410 ELECTRICAL MACHINERY 



The plug fuse is designed to overcome this possibility; 

 it consists of a short string fuse mounted in a porcelain 

 plug fitted with a screw base like an incandescent lamp base. 

 The cover to the plug is made of mica so that it may be 

 seen whether or not the fuse has blown. The plug fuse is 

 illustrated in Fig. 267. 



Cartridge Fuses. It is not permitted by the National 

 Board of Fire Underwriters to use plug fuses for currents 

 greater than 30 amperes. For larger 

 sizes the cartridge fuse must be used. 

 This consists of a tube made of fiber, 

 filled with borax, infusorial earth, or 

 similar substances through the center 

 of which the fuse ribbon passes. The 



two ends of the paper tube are fitted 

 FIG. 267. A Plug . , , . 



Fuse, Such as is Wlth c PP er ends to which the ends of 



Used in the Dif- the fuse are soldered. Short copper 



ferent Circuits in blades are fastened to these copper caps 



House Wiring. m the larger sizes and these fit into 



copper clips on the fuse block. When 



such a fuse blows, the arc is confined and smothered by the 



substance with which the fiber tube is filled. 



In order to detect whether or not such a fuse is blown a 

 tell-tale is provided. This consists of a very small fuse, 

 soldered to the copper terminals so that it is in parallel 

 with the main fuse. This small fuse however, for a short 

 way passes on the outside of the paper tube, so that it can 

 be seen. When the main fuse blows of course the little 

 one immediately melts and so gives evidence of the blowing 

 of the main fuse. Such fuses are generally called N.E.C.S. 

 fuses, meaning that they are designed in accordance with 

 the National Electric Code Standard. 



For high tension circuits, say several thousand volts, 

 a special type of fuse is used, called an expulsion fuse. A 

 long thin fuse is fastened in a tubular container (of some 

 insulating material) open at one end. When the fuse blows 



