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the marble slabs join. These angle iron bars run down to the floor and thus form the legs of the structure. If the board is so small that it can be made of a single slab
Another method of fastening. there will be only two angle bars, one at each side. At the bottom of the board the vertical angle bars are tied together by means of a horizontal bar which is sometimes
Use of angle bars.
secured in the way shown in Fig. 35, or more clearly in the enlarged detail, Fig. 36, and sometimes as shown in Fig. 37. If the construction of Fig. 36 is used the angle iron cannot be seen, except by its narrow edge, but with the construction of Fig. 37 the angle can be readily seen
from a short distance, as it projects its full width below the marble slab. With the latter construction the marble can be made to rest directly against the vertical angle bars A, but with the arrangement of Fig. 36 there will be a clear space equal to the thickness of the side of the Details of fastening.
angle iron, and unless the bar is rolled with sides of uniform thickness it will be necessary to chamfer away the edges of the slab to obtain a bearing anywhere except at the corner. The top of the switchboard frame is finished off with an angle bar run even with the top of the slab. If the construction of Fig. 36 is used the angle is located as in Fig. 35, that is, with one side between the vertical
Figure 37. Another detail of Figure 35
angle bars and the slab, and the other side resting on top of the verticals. With the construction of Fig. 37 the top angle is placed so as to rest on top of the slab, and the verticals are carried the width of the angle above the slab.
While angle iron is very generally used for the framing of switchboards, it is not universal. In some instances
Uprights of I-beams.
I-beams are used for the uprights, the arrangement being as shown in Fig. 38, which represents a plan in section through a beam located opposite a joint in the marble. At the side of the board the I-beam is placed flush with the edges. The use of I-beams cannot be said to be as desirable as the angle iron, because it is more expensive and not so substantial, so far as the slabs are concerned, owing to the fact that the bolt holes must be drilled near
to the edges, and, therefore, are more liable to break out.
The bottom of a switchboard should be of such height that the attendant can see the lower instruments or switches without inconvenience, and the top should be of such a height that the highest instruments can be reached from the floor. If space is limited the bottom of the board may not be over a foot and a half from the floor, but two feet should be allowed whenever possible. Height of instruments.
If the board cannot be made of such width as to permit of placing all the switches that have to be handled within easy reach of a man of average stature, then the board should be made in two stories, with a platform from which the device upon the upper part may be manipulated.
The space between the back of the board and the wall should be four feet or more, and if possible the location should be such that there is an abundance of daylight back of the board, so that connections may be made when necessary without the aid of artificial light. Artificial lights should also be provided so as to facilitate work after dark. If there are windows back of the board they should be so arranged that rain and snow cannot enter through them and reach the board, and in damp weather they should be closed tightly. A perfectly watertight ceiling should be constructed over the entire switchboard so as to obviate all danger of injury by water falling from overhead.
Switchboard fronts are made of marble and marbleized slate, both of which are suitable for the purpose, when free from seams and metallic veins. Marbleized slate is much superior to the plain stone, as the enamel makes it waterproof. Polished marble is also better than the rough stone, on account of the polish rendering the surface more impervious to moisture. When there is much dampness in the air there is a liability of more or less leakage in the switchboard if the voltage is high. This can be reduced by varnishing the back of the slabs after they have been well dried out by heat.
Fig. 39 shows the back of a switchboard arranged for two generators connected in the three-wire system. The