[0001] This invention relates to protective electric fuses, that is to say devices of the
kind comprising a member of readily fusible electrically conducting material which
is arranged to be connected in series with the circuit to be protected and, on the
passage of excessive current, is arranged to melt and thereby interrupt the current
path.
[0002] The invention is more especially, though not exclusively, concerned with such fuses
which are incorporated within electric incandescent lamps.
[0003] According to the invention, in a protective electric fuse of the kind referred to
the fusible member is formed with at least one region of relatively smaller cross-section
between two regions of relatively larger cross-section, and the fuse incorporates
a quantity of electrically insulating material in the form of grains adhering to each
other around the or each region of smaller cross-section, and covered by a protective
coating.
[0004] On the passage of an excess current, as by the occurrence of a short circuit, the
fuse will overheat and rupture at a region of smaller cross-section because of the
greater current density at that region.
[0005] The granular material serves to provide a large surface area to cool the hot metal
vapour evolved on rupturing the fuse, provides that the metal film formed by condensation
of the metal vapour is not continuous from end to end and conveniently comprises or
contains a substance which, when heated by the arc which is formed when the fuse operates,
liberates an electro-negative gas, such as fluorine. Calcium fluoride is one such
material.
[0006] The protective coating serves, not only to protect the volume of granular material
from physical damage, but also to retain the hot gases and plasma formed in the course
of fuse operation. Preferably therefore the coating extends beyond the granular material
into contact with the surfaces of the regions of the fusible material of relatively
larger cross-section. In such a case it is desirable to arrange for, or to compensate
for the lack of, an expansion match between the materials of the coating and the fusible
member. Suitable materials for the coating may be vitreous, for example similar to
those used for coating wire-wound resistors.
[0007] Fuses in accordance with the invention have the advantage that they can readily be
fabricated by a continuous manufacturing process. For example in one such process
a series of waisted regions are formed along a continuous length of said readily fusible
material, initially having a cross-section equivalent to that of said regions of larger
cross-section, each of the waisted regions is coated with an appropriate quantity
of the granular material, each deposit of granular material is covered by said protective
coating, and the continuous length of fusible material separated into individual fuses.
[0008] Conveniently the fusible member is in wire form. Apart from mechanical methods, such
as drawing, swageing, pressing and ring-rolling, there are chemical and electrochemicals
techniques which can usefully be employed to form the waisted regions. For example
the member may be in the form of a Wollaston wire which has a central core of a chemically
resistant alloy surrounded by a layer of a metal or alloy which can readily be dissolved
in an etching bath. The parts of the wire which are intended to remain at the full
diameter are provided with a protective coating, such as a lacquer, which may be applied
by any suitable technique, and following the etching away of the coating in the intervening
region the lacquer is removed, the process being completed by the application of the
granular material and protective coatings to the waisted regions, and the separation
of the assembly into individual fuses.
[0009] The end regions of the fusible member should be of sufficiently large cross-section
that they do not melt under any conditions of the fuse's operation, including short-circuit
current clearance, and should also be sufficiently strong to support the weight of
the fuse and any electromagnetic forces produced. They should also be readily connectable
into the circuit to be protected, as well as playing a useful part in conducting away
the heat dissipated under normal load current in the thinner intervening part of the
member.
[0010] The dimensions of the region of smaller cross-section will be determined mainly by
the required operating characteristic of operating time versus current, and by the
length of arc column which is required to introduce impedance into the circuit. The
region need not, however, be of uniform cross-section along its length, as certain
kinds of non-uniformity can provide useful characteristics, such as causing multiple
arcs to form in series, so yielding an increased voltage drop.
[0011] Fuses in accordance with the invention are especially suited for incorporated into
electric incandescent lamps.
[0012] The fabrication of the fuses by a continuous technique significantly reduces the
cost of manufacture, compared with that required to produce existing types of fuses
employed for such a purpose.
[0013] One fuse in accordance with the invention for use in an electric incandescent lamp
will now be described by way of example with reference to Figures 1 and 2 of the accompanying
drawing, in which
Figure 1 shows a sectional view of a lamp incorporating the fuse, and
Figure 2 illustrates an enlarged sectional view of the fuse itself.
[0014] Referring to Figure 1, this shows part of the envelope 1 of an electric incandescent
lamp, closed in the usual manner by a pinched foot tube 2 through which are sealed
a pair of conductors 3, 4, these being connected internally to the lamp filament 5.
[0015] The lamp is fitted with a cap 6, and one of the conductors 3 extends through an insulating
filler 10 closing the base of the cap and is secured externally by a solder deposit
7 which provides one of the lamp contacts. The other conductor 4 is connected, as
by clamping, to one end of a fuse F constructed in accordance with the invention,
the other end of the fuse being similarly connected to a further length of wire 8
which extends through the insulating filler 10 and is secured by a further deposit
of solder 9 which forms the second lamp contact.
[0016] The fuse, which is shown in greatly enlarged form in Figure 2, comprises a length
of wire 11 of a suitable metal or alloy having a waisted central portion 12 between
two thicker end portions 13. A volume of a non-electrically-conducting granular material
14 is packed around the wire 11 so as to completely enclose the waisted portion 12.
The surfaces of the particles are such that where they touch they adhere. The adhesion
may be of the kind that is promoted by one of various outside influences, such as
heat, ionising radiation or ultraviolet light.
[0017] On the passage of excessive current, as may be caused by a short circuit, the wire
will melt at the waisted portion which is surrounded by the particulate material.
[0018] The particles of the latter are conveniently formed of calcium fluoride which will
liberate fluorine when heated by the arc produced following the rupturing of the fuse
wire, the fluorine being an electro-negative gas which reduces the ionisation of the
arc plasma and rapidly extinguishes the arc. Other materials which liberate an electro-negative
gas when heated may, however, be used.
[0019] The fuse is completed by a gas-impermeable coating 15 which completely encompasses
the particulate material and extends beyond it into contact with the surfaces of the
end portions 13 of the fuse wire 11. The coating, which is conveniently of a vitreous
material as commonly used for coating wire-wound resistors, serves to protect the
particulate material and to contain the hot gases and plasma formed in the course
of the fuse operation.
[0020] Fuses as above described can conveniently be formed by a continous process, a length
of wire, initially of a diameter corresponding to that of the end portions 13, has
a series of waisted portions 12 formed in it, each of the waisted portions being coated
with an appropriate quantity of the particulate material 14 which is then surrounded
by the coating 15, the wire then being separated into the individual fuses. The wire
conveniently comprises a form of Wollaston wire which has a central core of chemically
resistant material surrounded by a layer of material which can readily be dissolved
by an etching fluid. The parts of the wire which are intended to remain at full diameter
are initially coated with a protective lacquer, which may be applied where needed
by an electronically controlled sprayer. Following the removal of the unprotected
regions of the outer layer the lacquer is removed, the intervening waisted portions
are coated with the calcium fluoride particles and the protective coatings, and the
continuous length of wire separated into individual fuse elements.
[0021] Although a fuse of the form above described is particularly suitable for use in electric
incandescent lamps, it could also be used to advantage on printed circuit boards,
and, with the addition of suitable connecting means, for general use.
1. A protective electric fuse of the kind referred to wherein the fusible member is
formed with at least one region of relatively smaller cross-section between two regions
of relatively larger cross-section, and the fuse incorporates a quantity of electrically
insulating material in the form of grains adhering to each other around the or each
region of small cross-section, and covered by a protective coating.
2. A protective electric fuse according to Claim 1 wherein the granular material comprises
or contains a substance which, when heated by the arc which is formed when the fuse
operates, liberates an electro-negative gas.
3. A protective electric fuse according to Claim 2 wherein the granular material comprises
or contains a substance which, when heated by the arc which is formed when the fuse
operates, liberates fluorine.
4. A protective fuse according to Claim 3 wherein the granular material comprises
or contains calcium fluoride.
5. A protective electric fuse according to any preceding Claim wherein the protective
coating extends beyond the granular material at each end into contact with the respective
region of the fusible material of relatively larger cross-section.
6. A protective electric fuse according to any preceding Claim wherein the cross-section
of the region of relatively smaller cross section is not uniform along its length.
7. A protective electric fuse according to any preceding Claim incorporated in an
electric incandescent lamp.
8. A method of manufacturing a fuse in accordance with any preceding Claim comprising
the steps of forming a series of waisted regions along a continuous length of said
readily fusible material initially having a cross-section equivalent to that of said
regions of larger cross-section, coating each of said waisted regions with a quantity
of said granular material, covering each said deposit of granular material with said
protective coating and separating the continuous length of fusible material into individual
fuses.
9. A method according to Claim 9 wherein the fusible member is in the form of wire
and the waisted regions are formed by a mechanical, chemical or electrochemical technique.
10. A method according to Claim 10 wherein the fusible material is in the form of
Wollaston wire, and the method includes the steps of coating the parts of the wire
which are to form said regions of larger cross-section with a protective covering,
dissolving away the outer layer of the wire between said coated regions by etching,
to form said waisted region, removing the protective covering, applying said granular
material and protective coatings to the waisted regions, and separating the assembly
into individual fuses.