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More than one circuit.
board is less complicated than when there are three divisions; and three divisions cause less complication than four, and so on, for any greater number. In Fig. 12 is illustrated the manner of making the connections when the external circuits are divided into two systems, and from this diagram, the method employed with a greater number of systems can be readily understood.
From the circuit breaker c b the current passes to the rop center contact of the main switch S by wire j, and by wire ; to the ammeter A M and thence by wire j" to the lower center contact of the same switch. The main switch is of the type called double-throw, that is, it can be thrown to the right or left. When thrown to the right the current passes to the right side contacts, and from here to the B busses at the top of the switchboard. If the switch is thrown to the left it will connect the center contacts with those on the left side, marked i, and then the current will pass to the lower busses, marked A. The small switches, si s2 are shown connected with the upper busses B, and the switch s3 is connected with busses A. The switch, s s, is provided for the purpose of connecting the two sets of busses, and when it is closed the generator will feed into the entire system, regardless of what direction the main switch S may be in; but when ss is open the current will pass to the busses with which S is connected.
From the foregoing it will be seen that by this arrangement we can connect the generator with the upper or the lower busses independently, or we can connect it with both by closing the s s switch.
Buildings in large cities that are provided with a lighting plant are, as a rule, arranged so that current may be
Current from street mains.
drawn from the street mains should the machinery get out of order or should it be desired to shut down for any cause. The street mains are always arranged upon the
three-wire system, while, as a rule, the plant in the building operates upon the two-wire system. The latter system is preferred for private installations, owing to the
Balancing the three-wire system.
fact that the lights can be turned on or off without any danger of unbalancing the system, while with the threewire system such unbalancing is possible, and in small installations very probable. When a building is wired so as to be supplied from the street mains upon the threewire system, and from the house plant upon the two-wire system, the arrangement of the switchboard, for a simple case involving the use of but one generator, is about as shown in Fig. 13. In this diagram, G represents the generator located in the building, and L represents the three wires leading in from the street mains. The generator connects first, with the circuit breaker, cb, and then with the two-pole switch, S. From the right upper contact of this switch a wire, i, runs out and branches into the two parts, j %, which are connected with the top and bottom contacts on the left side of the double-throw switch Sz. The wires L from the street mains are connected with the contacts on the right of this S, switch. When the generator is in use, the current from binding post b passes to busses i and 3, and the current from post a passes to bus 2. It will thus be seen that if bus 2 is negative, busses I and 3 will be positive. When the street wires are to supply the current, the switch S, is thrown in the direction shown in the diagram—that is, to the right—and then each one of the three wires connects with one of the busses.
When the street service is used, the current flowing through the central wire, or bus 2, which is the neutral wire, is much smaller than that through the other wires, and will be nearly zero if the system is properly balanced. When the current is supplied by the generator G, the center wire will carry as much current as the
Switches and circuit breaker.
other two combined; hence the center wire must be made of double the cross section. Thus it will be seen
that, while in a regular three-wire system the neutral wire is smaller than either of the other two, in some cases
Connecting two machines.
being only one-third the size, a building wired for twowire house supply, and three-wire street supply will have the neutral made of a cross section equal to the other two wires combined.
In Fig. 13 the generator is represented by a simple square, with circles a b c d to represent the terminals of the armature and the field coils respectively. This construction is used for the purpose of simplifying the drawing, and for the same reason, the connection between c d a and the regulator R are omitted. In Fig. II the wires from the generator are shown connected with the circuit breaker, but they can also be arranged as in Fig. 14 so as to have the current pass first through the main switch and then through the circuit breaker. With some types of circuit breakers, this arrangement is necessary.
Operating Generators in Parallel.
When it is desired to operate two or more generators in parallel, that is, so as to feed into the same system of external wires, the simplicity of Fig. II cannot be preserved, for then it becomes necessary to provide means that will prevent one of the generators from taking more than its share of the load. If the machines are of the simple shunt-wound type, nothing can be done to equalize the work between them, except to adjust the field regulators so that all the generators develop the same voltage; and if they are all of the same size and design, so