To install a cable into a conduit, it must be pulled from one end of the conduit to the other with a strong wire. When a cable is pulled into a conduit, there are maximum pulling tensions that the cable can withstand without damage. There are various types of pulling equipment that can be used to pull a cable into a conduit. Each different type of pulling equipment has a maximum pulling tension (or pulling force) that it can withstand. The configuration of the conduit, the type of cable that is to be installed, and the types of pulling equipment that are chosen for the installation should all be evaluated so that damage to the cable or to any installation equipment does not occur. Calculations to determine the maximum pulling tensions that could occur with various conduit configuration and pulling equipment combinations are performed during the design phase of an installation. These calculations are evaluated to ensure that maximum pulling tensions are not exceeded during a cable installation pull.
There are various ways to reduce the pulling tension for a given cable installation: the rigging equipment can be varied, the size of the conduit can be increased, the conduit configuration (e.g., turns or angles) can be altered, or the pull point frequency can be changed. This section of the Module provides information on the following topics that are pertinent to determining cable pulling requirements:
o Rigging Procedures o Cable Pulling Parameters Rigging Procedures
During the design phase of the wire or cable installation, once the installation type (e.g., conduit) and the cable route have been chosen, the Electrical Engineer selects a rigging method and then performs a pulling tension calculation. If the pulling tension calculation indicates that maximum tensions could be exceeded by the cable pull, design changes are made. Before the cable pulling parameters and pulling tension calculations are described, a description of the cable rigging equipment and methods is necessary.
The method that is used to rig the cables for pulling in above-grade conduit and cable tray systems depends on the length of the pull and the size of the conductors to be installed. Smaller conductors that are installed in a short run can most likely be pulled in by hand. Pulling equipment will probably be required to install larger conductors or to install conductors in long runs so that a constant pulling tension can be maintained on the cables.
Suitable pulling equipment that is in good working condition should be on hand for the pulling operation. Hydraulic pulling equipment that has smooth, variable-speed control is
a good choice for cable installations in above-grade conduit installations. To ensure that the maximum allowable pulling tension for the installation is not accidentally exceeded, a steady pulling rate should be maintained, whenever possible, during the pull. Cable pulling speed should never exceed 15 m/min (50 feet/min) and, if at all possible, the cable should not be pulled slower than 4.5 m/min (15 feet/min).
If the cables will be installed in a grouped conduit run, the conduit that will be used for the installation of a single cable should be identified throughout the entire length of the run to avoid cable crossovers during the installation process. As much as possible, the same relative position in the group should be maintained throughout the run. In general, the longer cables should be installed in the lower raceways, and the shorter cables should be installed in the upper raceways to facilitate the ease of installation.
Before the pulling operation begins, the direction of the pull should be checked to ensure that it is the direction that results in the minimum pulling tensions and sidewall pressures. Care must also be taken when moving the cable reels into their proper positions.
If the pull and cut method of installation is used, cable damage can occur during the setup phase of a cable pull. The pull and cut method is most often used for pulls at several different locations.
During cable installation, once the protective covering is removed from the reel, the cable is particularly vulnerable to mechanical damage.
The cable reels must be supported by an axle at the installation location so that the cable can be pulled with minimum friction. The pulling tension equations should take into account any friction that results from the setup at the feed end of the conduit so that the maximum allowable installation tension is not exceeded.
To measure the pulling tension that is applied to the cables, a dynamometer is often used during the pulling operation. The dynamometer can be connected at the feed end of the conduit or at the discharge end of the conduit. An idler is attached to the dynamometer at the feed or discharge end of the conduit, and the pulling line and/or cable is routed over the idler to allow the tension that is applied during the pulling operation to be measured. Figure 9 shows a method that could be used to attach the dynamometer at the discharge end of the conduit. The equation that is used to determine the tension on the cable in the conduit is also shown in Figure 9. If the dynamometer is not zeroed with the idler attached, the weight of the idler must be subtracted from the meter reading.
Figure 9: Dynamometer Used to Measure Pulling Tension
When several single-conductor cables are to be pulled into a conduit or duct, the cable reels should be set up in tandem, and all of the cables should be simultaneously pulled into the conduit or duct. The cables should be continuously trained into the conduit in such a manner that the cable will not drag on the edge of the conduit. The cables should be fed into the conduit by hand or, for large conductors, by a large diameter sheave (pronounced "shiv").
In addition to reducing friction, selection of the correct diameter sheave for the job ensures that the minimum-bending radius of the cable is maintained. Although quality sheaves are generally treated as frictionless (no effect on pulling tension), extreme care and good judgement should be exercised in their use. Short cable bends, sharp edges at the feed to the raceway, and cable crossovers should be prevented at the point where the cable enters the raceway. The Electrical Engineer should ensure that the diameter of the sheave that is used for the installation is large enough that the sidewall pressure that is exerted on the cable does not exceed limits. Sidewall pressure (SP) is defined as the crushing force that is exerted on a cable as it is pulled around a bend section of conduit or duct.
Supports should be used to alleviate stress on the cables where they enter the conduit or cable tray. Cables should be laid out or pulled into cable tray runs. When cable is pulled through a cable tray, cable rollers, pulleys, and sheaves should be used to prevent cable damage. If the pulleys, sheaves, and rollers that are used for cable installation in cable trays are undersized, the cable can be damaged if it bends below its minimum allowable bending radius. If the rollers that are used for the installation are too widely spaced, the cables can be damaged from abrasion against the cable tray rungs. Rollers should be separated by a maximum distance of 3 m (10 feet) during cable pulls in cable trays. Sharp points in the cable tray, such as bent or burred metal, dropped tools in the cable tray, and reversed bolts (heads on the outside of the cable tray rather than on the inside of the cable tray), can cause extensive cable damage during the cable pull, and they can lead to early failure of the installed cables. The maximum pulling tension equations that are listed in the following sections should be followed for both conduit and cable tray systems.
The remainder of this section of the Module provides information on the following topics that are pertinent to cable rigging procedures:
o Pulling Grips o Pulling Lines o Duct Lubricating
Pulling Grips
Pulling grips are used to fasten the pulling line to the cable that is to be pulled into the raceway. There are two basic types of pulling grips that are frequently used for cable installations: basket grips and pulling eyes.
A basket grip is a flexible metal device that slips over the end of the conductor that is to be pulled; a pulling line is attached to the basket grip so the cable can be pulled through the raceway. The basket grip has a web-like grip that tightens as tension is applied to the pulling line. Long basket grips should be used to pull type MC cables. To use the basket grip, the armor is removed for a short distance, tape is applied over the armor and onto the conductors, and the basket grip is placed onto the cable to allow the grip to squeeze both the armor and the conductors. Figure 10 shows how a typical metal basket grip is installed onto a cable. To avoid cable twist during the pulling operation, a swivel has been installed onto the pulling line where it attaches to the basket grip.
Figure 10: Basket Grip on Cable
Because the conductor and the cable insulation are stressed during the pulling operation, the parts of the cable that are directly affected by the basket grip should be removed before a splice or termination is installed. Sufficient slack must be present at the pulling end so that the last 600 mm (2 feet) of cable beyond the basket grip can be removed before the cable is spliced or terminated.
Pulling eyes are sometimes used to pull larger sizes and long sections of cable; pulling eyes allow for a higher pulling tension than basket grips. A pulling eye is a steel eye that is usually fastened directly to the cable conductors. Many manufacturers supply a pulling eye or pulling bolt to the leading end of the cable that is on the cable reel. Once the pulling operation is complete, the pulling eye should be removed from the cable. Figure 11 shows how the cable conductors are securely fastened and solder-wiped to the shank of the pulling eye. In Figure 11, a swivel is used to connect the pulling line to the pulling eye to avoid cable twist during the pulling operation.
Figure 11: Pulling Eye on Cable
Pulling Lines
A pulling line is used to pull the cable through the conduit or cable tray during the cable installation. Pulling lines can be made of rope or wire, and they are provided in various sizes. The type of pulling line that is used for a given installation depends on the size of the conductor to be pulled, the type of pulling grip to be used, and the length of the pull.
The pulling line is typically drawn into a conduit with a steel fishing wire or "snake." Fishing wire is a tempered-steel wire that has a rectangular cross-section. Although the flat rectangular fishing wire is preferred because of the ease with which it can be run through a completed conduit system, galvanized-steel wire can also be used for fishing. Any size wire from #14 up to #6 can be used for fishing. When the fishing wire is drawn through the conduit, the pulling line is attached to the fishing wire. The fishing wire is then withdrawn from the conduit so that the pulling line is completely drawn through the conduit system. Fishing wire is not required to pull cables through cable tray systems.
After the pulling line is completely drawn through the affected raceway, the pulling line is firmly attached to the pulling grip and to the pulling device through an idler. The idler is attached to a dynamometer to allow the tension on the pulling line to be evaluated throughout the pull.
Duct Lubricating
To reduce friction during the pulling operation, lubrication should be liberally and continuously applied to conduit. Increased friction during pulling can also cause an increase in the pulling tension that is applied to a cable during its installation. The use of minerallac #100 or equal lubricant is recommended for duct lubrication in Saudi Aramco above-grade conduit installations.
Duct lubricant should not be applied to the first and last 15 meters (50 feet) of the cable for reasons of convenience and cleanliness in splicing operations. Duct lubrication is not required during cable tray installations.
Cable Pulling Parameters
Cable pulling parameters include maximum pulling tension and sidewall pressure calculations. The tension calculations take into account the maximum allowable tension that can be withstood by the pulling device and the conductors. The sidewall pressure calculation takes into account the stress effects that a bend in the conduit or cable tray has on the cable during the pulling operation.
The general procedure that is used to determine the cable pulling tensions and parameters is to calculate the pulling tensions for the entire length of the pull and then to determine whether the sidewall pressure is too great at the conduit bends. When the pulling tensions are calculated for the pull, the calculations are performed twice: once for a pull in one direction and again for the pull in the opposite direction. This section of the Module describes cable-pulling parameters in terms the following topics:
o Maximum Pulling Tension o Sidewall Pressure
o Rigging Method Effects Calculations
Maximum Pulling Tensions
After the size of the raceway has been determined (based on allowable fill, the jam ratio, and cable clearance), the maximum tension for the pulling device and the maximum tension that can be safely applied to the conductors should be calculated. The most limiting maximum tension is used as the maximum allowable pulling tension (Tm). Next, the pulling tension (T) that is actually required to pull the cable through the raceway is calculated and compared to the maximum allowable pulling tension. If the actual pulling tension that is calculated for the pull exceeds the maximum allowable pulling tension, the conditions of the pull should be changed.
When the actual pulling tension exceeds the maximum allowable pulling tension for the installation, the following actions should be considered:
o Increase the bending radii that will be used for the conduit installation.
o Reduce the number of bends in the conduit.
o Perform a reverse pull.
o Perform the pull in stages.
o Decrease the length of the pull.
The maximum allowable tension for the pulling device (Tdevice) is dependent on whether the pulling device is a pulling eye or a basket grip:
o The maximum pulling tension that can be applied when a pulling eye is used should not exceed 22 kN (5000 lbf) per cable (assuming that each cable has its own pulling eye) for single-conductor cables or 27 kN (6000 lbf) per cable for multiple-conductor cables.
o The maximum pulling tension that can be applied when a basket grip is used over the outer jacket of a cable should not exceed 4450 N (1000 lbf) per cable (assuming that each cable has its own basket grip) in any case.
The maximum allowable tension for the cable (Tcable) is dependent on whether single conductor cables or multiple conductor cables are to be used in the installation. Different conductor sizes can be pulled at the same time, but a simultaneous pull is not recommended if the dimensions of the conductors are significantly different. If different sizes of conductors are to be pulled into the same run of conduit, care must be taken not to exceed the maximum pulling tension of any one cable during the pull.
Because a pulling rope under tension could possibly cut previously existing cables in conduit systems, pulling additional cables into an existing conduit is generally not recommended. However, consideration should still be given to whether conductors that are of different sizes should be pulled at the same time for a given installation. The equations that are used to determine the maximum allowable cable tension are as follows:
o The maximum pulling tension that can be applied to a single-conductor (copper conductor) cable is calculated through use of the following equation:
mm) (in AREA mm
71 N
=
cmil) (in AREA cmil
0.008 lbf T =
2 2
cable
•
•
o The maximum pulling tension that can be applied to multiple conductors when there are three or less conductors (for cables that are in parallel, multiplexed, or are multiple-conductor cables) is calculated through use of the following equation:
= T
T
cableΣ
cWhere Σ Tc is the summation of the maximum pulling tension for each individual cable.
o The maximum pulling tension that can be applied to multiple conductors when there are more than three conductors (for cables that are in parallel, multiplexed, or are multiple-conductor cables) is calculated through use of the following equation:
0.8 T
T
cable= • Σ
cWhere Σ Tc is the summation of the maximum pulling tension for each individual cable.
Once the maximum allowable tension on the pulling device and the maximum allowable tension on the cable have been determined, the two values are compared, and the lowest of the two values is designated as the maximum allowable pulling tension for the installation (Tm). Next, the actual pulling tension for the installation is calculated and compared to the maximum
The configuration of the cables that are being pulled into a conduit system affects the pulling tension calculations; the weight correction factor (w) is a calculated value that is used in the tension equations to account for the effect of cable configuration.
Figure 12 shows the various cable configurations that can occur for a single-cable installation, a dual-cable installation, a three-cable installation (cradled or triangular configuration), and a three-cable installation for more than three cables (complex configuration).
The three-cable installation configurations are of special interest.
Based on the result of the jam ratio (recall that the jam ratio is equal to 1.05 times the D/d ratio), a three-cable installation will have a cradled configuration or a triangular configuration. If the jam ratio is less than 2.5 or if assembled cables (triplexed cables) are to be pulled, the installation will result in a triangular configuration.
Figure 12: Cable Configurations
The equations that are used to calculate the weight correction factor are selected based on the configuration of the cables for the installation. The weight correction factor equations are as follows:
o The weight correction factor for a single-cable configuration (including the case of a multiple-conductor cable) is as follows:
w = 1
o The weight correction factor for a dual-cable configuration is as follows:
o The weight correction factor for three cables with a triangular configuration is as follows:
o The weight correction factor for three cables with a cradled configuration is as follows:
o The weight correction factor for complex cable configurations is as follows:
ww = 1.4
Another factor that is of critical importance to the tension equations is the cable weight (W). Cable weight is the weight per unit length of the cable that will be pulled through the raceway.
When cables are to be installed in parallel (at the same time), the cable weight factor that is used in the pulling tension equations
When cables are to be installed in parallel (at the same time), the cable weight factor that is used in the pulling tension equations