These sleeves are furnished by the manufacturers at a nominal charge per foot. They are about four inches in length and fit over the ends of the abutting pipes, so that they make tight joints and give perfect alignment.
Although its employment is not permitted where fireproof regulations are in force, fibre conduit is now being extensively used in other places, and is giving satisfactory service. It is not affected by moist earth and is impervious to the action of acids, alkalies, and gases. As it is not subject to expansion and contraction, leakage is practically eliminated, and since it is a very good insulator, troubles due to stray currents are reduced to a minimum. It is extremely light, comparatively non-breakable, and can be accurately laid at the rate of 12,000 duct feet per day by a gang of common laborers, consisting of two layers and three helpers.
Fig. 986.—Screw joint type of fibre conduit. This method of connection will form a tight line and is suitable for running under the lawns of private houses and parks, under the streets of towns and villages, and in other places where the cost of building electric subways is prohibitive.
Edison Tube System.—Of the various built in or solid underground conduit systems other than those already described under wooden conduit systems, the most satisfactory are the Edison tube system, the Crompton naked conductor system, the Kennedy system, which is a modification of the Crompton and the Callender systems.
Ques. Describe the Edison tube system.
Ans. It consists of a series of iron tubes or pipes containing one or more copper conductors which are placed therein before each complete section or pipe leaves the factory, so that they only need to be joined together to form a continuous line of underground conduit with conductors in place. The arrangement of wires and the details of the Edison tube system are shown in figs. 987 to 989.
Fig. 987.—Cross section of Edison "feeder" tube. This runs from the power station to the centers of distribution, and contains two principal conductors and a smaller conductor to serve as a neutral wire, and also three insulated cables of seven strands of No. 19 B. W. G. wire each. These cables form independent circuits and enable the voltages at the distant end of the feeder to be read at the central station. For this reason they are commonly called pressure wires.