Bars often better than wires.

electro-chemical operations. If we estimate the cross section of the conductors at the rate of 1 square inch per 1,000 amperes, then the range of cross sections for the switchboards of industrial installations will be from 1 to 15 or 20 square inches. Unless the current is used for chemical processes the cross sections will not very often run beyond four or five inches, for when the power transmitted is very great the voltage is increased, and thus the current strength reduced.

For any size of conductor greater than one-half of a square inch, flat bars can be used to better advantage than round wires, and they are more desirable from an electrical point of view, as they present a greater radiating surface for the same cross section, and therefore will not heat up to the same extent with a given current strength. The general arrangement of these connecting bars is shown in Fig. 27, which represents the back of a switchboard arranged for four generators, each one having a capacity of 1,000 amperes, making 4,000 amperes as the total capacity of the board. The connections between the generators and the board are shown in single lines, but these are made with cables, or bars, whichever may be the most convenient. The conductors I 2, of each generator, run to the lower terminals of circuit breakers, which are marked C B1, C B2, etc. The conductors marked 3 are the equalizer connections, and run to the lower center terminals of the switches marked S1, S2, S3, S4. The bar D is the equalizing bus, and the bars C and E are the positive and negative busses, and to these the current passes from the outside terminals of the switches, these being connected with the center and top terminals of the circuit breakers by the bars b b. This board is

How bus bars are arranged.

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arranged so as to distribute the current of the generators, or to obtain a supply from the street mains. The switch S, is for the purpose of connecting the bars E, E3, C2, which are the main distributing busses, either with the generator busses C E D or with the street mains, which come to the three terminals in the vertical row marked SS. The switch S, is double throw, and when thrown to the left connects the generators and the main busses, and when thrown to the right connects the street mains with the main busses. The main busses, at both ends, connect with secondary busses E Es C3 by means of the bars marked d d d and d' d' d'. From these secondary busses the current passes by means of the distributing switches S' to the external circuits LLL.

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The ammeters are of the shunt type and derive current from the top and bottom terminals of the ammeter shunts marked St. The voltmeters are connected with the lower terminals of the circuit breakers or with any convenient points on the wires I 2 leading from the generator. As the ammeters and voltmeters take very small cur

Figure 28.

bus bars.

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Eg

rents, the connecting wires are small Cross section showing flexible cables.

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Current that must be carried.

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As a rule all the connecting bars are made straight and are run at different distances from the back of the board, so that they may cross each other and leave a sufficient intervening space to render accidental contacts improbable. The cross section, Fig. 28, shows the location of the main bus bars E2, E, and C2, and also of the secondary busses E1, E, and C. The position of the connecting bars d d d and d' d' d' is also shown. Owing to the fact that the different bars are run in separate planes, the studs which form the terminals of the switches and circuit breakers and to which the connecting bars are attached, must be made of different lengths or else be long enough to reach the most distant bars. The latter construction is generally followed, and in order that the studs may be adapted to hold bars at different distances from the back of the board, they are threaded throughout their entire length, and are provided with three nuts, one of which clamps the stud to the board, the other two holding the bar, as is shown in Fig. 29.

The generator busses E D C have to carry currents of 2,000 amperes, owing to the fact that the switch S, is located at the center. If this switch were placed at one end, then these bars would have to be of sufficient size to carry 4,000 amperes. Thus it will be seen that by properly locating the switches the size of the connecting bars can be reduced. The main busses E, E, and C, are also arranged so that one-half the current runs in each direction; therefore, they need only be of sufficient capacity to carry 2,000 amperes. The bars leading down from C D E, and up from E, E, and C, to the throwover switch S, must be of sufficient size to carry the whole current. The bars d d d and d' d' d', and also the sec

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Clamps for bus bars.

ondary busses, are made of a cross section sufficient to carry 1,000 amperes. From the foregoing we see that the bars connecting with switch S, should have a cross section of 4 inches, but as they are very short we can

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d

d

g

Figure 29. Method of clamping bus bars.

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safely reduce it to 3 inches. The generator busses C D E and the main distributing busses E2 E, C2 must have cross sections of 2 inches, and the bars d d d and d' d' d' and the secondary busses E, E, C, must have cross sections of I square inch.

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Figure 30.

Increasing radi. ating surface.

Radiating heat of current.

When the cross section of the bars is small they are made of a single bar, but when the cross section is large they are generally built up of several thin bars separated from each other as shown in Figs. 30 and 31, the latter being an enlarged section through a a of the former. This construction is resorted to for the purpose of reducing the cross section. When the current to be carried by a conductor is small the size of the bar required may be too small to give the necessary stiffness, and on this account the carrying capacity is estimated at the lowest limit, but when the current is large there is no lack of mechanical strength, and then as the size of the bars begins to seriously increase the cost of the structure, the carrying capacity is estimated at the highest limit. For example, if the current is, say, 500 amperes, the bars may be calculated on a basis of 600 amperes to the square inch of cross section, but if the current is 4,000 amperes the bars will be estimated on the basis of 1,000 amperes per square inch. In the latter case the increased current density will result in generating more heat, and in order that the temperature of the bars may not rise to such a degree as to materially increase the electrical resistance they are laminated so as to increase the radiating surface; hence the construction shown in Figs. 30 and 31.