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90 HANDBOOK OF PROJECTION FOR
have the system unbalanced by y2 ampere, with \l/2, l/2, 1 ampere flowing respectively in wires E, D and F between the lamps and the generators, and generator A producing l/2 ampere more of current than generator B.
Insofar as current flow be concerned that is the way the three-wire system operates, but there are some points in connection with it which are very puzzling to the novice and more or less so to some more experienced men.
Fig. 11 is the diagrammatic representation of several house circuits fed by a three-wire service circuit, each wire of which is fused at 60 amperes. Between the neutral (central) wire and the upper wire, circuits A, B and C are connected, each of which is connected to apparatus using just 10 amperes. Between the neutral and the lower wire circuits D, E and F are connected the first two using 10 amperes each. Circuit F is idle.
Question : Would it be possible to attach a 25-ampere projection arc to circuit F, when the three main wires are only fused to 60 amperes and the circuit already loaded as shown?
The novice will probably answer, "No, the circuits are already using 50 amperes, and the addition of a 25-ampere arc would overload the fuses." The novice would, however, be in error, because the circuits are not using 50 amperes, but only 30, 10 of which are handled individually by the generator attached between the neutral and the upper wire. Circuits A, B, D and E will burn in series, as has already been explained, so that instead of 40 amperes at 110 volts, the lamps or motors on circuits A, B, D and E will work in series on 220, and only a total of 20 amperes will flow.
Circuit C will use 10 amperes at 110 volts, just as though wire 3 did not exist, as long as circuit F is idle. This will have the effect of causing the upper wire to carry 30, the lower 20 and the neutral to carry 10 amperes, so that the upper fuse will carry 30 amperes, the neutral fuse 10 amperes and the lower fuse 20 amperes. Under that condition the system is unbalanced 10 amperes, and the generator attached to the neutral and upper wire is carrying that much more load than is the generator attached to the neutral and lower wire.
Suppose we now connect a 25-ampere projection arc to circuit F. Circuit C now burns in series with circuit F to the extent of 10 amperes, but 15 amperes of the 25 must come from the generators over neutral wire, so that we now have the following condition : The upper fuse carries 30, the center fuse 15 and the lower fuse 45 amperes, and the generator