Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELFCTRIC FUSES EMPLOYING COMPOSITE
METAL FUSE ELEMENTS
Field of the Invention
The present invention is concerned with imporovements
in or relating to electric fuses employing composite metal fuse
elements, and especially to such fuses of high voltagP current
limiting type.
Review of the Prior Art
It is known practice to protect an electric circuit by
means of two different fuses, one of which is a current limiting
fuse that will interrupt fault currents from its maximum
interrupting rating to its minimum interrupting rating, and the
other of which is a so-called weak lin]c expulsion fuse that will
interrupt fault currents a value from slightly above the minimum interrupting
current rating of the current limiting fuse. Obviously it is desirable to eliminate
the practice of using two fuses, but the design of fuses for
interrupting low currents just above (e.g. two times or more)
the maximum current rating of the fuse has been a constant problem
to the fuse designers, and has added substantially to the
complexity, size and cost of the fuses.
Fuse elements for such fuses commonly consist of one or
more strips or ribbons of metal mounted in a suitable casing, and
the design of such a fuse element requires the careful choice of
different parameters among which are the metal from which the
element is made, the dimensions of the strip or ribbon, whether
or not the strip is notched or provided with eutectic spots
(Metcalf effect) along its length; whether or not the ele~ent is wound on a
ceramic or deionizing gas producing core; whether or not the element consists of
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tw~ differentn~tals oonnected in series, and ~e choice of the matPrial
surrounding the el~t. In a specific ~xample, the ribbon may be of silver
and provided alony its length wi~h up to about 100 notches,or holes,
each of which i5 the potential site for melting and the initial
formation of an arc; the element is completely buried in quartz
sand which acts to abso.rb the energy generated by the arcs, and
also to receive the melted element material.
The choice of the me~al to be used is always difficult,
since each metal usable in commercial practice has its own
advantages and disadvantages. For example, silver has a desir-
able high conductivity and resistance to oxidation, but has a
high melting point ~960C), and a high heat of evaporation and
is costly. When spots of tin are soldered along the silver
element to make use of the so-called Metcalf or M-effect a
eutectic alloy is formed, the meltinq temperature being lower at
the spot (approximately 230C) to make the fuse applicable for
low current operation, but such spots~ exhibit with time a non-
reversible change under the effect of non-melting current flows
that can lead to damage of the fuse. Additionally, while the spot
initiates a single melt and subsequent arc at its location,
approximately 700C greater temperature is required to result
in further melting of the silver sufficient to interrupt the high
voltage circuit. The added time required for the small overcurrent
to produce the much higher element temperature limits the effect-
iveness of the d~sign.
Cadmium is a low melting point metal (321C) with a vexy
low temperature of evaporation (750C). It has an excellent
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arc extinguishing characteristic and therefore it is widely
used in electrical contacts. Moreover, it has very high burn-
back rate and is very convenient for interruption of low
currents. Cadmium has low conductivity and current carrying
capacity while the resultant cadmium ox.ide is a very good
insulator.
Zinc is a low melting point metal (419C) that is
resistant to oxidation, has a high burnback rate and has a non-
linear coefficient of resistivity, which is useful, but has a
conductivity 3 - 4 times lower than that of silver. Other metals
and alloys thereof show some disadvantages when all of the
necessary characteristics are evaluated.
Alum.inum has a high current capacity and low melting
point (.658C) and the oxide produced is non-conductive, which are
all desirable, but the oxide film prevents disbursement of the
melted metal into the surrounding sand and the melting charac~ex-
istic for low currents applied for long t.imes becomes inconsistent.
It is therefore an object of the present invention to
provide a new electric fuse employing a new composite mat~rial
as the fuse element.
In accordance with the present invention there is
provided an electric fuse for use in circuits of at least 1000
volts and of the current limiting type, comprising:
a tubular housing of insulating material;
two spaced terminals mounted on said housing for .
connection of the fuse in an electric circuit;
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at least one metal fuse element mounted within the
housing with the two ends of each element connected respectively
to the said two terminals to form a respective conducting path
therebetween;
each fuse element being embedded in and surrounded
by silica sand disposed within the housing;
characterized in that:
each fuse element comprises a cadmium portion and an
aluminum portion, each of which provides a corresponding
continuous current carrying path between the said terminals;
each metal being pxesent in the fuse element in an
amount not less than 3% by ~olume of the total;
t.he said portions: being bonded to one another at
adjoining contacting surface~ to constitute a composite metal
element wherein t~e cadm~um portion due to its lower melting
point will melt before the aluminum portion to increase the
current density through the unmelted aluminum portion.
Description of the Drawin~s
Particular preferred embodiments of the invention
2Q will now be described by way of example, with reference to
the accompanying diagrammatic drawings, wherein:
Figures 1 to 4 are respective perspective views
of preforms from which fuse elements of the invention can
be formed~
Figure 5 is a perspective vie~ of one form of
fu~e con~tructed accoxding to this invention~
Figure 6 i5 a longitudinal cross-sectional vie~
of the structure of Figure 5, parts the.reof being shown
broken away as necessary for clarity of illustration,
and
Figure 7 is an enlarged view depicting the specific
details of construction of the fuse elements shown in Figure 6.
2Q
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Description of the Preferred Embodiments
A fuse element for use in an electric fuse of the
invention consists of a-t least two separate metals, each of which
is present in the form of at least one so-called separate fuse
element portion, and preferably the different fuse element
portions are metallurgically bonded to one another at their
adjoining surfaces where they contact one another to form in
effect a composite metal body.
For example, referring especially to Figure l, the fuse
element preform illustrated therein consists of two thin flat
portion~ lO and 12, each of which has the form of a thin flat
strip or ribbon having two parallel wider faces and two parallel
narrower faces or edges. The two strips are placed face to face
and metallurgically honded to one another by, for example, co-
extrusion, cold rolling or by hot rolling at below the meltingtemperature of the lower melting material. In another method the
portion of lower melting temperature metal is formed by casting
against the body portion of the higher melting temperature metal,
the resulting composite body then being extruded, cold or hot
rolled, etc.
In another embodiment illustrated by Figure 2, more than
two separate portions are employed (three in this example); the
metal of the portions lO and 14 can be the same, in which case
the portion 12 is sandwiched between two identical other portions r
or they can be of different metals. In the embodiment illustrated
by Figure 3 one portion consists of a plurality of uniformly-
spaced metal wires or rods lO which are enclosed in the second
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body portion 12 by cas-ting the latter metal around them. The
r~sultant rod or wire can then be rolled or extruded as required.
In the embodiment illustrated in Figure 4 a single body
portion 10 is enclosed by the other metal portion 12. The
metallurgical bonding of the two body portions at their abutting
surfaces is further increased by hot rolling the cast body.
Each of the preEorms illustrated is processed, for
example, to give it the specific dimensions necessary for fuse
elements; notching and mounting the element between a pair of fuse-
terminals; and embedding the element in a sui~able surroundingmedium, such as quartz sand, in a suitable container.
Referring now to Figures 5 to 7, there is illustrated
therein an electric fuse consisting of a tubular housing 16 of an
insulating material, provided with end caps 18 and 20 of a
suitable conducting material at each end thereof. Outer caps
22 and 24 are secured about the end caps 18 and 20 respectively by
a press fit and are secured to the tubular housing 16 by cement
layers 26 and 28 respectively. An end terminal sleeve 30 and an
end terminal cap 32 are fastened respectively to the inner surfaces
of end caps 18 and 20, and the housing is filled with a granular
filler consisting of silica sand 34. Disposed within the housing
of the fuse and embedded within and supported by the sand filler are
a plurality (5 in this embodiment) of coaxial helical fusible
elements 36 through 44, each of which has its two ends connected
respectively to the terminal 3Q and 32.` As is apparent from
Figure 3 the helical fusible elements are each provided along its
length with a large number o~ spaced notches 46
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, . . .
It is found that in meeting the special requirements
of a fuse element, the properties of this composite metal
element are not simply the mean values for those of the two
constituents, but are complex and differ in important respects
thereErom. The temperature/time charact~ristics of the composites
of the invention are characteri~ed by two different stages. The
initial stage is a normal exponential increase of temperature
with time as the fuse is subjected to its normal load current.
Wllen an overload is present the temperature of course increases,
and upon xeaching the melting temperature of the low~r melting
component, there will be a rapid increase of temperature with
time, due to a reduction in the cross sectional area of the
element caused by successive melting of the lower temperature
component and consequent increase in the current density through
the rernaining component. This marked increase in characteristic
at a specific point permits a more accurate predetermination of the
fuse melting characteristic, without this characteristic
being unduly afEected by aging or pre-melting temperature changes
of the fuse element, resulting for example, from current surges
pa~sing through it. The temperature/time characteristic of the
composite is therefore al~ays reversible up to the temperature
at which melting of the lower melting component begins , whereas
by comparison the characteristic of a silver element with a tin
eutectic spot was found to be irreversible thus leading to
eventual damage.
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A
The metals employed in a composite fuse element of the
invention are specified as being different as to their electro-
thermal properties, by which are meant any one or more of their
characteristics; resistivity, thermal conductivity, melting
point, boiling point, heat of fusion, and heat of evaporation.
It will be understood that different metals may have such similar
electrothermal properties as not to be suitable for application
of the invention. The different portions of the element have as
intimate an interface as possible, in order to obtain the best
possihle electrical and thermal conductivity between the
metals without having the undesirable interaction of two metals
during the premelting time.
Each metal present in the composite should be present
in an amount not less than 3% by volume of the entire element
body, since otherwise there will not be sufficient present to
significantly affect the properties of the composite. It will
be apparent that each metal must be present in the form of a
separate body or plurality of bodies that will extend through
the intend~d melt and arc zone of the fuse element in the
direction of flow of the current therethrough~
When two metals are employed in the composite one of
them ~ill be of high conductivity and high melting point namely
aluminum in the preferred combination, while the other is of
low melting point namely cadmium, so that element mel~ing i5
initia~ed at any and all locations along the element which
reach the melting temperature of the low melting point constituent
starting, of course, at the notches 46.
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It is found that a fuse element of the form illustrated by
Figure 2 is preferred, in which a high meltiny temperature
strip is sandwiched between two low melting temperature strips.
It is also found that there is a preferred ratio of width to
thickness of each strip, and this should be about 10:1, and
may of course vary between say ~:1 and 12:1. An 80 amp fuse
as described above will typically employ 12 helical elements
connected in parallel each measuring about 2.5mm by 0.2mm.
The silica sand filler 34 preferably is in the form
of approximately spherical ~rains of random size within a given
range. These grains preferably are composed of at least 99~
silica and approximately 98~ of the grains are retained on sieve
mesh size 100 while approximately 2~ of the grains are retained
on sie~e mesh size 30, Approximately 30% of the grains are
retained on sieve mesh size 40 while approximately 75~ are
retained on sieve mesh size 50. The pellets are identiied as
109 G.S.S~
In the e~ent of the occurrence ofa high magnitude
fault current such as many times rated load current, the fusible
elements 36-44 melt practically simultaneously at all of their
reduced sections 46 to form a chain of arcs. These arcs
quickly lengthen and burn back from their roots. The energy of
the arc in the form of heat is absorbed by the filler material
in the granular form 34. The exchange of energy between the arcs
and the filler material is influence~ by the surface area of
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filler grains which is exposed to the arcs. The ~reater the area
of this exposure the more efficient is the exchange of energy.
This factor is facilitated by the use of the filler described
and by the fact that the fusible elements are of ribbon -form
and that they are arranged as multiple elements rather than as
one single element, although the invention in its broader aspects
is not limited to a fuse using a plurality of parallel connected
fuse elements.
Since the invention is concerned with high voltage
currents of 1,000 volts and abov~, it is herein categorized as
a high voltage fuse.
A fuse constructed according to this invention is well
suited for use in protecting circuits and their connected apparatus
such as transformers, capacitors, switchgear and the like. By
the invention a fuse is provided which is capable of effective
fast acting current limiting action for currents of high magnitude
and which also operates reliably for low currents which are but
slightly in excess of the normal rated current of the fuse due in
part to the fact that the fusible elements may be raised by
relatively low fault currents to temperature levels approaching
melting without establishing an excessively high overall fuse
temperature~
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It will also be noted that the preferred illustrated
fuse is of coreless design which is to be preferred~ In addition
to their expense, cores are objectionable because contact with
the fusible element reduces the area over which energy exchange
between the arcs and the filler material can take place. Since
the interrupting process requires that most of the arc energy
be transferred to latent heat of fusion of the filler material
an~ reduction of the area of contact with the filler material
is undesirable. Moreover, the areas of contact between the
elements and core can produce high temperatures in the core.
The ceramic materials commonly used exhibit marked reduction
in their insulating properties at such elevated temperatures.
This reduction in insulating property of the core results in
a non-uniform voltage distribution across the fuse in the
period following arcing.
Under certain transient current conditions, an
appreciable temperature rise in the fuaible elements may occur
and may cause a de~ormation of the fusible elements. Repeated
heating and cooling cycles may impose increasing tensile load
on the fusible elements since they may not straight~n out due to
construction of the sand. If movement of the elements is
possible, as in a coreless construction~ thls tension may be
relieved. In elements wound on a core, the opportunity for
relieving tension is severely restricted and mechanical failure
due to this increased tension may occur, since the increases may
be sufficient to break the fusible element, particularly at the
reduced cross section notches.
