Note: Descriptions are shown in the official language in which they were submitted.
35~;~
Case No. 274-1027-00
TIME LAG ELECTRICAL FUSE
Background of the Invention
This invention relates to an improvement in
protective devices for el,ectrical circuits, and more
particularly to an improvement in electrical fuses.
S Electrical fuses are provided for interrupting
electrical circuits ~ubj ected to various types of abnormal
increased current flow. An abnormal increase in current
flow up to six times the normal current flow is considered
an overload current. Current flows of still higher
magnitudes are considered short circuit currents., ~ut for
the inclusion of a fuse in a circuit, short circuit
currents in the circuit may be limited very little in
magnitude by the remaining circuit elements. Depending on
the electrical circuit to be pro-tected, separate
electrical fuses have been provided in the past for
protecting circuits against overload currents and short
circuit currents. E`urther, in the case of overload
; currents, it has been found desirable to control the time
' delay, following the initiation oE the overload current,
after which the fuse interrupts the overload current flow.
The limiting o~ such time delay may be important to
~ protect the circuit elements, while lengthening the time
`~ delay may be desirable in circuits which are expected to
experience temporary overload currents of limited
duration. Such overload currents may be experienced in
motor starting and upon first energizing a transformer~
In electrical circuits which must be provided with
protection with respect to both overload currents and
short circuit currents, such protection has sometimes been
provided in the past by separate fuses, one providing the
~, protection against overload currents, and the other
providing the protection against short circuit currents.
Fuses have been constructed which provide both types of
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protection. However, the construction of some such fuses
have been quite comple~ and therefore quite costly.
Further, when such a dual purpose fuse is deslgned to meet
particular overload and short circuit current interrupting
05 specifications, particular components of the fuse would
have to be changed to vary the overload or short circuit
current interrupting characteristics.
Summary of the Invention
The present invention provides an improved
composite fuse which comprises in combination a time delay
overload and a short circuit device connected electxically
in a series circuit arrangement. The composite fuse has
enhanced operating characteristics which are not provided
by either of the constituent devices separately. In
accordance with the present invention, an electrical
heating means is connected in series with and located in
immediate proximity to an overload time delay fuse device~
In the presence of an overload current, the heating means
provides increased heat transfer to the overload fuse
device, so as to provide quicker interruption by the
overload device in a desired current range. A short
circuit fuse device connected in series with the time
delay overload device provides the desired interruption of
short circuit currents. Further, by containing the time
delay overload device and the short circuit device in an
insulating housing containing electrically insulating fill
material, a composite fuse construction is provided which
will reliably operate at a voltage higher than that for
which either of the component fuse devices are rated.
By way of example, a time delay fuse may be
particularly designed to conduct 200% of its rated current
for 12 seconds before opening to interrupt current flow in
the circuit which it protects. ~owever, the same time
delay fuse would conduct 500~ of rated current for only
1/10 of a second before opening to interrupt the protected
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circuit. In a typical time lag fuse application, it is
desirable for the fuse to conduct 500~ of rated current
for 10 seconds. If it is desirable to provide a time lag
fuse, rated at 10 amperes, which will conduct 500% of
05 rated current or 50 amperes for 10 seconds, it has been
found that a time delay fuse rated at 25 amperes will
provide the desired interruption of 500~ of rated current,
i.e. 50 amperes, after 10 seconds delay. However, the 25
ampere time delay fuse would not open for a 135~ or 13.5
ampere overload current. In order to provide the desired
current interruption at 135~ of rated current, an
electrical heating means is connected in series with the
time delay fuse rated at 25 amperes. The electrical
heating means is located in immediate proximity to the
time delay fuse. The electrical heating means is designed
to transfer sufficient heat to the time delay fuse to
cause it to open as desired for 135% overload currents,
i.e. 13.5 amperes. However, the heat transfer path
between the electrical heating means and the fuse is such
that with overload currents above 500%, and with short
circuit currents, the heat transfer from the electrical
~ heater to the time delay fuse is sufficiently delayed,
; that it does not have a significant effect on the time
delay before fuse operation interrupts the circuit.
Commercially available time delay fuses having
current ratings in the desired range for use in time lag
fuses in accordance with this invention typically have
voltage ratings below 250 volts, some as low as 32 volts,
for larger current ratings. However, the desired voltage
rating of the composite time lag fuse of this invention is
typically over 250 volts, and may even be as high as 600
volts. By connecting the time delay fuse and electrical
heating means in series with a short circuit fuse device
in an end to end or overlapping relationship, all within
an elongated insulating housing filled with an
electrically insulating arc suppressing medium, it is
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02 possible to ~rovide a time lag fuse having the desired
03 higher voltage rating, such as 250-500 volts, while using
04 a time delay fuse having a much lower vo:Ltage rating, such
05 as 32 to 250 volts.
06 By constructing the composite time lag fuse
07 utilizing both a readily available overcurrent time delay
08 fuse and a readily available short circuit fuse, it is
09 possible to provide the desired overload and short circuit
fusing characteristics in a device requiring a lesser
:11 number of specially made components and at a lesser cost
12 than would be experienced, were the readily available
13 overload time delay and short circuit devices not
14 utilized.
It is therefore an object of the present
~16 invention to provide an improved composite time lag fuse
17 which provides desired overload time delay and short
18 circuit interruption characteristics, which is assembled
~19 from separate overload time delay and short circuit
devices.
~1 A further object of the invention is to provide
~:22 a composite fuse device utilizing separate overload time
.23 delay and short circuit devices which has enhanced time
24 lag operation characteristics.
A still further object of the invention is to
26 provide a composite time lag fuse comprising separate
~27 overload time dela~ and short circuit elements which is
28 capable of operating at voltages greater than thGse of
29 either of the separate composite devices.
More particularly, a preferred embodiment of the
31 invention is a time lag fuse comprising a time delay fuse
32 and at least one electrical resistance heating structure
33 connected in a series circuit with the time delay fuse,
34 and located in intimate heat transfer relationship with
the time delay fuse. A short circuit fuse is connected in
36 series in the series circuit, the short circuit fuse being
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02
03 enclosed in a first elongated insulating housing having
04 first and second ends. First and second electrically
05 conductive end caps are secured to and close the first and
06 second ends respectively of a ~irst insulated housing.
07 The first and second electrically conductive end caps are
08 connected in the series circuit. A second elongated
09 hollow insulating housing having first and second ends,
encloses the time delay fuse, the electrical resistance
11 heating structure and the short circuit fuse. Third and
12 fourth electrically conductive end caps are secured to and
13 close the first and second ends respectively of the second
14 insulating housing. The second circuit is connected
between the third and fourth end caps. The end caps are
16 connected in series wi-th a circuit to be protected, such
17 that overload current flow in the at least one electrical
18 resistance heating structure will result in the generation
~;19 and transfer of heat from the at least one electrical
resistance heating structure to the time delay fuse, to
21 cause the time delay fuse to interrupt, after a
22 predetermined time delay, an overload current flow in the
23 circuit to be interrupted, which overload current is of a
24 lesser magnitude than would otherwise be interrupted by
the time delay fuse.
26 Brief Description of the Drawin~s
27 FIGUR~ 1 is a side view of a prior art time
28 delay fuse which is utilized in the composi-te fuse
29 construction of the present invention,
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FIGU~E 2 is a side view of the prior art time
delay fuse shown in FIGURE 1, provided with heating coils
in accordance with the present invention;
F~GURE 3 is a partial cross sectional view
05 showing the composite fuse of the present invention
incorporating the subassembly of FIGURE 2,
FIGURE 4 is a sectional side view of a second
embodiment of the present invention;
FIGURE 5 is a sectional side view of a third
embodiment of the present invention;
FIGURE 6 is a sectional side view of a fourth
embodiment of the present invention;
FIGURE 7 is a sectional side view of a fifth
embodiment of the present invention;
FIGURE 8 is a sectional side view of a sixth
embodiment of the present invention;
FIGURE 9 is a sectional side view of a seventh
embodiment of the present invention;
FIGURE 10 is a sectional side view of a eighth
embodiment of the present invention.
Detailed Description of the Preferred Embodiment
Throughout the FIGURES of the drawings, similar
components of the various embodiments are identified by
the same reference numerals.
Referring to the drawing, and initially to
Figure 1, there is illustrated a slow blow or time delay
type fuse generally designated by the reference numeral
10. Fuses of this type are commercially available from
the assignee of the present application, being sold under
the model type designators "MDL" and "MSL". The operating
element of the fuse 10 includes a body 12 of fusing alloy,
which is contained within a supporting member 14, which
may be formed of brass or copper. A pair of conductive
elements 16 and 18 are embedded in a spaced apart
relationship in the body 12 of fusing alloy. A tension
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spring 20 surroundin~ the conductor 16, engages at its
right end enlarged portion 22 of the supporting member 14.
The fuse components just described are placed in a glass
tube 24 with the conductor 18 being electrically and
05 physically connected to a conductive end cap 26 on the
right end of the glass tube ~4. The conductor 16 is
electrically and mechanically connected to conductivè end
cap 28 on the left end of the glass tube. An electrical
circuit is thus formed between end caps 26 and 28 by the
10 conduc~ors 16 and 18, and the body 12 of fusing alloy. The
left end of tension spring 20 is secured in the region of
end cap 28, such that a force is provided on supporting
member 14, urging it to the left. When an overload
current flows through the conductors, 16 and 18, and body
12 of fusing alloy for a sufficient time to raise the body
of fusing alloy 12 to its melting temperature, the fusing
alloy will lose its grip on the conductors 16 and 18,
permitting the spring 20 to pull the supporting member 14
to the left. As a result of the displacement oE
supporting member 14, the fusing alloy bridge between the
inner ends of conductors 16 and 18 will be removed, thereby
opening the electrical circuit between end caps 26 and 28.
It has been found desirable to provide a time
lag fuse which will open at a predetermined low overload
25 current, i.e. for instance 135~ to 200%, in a shorter
period of time than the fuse 10 of FIGURE 1 is designed to
open. This desire is met, in accordance with the present
invention, by providlng the fuse 10 of FIGURE 2 with a
pair of heating coils 30 and 32. An insulated wire 34
comprising a center conductor 35 and an insulating cover
37 is ~ound around the glass tube 24 just to the left of
the end cap 26, so as to encircle the conductor 18. One
end of conductor 35 is soldered or welded at 36 to end cap
26. Similarly, at the left end of FIGURE 2, an insulated
wire 38 comprising a center ~onductor 40 and an insulating
cover 42 is wound around the glass tube 24, so as to
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encircle the conductor 16. The conductor 40 is attached
to ~he end cap 28 ~y soldering or welding at 44. Two
heating coils 30 and 32 are provicled, rather than just one
longer one, so as to maintain the necessary spacing of
- 05 conductive elements to prevent undesirable voltage
: breakdowns between conductive elements of the fuse
- assembly.
The subassembly, shown in FIGURE 2, is assembled
into a time lag fus~ 45, constructed in accordance with
this invention, as shown in FIGURE 3. An insulating
housing 46 of sufficient internal volume to enclose the
subassembly of FIGURE 2 is provided with conductive end
caps 48 and 50~ The subassembly of FIGURE 2 is centered
wi-thin the lnsulating housing 46, with conductor 35 being
electrically and mechanically secured to end cap 50 by
soldering or welding material 52. Similarly conductor 40
is electrically and mechanically connected to end cap 48
by soldering or welding material 54. That portion of the
space within the insulating housing 46, which is not
occupied by the subassembly shown in FIGURE 2, is filled
with an electrically non-conductive granular fill material
~ 56, such as sand.
: The time lag fuse assembly 45, shown in FIGURE
3, provides the desired time lag fuse performance
characteristics which are not provided by the fuse 10 of
FIG~RE 1. The conductors 35 and 40 are formed of
resistance wire, chosen to generate, for transfer to the
fuse 10, a predetermined amount of heat at a particular
overload current so as to.cause the fuse 10 to open faster
than it would with the same overload current flow without
the heating coils. A further enhancement of the operating
characteristics of the composite time lag fuse 45, shown
in FIGURE 3, over the time delay fuse 10, shown in FIGURE
1, is provided by the housing 46 and the granular fill
material 56. The provision of the housing 44 and the
electrically insulating granular fill material 56 ensures
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proper operation of the device at higher voltages than
would be permitted by the fuse 10 alone, as shown in
FIGURE 1.
As compared to the fuse 10 of FIGURE 1, the
05 composite time lag fuse construction of FIGURE 3 will
provide the desired time delayed opening of the circuit
for overload currents in the approximate range of 135 to
600 percent of rated current. As overload currents
continue to increase above 600 percent, the heating coils
have a lesser and lesser effect in causing the fuse to open
the circuit sooner, wherein the overload current flow
through the body 12 of fuslng alloy will cause it to melt
prior to the heat transferred from the coils 30 and 32
being effective to heat the body 12 of fusing alloy.
However, the composite time lag ~use 45, shown in FIGURE
3, is not provided with enhanced short circuit current
interruption capabilities.
Referring to FIGURE 4, a preferred embodiment of
the present invention is shown. A time delay fuse 10 of
; 20 the type shown in FIGURE 1, is again used as the overload
current trigger element. A bare wire 58 is soldered or
welded to the end cap 28, as shown at 60, and extends to
the left through an insulating member 62. The insulating
member 62 together with a formed cylindrical insulating
member 66 closes the left end of the insulating housing
46. End cap 48 is electrically and mechanically connected
to the left end of conductor or wire 58 ~y solder or weld
material 68. At the right end of the assembly, an
insulated resistance heating wire 70 is wound around the
glass tube 24 to overlay the conductive element 16, which
is shown on the left in FIGURE 1. One end of the
conductor of insulated resistance heating wire 70 is
soldered or welded to the end cap 28 at 72. The other end
extends to the right to be connected electrically and
mechanically by solder or weld material 74 to both the end
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cap 50, and to a metal layer 76 formed on the right side
of an insulating washer 78. The embodiment of the
invention, shown in ~IGURE 4, offers not only the
advantages of the embodiment of the invention shown in
05 FIGURE 3, but also improved short circuit current
interruption capabilities. The material and cross-
sectional area of bare wire 58 is selected to provide the
desired short circuit current interrupting capabilities.
With the provision of only one resistance heating wire 70,
rather than two, it is possible to obtain different, yet
equally desirable heat transfer characteristics so as to
provide current interruption for a predetermined
percentage overload current with a preselected time delay.
As compared to using the trigger element or time delay
fuse 10 alone, higher voltage operation is possible due to
the use of the short circuit wire 58 and the longer
housing. The voltage rating of the composite time lag
fuse of FIGURE 4 is determined by the electrical
characteristics of trigger element 10, the short circuit
wire 58, and by the length of the insulating housing 46.
Another preferred embodiment of this invention
is shown in FIGURE 5. In addition to a time delay fuse
10, similar to that shown in FIGURE 4, a short circuit
fuse 80 having desired short circuit current interrupting
characteristics is utilized. In the preferred embodiment
shown in FIGURE 5, a finer resistance heating wire 82 is
utilized, 50 as to provide more heat at lower overload
currents. An insulating support member 84 is provided
with a notch which engages the insulating tube 24, and is
positioned to confine the heating wire 82 in the desired
position on insulat ng tube 24. One end of the resistance
heating wire 82 is connected to the right end cap 28 by
solder or a weld, as shown at 86. The other end of the
resistance heating wire 82 extends to the right, through
an aperture 88 in support member 84 to be soldered or
welded, as shown at 90, to the composite fuse end cap 50.
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Short circuit fuse 80 is provided with a right end cap 92
and left end cap 94. The right end cap 92 is electrically
and mechanically connected to the left end cap 26 by
soldering or welding at 96. As is the case in previously
05 discussed embodiments, the left end cap 94 is electrically
and mechanically connected to the left end cap 48 of the
composite fuse by solder or welding material 98. The
; comp~site fuse shown in FIGURE 51 not only provides the
enhanced overload current interrupting capabilities of the
embodiment shown in FIGURE 4, but further provides the
enhanced short circuit current interrupting capability
co~tributed by the fuse 80. The short circuit current
interrupting capabilities of the fuse 80 are enhanced, in
terms of voltage capabilities, by enveloping the fuse 80
in the granular electrically insulating fill material 56.
The granular fill material 56 enveloping the fuse 80
permits it to be dependably opera-ted at higher voltages
than it would if used in air.
The preferred embodiment shown in FIGURE 6 is
similar to that shown in FIGURE 5, except that the short
circuit current interrupting element is shown as a wire
: lbo, which is soldered or welded to end cap 26 at 102 and
to the end cap 48 at 104. Again, the embodiment shown in
FIGURE 6 provides all the enhanced characteristics with
respect to time delay for overload currents in the range
of 135 to 600%, of the device shown in FIGURE 5, but
further provides the enhanced short circuit character-
istics attributable to the use of the short circuit wire
lO0, as set forth for the embodiment shown in FIGURE 4.
The embodiment shown in FIGURE 7 combines
features of the embodiments shown in FIGURES 4 and 5. As
compared to FIGURE 4, this embodiment provides the
enhanced characteristics with respect to interrupting
short circuit currents which are attributable to the short
`~ circuit fuse 80.
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Wherein this invention contemplates a composite
fuse construction combining the desired overload current
interruptive characteristics of one fuse and the short
circuit current interruptive characteristics of
05 another fuse, both enclosed in a housing which provides
an ~nhanced voltage rating,. still other embodiments of the
invention are contemplated. Referring to FIGURE 8, the
use of still another type of time delay fuse 106 is
illustrated, The time delay fuse 106 is described in U.S.
Patent 4,517,544 - Spaunhorst, assigned to the assignee of
the subject application. The time delay fuse 106 includes
an electrically insulative cylindrical core 108 around
which is wrapped a uninsulated fusible wire 110. The
short circuit fuse 112 is shown as a fusible ribbon having
an area of reduced cross section 114.
- In the embodiment of this invention shown in
FIGURE 9, the time delay fuse 10 is shown to include a
fusible ribbon 116, having a portion loaded with a
conductive material 118, which upon heating forms a
amalgam with the ribbon 116, to provide opening at the
desired temperature. The short circuit fusible member 120
is shown to be a fusible ribbon having a plurality of
portions of reduced cross section formed by providing
holes 122 in the ribbon.
Finally, the embodiment of the invention shown
in FIGURE 10 is similar to that shown in FIGURE 8, except
that a fusible cylindrical wire 124 is provided in place
of the fusible ribbon 116. Further, in embodiments shown
in both FIGURES 8 and 10, the resistance wire heating coil
is formed of uninsulated wire, with the turns formed in a
spaced apart relationship upon the glass housing of the
time delay fuse.
The embodiments of the present invention
described herein present the preferred embodiments of the
invention. However, it is to be understood that changes
and modifications thereto are wlthin the intent and spirit
of the present invention,
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