Indicating or Striker Pin for Electric Fuses

Indicating pin fuses include a biased pin that moves in a direction longitudinally thereof in response to blowing of a fuse to indicate that the fuse has blown. Striker pin fuses include also a pin that moves in response to blowing of a fuse, but the object of a striker pin is to perform some mechanical work in response to blowing of a fuse, e.g. to unlatch the latch mechanism of a circuit breaker. Both indicating and striker pin fuses may have identical design features, except that striker pin fuses must generally have a stronger biasing force, e.g. a stronger spring, since their striker pin is supposed to perform work, while an indicating pin is not supposed to perform work.

Indicating and striker pin fuses include an elongated means, such as a pin, that is responsive to blowing of the fuse. Normally the indicating or striker pin is restrained, or held captive, by the restraining wire or fusible element of the fuse. The fusible element is normally embedded in a pulverulent arc-quenching filler and the point where initial fusion of the fusible element occurs is normally unknown, i.e. its location is unpredictable. As long as the fusible element is straight, or V-shaped, the fact is of no consequence that the point of initial break formation is unknown because under such conditions the force biasing the indicating or striker pin is sufficiently high to move an interrupted fusible element inside of a body of pulverulent arc-quenching filler. This is no longer possible where the fusible element exerts a high resistance against the drawing force, i.e. the force tending to displace it, e.g. where the fusible element is wound helically. It is, therefore, the principal object of the present invention to provide electric indicating and striker pin fuses that operate properly though their fusible element is precluded, e.g. by friction, from sliding inside the pulverulent arc-quenching filler. Another object of the invention is to adapt conventional indicating and striker pin fuses, such as disclosed, for instance, in U.S. Pat. No. 3,697,916; 10/10/72 to Richard A. Belcher et al for ELECTRIC LOW-VOLTAGE FUSE for application in fuses whose fusible element that controls the operation of the indicating or striker pin is wound helically, or precluded for other reasons from moving inside the pulverulent arc-quenching filler into which it is embedded.

The closest prior art known to me dealing with similar problems are U.S. Pat. No. 3,832,665; 08/27/74 to Richard A. Belcher for BLOWN FUSE INDICATOR FOR HIGH-VOLTAGE FUSES, and U.S. Pat. No. 4,001,749; 01/04/77 to Frederick J. Kozacka for ELECTRIC FUSE FOR ELEVATED CIRCUIT VOLTAGES.

Fuses according to the present invention include a first fuse terminal and electrically conductive elongated means responsive to blowing of a fuse movable in a direction longitudinally thereof. Said elongated means is in conductive relation with said first terminal. Said electrically conductive elongated means is biased by spring means in a direction longitudinally thereof. A partition of electric insulating material extends transversely to said elongated means and has an aperture for the passage of said elongated means, and said elongated means project through said passage. A second fuse terminal is arranged in spaced relation from said first fuse terminal. A fusible element is conductively connected with one end thereof to said second terminal and engages with the other end thereof said portion of said elongated means projecting through said passage. The point of engagement of said fusible element and said elongated means is the point of highest resistance of said fusible element so that fusion of said fusible element will occur at said point.

FIG. 1 is a vertical section of a fuse embodying the present invention taken along I--I of FIG. 2 and showing the part of an indicating or striker pin adjacent one of the terminals of an electric fuse;

FIG. 2 is a section along II--II of FIG. 1; and

FIG. 3 shows on a smaller scale than FIG. 1 the end of the indicating or striker pin mechanism adjacent the opposite terminal of the fuse.

Indicator or striker pin devices are being manufactured in two types. In one type the indicator or striker pin device is in itself a complete structure which is shunted across a fuse to indicate whether or not the fuse has blown. In the other type the indicator or striker pin device forms an integral part of a fuse structure whose state it is supposed to indicate. The drawing illustrates an indicator or striker pin device of the first mentioned type.

The structure shown in the drawing, and more particularly FIGS. 1 and 3, includes two plug type terminal elements 1,2 of which each plugs one of the ends of a casing 3 of electric insulating material. Casing 3 and terminal plugs 1,2 are tied together by fastener means, as for instance, roll pins, not shown in the drawing. Terminal element 1 is provided with a coaxial projection 1a forming an integral part of terminal element 1. Arranged inside a bore 1b inside part 1,1a and extending throughout its length is a metal tube 4 press-fitted into part 1a. The length of tube 4 slightly exceeds that of part 1a. Inside of tube 4 arranged in coaxial relation thereto is an electroconductive elongated means or rod 5 responsive to blowing of a fuse and movable in a direction longitudinally thereof. The upper end of elongated indicating or operating means 5 holds in position a cap 5a against which the upper end of helical spring 6 rests. Interposed between tube or tubular member 4, on the one hand, and cap 5a and spring 6, on the other hand, is a member 7 in part cylindrical and in part spherical that has a hole 7a at the apex of its spherical portion. Rod or indicating or striking member 5 extends through hole 7a into chamber 8 formed by tube 4 and closed on its lower end by a partition 9 of electric insulating material. Partition 9 is provided with an aperture 9a through which elongated part 5 projects. The lower end of elongated part 5 is flattened, projects through aperture 9a and is provided with a hole or opening 5b. The upper end of wire 10 is threaded through hole or opening 5b and bent to form a hook. This serves the dual purpose of maintaining the fusible element 10 in electrical contact with elongated pin means 5 and establishing the point of highest resistance of the current-path extending from terminal element 1 to terminal element 2. The lower end of wire 10 is anchored in terminal 2 by an anchor member 11 that is tubular and plugged by a plug member 12. Plug member 12 and anchor member 11 allow filling of fuse casing 3 with a pulverulent arc-quenching filler 13, e.g. quartz sand. Arc-quenching filler 13 must be prevented from entering into chamber 8 by way of aperture 9a. To this end chamber 8 is filled with a pasty substance. Silicone grease is particularly suitable for filling chamber 8 because relatively high temperatures may be encountered in electrical equipment of the kind under consideration. The pasty substance must preclude filler 13 from entering into chamber 8 but must allow movement of elongated member 5 therein in a direction longitudinally thereof.

The bore 1b below partition 9 is provided with a tubular sleeve 14 of electric insulating material that prevents shunting out of any portion of fusible element or wire 10 by engagement with the wall of bore 1b of terminal 1.

The current path in the device which has been described above is as follows: terminal 1; tubular metal member 4; part 7; spring 6; cap 5a; elongated member 5; opening 5b; wire 10, and terminal 2. For reasons of clarity the point of engagement of helical spring 6 and member 7 has not been shown. Since spring 6 expands, its lower end must necessarily engage part 7 and thus close the current path from terminal 1 to terminal 2. The point of engagement of fusible element or wire 10 and the portion of part 5 projecting through passage 9a of partition 9 is the point of highest resistance of fusible element 10. That point is the point at which wire 10 is threaded through hole 5b. Consequently fusion of element 10 will take place at this specific point of the current path through the fuse.