The invention relates to electric fuses and in particular to fuses to be used under marginal conditions both in respect of the steady state ope¬ rating current and of the operating voltage and in particular to fuses with an extremely fast action and current limiting operation. Such ope¬ rating conditions are generally- associated with fuses used for the pro¬ tection of semiconductor devices.

It is the object of the invention to indicate a design that permits to ex¬ tend these marginal conditions within given limitations in size and with¬ out increasing the power loss encountered.

The invention is generally limited to enclosed fuses with fuse conductors embedded in a porous filler, and where the fuse conductor consists of successive parts of different metals in the direction of current flow. Such bi-metallic fuses are known to the trade and used because they allow advantages of an economic nature without necessarily being associated with any inferior performance. However, it is also the object of the invention to achieve a significantly superior performance under marginal operating conditions without sacrificing the economic advantages known from prior art.

Prior art includes fuses where the current successively passes part of the fuse conductor made of copper, then a part made of silver, and finally again a part of copper, the parts so arranged that the fuse element, name¬ ly the part intended to melt and open the circuit, is situated entirely in the silver part, while the parts that primarily serve to conduct elec¬ tricity and, the heat created in the fuse element are made from the less expensive material copper. This results in a fuse that by and large be¬ haves like a fuse with the entire fuse conductor made of silver.

A typical example of such prior art and an example which is also the con¬ struction coming closest to the present invention as a forerunner is that disclosed in US patent 2,781,434. With a knowledge of the present inven¬ tion, the prior art so disclosed superficially resembles the present in¬ vention, but a closer study will reveal that this is not the case.

OMPI

For instance, said patent disclosure says (col. 1, line 24ff), '.... and which are nevertheless capable of limiting major fault currents at_ least to the same extent as comparable fuses having links consisting entirely of silver. ' .

Here the expression 'at least' might appear to be anticipating the pre¬ sent invention, but it becomes obvious from the rest of the disclosure that this 'at least' should ^* be ^' read in the sense of 'up to' because there is nowhere else in the description the slightest mention of any possible improvement in performance. Only the economic advantages are disclosed and claimed.

Moreover, it is cautiously mentioned that even i_f the arc should burn into the copper part, it will not be detrimental to the performance. For in¬ stance (col. 3, line 42ff), 'The silver strip 2 does not need to be as long as the required back-burning distance of the link * , and (line

47 ff), 'For reasons of economy the lenght of the silver strip 2 ought to be less .......'.

Thus reading claim 7 of the above identified prior art in connection with the citations from the disclosure, the conclusion is, that this claim is added to be sure to include the most economic design and not with any idea of a deliberate preference ^' for this version, mentioned at the very last, because it might involve any specific advantage of performance. This indicates that the effect achieved by a fuse according to the present in¬ vention is in fact an unexpected effect, that hitherto has remained un¬ noticed and therefore neither disclosed, nor claimed in any prior art.

In contrast to this it is disclosed and claimed that by following the pre¬ sent invention significant advantages can be realized.

By a design that impels the arc to burn predominantly between copper elec¬ trodes instead between silver electrodes superior performance is achieved.

The fact that the conditions preferred according to the invention pretty well coincides with the maximum savings in expensive silver is an inci¬ dental, but most welcome supplementary advantage.

OMPI

.- -

The facts of physics that make the invention work are associated with the desire for the highest possible arc voltage once the interrupting arc is established. Careful investigations using two sets of fuses, identical except for the metal used for the fuse conductor, have shown that in the case where the metal is copper, the arc voltage will reach a higher value, reach it earlier, and retain it longer that in the case where the metal in the fuse conductor is silver.

However, the obvious solution of just using copper in stead of silver is only possible in the cases where the only duty of the fuse is to inter¬ rupt high values of overcurrent. If the fuse is also required to carry- current of a value in the neighbourhood of its marginal current carrying capacity for a long time, the temperature at the fuse element would be so high that it would be subject to fast deterioration due to progressive oxidation resulting in unwanted operation of the fuse.

The invention explores prior art in the light of this hitherto unnoticed advantage of copper by locating the thermally highly stressed fuse element in the part made of silver, while at- the same time locating the part made of copper as close to the fuse element as compatible with an avoidance of the tendency to be subject to progressive oxidation.

Under such circumstances the arc will quickly burn back through the silver and come to the copper part, and the desired high and lasting arc voltage will be realized.

It is clear that the full advantage of the invention is only obtained when it is needed, namely in case of marginal values of the system volt¬ age, where it is decisive for a successful operation that the arc voltage is both high and sustained. However, it is those marginal cases that de¬ cide how high a current rating and how high a voltage rating it is pos¬ sible to assign to a fuse, and it is clearly demonstrated that a fuse according to the invention can be assigned significantly higher ratings than fuses of similar dimensions, but using conventional fuse conductors of silver.

For better understanding the invention reference may be had to the accom¬ panying drawings, wherein:

FIG. 1 is a plane view of a fuse conductor in one metal and according to prior art.

FIG. 2 is a plane view of a fuse conductor in two metals demonstra¬ ting the difference between prior art and the invention.

FIG. 3 is a plane view of a fuse -conductor for use at a higher value of voltage. - ^* -

FIG. 4 is a plane view of a fuse conductor according to the invention but for different working conditions.

FIG. 5 is a detail of FIG. 4.

The fuse conductor of FIG. 1 consists of a thin plate of silver intended to carry the current in the direction coinciding with its largest dimen¬ sion. Across this direction of current flow, fuse elements have been cre¬ ated by punching openings in the conductor. In FIG. 1 there are a number of holes in a row, namely four, thereby creating five instances of redu¬ ced cross-section forming five fuse elements at the locations indicated by (1), and this pattern of fuse elements is repeated five times along the lenght of the fuse, so that the fuse can be adapted to a voltage ra¬ ting five times that of a single fuse element.

It is also part of the prior art to let the series of openings across the fuse conductor begin with 'half openings, i.e. with the center of the opening coming essentially in the contour of the edge. In case of only two half openings we have the equivalent of a notched fuse conductor. The openings can.have any suitable shape besides the simple circular shape shown, and in stead of openings it is also known to use reductions of the thickness of the conductor to constitute the fuse element, - or to use any other practical way of providing reduced cross-section.

The fuse conductor shown in FIG. 2 can be conceived as simply according to prior art as disclosed in US patent 2,781,434, but depending on the di¬ mensions selected it can also be according to the present invention. For simplicity it only shows the fuse elements (1) formed by the punching of circular openings (2) in the part made of silver (3) located between the

outer parts of the fuse conductor made of copper (4), but special atten¬ tion should be directed to the location of the transitions (5) between silver and copper.

The critical dimension defining the invention is the distance between the center of the fuse elements (1) and the transitions (5), because this distance shall be long enough to ensure that the temperature of any part of copper does not exceed the.value of temperature that leads to harmful progressive oxidation, while on the other hand, the amount of silver lo¬ cated in between must burn away before the arc can be established between foot-points on copper.

The invention centers on the means available to achieve the desirable but hitherto unnoticed effect resulting from an early transition of the arc from silver to copper.

As the flow of electric current and the flow of heat in a metal to a great extent coincide, the object of design must be to achieve the reduc¬ tion of- cross-section in ^' as short a distance as practical. An important means to that end is to increase the number of openings in a given width of conductor, i.e. to have many small openings in stead of a few large ones, and to locate the fuse elements essentially midway in between the transition lines (5). Further improvements can be achieved by proper de¬ viations from the simple circular form in various ways generally known to the man skilled in the art.

The degree to which this critical distance can be diminished also depends on the operating temperature of the fuse element. The most unfavorable case is that of the temperature of the fuse element being almost the mel¬ ting point of silver.

However, operation that close to the melting point is not a normally ad¬ visable practice, and therefore it is more ^* usual to assign to the fuse a certain value of current, a rated current, which is a limiting value up to which limit the manufacturer of the fuse can assure proper operation such limitations allow the safe lowering of the length of current flow in the silver and thereby increases the benefit of the invention.

-£ ^j R£ c_,:?ι

The fuse conductor shown on FIG. 3 is made according to the invention. It shows the use of a higher number of openings with inter¬ vening fuse elements across the width of the fuse conductor and also the use of several bands of fuse elements over the length, making it possible to design the fuse for any desired value of voltage.

The fuse conductor shown on FIG. 4 is similar to that of FIG. 3 except that the length of silver is shown to be less than the diameter of the openings. A detail is shown enlarged on FIG. 5.

It should be mentioned that the shown use of two or more different metals in the fuse conductor does not relate to the use of. the so-called M-ef¬ fect, where one metal at a certain temperature forms an alloy with an¬ other metal, thereby altering the electrical and other physical proper¬ ties of the fuse conductor. However, there is no reason why this M-effect cannot be compatible with and used in combination with a fuse conductor according to the invention.

So far the invention has been explained only on the basis of prior art, namely the use of composite fuse conductors with the metals silver and copper, but similar advantages can .be had from other combinations of dif¬ ferent metals.

Of such other metals special mention should be made of aluminium. In con¬ trast to copper, aluminium will not be subject to harmful progressive oxidation because the first oxide film formed will be mechanically strong and impervious to oxygen and therefore protect against further oxidation. This implies that in case aluminium is used in place of silver and to¬ gether with copper as explained, the fuse element can assume a tempera¬ ture even above the melting point of aluminium, the molten aluminium being retained within the oxide film until it bursts. However, because of the lower temperature it can be used with less distance to the copper, and because aluminium at the temperature of the arc reacts with quartz, a frequently used component of the porous filler of the fuse, the burn- back velocity in aluminium will be higher than for silver. Both of these properties lead to a faster transfer of the arc foot-points to copper to realize the effect according to the invention.

OMPI

It is also foreseen thet for special applications it can be advantage¬ ous to use the combination of silver with aluminium in stead of copper, because the faster burn-back of aluminium will lead to a fast growth of arc voltage due to elongation, while the more precise operation of sil¬ ver in the opening of small overloads warrants the use of silver for the location of the fuse elements.

In general the invention opens" the possibility to exploit many more com¬ binations of different metals, each with its particular advantages in the two parts of the bi-metallic fuse conductor. Such other combinations will occur to the man skilled in the art when the desired special proper¬ ties of the fuse and its operating conditions are specified.