OVERHEAD CONSTRUCTION.
Fig. 64.
Trolley Wire. The trolley wire is suspended from the span wires or brackets in such a way as to permit of an uninterrupted passage of an upward pressing trolley wheel underneath it. The trolley wire itself may be either round, grooved, or figure 8 in section. Where a round wire is used, No. 00 B. & S. gauge is the most common size. Figure 8 wire, so called from its section, which is shown in [Fig. 64], is designed to present a smooth under surface to the trolley wheel, which will not be interrupted by the clamps or ears used to support it. Clamps are fastened to the upper part of the figure 8. The grooved wire is rolled with grooves into which the supporting clamps fasten. This wire also presents a smooth under surface to the trolley wheel.
Fig. 65. Trolley Wire Clamp and Ear.
Trolley-Wire Clamps and Ears. The trolley is supported either by clamps or by soldered ears. One type of clamp grasps the wire by virtue of screw pressure. A soldered ear is shown at E, [Fig. 65]. This ear has small projections at each end, which are bent around the wire to assist the solder in holding the wire to the ear. Another form of ear, used to some extent, holds the wire by virtue of having the edges of the groove offset or riveted around the wire.
The ear or clamp screws to a bolt which is insulated from the metal ear through which passes the span wire. A cross-section through a common type of trolley-wire hanger is shown in [Fig. 66]. Here there is an outer shell of metal, which is adapted to hook to the span wire. In this shell is an insulating bolt, that is, a bolt surrounded with some form of insulating material which is very strong mechanically and not likely to be cracked by the hammering action of the passing trolley wheel. Most of the insulating compounds used in making trolley-wire insulators are trade secrets. Another kind of insulator called the “cap and cone” type is shown at C, [Fig. 65]. In these insulators, the metal part B which fastens to the span wire does not completely surround the insulation C. Wood has sometimes been used for the insulation of trolley-wire hangers.
Fig. 66. Cross-Section Trolley Wire Hanger.
Span Wires. In city streets, the trolley wire is commonly suspended from span wires stretched between poles located on both sides of the street. These span wires are of ¼-inch or ⅜-inch galvanized stranded steel cable. In order to add to the insulation between the trolley wire and the poles at the side of the street, what is called a strain insulator is placed in the span wire. This is an insulator adapted to withstand the great tension put upon it by the span wire. One of these is shown in [Fig. 67]. Means are usually provided for tightening the span wires as they stretch and as the poles give under the strain. The insulator in [Fig. 67] has a screw eye for that purpose.
Fig. 67. Strain Insulator.
Fig. 68. Overhead Construction.
Brackets. In the bracket type of overhead construction, a trolley wire is fastened to brackets placed on poles near the track. This construction is used on suburban and interurban lines where the presence of poles near the track is not objectionable. It has been found that a rigid connection of the trolley wire to a bracket is likely to result in the breaking of the trolley-wire insulators. For this reason the brackets now commonly used provide for a flexible suspension of the trolley-wire hanger from the bracket. A bracket employing such flexible construction, made by the Ohio Brass Company, is illustrated in [Fig. 68].
An example of standard straight-line bracket construction is shown in [Fig. 69].
Feeders. Where additional conductivity is needed beyond that furnished by the trolley wire itself, feeders are run on insulators along the poles at the side of the track. Such feeders are connected to the trolley wire at regular intervals. Where span-wire construction is used, the feed wire may be substituted for the span wire at the pole where the connection between feed wire and trolley wire is made. In such a case, of course, a trolley-wire hanger is used which has no insulator, so that the current feeds directly through the hanger. Another method is to run the feed connection parallel with a span wire and a short distance from it.
Fig. 69. Standard Straight Line Construction.
Section Insulators. Section insulators are usually placed in the trolley wire at regular intervals. Such a section insulator is shown in [Fig. 70]. Its purpose is to insulate one section of trolley wire from the next, so that in case the trolley wire of one section breaks, or is grounded in any other manner, that section can be disconnected and the other sections on either side kept in operation. In large city street-railway systems, each section of trolley wire usually has its own feeder or feeders, independent of the other sections. This feeder is supplied through an automatic circuit breaker at the power house. In case a certain section of trolley wire is grounded the large current that immediately flows will open the circuit breaker supplying that section; but, unless the ground contact is of an extremely low resistance, it will not affect the operation of the other feeders. Should it be of sufficiently low resistance to cause all the generator circuit breakers to open, it would, of course, interrupt the entire service temporarily; but usually the circuit breaker on any individual feeder will cut that feeder out before all the circuit breakers will open.
Fig. 70. Section Insulator.
High-Tension Lines. Where high-tension alternating-current wires are run, as in the case where the road is of such length as to require the establishment of several substations, these high-tension circuits are usually carried some distance above the 500-volt direct-current trolley and feeders. An example of interurban overhead construction is shown in [Fig. 69]. Here the high-tension wires are carried on large porcelain insulators of a size necessary for 26,000 volts. These insulators are placed 35 inches apart. High-tension wires are kept so far apart because of the danger that arcs will in some way be started between the lines, as the high-tension current will maintain an extremely long arc. The blowing of green twigs across the lines, or birds of sufficient size flying into the lines, is likely to establish arcs which will temporarily short-circuit the line. The greater the distance apart of the wires, the less danger that such things will occur.
Both glass and porcelain insulators are successfully used on lines of very high tension. Glass is the cheaper and porcelain has the greater mechanical strength.
High-tension wires are usually of hard-drawn copper or of aluminum made up in the form of a cable of several strands. Aluminum is lighter for a given conductivity than copper; and, at the market price controlling at the present time, is cheaper. It is, however, more subject to unevenness of composition, which leaves weak spots at certain points in the wire; and that is the reason why aluminum is now always used in the form of a stranded cable rather than as a single conductor. Aluminum, being considerably softer than copper and melting at a lower temperature, is more likely to be worn through as a result of abrasions or to be melted off by a temporary arc. These slight objections are balanced against its smaller first cost as compared with the cost of copper.
The calculation of the proper amount of feed wire for a given section of road is somewhat similar to the calculation of electric light and power wiring as already outlined. It is first necessary to estimate approximately the amount of current required at different portions of the line. The amount of drop to be allowed between the power house and cars must be decided arbitrarily by the engineer. A drop of 10 per cent is probably the one most commonly figured upon in designing feeding systems. The resistance in ohms of the copper feeders required to conduct a given current with a given loss in volts, can be calculated by dividing the volts lost by the current, according to Ohm’s law. By the aid of a table which gives the conductivity of various sizes of wire according to the methods outlined in connection with “Electric Wiring,” the proper number and size of the feeders can be determined. The most difficult thing to determine is the load that will be placed upon any section of the line. Of course, there will be times when cars are bunched together owing to blockades. It is out of the question to provide enough feeder copper to keep the loss in voltage within reasonable limits at such times. The ordinary load upon any feeder is used as the basis of calculation in most cases. The amount of current required per car depends on the weight of the car and the character of the service. This will be taken up later under the head of “Operation.”