GB1558666A

GB1558666A - Cartridge fuse

Info

Publication number: GB1558666A
Application number: GB4371875A
Authority: GB
Original assignee: Brush Power Equipment Ltd
Current assignee: Brush Power Equipment Ltd
Priority date: 1976-10-25
Filing date: 1976-10-25
Publication date: 1980-01-09

Description

(54) CARTRIDGE FUSE (71) We, BRUSH POWER EQUIPMENT LIMITED, a British Company, of Bridgend Industrial Estate, Bridgend, Mid-Glamorgan, South Wales, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an electric fuse and particularly but not exclusively to a current-limiting cartridge type of fuse in which a fuse element is, or more than one fuse elements connected in parallel are, enclosed within an insulating tube filled with a refractory powder acting as an arcextinguishing medium. The or each fuse element is usually either a flat strip or a round-section wire of a suitable fuse metal or alloy, e.g., silver.

Where the fuse element, whether a flat strip or a round-section wire, is of uniform cross-section along its length, there are two serious disadvantages where short circuits passing heavy currents are to be broken. One is that the whole length of the fuse element is melted simultaneously, resulting in an objectionably high voltage being developed, and the other is that the current is not greatly limited. Both these disadvantages are avoided by the well-known expedient of providing local reductions in cross-sectional area, the purpose of which is to ensure that initial melting of the fuse occurs at the points of reduced cross-sectional area where the current-density is higher than in the remainder of the strip of normal crosssection. As the length of the or each point of reduced cross-sectional area is short, the length of an arc produced there will also be short and so the voltage developed will be correspondingly low. As the arcs burn back from the or each point of reduced crosssection, the voltage, developed will increase but at no time during the arcing period of the fuse will the voltage be as high as that which would have been produced if the reductions had not been provided. The high current-density at the points of reduced cross-section results in fast melting there. If the melting occurs before the current has reached its maximum cyclic value, the current is limited to a low value.

Except for comparatively high currents produced in a fuse when a short-circuit occurs, a strip provided with points of reduced cross-section, as hereinbefore described, will take longer to melt than a strip having a uniform cross-sectional area equal to that at the points of reduced crosssection throughout its length. This is generally an advantage, as will be described hereinafter. However for certain faults producing lower currents it is desirable to decrease the time taken for the strip to melt.

An object of the invention is to provide a fuse having a strip, or a plurality of strips connected in parallel, of the kind having points of reduced cross-sectional area in which the or each strip is so modified as to enable it to melt more quickly for certain low fault currents.

According to the invention, an electric fuse includes a fuse element, or a plurality of fuse elements connected in parallel, extending between a pair of end connectors by which the fuse is, in use, connected in a circuit to be protected by the fuse, the or each fuse element being a fusible strip having intermediate its ends a first portion of reduced cross-sectional area produced by forming in at least one of the longitudinal edges of the strip a first notch having a longitudinal edge extending longitudinally of the strip, and a plurality of second portions of reduced cross-sectional area spaced apart in the length of the strip and produced by forming second notches either in a longitudinal edge of the strip or in both a longitudinal edge of the strip and in the longitudinal edge of a notch in the same longitudinal edge of the strip and forming said first portion of reduced cross-sectional area, the distance in the longitudinal direction of the strip between the ends of each second notch being shorter than the length of the longitudinal edge of the or each first notch and the minimum crosssectional area of each second portion being less than the cross-sectional area of said first portion.

Conveniently said second notches are spaced apart uniformly along the length of the strip and the distance between the ends of the or each first notch is at least equal to the spacing between adjacent second notches.

The cross-sectional area of said first portion is preferably approximately three times the minimum cross-sectional area of each said second portion.

The strip may be of uniform rectangular cross-section except at the positions of the notches forming said first and second portions of reduced cross-section, the notches being formed in both longitudinal edges of the strip.

By way of example, three alternative fuse elements, in accordance with the invention are now described with reference to the accompanying drawings, in which: Figures 1, 2 and 3 are respectively plan views of the three elements; Figure 4 is a graph illustrating the function of a fuse element in accordance with the invention, and Figure 5 is a diagram of a typical circuit in which a fuse incorporating one or more fuse elements according to the invention may be included.

The fuse element provided by this invention is a strip of fusible metal or alloy, e.g., an alloy containing silver, of uniform rectangular cross-section, except where the aforesaid portions of reduced cross-section are formed.

Strip 1, shown in Figure 1, has a uniform thickness throughout its length but has its width (i.e., the dimension which is vertical in the drawing) reduced along a first portion of reduced cross-section G at a position at or near to the center of the element by stamping two rectangular notches in the edges of the strip. The portion G is of uniform width along its length. The parts of the strip at each end of portion G are each formed with a plurality of second portions of reduced cross-section F by stamping notches in the edges of the strip. The width of each portion F is the same, but is less than the width of the portion G. This is an essential feature of the strip and the reason for it will be explained hereinafter. The hatched areas H and J illustrate modifications of the strip and will be described later.

The second strip 2, shown in Figure 2, is similar to that shown in Figure 1, except that the strip is formed with the portions F at equal spacings along its whole length. The position of the portion G and its length are such that each end of the portion G coincide with a portion F. Alternatively the position of G may be such that its ends do not coincide with portions F and in that case there would be one portion F formed within the length of the portion G.

The third strip 3, shown in Figure 3, is similar to the strip 2 shown in Figure 2, except that the portion G is longer and its ends are positioned intermediate a pair of adjacent portions F so that at least two portions F are formed with the length of the portion G.

Typical dimensions of the strip shown in Figures 1 to 3 are: Thickness of strip 0.1 mm Width of uncut strip 1.25 mm Width of strip at portions F 0.25 mm Width of strip at portion G 0.75 mm Length of portion G in Figures 1 and 2 15 mm Length of portion G in Figure 3 30 mm Spacing of portions F 15 mm The width and thickness of the strips can vary over a wide range, dependent on the current rating required. Similarly different numbers of the strips may be connected in parallel between the end connections of a fuse. Also the length and width of the portion G may vary from the foregoing dimensions. For example, the length of the portion G may vary between 15 and 30 mm, where the other dimensions given remain unaltered.

The strip or strips may be mounted at their ends in any known way for currentlimiting fuses of the cartridge type and are enclosed within an insulating tube filled with a refractory powder acting as an arcextinguishing medium. Fcr example as shown in Figures 10 to 13 of Patent Specification No. 1,294,085.

The relation between the current through a fuse element and the time taken to effect melting of the element is conventionally presented as a characteristic plotted with current as abscissa and time as ordinate, logarithmic scales being used on both coordinate axes. Such a characteristic for a typical fuse element strip of uniform crosssectional area except for portions of equal reduced cross-sectional area spaced apart along its length is shown as 'Fuse B" in Figure 4. A corresponding characteristic for a similar fuse element strip of uniform crosssectional area equal to that of the portions of reduced cross-sectional area in Fuse B, but not having portions of reduced crosssectional area is indicated by a dash-dot line as "Fuse A" in Figure 4. It will be seen that except for large currents greater than a current 1D Fuse B will melt more slowly than Fuse A.

Figure 5 is a diagram of a typical circuit in which a current-limiting cartridge fuse such as Fuse B may be employed. The diagram shows a step down transformer 4 with a high-voltage fuse 5, such as Fuse B, in the input line 6 and a low voltage fuse 8, of a kind which will melt before Fuse B, in the output line 7. When a short circuit occurs in the output line 7, the low voltage fuse must melt before the high voltage fuse in a time, typically, of the order of 10-' seconds. A fuse having one or more fuse elements such as Fuse B, having points of reduced crosssection, is superior to Fuse A having one or more fuse elements of uniform crosssection, where used as the high voltage fuse, as the melting time for Fuse B is longer than for Fuse A for a current such as I" in Figure 4, which will occur with a short circuit in the output line 7. A short circuit in the input line 6 results in a much higher current, shown as ID in Figure 4 and it is desirable that the high voltage fuse shall melt as quickly as possible. Thus a fuse element such as Fuse B in which the or each fuse element has points of reduced area is just as suitable as Fuse A in which the or each fuse element is of uniform cross-sectional area.

Fuse B however is not as suitable in the event of a high resistance fault indicated at 3 or in the event of a winding fault in the transformer resulting in a current 1E in Figure 4. In these circumstances it is desirable that the high voltage fuse shall operate reasonably quickly. Thus in accordance with the invention, a fuse element strip of the kind used in Fuse B is provided with a first portion G of reduced cross-sectional area but of larger crosssectional area and longer length than the second portions F of reduced crosssectional area. Fuse element strips as provided by this invention are shown in Figures 1 to 3 and the time/current characteristic is indicated at "Fuse C" in Figure 4. For heavy currents such as current ID in Figure 4, melting occurs simultaneously at all the second portions F, as the rate of development of heat at portion G is less than that at each portion F, and so Fuse C behaves similarly to Fuse B at current I,.

However, for lower values of current, such as 1E in Figure 4, the reductions of cross-sectional area at portions F have little effect, as the portions F are of short length, thus allowing considerable conduction of heat to the un-notched parts of the strip. At the portion G, however, which is considerably longer than each portion F, there is little heat conduction along the strip from the portion G and consequently fusion occurs at the portion G in a much shorter time than with Fuse B. Thus the time/current characteristic for a fuse element strip provided by this invention and shown in Figures ! to 3. is typically as indicated as Fuse C in Figure 4.

In any of the fuse element strips shown in Figures 1 to 3, the first portion G, may be surrounded by a heat-insulating jacket, for example as shown by N' in Figures 7 and 8 of Patent Specification No. 1,294,085. The purpose of such a jacket is to reduce dissipation of heating into the arcextinguishing medium, thereby to reduce further the melting time for currents less than that corresponding to melting time of 10-' seconds. This is indicated in Figure 4 by the broken line 'K'.

Further modification of the time/current characteristic may be made by adding a small amount of a low melting point alloy of the strip as described in Patent Specification No. 1,294,085. For example, referring to Figure 1, a small amount of the alloy may be placed at position H, on an un-notched part of the strip, as indicated by hatched area in Figure 1. The effect of this is to modify the time/current characteristic as shown by broken line H' in Figure 4, thereby reducing the melting time for a low fuse current where the melting time is of the order of 100 seconds. Alternatively, a small amount of the alloy may be placed on the portion G and J, where the temperature is higher than at H, as shown by hatched area in Figure 1, thereby modifying the time/current characteristic as shown J' by broken lines in Figure 4, to produce lower melting time for a fuse current where the melting time is of the order of 10 seconds.

WHAT WE CLAIM IS: 1. An electric fuse including a fuse element, or a plurality of fuse elements connected in parallel, extending between a pair of end connectors by which the fuse is, in use, connected in a circuit to be protected by the fuse, the or each fuse element being a fusible strip having intermediate its ends a first portion of reduced cross-sectional area produced by forming in at least one of the longitudinal edges of the strip a first notch having a longitudinal edge extending longitudinally of the strip, and a plurality of second portions of reduced cross-sectional area spaced apart in the length of the strip and produced by forming second notches in either a longitudinal edge of the strip or in both a longitudinal edge of the strip and in the longitudinal edge of a notch in the same longitudinal edge of the strip and forming said first portion of reduced cross-sectional area, the distance in the longitudinal direction of the strip between the ends of each second notch being shorter than the length of the longitudinal edge of the or each first notch and the minimum crosssectional area of each second portion being

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. as Fuse B may be employed. The diagram shows a step down transformer 4 with a high-voltage fuse 5, such as Fuse B, in the input line 6 and a low voltage fuse 8, of a kind which will melt before Fuse B, in the output line 7. When a short circuit occurs in the output line 7, the low voltage fuse must melt before the high voltage fuse in a time, typically, of the order of 10-' seconds. A fuse having one or more fuse elements such as Fuse B, having points of reduced crosssection, is superior to Fuse A having one or more fuse elements of uniform crosssection, where used as the high voltage fuse, as the melting time for Fuse B is longer than for Fuse A for a current such as I" in Figure 4, which will occur with a short circuit in the output line 7. A short circuit in the input line 6 results in a much higher current, shown as ID in Figure 4 and it is desirable that the high voltage fuse shall melt as quickly as possible. Thus a fuse element such as Fuse B in which the or each fuse element has points of reduced area is just as suitable as Fuse A in which the or each fuse element is of uniform cross-sectional area. Fuse B however is not as suitable in the event of a high resistance fault indicated at 3 or in the event of a winding fault in the transformer resulting in a current 1E in Figure 4. In these circumstances it is desirable that the high voltage fuse shall operate reasonably quickly. Thus in accordance with the invention, a fuse element strip of the kind used in Fuse B is provided with a first portion G of reduced cross-sectional area but of larger crosssectional area and longer length than the second portions F of reduced crosssectional area. Fuse element strips as provided by this invention are shown in Figures 1 to 3 and the time/current characteristic is indicated at "Fuse C" in Figure 4. For heavy currents such as current ID in Figure 4, melting occurs simultaneously at all the second portions F, as the rate of development of heat at portion G is less than that at each portion F, and so Fuse C behaves similarly to Fuse B at current I,. However, for lower values of current, such as 1E in Figure 4, the reductions of cross-sectional area at portions F have little effect, as the portions F are of short length, thus allowing considerable conduction of heat to the un-notched parts of the strip. At the portion G, however, which is considerably longer than each portion F, there is little heat conduction along the strip from the portion G and consequently fusion occurs at the portion G in a much shorter time than with Fuse B. Thus the time/current characteristic for a fuse element strip provided by this invention and shown in Figures ! to 3. is typically as indicated as Fuse C in Figure 4. In any of the fuse element strips shown in Figures 1 to 3, the first portion G, may be surrounded by a heat-insulating jacket, for example as shown by N' in Figures 7 and 8 of Patent Specification No. 1,294,085. The purpose of such a jacket is to reduce dissipation of heating into the arcextinguishing medium, thereby to reduce further the melting time for currents less than that corresponding to melting time of 10-' seconds. This is indicated in Figure 4 by the broken line 'K'. Further modification of the time/current characteristic may be made by adding a small amount of a low melting point alloy of the strip as described in Patent Specification No. 1,294,085. For example, referring to Figure 1, a small amount of the alloy may be placed at position H, on an un-notched part of the strip, as indicated by hatched area in Figure 1. The effect of this is to modify the time/current characteristic as shown by broken line H' in Figure 4, thereby reducing the melting time for a low fuse current where the melting time is of the order of 100 seconds. Alternatively, a small amount of the alloy may be placed on the portion G and J, where the temperature is higher than at H, as shown by hatched area in Figure 1, thereby modifying the time/current characteristic as shown J' by broken lines in Figure 4, to produce lower melting time for a fuse current where the melting time is of the order of 10 seconds. WHAT WE CLAIM IS:

1. An electric fuse including a fuse element, or a plurality of fuse elements connected in parallel, extending between a pair of end connectors by which the fuse is, in use, connected in a circuit to be protected by the fuse, the or each fuse element being a fusible strip having intermediate its ends a first portion of reduced cross-sectional area produced by forming in at least one of the longitudinal edges of the strip a first notch having a longitudinal edge extending longitudinally of the strip, and a plurality of second portions of reduced cross-sectional area spaced apart in the length of the strip and produced by forming second notches in either a longitudinal edge of the strip or in both a longitudinal edge of the strip and in the longitudinal edge of a notch in the same longitudinal edge of the strip and forming said first portion of reduced cross-sectional area, the distance in the longitudinal direction of the strip between the ends of each second notch being shorter than the length of the longitudinal edge of the or each first notch and the minimum crosssectional area of each second portion being

less than the cross-sectional area of said first portion.

2. An electric fuse as claimed in Claim 1 in which the or each fuse element has said second notches spaced apart uniformly along the length of the strip and the length of the longitudinal edge of the or each first notch is at least equal to the spacing between adjacent second notches.

3. An electric fuse as claimed in Claim I or 2 in which the cross-sectional area of said first portion of the or each fuse element is approximately three times the minimum cross-sectional area of each said second portion of the or each fuse element.

4. An electric fuse as claimed in any preceding claim in which the strip constituting the or each fuse element is of uniform rectangular cross-section except at the positions of the notches forming said first and second portions of reduced crosssection, the notches being formed in both longitudinal edges of the strip.

5. An electric fuse according to Claim 1 in which the or each fuse element is constructed substantially as described herein and shown in Figures 1, 2 or 3 of the accompanying drawings.