This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
Fig. 29 shows the three-wire system. The two dynamos d and d1 are connected in series, the positive lead of one being joined to the negative lead of the other. From the remaining terminals of the machines, the main leads a b and c f are brought out, and the third or neutral wire e h is connected to the short lead, already mentioned, which joins the two dynamos. The lamps are connected between this neutral conductor and either of the outer mains. It is, therefore, necessary that the E. M. F. of each dynamo should be the same, in order that lamps of one voltage may be used throughout. When, as in the figure, there is an equal number of lamps on each side of the system, it will be observed that no current will pass through the neutral wire. Considering the outer wires as the main conductors, this becomes a 200-volt system, if the lamps used are for 100 volts; for each of them, such as l and l1, would require an E. M. F. of 100 volts at its terminals m, n and n, o, making a total E. M. F. between m and o of 200 volts. The advantage of such an increase in voltage will be pointed out later. It is not possible in practice to have the two sides of the system always perfectly balanced, and when the current required for one side is greater than that used on the other side, the difference in amount will return by way of the third wire. This extra current changes in direction of flow, according as one side or the other has the larger number of lamps burning.
It will be seen that the three-wire system is quite like the multiple-series with groups of two lamps. There is, however, the important difference that when a lamp on the three-wire system burns out or is switched off, none of the remaining lamps are affected.