For a local installation the three-wire system is not a good choice, as two dynamos must be constantly running; but when there is a three-wire city supply available, a combination wiring for the two systems may be put in, the private plant or the city mains being used as may be convenient. This arrangement is known as the flexible two-wire system, and is almost identical with the three-wire system, the only difference being that the neutral wire N, Fig. 30, is made double the size, in sectional area, of each of the outer wires. When used on the two-wire multiple-arc system, the connection to the street mains m, m, m is broken, and the two outer leads are joined together and to one terminal of the dynamo in the building, and the middle wire to the other terminal. This may be accomplished by means of a special switch S, as shown in the figure. The upper contact-blocks 1, 2, 3 are connected to the street mains, and the lower two, 4 and 5, are joined together by a copper bar c. The several contacts 6, 7, 8 are connected to the three blades of the switch, which are insulated from each other. When the switch is thrown upwards, the mains a, N, b are put in communication with the street mains m, m, m; when it is thrown downwards, the outer mains a, b are joined together through the circuit 6-4-5-8; and, the other switch being closed, the leads d, e from the dynamo D supply current to the lamps throughout the whole system, the heavy wire N acting as a return path for the current passing through the two outer mains a, b.

40 Flexible Two Wire System 479

Fig. 30.

41. There is an important consideration to be remembered as regards the E. M. F. of the dynamo. If lamps taking 110 volts are used, the E. M. F. across the mains a, b for the three-wire system will be 220 volts. If the drop of potential (Art. 14) be 5 per cent. from street mains to lamps, there will be a loss at full load of about 11.5 volts, being 5.75 volts in each main a and b, the neutral wire taking no current. On changing over to the two-wire system, there will be the same percentage of loss in each of the mains a and b, and an equal loss in the return wire N, being a total drop of 11.5 volts. Therefore, the dynamo must furnish an E. M. F. of 110 + 11.5 = 121.5 volts, although the pressure on each side of the three-wire system, at the street mains, is only 110 + 5.75 = 115.75 volts. In this case, however, the neutral wire carries practically no current, the drop being confined to the outer mains.