Possibility of damage.

might short circuit some of the other conducting bars and result in a serious loss.

Since the accidental contact of conducting bars that should be insulated from each other can result in the destruction of the generators, it follows that no construction is proper that is in any way uncertain, and on this account the arrangement of Fig. 34 cannot be looked upon as desirable. It costs less than the design Fig. 33, but in the long run may cost several hundred or possibly thousand times more.

The conductor bars of switchboards are made of copper of the purest kind. Copper castings should never be used unless the pieces are very short, owing to the fact that castings are not made of pure copper, and their resistance is from about one and a half to several times as great as that of the pure metal. The relation between the width and the thickness of the bars has to be determined by the amount of surface required where two bars are joined, and also by the thickness necessary to afford the proper stiffness. A straight bar is made in one piece, but where bends are required they are made by lapping one bar upon another at the proper angle. In some cases it may be found advantageous to heat the bars and bend them into the desired shape, but generally this procedure is not satisfactory, as there is considerable difficulty in handling these crooked pieces when they are entwined with each other on the back of the board. If the points between which the bars are supported are far apart the thickness must be increased to prevent vibration, but bars as thin as 1/4 inch may be as long as 4 or 5 feet between supporting points, and if bars that are to be maintained separate from each other are held in position by means of plates of

Current in "lapped" bars.

glass or porcelain, where they cross each other, the distance between supports can be nearly doubled. When glass or porcelain plates are placed between bars the plates should be of a thickness equal to the space between the bars, and the bars and plates should be securely held by suitable clamps.

When one bar is lapped on another the surfaces in contact should be at the rate of about square inch for each 150 amperes of current for small capacities, and for very strong currents the surface may be reduced to 1 inch per 200 amperes.* Two bars 2 inches wide lapped on each other at right angles will have a contact surface of 4 square inches and will conduct about 700 amperes; therefore, the thickness should be about 3 inch, which would give a cross section of 3/4 inch. The contact surfaces should be as true as they can be made without going to the expense of scraped surfaces, and the bars should be held together with iron bolts or screws, the latter being used for small bars. The number of screws should be not less than two in any case, otherwise the joints may twist around and become loose. It is better to use four small bolts than two large ones, as, with the increased number the pressure is more evenly distributed and the contact of the surfaces is made more complete. Iron bolts are better than brass or copper, not only because they are stronger, but also because they do not expand as much as the copper bars with increase in temperature. Therefore, the warmer the joint becomes the more perfect the contact, as the difference in expansion increases the pressure.

The supporting frame of a switchboard is a very simple structure, consisting of vertical angle irons placed at the sides of the board, and intermediate angles located where

By amalgamating the contacts on bus bars or switches with the Harold Brown process and using a thin layer of Plastic Alloy, the contact surfaces may be reduced to one square inch per 1000 amperes without danger of heating and a permanent joint of very low resistance obtained.

80

Switchboard construction.

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

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.

A

d

B

Figure 36. A detail of Figure 35.

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

A

C

d

B

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