The basic objective of stringing overhead distribution is to install the conductor from one point to the other in the best possible condition and in the safest, most economical manner. To accomplish this job, the proper equipment, tools, and training are most important.
Four Key Components
There are four key components used in an overhead distribution stringing job and are listed below in order of importance: the tensioner, pulling line (bull line), stringing blocks, and the puller. These four key components should work together, and if any one of these components is deficient in design or performance, it directly affects the other three. If the stringing blocks are of poor quality and do not roll efficiently, then they directly affect the amount of pulling capacity needed to install the conductor. This would then require a larger capacity puller.
The pulling line is also affected and could possibly cause it to fail due to increased loads above its safe working load. The tensioner will also be required to create more tension, thus causing additional stress on the hardware, structures, and the most important component – the conductor. So, you see how all four components work together and one deficiency can cause multiple problems.
The first step in the selection of the equipment would be to use the IEEE Guide formula to determine the amount of tension it takes to pull this conductor in and have adequate capabilities with some reserve. There are two formulas used. One formula, T1, is used to establish the amount of tension required to support the conductor in one span. The TMax formula is used to determine the maximum amount of tension that is needed to pull the conductor in. The formulas are as follows where:
T1 = the tension required to support 1 span (static condition)
W = weight per unit length of the conductor
L = span length
D = sag during the stringing phase
TMax = the maximum tension required to pull the conductor
.98 = efficiency of stringing blocks
n = number of supports or blocks
T1 = WL2 ÷ 8D
TMax = T1 ÷ .98n
In consideration of the first piece of equipment, the tensioner, a single trailer capable of carrying the reel size and weight of the conductor should also have a set of multi-groove bullwheels that would allow the conductor to be tensioned and not come directly from the reel. However, if the tensions are acceptable on the reel itself, then a hydraulically controlled puller/tensioner is a very practical approach to tensioning the conductor, considering the physical condition of the reel itself.
Although bullwheel type pullers are used in some countries for various reasons, two like machines called puller/tensioners work in concert with each other for the best and smoothest installation and control. In the United States the most common puller is a drum type wherein a large winch is used with the pulling line or bull line. Traditionally, the use of a parallel lay rope, namely Uniline, has the least elongation, is derived with the highest strength to size ratio, is the longest lasting rope, and should be used on drum type pullers.
The third piece of equipment used in the distribution stringing operation are commonly referred to as blocks. Good stringing blocks use ball bearings to allow the sheave to spin efficiently, usually at only a 2% loss per block. This loss is due to the constant conductor bending and straightening as it passes over blocks. It is common to use a larger diameter block at the lead and dead-end poles, as well as at severe angle points in a pull. The stringing blocks are usually installed when framing the poles, crossarms, hardware, and insulators.
Once the poles in a given pull have been installed, a pilot line system, commonly called a Spider System, is placed on the pole at the conductor tension end of the pull itself. It is advantageous to use a single Spider Unit consisting of a brake. This is chained to the pole, along with a drum with typically 3,000 or 6,000 ft. of pilot line, all four mounted on the lead pole. These four ropes can be taken from pole to pole as they are framed with distribution blocks. When the last pole is framed there will be four continuous ropes of different color, from one end of the pull to the other. Using this system instead of individual threader ropes allows the pulling line to be pulled back individually one at a time in order to string the phase conductors and finally the neutral.
These also allow the stringing of the main pulling rope under tension and therefore keep it out of any existing underbuild or obstructions below. Once the pulling rope is installed and attached to the conductor with a grip and swivel, each phase can be pulled through individually and caught off and brought up to sag as the pulling rope is returned to the tensioner for the second pull. Once the 3-phase conductors have been pulled and sagged, the neutral can be pulled in the same manner at which time the job is complete, as far as the installation of the conductors is concerned. All that remains is the clamping in of the conductors in the insulators and removal of the stringing blocks to finish the job.
Need some guidance for your crew or maintenance team? Sherman+Reilly provides in-service, classroom, and on-site training customized for your equipment and your team from operations to preventative maintenance. Contact us at 423-756-5300 or click below to learn more about our training options.
Learn more about how to pull conductor with a four drum rope rig by watching our whiteboard video. Click below to go to our YouTube channel: