Ques. For what service are "snap" switches suitable?
Ans. They are used on circuits containing lamps in comparatively small groups, and other light duty service.
Ques. What is a quick break switch?
Ans. A form of switch in which the contact pieces are snapped apart by the action of the springs, as shown in [fig. 463], so as to make the duration of the arc as short as possible.
The current allowed in each branch circuit of an electric lighting system is limited by the insurance rules to a maximum of 660 watts equivalent to 12 lamps of 16 c.p. each at 110 volts. They are also employed to control lamps in groups in theatres and other places where many lamps are turned on or off at about the same time.
Fig. 467.—Spool of fuse wire. The wire is usually made of an alloy of tin and lead, such as half and half solder. Bismuth is frequently added to the alloy to lower the melting point For half and half solder the melting point is 370° Fahr. The quickness with which a fuse will melt after the current has reached the limit depends upon the specific heat and latent heat of the metal. The current required to "blow" a fuse increases somewhat with the age of the fuse owing to oxidation and molecular changes. Fuses are sometimes rated according to the number of amperes to be taken normally by the circuit they are to protect. Thus, a 10 ampere fuse is supposed to protect a circuit whose regular current should not exceed 10 amperes, and to blow if the current rise to say 12 amperes. The Underwriters' rule requires that the rating be about 80% of the maximum current it can carry indefinitely, thus allowing about 25% overload before the fuse melts. The fusing current varies considerably according to circumstances. The temperature of the surrounding air or other substances affects the melting current greatly, because the rate at which heat from the fuse will be transferred to the surroundings depends upon the difference of temperance between them and the fuse. Hence a fuse in a warm place will be melted by a smaller current than a similar fuse in a cold place. For a similar reason, a fuse in an enclosed place where there is little chance for the heat to be dissipated, will melt with a smaller current than the same in an open place. If the current increase gradually to that which would ordinarily melt the fuse, the high temperature makes the fuse wire oxidize rapidly; this sometimes makes a sort of tube of oxide which will not break even after the fuse wire inside has melted, and so the fuse carries more than its rated current. Open fuses are so unreliable that circuit breakers are preferable for large currents; when fuses are used, the enclosed type as shown in figs. 468 to 470, is usually the more desirable.
Fuses.—All circuits subject to abnormal increase of current which might overheat the system, should be protected by fuses which will melt and thus open the circuit. A fuse is simply a strip of fusible metal, often consisting of lead with a small percentage of tin, connected in series in the circuit.
Experiments have shown that for large fuses, a multiple fuse is more sensitive than a single one. A one hundred ampere fuse may be made by taking four wires of twenty-five amperes capacity. A fuse block may be overloaded, not because the metal of the terminals is not of sufficient cross section to carry the current, but because of insufficient area of, contact, or loose contact of fuse and wires; the overload thus caused results in heating and frequently melts the fuse.
