CA1052425A - Expulsion type current limiting fuse device - Google Patents

Abstract

ABSTRACT: A current-limiting device is joined in series with an expulsion fuse to provide current interruption for a full range of fault currents. The expulsion fuse portion is joined to the current-limiting device in such a way as to reduce the overall length of the combination by allowing the operative elements of the expulsion fuse portion to move into the current-limiting fuse portion upon operation of the expulsion fuse portion.
Also, in those instances where it is desirable to use an exhaust control device in conjunction with the expulsion fuse portion, the exhaust control device can be positioned within the current-limiting device to reduce the overall length of the device.

Description

~ q- S~,5f~d~ 3 T~e pxese~t inyent~,on xelates to h;igh Yaltage power fuses and ~ore paxtiucl~,xl,~ t~ the com~ination o~ current~limiting deYices ~ith expuls~on ~uses to obtai,n a desirable full range oP
ope~ating characte~istics~
~xpulsion ~uses are well known in the art and are deslyned to interrupt relativel~ low leyels of current. However, expulsion fuses are not as suitable for high current interruption as current-limiting fuses ~or several reasons. For example, the forces resulting from gas pressure caused by fuse operation for high current leyels are extremely large, necessitating expensive housing designs to withstand the forces created by expulsion fuse operation at high current leyels. In addition, `
expulsion fuses designed to operate at high current levels can ', produce objectionable noise, hazardous gases and shrapnel during - 15 operation. Further, for cost and size of package, current~limiting devices are more suited for high current interruption.
High currents can be limited by a current-limiting `"~
fuse arrangement which operates to limit prospective extremely `
high fault currents to a much lower level~ However, current~
limiting devices are inherently unsuited for interrupting current ' flow at low current levels. ', It has been previously recognized that current-limitlng ,'~ ' fuses and expulsion type fuses can be connected serially to provide a greater range of current interruption capabilities. For example, , U.S. Patent No. 2,917,605 ~ Fahnoe recognizes that current- ' limiting fuse elements can be associated ~ith expulsion type ~, fuse elements to proYide a broad spectrum o~ current interruption ,~, , cap~bilities. Howe~ex, such prior art combinations haYe resulted ', in an undesixable lon~ fuse deyice since the current~limiting '~
fuse and the expuls~on fuse have been serially ~oined. This .
, ~. ' - , , increased length ~eclui~es ~ore expensi~e and bulk~ fuse mounting e~uipment and tends to ~reclude utilizat~on of the conventional fuse mounting structure.
Therefo~e, it would be a highl~ desirable advance in the ~rt to proYide a Xuse deYice that c~bines the high current-limit~ng properties o~ a cur~ent-limiting fuse and the low current interrupting properties of an expulsion fuse in a relatively short hous~ng so th~t the most efficient operating range of both deyices can be optimized to provide for efficient interruption across a greater range of currents.
A fuse deyice in accordance with the present invention -comprises the utilization of an expulsion fuse means and a current-limiting means that are joined together so that the internal por~
tions of the current-limiting means can xeceive the operative portions of the expulsion fuse means thereby reducing the overall length of the fuse device.
More specifically, a fuse device for interrupting current flow between two points of an electrical circuit in accordance with the present invention comprises a hollow first housing formed of an electrically insulating material having a first end plate connected to one point of the circuit sealing one end of the first housing and a second end plate sealing the other end of the first housingO The second end plate has an aperture opening at the center thereof through which a hollow thimble is mounted so ~;
that the thi~ble is positioned within the first hollow housing.
fuse element connects the first and second end plates. The fuse element will fuse when a predetermined current level is exceeded. A hollow second housing ~ormed of an electrical insulatlng material is mounted at one end to the second end plate around the cperture. A second electrical terminal connected to ~2-.~" '' .

another ~oint of the electxical circ~lit is moun-ted on the other end of the secolld hous~ng~ ~ moY~ble arcing xod is movably mounted ~ithin the second hous~ng and a fusible wiXe connects the arcing xod and the third end ~late. The fus~ble wire Will fuse when a second predetermined current level is exceeded. Contact means 5 are provided connecting the second end plate and the arcing rod ;
so that a circuit is completed between the first end plate and second terminal. Spring means are provided within the second housing for urgin~ the arcing rod towards the hollow thimble so that the arcing rod will be rapidly moved into the hollow thimble within the first housing when the fusible wire fuses.
An insulating material such as quartz sand as is well known in the art is placed in the first housing to assist in the ~ ;
current-limiting function of the current-limiting portion of the fuse device~ A hollow annular liner, foxmed of a material capable of producing an arc-quenching gas when exposed to an electrical aXc, is positioned in the second housing around the arcing rod.
Thus, the liner facilitates the extinguishment of the art after ` -the fusible wire fuses.
The fuse element in the first housing may be an essentially flat ribbon having two opposite flat sides. This fuse element may be helically wound in an edge-wound configuration so that the flat sides of the fuse element are essentially perpendicular to the center line between the first and second end plates.
Suitable mounting brackets may be affixed to the first and third end plates for mounting the fuse device to electrical circuit elements.
Thus, it is a primary object of the present invention to ~
pxovide a fuse deyice combining the relati~e advantages of a ~-current~limit1ng de~ice and an expulsion fuse so that a wide range ~ . .
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of curxent interruption perfoxmance can be proYided in a relatiyel~ short hous~n~
These ~nd othex objects, advantages and ~eatures shall here~nafter appea~ in the ~ccomp~n~ing dr~wings.
F~GUR~ 1 is a cross-sectional side View of a preferred embodiment of the present invention.
~IGU~E lA is a cross-sectional vie~ of the embodiment shown in FIGURE 1 taken substantially along line lA-lA in FIGURE 1.
FIGURE 2 is a cross-sectional side view of an alterna-10 tive embodiment of the present invention. ;
~ IGURE 2A is a cross-sectional view of the alternative embodiment shown in FIGURE 2 taken substantially along line -2A-2A in FIGURE 2.
FIGURE 2B is a cross-sect1onal view taken substantially along 2B-2B ln F~GURE 2 FIGURE 3 is a cross~sectional side view of another altexnative embodiment of the present invention.
With reference to FIGURE 1, fuse device 10 comprises current-limiting portion 11 and expulsion portion 13. Current-20 limi~ing portion 11 comprises a first cylindrical housing 12 -~
formed of an electrically insulating material having the ends thereof sealed by first end plate 14 and second end plate 16.
Formed in the center of second end plate 16 is aperture 18 surrounded by flange 20 that is formed perpendicular to the surface of second end plate 16. Mounted to first end plate 14 is mounting terminal 22 having an opening therein for mounting fuse device 10 to an appropriate portion of an electrical circuit.
Mounted to second end plate 16 and extending through aperture 18 is hollow thimble 24 com~ised of insulating material, ~hich extends through first cylindrical housing 12. Hollow ~4~

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.. ,. ,~ , ~'D~ ''3 ;, thimble 24 is ~entagonal in cxoss section and has ~ hollow circulax inte~io~ poXtion. ~ounted in gXooYes at the corner apexes of the hollo~ thimble 24 (see FIGURE lA~ are suppcxt strips 26. Fox~ed on Support stri~s 26 are tongues 28 (see FIGURE 1) which support a fusible element 30 that is helically wound around support strips 26. Fusible element 30 is connected at one end to first end plate 14 and at the other end to second end plate 16. Fusible ele~ent 30 has formed therein indentations 31 along the edge thexeof to provide initial fusion points and also to facilitate winding of the element.
Fixst housing 12 is filled with a suitable insulating material such as ~uartz sand 32 that assists in the current-limiting function of the current-limiting portion 11 when fusible element 30 fuses.
Mounted to the flange 20 on second end plate 16 is second hollow housing 34 formed of an electrical insulating ;
material, Mounted over the opposite end of second hollow housing 34 i5 third end assembly 36 which seals second hollow housing 34.
Third end assembly 36 comprises ferrule 38 partially inserted into second hollow housing 34 over which is mounted cap 40. Mounted within ferrule 38 is contact member 48. Also mounted externally to ferrule 38 is mounting terminal 42.
Positioned within second hollow housing 34 is annular liner 44 formed o~ a material that will produce an arc quenching ;~
gas when exposed to an elèctrical arc. Annular liner44 has a hollow interior in which is positioned arcing rod 46. Arcing rod 46 is connected at one end to contact member 48 b~ fusible wires 50 ~nd 52. Fusible Wire 52 is made of a nichrome and fusible wire 50 is made of silver. ~ilyex wire 50 is the main current carrying ele~ent and fuses in xesponse to heatin~ when a predetexmined .

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current level is exceeded. Nichrome wire 52 is provided to ~ithstand force~s e~erted upon the a~cin~ rod so that the silver wire 50 does not h~e ~n~ tensile foxce exerted upon it. Nichrome wire 52 fuses immed.~ately upon the fusion of silver wire 50.
Connected to the opposite end of arcing rod 46 is con-tact button 56 and posit~oned immediately adjacent to contact button 56 is spring insulator 58. Spring cup 60 is mounted within second houslng 34 and electrically engages flange 20 of second end plate 16 and also elect.rically engages contact button 10 56 connected to arcing rod 46. ~ spring 62 is mounted within spring cup 60 and engages spxing insulator 58~ Spring 62 is compressed in the position shown in FIGURE 1 so that arcing rod 46, contact button 56, and sprin~ insulator 58 are all urged to~ard the left as shown in FIGURE 1 thereby causing tensile 15 stress to be exerted upon nichrome wire 52.
- Fusible element 30 is designed so that when a pre-determined current flowing through fusible element 30 is exc~eded, fusible element 30 will fuse initially at indentations 31 on the edge of fusible element 30. If a severe and high 20 magnitude fault occurs in the circuit being protected by the fuse device 10, fusible element 30 will almost instantaneously fuse and vaporize the silver material from which fuse element 30 is ,`:
fabricated. Fusible element 30 thus acts in such a way as to effectively insert a high resistance between the two end plates -~
25 thereby limi.ting the current to a fraction of the potential fault ;
current that would otherwise flow.
~ilver wixe 50 and nichrome wire 52 are d~si~ned to ef~ectivel~ operate to intexrupt curXent rlow at much lower current leyels than the cuxrent-limiting portion 11 of fuse deyice 10.
The current-limiting portion 11 of fuse device 10 , ~6- :

operates mo~st effectiyel~ to inte~xupt high c~rrent levels, whereas e~pulsion fuse poxtion 13 of ~use dey~ce lO operates most effectivel~ to intexrupt lowex current leYels.
After silYer wire 50 and nichrome wire 52 fuse, spring -~
62 moVes areing rod 46 through the opening in linex 44 until the end of arcing rod 46 has moved all the way into thimble 24. When silvex wire 50 and nlehrome wire 52 fuse, an arc forms between ~reing rod 46 and eontact member 48. However, as areing rod 46 moves rapidl~ through liner 44 and the are is "stretehed", the are eomes in contaet with annular liner 44 Whieh is formed o~ a material which emits an arc-quenehing gas when exposed to an eleetrieal arc. Thus, the rapid combination of the movement of ~
areing rod 46 and the emission of are-quenching gas from liner i :
- 44 tends to extinguish the arc thereby interrupting current flow. ;
~ne uni~ue feature of the present invention is the eombination of a current-limiting fuse portion and an expulsion t~pe fuse portion so that the moving parts of the expulsion type ;
fuse, namely arcing rod 46, contact button 56 and spring insulator 58 move into a space within the current~limiting portion of the ~use, i.e., thimble 24. It would ordinarily be expected that it would be impractical to permit metallic components to enter into or along the length of the current-limiting portion of a fuse device because this might limit the voltage withstand capabilities of the eurrent-limiting portion of the fuse device.
However, it has been discoyered that this unique eombination does not affeet the voltage withstand eapabilities of the eurrent-limiting portion of the fuse deyice. For example, at overlo~d or low ~ault curxent levels, only the expulsion fuse portion, i.e. ! onl~ s~ilYer wire 50 and nichrome wixe 52 fuses ' ' . , 'f, . . .
and fusible element 30 remalns intact. In this instanee, no :
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-Yolta,ge ~p~eaxs ac~ss ~l,rst a,~d seco~d end Rla~es 14 and 15 and the pXesence o~ the ~rc~ng ~od 46 ! con~act blltton 56r and spring ~nsula,tor 58 ~th~n th,im~le 24 has a limited, i~nconse~uential e~ect upon the vo~ta~e wi,thstand capabillties o~ the current-limiting portion of the ~use device 10.
,~t lntermediate current levels, where the current is high enou~h to melt the fusible element 30 as well as silver wire 50 ~nd nichrome wixe 52, the movement of the fuse components ~; , into thimble 24 still produces no detrimental effect because either the expulsion fuse portion of the fuse device 10 is suffi-cient to interrupt current flow without assistance from the current-limiting portion, or at currents of this level, the '' maximum arc voltage produced by the current-limiting section is relatively low, tending to be inversely proportional to the excursion of the fuse arcing rod at the time crest voltage occurs.
Thus, the withst~nd capabilities of the current-limiting portion enclosed in housing 12 are not affected, and the thimble is thus '~ ~;
not subjected to severe dielectric stress. '~-~
At high fault currents, interruption is substantially brought about by the current-limiting section. The current is rapidly forced to a low level when fusible element 30 fuses, and is thus interrupted before a significant motion of fuse arcing ,~
xod 46 occurs. Thus, during the portion of time that maximum voltage exists between first and second end plates 14 and 16, the metallic fuse components, i.e., arcing rod 46 and contact button '~
56, do not extend far enough into thimble 24 so as to reduce the voltage withstand capabillty and cause dielectric failure of the thimble wall between the element 30 and the metallic use parts.
~ith reference to FIGU~E 2, an alternative embodiment of the present inYention is illustrated. Fuse device 210 comprises `

current-limltin~ po~tiQn 211 and ex~ulsion portion 213. Current- ;
li~iting portion 2ll com~riseS ~irst c~lindrical housin~ 212 sealed at one end b~ first end plate 214 and sealed at the other end o~ second end plate 216~ Second end pla,te 216 has formed `~
therein aperture 218 ~round which is formed flange 220. A
mountin~ terminal 222 is mounted to first end plate 214 for mounting the fuse device to appropriate portions of an electrical circuit.
Positioned within first cylindrical housing 212 is hollow thimble 224. Hollow thimble 224 has a pentagonal cross section and supported at grooves at each corner of the pentagonal cross section and supported at grooves at each corner of the pentagonal hollow thimble 224 are support strips 226 ~see FIGURE 2A).
~ongues 228 are formed on support strips 226 tsee ~IGURE 2) and support a fusible element 230. Fusible element 230 has indenta-tions 231 formed around the edge thereof to facilitate hexagonal windiny of the fusible element 230, and also to provide a reduced area for initial ~usion of fusible element 230. First cylindrical housing 212 is filled with a suitable insulating material such as quartz sand 232 that assists in the current-limiting function of the current limiting portion 211. Mounted to flange 220 around aperture 218 is second hollow housing 234. Partially inserted into the other end of second hollow housing 234 is third end assembly 236 comprising cylindrical portion 238, sealing plate 25 240 and mounting terminal 242.
Positioned within second hollow housing 234 is hollow annular liner 244 which is formed of ~ mate~ial that produces an arc~uenching yas when exposed to an electrical ~rc. Positioned ~thin the hollow portion of annular portion 244 is arcing rod 246~ -~
A contact member 248 ~see FIGURES 2 and 2B) is electrically-' ~ .~3~
connected to fl~n~e 22Q o~ sec~nd end pl~te 216. Axcing rod 246 is connec!ted to contact membe~ 248 by silyer wire 250 and nichxome wire 252. ~ilverwire 25Q and nichxome wire 252 are ,'.
designed to fuse in response to heating when a predetermined curXent leyel is exceeded~ ~
A ca,ble 256 is connected between arcing rod 246 and cylindrical portion 238 of third end assembly 236 so that a positive electrical connection is maintained between between the arcing rod 246 and the third end ~ssembly 236~ A spring 258 is ~.
connected to arcing rod 246 and to sealing plate 240. Spring 258 is stretched so that it tends to ur~e arcing rod 246 towards :
the right as viewed in FIGURE 2. However, as long as nichrome ~,' '' wire 252 remains intact, arcing rod 246 is reta,ined in the ~, position illustrated in ~IGURE 2.
Thimble 224 forms an exhaust control chamber that may be filled with any suitable condensing media such as alumina particles 260 which act as a muffler to condense hot arc gases when silver wire 250 and nichrome wire 252 are fused during the operation of expulsion portion 213. ~ perforated cap 262 retains ' alumina particles 260 within hollow thimble 224, but allows the entry of arc gases during the operation of expulsion portion 213.
The ~IGURE 2 embodiment operates in substantially the '`,i`~:' same manner as the FIGURE 1 embodiment except that the arcing xod mo~es away from the current-limiting portion 211 during '~
25 operation of the expulsion portion 213. Thus, there is no ' movement of metallic ~embers into the current~limiting portion 211 during operation of the fuse deYice 210. The muffler portion '~, comprising alumina pa~ticles 260 ls positioned within the cur~ent-limitin~ portion 211 to facilitate the condensation of hot arc gases thereby permit~ing operation of the expulsion r~10--. . . ', .

~ortion 213 ~lthout discharge of hot gases to the ambient atmosphere~
F2GUXE 3 illustxates a third embodiment of -the present invention~ In this e~bodiment ~use device 310 comprises two current-limiting portions 312 and 314 connected to opposite ends o~ an expulsion portion 316. It can be readily observed ~`
~ithout detailed explanation that the FIGURE 3 embodiment comprises a combination of the FIGURE 1 and FIGURE 2 embodiments so that ~:
the arcing rod 320 moves into thimble 322 in current-limiting portion 312 and the arc ~ases are condensed in muffler 324 in current-limiting portion 314 upon operation of expulsion portion :
316. The FIGURE 3 embodiment provides increased current inter-xupting capabilities at high current levels since the separate : :
current-limiting portions 312 and 314 need to only be capable of providing one-half the total interrupting capabilit~ of the fuse device 310 at high current levels. ~ ;

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Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A current-limiting fuse device for interrupting current flow comprising:
(1) an expulsion fuse means comprising at least one fusible element for optimumly inter-rupting current flow in a first current range, said expulsion fuse means having a movable arcing rod that moves when said expulsion fuse means inter-rupts current flow, said expulsion fuse means producing hot arc gases when said expulsion fuse means interrupts current flow, (2) in combination with at least one current-limiting means for optimumly interrupting current flow in a second current range, said current-limiting means containing a hollow thimble means disposed in the interior portion thereof, said expulsion fuse means and at least one of said current-limiting means being joined together and arranged in a position such that said hollow thimble means of the current-limiting means receives at least one of said movable arcing rod and said hot arc gases, without adversely affecting the voltage withstand capabilities of said current-limiting means, thereby reducing the overall length of said fuse device.

2. A current-limiting fude device for interrupting current flow comprising:
an expulsion fuse means comprising at least one fusible element for optimumly inter-rupting current flow in a first current range, said expulsion fuse means having a movable arcing rod that moves when said expulsion fuse means interrupts current flow; and a current-limiting means for optimumly interrupting current flow in a second current range, said current-limiting means containing a hollow thimble means disposed in the interior portion thereof;
said expulsion fuse means and said current-limiting means being joined together and arranged in a position such that said hollow thimble means of the current-limiting means receives said movable arcing rod when said expulsion fuse means interrupts current flow without adversely affecting the voltage withstand capabilities of the current-limiting means thereby reducing the overall length of the fuse device.

3. A current-limiting fuse device, as claimed in claim 2 wherein said current-limiting means comprises:
a hollow first housing formed of an electrically insulating material;
a first conductive end plate sealing one end of said first housing connected to the first point of an electrical circuit;
a second end plate sealing the other end of said first housing, said second end plate having an aperture formed through the center thereof;
a fuse element connecting said first and second end plates, said fuse element formed of a material that will fuse when a first predetermined current level is exceeded;

a hollow thimble mounted to said second end plate through the aperture so that said thimble is positioned within said first hollow housing.

4. A current-limiting fuse device, as claimed in claim 3, wherein said expulsion fuse means comprises:
a hollow second housing formed of an electrically insulating material mounted at one end to the second end plate around said aperture;
an electrical terminal connected to the other end of said second housing, said electrical terminal connected to the second point of the electrical circuit;
said movable arcing rod being positioned within said second housing;
a fusible wire means connected between said arcing rod and said terminal, said fusible wire means formed of a material that will fuse when a second predetermined current level is exceeded;
contact means connecting said second end plate and said arcing rod so that a circuit is completed between said first end plate and said terminal;
spring means within said second housing for urging said arcing rod towards said hollow thimble so that said arcing rod will be rapidly moved into said hollow thimble within said first housing when said fusible wire means fuses.

5. A fuse device, as claim in claim 4, wherein the first hollow housing is filled with an insulating material.

6. A fuse device, as claimed in claim 5, wherein said insulating material is quartz sand.

7. A fuse device, as claimed in claim 4, 5 or 6 further comprising an annular liner having a hollow center positioned around said arcing rod in said second housing; said liner formed of a material capable of producing an arc-quenching gas when exposed to an electrical arc.

8. A fuse device, as claimed in claim 4, 5 or 6 wherein said fuse element is an essentially flat ribbon having two opposite flat sides and said fuse element is helically wound so that said flat sides of said fuse element are essentially perpendicular to a center line between said first and second end plates.

9. A fuse device, as claimed in claim 4, 5 or 6 further comprising mounting terminal means attached to said first end plate for mounting said fuse device.

10. A fuse device, as claimed in claim 4, 5 or 6 wherein said fusible-wire means comprises a silver wire and a nichrome wire.

11. A current-limiting fuse device for interrupting current flow comprising:
an expulsion fuse means comprising at least one fusible element for optimumly interrupting current flow in a first current range, said expulsion fuse means producing hot arc gases when said expulsion fuse means interrupts current flow; and a current-limiting means for optimumly interrupting current flow in a second current range, said current-limiting means containing a hollow thimble means disposed in the interior portion thereof;
said expulsion fuse means and said current-limiting means being joined together and arranged in a position such that said hollow thimble means of the current-limiting means receives said hot arc gases when said expulsion fuse means interrupts current flow without adversely affecting the voltage withstand capabilities of the current-limiting means thereby reducing the overall length of the fuse device.

12. A current-limiting fuse device, as claimed in claim 11 wherein said current-limiting means comprises:
a hollow first housing formed of an electrically insulating material;
a first conductive end plate sealing one end of said first housing connected to the first point of an electrical circuit;
a second end plate sealing the other end of said first housing, said second end plate having an aperture formed through the center thereof;
a fuse element connecting said first and second end plate, said fuse element formed of a material that will fuse when a first predetermined current level is exceeded;
a hollow thimble mounted to said second end plate through said aperture so that said thimble is positioned within said first hollow housing.

13. A current-limiting fuse device, as claimed in claim 12, wherein said expulsion fuse means comprises:
a hollow second housing formed of an electrically insulating material mounted at one end to said second end plate around said aperture;
an electrical terminal connected to the other and of said second housing, said electrical terminal connected to the second point of the electrical circuit;
a movable arcing rod positioned within said second housing;
first contact means electrically connected to said second end plate and positioned over said aperture, said contact means having openings there-through communicating with the hollow portion of said thimble;
fusible wire means connecting said contact means and said arcing rod, said fusible wire means formed of a material that will fuse when a second predetermined current level is exceeded;
second contact means connecting said arcing rod and said electrical terminal so that a circuit is completed between said first conductive end plate and said electrical terminal;
muffler means within said hollow thimble for condensing arc gases resulting when said fusible wire means fuses;
spring means within said second housing for urging said arcing rod away from said contact means so that said rod will move rapidly away from said contact means when said fusible wire means fuses.

14. A fuse device, as claimed in claim 13, wherein said muffler means comprises activated alumina particles.

15. A fuse device, as claimed in claim 13, wherein said first hollow housing is filled with an insulating material.

16. A fuse device, as claimed in claim 15, wherein said insulating material is quartz sand.

17. A fuse device, as claimed in claim 13, 14 or 15 wherein said fuse element is an essentially flat ribbon having two opposite flat sides and said fuse element is helically wound so that said flat sides of said fuse element are essentially perpendicular to a center line between said first and second end plates.

18. A fuse device, as claimed in claim 13, 14 or 15 further com-prising mounting bracket means attached to said first end plate for mounting said fuse device.

19. A fuse device, as claimed in claim 13, 14 or 15 further com-prising an annular liner having a hollow center positioned around said arcing rod in said second housing; said liner formed of a material capable of producing an arc-quenching gas when exposed to an electrical arc.

20. A fuse device, as claimed in claim 13, 14 or 15 wherein said fusible wire means comprises a silver wire and a nichrom wire.

21. A current-limiting fuse device for interrupting current flow between a first and second point of an electrical current comprising:
a first current-limiting means comprising a hollow first housing formed of an electrically insula-ting material;
a first conductive end plate sealing one end of said first housing connected to the first point of -the electrical circuit;
a second end plate sealing the other end of said first housing, said second end plate having a first aperture formed through the center thereof;

a first fuse element connecting said first and second end plates, said first fuse element formed of a material that will fuse when a first predetermined current level is exceeded;

a hollow first thimble mounted to said second end plate through said first aperture so that said thimble is positioned within said first hollow housing;
in combination with an expulsion fuse means comprising a hollow second housing formed of an electrically insulating material mounted at a first end to the second end plate around said aperture;
first contact means mounted at the other end of said second housing, said first contact means having openings therethrough;
a movable arcing rod positioned within said second housing;
a fusible wire means connected between said arcing rod and said first contact means, said fusible wire means formed of a material that will fuse when a second predetermined current level is exceeded;
second contact means electrically con-necting said second end plate and said arcing rod;
spring means within said second housing for urging said arcing rod towards said first hollow thimble so that said arcing rod will be rapidly moved into said first hollow thimble within said first housing when said fusible wire means fuses;
and a second current limiting means com-prising a hollow third housing formed of an electri-cally insulating material;
a third conductive end plate sealing one end of said third housing connected to the second point of the electrical circuit;

a fourth end plate sealing the other end of said third housing, said fourth end plate having a second aperture formed at the center thereof;
a second fuse element connecting said third and fourth end plates, said second fuse element formed of a material that will fuse when a third predetermined current level is exceeded;
a hollow second thimble mounted to said second end plate through said second aperture so that said thimble is positioned within said third hollow housing;
said hollow second housing being mounted at a second end to said fourth end plate around said second aperture;
muffler means within said second hollow thimble for condensing arc gases resulting when said fusible wire means fuses.