US 3838376 A
In an electric fuse a fuse element of metal surrounds at least partly another material which has its boiling point near the fusing point of the fuse element. In one embodiment the fuse element consists of aluminium and the surrounded material consists of cadmium.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Norhnlm Sept. 24, 1974 ELECTRIC FUSES  Inventor: Olav N0rhlm, l-lerlev, Denmark  References Cited  Assignee: Aktieselskabet Laur. Knudsen FOREIGN PATENTS OR APPLICATIONS Elektricitets Selskab, Copenhagen, 626,530 8/1961 Canada 327/160 Denmark Primary Examiner-J. D. Miller Flledi 2, 1973 Assistant Examiner-Fred E. Bell 21] APP] NO: 329 197 Attorney, Agent, or Firm-Watson, C016, Grindle &
Watson  Foreign Application Priority Data  ABSTRACT 1972 Denmark 497/72 In an electric fuse a fuse element of metal surrounds at least partly another material which has its boiling  111.8. Cl 337/296, 337/160, 337/290, point near the fusing point of the fuse element In one 51 1 Cl 337/292 embodiment the fuse element consists of aluminium E 5 z' gg and the surrounded material consists of cadmium.
111111111 l. 1 33 7 2951 29 4 Claims, 3 Drawing Figures ELECTRIC FUSES BACKGROUND OF THE INVENTION The present invention relates toan electric fusehaving at least one fuse element of metal which at least partly surrounds another material and which fuses as a result of the heat generated by the passage of current whereby the electrical circuit is broken. In particular the inventionrelates to such fuses where the fuse element consists of metal having a substantial affinity to oxygen.
In a normal sequence of events in an electric fuse the electrical circuit will be open practically at themoment when the fuse element reaches the fusing point because the surface tension of the molten metal or other forces breakthe metallic continuity.
However, fusing at a relatively low current value will take comparatively long time due to the inevitable cooling, and if the metal has a considerable affinity to oxygen, this will, in connection with the long time and hightemperature, result in a heavy oxidation. The oxide film thereby formed may then obstruct opening of the circuit since the current continues through the molten metal which is confined within the oxide film. This delays the breaking and the delay may be of such a magnitude that the breaking does not take place until themoment when the metal exceeds its boiling point and its vapour pressure bursts the oxide film.
Obviously this phenomenon is most likely with fuses wherein the fuse element is supported by sand but it may also occur in cases where the fuse element is disposed freely in the air.
Likewise it is understandable that the phenomenon is particularly pronounced in case of fuses where the fuse element consists of aluminium which has both a very strong affinity to oxygen and an oxidation product with a great mechanical strength, but as an example it is also both theoretically possible and has been experimentally proved for copper. It is to be expected to occur for all metals with the exception of the really precious metals, such as gold, silver, platinum, etc.
It has also been proved that said phenomenon occurs in case of metals coated with a layer of precious metal unless this coating is so tight that it is practically unimpervious to oxygen at the temperatures to be taken into consideration. On the other hand, however, and oxide film in itself may be practically unimpervious within a wide temperature range. This applies to aluminium, for example, which in the temperature range up to about 400C is sufficiently well protected so that it does not cause troubles in practice while in the case of copper, for example, there is no such lower limit within a temperature range of significance.
Thus, for a particular practical form of a fuse element of aluminium it was found that within the temperature range from about 450C up to the fusing point at 660C an oxide layer may develop which layer may cause inconvenience if the fuse element is in the said temperature range for a sufficiently long time. For the fuse element in question the critical time is of the order of one second to some seconds depending on the circumstances.
When the described phenomenon occurs it may cause inconvenience not only by giving rise to an undesired delay but also by the fact that at the moment when the circuit is opened and the arc is established both the fuse element and its surroundings are at much higher temperature. Thus, the fusing point of aluminium is about 660C while its boiling point is about 2,000C.
SUMMARY OF THEINVENTION The object of the present invention is to eliminate the above mentioned drawbacks in such a manner that the fuse element will open the electrical circuit very quickly after the fuse element has reached its fusing temperature.
According to the invention the said other material has its boiling point near the fusing point of the fuse element. Thus, when the fuse element has a cavity containing said other material, e.g., another metal, the latter will reach a sufficient vapour pressure to burst an oxide film, if any, on the fuse element at a temperature which is close to the fusing temperature of the fuse element, on principle slightly higher.
It is expedient when this invention is carried out that the other material disposed within the fuse element does not otherwise interfere with the function of the fuse. Therefore the surrounded material is preferably a material which will not react in a disturbing manner either physically or chemically with the fuse element at a temperature below the fusing temperature of the latter. Thus, at temperatures substantially below the fusing point of the fuse element it must not react chemically with the fuse element or form an alloy with it or migrate into it by diffusion. However, there is no hindrance to making use of such processes in the same fuse, such as it is generally known to do, if this is found desirable.
The invention should not be confused with the well known method generally known under the term metallurgical effect or M-effect, where a different metal in or on the fuse element or in close proximity thereto at a temperature lower than the fusing temperature of the fuse element causes such changes that the fusing is accelerated resulting in a substantial change of the electrical properties of the fuse.
Neither should the invention be confused with the likewise known methods where a material different from the fuse element at a temperature lower than the fusing temperature of the fuse element reacts chemically with the latter and thereby causes destruction thereof and consequently opening of the circuit.
Experiments have shown that in connection with a fuse element of aluminium the metal cadmium (Cd), or an alloy mainly containing cadmium may be used for practising the invention because no disturbing influence on the fuse element has been proved at temperatures below the boiling point of cadmium which is about 760C, i.e., about C above the fusing point of the fuse element but also more than l,300C below the boiling point of aluminium. Of course the fact that these two metals have been mentioned does not mean that the invention is limited to this combination in as much as the principle applies to many other combinations of materials.
In order to prevent the surrounded material from impairing the function of the fuse as such the surrounded material in a cross-section transverse to the current direction should not exceed one fourth of the total crosssectional area of the fuse element.
Experiments have also shown that in order to obtain the desired effect it is not necessary that the fuse element completely surrounds the said other material on all sides and according to an ambodiment of the invention the surrounded material may be placed in a passage completely or partly open at both ends, said passage running substantially transversely to the direction of the electric current. This embodiment may be technologically advantageous in many cases.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a part of a cylindrical fuse element according to one embodiment of the invention,
FIG. 2' shows a fuse element according to a preferred embodiment of the invention, and
FIG. 3 shows a practical construction of a fuse strip provided with the fuse element of FIG. 2.
DETAILED DESCRIPTION In FIG. 1 a cylindrical fuse element 1 surrounds another material 2 which is shown in dotted line.
In FIG. 2 a fuse element 1 of rectangular crosssection is shown but in this case the added material 2 is in the form of a filled passage running all through the fuse element so that the material 2 is not completely surrounded by the fuse element.
In FIG. 3 a practical constructions is shown in which the fuse element 1 is formed by providing notches 4 in a metal strip 3 in such a manner that the surrounded material 2 will be located in the fuse element 1 thereby formed. In the same strip further fuse elements, e.g., 1
formed by notches 4', may be provided which further fuse elements may be carried out in accordance with the invention or in another way.
'1. Electric fuse having at least one fuse element of metal which at least partly surrounds another material, characterized in that said other material has its boiling point near the fusing point of the fuse element and the surrounded material is a material which will not react in a disturbing manner either physically or chemically with the fuse element at a temperature below the fusing temperature of the latter.
2. Electric fuse according to claim 1, characterized in that the fuse element consists mainly of aluminium and the surrounded material is cadmium or an alloy consisting mainly of cadmium.
3. Electric fuse according to claim 1, characterized in that in a cross-section transverse to the current direction the surrounded material does not exceed one fourth of the total cross-sectional area of the fuse element.
4. Electric fuse according to claim 1, characterized in that the fuse element has a passage therein, which passage extends in a direction substantially transverse to the direction of the electric current and is at least partially open at both ends, and the surrounded material is placed within the passage.