GB2297003A - Fuse assemblies - Google Patents

Description

f FUSE ASSEMBLIES 2297003 The present invention relates to fuse assemblies

having a plurality of fuse conductors. In particular, though not necessarily, the present invention is applicable to high voltage fuses in which the fuse conductors are printed onto a tubular insulating support.

Where it is required to provide "full range" fuses, i.e. fuses which must be capable of blocking all currents that cause melting of the fuse conductors, fuses for which the speed of operation is not critical, of the type described in EP-0117582-Al, have found considerable cylindrical of which are printed a plurality of helically extending conductors. Under normal operating conditions the fuse current is spread equally amongst the plurality of fuse conductors. Notches of reduced thickness are provided in each of the conductors such that for low over-currents, i.e. currents in the region of 1 to 10 times the rated current, the notches will fuse individually and the entire fuse will blow only if the over- current is maintained for a significant period of time, e.g. greater then one second. This inverse operating characteristic is desirable in order that the fuses be compatible with other protection devices and with the requirements of a high voltage system.

EP-0123331-Al describes end-caps suitable for use in providing electrical connections to a fuse of the type described in EP-0117582-Al. The fuse is designedto success. These fuses generally comprise a quartz glass support body on the outer surface achieve a desired over-current vs fuse time relationship.

A computer based model for predicting the operational behaviour of fuses of the above discussed type is given in an article entitled "Analogue Simulations of Heat Flow in a High Voltage Fuse" by J G J Sloot in Proceedings of the Third International Conference on Electrical Fuses and their Applications, May 11 to 13, 1987.

Whilst the aforementioned computer simulation technique may assist in optimising the design of multiconductor fuses, it remains difficult to fabricate such fuses which have time-current operating characteristics which comply with recognised standards. In particular, it remains difficult to provide a fuse in which the clearance of a fault condition in the low voltage terminal zone of a distribution transformer is sufficiently fast.

It is an object of the present invention to overcome or at least mitigate the disadvantages of known multiconductor fuses. In particular, it is an object of the present invention to provide such fuses which have full range performance with pre-arcing tine characteristics which allow the clearance of low over-voltage fault conditions within recognised standards, e.g. VDE 0670/402 and ESI 12-8.

It is a further object of the present invention to provide a full range fuse which has a predictable arc burnback characteristic during the arcing period of fuse operation.

According to a first aspect of the present invention there is provided a fuse sub-assembly comprising an elongate support body of insulating material and a multiplicity of fusible conductors supported by the support body and extending over the support body, wherein the conductors of one or more group of adjacent fusible conductors are merged together at at least one intermediate region along their lengths so as to form a bridge.

Preferably, the conductors of each of said groups are merged at at least two intermediate regions along their lengths, and more preferably at two intermediate regions.

Preferably each of said groups consists of at least three fusible conductors, and more preferably of three fusible conductors.

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

Preferably, each of the fusible conductors has a plurality of notches formed along its length, where each notch is a section of reduced width. The notches are arranged such that they melt prior to melting of the body portions of the fusible conductors.

Preferably, a spot of low-melting point metal/metal alloy, for example tin or tin-silver, is provided in thermal and electrical contact with some of the bridges and more preferably with at least one of the bridges associated with a group of fusible conductors.

In a preferred embodiment of the fuse sub-assembly of the above first aspect of the invention, the support body is substantially tubular, for example a hollow cylinder, and is preferably of quartz glass. Preferably, the conductors extend helically around the outer surface of the tube. The conductors may be screen printed onto the surface of the tube.

According to a second aspect of the present invention there is provided a fuse comprising an outer casing, a fuse sub-assembly according to the above first aspect of the invention contained within the outer casing, a filler disposed within the casing and substantially surrounding the subassembly, and a pair of electrical contact means making electrical contact with respective ends of the plurality of fusible conductors to enable external electrical connections to be made to the fuse.

For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 shows a perspective view of a fuse subassembly comprising a multiplicity of fuse conductors; Figure 2 shows the fuse conductors of the sub-assembly of Figure 1 opened- out; Figure 3 shows the relative dimensions of a group of conductors of the sub-assembly of Figure 1; and Figure 4 shows the time-current operating characteristic of the fuse subassembly of Figure 1 and also the tine-current operating characteristics of a known multiconductor fuse sub-assembly.

There is shown in Figure 1 a partial section of a fuse sub-assembly for incorporation into a full range high voltage fuse. The sub-assembly comprises a hollow cylindrical quartz glass tube 1 on the outer surface of which are provided a number of helically extending fuse conductors 2 (for clarity the conductors are shown complete only on the right hand side of the fuse sub-assembly of Figure 1). The conductors extend between opposed end regions 3, 4 of the quartz tube and in practice are screen- printed onto the outer surface of the quartz tube by a process such as that described in NI, 8801355. NI, 8801355 also disclosed a method of increasing the thickness of the printed conductors, in order to increase voltage capacity, by electroplating. The conductors 2 merge adjacent each end of the tube 1 into a band of conductive material 5, 6 which is screen printed onto the tube at the same time as the conductors. The bands 5, 6 and conductors 2 are of a fusible material such as silver.

In order to construct a fuse using the sub-assembly of Figure 1, the subassembly is inserted into a cylindrical outer casing which is closed at both ends by end-caps (not shown) which make electric contact with the conductive bands 5, 6 of the sub-assembly. External electrical contacts are made to the fuse conductors via the end-caps and in turn the conductive bands.

Figure 2 shows the conductors 2 of the sub-assembly "opened-out" in order that their geometry can be seen more clearly.

As can be seen from Figure 2, the fuse conductors 2 are formed into a plurality of groups of three adjacent fuse conductors 2 and each group is merged together at two regions near respective ends of the conductors to form "bridges" 8. In the particular arrangement shown in Figure 2, there are fifteen conductors arranged into five groups. The current density at each bridge is determined by the number of merging conductors and the cross-sectional area of the bridge but is typically three times the normal current density in each conductor and approximately the same density as at regular notches 9 in each conductor. The length of each bridge is significantly longer than that of notches 9 and heat transfer from the bridge centres by conduction is considerably less than at a notch.

Figure 3 shows in more detail an end region of one of the groups of conductors 2 associated with the dimensions marked on the Figure in millimetres. The thickness (radial with respect to the quartz tube) of the conductor is typically 6 to SOmm and the spacing between adjacent conductors is at least as great as the width of the conductors, i.e. 1.Omm or more.

Notches 9 are formed in the sub-assembly of each conductor 2 to improve the arcing characteristics and to alter the current-time performance of the sub-assembly. The notches 9 have a width of between one half and one fifth of the width of the body of the associated conductor 2, e.g. 0.25mm as shown in Figure 2. The notches 9 vary in length between 0.5mm and 2.5mm and there are between 2 and 12 notches in each conductor 2.

The provision of bridges 8 for each of the groups of conductors 2 also improves the time-current performance of the sub-assembly. With the above described geometry, for pre-arcing times, i.e. the time taken for all the bridges to melt, less than typically ims, arc initiation occurs simultaneously at both the notches 9 and the bridges 8. For pre-arcing times typically in the ims to 1 second region, arc initiation occurs at the bridges 8 before the notches 9 due to the low heat loss from the centres of the bridges relative to the heat loss from the notches. The period between arc initiation at the bridges and the notches increases with increasing values of pre-arcing time (heat losses increase with time) giving a significant improvement in the time-current characteristics of the subassembly.

The application of a spot of tin 10 (known as a 'Itinspot" or the "Meffect") to the centre of a bridge 8 further improves the pre-arcing characteristics for times approaching one hour. This is because a tinspot melts at a temperature below that at which the silver conductors 2 alone would melt and the molten tin alloys with the silver to produce an alloy having a melting point less than that of the silver. For pre-arcing times nearing the minimum melting condition, the thermal gradient pattern along a conductor 2 changes as heat transfer to the fuse end terminals becomes more significant, the temperature being a maximum near the fuse centre and falling off towards the end regions. By positioning the tin-spot over the bridges and near the ends of the fuse, a longer pre-arcing time is obtained than would be achieved with the tin-spot located at the fuse centre, as is the case with conventional multiconductor fuse sub-assemblies, enabling higher current ratings to be achieved.

The location of the tin-spot near the fuse ends allows the notched sections to reach greater temperatures than would otherwise be attained before melting at the tin-spot occurs. This is known to assist the arc "burn-back" process by reducing the arc energy required to melt the silver, thereby aiding the arcing performance.

In the embodiment shown in Figure 2, a tin-spot is applied to one bridge of each group of conductors 2 in an alternating manner between each end of the fuse subassembly.

A further advantage of the multiconductor bridge configuration is the improved arc control that is achieved when the fuse operates under low current operating conditions, i.e. 1 to 10 times the rated current. With conventional multiconductor configurations either the tinspot or the notches have to melt in all the conductors before commencement of arcing. Commutation of the arc then occurs between conductors at random until the final conductor develops sufficient arc-voltage to suppress the current. With the multiconductor bridge configuration arcing will commence when one bridge in all conductor groups have melted. Commutation of the arc will then continue within one conductor group at the notched sections until it is forced to commutate to another conductor group. This process continues until the final conductor group clears the current.

The dual step process provides improved control of the random nature of the commutation process and reduces the arc energy developed in the 'final' conductor to clear the circuit.

Figure 4 shows the time-current characteristics the multiconductor fuse (A) of the type described in 0117582-A2 and also for a fuse (B) employing multiconductor bridges described above. it Will f or EPthe be apparent that for a given over-current, in the region below about 500amps, the use of conductor bridges linearises the time-current curve and reduces the pre-arcing time by up to an order of magnitude.

It will be appreciated that modifications may be made to the above described embodiment within the scope of the invention. For example, the support body may be planar instead of tubular. Also, the tin-spot may be a spot of tin-silver alloy.

Claims (9)

1. A fuse sub-assembly comprising an elongate support body of insulating material and a multiplicity of fusible conductors supported by the support body and extending over the support body, wherein the conductors are merged together at at least one intermediate region along their lengths so as to form a bridge.

2. A fuse sub-assembly according to claim 1, wherein the conductors of each of said groups are merged at at least two intermediate regions along their lengths.

3. A fuse sub-asembly according claim 1 or 2, wherein each of said groups consists of at least three fusible conductors.

4. A fuse sub-assembly according to any one of the preceding claims, wherein the cross-sectional area of each of the bridges corresponds substantially to n times the cross-sectional area of the notches, where n is the number of conductors in the corresponding group.

5. A fuse sub-assembly according to any one of the preceding claims, wherein each. of the fusible conductors has a plurality of notches formed along its length, where each notch is a section of reduced width, wherein the notches are arranged so that they melt prior to melting of the body portions of the fusible conductors.

6. A fuse sub-assembly according to any one of the preceding claims, wherein a spot of low-melting point metal/metal alloy is provided in thermal and electrical contact with at least one of the bridges associated with each group of fusible conductors.

7. A fuse sub-assembly according to any one of the preceding claims, wherein the support body is substantially tubular, the conductors extending helically around the outer surface of the tube.

8. A fuse comprising an outer casing, a fuse sub-assembly according to any one of the preceding claims contained within the outer casing, a filler disposed within the casing and substantially surrounding the subassembly, and a pair of electrical contact means making electrical contact with respective ends of the plurality of fusible conductors to enable external electrical connections to be made to the fuse.

9. A fuse substantially as hereinbefore described with reference to the accompanying drawings.