Fig. 2,240.—Inside view of end ferrule of Noark enclosed fuse. Two prongs O and V, which are a part of the knife blade K, pass through the square holes in the ends of the ferrule R, and are riveted to the anchor plate T. The object of this plate is to stiffen the structure and to increase the current carrying capacity of the metal between the holes, also to permit of proper alignment of the plates. In each ferrule is placed a vent screen, composed of reticulate material, such as cheese cloth. The fuzz between the threads of the cheese cloth prevents the escape of the granular material through the vent holes A, but when the fuse operates, allows free egress of the air, thereby permitting the vapor formed upon the operation of the fusible element to quickly and freely pass through the interstices of the filling material and become cooled, eliminating any possibility of flame issuing from the ends of the tube.
The object of the powdered material is to assist in extinguishing the arc formed when the fuse blows.
Ques. What is a magnetic blow out fuse?
Ans. An enclosed fuse which is subject to the action of a magnetic field produced by the current, the magnetic field tending to blow out the arc when fusing occurs.
Ques. What is a quick break fuse?
Ans. One having a weight suspended from its center, or springs attached to its ends so that the arc formed at fusing is quickly attenuated and extinguished.
Ques. What is the disadvantage of a fuse as compared to an oil switch circuit breaker?
Ans. When a fuse blows, the arc causes oscillations in the line, which cause excessive rise of pressure under certain capacity conditions, whereas this disturbance is reduced to a minimum with an oil switch.