Scientific American Supplement, No. 821, September 26, 1891

by Various

Every failure can be traced to poor work in the original installation or to the use of a cheap cable, both causes being due, generally, to that false economy which looks for too quick returns. A poorly insulated line wire and a poorly insulated cable are two very different things. However, it is a fact that by the use of a good cable it is not difficult to construct an underground system for light, power, telegraph or telephone uses that will be superior to overhead lines in its service and in cost of maintenance. The ideal underground system must have as a starting point a system of subways admitting of the easy drawing in and out of cables and affording means of making subsidiary connections readily and with the minimum of expense and interruption of service. This is practically accomplished by a subway consisting of lines of pipe terminating at convenient intervals, say at street intersections, in manholes, for convenience in jointing and in running out house connections. These pipes, or ducts, as they are called, should be for two kinds of service; the lower or deeper laid lines for the main or trunk circuits, and a second series of ducts laid nearer the surface, running into service boxes placed near together for lines to "house to house" connections. In some cities where it is allowed to run overhead lines, the plan of running but one service connection in a block is followed, all customers in the block being supplied from a line run over the housetops or strung on the rear walls.

This makes unnecessary all subsidiary ducts except a short one from the manhole to the nearest building in the block, and effects a considerable saving in pipe, service boxes, cables and labor. The manholes should have their walls built up of brick, the floors should be of concrete, and there should be an inside lid which can be fastened down and the manhole thus made water-tight.

For ducts wood, iron or cement lined pipe may be used. To preserve the wood it is generally treated with creosote, which, in contact with the lead cover of the cable, sets up a chemical action, resulting in the destruction of the lead. Wood offers but little protection for the cable, as it is too easily damaged and broken through in the frequent street openings made by companies operating lines of pipe in the streets, and as one of the main purposes of a subway is that of a protection to cables, wooden ducts have little to recommend them except their cheapness.

Iron pipes are either laid in trenches filled in with earth or are laid in cement. Iron pipe will of course rust out in time, and if absolute permanence in construction is desired, should be laid in cement, for after the pipe rusts out, the duct of cement is still left. However, if we are going to the expense of laying in cement, it would be much preferable to use cement lined pipe, which is not only cheaper than iron pipe, but makes the most perfect cable conduit, as it affords a perfectly smooth surface to draw the cable over and give a good duct edge.

It is not necessary, however, in small installations of cable, especially where additional connections will not be of frequent occurrence, to go to the expense of subways, for cable may be safely laid in the ground in trenches filled in with earth, or can be inclosed in a plain wooden box or a wooden box filled with pitch.

There are, of course, many localities where, if the cable is laid in contact with the earth, a chemical action would take place which might result in the destruction of the cable.

Underground cables are of the following classes: 1. Rubber insulated cables, insulated with rubber or other homogeneous material. 2. Fibrous cables, so called from the conductors being covered with some fibrous material, as cotton or paper, which is saturated with the insulating material, paraffine, resin oil, or some special compound. Under this latter head is also included the dry core paper cables.

The first thing to do is to get the cable drawn into the ducts, and on the proper accomplishment of this depends to a great extent the success or failure of the whole installation. Probably the ducts have been wired when the subway was constructed, but if not a wire must be run through as a means of pulling in the draw rope. There are several kinds of apparatus for getting a wire through a duct--rods, flexible tapes, mechanical "creepers," etc.; but probably the best is the sectional rod. This simply consists of three or four foot lengths of hard wood rods, having metal tips that screw into each other. A rod is placed in a duct at a manhole, one screwed to that, both are pushed forward, another one added and pushed forward, and so on until they extend the entire length of the duct. Then the wire is attached and the rods are pulled out and detached one at a time and with the last rod the wire is through. At least No. 14 galvanized iron or steel wire should be used, for any smaller size cannot be used a second time, as a rule. In starting to pull in the draw rope a wire brush should be attached to the wire and to this again the rope, and when the brush arrives at the distant end of the duct it very likely will bring with it a miscellaneous collection of material which for the good of the cable had better be in the manhole than in the duct.

The reel or drum carrying the cable should be mounted on wheels or jacks and placed on the same side of the manhole as the duct into which the cable is to be drawn, and must always be so placed that the cable will run off the top of the reel.

There are several methods of attaching the draw rope to the cable. As simple and strong a method as any is to punch two of these holes through the cable, lead and all, and attach the rope by means of an iron wire--some of the draw wire will do--run through these holes. Depending on the length and weight of cable to be pulled it can be drawn either by hand or by a multiplying winch. The rope should run through a block fastened in the manhole in such a position that the rope shall have a good straightaway lead from the mouth of the duct.