Figs. 930 to 932.—Pole line construction tools. Fig. 930, split wooden handle post hole auger; fig. 931, cant hook; fig. 932. socket peavey.
Methods of Setting Wooden Poles in Unsuitable Soil.—In places where salt is plentiful and cheap, such as the Great Salt Lake region in Utah, it has been found that the liberal use of salt mixed with the dirt filling tamped in around the foot of the pole is very effectual in preventing decay below the soil line.
Where poles have to be planted in low, swampy ground, or where the climatic conditions are such that timber decays rapidly, it has been found advantageous to place the poles in concrete settings. This method is extensively employed in various parts of the Southern States, square poles being placed in settings about 7 feet deep and 3½ feet square. In very soft ground the employment of a concrete setting is sometimes impracticable. In such cases piles are driven deep into the soil, and the pole bolted to the part of the pile extending above the ground.
Reinforced Concrete Poles.—The strongest point in favor of concrete poles is their durability. Untreated wooden telephone and telegraph poles have to be replaced by new poles about every six or seven years, depending on the percentage of moisture in the soil, the drier the soil, the longer being the life of the pole. Concrete poles are not affected by soil conditions, and if properly made will last indefinitely.
Figs. 933 to 935.—Glass insulator and insulator pin and bracket. The insulator here shown is of the pony double petticoat type. Insulator pins are used with cross arms, brackets are attached direct to the pole.
One form of reinforced concrete pole consists of a skeleton frame work of four corrugated iron rods covered with ordinary concrete. The pole is octagonal in shape, 30 feet long, and provided with mortises for cross arms, the latter being fastened in place by means of iron bolts. It is stated that they are less expensive than pine poles, and that each pole can be manufactured at the point on the line at which it is to be installed or planted.
In Canada, reinforced concrete poles are made square on account of the ease of making, and also on account of the steel economy permitted thereby. All poles are made at the point of erection. They are moulded in wooden forms, in a horizontal position, the top side being left open and finished with a trowel. The concrete is composed of one part of Portland cement, two parts clean sharp sand, and four parts broken stone. A 35 foot pole for ordinary line work weighs about 2½ tons and a 50 foot pole about 5 tons.
Cross Arms.—The familiar cross arms for stringing wires are usually attached to the poles before they are erected. They are commonly made from yellow pine wood, generally 3¼ x 4¼ inches, and are freely coated with good mineral paint as a preservative. Attachment is made to the pole by cutting a gain one inch deep and of sufficient breadth to allow the longest side of the cross arm to fit accurately. It is then secured in place by a lag screw, with a square head, so that it may be driven into place with a wrench.
Fig. 936.—Cross arm which carries the insulator pins. The standard cross arm is 3¼ x 4¼ inches, double painted, and bored for 1½ inch pins and two ½ inch bolt holes. Telephone arms are 2¾ x 3¾ inch, bored for 1¼ inch pins and two ½ inch bolts.
The cross arm is further secured to the pole with braces. These are flat strips of wrought iron or low carbon steel, 30 inches long, ¼ inch thick and 1¼ inches wide, according to standard specifications. Holes are bored at points one inch from either end, one for attaching to the pole, the other for attaching to the cross arm; two braces forming a triangle with the cross arm for the base and with the apex at the point of connection to the pole. Like all other iron work used on pole lines, the braces are carefully "galvanized," so as to stand three immersions of one minute each in a saturated solution of copper sulphate without showing copper deposits, the color being black at the completion of the test.
Before the cross arm is set in place the gain is carefully painted with white lead. As it is important that cross arms on a line of poles, particularly when there are several on each pole, should be at equal distances from the ground as well as being uniformly spaced, it is necessary that some measuring instrument should be used to accomplish this. Such an instrument is the ordinary template, which is a length of board carrying a pointed block at one end, to correspond exactly with the top of the pole, and also cross cleats nailed at precisely the same intervals below it as it is proposed attaching the cross arms. The template, laid upon a pole, shows where to cut the gains.
In planting the poles it is customary to so arrange them that the cross arms on alternate poles shall face in opposite directions, for the purpose of equalizing the strain on the line. On curves, however, all cross arms are placed on the side of the pole facing the middle of the curve.
Ques. What provision is made for attachment of the wires?
Ans. The cross arms are bored with holes for the insertion of the insulator pins, which are made of locust wood and threaded at the upper end to receive the glass insulator.
The cross arm is made of such a length as to accommodate the number of pins to be inserted. An arm for two pins is made three feet long, according to the standard usually followed, with holes for the pins at center points three inches from either end and a space of 28 inches between them in the center.
Ques. How must electric light and power wires be placed when wired on telephone or telegraph poles?
Ans. They must not be put on the same cross arm with the telegraph, telephone, or similar wires, and when placed on the same pole with such wires the distance between the two inside pins of each cross arm must not be less than twenty-six inches.
Fig. 937.—Portable platform with rigging as used by linemen in wiring and making repairs.
Poles for Light and Power Wires.—In selecting the style of pole necessary for a certain class of work, the conditions and circumstances should be considered. Poles may be divided into three classes, the size of wire to be carried being one of the important considerations.
First Class.—Main line of poles should range in length of from 30 to 35 feet with 6 inch tops. The height of trees, of course will have to be considered in many cases.
Second Class.—Town lighting by arc lights. All poles should have at least 6 inch tops. The corner poles should have 6½ inch tops, and wherever the cross arms are placed on a pole at different angles, the pole should have at least a 6½ inch top. A 30 foot pole is sufficiently long for the main line, but it would be advisable to place 35 foot poles on corners.
Third Class.—Where heavy wire, such as No. 00, is used for feeder wire, the poles should have at least 7 inch tops. Where mains are run on the same pole line the strain is somewhat lessened, and poles of smaller size will answer.
Cull Poles.—All poles that are smaller at the top than the sizes agreed upon, are troubled with dry rot, large knots and bumps, have more than one bend, or have a sweep of over twelve inches, should certainly be classed as cull poles. Specifications for electric light and power work should be, and in many cases are, much more severe than those required by telegraph lines. A cull pole, one of good material, is the best thing for a guy stub, and is frequently used for this purpose. A cedar pole is always preferable to any other, owing to the fact that it is very light in comparison to other timber, and is strong, durable, and very long lived.
Pole Setting.—In erecting poles, it seems to be the universal opinion of the best posted construction men that a pole should be set at least five feet in the ground, and six inches additional for every five feet additional length above thirty-five feet; also additional depths on corners. Wherever there is much moisture in the ground, it is of much value to paint or smear the butt ends of the pole with pitch or tar, allowing this to extend about two feet above the level of the ground. This protects the pole from rot at the base. The weakest part of the pole is just where it enters the ground. Never set poles further than 125 feet apart; 110 feet is good practice.
Painting.—When poles are to be painted, a dark olive green color should be chosen, in order that they may be as inconspicuous as possible. One coat of paint should be applied before pole is set, and one after pole is set. Tops should be pointed to shed water.
Spacing the Poles.—In general, the spacing of poles, like their dimensions, is regulated by the weight of the lines they are designed to carry—the heavier the lines the greater the number of poles. The spacing of poles also depends on their liability to injury from storms and wind in any given locality, and the nature of the service. Poles for a telephone line may be spaced twenty to fifty to the mile—that is, from about 260 to 100 feet apart.
Figs. 938 to 941.—Pole line construction tools. Fig. 938, pike pole; fig. 939, raising fork; fig. 940, mule pole support; fig. 941, jenny pole support.
Erecting the Poles.—Since each pole on a properly constructed line is sawed to the right length and carefully shaped before it is finally inserted in the ground, it is necessary that the holes be dug to as nearly the required depth as possible. Holes for poles are dug very little wider than their diameter at the butt, and the depth is usually computed according to the nature of the soil and the weight of the proposed line. Excavation, while sometimes accomplished with patent post hole augers, or even dynamite, is usually done with a long handled digging shovel, and the earth removed with a spoon shovel, such as is shown in fig. 921.
Fig. 942.—Guy anchor log in position.
Fig. 943.—Stombaugh guy anchor. It is made of cast iron and can be screwed into the ground like an auger.
Wherever required by the nature of the soil, a "grouting" or foundation of loose stones is formed in the bottom of the hole, and, in marshy or springy ground, a base of concrete and cement is laid, with filling of the same material around the pole, when raised.
Ques. How are the poles transported to the holes?
Ans. They are rolled or carried on hooks similar to those used for carrying blocks of ice, except for a long handle for lifting the load at either side.
Fig. 944.—Method of raising a pole. When the pole has been properly placed, it is seized by several linemen. As soon as the top of the pole is raised high enough to permit the pikes to be thrust into the pole, it is then raised to a vertical position. At about 50° the butt end slides into the hole. The earth is then filled in around the pole and firmly tamped down. Eight or ten poles are about as many as can be set by the average gang in a day.
Ques. How are the poles raised and placed in the holes?
Ans. A piece of timber is inserted in the hole as a slide to prevent crumbling of the earth as the pole is slid into place. The end is raised by hand sufficiently to allow the "dead man," or pole hoist, to be placed beneath, and this is moved along regularly as the pole is lifted with pike poles, until it slides into place through the force of gravity.
Fig. 945.—Method of pulling an anchor into place before the guy wire is fastened to the top of the pole, thus obviating the liability of pulling the pole out of plumb.
This accomplished, the pole is held in a perpendicular position by pikes in the hands of assistants, or planted in the ground around it, while the earth is carefully shoveled into the hole and thoroughly packed down with a tamper.
Guys for Poles.—Where poles are subject to severe strains which might throw them down and break the wires, guy cables are largely employed, these being attached near the top and secured either to the base of the next pole, to a suitable guy stub or post, or to a guy anchor, which is buried about eight feet in the earth and held down by stones and concrete.
Ques. Under what conditions is it necessary to guy poles?
Ans. They are guyed at corners in order to thoroughly secure the poles so that no strain may come on the cornerwise span. It is also necessary to guy a line where it is to be deflected from a straight path, as when rounding a hill, water course or railway curve, in order to neutralize the pull of the wires, tending to incline the poles toward the center on which the arc is described; also when descending a hill.
Figs. 946 to 948.—Methods of guying corner poles. The proper guying of corner and terminal poles is especially important; on corners and curves, the guys should be stronger and more frequent and should be placed on the outer side as shown in the diagrams.
Fig. 949.—Head and foot guying of a pole line in descending a hill.
Guy Stubs and Anchor Logs.—In guying a line under such conditions, each pole is connected by a suitable cable to a guy post or "stub," or to an anchor log. Standard rules specify stubs between 18 and 25 feet, with exact limits as to circumference measures at the top and at a point 6 feet from the butt, according to the kind of wood used.
Figs. 950 to 952.—Lineman's tools. Figs. 950 and 951, Eastern pole climbers, with and without strap for attaching to legs; fig. 952, portable vise with strap for pulling up the slack in splicing.
Thus, guy stubs of cedar or juniper, either 18 or 25 feet in length, must have a circumference of 22 inches at the top and of 32 inches 6 feet from the butt; stubs of chestnut must measure 24 inches in the first, and 34 in the second, while those of cypress require 28 in the first, and in the second, 39 inches for an 18 foot length, and at least 41 for a 25 foot length. In planting guy stubs the same rules are followed as hold for poles, every means being adopted to promote security of construction except that the stub is raked or tilted against the strain on the guy cable.
Wiring the Line.—The erection and guying of the poles of a line as well as the attachment of the cross arms and the screwing on of the insulators are completed before the stringing of the line is begun. It is particularly essential that the pull on poles of a given line be accurately calculated, and that each one be guyed accordingly before the line is strung, in order to avoid the danger of an undue strain upon the wires in attempting to rectify the condition afterward. It is a good working rule that the wires should be subjected to no stress other than the weights of their own spans after they have been attached to the poles.
Figs. 953 and 954.—Pay out reels. Fig. 953, type used for telephone or telegraph work; fig. 954, type used for electric light work.
Ques. Describe how the wires are strung.
Ans. In stringing the lines, either one or the full number of wires may be put up at the same time. When one line only is to be strung, the operation consists simply in reeling the wire and running it off from a hand reel, such as is shown in fig. 953 or 954. At each pole the wire is drawn up to its place, pulled out to the desired tension, and attached to the insulator.
In the operation of stringing a number of lines at once, the method is different. The reels are placed at the beginning of a section, each wire being inserted and secured through a separate hole in a board, which is perforated to correspond with the spacing of the insulators on the cross arms. A rope is then attached to this running board, which is drawn by a team of horses through the stretch to be wired, being lifted over each pole top in turn. When a certain length has thus been drawn out the wires are drawn to the required tension between each pair of poles and secured to the insulators.
Fig. 955.—One form of "come along." The wire is inserted between jaws and is held fast when tension is applied to the ring.
Fig. 956.—An improved form of "come along" or wire stretcher. The jaws which grip the wire are smooth and remain parallel in closing, thus the wire is not scratched or indented, as with circular jaws having teeth.
Ques. How much tension must be put upon the wires?
Ans. In applying tension to the wires as they are strung on the poles, it is the rule to allow some sag. The amount of sag to be allowed varies with different line hangers.
A typical case quoted by one or two authorities gives a sag of four inches at the center of each 130 foot span for a given size of wire, at a given temperature. A more general rule is to make the tension on a wire as it is drawn up between each pair of poles equal to one-third of its breaking weight. Thus No. 10 B.& S. gauge, would be drawn to about 163 pounds, and No. 12 to about 102 pounds. The temperature at the time of stringing and the distance between the poles are, however, important considerations in applying tension and allowing for sag. Thus, one construction company specifies a dip of 10 inches in summer and 8 inches in winter for spans of 130 feet, or 40 poles to the mile. Several authorities specify figures about as given in the above table for No. 14 iron or copper wire.
Fig. 957.—Wireman's "come along" with hook and tackle.
| Span in Feet |
Temperature Fahr. | ||
|---|---|---|---|
| 30° | 60° | 80° | |
| Sag in Inches | |||
| 75 | 1¾ | 2½ | 3⅛ |
| 100 | 3 | 4¼ | 5⅜ |
| 130 | 5⅛ | 7 | 8⅝ |
| 150 | 6¾ | 9 | 11¼ |
Ques. How is the wire drawn out?
Ans. In drawing out the wire, it is customary to use a wire clamp, or "come along." This tool is attached to a block and tackle, or drawn in by hand, and, as soon as the proper force has been applied, the wire is held, while the lineman secures it to the insulator.
Fig. 958.—Lineman's block and fall with "come alongs" for stretching wire and holding same when making splices.
Figs. 959 and 960.—Approved method of attaching wire to an insulator; elevation and plan of insulator and tie. The line wire is first laid in the groove of the insulator, after which a short piece of the same size of wire is passed entirely around to hold it in place, then it is twisted to the line at either side with pliers.
Another contrivance for this purpose is the pole ratchet, by which the wire is drawn tight and held until attached to the pole.
Ques. How are the wires attached to the insulators?
Ans. An approved method is shown in figs. 959 and 960. Standard rules specify that all wires shall be tied to the side of the insulators toward the pole, except on the insulators next to the pole, where they are to be attached on the opposite side. On curves, however, it is required that all wires shall be arranged so that the strain shall be against the insulator and not on the wire.
Fig. 961.—American wire joint. This is a simple method of connecting the ends of the sections of wire by tightly twisting the ends around each other for a few turns; it is the standard Western Union wire joint.
Figs. 962 and 963.—McIntire sleeve and sleeve joint. An approved method of making the joints of telephone lines is by the use of some form of sleeve, such as is shown in fig. 962. This consists of two copper tubes of the required length, and of sufficient inside diameter, to admit the ends of the wires to be joined, fitting tightly. The tubes are then gripped with a tool, shown in fig. 964, and twisted around one another, so that the wires are securely joined and locked, as shown in fig. 963.
Ques. How are the wires spliced?
Ans. There are several methods of splicing wires. Fig. 961 shows the American wire joint, and fig. 963 the McIntire sleeve joint. In making a joint, the two ends are gripped by come alongs and drawn up to the proper tension with tackle as shown in fig. 958. The joint is then made as shown in the illustrations.
Transpositions.—In some classes of circuit, as for instance telephone lines, the current is often seriously affected by electrostatic induction from other lines, and also from power circuits, owing to the fact that the surfaces of the wires form, as it were, so many charging plates of an electrical condenser, with the intervening air as the insulating layer or dielectric.
Fig. 964.—McIntyre's twisting clamp for wires 00 to 16 B. & S. gauge.
Fig. 965—Method of making a "transposition." This is usually done by means of transposition insulators, which are either double insulators, one being screwed to the pin above the other, or else such caps as are shown in fig. 967. Such insulators are intended to act as circuit breakers, the particular wire to be transposed being cut and "dead ended," or tied around, on both the upper and lower grooves of the cap. The free end of each length is then passed back and around the insulator and twisted, or sleeve jointed to the other limb of its own circuit.
The telephonic current changes the pressure of its own charging surface as frequently as it alternates, and this fact in itself is amply sufficient to account for a vast weakening of the current before it reaches its destination. The only practicable method of overcoming this annoyance in pole lines is by the arrangement known as "transposition," which is, briefly, the practice of regularly shifting the relative position of the two limbs of each circuit as regards other wires in the same pole system, as shown in fig. 965.
For short lines and pole systems with only a few wires it is not necessary to transpose very frequently. On longer lines it has been found amply sufficient to transpose once every quarter mile; that is to say to change the relative position of the wires of the different circuits at posts situated about that distance apart. This does not mean, however, that each pair of wires is transposed so often, but that on ordinary sized systems, the transposition of some one circuit is amply sufficient to secure balanced relations and effectually counteract the effects of cross induction. It is a matter which must be carefully calculated and planned in each particular instance in order to secure the best advantages.
Fig. 966.—Telegraph and telephone line glass insulator.
Fig. 967.—Type of insulator used in making a transposition.
Insulators.—Glass and porcelain are employed almost universally for supporting overhead wires. Insulators made of these materials are superior to those made of other material such as hard rubber, or various compounds of vegetable or mineral matter, with the exception perhaps of mica insulators used on the feeders of electric railway lines.
Fig. 968.—Tree insulator. This type of insulator is especially useful in connection with temporary or repair work, or where the wires pass through trees having numerous branches. The illustration shows a Cutler tree insulator lashed to the trunk of a tree. It is made of a single piece of glass, and is provided with a slot which the wire cannot leave accidentally. The back of the device is concave and provided with ribs which prevent sliding. It can be readily slipped over wires already in place, is available for electric light circuit, and will take wires up to ½ inch, in diameter.
Figs. 969 and 970.—Overhead cable construction. In some cases, particularly on short lines exposed to inductive disturbances from power and other electrical circuits, it is usual to string the cables on poles such as usually carry the bare conducting wires. It is not necessary, however, to insulate the cable in any way; consequently it is merely hung to a supporting wire rope or cable, called the "messenger wire," being attached either with some form of hanger, such as is shown in figs. 969 and 970, or by loops of tarred marline. The marline is sometimes wound over the cable and messenger wire from a bobbin, but frequently it is merely wound on by hand. Cables used in such overhead construction consist of bundles of wires, the pairs twisted together. The size most often used is No. 19, B. & S., which is about .03589 inch in diameter, weighs 20.7 pounds, and has a specific resistance of about 8 ohms to the mile.
Glass insulators are generally used on low tension lines, and porcelain insulators on high tension lines, the latter type being usually stronger and less brittle. Porcelain is more expensive than glass, and its opacity prevents the detection of internal defects which would be readily observed through glass.
Fig. 971.—Clark's "antihum;" a device designed to prevent the humming of telegraph wires.
Ques. What is a petticoat insulator?
Ans. An insulator which has one, two or three deep flanges or "petticoats" around the base for the purpose of increasing the leakage path from the line to the pin.
Both glass and porcelain insulators may be the double or triple petticoat type which may be cast or moulded solid, or made in two or more parts which are subsequently cemented together.
Service Connections and Loops.—Whenever it is necessary to tap an overhead conductor for service connection, the method of connection will depend upon the character of the circuit. In the case of a parallel circuit, an extra insulator must be placed on the cross arm so as to prevent the service main putting a side strain on the main line conductor. In the case of a series circuit the main line conductor is usually dead ended at the nearest pole and a loop taken to the point of service, as shown in fig. 972.
Fig. 972.—Method of making a series "loop" service connection.
Fig. 973.—Parallel service connection. Service wires tapped to the main wires, are run to insulators on an auxiliary cross arm, thence to insulators on the side of the building, and through the drain tube to the service switch.
Fig. 974.—Joint pole crossing, showing wires of two lines crossing each other. Four guard wires (shown heavier than the others) extend for one span either side of the joint pole parallel to the wires of the lower circuits and protect them from contact in case of a break in the wires of the upper circuits. These guard wires are insulated. The minimum distance between high and low tension wires should be three feet. Five is better. The end guards, which prevent wires slipping off ends of cross arms and dropping on the lower wires, should extend about six inches above the level of transmission line.
Ques. What are service wires?
Ans. Wires which enter a building.
CHAPTER XL
UNDERGROUND WIRING
In large cities, the best method of running wires for all varieties of electrical power transmission is to place them underground. Many city authorities have made this method of wiring compulsory by law, because of the difficulty in approaching a burning building, the danger from crossed and falling wires, and the disfigurement of the streets where there is a network of overhead wires.
The expense of installing an underground system is very great in comparison with that of overhead construction, but the cost of maintenance is much less and the liability of interruption of service greatly reduced.
Underground Systems.—An underground system of electrical conductors is composed of three essential elements:
1. The conductor itself, which is almost invariably of copper;
2. The insulation, which is either in the form of a complete covering of insulating material, or simply insulated supporting points;
3. The tube or conduit, which constitutes the mechanical protection against the effects of the severe shocks, weather conditions, etc., to which the system is naturally exposed.
The various underground systems may be divided into three classes:
1. Lead encased cables laid directly in the ground;
2. Solid or built in systems;
3. Drawing in systems.
Ques. What may be said of the first mentioned construction?
Ans. Where cables are laid directly in the ground, the metallic covering, consisting usually of a lead tube, which is placed over the insulation is depended upon for mechanical protection. Such cables are largely used for short private lines and the first cost is less than that of the others, but in case of repairs it has to be dug up.
Ques. Describe the drawing in system.
Ans. In this construction the cables are drawn in after the conduits are built. The conduit of the drawing in system may consist of various forms of pipe or troughs of iron, earthenware, concrete, wood or fibre, while those of the solid or built in systems are composed of either iron tubes or concrete trenches.
Conduits.—The principal qualifications of a good conduit are freedom from disintegration by the action of fire, water, acids, alkalies, or electrolysis; second, a smooth interior surface so as to permit of the easy drawing in of the cables; and third, a design which will permit of its economical installation in crowded streets. There are numerous kinds of conduit of which may be mentioned:
1. Vitrified clay pipe conduits;
2. Vitrified clay or earthenware trough conduits;
3. Concrete duct conduits;
4. Wooden duct conduits;
5. Wooden built in conduits;
6. Wrought iron or steel pipe conduits;
7. Cast iron pipe and trough conduits;
8. Fibre conduits.
Fig. 975.—A few forms of vitrified clay pipe conduits; view showing single and multiplex types. The dimensions of each duct are about 3½ × 3½. The lengths vary from two to three feet.
Vitrified Clay Pipe Conduit.—Various forms of vitrified clay conduit appear to possess the qualifications, desirable in underground construction, to a higher degree than any other type. They are made in both single and multiple duct, as shown in fig. 975, the single type being about 3½ inches in diameter, or 3½ inches square, and 18 inches long. Multiple conduit is made in two, three, four, six and more sections, ranging from 2 to 3 feet in length.
Ques. For what conditions is the single conduit especially adapted?
Ans. It is most suitable for use where the sub-surface conditions are characterized by a great crowding of gas, water, and other pipes, as the conduits can be divided into several layers so as to cross over or under such pipes, and many other sub-surface obstructions which are present in the streets of large cities and towns.
Ques. What are the features of the multiple duct conduit?
Ans. It can be laid somewhat cheaper than the single duct type, especially in lines of about two to four ducts; it is, therefore, most suitable for use in outlying communities where the streets are comparatively free from many sub-surface obstructions.
Ques. How is the conduit laid?
Ans. In laying conduit, a trench is dug, usually sufficiently wide to allow the placing of three inches of concrete on each side of the ducts, and sufficiently deep to hold at least thirty inches of concrete on top of the upper layer of concrete forming the conduit, and to allow for three inches of concrete in the bottom. The trench is graded from some point near the middle of the block to the manhole at each intersection, or from one manhole to the next manhole, at a gradient not less than 2 inches to 100 feet.
Ques. How are single duct conduits laid?
Ans. The tiles of the several ducts are placed close together, and the joints plastered and filled with cement mortar consisting of one part of Portland cement to one part of sand. When the conduit is being laid, a wooden mandrel about four or five feet long, three inches in diameter, and carrying a leather or rubber washer from three to eight inches larger at one end is drawn through each duct so as to draw out any particles of foreign matter or cement which may have become lodged in the joints, and also to insure good alignment of the tiles, as shown in fig. 977.
Single duct conduits are usually laid by bricklayers. This fact accounts for the somewhat greater cost of the single over the multiple conduit which is usually laid by ordinary laborers. One good brick-layer and helper, however, will lay from 200 to 300 feet of single duct conduit per hour.
Practically the same standard of construction is maintained on all conduit lines from two ducts up to twenty-five ducts, as many of the smaller lines may extend for miles into the outlying districts, and contain transmission lines of the maximum working voltage.
Fig. 976.—Vitrified clay or earthenware trough conduit; this type of conduit consists of troughs either simple or with partitions, the latter type being shown in the figure.
Vitrified Clay or Earthenware Trough Conduit.—It consists of troughs either simple or with partitions as shown in fig. 976. They are usually made in tiles 3 or 4 inches square for each compartment, with wall about one inch thick. The length of the tiles ranges from two to four feet. Each of the two foot form duct troughs weighs about 85 pounds. When laid complete, the top trough is covered with a sheet of mild steel, about No. 22 gauge, made to fit over the sides so as to hold it in position, and then covered over with concrete.
Joints in Multiple-duct Vitrified Clay Conduit.—In laying multiple duct earthenware conduit, the ducts or sections are centered by means of dowel pins inserted in the holes at each joint, which is then wrapped with a six inch strip of asphalted burlap, or damp cheese cloth, and coated with cement mortar as shown in fig. 978. Economy of space and labor constitutes the principal advantages derived from the use of multiple duct conduit.
Fig. 977.—Method of laying single duct vitrified clay conduit. The tiles of the several ducts are placed close together as shown in the figure, and the joints plastered and filled with cement mortar consisting of one part Portland cement and one part sand.
Concrete Duct Conduits.—These are usually constructed by placing collapsible mandrels of wood or metal in a trench where the ducts are desired and then filling the trench with concrete. After the concrete has solidified, the mandrels are taken out in pieces, leaving continuous longitudinal holes which serve as ducts. Some builders produce a similar result by placing tubes of sheet iron or zinc in the concrete as it is being filled into the trench. These tubes have just enough strength to withstand the pressure to which they are subjected, and are, therefore, very thin and liable to be quickly destroyed by corrosion, but the ducts formed by them will always remain unimpaired in the hardened mass of concrete.
Wooden Duct Conduits.—In this type of conduit, the ducts are formed of wooden pipe, troughing, or boxes, and constitute the simplest and cheapest form of conduit. A pipe conduit consists of pieces of wood about 4½ inches square, and three to six feet long, with a round hole about three inches in diameter bored through them longitudinally. As shown by fig. 979 a cylindrical projection is turned on one end of each section, which, when the conduit is laid fits into a corresponding recess in one end of the next section. The sections are usually laid in tiers, those of one tier breaking joint with those of the tiers above or below.