ducts were reserved; the spacing between ducts is too small.

Reference standard: Duct mouths must be round and smooth, without projections, and spare duct mouths must be sealed. Reserve a suitable number of spare ducts along the route. The curvature at longitudinal joints must allow the cable to be pulled without damage. Before laying, draw a test mandrel through each duct to confirm there are no sharp points or obstructions. Give the duct bank a slope of at least 0.1% so that water drains to the manhole.

Figure 20. Cable duct bank built to a non-compliant standard.

(6) Cable supports built to a non-compliant standard

Defect: The supports received no anti-corrosion treatment and were not earthed.

Reference standard: Steel cuts must be free of rolled edges and burrs. Supports must be firmly fixed, level and plumb, and all supports must be earthed. Supports should preferably be welded to embedded parts in the trench wall, with the welds protected against corrosion; the installation must be firm, level and plumb, the crossarms of the same tier must be at the same level with a deviation of no more than 5 mm, and the last 50 mm of each crossarm must be bent upwards at 10°.

Remedy: Construct strictly to the specification.

Figure 21. Cable supports built to a non-compliant standard.

(7) Cable tunnel built to a non-compliant standard

Defect: The clear height and width do not meet the requirements; the crossarm ends are inclined at less than 10°; cables are not carried on the supports and are not arranged as required.

Reference standard: The clear height of the walkway in the tunnel must be at least 1.9 m; it may be reduced but not below 1.4 m in short sections where the tunnel crosses other ducts. The walkway on either side of the supports must be at least 1 m wide. Provide a longitudinal drainage ditch at the tunnel floor and at least two safety openings, spaced no more than 75 m apart. Where cables are carried on supports, power cables must be above control cables. Hang a marker plate every 100 m. The last 50 mm of each crossarm must be inclined upwards at 10°.

Figure 22. Cable tunnel built to a non-compliant standard.

(8) Direct-buried cable built to a non-compliant standard

Defect: There are stones and other hard objects in the trench; no concrete protection plate or bricks were laid over the cable; the burial depth is insufficient; the backfill is incorrect.

Reference standard: Use armoured cable for direct burial, and keep at least 0.7 m from tree trunks. The trench must contain no stones or hard objects; otherwise place a 100 mm layer of soft soil or sand. After laying, place a further 100 mm of soft soil or sand and then a concrete protection plate or bricks, extending 50 mm beyond both sides of the cable. Backfill preferably by hand with lime powder or coarse sand in layers not exceeding 300 mm. Burial depth at least 0.7 m, and at least 1 m when crossing farmland or a roadway, or below the frost line.

Figure 23. Direct-buried cable built to a non-compliant standard.

7.2 The cable itself

(1) Cable stored or placed incorrectly

Defect: The cable was placed in an unsuitable environment, without hard board or thin timber packing underneath.

Reference standard: Place cables stably. To prevent twisting and deformation, level the ground with hard board or thin timber. Keep cables away from walls and damp areas to prevent mildew, deformation, sheath detachment, paint cracking, blistering and fading.

Remedy: Construct strictly to the specification.

Figure 24. Cable placed without packing and against a damp structure.

(2) Cable penetration not sealed correctly

Defect: The wall penetration was not fire-sealed, or was sealed with non-fire-rated material, or the seal is less than 300 mm thick.

Reference standard: Seal the penetration with a combination of fire-stop bags, organic fire-stop compound, fire-stop boards and fire-retardant coating, to a thickness of at least 300 mm. Coat the cable and both faces of the seal with fire-retardant coating over a length of at least 1 m. Wrap at least 40 mm around the cable with organic fire-stop compound and fill the remaining voids tightly with fire-stop bags, to the criterion that no light passes through. Fix one fire-stop board on each side of the opening with expansion bolts.

Figure 25. Cable penetration without a proper fire seal.

7.3 Cable accessories

(1) Cable protection pipe installed incorrectly

Defect: The pipe was not cleared before installation; outdoor cable terminations and metal protection pipes were not earthed; the pipe mouth was not sealed or the seal is incorrect; the pipe is buried less than 100 mm deep in non-concrete ground.

Reference standard: Cables less than 2 m above ground must be protected by a cable protection pipe. The pipe bore must be at least 1.5 times the cable outside diameter. Clear and remove debris from the pipe before installation, then temporarily seal both ends. Outdoor cable terminations and metal protection pipes must be well earthed, and the open end of the pipe must be sealed promptly.

Figure 26. Protection pipe on a pole without a proper termination seal and earthing.

(2) Cable marker posts missing or wrongly placed

Defect: No marker posts were installed on the route, or the posts do not meet the requirements.

Reference standard: Direct-burial marker posts are normally precast reinforced concrete members, with the text and graphic markings cast in recesses and painted red. Space them 20 m apart on straight runs. Use a forward arrow on straight sections, an angled arrow at corners and a double-headed arrow at joints, facing the side from which operation and maintenance staff patrol. In green belts, install posts at straight sections, joints and corners of direct-buried routes.

Remedy: Install the marker posts as required.

Figure 27. Direct-buried route without cable marker posts.

Closing note

Cable laying is decided long before the first metre of trench is opened: by the route, the bedding, the pulling force and the clearances chosen on the drawing. The defects collected in Section 7 are almost all failures to follow those few requirements, and most of them only show up years later as a fault on the network. Laying the cable correctly the first time costs far less than finding it again.