Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Case SC-514fi-C-DIV
11~i6t~
Hlc~H-voLTA~E FI~SE HA~ING AN IMPROVED
MULTI~AP HIGH-VOLTAGE SWITCH
BACKGROUND OF THE INVENTION
__ _
5 Field of the Invention
.
The present invention relates to a fuse having an improved electric
switch. More specifically, the present invention constitutes an improvement of the
inventions claimed in commonly-assigned Canadian Patent Applications, Serial No.
341,244 filed December 5, 1979 and Serial No. 346,009, filed February 18, 1979, both
10 in the name of Otto Meister.
The '244 applicat;on relates to a circuit-protection device which includes
a first current path having a high continuous current-carrying rating. A pair of
normally electrically interconnected contacts are included in the first path. The
15 contacts are relatively movable apart along a fixed line of direction. When the con-
tacts are relatively movable apart, the electrical interconnection therebetween is
broken to open the first path. When the contacts are electrically interconnected, at
least one of them defines a portion of an enclosed chamber. An ignitable device, such
as a power cartridge, is included in the chamber for pressurizing it upon ignition
20 thereof to rapidly drive the contacts apart. A second current path is in electrical
shunt with the contact and physically surrounds the first path. The second path may
include a fuse or a fusible element which may be either current limiting or non-
current limiting. The first path normally shunts away from the fusible element the
majority of the current passing through the device. Only when the contacts move
25 apart is current commutated to the fuse which then interrupts such current.
Preferably, the power cartridge is ignited to move the contacts apart in response to
the occurrence of a fault current or other overcurrent in a circuit in which the device
is connected. As more fully explained in the '244 application, in this way current-
limiting fuses, which may have high fault current-interrupting ratings, but which can
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carry only low continuous currents, may be used to protect circuits having high con-
tinuous currents because the first path (including the contacts) and not the fuse,
normally carries the majority of the current in the circuit.
Brief Discussion of the Prior Art
A fault current (used herein to mean any undesirable overcurrent)
impresses rather stringent thermal and mechanical stresses on high-voltage electrical
systems and on apparatus used in such systems. The severity of the thermal stress is
known to be generally proportional to the product of (1) the square of the Eault cur-
rent, and (2) time--that is, I2t. The severity of the mechanical stress is generally
proportional to the square of the peak or crest value achieved by the fault current.
Thermal stresses are generally manifested in the burning down of, or other thermal
damage to, lines, cables, and equipment. Mechanical stresses are manifested in the
deformation of bus work and switches in damage to items, such as transformers orreactor coils, due to the extremely high magnetic forces generated by a fault current.
Current-limiting fuses including the so-called silver-sand variety are
well known expedients for limiting the magnitude of fault currents. See the following
commonly-assigned United States Patents: 4,063,203 to Bernatt; 4,057,775 to Biller;
4,035,753 to Reeder; 4,028,656 to Schmunk and Tobin; 4,011,537 to Jackson and Tobin;
snd 4,010,438 to Scherer. In interrupting a fault current, a current-limiting fuse
limits both the peak thereof and I2t to tolerable levels, thereby minimizing thermal
and mechanical stresses. These tolerable levels of peak fault current and I2t are often
termed the "let through current" or simply "let through." As is well known, current-
limiting fuses, particularly at higher voltages, have relatively low continuous current
ratings which impose limitations on the applicability thereof.
As electrical systems have expanded and electrical consumption has
increased, continuous current in such systems has also increased. Because of the low
continuous current rating of conventional silver-sand current-limiting fuses, such fuses
have limited applicability in the systems. The low continuous current rating of cur-
rent-limitng fuses is apparently inherent. Most known current-limiting fuses cannot
meet both the requirements of low "]et through" and high continuous current rating
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without some modification or the addition of some special apparatus. Further, fault
current levels have begun to exceed the capabilities of ex;sting switchgear. If, in
order to avoid the occurrence of increased fault currents, electrical systems are
arranged so that they contain individual sections having ]ow available fault currents,
5 or if current-limiting reactors, high-impedance transformers, or the like are used,
certain disadvan$ages may nevertheless result. ~or example, sectionalizing and the
use of current-limiting reactors are uneconomical and may render satisfactory voltage
regulation difficult to achieve. These techniques also usually result in an overabund-
ance of idle reserve in the electrical system. Thus, unless an economical and reliable
10 current-limiting fuse having a high continuous current rating becomes generally avail-
able, the only solution--a costly one--to the problems engendered by increased fault
current levels is to replace existing switchgear with gear having higher fault or over-
current withstand capabilities and higher interrupting capabilities. Accordingly, the
fault-limiting properties of current-limiting fuses have been, and remain, the subject
15 of great interest.
Twenty or so years ago, a device, sometimes referred to as an
"Is-Limiter," was developed by Calor-Emag Corporation (now a division of Brown
Boveri of West Germany). The Is-Limiter is constructed with a high continuous-cur-
20 rent-capacity, main conductive path which is electrically paralleled with a more or
less standard current-limiting fuse~ The current-limiting fuse may be of the well
known silver-sand type having a silver fusible elem ent surrounded by a fulgurite-
forming, arc-quenching medium, such as silica or quartz sand. The main conductive
path of the Is-Limiter includes a so-called "bursting bridge" which, upon detonation of
25 a chemical charge contained therewithin in response to & fault current, renders the
main conductive path discontinuous and rapidly transfers or commutates the current in
the main conductive path to the current-limiting fuse.
The bursting bridge is comprised of a pair of tubular sections, each open
30 at one end and containing a series of longitudinal slots over the majority of its
length. The open ends of the tube sections are joined along a brazed, weak interface
to enclose the chemical charge. Detonation of the chemcial charge breal~s the weak
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interface, blowing up the bursting bridge, and bending fingers defined between the
slots of each section out and bac~ in a "banana peel" configuration; this renders dis-
continuous the main conductive path. See U.S. Patent 2,892,062 to Bruckner, et al.
This discontinuity in the main conductive path transfers or commutates the current to
5 the current-limiting fuse, which current is then interrupted in a conventional manner
common to silver-sand current-limiting fuses. The chemical charge is detonated by
means of a pulse transformer or other device contained in one of two insulators which
mounts the current-limiting fuse and the main conductive path, each of which is
housed in its own independent, individual, insulative housing. When the bursting bridge
10 is blown apart, an arc typically forms between the tube sections. Sometime there-
after, the arc voltage is sufficiently high to commutate the current to the fuse so that
interruption thereby may occur.
If not properly fabricated, the bursting bridge may not fully open.
15 Further, it has been found that the gap between the bent back fingers of the tube
sections may be contaminated or ionized by the chemical charge or the arc. Specif-
ically, when the chemical charge detonates, hot ignition products--gaseous and solid
--fill the gap. These ignition products lower the dielectric strength of the gap. So
too, the action of the arc--the formation of which itself involves ionization of gas in
20 the gap-- on metallic or non-metallic materials in the vicinity thereof produces
ionization of the gap, further lowering the dielectric strength thereof. Such
ionization, due to either or both causes, may permit the arc to persist or may lower its
voltage, thus slowing or preventing commutation of the current to the current-limiting
fuse. It has also been found, however, that the dielectric strength across the gap may
25 recover, or at least increase rather quickly after about 200 microseconds. Therefore,
the current-limiting fuse of the Is-Limiter must be designed and constructed as to (a)
overlap the "dead time" of the bursting bridge until the 200 microsecond time passes
and then (b) limit and interrupt the current. Following the initial 200 microseconds,
voltage stress across the gap has been found to be rather low due to the lower resis-
30 tance of the fusible element as compared to that of the gap. Thus, the Is-Limiter is a
current-limiting device combining a fast-acting switch having a high continuous cur-
~166~71
rent capability, but poor current-interrupting capability with an electrically parallel
current-limiting fuse having a low continuous current capability but high current
limiting and interrupting ability.
Several disadvantages of the Is-Limiter should be noted. First, the
current-limiting fuse and the main conductive path form two separate elements in
their own separate housings. This arrangement is not only somewhat clumsy and
difficult to manipulate during replacement or initial placement, but increases material
costs due to the duplication of certain elements such as housings, end ferrules, con-
10 ductors, and the like. This first disadvantage of the ls-Limiter is obviated by the
invention claimed in the '244 patent application, wherein a high continuous-current
capability, fast-acting switch, and an electrically parallel current-limiting fuse are
contained in the same housing. A second disadvantage of the Is-Limiter relates to the
fact that speed of commutation of the current in the main current path to the cur-
15 rent-limiting fuse may be slowed by the relatively high inductance of the main con-
ductive path and current-limiting fuse combination. This second disadvantage of the
ls-Limiter is also obviated by the invention claimed in the '650 patent application by
surrounding the main current path with the current-limiting fuse to minimize the
inductance of the combination, as described more fully in that patent application.
A third disadvantage of the Is-Limiter is that there is a practical limi-
tation to the length of the gap that can be formed by the bursting bridge. Specifi-
cally, only so much chemical charge may be confined within a practical volume of the
bursting bridge to ensure that the fingers defined by the slots into the two tube
25 sections are sufficiently blown outwardly and bent backwardly. That is, the tube
sections could be greatly elongated and filled with a chemical charge of larger size so
that the detonation bends back fingers of increased length. Both the increased size of
the charge and the length of the fingers, however, require a larger housing of higher
burst strength, adding to the cost and inconvenience of the overall device. This third
30 disadvantage of the Is-Limiter is obviated by the invention claimed in the '244 patent
application. Specifically, rather than including a bursting bridge, the high-speed
switch of the invention of the '244 patent application comprises a pair of normally
-- 5
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electrically connected contacts which are driven apart along a fixed line by the
ignition of a power cartridge. In this way, the switch of the '244 patent application
does not depend upon the fracturing (blowing apart) and peeling back of portions of the
main current path as is the case with the Is-Limiter; rather, the contacts are
5 positively driven and moved apart, ensuring that a large gap is opened therebetween.
See also German Offenlegungsschrift, 1,094,244 published August 6, 1970 and a related
article entitled "Ultra-High Speed Protection Device--Fuji Ultrup Fuse" in the
Electric Review, Vol. 18, No. 1 (1972) Pages 49-51.
A fourth disadvantage of the Is-Limiter, alluded to above, relates to the
fact that some coordination between the operation of the current-limiting fuse and
the dielectric recovery of the gap formed between the tubular sections of the bursting
bridge may be necessary. Due to the vagaries of fault current conditions in high-
voltage circuits, this coordination may prove difficult to achieve. Both the invention
15 of the '~44 patent application, as well as the device of the German Offenlegunges-
schrift and the Fuji article suffer from a similar disadvantage. Simply stated, the
need to await the dielectric recovery of the gap is due to contamination of the gap by
the ignition products of the chemical charge and the formation of the arc, as
described earlier. Even ignoring gap contamination bg the ignition products of the
20 ¢hemical charge, there is evidence that where a single gap is opened, RS occurs in the
prior art devices so far discussed, a sufficiently high arc voltage may not always
predictably exist at an early enough time to transfer current to the fuse to ensure
appropriate fault current limitation and interruption. Moreover, even where current
does transfer to the fuse, the operation of the fuse may involve an arc voltage suffic-
25 iently elevated to retransfer current to the main path and defeat the protectivefunction of the device.
The above-described need for coordination insofar as it is due to di-
electric recovery problems or gap contamination has been at least partly solved by the
30 invention of commonly-assigned Canadian patent application, Serial No. 346,009, filed
February 18, 1979 in the name of Otto Meister. In that invention, which constitutes
an improvement of the invention of the '244 patent application, one of the contacts
-- 6
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mounts a piston. The piston is preferably insulative and may be made of an ablative,
arc-extinguishing material. The piston is also configured so that when the power
cartridge is ignited to pressurization the chamber and drive the contacts apart, the
piston is forced into intimate contact with the walls of a sleeve-like liner, also pref-
5 erably made of an ablative arc-extinguishing material. In this way, the contact which
mounts the piston is isolated from the ignition products of the power cartridge and
other gap contaminants. Also, the engagement between the piston and liner constricts
and subjects to the action of deionizing, arc-extinguishing gas any arc that forms
between the contacts following their movement apart similar to so-called trailer-liner
10 interrupters. Such constriction and arc-extinguishing gas tend to elevate the arc
voltage or extinguish the arc (or both), either of which increases the likelihood that
current will be commutated to the fuse. Further, the isolation of the contact by the
piston from contaminants--whether produced by the power cartridge or the arc itself
--tends to ensure that the gap has a high dielectric strength as the fuse operates, thus
15 inhibiting retransfer of the current from the fuses to the main path. Thus, the inven-
tion claimed in the '009 patent application takes long strides toward solving the coor-
dination and dielectric recovery problems of the ls-Limiter, the device of the '244
application and the devices of the German Offenlegungsschrift and the Fuji Electric
Review.
Nevertheless, the invention claimed in the '009 patent application, as do
earlier inventions, depends for current commutation to the fuse on the opening of a
single gap. It has been postulated that even where the invention of the '009
application is used, a single gap may not reliable ensure current commutation of the
25 type resulting in appropriate current limitation and interruption for at least two
possible reasons. ~irst, the arc in the single gap both contaminates the gap due to its
effect on metal parts and erodes the piston and the liner of the '009 application where
such are used. This erosion may permit contaminants to be distributed across a
portion of the entire gap and to be present in the vicinity of both contacts. In this
30 event, the contaminants may prevent sufficient elevation of the arc voltage to appro-
priately commutate current to the fuse. Second, even if the current is commutated to
the fuse--either by a sufficiently high arc voltage or by extinguishment of the arc
i667i
through the action of the piston and the liner--when the fuse operates to limit and
interrupt the fault current~ a sufficiently high arc voltage rnay occur so that current
in the fuse attempts to retransfer to the main path. If the single gap is sufficiently
contaminated for any reason, an arc may reform therein which may result in
5 retransfer of the current to the main path and a failure of the switch-fuse
combination to perform its intended function.
Accordingly, the present invention constitutes, in general, an improve-
ment of the inventi~n claimed in the '244 and '990 patent applications and, more
10 specifically, a solution to the coordination and dielectric recovery problems residing in
prior art deviees. Additional background and discussion of the prior art is more fully
set forth in the '244 and '009 applications, which are specifically incorporated by
reference hereinto.
SUMMARY OF THE INVENTION
The present invention contemplates a high-voltage fuse having a high
continuous current rating. The fuse includes a first current path which has a high
contlnuous current-carrying capacity. A second current path includes a fusible
element therein. A first facility is capable of opening a first insulated gap in the first
20 path; an arc forms in this first gap. A second facility connects the second path in
shunt with the first path as the voltage of the arc elevates. This transfers current
from the first path to the second path. A third facility opens a second insulated gap in
the first path after the current transfers to the second path. Accordingly, the second
gap is uncontaminated. As a consequence, as the back voltage of the fusible element
25 elevates, retransfer of the current to the first path is prevented by the presence of
both the first gap and the second uncontaminated gap in the first path.
The fusible element may be current-limiting so that the high-voltage
fuse is a current-limiting fuse having low let-through. Moreover, the second path may
30 surround the first path and an insulative housing may enclose both paths.
~16667~
In a specific embodiment, the first facility includes a pair of normally
electrically interconnected contacts separable along a first line of direction to form
the first gap, which is electrically insulated as the contacts separate, and an ignitable
device, ignition of which separates the contacts. The third facility includes a terminal
which is electrically connected to one contact when the contacts are interconnected.
The second gap forms between the terminal and the one contact as the contacts
separate and is electrically insulated as the contacts separate.
BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 depicts a side elevation of the exterior of a high voltage fuse in
accordance with the principles of the present invention; the device, which includes a
fuse section and an improved switch section according to the invention, is mounted
between a pair of insulators, one of which is partially sectioned to generally depict a
sensing and triggering unit contained therein;
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S~'n FIGURE 2 is a partially sectioned, side elevational view of a generalized
;-
switch section depicting certain novel features in accordance with the present
invention; the switch section is closed and may constitute a portion of the fuse shown
in FIGURE l; and
,
'; 20
t'' FIGURE 3 is a partially sectioned, side elevational view of a preferred
alternative embodiment of a switch section usable with the fuse of FIGURE 1
according to the principles of the present invention.
DETAILED DESCRIPTION
Referring first to FIGURE 1, there is shown a general exterior view of a
novel electrical fuse 10 in accordance with the principles of the present invention.
The novel fuse 10 may be usable at high voltages and may include a high-voltage fuse
section, generally indicated at 12, and a novel high-voltage switch section, generally
30 indicated at 14, both contained within an outer, elongated, insultive housing 16. The
fuse section 12 may be either current limiting or non-current limiting, although the
former is preferred. The outer housing 16 may include a plurality of leakage-distance-
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increasing skirts 28, as is well known7 and may be made of porcelain or other
insulative material, such as molded cycloaphatic epoxy resin. The outer housing 16
may surround an inner housing 19 (FIGURE 3) preferably made of glass-fiber-
reinforced epoxy. The switch section 14 may be contained by the inner housing 19,
5 while the fuse section 12 may be located between the housings 16 and 1~, as described
more fully below.
Extending from one end of the housing 16 is a first terminal 20 which is
connected to various elements within the housings 16 and 19 in a manner to be
10 described below. Extending from the outer end of the housing 16 is a second terminal
22 which is also connected to elements within the housings 16 and 19. The terminal 20
may be detachably connectable in any convenient fashion to a mounting facility 24
which may be formed integrally with or otherwise suitably connected to a cable or line
attachment facility 26. One cable or line (not shown) of a circuit (not shown) to be
15 protected by the fuse 10 is attached in any convenient manner to the attachment
facility 26. The mounting facility 24 and the cable-attachment facility 26 are sup-
ported by and are attached to a support insulator 28 formed of porcelain or other
convenient insulative material, such as cycloalphatic epoxy resin. The insulator 28
may include a plurality of leakage-distance-increasing skirts 30 and is supported by a
20 common base 32 which may be a structural steel member or the like.
The other terminal 22 may have any convenient configuration, the
inverted L-shape depicted in FIGURE 1 being one example thereof. The terminal 22 is
detachably engageable by a mounting facility 34. If the terminal 22 has a generally
25 circular cross-section, the mounting facility 34 may comprise a plurality of contact
fingers 36 (only two are shown~, spring-biased into intimate engagement with the
terminal 22 by one or more garter springs 38. The mounting facility 34 may be molded
in as an integral part of an insulator 40 which may be made of porcelain, a cyclo-
alphatic epoxy resin, or other suitable insulative material. Also contained within the
30 insulator 40 may be a conductor 42, which is continuously connected to the fingers 36
as at 43, and which is connectable to another cable or line (not shown~ of the circuit
(not shown) being protected by the fuse 10.
- 10 -
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A sensing nnd triggering unit 44 generates appropriate output signnls on
output conductors 46 for a purpose to be described below in response to the condition
of current in the conductor 42, which may be sensed by a current transformer 48. The
unit 44 and the transformer 48 may be integrally molded into the insulator 40. The
5 current transformer 48 and the sensing and triggering unit 44 are interconnected by
appropriate leads 50. The output conductors 46 of the sensing and triggering unit 44
may pass through both a portion 51 of the insulator 40 and an appropriate detachable
clamp member 52 surrounding the terminal 22. The output conductors 46 may enter
the interior of the housings 16 and 19 through the terminal 22 which may be hollow or
10 bored out for this purpose. The insulator 40 may include a plurality of leakage-
distance-increasing-skirts 54 and is attached to the common base 32. The present
invention contemplates that the unit 44 and/or the transformer 48 may be located
other than in the insulator 40. For example, the unit 44 may be within the housing 16
or in a separate housing (not shown) attached to or formed integrally with the housing
15 16. In this latter event, the structure of the terminal 22, the mounting facility 34, and
the insulator 40 may well vary from that depicted in FIGURE 1.
The insulators 28 and 40 on the one hand, and the fuse 10 on the other
hand, as shown in FIGURE 1, have respective vertical and horizontal orientations.
20 Any of these components may be mounted in any other desired orientation, as should
be obvious. The unit 44 and the transformer 48 may be reusable; only the fuse section
12 and the switch section 14 and their common housing 16 require replacement
following operation of the fuse 10.
~, 25 Referring now to FIGURE 2, there is shown one generalized embodiment
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~i of the switch section 14 in accordance with the principles of this invention. The fuse
section 12 usable with the switch section 14 to form the fuse 10 of the present inven-
s tion, is shown only schematically in FIGURE 2.
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The switch section 14 is included in a first current path 58 and has a pair
of separable contacts 60 and 61. The contact 61 comprises a conductive metal tube 62
stationarily held by or mounted to an insulative cylinder 64 which is preferably fiber-
glass-reinforced epoxy. The contact 60 comprises a movable conductive metallic rod
~, 66.
The contacts 60 and 61 are normally electrically interconnected by an
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annular or disk-like metallic member 68, having the general form depicted, which is
. .,
f~ normally attached between one end of the tube 62 and one end of the rod 66. As is
~ 10 disclosed more completely in the '009 application, the member 68 may take the form
:~ of a ring-like silver diaphragm, as shown in detail in FIGURES 6 and 7 of the afore-
mentioned application. An enclosed chamber 70 which contains a power cartridge 72~; is formed by a plug 73 mounted in or integral with the tube 62 and an insulative trailer
or rod 74 preferably made of an ablative, arc-extinguishing material. The trailer 74 is
15 attached by a connector 77 to the rod 66 and is mounted for conformal movement
within a bore 78 defined within the tube 62. Pressurization of the chamber 70 by
; ignition of the power cartridge 72 pushes the trailer 74 rightwardly, which in turn
~IJ moves the rod 66 rightwardly.
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. ~ 20 The contact 60 is normally electrically connected to a terminal 80 by a
r~; metallic member 82 attached as convenient by a connector 84 to the rod 66. The
~, member 82 may take a form similar to that of a metallic member 68. The terminal 80
may take the form of a conductive metallic tube 85 which may be stationarily he]d by
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~j the insulative cylinder 64, in the same manner as the tube 62.
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The rod 66 is covered with an insulative sleeve 86, preferably made of an
ablative, arc-extinguishing matérial. The sleeve 86 and the rod 66 are conformally
,
movable through a bore 88 formed in a stationary sleeve 90 of an insulative material,
,
~;1 preferably an ablative, arc-extinguishing material. The sleeve 90 may be stationarily
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~ 30 mounted by the insulative cvylinder 64. A bore 92 formed in the tube 85 is lined with a
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sleeve 94 of an electrically insulative material, preferably an ablative, arc-extin-
guishing material which defines a bore 96. The bore 96 has substantially the same
cross-sections as, and may conformally receive, the rod 66 with the sleeve 86 thereon.
The fuse section 12 is in a second current path 98 which may be
connected in shunt, as shown only schematically, to the first path 58 at points 100 and
102. As disclosed in the '244 and '009 applications, the second path 98 rnay be
~-~ supported at least in part by the cylinder 64, which may constitute a part of the inner
. housing 19 (FIGURE 3) and around which a fusible element (not shown) of the fuse
10 section 12 may be wound. The entire structure of FIGURE 2 may be included within
G., the housing 16 of FIGURE 1.
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In operation of the fuse 10 which includes the switch section 14 of
FIGURE 2, the first current path 58 comprises, in order, the tube 62 (the contact 61),
the metallic member 68, the rod 66 (the contact 60), the metallic member 82, and the
tube 85 (the terminal 80). The tubes 62 and 85 may constitute or form a part of the
terminals 22 and 20, respectively, of FIGURE 1. When it is desired to commutate
current from the first current path 58 to the second current path 98, the power car-
~ tridge 72 is ignited. Ignition of the power cartridge 72 pressurizes the chamber 70 to
n~ 20 move the trailer 74 and the rod 66 with the sleeve 86 thereon rightwardly. Such
,'; .
rightward movement breaks the normal electrical interconnection between the
contacts 60 and 61 by ripping, tearing, or otherwise rendering discontinuous the
member 68 as at a shoulder or necked-down area 68a to open a first gap between the
~ contacts 60 and 61. A peripheral portion 68b of the member 68 remains attached to
C~ 25 the tube 62, while a central portion 68c thereof remains attached to and is carried
rightwardly by the trailer 74 and the rod 66 into the bore 88. This rightward
~.,
movement also rips, tears, or otherwise renders discontinuous the metallic member 82
as at a shoulder or necked-down area 82a, opening a second gap between the rod 66
~^ and the tube 85. A peripheral portion 82b of the member 82 remains attached to the
., 30 tube 85, while a central portion 82c thereof remains attached to and is carried right-
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wardly by the rod 66 into the bore 96.
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Thus, the switch section 14 in FIGURE 2 opens two gaps in order to
ensure commutation of current to the second current path 98. A first arc (not shown)
forming in the first gap between the rod 66 (or the central portion 68c of the member
68) and the tube 62 (or the peripheral portion 68b of the member 68) is constricted
._,
~ 5 between the trailer 74 and the sleeve 90 into which the trailer 74 moves. Also, the
~.
contact 60, comprising the rod 66, is shielded to some extent from the ignition
~r products of the power cartridge 72 by the trailer 74 and its conformal movement
,:
through the bore 88 of the sleeve 90. Further, if the trailer 74 and the sleeve 90 are
made of ablative arc-extinguishing material, the voltage of the first arc is elevated
10 and the arc may be ultimately extinguished by the action thereof. Thus, the first gap
. ~
"-^ is insulated by the trailer 74 and the sleeve 90 which are interposed between the
contacts 60 and 61 as the gap is opened.
The above-described rightward movement of the rod 66 telescopes it and
lS the sleeve 86 thereon into the bore 96 of the sleeve 94. This results in the opening of
a second gap between the contact 60 (for the central portion 82c of the member 82)
and the terminal 80 (or the peripheral portion 82b of the member 82), because of the
interpositioning of both sleeves 86 and 94 between the contact 60 and the terminal
j ~ 80. The telescoping of the sleeves 86 and 94 also shields the end of the rod 6fi
~ 20 carrying the central portion 82c of the metallic member 82 from any of the ignition
,j 5 products of the power cartridge 72 which may be in the vicinity of the peripheral
,......
portion 82b of the member 82. A second arc which may form between the rod 66 and
the tube 85 is constricted by the sleeves 86 and 94, which, if made of the preferred
arc-extinguishing materials, will elevate the voltage of the second arc and may extin-
25 guish it.
The combined action of all of these events--the formation of the two
gaps, the insulation of both gaps by the trailer 74 and the sleeves 86, 90, and 94, and
t;
the constriction, voltage elevation, and extinguishment of the arcs which may form in
30 each gap--ensure that when the switch section 14 opens the first current path 58,
L? current is commutated to the second current path 98 and to the fuse therein.
/ .
f: - 14
Referring now to FIGURE 3, there is shown a preferred embodiment of
the present invention. In the switch section 14' of FIGURE 3, two insulated gaps are
opened. Also, electrical connection of the second path 98 to the first path 58 is timed
with respect to the formation of the two gaps to ensure improved operation of the
fuse 10. Where possible, the same or similar reference numerals as used in FIGURE 2
t- .
have been used in FIGURE 3 to denote the same or similar elements.
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L~ In FIGURE 3, the contact 61 comprises a tube or cup-shaped member 62t
which may be attached to or formed integrally with an end member 104. The tube 62'
lO and the end member 104 together define the chamber 70 for the power cartridge 72 in
~: combination with the trailer 74' which in FIGURE 3 has a configuration more like a
~,- piston than the trailer 74 in FIGURE 2, and in that regard, resembles the piston
depicted in ~he '009 application. The end member 104 may be formed integrally with
or otherwise electrically connected to the terminal 22 depicted in FIGURE 1. Through
15 the end member 104 may pass the output conductors 46 of the sensing and triggering
unit 44, also shown in ~IGURE 1.
As noted immediately above, the trailer 74' of PIGURE 3 has a piston-
like configuration and conformally moves through the bore 78 formed in the tube 62'.
20 As in FIGURE 2, the trailer 74' is attached to the contact 60. In FIGURE 3, the
contact 60 comprises a metallic conductive tube 106 which has a greater diameter at
the left thereof than it does at the right thereof. The tube 106 is connected to or
~":J
formed integrally with a body member 108, also made of a conductive material to
which is attached, as convenient, the trailer 74'. A diaphragm 68', which may be?,~
c~ 25 similar to the diaphragm 68 in FIGURE 2, normally electrically interconnects the tube
~,J
62' and the body member 108. In this way, the contacts 60 and 61 are normally elec-
trically interconnected. Movement of the trailer 74' due to pressurization of the
chamber 70 by the power cartridge 72 moves such trailer 74', as well as the body
member 108 and the tube 106 rightwardly.
- 15 -
7~
The larger diameter portion of the tube 106 is lined with an insulative
sleeve 110, preferably made of an ablative, arc-extinguishing material. The smaller
diameter portion of the tube 106 is preferably unlined and defines a bore 112 which is
continuous with and shaped similarly to a bore 114 defined by the sleeve 110.
The tube 106 also carries on its exterior a sleeve 86' somewhat differ-
ently configured than, but serving a similar function to, the sleeve 86 in FIGURE 2.
~,
As shown in FIGURE 3, the sleeve 86' extends somewhat to the right of the tube 106.
, Carried by or embedded in the sleeve 86' is a annular conductive member or bridge 116
which serves a function described below. Both the sleeve 86' and the bridge 116 are
~;,
,, ~ carried by and move with the tube 106.
~,
An insulative cylinder 64', similar to the cylinder 64 in FIGURE 2, is
attached to or mounts both the tube 62' and the end member 104. The cylinder 64'
15 also carries the insulative sleeve 90 preferably made of an ablative arc-extinguishing
material. The trailer 74', the body member 108, and the tube 106 with the sleeve 86'
thereon, as well as the bridge 116 carried by thé sleeve 86', are conformally movable
through the bore 88 defined by the sleeve 90.
2~ 20 In FIGURE 3, the terminal 80 is constituted somewhat differently than it
~j is in FIGURE 2. Specifically, the terminal 80 comprises a conductive rod-like member
118 terminating in an enlarged head 120. The head 120 is conformally movable
throu~h the bores 112 and 113 as the contact 60 moves rightwardly, and is normally
maintained in sliding electrical engagement with the bore 112 defined by the tube
~,...
,~ 25 106. The outside of the rod 118 is covered by an insulative sleeve 122, preferably
made of an ablative, arc-extinguishing material. The rod 118 may be mounted to or
~,~
formed integrally with an end member 124, which may in turn be connected to or
formed integrally with the terminal 20 depicted in FIGURE 1. The housing 16
depicted in FIGURE 1 may be maintained around the elements thus far described by
30 end ferrules 126 which are attached between such housing 16 and the respective end
members 104 and 124. The cylinder 64' may serve as a support about which the fuse
section, shown only schematically at 12, may be wound. If the fuse section 12 is the
- 16 -
66~1
preferred silver-sand current~limiting variety, its element may be supported by the
cylinder 64' and is surrounded by sand occupying the volume defined between the outer
housing 16 and the cylinder 64' acting as the inner housing 19.
In the operation of the device depicted in FIGURE 3, which contains the
switch section 14' thus far described, a normal first path 58 exists between the ter-
minals 22 and 20 in FIGURE 1 as fonows: the member 104, the tube 62', the
~,~
~ diaphragm 687, the body member 108, the tube 106, the interface between the tube 106
~ and the head 120, the rod 118, and the end member 124. Upon ignition of the power
a' 10 cartridge 72, the trailer 74' and the contact 60 are moved rapidly to the right. As
~ described with reference to FIGURE 2, rightward movement of the contact 60 severs,
s,~ .
tears, or rips the diaphragm 68, breaking the normal electrical interconnection
between the tube 106 and the tube 62'. The piston 74', because of its configuration,
ensures efficient rightward movement of the contact 30 and, also similar to FIGURE
v- 15 2, isolates the tube 106 and the body member 108 from the ignition products of the
power cartridge 72 and constricts any arc forming between the tube 62' and the body
member 108. If the trailer 74' and the sleeve 90 are made of the preferred arc-extin-
guishlng material, any arc forming between the contact 50 and 61 has its voltage
elevated and may be extinguished. Thus, a first gap, similar to that described with
reference to ~IGURE 2, is~formed between the tube 62' and the body member 108.
The first gap is insulated by the interposition of the trailer 74' and the sleeve 90
between the contacts 60 and 61. In the embodiment of FIGURE 3, a second gap is
formed, but not immediately.
,,:,.
r
4~ 25 Specifically, during the initial portion of the rightward movement of the
C; contact 60, sliding electrical contact between the head 120 and the wall 112 of the
tube 106 prevents formation of the second gap. Ultimately, the head 120 engages the
bore 114 defined by the sleeve 110. At this point, electrical connection between the
head 120 and the tube 106 is broken; immediately thereafter, the rightward portion of
the bore 114 of the sleeve 110 engages the sleeve 122 on the rod 118. Thus, only after
a certain amount of rightward travel of the contact 560 is a second ins~ated gap
opened between the contact 60 and the terminal 80.
iL!~
The switch section 14' of FIGURE 3 preferably delays the connection of
the second path 98 to the first path 58 until sometime after the first gap is opened,
but before the second gap is opened. Specifically, in FIGURE 3 the second path 98
may have one point of connection 100 directly to the contact 61 as shown. The other
5 point of connection 102', however, is normally not electrically connected to the first
current path 58. As depicted in FIGURE 3, the second connection point 102' may take
the form of a stud 128 connected to one end of the second path 98 and passing through
the cylinder 64' and the sleeve 90 as shown. A second stud 130, which similarly passes
through the cylinder 64' and the sleeve 90, is diametrically opposite the stud 128 and is
10 electrically connected by a conductor 132 to the end member 124, as shown. In the
embodiment of FIGURE 3, after the first gap is opened and insulated between the
contacts 60 and 61, but while the contact 60 and the terminal 80 are still connected,
the bridge 116, which is being carried rightwardly due to rightward movement of the
contact 60, simultaneously engages the studs 128 and 130. At this time, the second
15 path 98 is electrically connected in shunt to the first path 58 between the end member
124 and the end member 104. At the time of the connection of the second path 98 to
the first path 58, the action of the trailer 74' and the sleeve 90 has, as described
above, either elevated the voltage of, or extinguished the arc in the first gap. Thus,
when the second path 98 is connected in shunt with the first path 58, current formerly
20 in the first path 58 is commutated to the second path 98. The fuse section 12 in the
second path 98 begins to operate, that is, to interrupt the commutated current in
either a current-limiting or a non-current-limiting mode. As is well known, fuse
operation typically involves the formation of one or more arcs, the voltage of which
called back voltage or arc voltage elevates as arc-extinguishing action occurs. As the
25 fuse section 12 operates, the second gap is opened by the action of the sleeves 86' and
122. If the current has commutated to the second path 98, the second gap is opened at
a time when no current is in the first path 58 and no arc forms in the second gap.
Thus, the second gap is virtually uncontaminated by either the ignition products of the
power cartridge 72 or by the action of any arc therein. Accordingly, the elevated
30 back voltage of the fuse 12 is unable to retransfer current to the first path due to the
high dielectric strength thereof effected by the presence of two gaps therein, one of
which is substantially uncontaminated.
-- 18 -
~66ti71
It should be obvious to those skilled in the art that the formation of the
second gap and the connection of the second path 98 to the first path 58 may be
adjusted to occur at any predetermined time after the formation of the first gap.
Thus, depending on the voltage and current of the circuit to which the device lû is
5 connected, commutation of the current and prevention of current retransfer can be
ensured by appropriate selection of the dimensions and relative positions and configur-
ations of the tube 106, the rod 118, the sleeves 86', 110 and 122, the brid~e 116, and
the studs 128 and 128.
The preferred embodiment of FIGURE 3 also contemplates that it may
not be necessary or possible to form the second gap when no current is in the first
path 58. For example, the second gap may be formed at a time when an arc is still
present in the first gap (and current is still in the first path 58) either by design or due
to the inability of the trailer 74' and the cylinder 64' to extinguish the arc, given the
15 voltage and current conditions of the circuit to which the fuse 10 is connected. In this
event, the formation of the second gap may act in aid of the first gap by extinguishing
or raising the arc voltage of a second arc formed therein. Moreover, the connection
of the second path ~8 in shunt with the first path 58 may be timed to precede or
follow the formation of the second gap, again in view of the voltage and current
20 conditions of the circuit.
Various other changes may be made in the above-described embodiments
of the present invention without department from the spirit and scope thereof. Such
changes, as are within the scope of the claims that follow, are intended to be covered
25 thereby.
-- 19 --
