Cable Pulling Procedure and Following Conditions in Field

Cable Pulling Procedure and Following Conditions in Field

Cable pulling procedure and following conditions in the field during cable pulling time is alternately referred as what the cable pulling right procedure is?; and what conditions should be followed?, in field level during cable lay due to safe cable pulling. Cable manufacturer have handbooks or cable catalogue that describe the methods of safe cable pulling procedure and conditions.

This article not described details theoretical and mathematical calculation; just tried to make sense basic knowledge for workforce related to cable pulling in field level to pull cable perfectly and safely as well as understand why should follow the cable pulling conditions.

Keywords: 

Discussed keywords in this article are History of Cable Installation, Maximum Allowable Pulling Tension on Conductors, Pulling Tension Calculations, Coefficient of Friction, Sidewall Bearing Pressure, Pulling Multiple Cables in a Duct or Conduit, Weight Correction Factors, Pulling Tensions, Cable Pulling Direction.

History of Cable Installation:

In the book of “Electrical Power Cable Engineering” edited by William A. Thue cited the history of initial cable installation procedure as- Thomas A. Edison installed his earliest cables in New York City in 1882. The cables were placed in iron pipes in the factory and then were spliced together in the field every 20 feet in an egg-shaped splice casing. Other systems, such as by Brooks. Callender, and Crompton, were installed by 1835 where they also used short sections of iron conduit. American Bell Telephone Company installed the first flexible communication cables in 1882 and 1883 where cables were pulled into the conduit in the field. “Pump logs” were first used for water supply lines, but were used in 1883 in Washington, DC, for telegraph cables. Tree logs were hollowed out, the exterior was trimmed to make them square, and the entire log was treated with creosote. These became the conduits of choice! So began the duct and manhole systems with the need to pull cables.

Sponsored:

Maximum Allowable Pulling Tension on Conductors:

Maximum allowable pulling tension on conductor is an important factor during cable pulling. We cannot apply any tension to pull the cable that have an impact on the cable type of metal, temper, and factors of safety. The pulling tension limits must be set based on the central conductor of the cables and considering the mechanical strength of all components of cable.

Pulling Tension Calculations:

Considering the basic factors the cable pulling tension calculation formula is as follows which allowing for weight of cable, length of cable, coefficient of friction with grabbing the outer layer of cable.

Cable Pulling Tension Calculator Formula for straight horizontal duct-

T=WxLxf

Where,

T = Tension in pounds

W = Weight of one foot of cable in pounds

L = Length of pull in feet

f = Coefficient of friction for the particular duct material and outer layer of the cable.

Coefficient of Friction:

The friction during cable pulling time is not same for all time; it varies on duct type, number of cables in duct, cable formation type, cable trench soil condition etc.

Sidewall Bearing Pressure is one of the most considerable factor during cable pulling around a bend or curve area. Cable trench wall in bend or curved area tends to apply force against pulling tension. The angle of bending denotes the degree of pressure.  Sidewall Bearing Pressure can express as formula below:

SWBP = T_o/R

Where,

 SWBP = Force in pounds per foot

T_o = Tension coming out of the bend in pounds

R= Radius of the inside of the bend in feet

Pulling Multiple Cables in a Duct or Conduit:

Frequently we pull multiple cables in a duct or conduit, consider three cables are in flat cradled formation where outer cables push in on the inner cable. Again, consider three cables in triangular formation, in this situation the top cable is riding on the two lower cables.  As a result it seems to be heavier than it actual weight. Due to this behavior “weight correction factor” is need to calculate the cable pulling tension.

Weight Correction Factors:

For the typical case of three cables of equal diameter and weights in a conduit of given size; the weight correction factor is higher for the cradled configuration than the triangular configuration.

Configuration of Three Cables	Weight Correction Facto
Triangular	1.222
Cradled	1.441

Around the bend or curve area the center cable may try to pass between the outer two cables. This may seems as a cable Jamm inside the duct. Cable Jamming increases the pulling tension many times.

Pulling Tensions:

It is fact that we have to come across the bend or curve in cable runs. The important point is that the friction and sidewall bearing pressure around that bend increase the tension coming out of the bend in respect to the tension on the cable coming into the bend.

T_o=T_IN e^cfa

Where,

  T_o= Tension going out of the bend

T_IN = Tension coming into the bend

c = Weight correction factor

f = Coefficient of friction

a = Angular change of direction in radians

Cable Pulling Direction:

There are always two possible directions that a cable can be pulled for any run just as a cable always has two ends; but notable that selection of cable pulling direction in curvy or bend area is very important and this is the core point of the article “Cable Pulling Procedure and Following conditions in Field” that trying to express today. 

Sponsored:

We cannot just start pulling cable from any side, any direction. The degree of pulling tension, the capacity selection of pulling grip depends on the direction of cable pulling. After all, safe cable pulling avoiding cable damage during pulling time can avoid just select the right direction of cable pulling. Following demonstration will make sense clear:

Consider a typical cable span showing in figure that having weight 6 pounds per foot and coefficient of friction is 0.5.

Consider Pulling Direction A to D

Tension at point A= 0

Tension at point B= 500x6x0.5=1500 pounds

e^{cfa  =}  e^{1x0.5x1.5708  =2.19}

Tension at point C= 1500x2.19=3285 pounds

Tension at point D=10x6x0.5 + 3285= 3,315 pounds

Consider Pulling Direction D to A

Tension at point D= 0

Tension at point C= 10x6x0.5=30 pounds

e^{cfa  =}  e^{1x0.5x1.5708  =2.19}

Tension at point B= 300x2.19= 65.7pounds

Tension at point A=500x6x0.5 + 65.7= 1,565.7 pounds

Look at the pulling tension, just double for the same cable in the same duct due to the change of cable pulling direction.

Hope the “cable pulling procedure and following conditions” in field level during execution the project will helpful for the persons who are concerns. Now it’s your turn to calculate the cable pulling tension and select the cable pulling right direction. 

What Happen If Cables Placed In Magnetic Metal Conduit

What Happen If Cables Placed In Magnetic Metal Conduit 

Do we know what happen if cables in a magnetic metal conduit? Yes, at least we know that in any circumstances it should not be installed individual phase of an ac (alternating current) circuit in separate magnetic metal conduit. 

Most of us we follow these important rules for cable installation, but everyone can’t explain why we should follow this; what happen if individual ac phase placed in a separate metal conduit.

Yes of course, some of us are little advanced; we know we don’t place individual ac phase in metal conduit because of excessive heat may occur in that situation. 

Anyway, separate phases should not pass through magnetic structures because overheating can occur due to excessive hysteresis and eddy current losses.

Already question is arisen, is it acceptable to passes all the phases together through in a magnetic conduit? Yes, it is wise to passes through all the phases in any magnetic enclosure simultaneously so that maximum cancellation occurs of the resultant magnetic field. 

This procedure huge reduces the magnetic field, though increases the skin and proximity effect of cable. Because of proximity of magnetic materials some losses occurs in conductor just placing the cable near the magnetic materials, amount of losses depends on volume of materials.

Sponsored:

Skin Effect:

In an ac circuit, current tends to flow through over the outer surface of conductor than the inner side, means that current density is greater near the outer surface of the conductor. This effect is called skin effect. 

Due to skin effect, a long cable center axis is surrounded more magnetic field than surface, as a result inductance increase toward the center. Finally resistance increase and conductance decrease in the center than surface.

 Proximity Effect:

If ac conductors are placed in closely, there is a tendency for the current to shift to the portion of the conductor that is away from the other conductors of that cable. This is called proximity effect. 

So, we can say: Insulated conductors should not be installed in metallic conduits and also not run close to magnetic materials. For example- high voltage underground power cable frequently need to passes along the metallic materials like gas pipe line, water pipe line, etc. which may dangerously electrified by induced voltage if not properly grounded both of metallic pipe and cables metallic sheath.