These pipes are laid in cement or concrete in the same manner as plain iron pipe, and are given a coating of tar on the outside to prevent rusting.
Cast Iron Pipe and Trough Conduit.—Cast iron pipe for underground conduits is similar to ordinary wrought iron pipe, except that it is thicker. The additional thickness is necessary to make the strength equal to that of wrought iron; it is therefore heavier to handle and more expensive.
Ques. Describe a cast iron trough conduit.
Ans. It consists of shallow troughs of cast iron in six foot lengths, laid directly in the earth so as to form a system of continuous troughing in which the conductors are placed and then covered over by cast iron covers which are bolted to the trough.
Ques. What advantages does this form of conduit possess over the duct type?
Ans. First, the cables can be laid directly in place, thus eliminating any chance of injury during the process of drawing in, and second, the cables are easily accessible at any point by simply removing one or two of the sectional cast iron covers, thus permitting of their being readily inspected and repaired.
Fig. 984.—Fibre conduit. It consists of pipes made of wood pulp, having about the same thickness as cast iron pipe. Slip joint conduit for electrical subways is three inches inside diameter. The socket joints keep the lengths centered and make it easier to lay than a mere butt joint. It is laid in cement like iron pipe.
Branch connections can be made with greater facility than in the case of any duct system, so that it is especially suitable for distribution systems were it not for the fact that it is so expensive as to be practically prohibitive.