Arrangement of Instruments and

Switches.

In what follows, it is proposed to explain the general arrangement of the switches, instruments and other apparatus, and also to show how they are connected with each other and with the generators and the external circuits. No general rule can be given for the location of the various devices or for the circuit connections, owing to the fact that in each case the conditions are different; therefore, each particular board must be treated as an independent problem, and the disposition of the parts must be such as to best meet the particular requirements. Since no general rules can be laid down, the only course of procedure left is to present practical examples of boards that have actually been made, and by explaining the conditions they have to fulfill, throw light upon the subject that will serve to show the proper course to pursue to obtain the best results. In some cases the conditions to be fulfilled are few and easily met, while in others the conditions are numerous and of necessity involve a considerable amount of complication. If we have two generators, and desire to connect them to feed into a single system of circuits, the arrangement of the switchboard becomes a decidedly simple matter. On the other hand, if we have two generators, and desire to so arrange them that they may feed into several independent circuits, and be so disposed that either one of the generators may feed any one of the

A simple switchboard.

distributing circuits or all of them, and further, that one machine may feed some of the circuits while the other feeds the balance, then we will have a complicated arrangement, and may have to exercise our ingenuity to the fullest extent to avoid the use of unnecessary switches, etc., and to avoid too great a complication in the circuit connections.

Fig. 18 shows an arrangement of switchboard for two generators, that may be classified as belonging to the simple type. In this case the generators are required to feed into a single system of distributing circuits, and the only other condition to be fulfilled is that the board must be so arranged that if for any cause the generators fail to operate, current from the street mains may be turned on at once. From the explanation which follows, the manner in which these results are attained will be understood.

As was explained in the last section, it is necessary when two or more generators are operated in parallel, to connect the entering ends, as well as the leaving ends, of all the series coils, so that the current may divide equally between all the generators. In Fig. 18 the equalizing connection runs to the center contact of the main switches S and S'. The connections from the ends of the series coils run to the right-hand side contacts of these switches, and the connections from the left-hand armature terminals connect with the left-hand contacts. If switch S is opened, the left-hand generator will be cut out of the circuit, and if switch S' is opened, the righthand generator will be cut out. When either one of these switches is closed, its respective generator is connected with the circuit, the center contact, which is the

equalizing connection, connecting with the center bus E, which is called the equalizing bus. If the two switches are closed, the two generators will be connected with the circuit, and their equalizing wires will be connected

Equalizing bus bars.

through the bus E. The current from switch S' passes to the ammeter on the right side of the board through the wires c and d, and the current from switch S passes to the other ammeter through wires c' and d'. The voltmeter can be connected with either generator by means of the switch s, and also with the busses N and P, as is clearly shown by the wires v v, v' v', v" v". Under the ordinary running conditions, the voltmeter is connected with the busses, and the switch s, when turned to make this connection, disconnects the wires leading to the two generators. If one generator is running, and it is desired to throw the other into service, it must be set in motion and its voltage brought up to equality with that of the running generator before its main switch is closed. Suppose the right-hand generator is in use, and that we desire to start up the other one, then we set it in motion and move the voltmeter switch s so as to connect alternately with each generator, and when both show the same voltage, the S switch is closed.

The main switches S and S' connect the generators with the lower set of busses, but if the switch S" is open, the current cannot pass to the external circuit. When this switch is closed to the left side, as shown in the diagram, the current from bus N will pass to the top and bottom busses of the upper set, and the current from the P bus will pass to the center bus of the upper set. With this connection, therefore, the current will pass out to the external circuit through the center bus and return to the two outside ones. The object of this arrangement is to render it possible to operate the lamps or other apparatus upon the circuit from the street mains as well as from the generators. The street service being arranged upon the three-wire system,

About safety fuses

the circuits supplied through the S' switch must be arranged for three-wire distribution. Should it become necessary to shut down the generators for any reason, all that would be required to connect the system with the street mains would be to throw the S" switch over to the right side, and then the three street mains would be connected with the three upper busses, each main being connected with a separate bus bar.

In the diagram, Fig. 18, no circuit breakers are shown, owing to the fact that in this case the generators are protected by simple safety fuses. These fuses can be mounted upon the switchboard or at any other convenient point; but when upon the board they are placed behind and not in front, as they are not attractive in appearance, and when they act they are liable to deface the surrounding parts by covering them with molten metal unless they are of the enclosed type. It is very seldom that a board is arranged so as to depend wholly upon safety fuses—a regular magnetic circuit breaker is invariably provided in the most modern installations. A magnetic circuit breaker is more effective than a fuse for sudden rises of current strength, as it will act instantly, while the fuse will not respond to a sudden rise in current strength if it lasts only for an instant, owing to the fact that it requires some time for a current to heat the fuse wire up to the melting point. Fuses are relied upon almost exclusively for the protection of branch circuits, but for the main line the proper thing is the magnetic circuit breaker.

In Fig. 18 the circuit connections are such that the distributing system can be fed from the two generators, or from the street mains, by turning the switch S" in one direction or the other. When the street connection is not