After the most appropriate type and size of conductor, pole, and poletop hardware have been selected for a specific application and the poles, poletop hardware, and any necessary guys have been properly installed, the conductor must be
strung. This involves placing the conductor in position, tensioning the conductor so that the tension does not exceed a certain percentage of its ultimate strength, and then fixing the conductor at each pole. Tensioning the conductor is referred to a "sagging"
because tension and sag are directly related to each other; the proper horizontal tension “H” is generally determined by measuring sag "S"
(Fig. 25).
Before any work on stringing and sagging the conductor can commence, the appropriate sag tables must be obtained. For the more conventional types of conductor (ACSR, multiplex or ABC, etc.), the
conductor manufacturer should be able to provide these. Examples are found in Appendix 8. If other conductor is used, it is necessary to establish the maximum tension that should not be exceeded and to calculate the sag associated with that tension (see next section). For example, in the U.S., the NESC limits the tension on ACSR conductor to 35 % of ultimate strength at 16 ºC when it is initially strung and carrying no ice or wind loading.
Placing the conductor in place can be fairly straightforward with small conductor. However, as larger and heavier conductor is used, more care must be exercised. This section deals primarily with such
conductor, although some points are common for all conductors.
* Trying to balance loads along a distribution line means that, as one proceeds along that line, loads are connected to each phase conductor in such a way that the currents in these conductors at each point along the line are as close to equal as possible. This leads to negligible current in the neutral conductor (in cases when there is such a conductor, see Fig. 14b and 14c) and minimizes voltage drop along the line.
Ls s wc
H H
Fig. 25. Basic terms associated with sagging a conductor.
Box 5. Estimating conductor size.
In the initial planning process, it is often necessary to obtain an initial estimate of conductor size for a specific project. The graphs below provide a quick way for using equations found in Table 8 to determine the conductor size required to keep the maximum voltage drop to within a desired range.
To estimate conductor size for a stretch of distribution line operating at a nominal consumer voltage of 230 V (see p. 77 for definition) with loads balanced along the line, a conductor equivalent spacing of 0.30 m, and a frequency of 50 Hz , either of the following three numbers will be required, depending on the actual situation:
4. If the load is concentrated at the end of the line, multiply the peak load (kW) by the length of the line (km) to get the kW·km loading, k’.
5. If the load is relatively evenly distributed over the entire length of the line, sum all the peak coincident loads (kW) and multiply this by half the length of line to get the kW·km loading, k’.
6. If the load is unevenly spread, sum the products of each load (kW) and its distance (km) from the beginning of the line to get the kW·km loading, k’.
To determine conductor size for a single-phase, two-wire system under the conditions mentioned above, look up the value k = k’ on the appropriate graph (determined by the average power factor of the loads served), move vertically until the desired voltage drop is reached, and then move horizontally left (for alu-minum conductor) or right (for copper conductor) to determine the value.
For perfectly balanced systems:
For a single-phase, three-wire system, use the value k = k’/4 and follow the steps noted in the previous paragraph. For a three-phase, delta system, use the value k = k’/2 and follow the same steps. For a three-phase, wye system, use the value k = k’/6 and follow the same steps.
For systems with a 50 % load unbalance:
For a single-phase, three-wire system, use the value k = k’/2.3 and follow the steps noted in the previous paragraph. For a three-phase, delta system, use the value k = k’/1.8 and follow the same steps. For a three-phase, wye system, use the value k = k’/4 and follow the same steps.
In preparing the graphs below, a distribution voltage of 230 V was assumed. To determine conductor size for another operating voltage, first determine the value of k as described above. Take this value of k and multiply it by (230/E)2, where E is the nominal voltage being used by a single-phase consumer (defined on p. 77). Use this modified value of k and proceed to use the appropriate graph to determine necessary conductor size. Note that, for a given conductor, if the distribution voltage were reduced by half to 115 V, the load that could be served by this same line would be reduced to one quarter of the original load served at 230 V.
In preparing the graphs below, an equivalent spacing of distribution conductors of 0.30 and a frequency of 50 Hz were assumed. If either of these parameters are different for a specific situation, the impedance of the line x changes somewhat, but this will generally change the graphs only slightly. If a more precise value is desired, the equations in Table 8 can be used. The resistance and reactance for a specific conductor can be obtained for the graphs and equations found in Appendix 6 (beginning with p. 224).
0 Conductor area. Al (mm 2 )
0
Conductor area, Cu (mm 2 ) 10 % Conductor area, Al (mm 2 )
0
Conductor area, Cu (mm 2 )
2 % 4 % 6 % Conductor area, Al (mm 2 )
0
Conductor area, Cu (mm 2 ) Power factor = 0.6
2 % 4 % 6 % 8 %
10 % 1 %
In planning for the stringing and sagging of larger conductor, it is necessary that sagging be done within several hours of pulling the conductor. This is because the conductor will begin creeping as soon as it is off the ground and the required sag will start to change. Creeping is the elongation of conductor under tension. As tension is applied to the conductor, it stretches and will continue to stretch until a balance between tension and the materials strength is reached. This process may take several years. With new conductor, if sagging is not completed within the recommended time period, it becomes impossible to accurately calculate the sag from sag charts, which only indicate "initial" sag and "final" sag. For
example, it can be seen from Appendix 8 that, while the proper sag for a 70-m span of #2 ACSR triplex at 25 °C is 0.53 meters for new conductor, it increases to 0.75 m after creeping has completed. If there is too much time between pulling the conductor and tensioning or sagging this conductor, the required sag will be at some unknown value somewhere between these two sag values.
Sag
The sag in a conductor is determined by the weight and tension of the conductor and its span. The relation between these three parameters for a given conductor is illustrated in Fig. 26. If one assumes that under a given tension, a conductor with a span of "L" has a sag of "S" (Fig. 26a), then keeping the same sag (and therefore ground clearance) while increasing the span will require placing the conductor under increased tension (Fig. 26b). If the tension then exceeds the allowable value, it can be decreased for this increased span by increasing the sag (Fig. 26c). If this reduces clearance to too low a value and the longer span is necessary, then a longer pole would be required.
For a given conductor type and size, the sag depends on the span according to the following relationship:
(a)
H H
(b)
2H 2H
4H 4H
(c)
same sag
doubling span same span doubling tension
S
S
S/2
2L
L L
Fig. 26. For the same span, the sag is inversely proportional to the tension, see (a) and (b). To maintain the same sag, the tension in a conductor is proportional to the square of the span, see (a) and (c).
H
H = horizontal force at pole (either kg or N but must be the same as used in weight of conductor above). This is approximately equal to the tension in the conductor.
Handling and inspecting the conductor
When receiving conductor to be used on a project, the reel and any protective covering on the conductor should be inspected for damage. A broken reel or damaged covering may indicate improper handling and possible damage to the conductor. When handling larger reels manually, they should be kept in an upright position and rolled. If necessary, ramps should be used to facilitate loading and unloading.
During warehousing and transport, reels should be kept in an upright position at all times; otherwise, the lays of the conductor may overlap, causing possible damage to the conductor or delays in the stringing process.
Preparation for stringing
Prior to stringing the conductor, the route of the line should be inspected to ensure all is ready for the pull. The right of way should be inspected for obstacles that may damage the conductor or complicate the stringing. If any obstacles cannot be removed, rigging may be required to ensure that the conductor is not damaged during the pulling process.
For larger conductor that is heavier and bulkier and involves handling greater forces in the stringing and sagging process, pulleys should be temporarily installed on each pole and inspected to ensure that surfaces are smooth and roll freely.
The reel should be properly located with a stable base and positioned on the reel stand so that the conductor will unwind from the bottom. A leader line (rope) several dozen meters long is attached to the beginning of the conductor, usually by means of a wire mesh grip (Fig. 27), and threaded through the first pulley. This provides added safety to the pulling team, facilitates threading the conductor through the pulleys, and helps protect the conductor during the installation process.
All guys should be installed and checked and poles inspected for proper positioning before pulling the conductor.
Pulling the conductor
The conductor should not be payed out (removed) from a reel or coil that is not free to rotate; otherwise, each turn removed will leave one complete twist in the conductor that could eventually cause kinks. In all
Fig. 27. These flexible grips are comprised of a tubular steel mesh that is fit over the end of the cable.
Under tension, the mesh tightens on the cable, increasing its grip as ten-sion is increased. But it is easily removed once the tension is relieved.
cases, the reel should be mounted so that it is free to rotate. But it should not be allowed to spin freely, because this can cause the conductor to tangle on the reel, possibly damaging the conductor and delaying the process. It is also important to make sure the insulation is not damaged by dragging the multiplex over the ground or sharp objects, to avoid having vehicles cross or animals walk over the conductor, and to avoid kinks while paying out the conductor.
A couple of approaches for paying out the conductor from a reel are possible:
• The conductor can be paid out along the ground from a rotating reel that is moved down along the line, carried either by a vehicle or by a group a individuals, depending on access to the
distribution line and the weight of the reel. Alternatively, the reel can be placed in the most accessible point nearest the section being constructed and the conductor pulled along the line. To avoid any damage to the insulator or conductor by dragging it across the countryside, villagers can hold on to the conductor at appropriate intervals, each carrying his or her section until it can be place under its final resting place along the line. The conductor would then be carefully raised onto the insulators for final tensioning.
• With conventional distribution lines, pulleys are hung from the location on each pole where the conductor is to be mounted. A rope would then be passed through the pulleys and the end tied to the conductor on a fixed but rotating reel located at the end of the section being worked on. This rope would then be pulled over consecutive pulleys toward the beginning of the section, pulling the conductor along with it.
Once the conductor has been pulled, it should be deadended at one end so that sagging can begin with minimum delay. To minimize delay, the specific span to be sagged within the entire section being pulled should have been selected before pulling the conductor and measuring the temperature. In this way, the sag is known and work on getting the proper sag can proceed immediately.
To facilitate temporarily holding the conductor along its length, any of a variety of grips can be used.
These hold the conductor while tension is applied, but they are easily removed once the tension is released (Figs. 28 and 29).
Before the conductor is tensioned, any pole that would be subjected to an unbalanced force must be suitably guyed to counteract the tension in the conductor that may cause the pole to otherwise bend over and break. This may be accomplished
Fig. 28. Grips of a wide variety of designs are used to temporarily hold a conductor.
Fig. 29. A grip is being used to tension a
conductor passing through a pulley in a system in rural Nepal.
by placing permanent guy wires and anchors in cases where this unbalanced force remains after the line has been fully strung. In cases where unbalanced conductor forces on a pole will disappear once the entire length of the line has been completed, a temporary guy wire firmly fixed to the bottom of a trees, a fence post, etc., can be used.
Sagging the conductor
The correct tensioning or sagging of the conductor is one of the most important phases of distribution line construction, especially for larger conductor sizes, and effects its reliability and longevity. If a conductor is sagged too tightly, it will cause the structure and conductor to fatigue. If all the conductors along a span do not have the same sag, the wind can cause them to slap together, causing outages and damage to the conductors. If it is sagged too loosely, it can become a hazard to the public because of the reduced clearance.
To determine the proper sag for a given span, it will be necessary to measure both the span and the temperature of the conductor at the time of sagging. To measure the temperature of the conductor, a thermometer should be placed directly against a piece of the conductor raised to poletop level. It should not be placed in direct sunlight as this will give a false reading. The temperature should be noted after the readings no longer change significantly.
As can be seen by referring to typical sag tables in Appendix 8, the temperature is important. Sag can change considerably with changes in temperature because the conductor expands and contracts as the temperature increase or decreases. For example, at an early morning temperature of 16 °C, a 70-m span of #2 ACSR triplex should have a sag of 0.48 m. But should conductor temperature rise to 32 °C in the middle of the day, this sag will increase about 20 % to 0.58 m. Close attention must be paid to the conductor temperature at the time of sagging.
For a given span and temperature, reference to sagging tables such as the ones in Appendix 8 will give the required sag under these conditions. The initial sag chart should only be used with new conductor that has never crept. (The final sag chart is to be used on conductor that has been removed from other lines and reinstalled. This chart is also used to check sag on existing line.)
After one span of a section of conductor has been sagged, it is not necessary to sag every span. Assume that the conductor has been pulled over freely rotating pulleys. Then, if the conductor is one span is
deadend
pulley timing
rope
span being sagged
cable reel
Fig. 30. Once a section of line has been pulled over pulley, properly sagging one span properly sags the entire section. Timed sagging is one method for sagging longer spans. In stringing the conductor in this case, the rightmost pole should be temporarily guyed toward the right because the conductor under tension tends to force that pole to the left.
properly tensioned (as determined by its sag), the tension will be the same in every span in that section (Fig. 30). (Note that if the pulleys are freely rotating, the tension of the conductor in each span will be the same, but the sag will be different if the spans are of different lengths.)
As noted above, properly tensioning the conductor is necessary. Tensioning the conductor is more easily done indirectly by measuring the conductor's sag "S" in meters rather than directly measuring its tension
"T". The two methods for doing this are the (1) sighting method and (2) the timing method.
Sighting method
This direct method of sagging is the easiest, especially for multiplex and short spans. It requires nailing a lath or small board horizontally to the pole at either end of the span being sagged.
These are nailed below the final resting place of the conductor (on insulators) at a distance equal to the required sag for that conductor, span, and temperature. Someone on the pole sights from one lath to the next and the tension of the conductor is adjusted so that the lowest point along the
conductor coincides to the person's line of sight
(Fig. 31). This is usually more easily accomplished if the person sighting the sag is back one span and is not on the same pole as the lath.
Timing method
If a conductor is struck at one end of a span, a wave is initiated and travels down the span, bounces off the far support and returns back to the beginning. The time that this takes depends only on the sag in the conductor and not on other variables, such as span length, conductor type or size, and temperature. This fact can therefore be used to indirectly measure the sag of a conductor.
Assume that the section shown in Fig. 30 is to be sagged. For this purpose, a light rope is thrown over the conductor, a meter or so from the end of the span. If a section of several spans are being sagged, a middle span should be sagged. A wave is created by briskly jerking once on the rope, at the same time that the stopwatch is started. Each return waves can be felt as it passes the lightly held rope and is reflected back for its next trip down the span. This continues until the wave damps out sufficiently so that it can no longer be felt. The time for 3, 5, or 10 return waves is measured. The larger the number of returns that are clearly discernible, the better the accuracy. With longer conductors, the wave may dissipate more quickly, in which case a fewer number of returns might be timed.
From the recorded time for the wave to complete a given number of return trips, the existing sag can then be calculated: N = number of return waves
A graphical solution to this equation is found in Fig. 32.
line of sight
lath lath
eye sag
Fig. 31. Using two wooden strips (laths) to sight low point of span.
For the above equation to be correct, the
For the above equation to be correct, the