« НазадПродовжити »
Different kinds of contacts.
rupted. To obviate the objection of the quick break and at the same time protect the switches from the destructive action of proionged sparks, the type of switch shown in Figs. 116, 117, 118 is used. In this switch two sets of contacts are provided, one being made of copper and the other of carbon blocks. The first set of contacts is for the purpose of conducting the current with the least resistance possible, and the second set is to receive the spark at the instant when the current breaks.
Switches of the type shown in Figs. 116, 117, 118 are made in many designs, but they are not used as extensively as formerly, except in connection with circuit breakers. For although they prevent the destructive action of the sparking upon the switch blades and contacts, they are rather bulky and more liable to get out of order than the plain type.
The stationary contacts N, and the blades S of switches should be made so as to fit well, and also so that when the blade is lowered it will not be caught upon the ends of the jaws. Some manufacturers provide against the latter difficulty by bending the ends of the jaws outwardly, as in Fig. 119, while others leave them straight as in Fig. 120, and bevel corners of the blade. The most important point about the jaws and of switches is to make them so that they will fit well. If the jaws are made long, they may apparently fit well when in reality they do not. To obtain a good fit, the part b to which the jaws are fasened should be of the same width as the blade S. The jaws should be made of sheet copper rolled hard so as to be springy. They should be bent at the upper edge of b, but only enough to slightly reduce the width of the opening at the top ends. With such construction, if S and b are of the same Examples of switches.
width, the jaws will stand parallel when the blade is in place, and there will be a bearing from edge to edge of S. If S is wider than b, as shown on Fig. 121, the effect will be as shown in Fig. 122 when the blade is in place; and if S is narrower, as shown in Fig. 123, the fit when
Fig. 128. Two Crouse-Hinds switches. the blade is in place will be as illustrated in Fig. 124. The thickness of the jaws should be sufficient to make them decidedly stiff, so that they may have to inward a very small amount to produce the necessary clamping pressure. Some switch jaws are made of the form illus
Oil break switches.
trated in Fig. 125, this construction being used under the erroneous impression that if there is sufficient elasticity, a bearing can be obtained over the entire width of the switch blade regardless whether the fit is good or bad. This notion is not correct; the best results are obtained with short jaws, but the perfect fit is a matter of accurate workmanship.
As the resistance of the contact surface of the switch jaws is much greater than that of the body of the conductors, an extra amount of heat is developed at these points, and for this reason it is desirable to have as much metal near the contacts as possible. One way of increasing the bulk of metal is shown in Fig. 126. It is very commonly used in switches of the side-throw type, in which case the thick side of the jaw rests upon the surface to which the switch is attached, and the spring is placed on top. Some of the commercial forms of knife switches are shown in Figs. 126 to 129.
For the high voltages used in alternating current circuits, it is necessary to devise means whereby the sparking produced when the switch is opened may be reduced to small magnitude, if this construction is not employed, then the contacts must be of such form that they will withstand heavy sparking without being seriously injured. To prevent the sparks from attaining a destructive magnitude, several expedients are resorted to, one of which is to place the contact point in a tank of oil. When switch is so arranged, the spark at the instant of opening the circuit is very small because the oil is a very high insulator, and being liquid, it immediately falls into the space left by the switch contacts, and thus, by interposing an insulator or very high resistance in the break between the contact points, the flow of the current is at
How they work.
once stopped. Even with an e. m. f. of two or three thousand volts, the spark produced when the contacts are separated under oil are very short, probably not more than a sixteenth of an inch. Such small sparks are necessarily of short duration, so that the burning effect upon the switch terminals is very slight.
Some switches are arranged so that the contacts are separated under oil. This construction is illustrated in the vertical elevation Fig. 130. The box which holds the oil, and within which the switch is placed, is made of iron. and varies in dimensions according to the capacity
Power operated switches.
of the switch. For a switch of about 25 amperes, and an
There is also another plan in use for reducing the size of the spark. In this design the switch contacts separate within a narrow passage, and on account of the restricted size of it the arc formed cannot be of very large dimensions. The switch is operated by pulling out the handle and the frame that holds the switch contacts is guided by the posts as is clearly shown in the figures.
A very similar switch is shown in switchboards, Figs. 75 and 76, shown in the latter figure in the form of a cross above the rheostats.
Switches intended for carrying very strong currents are in some instances arranged so as to be operated by compressed air. A switch of this type is used at Niagara Falls. The operation is the same, except that a compressed air cylinder controls the movement. A very high resistance is interposed between the first and second set of contacts, so that when the first break, the current is immediately cut down by the increased resistance in the