Figs. 468 to 470.—D & W, enclosed "cartridge" fuses. Fig. 468, type for 3 to 60 amperes; fig. 469, type for 61 to 100 amperes; fig. 470, type for 101 to 1,000 amperes.
Ques. Where should fuses be placed?
Ans. They should be inserted wherever the size of wire changes or wherever there is a branch of smaller size wire connected, unless the next fuse on the main or larger wire is small enough to protect the branch or small wire.
Figs. 471 to 478.—Interior construction of D. & W. fuses. In the manufacture of these fuses, four types of fuse link are used according to capacity of fuse, and classified as: 1, air drum link; 2, flat link; 3, multiple link; 4, cylinder link. In the air drum link, figs. 471 and 472, a capsule provides an air space about the center of the link, the rate of heat conduction through the confined air being very slow, the temperature of that portion of the link rises rapidly with increasing current, rendering the blowing point practically constant; fig. 473 shows a section through the complete fuse. In the flat link, fig. 474, the section is reduced in the center, cutting down as far as possible the volume of metal to be fused. Figs. 475 to 478 show various form of multiple link construction. By subdividing the metal, increased radiating surface is obtained which permits a reduction in the volume of fusible metal necessary, and the metal vapor formed when the fuse blows on heavy overload is more readily dissipated. Figs. 477 and 478 show two forms of the cylinder link, the plain cylinder fig. 477, being used for low voltage and large current, and fig. 478, for certain high tension service. The corrugated cylinder presents more surface to the fuse filling than the plain type and secures a maximum radiating surface with resulting minimum volume of metal for a given current.
Ques. How should fuses be mounted?
Ans. They should be placed on a base of slate, porcelain, marble, or other incombustible material.
Ques. What is the objection to copper fuses?
Ans. They heat perceptibly soon after their rated capacity is passed. The melting temperature is higher than lead alloy.
Ques. Upon what consideration does the choice between switches and circuit breakers depend?