US3599137A

US3599137A - Current limiting device

Info

Publication number: US3599137A
Application number: US838238A
Authority: US
Inventors: Toshio Ito; Toshio Miyamoto; Yuichi Wada
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1968-07-05
Filing date: 1969-07-01
Publication date: 1971-08-10
Anticipated expiration: 1988-08-10
Also published as: GB1209020A

Abstract

Two opposite electrodes electrically interconnected through a current limiting material are united into a unitary structure with a solid insulation interposed between them. A flow of overcurrent through the electrodes causes the material to evaporate to isolate the electrodes from each other. Simultaneously, a portion of the current limiting material disposed on that side of one electrode remote from the other electrode evaporates to move a piston for a compressed gas-filled cylinder. Alternatively, the unitary structure can act as the piston. After the material has been restored to its original solid or liquid state, the moved parts automatically return to their original positions.

Description

United States Patent CURRENT LIMITING DEVICE 8 Claims, 4 Drawing Figs.

U.S. Cl 337/21, 337/! 14, 337/l59 Int. Cl ..H0lh 85/06 Field of Search 337/21,

151, 152, 158, 159,290,401, ma t 63,404, 405, 407,409,ll4,ll9,3l5,326,33l

[56] References Cited UNITED STATES PATENTS 3,501,730 3/1970 lto etal 337/159 3,28 l ,559 10/1966 Ebensteiner 337/403 2,505,193 4/1950 Loporto., 337/ll9 2,087,905 7/l937 French 337/119 Primary Examiner Bernard A. Gilheany Assistant Examiner Dewitt M. Morgan Anorneys--Robert E. Burns and Emmanuel] l Wato ABSTRACT: Two opposite electrodes electrically interconnected through a current limiting material are united into a unitary structure with a solid insulation interposed between them. A flow ofovercurrent through the electrodes causes the material to evaporate to isolate the electrodes from each other. Simultaneously, a portion of the current limiting material disposed on that side of one electrode remote from the other electrode evaporates to move a piston for a compressed gas-filled cylinder. Alternatively, the unitary structure can act as the piston. After the material has been restored to its original solid or liquid state, the moved parts automatically return to their original positions.

CURRENT LIMITING DEVICE This invention relates to a current limiting device, and more particularly to such a device of the type wherein a flow of overcurrent through a pairof opposite electrodes causes the evaporation of a current limiting material in the form of a solid, a liquid or a powder, normally electrically connecting both the electrodes, to electrically isolate the electrodes from each other, and wherein after the electrodes have been maintained isolated from each other fora predetermined interval of time, the evaporated material is liquidized or solidified to electrically reconnect the electrodes to each other.

Inthe past, fuse means have been widely used as current limiting devices. The fuse means have been fused in response to an overcurrent flowing therethrough leading to the necessity of replacing the broken fuse by a new fuse for each fusing.

Accordingly, it is a general object of the invention to eliminate the above-mentioned replacement of fuses by the provision of anew and improved current limiting device of the type capable of being automatically restored to its conductive state after it has been brought into its interrupted state in response to a flow ofovercurrent therethrough.

It is another object of the invention to provide a current limiting device, of the type as described in the preceding paragraph, comprising improved means for preventing one of two opposite electrodes, insulatingly bonded to each other, from permanently moving away from the other electrode in response to a high pressure produced upon evaporating the associated current limiting material.

7 It is still another object of the invention to provide a current limiting device of the type, as described in the preceding paragraph, comprising'pressure buffer means for preventing a high pressure, produced upon evaporating a current limiting material, from damaging the various parts of the device.

The invention accomplishes these and other objects that become apparent as the description proceeds by the provision of a current limiting device comprising a first metallic electrode; a second metallic electrode opposing to the first electrode to encircle at least one portion of the latter electrode in a spaced relationship; a solid insulation for bonding the first and second electrodes into a unitary structure in electrically insulated relationship; and a quantity of current limiting material normally electrically interconnecting both the electrodes and responsive to a flow of an overcurrent through the device to evaporate to electrically isolate both the electrodes from each other, the current limiting material being returned back to its conducting stated after a predetermined interval of time, characterized in that the quantity of current limiting material is disposed on that side of the second electrode remote from the first electrode, and in a bore extending through both the second electrode and the solid insulation to the first electrode.

In a preferred embodiment of the invention, the current limiting device may comprise a first metallic electrode in the form of a cup encircling one end portion of the first electrode in spaced relationship, a solid insulation for bonding both the electrodes into a unitary structure in electrically insulated relationship with the radial flange embedded in the solid insulation, a pressure buffer means disposed in spaced relationship on that side of the second electrode remote from the first electrode, and a quantity of current limiting material diaposed in a space formed between the second electrode and the pressure buffer means and in a bore extending through both the second electrode and the solid insulation to the first electrode wherein, the current limiting material, normally electrically interconnecting the electrodes, responds to a flow of overcurrent therethrough to evaporate to electrically isolate the electrodes from each other, while at the same time a pressure produced upon evaporating the current limiting material is absorbed by the buffer means.

Thepressure buffer means may advantageously include a cylinder attached the second electrode to a piston slidable in the cylinder, and a rod connected to the piston and projecting from the cylinder. A pressure produced upon evaporating the current limiting material is exerted upon the piston to move the latter against the spring force of a gas trapped within the cylinder on the side of the piston opposite the evaporative material.

In an alternative form of the invention the current limiting device may comprise a first metallic electrode in the form of a rod, a second metallic electrode in the form of a rod, a second metallic electrode in the form of a cup encircling one end portion of the first electrode in spaced relationship, a solid insulation for bonding both the electrodes into a unitary structure in electrically insulated relationship, a cylindrical housing closed at one end, the above-mentioned unitary structure being 7 slidably disposed in the housing with the second electrode facing the closed end of the housing to form a spacing therebetween, an apertured end cover closing the aperture on the cover, a quantity of compressive gas filling a space confined by the housing, the unitary structure and the end cover, and a quantity of current limiting material disposed in the spacing between the closed end of the housing and the second electrode, and in a bore extending through both the second electrode and the solid insulation to the first electrode, the current limiting material normally electrically connecting the first electrode to the second electrode and responding to a flow of overcurrent therethrough to evaporate to electrically insulate the electrodes from each other while at the same time moving the unitary structure toward the end cover of the housing against the spring force of the gas.

The invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a fragmental longitudinal sectional view of a current limiting device embodying this invention;

FIG. 2 is a fragmental longitudinal sectional view of a modification of the device illustrated in FIG. 1;

FIG. 3 is a fragmental longitudinal sectional view of another embodiment ofthe invention;

FIG. 4 is a view similar to FIG. 3 but illustrating a modification of the device illustrated in FIG. 3.

Referring now to the drawing and particularly to FIG. 1, there is illustrated a current limiting device constructed in accordance with the principles of the invention. A cylindrical housing 10 of any suitable metallic material such as copper, brass, stainless steel, or the like, if open at one end and closed at the other end except for a central bore for the purpose as will be described hereinafter. The housing 10 has a cylindrical hollow portion having a shoulder defining a transition between wider and narrower portions of the hollow portion of the housing. Within the wider hollow portion of the housing 10, a first metallic electrode 14, in the form of a cup, are disposed in opposite relationship. More specifically, the first electrode 12 extends through a cap 16 of any suitable electrically insulating material such as glass, epoxy resin or ceramic closing the open end of the housing 10, and through a retaining block 18 screw threaded into the open end portion of the housing 10, said electrode 12 extending until it projects into the cup 14 of the second electrode to form a gap therebetween. The first electrode 12 is provided with a radial flange 20 disposed on that portion thereof not projecting into the second electrode 14. The second electrode 14 is snugly fitted into the wider hollow portion of the housing to leave a gap 22 between the bottom thereof and the shoulder disposed between the wider and narrower hollow portions of the housing 10. The bottom of the second electrode 14 is provided on the outer periphery with a sealing member such as an O ring 24 and has a central bore extending therethrough and facing a recess formed on the adjacent end face of the first electrodel2 to be substantially material, except for a thin cylindrical space defined by and extending between the central bore on the second electrode, and i the recess on the first electrode for the purpose offixing both the electrodes in place within the housing. Then the gap 22, the bore on the second electrode 14, a recess on the first electrode l2 and the said thin cylindrical space between the bore and the recess is filled with a quantity of any suitable current limiting material 28 in the form of a solid, a liquid or a powder normally put in its conductive state to electrically interconnect the first and second electrodes, and responsive to a flow of overcurrent therethrough to evaporate into nonconductive state thereby to electrically isolate the first electrode from the second electrode. After a predetermined interval of time the material is solidified or liquidized as the case may be to be automatically restored to its original conductive state. Suitable examples of the current limiting material involve Na, NaK, K, Ga, Fe, Co, Ag etc. in the form ofa solid, a liquid or powder.

On the narrower hollow portion of the housing 10, that is to say, the right-hand portion of the housing as viewed in FIG. I, there is disposed pressure buffer means generally designated by the reference numeral 30. The pressure buffer means 30 comprises a piston member 32 slidably fitted into the narrower cylindrical interior of the housing 10, and a rod 34 integral with the piston 32 and movably extending through the above-mentioned central bore formed at the closed. end of the housing 10. A compressed gas 36 fills a compartment formed between the piston 32 and the closed end of the shell 10. The piston member 32 normally abuts against the current limiting material 28 disposed in the gap 22. The piston and its rod, 32 and 34, respectively, are operatively coupled to respective sealing members 24 to prevent the current limiting material28 and the compressed gas 36 from escaping from their prescribed spaces.

The arrangement thus far described operates as follows:

In the normal operation a current flows from the first electrode 12 through the current limiting material 28 to the second electrode I4 and thence to the associated load (not shown). If the current exceeds its predetermined magnitude, or if an overcurrent flows through the device, then it heats the current limiting material 28 to a temperature at which the current limiting material 28 evaporates instantaneously. The evaporated material 28 causes the first electrode 12 to be electrically isolated form the second electrode 14. The evaporation of the current limiting material 24, due to the overcurrent, simultaneously produces a high pressure in the gap-22 to move the piston 32 and the rod 34 in the right-hand direction as viewed in FIG. 1 while the gas 36 is further compressed to absorb the pressure. Thus, the piston 32 cooperates with the compressed gas 36 to serve as pressure buffer means. The right-hand movement of the piston 32 is accompanied by the similar movement of the rod 34 to drive an associated actuator (not shown).

It will be readily understood that upon evaporation of the current limiting material 28 a pressure is exerted at the recessed portion of the first electrode 12, urging the first electrode 12 to move outwardly of the open end of the housing 10. I

In order to prevent such movement, radial flange is disposed on the first electrode 12 as previously described. The sealing member 24, operatively coupled to the first electrode,

serves to prevent the evaporated current limiting material from entering any clearance between the internal wall surface of the housing 10 and the outer surface of the second electrode 14. After the first electrode 12 has been maintained in its insulated state with respect to the second electrode 14 for a predetermined interval of time the current limiting material 28 is automatically liquidized or solidified, as the case may be, whereby an electrical connection between the first and

second electrodes

12 and 14 respectively is again accomplished. At that time the compressed gas 36 functions to move the piston 32 toward the liquidized or solidized limiting material or in the left-hand direction as viewed in FIG. 1 until it again abuts against the latter, thereby aiding in reestablishing a good electrical connection betweenthe electrodes.

In this way, all the movable components are returned to their original positions, thereby permitting the device to be used again.

.In FIG. 2, wherein like reference numerals designate the components corresponding or identical to those shown in FIG. 1, there is illustrated a modification of the device as shown in FIG. 1. In an arrangement shown in FIG. 2, the cap 16 and the retaining ring 18 are omitted and the housing 10 is physically connected to the second electrode 14 to form only a cylinder for the piston 32 and the solid insulation. In other respects the arrangement is substantially identical to that illustrated in FIG. 1.

Referring now to Hg. 3, another embodiment of the invention is illustrated and comprises a cylindrical housing 50 made for example of the same material as the housing 10 shown in FIG. 1, said housing being closed at one end and open at the other end. Disposed within the housing 50 are first and second metallic electrodes, 52 and 54 respectively, a solid insulation 56 substantially identical in material and construction to the insulation previously described in conjunction with FIG. 1, and a gap 62 formed between the closed end of the housing 50 and the second electrode 54. A quantity of current limited material 68 is disposed in the gap 62 and connected to the first electrode 52 in the same manner as previously described in conjunction with Flg. 1. An annulus of any suitable electrically insulating material is formed integrally with the first electrode 52 and also rigidly secured to the exposed end face of the solid insulation 56. Another annulus 72 similar to the annulus 70 closes the open end of the housing 50 and the first electrode 52 extends movably therethrough a compressed gas 74 fills a compartment defined between the two annuli and the housing. Sealing members 64 such as 0 rings are operatively coupled to the second electrode 54, and the

annuli

70 and 72, respectively, for the same purpose as previously described in conjunction with FIG. 1. Unlike the arrangement as shown in FIG. 1, it is to be noted that, upon evaporating the current limiting material 68 the first and

second electrodes

52 and 54, the solid insulation 56 and the insulating annulus 72 of the first electrode 52 can operate the associated actuator (not shown). In other words, the

components

52, 54, 56 and 70 serve as a piston and its rod, such as the piston and

rod

30 and 32 shown in FIG. 1. In the arrangement illustrated in FIG. 3, the radial flange 20 on the first electrode 12, as shown in FIGS. 1 and 2, is omitted because the first electrode 52 is moved along with the solid insulation 56. After the evaporated limiting material has been returned to its original state the moved components are restored to their original positions as illustrated in FIG. 3 by the action of the compressed 'gas 74 leading to the readiness for the succeeding operation.

In FIG. 4, wherein like reference numerals designate components similar or corresponding to these shown in FIG. 3, a cylindrical housing 50 is formed of any suitable electrical insulating material rather than an electrically conductive material and is closed at one end with a metallic plug 50 in the form of a cup. On the other hand, a pair of

annuli

70 and 72 similar to those shown in FIG. 3 are made of any suitable electrically conductive material such as a copper alloy. The annulus 72 serves as leading electrode for the first electrode 52. In other respects, the arrangement as substantially identical to that shown in FIG. 3.

While the invention has been illustrated and described in conjunction with several preferred embodiments thereof, it is to be understood that various changes and modifications may be resorted to without departing from the spirit and scope of the invention. For example, the compressed gas may be replaced by a compressive spring disposed the piston rod 34, or the corresponding portion of the

first electrode

12 or 52 as the case may be.

I claim:

1. In a current limiting device, a combination comprising a first metallic electrode, a second metallic electrode having a cup shaped configuration including a bottom portion and a rim extending from said bottom portion, said rim surrounding through both said second electrode and said solid insulation means to said first electrode, said first and second quantities of material being in mutual contact to normally electrically connect said electrodes to each other, and wherein said quantities of current limiting material change state, under a current overload applied to said electrodes, to electrically disconnect said electrodes. 1

2. A current'limiting device as claimed in claim 1, wherein said first electrode includes a radial flange embedded in said solid insulation.

3. A current limiting device as claimed in claim 1 wherein said first electrode includes a radial flange embedded in said solid insulation means, and further comprising pressure buffer means for absorbing and storing energy released by a change of state of said current limiting material under a current overload applied to said electrodes, said pressure buffer means being spaced from said outer side of said second electrode to provide a gap therebetween, said first quantity of current limiting material being disposed within said gap.

4 A current limiting device as claimed in claim 1 wherein said first electrode includes a radial flange embedded in said solid insulation, and further comprising pressure buffer means for absorbing and storing energy released by a change of state of said current limiting material under a current overload applied to said electrodes, said pressure buffer means including a cylinder connected to said second electrode, a piston slidable in said cylinder'and spaced away from said outer side of said second electrode to provide a gap between said piston and said outer side, said first quantity of current limiting material being disposed within said gap, a rod connected to said piston and projecting outwardly from said cylinder for actuating movement, a compartment defined by said cylinder and a side of I said piston opposite said gap, and compressive means disposed within said compartment for urging said piston toward said second electrode, whereby said piston moves under the force exerted by said change of state of said current limiting material, and said compressive means absorbs said energy and returns said piston to its original position upon a second change of state of said material when it restores itself to its original state.

5. In a current limiting device, a combination comprising a first metallic electrode, a second metallic electrode having a cup shaped configuration including a bottom portion and a rim extending from said bottom portion, said rim surrounding at least one portion of said first electrode in a spaced relationship, solid insulation means for bonding said first and second electrodes into a unitary structure in an electrically insulated relationship, said insulation means and first and second electrodes having a bore passing therethrough and in communication with said first electrode, a cylindrical housing having said unitary structure slidably mounted therein, and having an end cover connected at one end thereof and spaced from said outer side of said second electrode to provide a gap between said end cover and said outer side, said gap being in communication with said bore, a quantity of current limiting material filling said gap and said bore to normally electrically connect said electrodes to each other, whereby said quantities of current limiting material change state under a current overload applied to said electrodes to electrically disconnect said electrodes.

6. A current limiting device as claimed in claim 5, wherein pressure buffer means are connected to said cylindrical housing at the other end portion thereof for resiliently urging said second electrode toward said current limiting materia and for absorbing and storing energy released by a change of state of said current limiting material under a current overload applied to said electrodes, and for slidably returning said unitary structure to its original position upon a second change of state of said material when it restores itself to its original state.

7. A current limiting device as claimed in claim 5 further comprising a rod connected to said first electrode and projecting from said other end of said housing for actuating movement upon change of state of said current limiting material.

8. In a current limiting device, a combination comprising a first metallic electrode, a second metallic electrode opposing said first electrode in a spaced relationship, solid insulation means for bonding said first and second electrodes into a unitary structure in an electrically insulated relationship, said insulation means and said second electrode having a bore passing therethrough and into communication with said first electrode, a first quantity of self-restoring current limiting material disposed on an outer side of said second electrode remote from said first electrode and in contact therewith, and a second quantity of current limiting material disposed in said bore extending through both said second electrode and said solid insulation means to said first electrode, said first and second quantities of material being in mutual contact to normally electrically connect said electrodes to each other, and pressure buffer means for absorbing and storing energy released by a change of state of said current limiting material under a current overload applied to said electrodes, said pressure buffer means being spaced from said outer side of said second electrode to provide a gap therebetween, said first quantity of current limiting material being disposed within said gap, whereby said quantities of current limiting material change state, under a current overload applied to said electrodes to electrically disconnect said electrodes, and said quantities restore themselves to their original state upon disconnection of said electrodes, and said pressure buffer means repositions said quantities in their positions occupied prior to said overload.

Claims

1. In a current limiting device, a combination comprising a first metallic electrode, a second metallic electrode having a cup shaped configuration including a bottom portion and a rim extending from said bottom portion, said rim surrounding at least one portion of said first electrode in a spaced relationship, solid insulation means for bonding said first and second electrodes into a unitary structure in an electrically insulated relationship, said insulation means and said bottom portion of said second electrode having a bore passing therethrough and in communication with said first electrode, a first quantity of self restoring current limiting material disposed on an outer side of said second electrode bottom portion remote from said first electrode and in contact therewith, and a second quantity of current limiting material disposed in said bore extending through both said second electrode and said solid insulation means to said first electrode, said first and second quantities of material being in mutual contact to normally electrically connect said electrodes to each other, and wherein said quantities of current limiting material change state, under a current overload applied to said electrodes, to electrically disconnect said electrodes.

2. A current limiting device as claimed in claim 1, wherein said first electrode includes a radial flange embedded in said solid insUlation.

4. A current limiting device as claimed in claim 1 wherein said first electrode includes a radial flange embedded in said solid insulation, and further comprising pressure buffer means for absorbing and storing energy released by a change of state of said current limiting material under a current overload applied to said electrodes, said pressure buffer means including a cylinder connected to said second electrode, a piston slidable in said cylinder and spaced away from said outer side of said second electrode to provide a gap between said piston and said outer side, said first quantity of current limiting material being disposed within said gap, a rod connected to said piston and projecting outwardly from said cylinder for actuating movement, a compartment defined by said cylinder and a side of said piston opposite said gap, and compressive means disposed within said compartment for urging said piston toward said second electrode, whereby said piston moves under the force exerted by said change of state of said current limiting material, and said compressive means absorbs said energy and returns said piston to its original position upon a second change of state of said material when it restores itself to its original state.

5. In a current limiting device, a combination comprising a first metallic electrode, a second metallic electrode having a cup shaped configuration including a bottom portion and a rim extending from said bottom portion, said rim surrounding at least one portion of said first electrode in a spaced relationship, solid insulation means for bonding said first and second electrodes into a unitary structure in an electrically insulated relationship, said insulation means and said bottom portion of said second electrode having a bore passing therethrough and in communication with said first electrode, a cylindrical housing having said unitary structure slidably mounted therein, and having an end cover connected at one end thereof and spaced from said outer side of said second electrode to provide a gap between said end cover and said outer side, said gap being in communication with said bore, a quantity of current limiting material filling said gap and said bore to normally electrically connect said electrodes to each other, whereby said quantities of current limiting material change state under a current overload applied to said electrodes to electrically disconnect said electrodes.

6. A current limiting device as claimed in claim 5, wherein pressure buffer means are connected to said cylindrical housing at the other end portion thereof for resiliently urging said second electrode toward said current limiting material and for absorbing and storing energy released by a change of state of said current limiting material under a current overload applied to said electrodes, and for slidably returning said unitary structure to its original position upon a second change of state of said material when it restores itself to its original state.