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Current that must be carried.
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 C., and also of the secondary busses E4, E, and Cz. 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 E2, 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
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
Figure 29. Method of clamping bus bars. safely reduce it to 3 inches. The generator busses C D E and the main distributing busses E. E, C, must have cross sections of 2 inches, and the bars d d d and d' d' d' and the secondary busses E. E; Cz must have cross sections of i square inch.
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 Figure 30. Increasing radis surface; hence the construction shown in ating surface. Figs. 30 and 31.
Laminated bus bars.
If Fig. 27 is closely examined it will be found that all the bars are supported by means of the connections made with each other and with the terminals of the switches and circuit breakers. Thus the generator busses C D E are held by the connections with the four main
generator switches and the connections with switch Sg. All the other busses and connecting bars are held by means of the connections with the distributing switches S', and also by the connection with the switch S., this latter attachment serving as a support for the center of the busses E2, E, and C2. The effort of the designer should be directed toward so disposing the switches and instruments with which electrical connections have to be made Insulation of switchboards.
that these connections will afford all the support the connecting bars require. There are very few instances in which a skillful designer cannot accomplish this result without in any way interfering with the proper location of the several devices. But when it becomes necessary to provide additional supports these must be of such a character as to insulate the bar from the switchboard, not only from the metallic parts thereof, but also from the slate or marble slabs which constitute the surface, although attachment to the marble slabs without insulation may be made if the voltage is not over 110 and the installation is of small capacity, say, under 100 horse-power. Marble and slate are generally good enough insulators to hold the circuit connections without further insulation, providing they are dry, but in damp weather their insulating qualities cannot be depended upon. On this account, for voltages over 500 the switch, circuit breaker and instrument terminals should be insulated from the board by means of sheet mica washers and tubes, as shown at. g k and j in Fig. 29.
In Fig. 29 the arrangement of the bus bars and other conductors on the back of the board is such that they can be securely held by means of the bolts that form the connections with the various switches, circuit breakers and other apparatus. There are very few cases in which it is not possible to obtain the necessary support in this way; when, however, this method will not give all the stiffness necessary to prevent the bars from coming in contact with each other by vibrations, or otherwise, they must be additionally supported by means of attachments with the marble slabs or the supporting frame of the latter.