EP0283414B1 - Fuse with high density ceramic casing and method of fabrication of that fuse - Google Patents

Description

The present invention relates to a high power current limiting fuse comprising an electrically conductive fuse element tightly surrounded by an envelope made of a high density rigid material (non-porous), in particular ceramic. The invention also relates to a method of manufacturing such a fuse.

Generally, a fuse is an electrical device which conducts a current and which interrupts this same current when it exceeds a predetermined value, thereby protecting an electrical circuit against a current of too great intensity. Very high intensity fault currents are therefore interrupted well before they reach their maximum amplitude. A fuse therefore limits the energy that could develop in a faulty electrical circuit and damage it.

Conventional high power current limiting fuses usually consist of a fiberglass or ceramic insulating tube closed at each end by metal covers. These covers constitute terminals which allow the connection of the fuse in an electrical circuit to be protected. These conventional fuses also contain one or more electrically conductive fuse elements which are in the form of wires or tapes and which are respectively connected at their two ends to the two covers. The fusible elements are metallic and contain, for example, silver, copper, aluminum, etc. They are also surrounded by an arc extinguishing agent, generally quartz sand that has been compacted and filling the insulating tube.

When a fault current flows through a fusible element, the metal which composes it reaches its point of fusion in places determined by its geometry. There then occurs an electric arc of current cutoff whose resistance increases to a value sufficient to develop a higher voltage than that of the source. As this arc voltage has the opposite polarity to that of the source, the fault current then decreases to a zero value. The characteristics of this current reduction are intimately linked to the nature of the arc extinguishing agent.

Because of the low thermal conductivity of quartz sand, and the partial filling (about 70%) by quartz sand of the interior space of the insulating tube, it results in a low heat dissipation during the production of the arc electric, which increases the cut-off time of the current by the fuse and the energy developed inside it. During the production of the arc, the metal forming the fusible element is vaporized and creates a pressure, which forms an arc channel in the quartz sand larger than the initial dimensions of the fusible element. The arc voltage then increases more slowly, which has the effect of increasing the power cut-off time.

In order to improve the thermal conductivity and the mechanical rigidity of quartz sand, the document US-A-3,838,375 (FRIND et al) to which the document DDA109 472 corresponds, and the document US-A-4 003 129 (KOCH et al) propose to introduce an inorganic binder into the quartz sand while retaining a certain porosity. The results obtained by this means are superior to those of conventional fuses having undergone conventional sand compaction.

Furthermore, document CH-A-209,745 describes an electric fuse comprising contact parts and a body made of insulating material provided with at least one channel containing a fusible element. The two pieces in question are held together using screws or rivets. The cavity in which the fusible element is placed is, in practice, filled with silica sand introduced through an opening provided for this purpose.

An object of the present invention is to improve the operating characteristics of a current limiting fuse, and more precisely still of a high power current limiting fuse, by replacing the quartz sand including or not an organic binder by a envelope of rigid high density material, in particular ceramic, which tightly surrounds the fusible element and which has a high dielectric resistivity to high temperature of the electric arc and high resistance to pressure and high temperature shocks caused by the arc.

More specifically, the present invention relates to a current limiting fuse comprising a fusible element, an arc extinguishing agent tightly surrounding the fusible element, this arc extinguishing agent having a high dielectric resistivity at high temperature d '' an electric arc occurring during the melting of the element, and a pair of terminals connected together via the fuse element, said terminals allowing the connection of the fuse element in an electric circuit likely to undergo a overcurrent; this fuse is characterized in that the arc extinguishing agent consists of an envelope of non-porous rigid ceramic having a central cavity completely filled by the fusible element so as to leave no vacuum, said rigid non-porous ceramic having a high resistance to internal pressure and high temperature shocks caused by said electric arc.

Preferably, the rigid non-porous ceramic constituting the envelope is a high density ceramic such as alumina, Al₂O₃, and Beryllium oxide, BeO, which also has high thermal conductivity and high specific heat which allow it quickly absorb the heat produced inside the enclosure by the electric arc.

As will be explained in more detail below, ceramics having a high mechanical resistance as well as a high resistance to the high temperature of the electric arc favor a more rapid rise of the arc voltage in comparison with the fuses of the prior art, which causes a much faster interruption of the fault current.

According to the present invention, there is also provided a method of manufacturing a current limiting fuse, as defined above, of the type comprising the insertion of a fuse element capable of conducting an electric current in an extinguishing agent d 'arc having a high dielectric resistivity at the high temperature of an electric arc occurring during the melting of said element, said insertion being carried out so that said arc extinguishing agent tightly surrounds said fusible element, and mounting subsequent of a pair of terminals connected together via the fuse element, said terminals allowing the connection of the fuse element in an electric circuit liable to undergo an overcurrent, method characterized in that one use a envelope made of a rigid non-porous ceramic as an arc extinguishing agent, said ceramic defining a cavity of the same shape and dimensions as the fusible element, so as to be completely filled without any vacuum by said fusible element during its insertion, said ceramic having a high resistance to internal pressure and high temperature shocks caused by said electric arc.

Preferably, the step of mounting the pair of terminals on the envelope includes metallization of this envelope at the two ends.

According to a preferred embodiment of the invention, the stage of production of the envelope comprises the production of two complementary parts made of rigid high density material and each having a surface of contact with the other of said two parts, the contact surface of one of these two complementary parts comprising a groove of the same shape and dimensions as the fusible element which is of elongated shape, and the step of inserting the fusible element consists of inserting this element inside the groove and assembling the two complementary parts by joining their contact surfaces.

According to another aspect of the present invention, a method of manufacturing a current limiting fuse comprises a step of producing an envelope made of a rigid non-porous ceramic which defines a cavity, this material having a high dielectric resistivity to high temperature as well as high resistance to internal pressure and high temperature shocks. This method of manufacturing a current limiting fuse further comprises steps of injecting a molten metal inside said envelope cavity to form a fuse element capable of conducting an electric current and designed to melt and thus interrupt said electric current when the latter reaches a given value, and mounting on the envelope of a pair of terminals connected together via the fuse element, and therefore allowing the connection of this element in an electrical circuit liable to undergo an overcurrent.

According to a preferred embodiment of this last method of manufacturing a current limiting fuse, the stage of production of the casing comprises the use of pieces of metal with high melting point to form the cavity of the casing .

A fiberglass or ceramic sheath can surround the envelope of the fuse according to the invention, thereby increasing the rigidity of the resulting fuse.

• La Figure 1 représente une coupe longitudinale d'un fusible selon l'invention, comportant une enveloppe en céramique rigide haute densité qui entoure de façon serrée un élément fusible;
• La Figure 2 a) représente l'état physique du fusible de la Figure 1, avant fusion de l'élément fusible;
• La Figure 2 b) représente l'état physique du fusible de la Figure 1, après fusion de l'élément fusible;
• La Figure 3 présente un oscillogramme typique du fonctionnement du fusible selon l'invention lors d'une interruption de courant;
• Les Figures 4, 5 a) et 5 b) sont des courbes qui démontrent les avantages du fusible selon l'invention par rapport aux fusibles de l'art antérieur;
• La Figure 6 illustre un premier mode de fabrication de l'enveloppe de céramique du fusible selon l'invention;
• La Figure 7 illustre un second mode de fabrication de l'enveloppe de céramique du fusible selon l'invention;
• Les Figures 8 a) et 8 b) illustrent un troisième mode de fabrication de l'enveloppe de céramique du fusible selon la présente invention; et
• Les Figures 9 et 10 illustrent des modes de fabrication du fusible selon l'invention, dans lesquels l'élément fusible est obtenu par injection de métal en fusion dans une cavité formée à l'intérieur de l'enveloppe de céramique.

The advantages and other characteristics of the present invention will emerge from the following non-limiting description of preferred embodiments thereof, given without limitation only with reference to the appended drawings in which:

• Figure 1 shows a longitudinal section a fuse according to the invention, comprising a rigid high density ceramic envelope which tightly surrounds a fusible element;
• Figure 2 a) shows the physical state of the fuse of Figure 1, before blowing of the fuse element;
• Figure 2 b) shows the physical state of the fuse of Figure 1, after blowing of the fuse element;
• FIG. 3 shows a typical oscillogram of the operation of the fuse according to the invention during a current interruption;
• Figures 4, 5 a) and 5 b) are curves which demonstrate the advantages of the fuse according to the invention compared to fuses of the prior art;
• Figure 6 illustrates a first method of manufacturing the ceramic envelope of the fuse according to the invention;
• Figure 7 illustrates a second method of manufacturing the ceramic envelope of the fuse according to the invention;
• Figures 8 a) and 8 b) illustrate a third method of manufacturing the ceramic envelope of the fuse according to the present invention; and
• Figures 9 and 10 illustrate methods of manufacturing the fuse according to the invention, in which the fusible element is obtained by injecting molten metal into a cavity formed inside the ceramic shell.

The preferably high power current limiting fuse F according to the present invention, as shown in longitudinal section in FIG. 1 of the drawings, comprises a metallic fuse element 1 in the form of a ribbon. The fusible element 1 comprises at least one reduction in width 2 (three of these width reductions being illustrated in FIG. 1 for example purposes) where an electric arc occurs when the fusible element blows at this location . Obviously, the parts of the element 1 reduced in width are the first susceptible to fusion. Because of their reduced cross-section, they heat up faster when subjected to an electric current.

The number of reductions in width of the ribbon forming the element 1, where electric arcs occur when the element 1 is fused at these locations, can vary and is conventionally selected according to the needs of the intended application. It is also well known to replace these width reductions with perforations made through the metal strip constituting the element 1.

The explanations below relate to a single electric arc for power failure. It is however obvious that these apply to each electric arc when the fusible element in the form of ribbon comprises several reductions in width or even several perforations.

• a) une très grande résistance aux chocs de pression interne;
• b) une très grande résistance aux chocs de température élevée;
• c) une grande résistivité diélectrique aux températures élevées; et
• d) une conductivité thermique élevée et une grande chaleur spécifique

The fusible element 1 is tightly surrounded by a rigid high density (non-porous) ceramic envelope 3 acting as an arc extinguishing agent. This rigid ceramic can be alumina with the chemical formula Al₂O₃, or beryllium oxide with the chemical formula BeO, which give excellent results. The rigid ceramic used in the manufacture of the envelope 3 may however be of various other compositions, provided that it is non-porous and has in particular the following qualities:

• a) a very high resistance to internal pressure shocks;
• b) a very high resistance to high temperature shocks;
• c) high dielectric resistivity at high temperatures; and
• d) high thermal conductivity and high specific heat

The ceramic envelope 3 must have sufficient dimensions to allow it to withstand the shocks of internal pressure and high temperature caused by the creation of the electric arc at the interruption of the current, without cracking or explosion, thus ensuring a great seal. . However, it can be of reduced size and reinforced by a cylindrical sheath 4 of ceramic or even of fiberglass.

The two ends of the casing 3 of the fuse F are metallized as indicated by the references 5 and 6. These metallizations are carried out conventionally directly on the ceramic. This results in two electrical terminals 5 and 6 which make it possible to connect the fuse F, more particularly its fuse element 1, in an electrical circuit to be protected liable to undergo an overcurrent. Obviously, during metallizations, the applied metal comes into contact with the ends of the fusible element 1, thereby connecting it between the terminals 5 and 6.

Figure 2 a) illustrates the physical state of the fuse F, before the fuse element 1 blows, that is to say in conduction. The fusible element 1 is then tightly surrounded by the ceramic envelope 3.

During the melting of the fusible element 1, the very high temperature of the electric arc of cut-off very quickly vaporizes the element 1 and forms at the place where the arc occurs (reduction in width of the metallic strip ) a pressure which must be maintained by the great tightness of the ceramic envelope 3. This pressure promotes a very rapid rise in the arc voltage, and when this reaches a value higher than the voltage of the source, an opposite current very quickly reduces the fault current to zero. The condensation of metallic vapors in the form of droplets on the ceramic walls provides good electrical insulation between terminals 5 and 6 of the fuse, i.e. between the terminals created by the ends of the fuse element 1 on each side of its melted and vaporized part.

Alumina, Al₂O₃, and Beryllium oxide, BeO, are ceramics which are particularly well suited for entering the manufacture of fuse F according to the invention. Indeed, these are capable of maintaining the pressure produced by the electric arc for a duration of less than 200 microseconds, which makes it possible to reach the peak value of the arc voltage. In the following milliseconds, the surfaces of these ceramics in contact with the electric arc are subjected to a high temperature as well as to a still high pressure and a slight part of these is brought to the melting point. A cavity slightly larger than the dimensions of the fuse element is thus formed under the combined effect of pressure and temperature, which promotes the decomposition of the gases produced and offers a greater dielectric distance between the terminals of the fuse created by the melting of element 1. The condensation of metallic vapors on the walls of these ceramics takes place, as already mentioned, in multiple metal droplets separated from each other by a distance which allows excellent dielectric strength when the arc goes out. The high dielectric resistivity of these ceramics at the high arc temperature also contributes to the rapid dielectric recovery of fuse F. In addition, their high thermal conductivity and their high specific heat allow these ceramics to rapidly absorb the heat produced by the ceramic. electric arc to thereby decrease the internal temperature of the fuse and help to reduce the power failure time.

Figure 2 b) shows the physical state of the fuse F after the element 1 has blown. The cavity formed at the location of the melted part of element 1 is relatively small, which made it possible to maintain the pressure at the melting point of element 1.

Figure 3 shows a typical oscillogram of the operation of a fuse F according to the invention. This oscillogram illustrates the very rapid rise of the arc voltage V following the melting of the fuse element 1. The instant at which this melting occurs is indicated by line B in Figure 3. The oscillogram further illustrates the very rapid breaking of the fault current I, the maximum value of which is represented by line A, which ceases to increase when the arc voltage V is at least equal to the source voltage S. The oscillogram of Figure 3 therefore demonstrates that the capacity of rigid high density ceramic to withstand the pressure and high temperature shocks, which allows the casing 3 to maintain the pressure at the point of creation of the arc at the cut of the current, ensures a rise very rapid arc voltage V compared to fuses of the prior art, from which comes the efficiency of interruption of the fault current I and, as will be explained in more detail below, a reduction of the joule integral (i integral as a function of the time of the term I²t) of the fuse F.

As also illustrated in Figure 3, the difference between the maximum value of the current represented by line A and that of the cut current at the time of the start of the arc represented by line B is less than 1%. When the fault current ceases to increase and reverses, the arc voltage V also ceases to increase. Consequently, with regard to the fuse F according to the invention, the rapid rise of the arc voltage V does not only mean a very rapid limitation of the fault current I, but also a limitation of the peak value of the voltage developed arc. Tests have shown that this overvoltage is greatly reduced compared to the fast fuses of the prior art using for example quartz sand with or without inorganic binder.

Figure 4 is a comparative series of curves which illustrate the operation of the fuse according to the invention compared to those of the prior art which use agglomerated or unglazed quartz sand as an arc extinguishing agent. It should be noted that the three fuses include substantially identical fuse elements.

In Figure 4, curve C illustrates the slope of a presumed fault current, applied at time t₀ to the various fuses. Curve C in fact represents a short-circuit current and its evolution as a function of time, if it were not interrupted. The fuse element of each fuse blows at the same time t₁.

Curve D in FIG. 4 illustrates the evolution of the current in a traditional fuse using quartz sand which has been compacted but not agglomerated as an arc extinguishing agent. This curve D shows that with such fuses, the fault current gradually increases after the fuse blows, then decreases slowly to reach a value of zero at time t₂ This phenomenon is due, as demonstrated by the curve E, at the rather slow increase in the arc voltage in such a fuse and also at the relatively low peak value of this arc voltage.

Curve R illustrates the evolution of the fault current as a function of the time obtained with the fuse described in United States patent no. 3,838,375 (FRIND and AL). This curve R clearly demonstrates that a fuse which uses as an arc extinguishing agent a quartz sand agglomerated using an inorganic binder provides better protection against overcurrents than a fuse using non-quartz sand agglomerated. Since the energy transmitted to the protected circuit corresponds to the integral as a function of time from t₀ to t₂ of the term I²t (joule integral), I representing as already mentioned the fault current, it It is therefore obvious that the fuse of patent no. 3,838,375 (FRIND and AL) considerably reduces the energy transmitted to the protected circuit, in comparison with those using non-agglomerated quartz sand as an arc extinguishing agent. This results from the much faster increase in the arc voltage of the fuse according to US patent no. 3,838,375 and the higher peak value of this voltage (see curve G in Figure 4). There is therefore an immediate and gradual reduction of the current through the fuse element, and this until the current is interrupted at time t₂.

The evolution of the fault current as a function of time in a fuse according to the invention is represented by the curve S in FIG. 4. The curve S clearly demonstrates the fundamental superiority of the fuse F according to the present invention. This improvement is provided by the high density rigid ceramic envelope for the various reasons explained above, and this without excessive increase in the arc voltage V (see Figure 3). Therefore, the appreciable reduction in the joule integral and the small increase in arc voltage have undeniable advantages of the fuse F.

In Figures 5 a) and 5 b), two fuses are compared, one using non-agglomerated quartz sand as arc extinguishing agent (corresponding to the curve on the left) and the other using a high rigid ceramic shell. density in accordance with the present invention (corresponding to the curve on the right).

In Figure 5 a) of the drawings, the curves H and I represent the evolution of the current in a fuse with non-agglomerated quartz sand, and in a fuse according to the invention, respectively, the two fuses having substantially identical fuse elements and the vertical lines 9 and 10 indicate the moment of fusion of the element fuse for both fuse models. The hatched part of the curve I shows the reduction of the total joule integral in the fuse F according to the invention.

In the case where the reduction of the joule integral is of secondary importance, the mass of the metallic fusible element 1 can be increased to delay the fusion. This procedure increases the maximum value of the cut current and thus increases the joule integral. In FIG. 5 b), the evolution of the current as a function of time is presented in two types of fuses, that according to the invention (curve K) and a conventional fuse with non-agglomerated quartz sand (curve J). The mass of the fuse element 1 of the fuse F according to the invention (curve K) has been increased compared to that of the fuse element of the conventional fuse so that the two fuses have identical total joule integrals. The fuse F according to the invention, (curve K) then offers a pre-arc joule integral of two to three times larger than that of the conventional fuse (curve J), which has an important advantage since the increase in the integral of total joule is zero. Note that in Figure 5 b), the vertical lines 11 and 12 indicate the time of melting of the fuse elements of conventional fuses and according to the invention, respectively.

It should be mentioned here that when determining the mass of the fusible element to obtain a certain integral of the total joule, account must be taken of the high thermal conductivity and the high specific heat of the high density rigid ceramic used. Indeed, the fuse element 1 being placed in contact with the ceramic, this ensures a lower temperature of the fuse element 1 in continuous service (conduction). When a fault current occurs, the melting of element 1 is delayed thanks to the large ceramic mass of the envelope 3 which absorbs and diffuses the heat.

Obtaining a larger pre-arc joule integral while retaining a low post-arc joule integral (Figure 5 b)) is sought after for certain applications and therefore has a definite advantage. Such an increase in the pre-arc joule integral makes it possible in particular to more effectively protect the circuits of motors and transformers without inadvertent operation of the fuse during switching on.

The fuse F according to the invention has another advantageous property, namely to be able to protect DC circuits. In fact, tests have confirmed that the breaking efficiency of a direct current of the fuse F is higher than that of the fuses of the prior art. An application of the fuse according to the invention for the protection of large power capacitor banks would therefore be possible. Another application of fuse F would be the protection of semiconductor circuits, thanks to its low Joule integral and its low arc overvoltage.

Another advantage of fuse F according to the invention is its high resistance to mechanical shock. It is well known that the resistance to mechanical shock of conventional high power fuses depends on the compacting of quartz sand or other non-agglomerated granular material surrounding the fusible element. Repeated mechanical shocks can indeed damage the fusible element (s), especially in conventional low-caliber fuses. As in fuse F according to the invention, all the parts form a rigid and compact mass, the rupture of the thin fusible elements is therefore avoided.

The manufacture of high density ceramic envelopes such as alumina, Al₂O₃, and Beryllium oxide, BeO, requires high pressure and temperature, more 1100 ° C. It is therefore impossible to directly insert the metal fusible element 1 during this manufacture, because of its relatively low melting temperature.

Consequently, ceramic pieces are rather formed beforehand with a space provided for receiving the fusible element 1, these pieces then being cemented together then subjected to firing at reduced temperature to form the envelope 3.

A first method of manufacturing the envelope 3 is illustrated in Figure 6 of the drawings. Two elongated complementary pieces 13 and 14 of high density rigid ceramic with a cross section in the shape of a half moon are first produced. A longitudinal groove 14ʹ is formed in the flat surface of the part 14, this groove matching the shape of the fusible element 1. When the element 1 has been inserted in the groove 14ʹ, the flat surfaces of the parts 13 and 14 are joined using an inorganic ceramic cement. The parts 13 and 14 thus joined together are then subjected to mechanical pressure in order to tighten them well one against the other, then baked in an oven at a temperature lower than the melting point of the metallic element 1. This results in a rigid and tight cylindrical envelope.

FIG. 7 illustrates a second method of manufacturing the ceramic envelope 3. A cylindrical rod 15 as well as a tube 16, both made of high density rigid ceramic such as alumina, Al₂O₃, and Beryllium oxide, BeO, are produced beforehand, the rod 15 comprising a longitudinal groove 15ʹ. The groove 15ʹ once again follows the shape of the element 1. When the metal element 1 has been inserted into the groove 15ʹ, the rod 15 -element 1 assembly is then slid inside the tube 16, such that 'indicated by arrow 49. The difference between the diameter internal of the tube 16 and the external diameter of the rod 15 leaves only a space between these rod and tube intended to be filled with an inorganic cement for appropriate ceramic. The assembly is then placed in an oven for cooking at a temperature lower than the melting point of the fusible element, in order to form a cylindrical envelope of waterproof and very rigid ceramic.

Another method of manufacturing the casing 3 of the fuse F according to the invention is illustrated in Figures 8 a) and 8 b) of the drawings. In this case, a tube 17 and several cylindrical elements 18 of short length are produced beforehand in a high density rigid ceramic. A groove having a longitudinal portion and a transverse portion is formed on the side and at one end of each cylindrical element 18. Again, the groove of each cylindrical element 18 follows the shape of the fusible element 1. The envelope of Figure 8 ceramic has the advantage of being able to separate two successive reductions in cross section 2 of the fusible element 1 by at least one cylindrical element 18 when these reductions in section are positioned in the geometric axis of the cylindrical envelope as as illustrated in Figure 8 b). In this way, the electric arcs occurring in the fuse F when the element 1 blows at the locations of these section reductions are separated from each other by at least one of the cylindrical elements 18. The cylindrical elements 18 are inserted end to end in the tube 17 with the fusible element 1 and connected together and to the tube 17 using an appropriate inorganic cement. The assembly is then subjected to firing at a temperature lower than the melting point of the metallic element 1 to form a rigid and tight cylindrical envelope.

Figure 9 illustrates two complementary parts 19 and 20 which, when assembled, form a cylindrical rod high density rigid ceramic. This rod is then inserted inside a cylinder 22 formed inside a cylindrical piece 21 also of high density rigid ceramic.

When assembled, the parts 19 and 20 define a cavity 28. Molten metal 23 can be injected into the cavity 28 to form the fusible element. Centrifugal force can be used to ensure that the molten metal completely fills the cavity, without leaving any voids. In Figure 9, the fusible element is in the form of a ribbon comprising several circular perforations.

The parts 19, 20 and 21 are combined using an inorganic cement, then heat treated to form a tight and rigid envelope. Parts 19 and 20 are joined using inorganic cement before metal injection. The assembly of the parts 19 and 20 thus joined with the part 21 and any heat treatment of these parts can take place either before or after the injection of metal. If the heat treatment takes place after the injection of metal, it must be remembered that this must be carried out at a temperature below the melting point of the metal of the fusible element.

The cylindrical part 21 has three cylinders such as 22 to receive three rods such as 19, 20, thereby forming a fuse with three identical fuse elements.

Metals with a high melting point such as tungsten can be used in the manufacture of the high density rigid envelope 3 to form the cavity necessary for the insertion of the fusible element 1. A tungsten ribbon or wire of the same shape that the fusible element is inserted into the ceramic during its manufacture. After forming and sintering the ceramic at high temperature and pressure, the tungsten tape or wire is removed and metal molten is injected into the cavity thus formed to form a fusible element.

10 illustrates the use of several tungsten wires to form several parallel filiform cavities of uniform cross section such as 29 inside a rigid high density ceramic rod 25. After the tungsten wires have been removed, molten metal 24 is injected into each cavity 29, to form a fusible element. Of course, the diameter of each cavity 29 is chosen according to the required characteristics of the fuse. Again, centrifugal force can be used to prevent the formation of any vacuum in the cavity when injecting the molten metal. The rod 25 can optionally be inserted into a cylinder 27 formed in a cylindrical piece of high density rigid ceramic 26, and connected to the latter with an inorganic cement either before or after the injection of metal. Again, the rod 25 and the cylindrical part 26 thus joined together are subjected to a heat treatment to form a rigid and sealed envelope, before or after the injection of molten metal.

As in the case of FIG. 9, the cylindrical part 26 is provided with three cylinders such as 27 to receive three rods such as 25 each containing several fusible elements.

According to the above teaching, it becomes easy to conceive that the fusible element 1 of the embodiments presented in Figures 6 and 7 can be manufactured by injection of molten metal.

Once the manufacture of the high density rigid ceramic envelope has been completed, this tightly surrounding the fusible element or elements, the two ends of this envelope are metallized to form two terminals (for example terminals 5 and 6 of the Figure 1) respectively connected at both ends of the fuse element or elements.

Thereafter, a cylindrical sheath such as 4 (Figure 1) can be placed on the ceramic shell. This sheath is made of ceramic or fiberglass and has the function of increasing the mechanical rigidity of the fuse F.

Claims (34)

1. A current-limiting fuse (F) comprising a fusible element (1), an arc-quenching agent closely surrounding the fusible element (1), said arc-quenching agent having a high dielectric resistivity at the high temperature of an electric arc produced upon melting of said fusible element (1), and a pair of terminals (5,6) interconnected through the fusible element, said terminals (5,6) providing for connection of the fusible element (1) in an electric circuit to be protected against an overcurrent, characterized in that said arc-quenching element consists of an envelope (3) made of non-porous rigid ceramic having a central cavity completely filled by the fusible element (1) in such a manner as not to leave any free space, said high density porous non rigid ceramic having a high resistance to shocks of internal pressure and high temperature caused by the production of said electric arc.
2. A current-limiting fuse according to claim 1, characterized in that said non-porous high density rigid ceramic further has a high thermal conductivity and a high specific heat to rapidly absorb the heat produced within said envelope (3) by the electric arc.
3. A current-limiting fuse according to claim 1, characterized in that said ceramic is alumina of formula Al₂O₃.
4. A current-limiting fuse according to claim 2, characterized in that said ceramic is beryllium oxide of formula BeO.
5. A current-limiting fuse according to any one of claims 1 to 4, characterized in that said fusible element (1) is elongated and in that said envelope (3) is formed of two complementary pieces (13,14; 15,16) each having a surface of contact with the other of said two pieces, a groove (14',15') having the same shape and dimensions as the fusible element (1) being provided in the contact surface of one of said two complementary pieces (14,15).
6. A current-limiting fuse according to claim 5, characterized in that said contact surfaces of the two complementary pieces (13,14; 15,16) are joined together.
7. A current-limiting fuse according to claim 6, characterized in that said contact surfaces of the two complemenraty pieces (13,14; 15,16) are joined together by means of an inorganic cement.
8. A current-limiting fuse according to any one of claims 1 to 4, characterized in that said fusible element (1) is elongated and in that said envelope (3) comprises a tubular portion (16) and a rod (15), said rod (15) having two ends and comprising a groove (15') interconnecting the two ends of the rod, said groove (15') having the same shape and dimensions as the fusible element (1), said rod (15) being mounted within the tubular portion (16) with the fusible element (1) being inserted in said groove (15').
9. A current-limiting fuse according to claim 8, characterized in that said rod (15) is fixed with the tubular portion (16) by means of an inorganic cement.
10. A current-limiting fuse according to any one of claims 1 to 4, characterized in that said fusible element (1) is elongated and in that said envelope (3) comprises a tubular portion (17) and a plurality of short cylindrical elements (18), said cylindrical elements (18) comprising grooves which follow the exact shape of the fusible element (1) and which are so positioned on said cylindrical elements (18) that the fusible element (1) follows a non linear course when inserted in the grooves of said short cylindrical elements (18) mounted end to end within said tubular portion (17) of the envelope (3).
11. A current-limiting fuse according to claim 10, characterized in that said tubular portion (17) and said cylindrical elements (18) are joined together by means of an inorganic cement.
12. A current-limiting fuse according to claim 10, characterized in that said fusible element (1) comprises a plurality of cross-section constrictions (2), and in that each pair of successive constrictions (2) are separated from each other by at least one of said short cylindrical elements (18).
13. A current-limiting fuse according to claim 10, characterized in that each of said short cylindrical elements (18) comprises two planar end surfaces substantially parallel to each other and a substantially cylindrical surface interconnecting together said two planar surfaces, and in that each of said short cylindrical elements (18) comprises a longitudinal groove made in its substantially cylindrical surface and a transversal groove made in one of its two planar surfaces, said longitudinal and transversal grooves communicating with each other.
14. A current-limiting fuse according to any one of the preceding claims, characterized in that it further comprises a sheath (4) covering the envelope (3) in order to increase the mechanical rigidity of the said envelope (3).
15. A current-limiting fuse according to claim 14, characterized in that said sheath (4) is made of glassfiber.
16. A current-limiting fuse according to claim 14, characterized in that said sheath (4) is made of ceramic.
17. A current-limiting fuse according to any one of the preceding claims, characterized in that said envelope (3) is metalized at two different locations thereof to form said pair of terminals (5,6).
18. A current-limiting fuse according to any one of claims 1 to 4, characterized in that said envelope (3) comprises a rod (19,20; 25) closely currounding at least one fusible element, and an external portion (21,26) defining a cavity (22; 27) to receive said rod (19,20; 25).
19. A current-limiting fuse according to any one of claims 1 to 4, characterized in that said envelope (3) comprises a plurality of rods (19,20; 25) each closely surrounding at least one fusible element and an external portion (21, 26) defining cavities (22; 27) to receive said rods.
20. A currenti-limiting fuse according to any one of claims 5 to 7, characterized in that said two complementary pieces (13, 14) are elongated and have a cross section in the form of a half-moon.
21. A current-limiting fuse according to any one of claims 5 to 7, characterized in that said contact surfaces of the two complementary (13,14; 15,16) are joined together to define a cavity having the same shape and dimensions as the fusible element (1), said fusible element being formed by injection of molten metal in said cavity.
22. A current-limiting fuse according to any one of claims 1 to 4, characterized in that said envelope (3) defines a cavity (28, 29) having the same shape and dimensions as the fusible element (1), said fusible element being formed by injection of molten metal 23,24) in said cariby (28,29).
23. A method of manufacturing a current-limiting fuse (F) according to claim 1, of the type comprising inserting a fusible element (1) designed to conduct an electric current (I) into an arc-quenching agent having a high dielectric resistivity at the high temperature of an electric arc produced upon melting of the fusible element (1), said insertion being carried out in such a manner that said arc given agent closely surrounds the fusible element (1), and subsequently mounting a pair of terminals (5, 6) interconnected through the fusible element (1), said pair of terminals (5, 6) providing for connection of the fusible element (1) in an electric circuit to be protected against an overcurrent, characterized in that use is made of an envelope (3) made of high density non-porous rigid ceramic as arc-quenching agent, said ceramic defining a cavity having the same shape and dimensions as the fusible element (1) so as to be completely filled by said fusible element with no free space left during its insertion, said ceramic having a high resistance to shocks of internal pressure and high temperature caused by said electric arc.
24. A method of manufacture according to claim 23, characterized in that said high density non-porous rigid ceramic further has a high thermal conductivity and a high specific heat in order to rapidly absorb the heat produced within said envelope (3) by the electric arc.
25. A method of manufacture according to claim 24, characterized in that said ceramic comprises alumina of formula Al₂O₃.
26. A method of manufacture according to claim 24, characterized in that said ceramic comprises beryllium oxide of formula BeO.
27. A method of manufacture according to any one of claims 23 to 26, characterized in that said step of mounting the pair of terminals (5, 6) comprises metalizing said envelope (3) at two different locations thereof.
28. A method of manufacture according to any one of claims 23 to 26, wherein said fusible element (1) is elongated, characterized in that:
       the envelope (3) is made of two complementary pieces (13, 14) made of said rigid ceramic and each having a half-moon shaped transversal cross-section defining a surface of contact with the other of said complementary pieces, the contact surface of one of the complementary pieces (13, 14) comprising a groove (14') having the same shape and dimensions as the fusible element (1); and
       said step of inserting said fusible element (1) consists in inserting said fusible element within said groove and in assembling together the two complementary pieces (13, 14), said assembly comprising (a) joining said two contact surfaces of the two complemenraty pieces by means of an inorganic cement, and (b) subjecting the so-joined complementary pieces to a pressure to press said two contact surfaces against each other, and to a heat treatment at a temperature lower than the melting point of the fusible element (1) for thereby forming a rigid and impervious envelope (3).
29. A method of manufacture according to any one of claims 23 to 26, wherein said fusible element (1) is elongated, characterized in that:
       the envelope (3) is made of two complementary pieces (15, 16) made of said ceramic on of said pieces having an annular cross-section and the other piece the shape of a hod sized to be inserted into said annular piece, each of said pieces having a surface of contact with the other of said pieces, the surface of contact of one of said pieces comprising a groove (15') having the same shape and dimensions as the fusible element (1); and
       said step of inserting the fusible element (1) consists in inserting said fusible element within said groove (15'), in inserting said rod-shaped piece into the other piece of annular cross-section (15,16) and in assembling together said two pieces (15, 16), said assembly consisting of (a) joining said contact surfaces of the two complementary pieces (15,16) by means of an inorganic cement, and (b) subjecting the so-joined two complementary pieces to a heat treatment at a temperature lower than the melting point of the fusible element (1) for thereby forming a rigid and impervious envelope (3).
30. A method of manufacture according to any one of claims 23 to 26, wherein said fusible element (1) is elongated, characterized in that:
       the envelope of ceramic is made of a tubular portion (17) and a plurality of short cylindrical elements (18), said cylindrical elements (18) comprising grooves which follow the exact shape of the fusible element (1) and which are so positioned on said cylindrical elements (18) that the fusible element (1) follows a non linear course when inserted in said grooves of the cylindrical elements (18) mounted end to end within said tubular portion (17); and
       said step of inserting the fusible element (1)    consists in (a) inserting the fusible element in said grooves of the cylindrical elements (18) and positioning end to end said cylindrical elements (18) along with the fusible element (1) within the tubular portion (17), (b) joining together said cylindrical elements (18) and tubular portion (17) by means of an inorganic cement, and (c) subjecting the low-joined cylindrical elements (18) and tubular portion (17) to a heat treatment at a temperature lower than the melting point of the fusible element (1) for thereby forming a rigid and impervious envelope (3).
31. A method of manufacturing a current-limiting fuse (F) according to claim 1, characterized in that it comprises the following steps:
       producing an envelope (3) made of non-porous high density rigid ceramic defining a cavity (28: 29), said ceramic having a high dielectric resistivity at high temperatures as well as a high resistance to shocks of internal pressure and high temperature:
       injecting a molten metal (23, 24) within said cavity (28, 29) of the envelope so as to completely fill this cavity without leaving any free space to form a fusible element (1); and
       mounting on said envelope a pair of terminals (5, 6) interconnected together through the fusible element, said pair of terminals (5, 6) providing for connection of the fusible element (1) in an electric circuit to be protected against an overcurrent.
32. A method of manufacture according to claim 31, wherein said non-porous rigid ceramic further has a high thermal conductivity and a high specific heat to rapidly absorb the heat produced within said envelope (3) by the electric arc.
33. A method of manufacture according to claim 31 or 32, wherein said step of producing the envelope (3) comprises the use of at least one piece of metal having a high melting point to form said cavity (29) of the envelope (3).
34. A method of manufacturing a current-limiting fuse (F) according to claim 31 or 32, wherein said fusible element (1) is elongated, characterized in that:
       said step of producing the envelope (3) comprises the production of two complementary pieces (13, 14; 15, 16) made of said ceramic and each having a surface of contact with the other of said two pieces, the contact surface of one of said two complementary pieces (14, 15) comprising a groove (14', 15') having the same shape and dimensions as the fusible element (1); and
       said step of producing the envelope further comprising assembling the two compementary pieces (13,14; 15, 16) by joining together said contact surfaces.

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