Cable Laying | The Importance of Cable Installation Instructions

Cable Laying

Usually the only people who take any notice of cables and wires are those who work with them. Most of us use the , watch television or surf the Internet without thinking about how the underlying infrastructure, the network of cables, has been assembled.

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Cables are rarely heeded; they are &#;simply there&#;.

General knowledge about cables is equally lacking. Very few people know what high technology is wrapped up in the inconspicuous black, grey or coloured wires. Indoor cables in particular are subjected to heavy handling by the user: office chairs are rolled over connection wires, computer patch cables are trodden on by people or nibbled by their amusing pets.

Outdoor cables are affected by huge differences in temperature and high bending and pulling forces while they are being laid. The thicker the cable the stronger it may appear. However thick as well as thin wires are subject to the same physical conditions and limits.

The following Blogs aim to raise aware­ness about cables and explain terms such as bending radius, temperature ranges or permissible tension. Furthermore you will find instructions and tips for correct and safe installation.

Parameters

Initially people have the idea, &#;We should/ must modernise or expand our network&#;.

An engineering consultancy is hired to transform the idea into a feasible plan.

Basic conditions and objectives are decided together with the customer. At the same time many technical parameters must also be considered along with the questions related to infrastructure.

During the realisation phase for the plans numerous trades work closely together. One of these is concerned with the laying of the physical network of wires or cables.

The installation company responsible for laying the cables must heed the following parameters:

• temperature range of the cable,
• bending radius of the cable,
• maximum tension of the cable,
• weight of the cable as well as
• storage and cutting.

Temperature Range

The temperature range of the cable is of great importance for both the user and fitter.

After all the cable is meant to function equally well in cold and hot temperatures. It is particularly during the fitting process that powerful mechanical forces act on the cable. The plastic used serves as the limiting element for the possible temperature range.

At overly warm temperatures the plastic becomes very soft and can change into a thermoplastic state (up to melting point), which causes irreversible changes in the cable.

At very cold temperatures, however, the material stiffens and becomes hard and inflexible. Here, too, irreparable damage can occur.

Tears in the sheath allow dampness and moisture in and impair the transmission rate.

Details about the permissible temperature range during laying and use (following successful fitting) can be found in the information sheets of the cable manufacturer. Since the mechanical strain on the cable in its laid form is significantly less, the permissible temperature range is greater than the range valid for the installation period.

The VDE gives the following values:

During Cable Laying

PE-sheath, from -20 ° C to + 50 ° C
PVC-sheath, from &#; 5 ° C to + 50 ° C

Before & After Cable Laying

PE-sheath, from -20 ° C to + 70 ° C
PVC-sheath, from &#; 5 ° C to + 70 ° C

 

 

 

 

 

Cable Bending Radius

Regarding the bending radius we distinguish between multiple and single bending (shaping into the final position). Multiple bending occurs mainly during the laying process.

Cables are laid under tension around deflector rolls. The particular stress of multiple bending lies in the alternating stress on the materials, which can be stretched several times as well as compressed during the laying process. To prevent permanent damage there are prescribed minimum bending radii of, for example, 10 x cable external diameter for multiple bending.

The stress on the material during final bending is not characterised by alternating stress.

The cable is bent into form a final time and stays in this position for the duration of its use. The minimum bending radius in this case is, for example, 7.5 x cable external diameter. During final bending the cable can, therefore, be bent more tightly.

Exact minimum bending radii for specific cables can be found in the information sheets of the cable manufacturer.

Tension

During laying of the cable particular attention must be paid to the maximum possible tension. The cable is very quickly damaged by the use of too much force and must then be replaced. The maximum possible tension depends in the first place on the overall cross section and the tensile strength of the conducting materials used.

For cables with steel tape or copper wire spiral armouring it is the internal copper conductors alone which determine the maximum tension! The armouring has no influence on the maximum tension or can possibly reduce it through additional weight.

For armouring with steel or steel profile wires, however, the tension is determined solely by the steel and steel profile wires.

Cable Weight

The cable weight of larger cable dimensions can take weights of up to more than 9 t/km (without the reel!).

Storage & Cable Cutting

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Cables for delivery as well as cut cables must generally be protected against moisture penetration. This best occurs through the use of shrink caps with fusible glue. Loose-fitting caps or temporary measures with plastic adhesive tape are not watertight and are unsuitable.

Moisture penetration leads to corrosion and deterioration of the transmission rate.

If two cable ends are to be connected with a sleeve, this must take place immediately and with protection against moisture and rain. For the period of the sleeve installation an installation tent must be erected.

Further Reading

Cable Pulling Equipment

Thorne & Derrick distribute an extensive range of Cable Pulling & Laying Equipment to enable the safe installation of fibre and copper cables within the telecommunications industry. Safely installed cables reduces operational and maintenance requirements to the network and reduced service interruption to telecom cables, wires, ducts, cabinets and exchanges &#; products include cable spiking tools, conduit rods, cable lubricant, cable socks and rollers.

Further Reading | Cable Drum Handling & Laying Cables | A Guide from Nexans

 

Nature conservation, natural objects, aesthetics and space make laying cables underground unavoidable and necessary. Underground cable laying has added benefits in the form of reduced transmission loss and reduced risk of service supply loss in cases of extreme weather.

What is underground cable laying?

In areas where space for cables is limited and crunched, especially the urban regions, underground laying of cables is an efficient method. Telecommunications or electric power can be transmitted through underground cables. Data is transmitted from one point to another using cable laid on the ground instead of the ones hanging from poles and towers in an underground cable system.

In comparison to overhead cables, underground cables are not visible to the naked eye and hence, retain the location&#;s aesthetic beauty. To ensure this, proper trenches need to be dug and the entire process needs to be carried out systematically to ensure success and minimum service requirements.

Benefits of underground cable laying

Climate change has made extreme weather conditions more common. In order to have an uninterrupted power supply and telecommunication in place, it is important that our infrastructure is planned and equipped to survive the harsh weather conditions. The following benefits make underground cable systems the preferred choice:

• Longer lifespan &#; Underground cables have a much greater life expectancy than aerial cables.
• Reduction in maintenance costs &#; Less exposure to nature&#;s fury like falling tree branches, strong winds and rain lead to fewer maintenance requirements.
• Accident prevention- Aerial cables can collapse on buildings and cars and put anyone near them at the risk of electric shocks. In this regard, underground cable laying is safer.
• Continuous service &#; Protected from external factors, underground cables provide uninterrupted power or service.
• Zero obstruction &#; Underground cable systems are completely out of sight and cause zero obstruction to properties.
• Minimum space requirement &#; Aerial cables require a lot of space for installing poles whereas underground cables require a much-limited band of land.

The Procedure & Installation of Underground Cable Laying

The effectiveness and efficiency of an underground cable system depend on proper cable laying, quality of cable joints and branch connections. There are three methods of underground cable laying. The procedure followed in each method is as follows:

Direct laying

This method requires digging a 1.5m deep and 0.45m wide trench which is then covered with a layer of sand. The cables are laid in the trench and covered with a 10 cm thick layer of sand. To protect against mechanical injury the trench is then covered with bricks and other materials.

If more than one cable is required to be laid in a trench then a horizontal or vertical inter-axial spacing of 30 cm is provided to prevent mutual heating.

Advantages:

• The simplest and cheapest method of underground cable laying
• The heat generated gets dispersed in the ground.

Disadvantages:

• High maintenance cost
• Pointing out accurate locations of faults is difficult
• Cable network alterations are difficult.

Draw in system

Ducts or conduits of cast iron or concrete or glazed stone with manholes are placed at suitable locations along the cable route. The manholes are used for pulling the cable in position.

Advantages:

• Manholes make additions, repairs and maintenance easy
• Low maintenance cost
• Mechanical protection lowers the occurrence of faults.

Disadvantages:

• Installation cost is high
• Low heat dispersion due to grouped cables.

Solid system

Underground cables are laid in open pipes or troughs along the cable route. The troughs are usually made of asphalt, stoneware or cast iron. Asphaltic compound is used for filling the troughs once the cable is laid in position.

Advantages:

• Strong mechanical protection

Disadvantages

• Skill and labour requirements result in high costs

Challenges of underground cable laying

The various challenges in the process of laying underground cables are as follows:

1. High installation cost: The initial installation cost for an underground cable system is high as costs for restoration of roads and surrounding infrastructure add to the total cost.
2. Permissions and approvals: Before starting with the digging process, a range of approvals need to be obtained from relevant authorities.
3. Cumbersome modifications: Inability to locate lines and particular sections makes making additions or modifications to the line tedious.
4. Obstacles in cable route: The cable route needs to be designed keeping in mind any obstacles that may be there in the route.
5. Insulation requirements: Layers of insulation are needed for underground cables to withstand the high voltage flows.

FAQs

What is the minimum depth for cable laying below the ground?

The minimum depth for cable laying below the ground varies according to the voltage capacity of the cable. The depth should be atleast 0.9m for 3.3kV to 11kV cables, 1.05m for 22kV to 33kV cables. The minimum depth of a low voltage cable should be 0.75m.

What is the cable laying method?

The dependability of a cable network depends on the laying method and attachment of fittings like cable end boxes, joint, branch connectors, etc. Laying method defines the approach that will be followed in terms of digging of trenches, fitting of ducts and positioning of cables. There are basically three methods of laying cables underground which are direct laying, draw-in and solid system.

How deep should the cable be buried underground?

The depth requirements vary according to the topography of the terrain. For road crossings and drainage ditches that are yet to be finished, the standard depth varies from 30 to 42 inches. If the ground remains frozen for extended periods of time, then the depth should be at least 30 inches. Roadway crossings require deeper digging with a standard depth of 42 inches.

What should be the normal depth of laying cables?

The normal depth of laying cables is influenced by climate conditions and terrain. It ranges from 30 inches to 42 inches depending on the conditions.

What is the advantage of direct laying of underground cables?

Direct laying of underground cable systems has the following benefits:

• Fast construction of cable network
• Low investment
• Fast heat dispersion
• Clean and safe method with cable protected from external disturbances.

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