do you know calculations?

thanks

Generally method is ;

LM = Tm/fW

where
LM = pulling length, feet (straight section)
TM = maximum tension, lb.
W = weight of cable per foot, lb.
f = coefficient of friction (usually 0.5)

for conductor


SINGLE CONDUCTOR:
T = S * A (pounds)

Where:
T = conductor tension, lbs
S = conductor stress, lbs/cmil (Table 1)
A = conductor area, cmil (Table 2)

MULTIPLE CONDUCTORS:

T = N * S * A (pounds) for 3 or less conductors
T = (0.8 ) * N * S * A (pounds) for more than 3 conductors

Where:
N = number of conductors

What is Maximum Permissible Pulling Length?

The Maximum Permissible Pulling Length refers to the longest length of a cable that can be safely pulled through conduits, ducts, or cable trays during installation without causing damage to the cable. It is a critical parameter in cable design, selection, and installation because improper pulling of cables over long distances can lead to mechanical stress, insulation damage, conductor deformation, and ultimately reduce the operational life of the cable.

Definition of Maximum Permissible Pulling Length

The maximum permissible pulling length is determined by the balance between the tensile strength of the cable and the pulling force applied during installation. Exceeding this length means that the pulling tension will surpass the cable's mechanical limits, which may cause:

• Stretching or elongation of conductors
• Damage to the cable insulation or sheath
• Deformation of the cable core
• Increased electrical resistance
• Reduction in cable performance and reliability

This concept is particularly important for long cable runs in large industrial, commercial, or utility installations where cables are often pulled through long conduits or buried underground.

Factors Affecting Maximum Permissible Pulling Length

Several factors influence the calculation of the maximum permissible pulling length:

• Cable Type and Design
  Different types of cables, such as power cables, communication cables, and fiber optic cables, have varying mechanical properties. The design of the cable, including the type of insulation and sheath material, affects its tensile strength and flexibility.
  
  
• Pulling Tension
  The pulling tension is the force applied to pull the cable through the conduit. This tension must not exceed the maximum pulling tension specified by the cable manufacturer. The pulling tension is influenced by factors such as friction, conduit bends, and the weight of the cable.
  
  
• Conduit Material and Condition
  The material and condition of the conduit affect the friction between the cable and the conduit walls. Smooth conduits reduce friction and allow for longer permissible pulling lengths, while rough or dirty conduits increase friction.
  
  
• Number and Degree of Bends
  Each bend in the conduit increases the pulling tension due to the added friction. The degree of the bend also matters; a 90-degree bend causes more friction than a 45-degree bend.
  
  
• Lubrication
  Using cable lubricants reduces friction between the cable and the conduit, allowing for longer pulling lengths. Lubricants should be compatible with the cable's sheath material to prevent chemical degradation.
  

Calculation of Maximum Permissible Pulling Length

The maximum permissible pulling length can be calculated using formulas that consider the pulling tension, friction coefficient, and weight of the cable. One common approach is to use the following equation:

L = T_max / [W × μ × (1 + K)]

Where:
- L = Maximum permissible pulling length (meters)
- T_{max} = Maximum allowable pulling tension (Newtons)
- W = Weight of the cable per unit length (Newtons/meter)
- \mu = Coefficient of friction between the cable and conduit
- K = Bending factor, accounting for the number and degree of bends in the conduit

Example Calculation

Consider a power cable with the following parameters:

• Maximum allowable pulling tension, T_{max} = 500 N
  
• Weight of the cable per unit length, W = 5 N/m
  
• Coefficient of friction, \mu = 0.35
  
• Bending factor, K = 0.2 (for multiple bends)
  

Using the formula:

L = 500 / [5 × 0.35 × (1 + 0.2)]}\)
  = \frac{500}{5 \times 0.35 \times 1.2}
  = \frac{500}{2.1}
  = 238.1 \text{ meters}

Therefore, the maximum permissible pulling length for this cable is approximately 238 meters.

Pulling Tension Limits

Manufacturers specify the maximum pulling tension for their cables based on the cable design and materials used. The two main methods of applying pulling tension are:

• Pulling by the Conductor
  In this method, the pulling tension is applied directly to the conductor using a pulling eye. The tension limit is determined by the tensile strength of the conductor material, typically copper or aluminum.
  
  
• Pulling by the Sheath
  In this method, the pulling tension is applied to the outer sheath of the cable using a grip. The tension limit depends on the tensile strength of the sheath material and the overall cable construction.
  

Preventive Measures During Cable Installation

To prevent exceeding the maximum permissible pulling length and damaging the cable, several preventive measures can be taken during installation:

• Use of Cable Lubricants
  Applying suitable cable lubricants reduces friction and makes it easier to pull the cable through conduits.
  
  
• Use of Rollers and Guides
  In open installations, rollers and guides can be used to minimize friction and mechanical stress on the cable.
  
  
• Avoid Sharp Bends
  Sharp bends in the conduit or duct should be avoided to reduce the bending factor and minimize pulling tension.
  
  
• Monitor Pulling Tension
  Using tension monitoring equipment ensures that the pulling tension remains within safe limits throughout the installation process.
  
  
• Adhere to Manufacturer Guidelines
  Always follow the cable manufacturer's guidelines regarding maximum pulling tension, bending radius, and permissible pulling length.
  

Standards and Guidelines for Cable Pulling

Several international standards and guidelines provide recommendations for cable installation, including maximum permissible pulling lengths and tension limits:

• IEC 60228 - Conductors of insulated cables
  
• IEC 60332 - Tests on electric and optical fibre cables under fire conditions
  
• NEC (National Electrical Code) - Guidelines for electrical installations in the United States
  
• IEEE Std 524 - Guide to the installation of overhead transmission line conductors
  

Consequences of Exceeding Maximum Permissible Pulling Length

Exceeding the maximum permissible pulling length during cable installation can lead to several issues:

• Mechanical Damage
  Excessive tension can cause mechanical deformation of the cable, including stretching of the conductors and damage to the insulation or sheath.
  
  
• Reduced Electrical Performance
  Mechanical deformation can increase the electrical resistance of the conductors, leading to higher energy losses and reduced efficiency.
  
  
• Shortened Cable Life
  Damage caused by excessive pulling tension can reduce the expected service life of the cable, leading to premature failure and higher maintenance costs.
  
  
• Safety Hazards
  Damaged cables can pose safety hazards, including the risk of electric shock, fire, and equipment failure.
  

Conclusion

The maximum permissible pulling length is a critical parameter in cable installation, ensuring that cables are installed safely without causing mechanical damage or compromising performance. By understanding the factors that affect pulling length, adhering to manufacturer guidelines, and implementing preventive measures during installation, engineers and contractors can ensure the long-term reliability and safety of cable systems. Proper planning and execution of cable installations not only prevent costly damages but also enhance the overall efficiency and lifespan of electrical and communication networks.