Switches and circuit breaker. other two combined; hence the center wire must be made of double the cross section. Thus it will be seen Current passes through main switch then to circuit breaker. 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. 11 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 Shunt machines in parallel. that the variation in pressure will be the same, practically, in all as the current strength changes, no one machine will be likely to take a portion of the load differing from its proper proportion by more than a few per cent. The way in which two shunt-wound generators are connected is shown in Fig. 15, only the generators and Two shunt wound generators in parallel. field regulators being represented, so as to avoid complication. Three, four or any other number of shunt machines would be connected in the same manner; that is, with all the a brushes connected with one bus bar, and all the b brushes with the other bus. Shunt-wound generators, however, are seldom used owing to the fact that they cannot maintain as uniform a voltage as the com Connected to equalize strength of current in series of coils of both machines. pound type. When two or more generators of this latter type are connected in parallel it is necessary to so connect the series coils that the strength of current passing through them will be the same in all the machines. This |