Overhead line power distribution
|Copyright. Do not change the body text, and be careful about reusing - we do not have clear permission to modify this work, and permissions for reuse are currently under discussion.
This content may be out of date. Take appropriate precautions before relying on it.
- 1 POWER DISTRIBUTION SYSTEM
- 1.1 MAP MAKING
- 1.2 SELECTING THE ROUTE
- 1.3 CLEARING THE ROUTE OF THE LINE
- 1.4 LOCATING POLE POSITIONS
- 1.5 CHOOSING THE POLE
- 1.6 POLE HAULING
- 1.7 POLE PREPARATION
- 1.8 TRIMMING
- 1.9 ROOFING
- 1.10 GAINING
- 1.11 PRESERVATIVE
- 1.12 DIGGING THE POLE HOLE
- 1.13 RAISING THE POLE (Pike Method)
- 1.14 POLE SETTING
- 1.15 GUYING THE POLE
- 1.16 DIGGING IN THE ANCHOR
- 1.17 INSERTING THE INSULATORS
- 1.18 FASTENING THE GUY TO THE POLE
- 1.19 TIGHTENING THE GUY AND FASTENING TO THE ANCHOR
- 1.20 JOINING LINE CONDUCTORS
- 1.21 MAKING A SPLICE JOINT
- 1.22 MAKING A SLEEVE JOINT
- 1.23 MAKING A COMPRESSION JOINT
- 1.24 STRINGING THE WIRE
- 1.25 SAGGING LINE CONDUCTORS
- 1.26 COPPER WIRE FOR DIFFERENT SPAN LENGTHS
POWER DISTRIBUTION SYSTEM
As in the previous section the emphasis of this section is doing. The PCTs will be digging pole holes, erecting the poles, and stringing the lines for the sample distribution system. They will also install needed equipment such as transformers, fuses, and lightning arresters.
In an effort to reduce the length of the training period it is recommended that the PCTs actually dig only one or two of the necessary holes for the poles and anchors. It is recommended that a professional crew be hired to dig the remaining holes with a truck mounted power auger. This operation would be observed by tie trainees and discussion of the methods used by the professional crew encouraged. The remainder of the installation should be handled by the PCTs.
The design of the distribution system should be the responsibility of the engineer in charge of the project. This section considers briefly the design of the system, in order to give the PCTs needed background knowledge.
The following describes the construction table. Such maps are valuable for village route of distribution lines, of serviceable layout plans, maps using and planning a plane the following tools and materials are needed: Plane table, Paper, Pencil, Ruler, Pins, Tape measure, Spirit level.
Lay out a one hundred foot interval on level ground, an uphill, and a downhill slope. If only a foot ruler is available, this may be used to mark out three or four feet on a stick, and this stick in turn used to measure the 100 feet. Being careful to work normally, the map maker then determines the number of paces over the 100 foot interval for each slope. By division, it is then possible to find a number of feet in an average pace or uphill, level, and downhill slopes.
The next step is to decide on a scale for the map. This is determined by judging the longest distance to be mapped and the size of the map desired, It should be noted that the map does not have to be made on a single sheet of paper but can be splice< together when completed. As an example, if one wanted a map 2 l/2 feet long to portray an area whose major distance is l/2 mile, 2640 feet, then a scale of 100 feet to the inch would be convenient.
Paper should be placed on the plane table and the plane table oriented on or near some principal feature of the map, that is, a path, road, creek, street, etc. A pin should then be placed vertically in the spot on the finished map where this location is desired. The plane table should be made level -by use of a spirit level, if available. The table should be rotated to a proper orientation, that is, so that the direction will appear on the finished map in the desired way. Now sight along the first pin to another principal feature which is visible from the table location (a bend in the road, a hill or any feature that will tie the map together), moving the second pin into the line of sight. A ruler may be used for this purpose if it has a sighting edge or even a couple of pins stuck into it. How draw a line in tile direction defined by the two pins. Measure the distance to the feature observed either by pacing or with a tape. Scale this distance along the line drawn, starting at the initial pin. Repeat this process for other principal features which may be seen from this location. When this has been done move the table to one of the points just plotted, selecting one which will enable you to move over the territory in a convenient fashion. For example, follow a lane or creek or some feature which ties things together. Set up the plane table over this point and reorient the table. Do this by putting pins into the map at the present and previous locations. Next rotate the table so that the pins line up with the previous location. This procedure in fact locates the line joining the two locations on the map in the same direction as the line exists in nature. Again from this new location map in the desired features which can be conveniently sighted.
In this way the entire region to be mapped may be covered in a systematic way. If gaps appear or if more detail is needed, you may go back and set up over some mapped feature , reorient the map by sighting on a second feature, and proceed to map in the detail.
An alternate procedure may be used in mapping features which are not going to be used as plane table locations in the mapping process. This involves drawing a line in the direction of each feature from two plane table locations. The intersection of these two lines corresponding to a single feature locates the feature on the map. As a result this avoids the necessity for measuring distances. Note, however, that it is impossible to avoid measuring the distances between plane table locations. If a spirit level is available, it is possible to level the plane table accurately, and using a ruler or other sighting device, relative elevation may be plotted on the map. A stick about six or eight feet long should be marked off in inches, and the person holding the stick vertically can, by moving his finger, identify to the person sighting, the distance up from the ground through which the line of sight passes.
A topographic map is a means of illustrating, through the use of contour lines, the shape of the ground surface. Many other kinds of numerical geophysical and geological data also lend themselves to the contouring method of expression. This exercise involves the determination of ground relief (topography) from points whose elevations above sea level are known. (You will need to know the basics of reading a topographic map.)
The method is called "contouring from spot elevations'. The might have been obtained by surveying with a plane table, although w3ern topographic maps are made much more easily and accurately by the stereoscopic plotting of air photo information.
A set of rules and hints in topographic contouring are: 1. All points lying on a contour are of the same elevation above (or below) mean sea level which is taken as the reference horizon. However, one contour need not satisfy all the points of equal elevation; e.g. adjacent hilltops of similar height might require separate, closed contours each showing comparable levels. Some contours may be cut by the edges of the map and appear, to be discontinuous but if the map be mad large enough every contour eventually closes on itself, becoming continuous. 2. With rare exceptions, the contour interval is constant for the map area and is defined as the vertical distance between successive contours. The contour interval is stated as part of the scale of the map so that the vertical dimension of the contoured surface has identical l0 foot, 20-foot, 50-foot and l00-foot intervals are common. The interval is selected to best show the shape of the surface at the desired horizontal scale without requiring an unnecessary, unreadable number of lines. The relief of the area to be mapped also influences the choice of the contour interval. 3. Contours do not cross. Such a situation would illustrate an impossible ground surface shape. Contours are closely spaced on steep slopes, and distantly spaced on gentle slopes. 4. Closed depression contours are hachured (See Fig. 4.2) on the lower side. They are used when all points within-the line are below the level of the line. Obviously they are only required to show depressions which are completely surrounded by high ground. Gullies and river valleys are not illustrated by depression contours. A depression contour takes its value from that of the lowest, topographically adjacent regular contour. 5. In contouring gullies and valleys, the contours vees in the upstream direction. Be careful to confine the stream to the lowest part of its valley by passing the stream through the notch of the vees. Fig. 4.3 Contours are broken where numbering is necessary, to improve readability. 6. The use of some degree of "artistic license" is recommended in contouring. Do not attempt to just satisfy the point data. Try to make the trend of a contour reflect the trend of its neighboring contours.
SELECTING THE ROUTE
The first step to be taken in the design or construction of any electric power line is to survey and map the country over which the line is to pass. With the map completed, the following principles should be used as guides in selecting the exact route: 1. Select the Shortest Route Practicable. The shortest line naturally is the cheapest, other things being equal. 2. Parallel Highways as Much as Possible. This makes the line readily accessible both for construction and for inspection and maintenance. 3. Follow Property Lines. This causes less damage to farmers' property and crops and often prevents legal squabbles. 4. Route in Direction of Possible Future Loads. If there is possibility of adding future loads. The route selected should be as close as-possible to the locations which will require electricity in the future. 5. Avoid Crossing Hills, Ridges, Swamps, and Bottom Lands. Lightning and storms are likely to hit lines on bills and ridges. Floods may affect lines in swamps and bottom lands.
CLEARING THE ROUTE OF THE LINE
Practically all lines will cross through some brush or timberlands. A line built in such terrain must have its route cleared before construction can be started. In clearing the route, all stumps should be cut low. All logs and brush should be cleared away for ten feet on either side of the pole line to make room for assembling and erecting poles and stringing wires. All dead limbs and branches near this cleared pole line should be cut down because a high wind may blow them into the line. Brush killing sprays may be sprayed on the base of shrubs and small trees to a height of 12 to 15 inches above ground.
LOCATING POLE POSITIONS
In locating poles, the following general principles should be kept in mind: 1. Select high places (avoid lowlands, swamps, etc.) 2. Keep "spans" uniform in length. ("Spans" are the distances between poles . This prevents the weight of the wire on one side from pulling the pole over). 3. Locate to give horizontal grade. (See Figure 4.4) Locate the poles on knolls or high places* so that shorter poles can be used to maintain the proper ground clearance at the middle of the span, (The ground clearance should be at least 18 ft. at middle of span). Avoid ravines and low places where the footing is bad.
In rolling country, the location should take into account the grading of the line. A well-graded line does not have any abrupt change, either up or down. The permissible difference in level between adjacent line poles is usually limited to 5 or 10 ft. This eliminates the necessity of using guys to counteract the strain of the different levels of line conductors. A difference of 5 ft. is allowed on spans of 150 ft., and 10 ft. on spans of 250 to 300 ft.
Special attention should be given to the location of poles where the ground washes badly. Poles should not be placed along the edges of cuts or embankments or along the banks of creeks or streams. When it becomes necessary to set poles on the edge of a cut, the pole should be set deep enough to protect the line in case the bank washes or crumbles away. After the exact pole positions have been fixed, drive a stake to indicate the center of the pole.
CHOOSING THE POLE
A pole or line support is simply a device to keep electric lines off the ground. Overhead electric lines are desirable for a number of reasons. It is safer to keep electric lines out of the hands of untrained people and roads and houses can be built beneath them. Any type of structure that keeps electric wire above ground is better than running the wire directly on the ground. The following list gives line support materials from the most desirable to the least desirable: 1. A 20 ft. standard wood pole (treated with wood Preservative). 2. A 20 ft. length of 4' x 4" lumber. 3. The corner of a building -wire mounted 12 ft. above ground, 4. A metal pole properly grounded (steel, aluminum, etc.). 5. A living tree.
Tree limbs falling into overhead wires cause the majority of low voltage power interruptions. For this reason do not use trees as line supports. If living trees must be used, trim the tree extensively almost to the point of stripping the tree to the trunk. Do not use a dead tree because it is very likely to be weak and rotten.
Poles can be hauled in several ways. They can be supported several feet below the mid point by a trailer, then towed behind a truck or jeep by securing the top of the pole to the vehicle. For shorter hauls a "timber hitch" (Fig. 4.5) can be tied around the butt of the pole and tie the rope to the yoke of an ox team, or a jeep.
The typical electric pole consists of high voltage wire supported on cross arms and low voltage wire mounted on racks of insulators below the cross arms. (Fig. 4.6)
Trimming is required if the pole is in an unprepared state. Trimming is the stripping off of all the bark and knots.
Roofing is the cutting of an angle on the top of the pole so that water, or perhaps snow or ice, will not collect on the top, thus preventing decay. In Fig. 4.7 there are several examples of roofs. Roofing is unnecessary if the entire pole is treated with wood preservative such as creosote.
Gaining is the notching of a pole so that a crossarm or other piece of hardware will mount flush against the pole.
A pole will last much longer if it is treated with a preservative. This will protect it from rot, and termites. The section of the pole that will be under ground level must be painted with a wood preservative. Ideally the whole pole should be so painted. Before a pole is erected as much preparation as possible should be done. The work is much easier to do on the ground with firm footing than it is in the air, after the pole has been erected. The preparations should include: trimming, roofing, gaining, boring of all needed holes for bolts, painting with preservative mounting all close fitting equipment, i.e. insulator racks, guy wire bolts, etc.
DIGGING THE POLE HOLE
The diameter of the hole is determined by the size of the large end of the pole. The hole should be large enough to allow plenty of space on each side of the butt of the pole for tamping the soil back into the hole. This requires at least 3 in. all around the butt. The diameter of the hole should be fairly uniform from top to bottom, How deep the hole should be is determined by the length of the pole and by the holding power of the soil or earth. The recommended depths of setting in soil and rock are given in Table 4.1 for various pole lengths from 20 to 50 ft.
RAISING THE POLE (Pike Method)
The piking method is the oldest method of raising poles. It gets its name from the so-called "pike pole" used by the men raising the line pole. A pike pole is a long pole with a steel spike on the end of the pole. A "jenny"', a sort of heavy shaft with a IJ on the top end, is also used to support the pole.
A "piking" crew always has one man at the butt of the pole and one man at the "jenny" --a "jennyman." The number of "pikers" depends upon the length and the weight of the line pole to be raised. Table 4.2 gives recommended crew sizes.
The first step in raising a pole using the piking method is to lay the butt end of the pole over the hole against a bump board or bar which rests on the bottom of the hole and extends over the top, as shown in Fig. 4.9. The board or bar protects the walls of the hole and prevents them from being caved in by the butt of the pole as the pole is raised.
Fig. 4.9 In the pole tree second support limb with step the pole a fork pole in is raised support, or the small by jenny, end hand and is The main placed on the initially a heavy duty of the man at the butt is to keep the pole from rolling. This is done by means of a cant-hook--See Figure 4.10.
In the third step the men stand side by side on either side of the top end of the pole. They then lift the top end of the pole as high as they can while the jennyman slides the jenny toward the butt. The jennyman supports the pole between lifts. In this manner they move along the pole until the pole is high enough to require the use of pikes.
The fourth step is to punch the pikes into the pole and prepare to raise the pole. As the pole is raised, the man carries the jenny forward always ready to support the pole if need be. The raising continues until "high pike" is called by one of the men. This means that the top of the pole is so high that he can no longer push it with his pike pole. The jennyman then sets the jenny to support the pole and while the other pike men hold it steady, first the man nearest the butt releases his pike and steps forward for a fresh lift closer to the butt. The other pike men follow in order and when all are ready they lift once again. The pole is raised in this manner until it drops into the hole.
When setting the pole after it has slid into the hole there are several things to keep in mind. First the pole should be faced. This is turning the pole with the cant hook, until the gains or the insulator racks are lined up facing the direction of the lines that will pass through them, The pole should then be straightened, or plumbed. This is to adjust the pole with the pikes until it is in a vertical position. This is done by the piking crew with a "foreman" standing away and giving directions. Once the pole is plumb the butts of the pikes should be jammed into the ground so that they support the pole without assistance.
Then the pole is ready to have the hole filled. In some soils there will be a need for cribbing as in Fig. 4.12. It cannot be stressed too much that the fill shoveled back in the hole must be well tamped, This is pounding down of the fill with rods until it is very hard packed. There should not be any soil left over if enough tamping is done.
Many utility accidents and fatalities are caused by linemen falling from line poles. With this as a preface, we can now discuss the proper way to climb a line pole. Climbing a line pole is extremely dangerous and the use of a ladder is much more desirable for the novice electric lineman. The ladder should be so placed that the distance from the base of the pole to the bottom of the ladder is 1/3 the distance from the base of the pole to the top of the ladder. (See Fig. 4.13). Before working on the pole from the top of the ladder, the ladder should be tied securely to the pole.
Teaching the techniques of climbing poles with a pair of "climbers" is beyond the scope of this manual. (instructor's Note: If these techniques are deemed necessary to the PCVs training, local utility personnel should be contacted and brought in to teach these techniques.)
GUYING THE POLE
Guys should be used whenever there is stress on a pole that tends to pull it out of line. Dead ends or corners should be guyed as to pull the pole over. Guys should also the weight of be used at the road lines or railroad tend crossing. "Crossarms" may need guying if there is an unbalanced pull on them. Examples of these are shown in Figs. 4.14.-4.19
Guy wire installed on a distribution line to counterbalance the pull of the "dead-ended" distribution wires. (Side view).
Wire guy installed on a terminal or end pole. (Top view).
Fig. 4.16 Guy installed at angle in line.
Fig. 43 Guying a corner pole
Fig. 4.m installed on crossarm.
Guying to strengthen pole line installed on steep grade. There are four steps in the installation of a guy: 1. Digging in the anchor 2. Inserting the insulators 3. Fastening the guy to the pole 4. Tightening the guy and fastening to the anchor
DIGGING IN THE ANCHOR
There are several types of anchors, some are illustrated in Fig. 4.20. Additional sections of pipe might be necessary to reach a depth giving the required holding power.
The most economical anchor when labor is inexpensive is the log type anchor. It is also the most effective anchor. Its installation will be described here. The installation of the others is explained in Kurtt's Handbook. A trench is dug a minimum of 4 feet deep. After having a hole bored in the log to pass the anchor rod through, the log is laid in the trench. The anchor rod is driven through the ground at the proper angle, and is secured through the log with a washer and nut. The trench is then filled and well tamped.
INSERTING THE INSULATORS
Insulators must be installed in a guy whenever there is the possibility of live wires falling and coming in contact with a guy. The insulator should be placed to insulate that portion of the guy from ground. The two sections of guy are looped through the separate parts of the insulator and the ends clamped to the guys.
FASTENING THE GUY TO THE POLE
The easiest method is to bore a hole for a bolt in the pole at the point the guy is to be attached. Loop the guy wire through the eye of an eye bolt and clamp the end to the guy. Then install this eye bolt through the pole. There are other pieces of hardware available for fastening a guy to a pole.
TIGHTENING THE GUY AND FASTENING TO THE ANCHOR
A wire grip is used with a block and tackle to pull the guy taut. The guy is tightened until the pole is pulled over slightly toward the WY* Then when the line conductors are strung later the pole will stand erect under the combined strain of the guy and the line. Once taut, the guy is looped through the anchor rod and clamped.
JOINING LINE CONDUCTORS
Line joints can be divided in three classes:
1) Splices. 2) Sleeve joints. 3) Compression joints.
Small-sized copper wires can be spliced, but the larger sizes of copper wire are usually joined by means of splicing sleeves or compression joints.
MAKING A SPLICE JOINT
In the case of covered wires, the two ends of the wires to be spliced should be scraped perfectly clean and free from insulation. The &res should be cleaned until they are bright. After the conductors are cleaned, they should be placed together until approximately 8 to 12 in. of the ends overlap each other for the smaller sizes and 12 to 18 in. for sixes No. 4 and larger (See Fig. 4.22)
It is easier to make a good splice If a clamp is used to hold the wires in place prior to twisting.
MAKING A SLEEVE JOINT
The best way to make a joint in medium-size conductors is by means of the so-called "splicing sleeve." It is a special connection that ensures good electrical and mechanical joints. The sleeve itself is a piece of single or double tubing. (Fig. 4.23) To make a sleeve joint, the ends of the wires should be scraped clean and bright. They are then inserted, one in each tube if a double tube is used, from opposite ends so as to lie side by side, The ends of the wires should project several inches beyond the ends of the sleeve. The ends of the sleeve are then grasped by two sleeve clamps or twisters. (See Fig. 4.24) The next operation consists in giving the conductors three and one-half or four turns. The twisting should be done from both ends. Sleeves should always be made of the same kind of material as the conductor they are to be used with. In making sleeve joints in iron wires, the sleeve should be tinned iron.
MAKING A COMPRESSION JOINT
A compression joint the sleeve, however, also the makes sleeve use is of a sleeve. Instead compressed with great of twisting force onto the conductor. This great force is brought about by the use of a hand-operated compression tool modeled after a bolt-cutter. The use of a die in compression makes the sleeve grip the conductor firmly.
To make a compression joint:
Clean the conductor ends thoroughly. Match the site of splicing sleeve to the size of the conductor. Match the die number to the sleeve number. Center the conductor ends in the sleeve. The specified number of indents must be made. Although the compression joint is one of the best electrical joints, the compression tool is a rather expensive piece of equipment.
STRINGING THE WIRE
When wire is installed on electric poles, all the wire is installed at one time. That is, if 3 conductors are to be put up, all three are put up at the same time. A truck with the three spools of wire loaded on the rear. end is used to pay out the distribution wires. The spools of wire are set up on the truck and unwind as the truck moves along.
In stringing wires on rack mounted insulators, the conductors are unreeled and passed through the rack. When the desired number of pole spans have been laid in place, the conductors are drawn UD and tied to the insulators. As many as 10.spans can be drawn up at one time in this manner. The regular "Western Union" tie is generally used (See Fig. 4.25). If the conductors are to be tied to the outside of the insulator, the Western Union is also used.
In turning corners and at angles in the line, the position of the line wires on the insulators will be determined by the direction of the strain. They should always be so placed that the conductor is pulled against the insulator and not away from it. Fig. 4.25 illustrates the correct positions for corner and angle construction. When a section of wire is strung to the last pole OF a pole line, the section of wire is "dead-ended" on that last pole. Fig. 4.27 shows a dead-ended pole.
SAGGING LINE CONDUCTORS
The line conductors expand in hot weather and contract in cold weather, so there should be some slack, or sag, between poles. The conductors should be sagged in accordance with the sag chart applying to the particular conductor used, the length of the span and the temperature prevailing. The sag should be adjusted in the middle span in short sections of line of live spans or less and at two or more spans in longer sections. Sagging is done just prior to tying the line conductors to the individual insulators or insulator bracket. Conductors can be sagged correctly only when the tension is the same in each span throughout the entire length. A simple and accurate method of measuring the sag is by the use of targets placed on the poles below the insulators, as shown in Fig. 4 .28.
COPPER WIRE FOR DIFFERENT SPAN LENGTHS
The targets may be a light strip of wood like a lath nailed to the pole at a distance below the conductor resting on the insulator equal to the desired sag. The lineman sights from one lath to the next. The tension on the conductor is then reduced or increased until the lowest part of the conductor in the span coincides with the lineman's line of sight. The recommended sag for cooper conductors is obtained from Table 4.4
(This page is based on information copied from Rural Electrification Systems prepared for the United States Peace Corps By: Volunteers in Technical Assistance, Inc. (VITA) 3706 Rhode Island Avenue Mt. Rainier., Maryland 20822 USA In accordance with Contract PC 251709 April, 1969.)