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Патент USA US2111628

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March 22, 1938.
H. c. HOBAN
2,111,628
ELECTRIC FUSE
Filed Jan. 15-, 1937
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HUGH CHARLES 'HOBAN
March 22, 1938.
H; c. HOBAN
2,111,628
‘ELECTRIC FUSE
Filed Jan. 15, 1937
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TEME-MINUTES——-———
M/VHWOR
HUGH CHARLES HOBRN
Patented Mar. 22; 1938 '
2,1 1
UNITED ‘STATES PATENT OFFICE
‘ ELECTRIC FUSE
Hugh Charles Hoban, Dartford, England, _assign=-_
or to W. T. Henley’s Telegraph Works Company
Limited, London, England, a British company
Application January 15, 1937;" Serial No. 120,748
In Great Britain January 21, 1936
8 Claims. (01. 200-131)
This invention relates to electric fuses which
comprise a fuse element in the form of a wire or
strip which makes connection at each end to a
terminal member. and is ruptured by the passage
CI of an excessive current.
To ensure that rupture
takes place only at a suitable point, generally at a
Both these constructional features, individu- .
ally and jointly result in the central portion run
ning at a higher temperature than it would other- ‘
wise do. éilonsequently the element fuses at a
current which more nearly approaches its work a
ing current.
The improved construction of fuse element is
it is usual to make the cross‘ sectional area of the not to be confused with elements of the well
part at which it is desired rupture shall take known bimetallic type in which there is present
lo place less than that of the remaining parts, genan insertion of a metal or alloy having a rela~ l0
erally the end or stem portions of the element. iively low melting poirr'?- Such an insertion is so
This has the effect of increasing the resistance . proportioned relative to the adjacent Parts of the
of that portion so that, under working condi- element that it will melt in the event of the sus
tions, it assumes a higher temperature than the
tained overload conditions. Nor is the new con
13 end portions with the result that the working struction to be confused with elements of the 15
current of the element more nearly approaches type in which a central fusible portion is con
_ the fusing current. This is an advantage since nected to stem portions of larger current carry
the fuse will blow at a smaller overload current ing capacity by means of an extremely thin film
than in the case of an element of uniform section.
of soldering metal. In the present case the in
point approximately midway between its ends,
‘20
It is an object of‘ the present invention to provide a fuse element of the type described in which
the ultimate fusing current of the element may
be made to approach Still more closely to the
working current or, in other words, to improve
3;, the fusing factor of a fuse element, the fusing
factor being equal to the ultimate fusing current
divided by the working current.
with this object‘in view we provide a fuse element comprising a central fusible portion and
30 stem portions, in which the central portion is
electrically connected to, and spaced apart to a
predetermined extent from, each stem portion by
an insertion of substantial thickness of a metal
or alloy having a relatively high thermal resist3;, ance, whereby the conduction of heat from the
fusible central portion to the adjacent portions of
high current carrying capacity is reduced. ~
With the same object in view ‘we provide a fuse
element which comprises a central fusible por40 tion of wire or narrow Strip and Stem portions of
relatively Wide Strip. at least Dart of each Stem
P01111011 being in the form of expanded metal’
that is to say, in the form of sheet metal cut and
bent or stretched to form a lattice.
Preferably
45 the Stet?‘ 1mm“? are formed mm a stnp of
sertions between the central and stem portions 20
are made of substantial and predetermined thick
Hess for/She purpose of e?eeting appreciable re
duction in the transmission of heat from the cen
tral portion, in which fusion takes place, to the
stem portions, and they are not intended to, and 25
are so proportioned that they do not, under any
circuit conditions with which they are designed to
cope, melt and break the junctions between the
centre and stem parts of the element before fu
Sion of the central portion occurs.
30
The construction of the junction between the
central and stem parts of the element is impor
tent. If the mass of the insertion metal or alley
is unduly large the thermal resistance instead of
being increased may even be deei‘eased- 0n the 35
other hand if it is too small the effect of the in
Sertion will be negligible Very satisfactm'y re
sults have been obtained by overlapping the cen
tral portion of wire for a distance along the stem
equal to about three times the diameter of the 40
wire and uniting the overlapping portions by an
insertion having a thickness of, approximately,
one. ?fth to one seventh the length of the over_
Ian
_ V
The invention will ‘be further described with 45
metal sht lengthwlse at 8' puniber of tpllaces inter-
the aid of the accompanying drawings which
mediate its ends and having he me a on one or
Show, by way of example’ fuse elements com
both sides of each slit pulled or pressed out of the
normal p1 ane (or other Surface) of the strip to
50 form a lattice. Such a strip has a considerably
higher super?cial area than has a plain strip of
the same cross sectional area“
It is therefore in
more intimate contact with the surrounding cooling medium and, consequently, facilitates the
as transfer of heat from the stem to the medium.
.
h
.
t.
d
structed 1n accordance with t e_1nven ion an a
cartndge type of fuse ?tredlwlth “number of 50
suck} elemeflts- _In the d1:awmgs
Figure 1 is a side elevation of one form of fuse
element. Figure 1A is a fragmentary elevation
partly in section and on an enlarged scale, of the
elements shown in Figure 1,
55
2,111,628
Figure 2 is a plan view of the element shown in
Figure 1,
-
-
Figure 3 is a cross-sectional view, taken on the
line IlI-III of Figure 1, of the element shown
therein.
Figure 3A is a cross-sectional view on an en
larged scale, taken on the line IIIA-IIIA of
Figure 1A, of the element shown therein,
Figure 4 is a perspective view of a second form
10 of fuse element,
Figure 5 is a side elevation of the element shown
in Figure 4,
'
Figure 6 is a plan view of the element shown in
Figures 4 and 5,
15
Figure 7 is a cross-sectional view, taken on the
line VII-VII of Figure 5, of the element shown
therein,
of these places the two outer parts (‘I or I) are
displaced in one direction out of the plane of the
strip and the intervening part (9 or III) is dis
placed in the opposite direction. The outer parts
‘I at one place are preferably displaced in the
opposite direction to the outer parts I at the
adjacent place, as is clearly shown in Figure 4.
Naturally, the number of groups and the num
ber of slits per group may vary and may be 15
more or less than in the example shown in the
drawings.
-
Figure 8 is a cross sectional elevation of one
half of a cartridge fuse ?tted with elements of
the form shown in Figures 4, 5, 6, and 7, and
Figure 9 is an end elevation of the fuse shown
in Figure 8.
Figure 10 is a graph of the relationship be
tween fusing current and time which demon
strates the advantage yielded by the improved
construction of element of the form shown in
Figures 1, 2, and 3.
tive to the strip and is preferably parallel with
the edge of the strip, as shown. The slits of each
group are shown to be in line with the slits of
the other group but this is not essential. It will
be seen that these slits 3 and l divide two short
lengths of the stem into three parts. At each
'
Referring ?rst of all to Figures 1 to 7 inclusive
it will be seen that the element comprises a
30 central portion I' in the form of a wire of circu
lar cross section and stem portions 2 in the form
of thin strips of metal of greater current carry
ing capacity. The central portion overlaps the
stem portions for a distance along the stem equal
35 to approximately three times the diameter of the
wire and these overlapping parts are united by
soldering them together in such a way that each
overlapping part of the wire is spaced apart from
the strip by the solder 3, a distance approxi
40 mately equal to half the diameter of the wire.
A convenient way of ensuring correct spacing is
to form a small upstanding projection on the
extremity of the strip, as shown at l in Figure 1.
The width of the insertion between the overlap
45 ping parts is determined by the diameter of the
wire, the soldering metal being allowed to form
a natural meniscus between the lateral surface
of the wire and the adjacent ?uxed surface of the
strip. It is preferred to form both the central
50 and stem portions of silver. In this case suitable
materials for the thermal resistance insertions
are pure lead solder, and a solder consisting of
an alloy of lead containing a small percentage
of tin.
55
In both examples of construction the central
fusible portion is shown as a wire of circular
cross-section. This form is advantageous in that
it reduces the area of the radiating surface of
the central portion to a minimum but it will be
obvious that central fusible portions of other
form, for instance, wire of noncircular cross
section or narrow metal strip, may Just as readily
be used if desired.
In the example of construction shown in Fig
ures 1, 2, and 3, the stem portions 2 are in the
form of thin ?at strips, but in Figures 4 to '7 in
_ elusive the stem portions 2 are of a modi?ed form
that gives improved heat dissipating properties.
70 In this form of construction, near the junction
of the stern and central portions, the stem has
two groups of slits, each group comprising two
slits, the slits of the one group being shown at
5 and those of the other group at 8 in Figure 6.
75 Each slit extends in a longitudinal direction rela
In Figure 8 is shown one half of a cartridge
fuse employing a number of elements similar to
that described with reference to Figures 4, 5, 8,
and '7; the other half of the fuse, being exactly
similar, has been omitted from the drawings.
The fuse comprises a barrel ii of ceramic mate
rial provided with a number of passages l2 ex
tending from end to end thereof. A metal cap I! 25
is positioned on each end of the barrel II and
is secured thereto by means of a cemented joint
I‘. This cap is provided with apertures corre
sponding to the passages II. In each
e
I! is disposed a pair of elements it. The cen
tral parts I of these elements are bunched to
gether and located in the centre of the passage
but the stems 2 diverge from one another so that
near the end of the passage they lie on oppo
site sides thereof. The extremities of the stems
pass through the apertures in the cap It and are
bent over as shown at It and held in contact
with the outer face of the cap by_ soldering or
welding and/or by means of an outer cap I‘!
which ?ts over the inner cap and carries the do
contact blades l8. The ends of the passages I!
are plugged with asbestos plugs l9 and an as
bestos pad is inserted between the bent over
ends of the elements I! and the outer metal
cap H, in order to reduce the transmission of
heat from the interior of the fuse to the con
tact blades.
The elements shown in, and described with
reference to, the drawings each have only a
single central fusible portion, but naturally each
element may be ?tted with two or more such
portions in cases where it is considered desirable.
The invention is particularly advantageous
when applied to fuse elements of cartridge fuses
where the element is embedded in a material
having a thermal conductivity that is low com
pared with that of the metal of the element.
Normally the major part of the heat generated
in the element will be conveyed to the terminals
of the fuse. By the improved method of uniting
the stem and central portions, this transmission
of heat may be reduced and the heat generated
be dammed in the centre part of the element with
the result that the fusing factor is considerably
decreased. This is clearly shown by the graphs
in Figure 10 in which Curve A shows the rela
tionship between fusing current and the time
taken to fuse for a normally constructed car
tridge fuse nominally rated at 300 amperes and
Curve B the relationship for a cartridge fuse with 70
an element of similar dimensions and rating but
having the central and stem portions united in
accordance with the invention, the stems in both
cases being of the form shown in Figure 1. It
2,111,628
will be seen that whilst the normally constructed
fuse requires approximately 95 minutes tofuse
at a current of twice its nominal rating, the im
proved fuse will blow in less than 12 minutes
under the same overload conditions.
In order to ‘demonstrate the greater heat dis
sipating properties possessed by the improved.
construction _- of stem, tests were undertaken on
two types of elements both formed of strip metal
10 of precisely the same dimensions. In both types
the width of the stem portions of the element
was 0.5 inches and the thickness 0.05 inches and
the width of the central fusible portion was 0.2
inches and the thickness 0.006 inches. In one
15 case the stems were of plain strip and in the
other case the stems were constructed precisely
as shown in-Figure 4. Acartridge fuse ?tted
with a pair of such elements with plain stems
fused with a current of 114 amperes in 17 minutes
2.0 41 seconds. A similar cartridge fuse fitted with
a pair of such elements having stems constructed
' precisely as shown in Figure 4 fused with the
I 3
prising a central portion of wire in which fusion
is initiated and stem portions of metal strip, said
central‘ portion overlapping each stem portion
for a. distance equal to about three times the
thickness of said wire, and means for uniting
the overlapping portions,'comprising‘an inser
tion which has a thickness of approximately one
fifth to one seventh the length of overlap and
is of metallic material having a relatively high
' thermal resistance.
10
4. A fuse element for an electric fuse, compris
ing a central fusible portion of wire orv narrow
strip and stem portions of relatively wide strip,
at least part of each stem portion being in the
form of expanded metal.
'
15
5. ‘A fuse element for an ‘electric fuse com
prising a‘ central fusible portion of wire or nar
row strip and stem portions, said stem portions
each ‘consisting of relatively wide metal strip hav
ing, near the central portion of the element, a 20
plurality of groups of parallel slits extending lon
gitudinally of the strip and having the metal on
both sides of each slit displaced from the normal
same current in 22 minutes 33 seconds. In the
former case. with an ambient temperature of ' surface of the strip to form a lattice.
26 20° C., the temperature of the fuse cap was '71.5°
6. A cartridge fuse comprising a fuse element 25
0., that of the fuse contact 64.2" C._and that of consisting of at least one central fusible portion
the circuit contact 51.5° C. -In' the latter case, of wire 'or narrowstrip and of stem portions
with the same ambient temperature, the tem
which are of relatively wide strip and comprise
peratures at corresponding placeswere consider
parts in thelform of expanded metal, a tubular
80 ably lower, being 62.6'' C. at the fuse cap, 585° C. member surrounding said element, terminal caps 30
at the fuse contact and 47.6° C. at the circuit ‘ mounted on the ends of said member, means for
contact, thus demonstrating that the element
electrically connecting the ends of said fuse’
having stems constructed in accordance with the
element to saidterminal caps, and a cooling me
invention dissipated considerably more heat into dium, in which the stems of said fuse element
the are quenching medium in which the element are embedded, contained in said tubular member.
was embedded. It will be appreciated that this
reduction in temperature of the circuit contacts
is particularly advantageous in cases where parts
of these contacts- are embedded in ‘insulating
40 compounds as is the case with fuse disconnecting
network boxes and the like.
'7. A fuse element for an electric fuse, com
prising a central portion of wire or strip in which
fusion is initiated, stem portions which are of
relatively wide strip and comprise parts in the
form of expanded metal, and means serving to 40
connect said central portion electrically to and
What I claim as my- invention and desire to - space it to a pre-determined extent from, each
secure by Letters Patent is;
1. A fuse element for an electric fuse, com-.
prising a centralportion in which fusion isini
tiated, stem portions and means serving to con
nect each end of said central portion electrically
to, and space it apart to a predetermined extent
. of said stem portions, each of said means-being >
an insertion of substantial thickness of a metallic
material having a relatively high thermal re 45
sistance.
_
8.‘ A fuse element for an electric fuse, includ
ing a central portion in which, under‘ any‘ cir
from, the, adjacent stem portion, each of said cuit conditions with which the fuse is designed‘
50 means being an insertionof substantial thick
to cope, fusion is initiated, a pair of stem por 60
ness of a metallic material having a relatively tions, and a pair of heat barriers, consisting of
high thermal resistance.
' ‘
metallic material of which the. thermal resistance
2. A fuse element for an electric fuse, com-v is ‘relatively high compared with that of the ,
prising a central portion of silver injwhich central and stem portions, one of said barriers
55 fusion is initiated‘, stem portions of silver and
means for electrically connecting said stem poix
tions to the ends of said central portion, each
being inserted between, and connecting together,_ 55
one end of said central portion and one of said ,
stem portions and the other of said barriers be
of said means- comprising an insertion of sub-' ing inserted between, and connecting together,
stantial thickness of metallic material having a the?oth‘er end of‘ said central portion and- the
thermal resistance considerably higher than that
of silver.
_
r
.
,
'
3. A fuse element for-an electric fuse, com
other‘of said‘ stem- portions.
'
noon CHARLES HOEAN. ;
60
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