ELECTRIC UNDERGROUND CABLES AND CONDUITS. In the first attempts to operate the electric telegraph, over 60 years ago, both in this country and in Europe, the wires were placed in cables underground; but owing to imperfection in the methods of in sulating the wires, as well as in the type of conduit or pipe employed, and of the manner in which the conduits were laid in the earth, the cables and conduits failed after compara tively short service and the use of overhead wires supported on poles was resorted to and became the universal practice, which continued almost without interruption for a quarter of a century.
Beginning about 1890, however, there has been a movement in all the principal cities of America and Europe to place all electric wires underground in order that the streets may be freed from the encumbering poles and overhead wires. In New York city, especially, the move ment to this end was carried on vigorously and persistently, with the result that for many years there has not been a pole supporting telegraph, telephone, electric light or trolley wire in any part of Manhattan borough, such wires all being placed in cables in conduits under the surface of the streets.
Electric Underground Cables.— The type of underground cable used for telephony, teleg raphy and electric light and power purposes varies greatly. For example, the conductors used in telephony have a diameter of .040 inch; those for telegraph purposes about .080 inch; those for electric light and power range from one-quarter of an inch to one inch and over in diameter. The smaller electric power wires are employed in high potential and .comparatively light current work; the larger wires in low tension and heavy current work. It is thus feasible to place about 400 telephone conductors, or 100 telegraph conductors, in one cable in a three-inch underground duct or pipe, while it is only practicable to place two, three, or, at most, five electric light or power conductors in a similar duct. The insulating material used for telephone conductors is usually a wrapping of tissue paper in narrow strips, laid on spirally over each conductor. The insulating material of telegraph underground cables is usually a rubber compound or strips of paper saturated with oils, the thickness of the wall of which Is about .038 inch. The insulating material of electric light and power cables is usually a rub ber compound, oil paper or varnished cambric, which is from one-eighth of an inch to nearly half an inch thicic, depending on the electric pressure to be withstood, which, in the case of low potential circuits, is about 220 to 600 volts, and in the case of high potential circuits may range from 1,000 to 30,000 volts. Gutta-percha,
which has been employed for the insulation of long submarine cables, is not used for under ground cables, owing chiefly to its low softening point under heat, 120° F., which temperature is not infrequently encountered in subways in cities.
Cables designed for underground work are encased in a lead envelope to protect the in sulating material from water, moisture and the effects of gases, acids, etc., in the underground conduits. For crossing rivers such cables are also armored with iron wires in addition to the lead covering, as a mechanical protection.
The term cable includes the conductor Ocore"), the insulating material, the lead cov ering and the armor when the latter is em ployed. Copper is practically the only metal used for the conductors of electric cables. Aluminum is not used because of its bulk for a given conductivity, which bulk is about 1.6 greater than copper. Tlie increased amount of insulating material and lead covering, as well as space in the conduits, that would be required in the case of aluminum for a given con ductivity would be virtually prohibitive of its use for underground cables.
The copper wire used in cables is drawn to the required size in the wire factory. If the wire is to be insulated with a rubber compound it is utinnedD to prevent any chemical action between the sulphur used in the rubber com pound and the copper. When the covering is paper, linen or fibre the wire is not tinned. The tinning process consists in passing the wire through a vat of molten tin. For electric light and power cables, when the conductors do not exceed .204 inch diameter, they are usually solid, or of one wire; above that size they are generally stranded to obtain flexibility. The wires are stranded in a stranding machine in one process, the wires being wound on reels, which are held on suitable spindles on the frame of the machine. A single wire is held in the centre of the frame and is slowly drawn through a guide. The wires for the first layer are wound spirally around the central wire; the wires for the second layer are held on another frame and are laid over the first layer in an opposite. direction, and so on for the additional layers required. The strand is wound upon a drum and is then ready for the insulating prOCCSS.