This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It is of the utmost importance that when cables have to be joined the joints should be most carefully made by skilled workmen. Bad joints may not become apparent at first, but sooner or later they are sure to do so, when the defect can often only be remedied by the destruction of elaborate decorations, such as frescoes or panelling.
Even in good work, when an installation proves faulty, the engineer always looks to the joints, and more frequently than not he discovers the fault by so doing.
Moreover, bad joints do not usually show themselves when tests are carried out upon the conductors, and it is always a wise plan, therefore, to have all the jointing work under the supervision of a thoroughly reliable man.
The method of jointing two lengths depends chiefly upon the size and construction of the cable.
To joint small wires of a single strand, the two ends are stripped of the protecting and insulating material to a length of about 2 inches each. The copper is thoroughly cleaned by scraping or rubbing with fine emery cloth, and the two ends are twisted together and soldered into a solid mass. The joint is then insulated by binding the stripped wire with two layers of pure rubber strip, and finally protected by a layer of prepared tape stuck on by means of rubber solution.
To joint a stranded cable of only 3 strands, the insulating material is cut back as with single wires, and the joint made in the same manner.
The method of jointing large stranded cables is different, and depends in some measure upon the number of the strands of which the cable is composed. Figs. 135 and 136 shows a joint between two cables composed of 7 strands each. The braiding is cut back to a distance of about 6 inches, and a shoulder is neatly formed with a sharp knife. The rubber insulation is also cut back and neatly tapered to a conical form about 1 1/2 inch in length. The copper wires are separated for a length of about 2 inches from the end, and carefully cleaned with emery cloth. The remainder of the exposed copper wires are twisted tightly together, and the central strand is cut out as close as possible to the point where the strands commence to separate, when the end of each cable will appear as shown in the upper part of Fig. 135. The strands are then "married"; that is to say, the separated strands of one cable are placed between the strands of the other cable, and the two twisted portions are pushed tightly together as shown in the lower part of Fig. 135. The separated strands of one cable are then twisted round the twisted strands, and vice versa, care being taken that the separated ends are twisted in the same direction as the twisted strands which they overlap. The joint now assumes the appearance shown in the upper part of Fig. 136, and is now ready for soldering. This should be performed rapidly, so that the insulation on either side of the joints may be affected as little as possible. Care should be taken that the solder thoroughly permeates the whole joint. Soldering is best performed with a soldering iron of good size, with a groove near the end of the bit in which the wires are placed during the soldering process. The joint is then laid in the groove, and resin is sprinkled upon it and the end of a strip of solder is placed upon that; the solder melts immediately and permeates the whole joint if the soldering iron is at the correct temperature. No other flux but resin should be used, as others, such as acid or "killed-spirit," cause the copper to corrode. When the joint has cooled any irregularities should be trimmed off with a file. The joint is then insulated in the following manner: - Rubber solution is thoroughly rubbed over the exposed metal and insulation in not too large a quantity. Pure rubber strips are then tightly bound in at least two layers over the whole length of the joint, the layers being made to adhere by the application of a small quantity of rubber solution. In the best work six layers are employed. A small quantity of rubber solution is applied to the outer layer of rubber strip, to cause the subsequent protecting coat of tape to adhere to it. This tape is specially prepared for the purpose, and is applied in two layers thoroughly amalgamated with rubber solution. The application of rubber solution well rubbed in to the outer layer of tape completes the joint, which then presents the appearance shown in the lower part of Fig. 136.
When cables consisting of a large number of strands are to be jointed it is found that these strands, by virtue of their number, become very unruly in the process of marrying. They are therefore bound together with fine copper wire at the points where the strands begin to separate.
When a tee joint is to be made in single wires, about 2 inches of the main cable and 2 inches of the branch cable are bared, the insulating material being treated as before described. The wires are cleaned, and about 1 inch of the branch is wound round the main cable, and soldered to it at the extreme end. It is found that if the whole coil of wire is soldered it becomes too stiff and brittle to permit the rubber strip to be bound round it or to enable the wires to be drawn into casing (see Fig. 137).
To join stranded cables in the tee form, about 5 inches of the main cable and 7 inches of the branch cable are bared. About 2 inches of the branch cable is twisted up, and the remainder of the exposed strands are divided into the form of a Y and laid side by side so as to form two flat strips, as shown at A, Fig. 138. The two horns of the Y are then wrapped round the main cables in opposite directions, as shown at B.