U.S. patent number 3,840,835 [Application Number 05/394,290] was granted by the patent office on 1974-10-08 for electric current-limiting low voltage fuse.
This patent grant is currently assigned to The Chase-Shawmut Company, I-T-E Imperial Corporation. Invention is credited to John G. Angelis, Frank W. Kussy, Kenneth W. Swain.
United States Patent |
3,840,835 |
Kussy , et al. |
October 8, 1974 |
ELECTRIC CURRENT-LIMITING LOW VOLTAGE FUSE
Abstract
An electric current-limiting low voltage fuse for back-up
protection of switching devices having relatively movable contacts
and being controlled by excess current tripping devices includes
low resistance fusible elements in ribbon form and a high
resistance fusible element in form of a helically wound wire. On
occurrence of major fault currents the low resistance and the high
resistance fusible elements fuse and vaporize sequentially. Fusion
and vaporization of the low resistance fusible elements limits the
peak let-through current to a value way below the peak of the
available short-circuit current. The relatively small current that
flows through the high resistance fusible element affords
sufficient time for the switching device to open the faulted
circuit completely.
Inventors: |
Kussy; Frank W. (Haverford,
PA), Swain; Kenneth W. (Hampton Falls, NH), Angelis; John
G. (Hampton, NH) |
Assignee: |
The Chase-Shawmut Company
(Newberryport, MA)
I-T-E Imperial Corporation (Springhouse, PA)
|
Family
ID: |
23558326 |
Appl.
No.: |
05/394,290 |
Filed: |
September 4, 1973 |
Current U.S.
Class: |
337/159; 337/162;
337/293; 337/164 |
Current CPC
Class: |
H01H
85/12 (20130101) |
Current International
Class: |
H01H
85/00 (20060101); H01H 85/12 (20060101); H01h
085/04 () |
Field of
Search: |
;337/159,161,162,163,164,293 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller; J. D.
Assistant Examiner: Bell; Fred E.
Attorney, Agent or Firm: Salzer; Erwin
Claims
We claim as our invention:
1. An electric current-limiting low voltage fuse for backup
protection of switching devices having relatively movable contacts
and being controlled by excess current tripping devices, said fuse
including in combination
a. a fuse tube of electric insulating material;
b. a pair of terminal elements closing the axially outer ends of
said fuse tube;
c. a unitary body of pulverulent arc-quenching filler inside said
fuse tube;
d. low resistance current-limiting fusible elements in ribbon form
conductively interconnecting said pair of terminal elements
submersed in said unitary body of filler and being arranged
symmetrically to opposite sides of a median plane of said fuse tube
to form separate current paths situated to opposite sides of said
median plane and relatively remote from said median plane; and
e. a high resistance fusible element in form of a helically wound
wire conductively interconnecting said pair of terminal elements,
submersed in said unitary body of filler and being arranged
relatively close to said median plane of said fuse tube inside of a
space bounded by said low resistance current-limiting fusible
elements so as to be sandwiched between said low resistance fusible
elements.
2. A fuse as specified in claim 1 wherein said helically wound wire
is of Nichrome.
3. An electric current-limiting low voltage fuse for back-up
protection of switching devices having relatively movable contacts
and being controlled by excess current tripping devices, said fuse
including in combination
a. a fuse tube of electric insulating material;
b. a pair of terminal elements in the form of a pair of plugs
plugging the axially outer ends of said fuse tube, the axially
inner end surfaces of said pair of plugs being provided with a pair
of juxtaposed bores;
c. a pulverulent arc-quenching filler inside said fuse tube;
d. low resistance current-limiting fusible elements in ribbon form
conductively interconnecting said pair of terminal elements,
submersed in said filler and being arranged relatively remote from
a median plane of said fuse tube; and
e. a high resistance fusible element in form of a helically wound
Nichrome wire conductively interconnecting said pair of plugs
submersed in said filler and being arranged relatively close to
said median plane of said fuse tube and being sandwiched between
said low resistance fusible elements, said pair of juxtaposed bores
in said axially inner end surfaces of said pair of plugs receiving
the axially outer ends of said helically wound wire of Nichrome,
and said bores further containing a mass of low fusing point metal
frictionally positioning said ends.
Description
BACKGROUND OF THE INVENTION
In view of the continuous increase of the magnitude of available
major fault currents in industrial and other systems it is
necessary, to an ever increasing extent, to protect electrical
apparatus, e.g. circuit breakers, by means of current-limiting
backup fuses.
It is possible to coordinate circuit breakers or other motor
starting equipment and current-limiting back-up fuses for the
latter either mechanically or electrically. In the first mentioned
instance blowing of the fuse results in the release of a striker
pin which operates a tripping bar which, in turn, unlatches the
latch mechanism of the circuit breaker. The above referred-to
mechanical coordination method has two limitations: It requires
additional parts and requires a special circuit breaker operating
mechanism. Conventional electrical coordination of a circuit
breaker and current limiting back-up fuses calls for certain values
of the let-through current, and more particularly certain i.sup.2.
t values thereof which cannot always be met. Either the tripping or
unlatching time of the circuit breaker may be too long to cause
tripping of the circuit breaker by the let-through current of its
back-up fuse, or the desired current-limiting action of the back-up
fuses could not be achieved if they were selected with the object
in mind of causing tripping of a given circuit breaker by the
let-through current thereof.
The present invention resolves these conflicts.
SUMMARY OF THE INVENTION
Fuses embodying this invention include in addition to the
current-limiting fusible elements a helically wound fusible element
of resistance wire which fuses at a later point of time than the
current-limiting fusible element, thus extending the duration of
current flow to cause tripping of a relatively slow automatic
switching device or circuit breaker. Both the current-limiting
fusible elements and the helically wound resistance wire are
submersed in the same or a unitary body of pulverulent
arc-quenching filler. The helically wound fusible element is
arranged close to a median plane of the fuse tube inside of a space
bounded by the current-limiting fusible elements and sandwiched
between the current-limiting fusible elements line 17, which form
parallel current paths arranged symmetrically in regard to said
median plane.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded isometric view of a fuse and fuse holder
combination embodying this invention and also showing a portion of
a molded case circuit breaker intended to be backed up on
occurrence of major fault currents;
FIG. 2 is a diagrammatic top-plan view of a fuse holder as shown in
FIG. 1 without any fuses therein;
FIG. 3 is a top-plan view of a fuse embodying this invention;
FIG. 4 is substantially a longitudinal section of the fuse of FIG.
3, and
FIG. 5 are traces illustrating the operation of the composite
circuit breaker and fuse unit shown in FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring now to the drawings, numeral 1 has been applied to
designate a tubular casing or fuse tube of a current-limiting fuse.
Fuse tube 1 is of an electric insulating material, e.g. melamine
glass cloth. The ends of casing 1 are closed by a pair of plug
terminals 2, which are press-fitted into casing 1. Steel pins 3a,
3b, 3c,3d project transversely through casing 1 into plug terminals
2, thus affixing firmly the latter to the former. Plug terminals 2
are conductively interconnected by current-limiting fusible
elements or fuse links 4 in ribbon form having a relatively low
electric resistance. The particular configuration of the fuse links
4 shown in FIG. 4 is more fully disclosed in U.S. Pat. No.
3,394,333 to P. C. Jacobs, Jr., July 23, 1968 for ELECTRIC FUSE
HAVING STRESS-REDUCING FUSE LINK MEANS. Each fusible element or
fuse link 4 is sandwiched between a pair of plates 4a of insulating
material for controlling the arc-voltage generated incident to
blowing of the fuse, as more fully explained in U.S. Pat. No.
2,964,604 to P. C. Jacobs, Jr. et al., Dec. 13, 1960 for CURRENT
LIMITING FUSES HAVING COMPOUND ARC-VOLTAGE GENERATING MEANS. The
axially inner surfaces of plug terminals 2 are provided with
grooves engaged by the axially outer ends of fuse links 4. The
aforementioned grooves are filled with soft solder to conductively
connect fuse links 4 and plug terminals 2. Fuse links 4 are shunted
by a high resistance restraining wire 6 for a blown fuse indicator
of the kind more fully disclosed in U.S. Pat. No. 3,391,369 to F.
J. Kozacka, July 2, 1968 for HIGH VOLTAGE FUSE. Fuse links 4 are
further shunted by a helically wound resistance wire 5 of Nichrome.
The ends of the latter are inserted into recesses or bores in plug
terminals 2 and conductively connected to the latter by masses of a
low fusing point metal. This may be a tin-free solder. It is not
possible to form a solder joint between the metal of which plugs 2
are made and the ends of Nichrome wire 5. However, the masses of
low fusing point metal surrounding the ends of Nichrome wire 5
exert such friction on wire 5 as to firmly position the ends
thereof inside the bores or recesses into which they are inserted.
This resistor 5 is our joint invention, but the fuse holder
described below is not our joint invention. It is claimed in a
copending patent application of Bernard DiMarco et al., filed Jan.
16, 1973, Ser. No. 324,193 for FUSE HOLDER WITH REJECTION FEATURE.
The upper plug terminal 2 is provided with a wire connector
generally indicated by reference numeral 7 and including the
set-screw 8. The lower plug terminal 2 is provided with a
substantially Z-shaped terminal strap 9 -- i.e., a connector strap
similar in shape to an inverted Z -- affixed to it by screws 10.
Connector straps 9 include web-portions extending parallel to the
axis of casing 1 and pairs of flange portions extending at angles
of 90.degree. in opposite directions away from the web portions.
The upper flange portion of each terminal strap 9 forms a lug
intended to engage a corresponding terminal in the molded case
circuit breaker 11 shown in FIG. 1. Reference numeral 12 has been
applied in FIG. 1 to indicate a screw intended to screw the upper
flange portion of terminal strap 9 against a cooperating contact
surface forming part of the molded case circuit breaker 11. The
molded case circuit breaker 11 is provided with a fuse holder or
pouch generally indicated by reference numeral 13. Fuse holder or
pouch 13 is a molding of a plastic or synthetic resin forming three
substantially tubular receptacles 13a for insertion of a fuse into
each of them. Each receptacle 13a may be provided with one or more
internal groove means extending in a direction longitudinally
thereof. One or more of the fastener pins 3a-3d for fastening the
lower plug terminal 2 (FIGS. 3 and 4) to casing 1 project radially
outwardly beyond the cylindrical surface of casing 1. Therefore a
receptacle 13a can only receive a fuse conforming to its shape and
size if there is a groove 14 juxtaposed to pins 3a-3d projecting
beyond the general outline of the fuse. It is possible to vary the
number of projecting pins and the position thereof, and to provide
fuse holders 13 which have grooves 14a whose number and position
correspond to the number and position of the pins of the particular
fuse that is intended to be accepted by the fuse holder.
The top-plan view of the fuse holder 13 shown in FIG. 2 is
diagrammatic. The fuse holder forms 3 receptacles 13a closed at the
bottom and open at the top. Each receptacle 13a is closed at the
front thereof and is provided with a slot 15 on the rear thereof
allowing the upper flange portion of terminal straps 9 to project
out of receptacles 13a into engagement with corresponding terminals
of molded case circuit breaker 11. Slots 13 and straps 9 determine
the position of each of the three fuses relative to the fuse holder
13, and thus a fuse can only be inserted into the fuse holder if
both have corresponding projecting pins 3a-3d and pin-engaging
grooves 1' . . . 4'.
In FIG. 2 the possible positions of four grooves 14a have been
indicated and numbered 1',2',3',4'. Considering a fuse holder 13
with a single groove having the position 1' and a fuse having one
single projecting pin 3a, pin 3a cannot engage the groove
positioned at 1' because the fuse is positioned relative to fuse
holder 13 by straps 9 of the former and the slots 15 of the latter.
A fuse having a single projecting pin 3a positioned as shown in
FIG. 3 can only be inserted into a fuse holder having groove means
positioned at 4'.
The resistor 5 of FIG. 4 makes it possible to achieve electrical
coordination of a circuit breaker and of its back-up current
limiting fuses even in instances where the circuit breaker is
relatively slow and where the let-through i.sup.2. t value of the
fuses is relatively small.
FIG. 5 explains more in detail the operation of shunt resistor 5.
FIG. 5 is a plot of current versus time, i being the trace of an
available short-circuit current i' being the trace of the actual
short-circuit current or, in other words, that of the available
short-circuit current as modified by the operation of the fuse. The
actual short-circuit current i' rises initially at the same rate as
the available short-circuit current i and the former decays rapidly
following melting and vaporization of the fusible elements 4. After
reaching an initial current zero the fault current limited by
resistor 5 continues to flow for a period of time sufficient to
cause tripping of the molded case circuit breaker 11.
For fuses embodying this invention the problem of interrupting the
current flowing through resistor 5 is not of a serious nature as
long as the current rating of the fuses is relatively low. This
problem becomes more serious, the larger the current-carrying
capacity or current rating of the fuse. Upon fusing of the resistor
5 series breaks exist in the molded case circuit breaker 11 and the
fuse F. It is desired to properly apportion the interrupting duty
to both aforementioned devices and to minimize the interrupting
duty of each. The interrupting duty of fuse F is much more severe
than that of a conventional back-up fuse because the presence of
resistor 5 results in a drastic increase of metal vapor incident to
blowing of the fuse and because that metal vapor has a relatively
low ionizing potential. Considering relatively high current ratings
of fuse F, the positioning of the helically wound resistors 5
relative to the other parts of the fuse becomes a factor of
considerable significance. To be more specific, in such instances
the resistor 5 ought to be arranged as shown in FIG. 4, i.e.,
resistor 5 ought to be arranged relatively close to a median plane
of the fuse tube 1 and the fusible elements 4 ought to be arranged
relatively remote from said median plane sandwiching resistor
winding 5. As a result of this configuration a zone of relatively
high and uniform temperature is formed adjacent the center region
of resistor winding 5. This results in a tendency of formation of a
very long initial break, or of series breaks, at the aforementioned
zone of relatively high and uniform temperature.
It may happen incident to some fault condition that fusible
elements 4 fuse and are vaporized by resulting arcing, that the
letthrough current causes opening of the backed-up circuit breaker
or other motor starting device, but that the resistor 5 remains
intact, i.e., is not melted and vaporized. Reclosing of the circuit
breaker or other motor starting device then causes re-energization
of resistor 5. Under such conditions it is necessary to effect
rapid interruption of the current flow through resistor 5 by
melting and vaporization of the latter.
It may be pointed out that a molded case circuit breaker or other
motor starting device and a back-up fuse may be designed from the
very outset to form a matched pair. In that case the problem
underlying this invention does not arise.
* * * * *