Method of avoiding arc prolongation on current interruption, particulary a fuse

#This invention relates to a fuse assembly that facilitates extinguishing of an arc discharge generated at the time when an overcurrent flows through a fuse element of the fuse assembly and the fuse element is then blown.

#As a fuse assembly, there is known a fuse assembly that comprises a hollow casing, a pair of spaced apart electrodes mounted to the casing, and a fuse link or fuse element disposed within the casing and bridged between the spaced apart electrodes. In such a fuse assembly, when an overcurrent, which ranges from several ten times to several hundred times larger than a rated value, flows through the fuse link, the fuse link is partially melted and then cut into two parts, and the current is resultantly cut off. At this time, an arc discharge is generated between the parts of the melted fuse link.

#As the two parts of the fuse element are melted by heat generated due to the arc discharge, a gap between the two parts of the melted fuse element becomes larger. As a result, the arc discharge will be spontaneously extinguished. Also, it is well known to persons skilled in the art that if an alternating current source is employed as a power supply for associated electrical equipment, the arc discharge tends to easily extinguish at the time when an AC voltage becomes zero.

#These and other objects and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which like reference numerals denote the same parts throughout the Figures and wherein:

#FIG. 1 is a graph that is of assistance in explaining a general behavior of an arc occurring immediately after a fuse element is blown due to an overcurrent flowing through the fuse element;

#FIG. 2 is a schematic sectional view of a tube having an inner concave surface provided at a first end thereof, which is of assistance in explaining general behaviors of shock waves incident upon the concave surface from a second end of the tube, reflected by the concave surface and converged into a focus;

#In order to facilitate understanding of the present invention, a general behavior of an arc discharge generated at the time when overcurrent flows through a fuse link of a fuse assembly and the fuse link is then melt or blown will be discussed hereinafter with reference to FIG. 1 that is disclosed in “Electric Fuses (1984), P. 38, A. Wright & P. G. Newbery, IEE, Power Engineering Series 2”. FIG. 1 shows that, during a pre-arcing period “a”, the current rises whereas the voltage across the fuse link is still low because the fuse link is not melted yet and sound. As shown in FIG. 1, at the end of the pre-arcing period “a”, the fuse link voltage suddenly rises. This is brought about by fusing of the fuse link and the conductance of an associated circuit. That is, it is conceivable that an arc discharge will occur at the end of the pre-arcing period “a”. Incidentally, it is known that metal of the fuse link is vaporized by only 40% of the metal at the end of the pre-arcing period “a”. When the occurrence of the arc discharge is continued from the end of the pre-arcing period to a ¼ cycle of AC where AC voltage approaches zero as shown in FIG. 1, the fuse assembly may be damaged. Therefore, it is strongly requested that the arc discharge should be terminated as soon as the pre-arcing period “a” has elapsed.

#Meanwhile, “Shock Wave (1998), Page 72, written by Kazuki Takayama and published by Ohmsha” describes that the arc discharge starting at the end of the period “a” brings about a sudden metal-evaporation of the fuse link metal at a substantially middle portion of an entire length of the fuse link, since after the period “a” has elapsed, the current goes stronger, the voltage goes higher, and arc heat generated due to the arc discharge is consequently high enough to melt the remainder of the fuse link explosively. As a result, pressure in a space of a fuse assembly casing in which the arc discharge is generated is increased, whereby shock waves are produced.

#In order to further facilitate the understanding of the present invention, a general behavior of the generated shock waves will be discussed hereinafter with reference to FIG. 2 that is disclosed in “Shock Wave Handbook (1998), Page 87, edited by Kazuki Takayama and published by Spuringer Verlag Tokyo”. FIG. 2 illustrates a cylindrical tube having a reflection wall provided at one end thereof. The reflection wall has an inner concave surface serving a reflecting mirror. FIG. 2 shows that shock waves which are incident from the other end of the cylindrical tube and travel toward the inner concave surface are reflected by the inner concave surface and then converged into a focus of the concave mirror. Incidentally, it is reported that, strictly speaking, the focus into which the shock waves are converged is not an optical focus but is an aerodynamic focus which is located closer to the concave mirror as compared with the optical focus. However, it is experimentally confirmed that the aerodynamic focus may be practically regarded as the optical focus.