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How they work.
this account switches of this construction are called double-break switches.
If the two sides of S in Fig 103, are not insulated from each other, the switch cannot be used as a doublepole. If the voltage of the current that passes through the two sides of a double-pole switch is low, the insulation need not be very heavy, but for an electro-motive
force greater than 110 volts the most substantial insulation must be provided. When a two-pole switch is used to connect a generator with the switchboard, the outgoing current passes through one of the blades S and the returning current passes through the other, and since it is much easier for the current to jump across from one side of the switch to the other than to make the journey through a
Carbon break switches.
weak point in the insulation. To properly insulate the sides of a multiple pole switches several types of construction are resorted to, one of which is shown in Fig.
Figure 118. 103. This consists in providing a block, which is made of hard rubber, fiber or hard wood, the former being used for the higher grade of switches and for higher volt
Sparks with heavy current.
ages. The sides S S are bent around at right angles at the ends and are secured to H by means of screws. Other constructions are shown in Figs. 105 to 110.
Figs. 108 and 109 show constructions for three-pole blades, and Fig. 110 is the type employed for switches of large capacity. When a strong current has to be transmitted through a switch, it is not sufficient to make the several parts of sufficient cross-section to carry the current without becoming overheated, but it is also necessary to provide a sufficient amount of contact surface, for if this is not provided the heat developed at the points of contact may be enough to absorb a considerable amount of energy and in extreme cases, may even result in burning out the switch, with more or less serious results. By providing three blades side by side for each pole, as in Fig. 110, the surface of contact is increased three-fold. The actual construction of large switches of this kind can be better understood from Fig. III, which is a switch made by La Roach & Co., of 3,000 amperes capacity.
When a switch is opened, through which a strong current is passing, a large spark is formed as the switch blades leave the contact, and the effect of this spark is to burn away the metal of both parts. This burning not only results in gradually consuming the working parts of the switch, but by roughening up the surface destroys the perfect contact. The trouble can be remedied by smoothing off the burned portions with a file, but this procedure, in the hands of a man who is not a good mechanic, soon spoils the switch by destroying the fit between the contact surfaces. The extent to which the parts are burned by the spark is dependent not only upon the strength of the current, but also upon the duration of the spark; therefore, if the time during which the Strain on insulation.
spark holds out can be reduced, its destructive effects can be reduced. To reduce the duration of the spark switches are made so as to move quickly after they break contact, and the way in which the result is accomplished can be understood from Figs. 112, 113 and 114. Switches of this kind are called quick-break or snap switches.
Fig. 115 shows a three-pole switch of the type shown in Fig. 114, the handle being raised to the point where the side bars are about to leave the contacts N. Quick action switches succeed in reducing the destructive effect of the spark upon the switch, but are not very desirable so far as the generator and motors in the circuit are concerned, owing to the fact that if the duration of the spark is reduced the strain upon the insulation of the generators and motors is increased. When a circuit is opened the current tends to keep on fiowing, and as the regular channel is disconnected, it tries to complete a circuit through some other path. If there are weak points in the insulation they will be pierced by the current in its effort to close the circuit. The action of an electric current in this respect can be compared to that of a current of water brought to a sudden stop. Now, in the case of the water current, if the flow is checked instantly the force exerted by the current becomes very great, but if the flow is stopped gradually the reverse is the case. With electric currents the result is exactly the same, if the current passes through wire coils wound upon a mechanism. If the current dies out slowly the kick, as it is commonly called, is slight; but an instantaneous stop may produce an enormous reaction, even if the current is small.
From this it will be seen that quick-action switches are objectionable because they increase the strain brought to bear upon the insulation when the current is inter