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Holding conducting bars.
In Figs. 32 and 33 a design is shown for holding the conducting bars from the framing. The first one of these figures is an elevation, showing a section through the conductor H, which is in a horizontal position, and the second figure is a plan. In both these figures the conductor is marked H and C represents the marble slab. The conductor H must be insulated from the clamps B and F, and from the bolts g, so that there may be no electrical connection between it and C or A. Since the object of the insulation is to separate, electrically, H from C and its supporting frame A it follows that if B is insulated from the bolt h, and from A, the insulation between H and B need not be provided ; that is, insulation at one point between H and A is just as good as at another. The construction shown, however, is the simplest and strongest. Insulation between H and A is required because A is generally not insulated from the ground, and even when it is A can be grounded accidentally very easily; hence, if there were no insulation between H and A there would be great liability of grounding the circuit. In a case where B is supported from the marble slab, it might be supposed that the insulation of H would be necessary, and it would be so far as C is concerned if the voltage were low, but insulation between H and d would still be required, for the head of d is upon the face of the switchboard, and if not insulated from the circuit damage might be done by accidentally connecting it with some of the switches..
If the voltage is not over 110 the insulating blocks D and E can be made of marble or slate, and be used alone, but for higher voltages sheet mica should be placed between them and H. In every case it is advisable to place a layer of asbestos between the clamps B F, and the blocks
Not as good connection.
D, and also between these and H, so that the pressure of the bolts g g may be distributed over a greater surface and not be liable to break the insulators. For the same reason it is well to make the surfaces of D and E cut away slightly around the edges, so that the pressure may be exerted against the center portion.
In some instances the conducting bars are supported from the framing of the switchboard in the
manner shown in Fig. 34, but this construction, as can be readily seen, is not mechanical since H is dependent wholly upon the strength of the marble or slate slab B. The slab B is secured to the framing A by the bolts C, and H is secured to B by the bolts d. As marble and slate are both brittle, it is not safe to depend upon the slab B to hold H, for if the pressure of the bolts e or d should be so applied as to produce a strain the slab might break and allow H to fall out of place, and in so doing it. 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 1 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 38 inch, which would give a cross section of 34 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.