## Cable Draw Pits

Last updated on 2023-06-27 3 mins. to read

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):

$T=W×L×f$, and      $L=\frac{T}{W×f}$

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.

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.