Underground Cables.—Electric light and power cables for use in conduit may be divided into two classes: moisture proof, and non-moisture proof, according to the character of the insulator. In the moisture proof cables, the insulation consists of some form of rubber, or of bitumen, and a metal sheath or covering, usually of lead, is provided to protect the cable from mechanical or chemical injuries. The non-moisture proof cables are insulated with paper impregnated with oil, wax, or resinous compounds.
Fig. 988.—Cross section of Edison "main" tube. A number of these tubes, which radiate from the center of distribution and loop the ends of the feeders together, have three conductors of the same size. These tubes are placed in the ground so as to bring the positive and negative conductors on one side of the center of the tube, and the neutral conductor on the other side. The mains are always laid with the neutral conductor adjacent to the curb line, and for convenience this side of the tube is commonly called the inside. The feeders are always laid with the positive conductor on the right hand side, as shown in [fig. 989].
Metal Sheaths on Underground Cables.—Metal sheaths are used on rubber covered cables to protect the insulating compounds from the deteriorating effects of electrolysis and various kinds of acids and gases which, under present methods of construction, are ever present in the underground conduits. It is a fact, however, that the lead sheath on a low tension cable, which is used as one side of a grounded circuit, has been, in some cases the cause of, instead of, cure for electrolysis. The proper cure lies in the omission of the sheath altogether, but as this is not practical except in the case of very large conductors, the best thing that can be done is to interrupt the continuity of the sheath by some form of insulating joint.
Fig. 989.—Method of laying Edison underground tube system. The tubes are laid in trenches about 30 inches deep and 20 inches wide at the bottom, each trench usually containing two lines of pipes—a main adjacent to the curb and a feeder on the outside. The copper rods forming the conductors are uniformly 20 feet, 4 inches in length and project from 2 to 3½ inches from each end of the pipes, which are connected together by means of coupling boxes. The coupling box usually employed consists of a two part egg shaped casting into which the ends of the pipes enter through water tight sleeves at the opposite ends of the oval. The projecting ends of the copper conductors are joined by short pieces of flexible cable with sockets on each end, which are drilled to fit easily over the conductor rods to which they are thoroughly soldered in order to make a perfect electrical joint. After the conductors have been thus properly connected the cover is bolted down on the lower half, and the whole of the interior of the box is filled with insulating compound through the small hole at the top of the cover, thereby completely insulating the copper conductors, the cable connectors, and the ends of the tubes. Finally, the hole in the cover is closed with a cast iron cap. These coupling boxes are also made in the form of tees for making branch connections, and in the form of elbows for turning corners, the ball ends attached to the tubes and the sockets into which they fit being designed to permit of variation in direction through an arc of 18 degrees on either side of the central position. Services or branches to the consumers' premises consist of short lengths of tube which tap the mains by means of three way or four way service boxes, the latter readily permitting the taking of two services from one joint. Services are never taken from the feeders, but the latter are brought to distributing boxes containing three copper rings to which the conductors are connected and branched out to one or more mains which are led out through fuses to supply the districts.
Pot Heads.—The upper end of a lateral cable is equipped with a discharge bell, which is commonly called a pot head. The purpose of a pot head is to hermetically seal the end of the cable and bring the conductors out in such a manner as to permit of their being conveniently connected to the primary service boxes.