و

Independent action.

brush and the other to the end of the series coil. The two sides of the breaker act independently of each other, and to close the circuit one side is closed at a time; hence the name "independent action." A circuit breaker cannot be used as a switch if it is not of the independent action type. By means of the switch S the generator on the left can be connected with the conductors hi, and by means of the switch S' it can be connected with j and k. As will be seen, e connects the center contact of the circuit breaker with conductor g, and d connects the top contact with conductor f. Therefore, if the S and S' switches are thrown to the left S will close the circuit between f and hand between g and i, while S' will close the circuit between f and k, through h', and between g and j, through i, When the S and S' switches are turned to the right, as shown in the diagram, the left-hand generator is cut out of the circuit, and the other one is cut in. If S is turned to the right and S' to the left the left side generator will feed into the conductors j k, and the right side generator will feed into conductors h i. The conductors hi lead to circuit breakers 1 and 2, and the conductors j k lead to circuit breakers 3 to 12, inclusive. Therefore, by throwing the S and S' switches in one direction or the other either generator can be connected with either set of circuit breakers; but whichever way the switches may be thrown both generators will be entirely independent of each other, and on this account no equalizing connection is required for this position. When it is desired to operate both generators together the switch S" is closed, and then the center blade of this switch connects the conductors c and c', which come from the equalizing terminals of the two generators. The left side of the switch con

Bus junction switch.

nects f with f', and the right side connects g and g'; thus through this S" switch the two generators are properly connected to run in parallel with the equalizer connection closed. As the office of the S" switch is to join the two generators it is called a "bus junction" switch.

From the foregoing explanations it will be seen that either one of the generators can be connected so as to feed the circuit h i or j k, or both, and that one machine can take part of the load while the other takes the balance; or both generators can be connected to work together, and all this is accomplished by the movement of the three switches S, S' and S".

The field regulators are placed by the side of the voltmeter V, so that the latter may be watched to note the effect produced upon the voltage of the generator when the regulator wheel is being moved. The two ammeters directly above the field regulators R R' indicate the current of the generators, while the other twelve instruments are placed in the twelve branch circuits controlled by the circuit breaker switches. Only one voltmeter is used, and, as in the case of Fig. 18 this is normally connected with the working circuit so as to indicate the voltage of the line, but when the generators are to be thrown into parallel connection it is used for the purpose of equalizing their electro motive forces, the small switch V S being provided for the purpose of connecting the instrument with either generator.

In comparing the switchboards illustrated in Figs. 18 and 25, we can see several marked differences; for example, the first is provided with an equalizing bus bar, while the second one is not; nevertheless, both are properly arranged to connect the equalizing wires of the gen

Current from street mains.

erators when the latter are required to operate in parallel relation. In Fig. 25 the center connecting blade of the S" switch plays the part of an equalizing bus. This arrangement is not adapted to Fig. 18, owing to the fact that the generators are not required to operate separately, but at all times feed into the same system of distributing circuits. Fig. 18 has a double set of bus bars, and before the current can pass to the distributing circuit it is necessary to close the S" switch; and this is true, whether the current is derived from the generators or from the street mains. Upon examining the diagram it will be found that the second set of bus bars become necessary in order to be able to take current from the three-wire street system. If the S1 switches were connected directly with the lower set of busses, they would operate well enough when taking current from the street; but when supplied from the generators they would not, for then all the lamps, or motors, connected between the E bus and the top one, P bus, would be practically connected with the same end of the line, and would therefore not receive any current worth mentioning, as the voltage acting upon them would be practically nothing. When the S" switch is thrown to the left it connects only the top and bottom busses of the lower set with those of the upper set, and as with this connection the center upper bus is the positive side of the generators and the top and bottom busses are the negative side, all lamps connected between the center and either of the two N busses will get the proper voltage when the current is derived from the generators, and also when the street supply is used.

In Fig. 18 the ammeters are connected in the circuit, so that all the current of each generator passes through its

Size of connections.

respective instrument; but in Fig. 25 the ammeters that measure the current of the generators are so arranged that only a portion of the main current passes through them. Tracing the lines rx and r' r' it will be seen that they lead to the ends of blocks marked St. These blocks are shunts and are for the purpose of diverting a portion of the current through the ammeter. The twelve ammeters in the branch circuits are arranged as in Fig. 18 so that the whole current passes through them. If we look at circuit breaker switch No. 3 we will find that its top terminal is connected with the No. 3 ammeter, the other terminal of this instrument passing directly out to the external circuit. The other side of this circuit comes down to the center terminal of the No. 3 circuit breaker. The ammeters in the branch circuit are connected so that the whole current passes through them, because they are of small capacity, compared with the instruments that measure the main currents. The connections on switchboards are generally made with bars that have a cross section of I square inch for each 1,000 amperes, and from this it can be seen that to connect ammeters of large capacity directly in the circuit would require the use of large conductors. When the shunts St are used, the current passing to the instrument need not be more than a small fraction of I per cent., hence it can be carried in small wires.

Fig. 26 shows the arrangement of circuit connections and instruments upon the switchboard of the Franklin Building, New York City. This board is one of the best examples of modern switchboard engineering practice. It is arranged to connect three generators with two systems of distributing circuits, one being used for lighting and the other for power. These circuits are also ar