Cycling resistant fusible element for electric fuses

A fusible element having an ability to withstand a large number of on-off cycles is provided. The element comprises a ribbon of copper without any M-effect causing means. The element is bent in a zig-zag shape to establish a plurality of contiguous planar sections enclosing obtuse angles with each other and forming straight edges of the loci of intersection of the planes defined by the planar sections. The straight edges are non-perforated to maximize their flexure strength. Each of the planar sections is provided with at least one point of reduced cross-section remote from the straight edges. The ribbon of copper is electro-plated with sulfamate of nickel forming a dull, ductile layer of nickel.

BACKGROUND OF THE INVENTION 
Up to now fusible elements which had to perform a high cycling duty were 
generally made of silver. The rising price of silver has raised the 
question whether any other less expensive metal than silver could be 
substituted for silver. Extensive experiments were carried out with 
fusible elements of copper and with fusible elements of aluminum. The 
results of these experiments were rather unsuccessful. Copper forms on 
account of its oxidation a brittle layer around the fusible element 
inconsistent with high cycling performance. 
Oxidation of copper occurs over a wide range of temperatures, beginning at 
room temperature and forming oxides that are not only brittle but also 
fissured, exposing also the underlying copper layer to oxidation. For this 
and other reasons, the cycling ability of prior art fusible elements of 
copper was extremely poor. Nor have fusible elements of aluminum been able 
to provide a satisfactory cycling performance. 
High cycling ability fusible elements in ribbon form were, therefore, made 
exclusively of silver. 
Prior art fusible elements designed to have a high cycling performance are 
disclosed, e.g. in U.S. Pat. No. 3,319,029 to P. C. Jacobs, Jr. for 
HIGH-VOLTAGE FUSE HAVING ZIG-ZAG SHAPED FUSE LINK; 5/9/67; U.S. Pat. No. 
3,394,333 to P. C. Jacobs, Jr. for ELECTRIC FUSE HAVING STRESS-REDUCING 
FUSE LINK MEANS; 6/23/68; U.S. Pat. No. 4,161,713 to P. C. Jacobs, Jr. 
for FUSIBLE ELEMENT FOR ELECTRIC FUSES HAVING A RELATIVELY HIGH VOLTAGE 
RATING AND A RELATIVELY HIGH CYCLING PERFORMANCE; 7/17/79; etc. All the 
fusible elements described in these patents were of silver. 
The present invention solves the problem of providing an inexpensive 
fusible element having a cycling-resist ability surpassing the cycling 
resist ability of any prior art fusible element. 
Another object of the invention is to provide fusible elements of copper 
that do not oxidize and have a high cycling ability. 
Other objects of this invention and advantages thereof will become more 
apparent as this specification proceeds. 
SUMMARY OF THE INVENTION 
A fusible cycling resistant element according to this invention includes a 
ribbon of copper without any M-effect causing means. The fusible element 
is bent in zig-zag shape to establish a plurality of contiguous planar 
sections enclosing obtuse angles with each other and forming straight 
edges at the loci of intersection of the planes defined by said sections. 
Said straight edges are non-perforated to maximize the flexural strength 
thereof, and said sections each have at least one point of reduced 
cross-section remote from said straight edges. Said ribbon of copper is 
electroplated with sulfamate of nickel forming a dull, ductile layer of 
nickel. The thickness of said layer is in the order of several ten 
thousandth parts of an inch., preferably 0.0002" to 0.00035". 
It has been found desirable in manufacturing such a fusible element that 
the process of bending the copper ribbon to zig-zag shape, or of crimping 
the copper ribbon, follows the electroplating step of the ribbon.

DESCRIPTION OF PREFERRED EMBODIMENT 
Reference numeral 1 has been applied to generally indicate a ribbon of 
sheet copper having a thickness in the order of e.g. about one tenth of an 
inch. The ribbon 1 is not provided with any M-effect causing means which, 
if present, would serve to reduce the temperature at which the ribbon 
would melt. This is necessary because an M-effect causing overlay fuses at 
temperatures which the fusible element of high cycling ability fuses 
should be allowed to reach, and because even partial fusion of such an 
overlay affects the time-current characteristic of the fuse. The ribbon 1 
is bent in zig-zag shape to establish a plurality of contiguous planar 
sections 2 enclosing obtuse angles .alpha. with each other and forming 
straight edges 3 at the loci of intersection of the planes defined by said 
sections. Edge 3 are non-perforated to maximize the flexual strength 
thereof. Sections 2 are provided with at least one point of reduced 
cross-section 5 remote from edges 3 formed as shown in FIG. 3 by two 
parallel current paths. Connector tabs 2a are provided on each end of 
fusible ribbon 1. Ribbon 1 is electroplated with sulfamate of nickel to 
prevent oxidation of the copper, providing a dull protective layer of high 
ductility. The thichness of the plating is in the order of ten thousandth 
parts of an inch, e.g. 0.0002 of an inch. 
In FIGS. 4 and 5 the same reference characters have been applied to 
indicate like parts as in FIGS. 1 to 3. Hence, FIGS. 4 and 5 call for a 
description only to the extent that parts in addition to those shown in 
FIGS. 1-3 have been shown therein. 
According to FIGS. 4 and 5 two fusible elements 1 are enclosed in a tubular 
housing 6. The ends of housing 6 are plugged by terminal plugs 7 from 
which blade contacts 8 project in opposite directions. Steel pins 9 
project through housing 6 into terminal plugs 7 to hold these two parts 
together. The axially inner end surface of plugs 7 are provided with 
groves 7a into which the ends of fusible elements 1 extend and wherein 
they are conductively connected by soft solder joints (not shown) to 
terminal plugs 7. Reference numeral 10 indicates a granular arc-quenching 
filler such as, e.g. quartz sand in which fusible elements 1 are embedded. 
The electroplating technology with sulfamate of nickel is well known in the 
electroplating art and, therefore, does not require any detailed 
description. Suffice it to state that nickel plating sulfamate baths are 
commercially available, and that nickel sulfamate has the chemical formula 
Ni (SO.sub.3 NH.sub.2).sub.2. 
The high cycling ability of fusible elements according to this invention is 
not soly attributable to their being electroplated with sulfamate of 
nickel. It is essential that the angle .alpha. between the sectors 2 be an 
obtuse angle because if that angle were an acute angle the cycling ability 
of the fusible elements would be greatly decreased by metal fatigue. It is 
also important that the edges 3 and the points of reduced cross-section 5 
be located on different parts of the fusible element so that the points of 
maximal stress which are the edges 3 are not weakened by the perforations 
4 by which the points of reduced cross-section 5 are established. 
It is further desirable, as mentioned above, to crimp the fusible element 
into zig-zag shape after it has been electroplated with sulfamate of 
nickel. The manufacture of such a fusible element hence includes the 
following sequential steps: Stamping perforations 4 into a planar ribbon 
of copper to establish a plurality of serially arranged points of reduced 
cross-section. Thereafter electroplating said ribbon with sulfamate of 
nickel to a thickness in the order of ten thousandth parts of an inch. 
Thereafter said strip of copper is bent between said points of reduced 
cross-section at obtuse angles to zig-zag shape. 
It is important to point out that the term electroplated with sulfamate of 
nickel, or in a sulfamate nickel plating bath, may have different meanings 
depending on whether it is used in the trade, or in a scientific 
publication. In the trade it means nickel electroplating produced in a 
sulfamate bath resulting in a dull appearance of the plated surface and a 
high ductility of the surface. It is in this sense that the above term is 
used in this context. 
It is known, however, to plating scientists that by varying the parameters 
of a standard sulfamate bath very different results from those ordinarily 
achieved with such a bath may be obtained. Thus it is possible to achieve 
with a nickel sulfamate solution extremely brittle rather than ductile 
overlays if the object to be plated is deposited in a nickel sulfamate 
solution at a high current density, i.e. a current density higher than 40 
Amps/dm.sup.2. It has also been reported that special processes in a 
sulfamate bath yielded bright plating. Such deviations from standard or 
established sulfamate bath procedures are not considered in this context. 
It is well known to use electroplated dull and ductile nickel layers for 
protection against oxidation. But this has not been done in any art akin 
to fuse technology, and under conditions not similar to those to which a 
fusible element in a fuse is subjected. This is apparent from what 
follows: 
Copper has a melting point which is much lower than the melting point of 
nickel and a vaporization point which is much lower than that of nickel. 
To be more specific, nickel has a melting point of 1450.degree. C., while 
copper has a melting point of only 1083.degree. C. Nickel has a 
vaporization point of 3075.degree. C. while copper has a vaporization 
point of only 2340.degree. C. Thus a solid outer envelope of dull ductile 
nickel is formed when the fuse blows which contains a liquid insert of 
copper. Due to its critical small wall thickness the outer envelope 
bursts, resulting in arc initiation without any significant time delay. 
Since what is contained within the envelope-forming nickel layer is, in 
essence, pure copper rather than various oxides thereof, the fusing 
i.sup.2. t of a composite fusible element according to this invention is 
relatively low. The formation of series breaks occurs before the i.sup.2. 
t value required for vaporization of the liquefied copper inside the outer 
nickel sheet occurs. 
It may be added that in carrying the invention into effect the difference 
in specific electric resistance of the outer nickel layer and the inner 
copper core cannot be disregarded. The specific resistance of pure nickel 
is in the order of 0.070 and that of pure copper in the order of 0.017. 
The nickel layer, in spite of its relatively small thickness and its 
relatively high specific electric resistance, forms a shunt of the copper 
core which affects the current-carrying capacity of the fusible element as 
a whole. This can, however, readily be compensated, and does not present a 
significant problem. 
Fusible elements accordings to this invention were tested to prove their 
superiority to other high cycling ability fusible elements and these tests 
confirmed the above claims in regard to their cycling performance.