Зображення сторінки
PDF
ePub

a rope through it in order to draw in the cable. One plan is simply to push through a steel tape or wire, which is provided with a rounded metal head to prevent the end from catching in the joints between the pipes. By means of this tape or wire, a small and then a large rope may be pulled through the duct. Another method is the so-called "rodding" of the ducts, a 20-minute operation, consisting in inserting one after another into a duct, short

Fig. 248. Rods for "Rodding Conduits."

rods of wood or steel, about 3 or 4 feet long and inch in diameter, which are connected together by screw or bayonet joints, as indicated in Fig. 248. When a sufficient number have been joined to reach from one manhole to the next, a small rope is attached to a ring in the last one, and the rods are then pulled through from the other end, being unjointed as they come out. A larger rope may then be drawn through, after which a steel scraper and a brush should be pulled through the duct in order to clean it, and remove any stones, tools, etc., that are often found in it and would be very likely to injure the cable. These are illustrated in Fig. 249.

Fig. 249, Scraper and Brush for Conduit.

After the duct has been cleaned properly, the end of the cable is attached to a heavy rope which has been drawn through the duct by attaching it to the cleaning implements. In attaching the cable to the rope, care should be exercised to avoid bringing undue strain on the copper conductor or its insulation during the operation of drawing in the cable. This may be done by putting a conical metallic head on the end of the cable, or by winding several iron wires spirally around the last foot or two of the cable, and forming these into a loop to which the rope is attached. For hard pulls and curved pipes the end of the cable is served, after removing about 18 inches of the lead and the insulation. The strands of the cable are then fanned out, and divided into four groups, and passed through a shackle as shown in Fig. 250; they are bent back on themselves, and bound tightly with spun yarn or wire. If the pull is to be extra hard, an iron wire may be also put through the

shackle, and driven through the lead sheath. This gives an excellent hold on the cable, distributing the strain over all the conductors, as well as to the lead covering.

[blocks in formation]

Having attached the cable through a swivel to a strong rope, it is drawn into the duct by means of an apparatus shown in Fig. 251. The cable unwinds from the drum D, as it is drawn by the winch W. It is evident that the cable should be somewhat smaller than

[blocks in formation]

the duct through which it is drawn, but the margin need not be great. For example, cables 2 inch in diameter can be drawn. through the standard 3-inch ducts. When several cables are to be put in the same duct it is much better to draw them all in at the same time; but it is possible to draw one into a duct already containing others, provided there is space enough. It is also possible to withdraw one or several cables from a duct without serious injury to them, in case repair or change becomes necessary.

Methods of Distribution from underground conduits constitute one of the most serious problems in connection with them. The main or trunk lines may be provided for by the various forms of conduit that have been described, but these do not readily allow branch connections to be made at frequent intervals to supply individual buildings.

It is decidedly objectionable to complicate and

weaken the construction of the main conduits by having side connections, so that the best plan is to keep those intact, and provide a subsidiary duct or conduit into which the conductors required for local distribution are run at one of the manholes. Such an arrangement is shown in Fig. 252.

[graphic]

Fig. 252. Hand-hole Distribution.

In Figs. 253 and 254 are shown iron subsidiary service boxes for use in connection with terra-cotta conduits.

The Edison system of distribution is well shown in Fig. 255 which illustrates also a distributing box and service box.

[graphic][subsumed][merged small]

The house vault, back yard, lamp-post, housetop, hand-hole methods of distribution are easily seen to refer to the way by which the service cable passes from the subway to the building. Street arc lamps may be supplied as indicated in Fig. 256.

In Baltimore there is a complete system of distribution, shown in Fig. 257, and consisting of two separate parts.

[graphic][subsumed][merged small][merged small]

For high-tension distribution there is a wrought-iron pipe duct with service boxes to transformer pits, consumers, etc., being entirely a subway, and entering buildings through cellars.

For low-tension mains there is a conduit system of wroughtiron tubes, cement-lined tubes, or terra-cotta duct, leading to pole terminals situated on each block, and thence overhead to

consumers.

VENTILATION OF UNDERGROUND CONDUIT SYSTEMS.

Considerable difficulty has been experienced in satisfactorily. ventilating subways carrying electric light and power conductors. When these systems were first introduced they were made as nearly as possible air-tight and the covers of manholes hermetically

-30

3'0" Radius

Fig. 256. Lamp-post Connection.

sealed. The porosity of the material forming the walls, etc., defeated this end, and allowed gas to enter the different parts of the system where it mingled with air. The blowing of a fuse or a spark due to some other cause would often ignite this mixture of air and gas, and destructive explosions sometimes resulted. To avoid this danger pumping equipments were arranged to remove the air from the subways. This at first seemed effective, as it tended to keep the air in the subway in motion, but it also in

duced gas to enter, due to the partial vacuum caused.

After abandoning this scheme a new arrangement was introduced known as the blower system. This was based on the idea that if

« НазадПродовжити »