Low Voltage Cables on Ladder or Tray

Note: for the spacing of supports for cables having an overall diameter exceeding 40 mm, the manufacturer’s recommendation should be observed.
* for flat cables taken as the dimension of the major axis.
† the spacings stated for horizontal runs may be applied also to the runs at an angle of more than 30º from the vertical. For runs at an angle of 30º or less from the vertical, the vertical spacings are applicable. 

Underground cable ducts are indispensable infrastructure components for the secure and efficient transmission of electricity, telecommunications and data services. The installation of these ducts requires planning, execution, and compliance with local regulations.

By considering the following factors when installing underground cable ducts, we can ensure that your infrastructure project is successful, durable, and efficient.

Planning, permits and regulations

A well-designed duct system is necessary for efficient cable installation and maintenance in the future. Consider factors such as depth, distance between ducts, and cable type when installing cables. In addition, plan for potential future system expansions and enhancements.

Before beginning any endeavour, it is essential to research and adhere to local regulations and obtain any necessary permits. These may include environmental factors, utility certifications, and right-of-way authorizations. Compliance with local regulations will facilitate a seamless installation process and prevent potential legal issues.

Before digging, identify and locate all existing subsurface utilities to prevent accidental damage to gas, water, and other utility lines. This will help assure worker safety and prevent expensive repairs or fines.

Installation, Soil Conditions and Materials

Soil conditions can have a substantial effect on the installation process and the durability of a duct system. Evaluate the soil to determine the optimal installation method, depth, and duct material in order to ensure the duct's stability and durability.

Select the proper duct material and installation method based on soil conditions, load requirements, and local regulations. Popular materials include PVC, HDPE, and fibreglass, and installation techniques include open trench, directional drilling, and microtrenching. Bed and backfill the ducts with suitable materials to protect them from damage caused by ground movement or large loads. This will aid in maintaining the duct system's integrity over time.

The distance between the ducts and the encircling materials largely determines the cables' current carrying capacity (Cable Sizing) and temperature. Maintain adequate separation between ducts in accordance with industry standards and local regulations to avoid this. In addition, choose bedding and backfill materials with high thermal conductivity, allowing for effective heat dissipation and preventing cables from overheating.

Install warning tapes or marker posts to alert future excavators of buried ducts and cables. This precaution helps prevent inadvertent damage during future construction or maintenance projects.

Design the duct system with adequate ventilation and discharge to prevent the accumulation of gases or water within the ducts, which can damage cables. This factor will aid in protecting the cables and extending their lifespan.

Draw Pits and Cable Pulling

Draw pits, also referred to as draw boxes or splice boxes, are indispensable components of an underground cable duct system. They facilitate cable installation, maintenance, and repairs along the duct route by providing access points at strategic locations. Consider the following when incorporating draw basins into your duct system:

1. Location: Place draw pits at regular intervals and at direction, elevation, or duct size changes. This makes cable pulling simpler, reduces cable tension, and reduces the risk of cable injury during installation.
2. Size: Ensure that the draw pit is large enough to accommodate the number of cables, splices, and any other apparatus necessary for cable installation or maintenance.
3. Design: Design draw pits with appropriate drainage, ventilation and secure access covers to protect cables and equipment from water and gas accumulation.
4. Material: Use concrete or polymer concrete to construct draw pits that can withstand large loads and resist corrosion.
5. Safety: Follow safety guidelines when working within draw pits, including proper ventilation, confined space entry procedures, and the use of fall protection equipment when necessary.

Use the proper cable-hauling equipment, techniques, and lubricants to prevent cables from being damaged during installation. This will aid in ensuring the performance and dependability of the installed cables.

Quality Assurance and Safety

Regularly inspect the installation process to ensure that best practices are followed, and the duct system meets the specifications. This will aid in ensuring a successful installation and reduce the likelihood of future issues.

Keep detailed as-built drawings and documentation of the underground duct system, including information on its location, depth, and cables. For future maintenance and expansion initiatives, this information will be invaluable.

Ensure that all personnel involved in the installation process are aware of potential dangers and adhere to appropriate safety procedures to reduce the likelihood of accidents or injuries. A safe workplace is crucial to the success of an endeavour.

Cable draw pits and duct access chambers provide access to subsurface utility services such as electric cables, telecommunication, and fibre optics.

For underground cable installation having drawing or pulling pits are essential.   Draw pits and temporary excavations along the cable route permit drawing and prevent over-tensioning cable. They facilitate large installation runs and aid in tension regulation during the draw.

Maximum Pulling Length

The maximum distance between draw pits should be determined when using them. This depends on several project-specific parameters; however, there are a few important factors to consider:

1. Cable type: Each cable has distinct physical properties, such as bending radius and tensile strength, determining the maximum distance between openings.
2. Trench conditions: The condition of the trench, its depth, and the type of soil can influence the friction between the cable and the trench, thereby influencing the drawing tension.
3. Route Complexity: Numerous turns or elevation changes may necessitate additional pits to manage the drawing tension and bending stress.
4. Cable weight:  The cable weight directly affects the force required to draw it. Generally, heavier cables require more tension management trenches.
5. Pulling equipment: The variety and strength of equipment used to pull the cable can also affect the distance between pits. Stronger apparatus may withstand extended pulls.

Given these factors, cable installers often use the following formula to estimate the maximum pulling tension or  length (distance between pits):

$T=W\times L\times f$, and      $L=\frac{T}{W\times f}$

Where,

• L is the length of the cable pull, m
• T is the maximum pulling tension, N
• W is the weight of the cable per unit length,
• f is the coefficient of friction between the cable and the conduit or trench.

Note: to conver from kG to N, multiply bby the acceleration due to gravity, approx. 9.81 m/s²

Once the maximum pulling tension has been calculated, it can be compared to the cable's specified tensile strength. If the anticipated pulling tension exceeds this value, reducing the distance between the pits or finding other means to reduce the tension is necessary.

Always incorporate a safety margin to account for unanticipated changes or uncertainties in the trench conditions. As a general rule, you should err on the side of caution and implement more pits than you believe are necessary.

Adjustments

For a curved section, the following multipliers are applied to the tension calculated for the preceding straight section.

Note: These multipliers are based on a friction coefficient of 0.5. If the friction coefficient, f, were different, the required multiplier touse is given by $M^{f/0.5}$, where M is the multiplier from the table.

To prevent damage to a cable caused by the pressure that develops when a cable is drawn around a bend under tension, the pressure must be kept as low as possible and not exceed specified values.
 

Sometimes it is necessary or desirable to run cables in parallel.  Reasons may include:

1. It is required to achieve the needed cable ratings
2. Installation of two smaller cables may be easier than one larger cable
3. The cost of buying smaller cables in bulk may make it more economical
4. It may make termination of the cable easier

In deciding to use parallel cables, there are some considerations the engineer needs to be aware of, particularly in current capacity, voltage drop and fault rating.

Current Capacity

In parallel cables, the phase current is divided between two or more conductors.  Typically in paralleling cables, the size and length of each cable are the same, and the current flow is equally divided between the parallel conductors. If the conductors (or lengths) are of unequal size, then the current will not be distributed evenly (i.e. the conductors will carry different currents).  Where currents in parallel conductors are unbalanced, it may be necessary to provide separate overload protection for each conductor. 

Voltage Drop

The calculation of voltage drop across a cable needs to consider the arrangement of parallel conductors.  For equally loaded conductors the impedance of each conductor is the same, and the calculation is relatively easy.  Where each conductor impedance varies, and there are different current flows in each conductor the calculation becomes more involved.

Fault Rating

For parallel conductors, the worse case fault may still be towards the fault source and may only apply to a single faulted parallel cable or conductor. Adding additional cables in parallel doe not necessarily increase the fault rating.  Generally, each individual parallel cable should be rated for the expected fault level.

Note: the through fault level may be increased (or decreased) by the use of parallel cables.