NL1002061C2 - Fuse assemblies. - Google Patents

Description

FUSE COMPOSITIONS

The present invention relates to fuse assemblies provided with a number of melting conductors. The present invention is particularly, but not necessarily, applicable to high voltage fuses, where the melting conductors are printed on a tubular insulating material support.

10 In those cases where it is necessary to provide "full-range" fuses, ie fuses capable of blocking all currents that melt the melting conductors, fuses where the operating speed is not critical are of the kind described in EP-0 117 582-A1, used with great success. These fuses generally comprise a cylindrical quartz glass support body, on the outer surface of which a number of spirally extending fusible conductors are pressed. Under normal operating conditions, the current is evenly distributed among the melting conductors present. Each of the conductors is provided with notches of a smaller thickness, such that with a small overcurrent, i.e. a current of the order of one to ten times the nominal current, the 25 notches will melt separately and the entire fuse will only Burn out if the overcurrent is maintained for a significant period of time, for example, for more than one second. This inverse operating characteristic is desirable for the fuses 30 to be compatible with other protection devices and with the requirements of a high voltage system.

EP-0 123 331-A1 describes closure caps suitable for use in making electrical connections to a fuse of the type described in EP-0 117 582-A1. The fuse is designed to obtain a desired ratio between the amount of overcurrent and the melting time.

1002061: 2

In an article entitled "Analogue Simulations of Heat Flow in a High Voltage Fuse" by J.G.J.

Closed in Proceedings of the Third International Conference 5 on Electric Fuses and their Applications (minutes of the 3rd International Conference on Electrical Fuses and their Applications) from May 11-13, 1987, a computerized model for predicting fuse behavior of the type discussed above.

Although the above-mentioned computerized simulation technique can help optimize the design of multi-conductor fuses, the manufacture of fuses with a time-current operating characteristic meeting the recognized standards remains a difficult matter. In particular, it remains difficult to provide a fuse in which the deblocking of an error condition in the low voltage connection area of a distribution transformer 20 occurs sufficiently quickly.

The object of the present invention is to eliminate or at least reduce the drawbacks of the known multi-conductor fuses. In particular, the present invention aims to provide such full-range fuses, with arc characteristics ("pre-arcing time characteristics") that unblock low span fault conditions within recognized standards, for example VDE 0670/402 and ESI 12-8, 30.

Furthermore, it is an object of the present invention to provide a full-range fuse that has a predictable arc backflow characteristic during the peak time of the fuse operation.

10 0 2 0 61. ' 3

According to a first aspect of the present invention there is provided a subassembly for a fuse, comprising an elongated support body of insulating material and a number of melting conductors supported by the support body and extending over the support body, the conductors of one or more connected to one another adjacent melting conductors in at least one intermediate region along their length are joined to form a bridge.

Preferably, the conductors of each of said groups are joined in at least two intermediate regions along their length, and more preferably still in two intermediate regions.

Preferably, each of the groups consists of at least three melting conductors, and more preferably still three melting conductors.

Preferably, the cross-sectional area of each of the bridges corresponds to n times the cross-sectional area of the notches, n being the number of conductors in the respective group.

Preferably, each of the melting conductors includes a number of notches along its length, each notch containing a portion of a smaller width. The notches are chosen to melt before the parts of the conductors melt.

Preferably, a tip containing a low melting point metal or metal alloy, for example tin or a tin-silver alloy, is arranged such that the tip is in thermal and electrical contact with some of the bridges, and more preferably still with at least one of the 35 bridges belonging to a group of melting conductors.

In a preferred embodiment of the sub-assembly according to 10 02 061.

4 the above-described first aspect of the invention, the supporting body is substantially tubular, for example consisting of a hollow cylinder made of quartz glass. Preferably, the conductors 5 extend spirally about the outer surface of the tube. The conductors may be screen printed on the surface of the tube.

According to a second aspect of the present invention, there is provided an outer casing fuse, with a subassembly according to the first aspect of the invention described above, wherein the subassembly is contained in the outer casing, with filler material disposed in the outer casing which substantially surrounds the subassembly, and with two electrical contact means which make electrical contact with respective ends of the melting conductors to enable external electrical connections to be made to the fuse.

20

To illustrate the present invention and to illustrate how it can be implemented, an embodiment of the invention will be discussed below with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view of a subassembly with a large number of melting conductors; Figure 2 shows the melting conductors of the sub-assembly according to Figure 1 in a deflected state;

Figure 3 shows the relative dimensions of a group of conductors of the sub-assembly of Figure 1; and 35

Figure 4 shows the time-current operating characteristic of the sub-assembly of Figure 1, as well as the time-current characteristic of a known multi-conductor sub-assembly of a fuse.

Figure 1 shows a partial cross-sectional view of a sub-5 assembly of a fuse intended to be included in a full range high voltage fuse. The sub-assembly comprises a hollow, cylindrical quartz glass tube 1, on the outer surface of which a number of spiral-extending melting conductors 2 are arranged (for the sake of clarity, the conductors are only on the right side of the sub-assembly of Figure 1 fully illustrated). The conductors extend between opposite end portions 3, 4 of the quartz tube, the 15 conductors in practice being screen printed on the outer surface of the quartz tube using a process as described in NL 8801355. In NL No. 8801355 also discloses a method of thickening, by electroplating, the printed conductors to obtain a higher voltage capacity. Near each end of the tube 1, the conductors 2 flow together in a strip of conductive material 5, 6, which is applied to the tube simultaneously by screen printing. The strips 5, 6 and the conductors 2 consist of fusible material, for example silver.

In order to produce a fuse using the subassembly of Figure 1, the sub-30 assembly is inserted into a cylindrical outer housing which is closed at both ends by means of caps (not shown) which make electrical contact with the conductive strips 5, 6 of the sub-assembly. External electrical contact is established via the closing caps and in turn the conductive strips.

In order to give a clearer picture of the geometry

10 0 2 0 3 f F

6 of the guides 2 of the subassembly, the 2nd in FIG. 2 are shown in "deflected" condition.

As shown in Figure 2, the melting conductors 2 are arranged in a plurality of groups of three adjacent melting conductors 2, the conductors of each group merging near the respective ends of the conductors to form "bridges" 8. In the specific configuration as shown in Figure 2, 15 conductors are arranged in five 10 groups. The current density at each bridge is determined by the number of confluent conductors and the cross-sectional area of the bridge, however, it is typically three times the normal current density of each conductor and is approximately as high as at the regular distances of notches in each guide. The length of each bridge is considerably greater than that of the notches 9, while the heat transfer via conduction over the center of the bridges is considerably less than at the location of a notch.

Figure 3 shows in more detail an end portion of one of the groups of conductors 2, the dimensions indicated in the figure being expressed in millimeters. The thickness (in radial direction to the quartz tube) of the conductor is typically 6-50 µm, while the spacing between adjacent conductors is at least as great as the width of the conductors, that is, 1 , 0 mm or more.

30

Notches 9 are provided in each conductor 2 of the sub-assembly for the purpose of improving the arc characteristics and changing the time-current performance of the sub-assembly. The width of the notches 35 is 1/2 - 1/5 times the width of the associated conductor 2, for example 0.25 mm in Figure 2. The length of the notches varies between 0.5 mm and 2.5 mm, while 10 0 2 0 61; 7 each guide contains 2-12 notches.

The provision of the bridges 8 for each group of conductors 2 also leads to an improved time-flow performance of the sub-assembly. With the geometry described above, at heights (ie "pre- arcing times"), that is, the time elapsing before all bridges have melted, of typically less than 1 ms, initiates arcing simultaneously at the notches 9 and at the 10 bridges 8 place. At heights of typically 1 ms to 1 second, the initiation of arcing occurs earlier at the bridges 8 than at the notches 9, due to the low heat loss at the center of the bridges relative to the heat loss at the notches 15 gene. The time elapsing between the onset of arcing at the bridges and the onset of arcing at the notches increases with increasing values of the heights (the heat losses increase with time), which is a 20 provides significant improvement in the time-flow characteristic of the sub-assembly.

By applying a tip of tin 10 (known as "tin tip" or the "M effect") in the middle of a bridge 8 25, the arc characteristic at pre-arcing characteristic up to one hour further improved. This is because a tin point melts at a temperature lower than the temperature at which the silver conductors 2 would melt alone, while the molten tin forms an alloy with the silver, the alloy having a melting point lower than the melting point of the silver. At peak times approaching the minimum melting state, the temperature gradient pattern along a conductor 2 changes as heat transfer to the fuse's terminal points increases, with the temperature near the center of the fuse increasing to a maximum value. and to the f 0 0 2 0 61.

8 end sections decreases. By applying the tin tip to the bridges, near the ends of the fuse, a longer peak time is obtained than if the tin tip were located in the middle of the fuses, as is the case with conventional multi-conductor sub-assemblies , making it possible to achieve a greater current range.

By arranging the tin tip near the ends of the fuse, the notched sections can reach higher temperatures than would otherwise be possible before the tin tip would melt. This is known to contribute to the "burn-back process" by reducing the arc energy required to melt the silver, thereby improving arc-forming performance .

In the embodiment shown in figure 2, a tin point is arranged alternately on one of the respective ends of the sub-assembly on a bridge of each group of conductors 2.

A further advantage of the multi-conductor bridged configuration is that better arc control is achieved when using the fuse at low currents, that is, one to ten times the rated current. In conventional multiconductor configurations, the tin tip or the notches in all conductors must have melted before arcing begins.

30

Then, the arc randomly commutes between conductors until the last conductor develops enough high voltage to suppress the current. In the multi-conductor bridge configuration, arcing begins when one bridge is fused in all conductor groups. Subsequently, the arc commutation continues within one conductor group at the notches, 10CP # 7 9 until the arc is forced to commutate to another conductor group. This process continues until the last conductor group unblocks the current.

This two-step process makes it possible to better control the random nature of the commutation process and reduces the arc energy developed in the "last" conductor to unblock the circuit.

10

Figure 4 shows the time-current characteristic for the multiconductor fuse (A) of the type described in EP-0 117 582-A2 and also for a fuse (B) using the multiconductor bridges described above.

It will be appreciated that at a given overcurrent, in the range of less than 500 Amp, the time-current curve due to the use of the conductor bridges is linearized, while the melting time is reduced by an order of magnitude.

20

It will be clear that within the scope of the invention changes are possible to the above-described embodiment. For example, instead of being tubular, the support body can also be made flat.

It is also possible that an alloy of tin and silver is used as the material for the tin tip.

1 0 02 061;

Claims (8)

1. A fuse subassembly comprising an elongated insulating support member and a plurality of melting conductors supported by the supporting body and extending over the supporting body, the melting conductors being joined in at least an intermediate region along their length so as to provide a bridge, each of the melting conductors having a number of notches along its length, and each notch having a portion of a smaller width, characterized in that the length of each bridge is so much greater than the length of the notches that the heat transfer via conduction over the center of the bridges is considerably less than the heat transfer at the location of a notch. A fuse sub-assembly according to claim 1, wherein the cross-sectional area of each of the bridges substantially corresponds to n-times the cross-sectional area of the notches, n being the number of melting conductors 25 in the respective group. A fuse subassembly according to claim 1 or 2, wherein a tip containing a low melting point metal or metal alloy is disposed such that the tip is in thermal and electrical contact with at least one of each of the group of melting conductors hearing bridges. A fuse subassembly according to any one of the preceding claims, wherein the support body is of substantially tubular design and the melt conductors extend spirally about the outer surface of the tube. A fuse sub-assembly according to any one of the preceding claims, wherein the notches are selected to melt before body parts of the melting conductors melt. A fuse subassembly according to any of the preceding claims, wherein the melting conductors of each of said groups are joined in at least two intermediate regions along their length. 15 A fuse subassembly according to any of the preceding claims, wherein each of the groups includes at least three melting conductors. 8. A fuse with an outer casing, with a sub-assembly according to any one of the preceding claims included in the outer casing, with filling material arranged in the outer casing, which substantially surrounds the sub-assembly, and with two electrical contact means, which electrical contact with respective ends of the melting conductors, to allow external electrical connections to be made to the fuse. 1002001