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means of taking care of three-wire systems. But these methods fail; in fact, the problem is practically impossible to solve if the neutral wire is grounded. This matter will be discussed further in the chapter on Detecting and Locating Faults.

Regarding the second advantage of grounding the neutral conductor, it may be said that it is well enough to use the earth to reenforce the conductance of the circuit, provided no serious difficulty results. But it is found that great damage is done by electrolytic action on gas, water, and other kinds of pipes, if large currents are allowed to flow promiscuously through the earth. In the case of electric railways with overhead trolleys, it is necessary to allow the current to pass into the track, or else adopt the double-trolley system, which is complicated, and not considered practicable for general use. But even for the trolley system the tendency is to demand more perfect bonding of the rails, and the use of return feeders to reduce the stray currents. In electric lighting there is no neces sity for intentionally grounding the circuit or any portion of it, but it is generally recommended for the secondary circuit of a transformer, as a safeguard in case the high-tension primary circuit accidentally connects with the secondary.

Insurance and fire department authorities have been vigorously opposed to grounding the neutral of a three-wire system, or, in fact, any part of an electrical circuit, their experience having led them to believe that it is the source of danger and trouble. This practice is not so strongly opposed at present, and is being more generally adopted, as nearly all central station officers would prefer to ground the neutral conductors.

Peculiar Conditions on a Three-wire System. The following cases may occur :

1. The dynamo or dynamos on one side of the system may be accidentally reversed, so that both of the outside wires are positive or both negative. In that case a motor or other device fed by the two outside conductors will receive no current; but lamps, etc., connected between the neutral and either of the outside wires, will have the usual voltage, which will be reversed on one side.

2. If one of the outside wires is open at B, Fig. 60, due to the blowing of a fuse or other cause, a motor, M (220-volt), beyond the break B, will receive some current at 110 volts through any lamps L that may be on the same side of the break as the motor,

and on the same side of the system as the break.

These lamps will light up when the motor is connected, but the latter will have comparatively little power.

3. If the neutral wire is open, a motor or other device connected to the outside wires will act as usual, but lamps on one side

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Figs. 60 and 61. Peculiar Conditions on Three-Wire System.

of the system will burn more brightly than those on the other side, unless the two sides are exactly balanced.

4. If one of the outside wires, Fig. 61, becomes grounded at P, a 110-volt lamp, L, or other apparatus, also grounded and connected to the other outside wire, will receive 220 volts, which is likely to destroy it.

FIVE-WIRE SYSTEMS.

The principle of the three-wire system may be extended, in order to effect a still greater saving of copper in electrical distribution. It would be possible, for example, to have a four-wire system requiring two-ninths as much copper as an equivalent two-wire circuit; but, for reasons to be given later, it has rarely, if ever, been tried. The five-wire system is employed in many places in Europe, but has not been introduced to any extent in this country. It may be operated with four dynamos, C, D, E, and F, as represented in Fig. 62; but the arrangements shown in Figs. 63 and 64 are more common. The second of these is similar to the three-wire system illustrated in Fig. 56, only one main dynamo, D, being required; and the total pressure generated by it, ordinarily about 440 volts, is subdivided by the four small equalizing machines or compensators, J, K, L, and M. These may consist of four separate machines mechanically connected together, or they may be made with all of their armature windings upon the same core and acted upon by one field magnet, in order to neutralize the effects of armature reaction. Fig. 63 shows a combination similar to the three-wire system represented in Fig. 51, a battery, N, P, Q, R, being utilized to

subdivide the voltage of the main dynamo D. The conductors are designated, as shown in Fig. 62, the two extra wires being called the "positive neutral" and the "negative neutral" respectively.

The comparative weight of copper required for. the five-wire system may be determined by reasoning similar to that used in connection with the three-wire diagrams (Figs. 47 and 48). But it can be arrived at more simply by considering that the current in each of the outside wires of a perfectly balanced five-wire system is one-quarter as much as in a two-wire circuit supplying the same number of lamps. Hence the drop is only one-quarter as great, assuming the conductors to be of the same size. But since with five wires there are four sets of lamps in series, the percentage of drop is as much, or in other words, each conductor need be only one-sixteenth as large for the same percentage of drop. Therefore the two outside conductors of the five-wire system weigh one-sixteenth as much as those of an equivalent two

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Figs. 62 and 63. Five-Wire Systems.

wire circuit, and the five conductors weigh

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as much, if all are made of the same size. By making each of the three intermediate wires one-half as large as each of the outside ones, the total weight is reduced to + 3 × 32 = 84, or less than one-eighth as much copper as the two-wire circuit demands. The various results that have been obtained may be recapitulated as follows:

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It is evident that similar systems having a greater number of wires might be designed, but they would be extremely complicated, and of very doubtful advantage. In fact, the desirability of a fivewire system.is questionable, since the use of 220-volt lamps enables three-wire circuits to be operated at 440 volts. A five-wire system calls for an even more perfect balance of load than is needed for three-wire circuits. This is secured by carefully dividing the lamps, etc., between the four parts of the system so that the loads may be as nearly equal as possible at all times. To this end all five wires should be carried wherever any considerable amount of energy is likely to be used, as represented at A in Fig. 64. If the demand for current is small, it is only necessary to run

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three wires, as shown at B. But in this case an approximately equal load, P, should be connected to the other side of the system, as near B as possible. For very small loads, E, F, R, and S, it may be allowable to put them on the separate parts of the systems, provided they are equally distributed, and not far apart, as represented. Motors should generally be supplied from the two outside wires (440 volts) as indicated at N; but if they are not large they may be connected to the + and 0 or to the 0 and conductors (220 volts) at B or P, and very small machines, such as fan motors, may be connected to adjacent wires (110 volts) at E or R. Arc lamps may be arranged as shown at F and S, or a suitable number may be put in series across the outer wires at A or B.

The flow of the currents and values of the potential in five-wire systems may be determined by extending the methods already explained in connection with three-wire circuits. Although apparently a complicated matter, a problem of this kind can be solved without much difficulty in most cases. In practice the current to be supplied is usually known, or its probable value may be assumed. A diagram similar to Fig. 65 should then be made, showing the arrangement of circuits and distribution of current. It is much simpler, and in most cases sufficiently accurate, to consider the lamps or other apparatus requiring energy to be located in groups, approximating as closely as possible their actual positions.

This enables the conductors to be divided into sections, in each of which the current is uniform, as represented in Fig. 65.

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determination of the amount and direction of the currents in the various sections is easily made. If 10 amperes are required at E and also at F, it follows that that amount of current will flow out on the wire, and half-way back on the wire, there being no current in the rest of this conductor. Since 5 amperes are required at G, one-half of the 10 amperes will flow out to that point, and the other 5 amperes will return to the dynamo through the O conductor, the function of the three neutral or intermediate wires being to carry the difference between the currents used in the adjacent portions of the system, whatever its amount and direction may be.

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The 5 amperes required at H are supplied by the tor, and the 5 amperes used at by the same current that flows through G, hence there is no current in the outer half of the

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