Current-limiting device

Abstract

 A current-limiting device includes a current-limiting material having first and second sides, and first and second electrodes structured for carrying current through the current-limiting material. Each of the electrodes has first and second sides, with the second side of the first electrode engaging the first side of the current-limiting material, and with the first side of the second electrode engaging the second side of the current-limiting material. The current-limiting device also includes first and second insulators, each of which has first and second sides. The second side of the first insulator engages the first side of the first electrode, and the first side of the second insulator engages the second side of the second electrode. The current-limiting device further includes first and second clips which engage different portions of the first side of the first insulator and the second side of the second insulator, and which compress the first insulator, the first electrode, the current-limiting material, the second electrode and the second insulator.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is related to commonly assigned, copending United States Patent Application Serial No. _/_,_, filed ______ _____, 2000, entitled "Current-Limiting Device Employing A Non-Uniform Pressure Distribution Between One Or More Electrodes And A Current-Limiting Material" (Attorney Docket No. 99-PDC-136).

BACKGROUND OF THE INVENTION

Field of the Invention

[0002] This invention pertains generally to current-limiting devices and, more particularly, to current-limiting devices including a current-limiting material which is engaged by electrodes.

Background Information

[0003] Current-limiting polymer compositions, which exhibit positive temperature coefficient (PTC) resistive behavior, and electrical devices comprising current-limiting polymer compositions have been widely used. See, for example, U.S. Patent No. 5,614,881. The current-limiting polymer compositions generally include conductive particles, such as carbon black, graphite or metal particles, dispersed in a polymer matrix, such as a thermoplastic polymer, elastomeric polymer or thermosetting polymer. PTC behavior in a current-limiting polymer composition is characterized by the material undergoing a sharp increase in resistivity as its temperature rises above a particular value known as the switching temperature. Materials exhibiting PTC behavior are useful in a number of applications such as, for example, electrical circuit protection devices, in which the current passing through a circuit is controlled by the temperature of a PTC element forming part of that circuit.

[0004] Electrical circuit protection devices comprising current-limiting polymer compositions typically include a current-limiting polymer device having two electrodes embedded in a current-limiting polymer composition. When connected to a circuit, the circuit protection devices have a relatively low resistance under normal operating conditions of the circuit, but are tripped, that is, converted into a high resistance state, when a fault condition or persistent overcurrent condition occurs. Under such conditions, when the circuit protection device is tripped, the current passing through the PTC element causes it to resistively self-heat to its switching temperature, T_s, at which a rapid increase in its resistance takes place.

[0005] The residual current, which flows through the current-limiting device, allows a series circuit breaker to absorb any stored residual energy (e.g., the majority of such energy is absorbed by the circuit breaker arc chamber during the switching transient and during recovery/reclosing to reestablish the power distribution system voltage) in the power distribution system. Typically, an external current-limiting device engages the load-side terminals of the circuit breaker. For example, a conductive polymer of the current-limiting device is coupled in series with the mechanical circuit breaker separable contacts, in order to limit fault current as those contacts open.

[0006] Previous materials used for current-limiting applications in conjunction with low voltage circuit breakers (e.g., less than about 600 VAC) generally consisted of a very brittle blend of conductive filler (i.e., carbon black) and a thermoplastic binder with two spring-loaded metal plates employed as electrodes. These electrodes serve to allow current to flow through the current-limiting material. In this arrangement, approximately 80% of the total device resistance in the low resistance state resulted from contact resistance, while only about 20% resulted from bulk material resistance.

[0007] Known high power prior art current-limiting devices, for example, up to about 600 VAC with a rated current of greater than several amperes (e.g., about 10 A to about 63 A), which employ current-limiting polymers (see, e.g., U.S. Patent No. 5,861,795), also employ a parallel electrical shunt (see, e.g., U.S. Patent Nos. 5,844,467; and 5,969,928) to protect the current-limiting material from overvoltage and from the stored system energy (e.g., generally magnetic system energy resulting from system inductance).

[0008] There is room for improvement in current-limiting devices which employ current-limiting material.

SUMMARY OF THE INVENTION

[0009] The present invention provides improvements in the operation of current-limiting devices by providing a current-limiting device which compresses a first insulator, a first electrode, a current-limiting material, a second electrode and a second insulator with first and second clips.

[0010] In accordance with the invention, a current-limiting device comprises a current-limiting material having first and second sides; first and second electrodes structured for carrying current through the current-limiting material, each of the electrodes having first and second sides, with the second side of the first electrode engaging the first side of the current-limiting material, and with the first side of the second electrode engaging the second side of the current-limiting material; first and second insulators, each of the insulators having first and second sides, with the second side of the first insulator engaging the first side of the first electrode, and with the first side of the second insulator engaging the second side of the second electrode; and first and second clips engaging different portions of the first side of the first insulator and the second side of the second insulator, and compressing the first insulator, the first electrode, the current-limiting material, the second electrode and the second insulator.

[0011] Preferably, each of the first and second clips includes first and second arms, with the first arm of the first clip engaging a first portion of the first side of the first insulator and the second arm of the first clip engaging a corresponding first portion of the second side of the second insulator, and with the first arm of the second clip engaging a second portion of the first side of the first insulator and the second arm of the second clip engaging a corresponding second portion of the second side of the second insulator.

[0012] As one aspect of the invention, the first and second electrodes are solely electrically connected by the current-limiting material.

[0013] As another aspect of the invention, the current-limiting material and a shunt electrically connect the first and second electrodes. The shunt may be a conductor which is electrically connected to the first and second electrodes. The conductor may include a pair of ends each of which is cinched to one of the first and second electrodes. The conductor may include one or more serpentine portions. A first serpentine portion may be parallel to the first arms of the first and second clips, and a second serpentine portion may be parallel to the second arms of the first and second clips.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

Figure 1 is a cross-sectional view of one type of a current-limiting polymer device utilizing spring pressure contacts for both electrodes;

Figure 2 is a cross-sectional view along lines 2-2 of Figure 1;

Figure 3 shows two plots of let-through current versus time for current-limiting devices which do and do not employ a parallel electrical shunt electrically connected between the electrodes to protect a current-limiting polymer;

Figure 4 is a side view of a current-limiting device including two electrodes electrically engaging a current-limiting polymer with a non-uniform pressure distribution in accordance with a preferred practice of the present invention;

Figures 5-8 are plots of let-through current versus time for various current-limiting devices in accordance with embodiments of the present invention;

Figure 9 is a plot of residual current versus spring rate;

Figure 10 is a cross-sectional view of a current-limiting device including two "money-clip" springs, two insulators, two electrodes, and a current-limiting material in accordance with an embodiment of the present invention;

Figure 11 is a plan view of the current-limiting device of Figure 10;

Figure 12 is an end view of the current-limiting device of Figure 10;

Figure 13 is a cross-sectional view of a current-limiting device including two "money-clip" springs, two insulators, two electrodes, a current-limiting material, and a wire shunt in accordance with another embodiment of the present invention;

Figure 14 is a plan view of the current-limiting device of Figure 13;

Figures 15 and 16 are end views of the current-limiting device of Figure 14; and

Figure 17 is a schematic diagram showing one use of the current-limiting device of Figure 10 in conjunction with a three-phase circuit breaker.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Referring to Figure 1, one type of a low voltage current-limiting resistance device 10, for providing electrical circuit protection for electrical apparatus, is shown. Within a suitable metal or plastic case 12, shown split into two parts, are metal bifold springs 14, resting on polyethylene terephthalate (Mylar) sheets 16, and supporting copper electrodes 18 on each side of a thin polymeric sheet of conductive current-limiting polymer composition 20, which may exhibit PTC behavior.

[0016] As shown in Figure 2, the springs 14, the electrodes 18 and vents 22 are further detailed. A wide range of other type springs, such as, for example, wave or compression springs, may be used to provide the contacting relationship between the electrodes 18 and the current-limiting polymer composition 20.

[0017] Figure 3 shows two plots 24 (with a shunt) and 26 (without a shunt) of let-through current versus time for two current-limiting devices which do (not shown) and do not (e.g., the device 10 of Figures 1 and 2) employ a parallel electrical shunt electrically connected between the electrodes to protect the current-limiting polymer (e.g., polyethylene). The sharp drop in current at point 28 of plot 26 is a result of a crowbar circuit (not shown).

[0018] Referring to Figure 4, a current-limiting device 30 is shown. Both electrodes 32,34 of the device 30 are pressed by different forces 36,37, in order to make suitable electrical contact with a suitable current-limiting material 38. The magnitude and the stiffness of the forces 36,37 (i.e., the spring rate, k, in pounds per inch), and the pressure distribution on the current-limiting material 38 are factors, which determine the overall current-limiting performance of the device 30. The electrodes 32,34 are structured for carrying current through the current-limiting material 38, with the electrode 32 electrically engaging a first portion 40 of the material 38, and the electrode 34 electrically engaging a second portion 42 of the material 38. The exemplary forces 36 and 37 (one of which might be zero) provide a non-uniform pressure distribution between one or both of the electrodes 32,34 and the current-limiting material 38.

[0019] The electrodes 32,34 are solely electrically connected by the current-limiting material 38. Preferably, the electrodes 32,34 are made of any suitably conductive metal, such as, for example, copper, or alloy or any such suitably conductive metal or alloy, which is plated in order to reduce or minimize oxidation. Suitable plating materials for the electrodes 32,34 include, for example, silver, nickel, gold, platinum, and other types of plating metals, which preferably maintain high conductivity over the life of the current-limiting device 30.

[0020] U.S. Patent Application Serial No. 09/406,534 (Attorney Docket No. 99-PDC-138), filed September 27, 1999, which is incorporated by reference herein, discloses a suitable epoxy based current-limiting material, such as 38, which is moldable, and not brittle upon cure so that it can be finished if necessary. This current-limiting material 38, when employed in combination with a suitable mechanism for providing a non-uniform pressure distribution, such as forces 36,37, between one or both electrodes 32,34 and that current-limiting material 38, does not require the use of a parallel commutation shunt electrically connected between such electrodes.

[0021] Examples of suitable types of current-limiting materials include thermoset (e.g., carbon black filled thermosetting resins), thermoplastic type current-limiting polymers, and elastomeric polymers. Preferably, the current limiting polymer is a mixture of readibly commercially available materials, such as epoxy resins, that are flexible and moldable, can be finished, are not brittle upon cure, and that are cuttable or punchable so they can be inexpensively volume-produced in long sheet form.

[0022] Such an epoxy based current-limiting material can be cast as a thin film (e.g., about 40 cm x 80 cm and between 0.05 cm and 0.5 cm, usually 0.13 cm (0.05 inch) thick), and then cut into smaller component pieces, for example, 6.1 cm x 4.0 cm x 0.12 cm thick (i.e., about 2.4 inch x 1.6 inch x 0.05 inch) without fracturing. Such electrically conducting material exhibits superior flexibility and punchability, electrical conductivity characteristics, and low let-through (i.e., the measure of effectiveness of the current limiter in reducing current and the duration of the current, typically less than 10 x 10³ A²-s), for use in a current limiting polymer device.

[0023] Such electrically conducting material consists essentially of the cured reaction product of: a resin component comprising a mixture of: 100 parts by weight of a short chain aliphatic diepoxide resin and 0 to 15 parts by weight of a bisphenol A epoxy resin, 80 to 150 parts by weight of conductive filler, and curing agent. Preferably the aliphatic diepoxide is the diglycidyl ether of an alkylene glycol, the bisphenol A epoxy resin is present in the range of 1 to 10 parts by weight to add strength to the material, and the curing agent is a borontrifluoride-amine complex. In some instances when no epoxidized bisphenol A epoxy is present a minor amount, about 2 to 20 parts by weight, of an epoxidized polybutadiene may be present.

[0024] As further disclosed herein (e.g., in connection with Figures 5-9, 10-12, and 13-16), the combination of: (1) the current-limiting material 38; (2) a suitable mechanical pressure on the electrodes 32,34; and (3) a non-uniform pressure distribution, allows the current-limiting material 38 to continue to conduct current during voltage holdoff (e.g., which occurs during the relatively flat portion 44 of the current plot of Figure 8 at I_R = 700 A) when the maximum system voltage is across the current-limiting material 38, but at a significantly reduced current let-through value.

[0025] By purposely designing a less than ideal switch, an external shunt is no longer needed for the current-limiting device 30. By having the current driven to a nominal maximum let-through value (e.g., approximately 500 A), the fault current is limited and the magnetic energy in the electrical circuit may, thus, be suitably dissipated. Since, unlike prior proposals, the exemplary current-limiting device 30 does not require a shunting resistance, there is a savings in cost, the package volume is reduced, and efficiency is increased. In contrast, an ideal switch transitions to a resistance that rapidly drives the fault current to zero, thereby causing a high transient voltage to appear across the current-limiting material and, thus, causing the stored magnetic system energy to destroy that current-limiting material.

[0026] The residual current in the current-limiting material 38 is controlled, without the need for a commutating shunt. The ability to continue to conduct current through the current-limiting material 30 depends upon the type of current-limiting material as well as the dynamics of the electrodes 32,34. For example, the spring rate, which provides the mechanical pressure on the electrodes 32,34, is employed to suitably hold such electrodes in electrical contact with the current-limiting material 38, and to control the residual current through such material during the recovery phase (e.g., the relatively flat portion 44 of the current plot of Figure 8 at I_R = 700 A).

[0027] Referring to Figure 5, for springs (e.g., compression springs) having a relatively low spring rate, the electrodes 32,34 of Figure 4 are easily lifted from the surface of the current-limiting material 38 during a switching transient. Initially, the fault or short circuit current is driven to zero. Then, after about 100 µs at that level (i.e., I_R = 0 A), the current-limiting material 38 begins to re-conduct the full current. In turn, after a brief period of full conduction, the current-limiting material 38 then re-transitions and the current again is driven to zero. Such a relatively low spring rate (e.g., k = 102 lbs./in.) has the effect of causing the electrode to mechanically oscillate on the surface of the current-limiting material 38. This results in the oscillatory effect on the current as shown in Figure 5.

[0028] The other extreme is shown by a relatively rigid structure (e.g., a wave spring; silicone rubber), which provides a relatively extremely high spring rate (e.g., k = 5000 lbs./in.). In this case, the electrodes 32,34 are not allowed to lift-off the surface of the current-limiting material 38, but are held rather firmly onto, but not embedded into (see, e.g., commonly assigned Application Serial No. ___/___,___ (Attorney Docket No. 99-PDC-137)), the current-limiting material 38 during the entire switching transient. This results in a relatively high residual current (e.g., I_R = 1600 A; I_R = 1904 A), as shown in Figures 6 and 7. With such higher values of residual current, there are corresponding greater values of let-through current.

[0029] Hence, one possible goal is to maintain a relatively low residual current and to minimize re-conduction. This allows for inductive energy to be safely dissipated. However, re-conduction per se does not cause damage to the current-limiting material 38, but only causes a minimal increase in let-through current.

[0030] As shown in Figure 8, a spring with a suitable spring-rate (e.g., k = 333 lbs./in. as provided by a bifold spring; k = 714 lbs./in. as provided by a wave washer) is selected to produce a suitable current waveform (e.g., I_R = 475 A of a bifold spring; I_R = 700 A of Figure 8; I_R = 750 A for a wave washer). This spring rate preferably produces a minimum let-through current value and does not result in re-conduction.

[0031] The gas pressure produced from the vaporization of the interfaces between the electrodes 32,34 and the current-limiting material 38 of Figure 4 during the switching transient is also important in obtaining the desired residual current. By controlling venting of the gas, the amount of force applied to the case (e.g., the case 12 of Figure 1), during the transient, also affects the residual current. Sealing the case 12 would, however, result in a greater force to such case and case rupture. In contrast, as shown in Table 1 and Figure 9, by venting the gas pressure and appropriately selecting the proper spring, the residual current is reliably controlled.

Table 1

Description	Spring Rate, k (lbs./in.)	Residual Current, IR (A)
Compression	102	0
BiFold	333	475
Wave Washer	714	750
Wave Spring	5000	1600
Silicone Rubber	6666	1904

[0032] The exemplary current-limiting devices disclosed herein employ mechanisms that provide a non-uniform pressure distribution and include a suitable spring having a predetermined spring rate, such as the exemplary spring rates of Table 1. As shown in Table 1 (and Figure 9), the predetermined spring rate is about 100 to about 7000 pounds per inch. Preferably, the predetermined spring rate is about 100 to about 700 pounds per inch, with a spring rate of about 300 pounds per inch providing minimum let-through current value without re-conduction. As discussed above, the selected spring rate is important in determining the resulting switching properties of the current-limiting devices. For example, spring rate determines the residual current, I_R, which has a large affect on the let-through energy.

[0033] For an exemplary spring, which is compressed 0.1 inch, with a spring rate of 333 pounds per inch, and with an electrode having a true contact surface area of 0.3 square inches, the resulting total pressure would be 111 PSI (i.e., 333 lbs./in. x 0.1 in./0.3 in.²).

[0034] The mechanical pressure distribution on the surface of the current-limiting material 38 is also important in determining the peak current and the device resistance. When the force is uniformly distributed over the entire electrode surface (e.g., 2.88 in.² in the exemplary embodiment), the pressure is relatively low (e.g., typically less than 20 PSI). This relatively low pressure typically produces high device resistance and increases the switching current.

[0035] With reference to Figures 5-8, the current-limiting material 38, the electrodes 32,34, and the forces 36,37 of Figure 4 are cooperatively structured for: (1) limiting a maximum residual let-through current (i.e., current after switching) to about 475 amperes to about 750 amperes (see Figure 8); (2) minimizing or eliminating re-conduction through the current-limiting material 38 (Figures 6-8); and (3) through appropriate selection (as shown in Figure 9), providing a predetermined residual let-through current through the current-limiting material 38, and a predetermined spring rate for the forces 36,37.

[0036] When the differential pressure is increased (e.g., to greater than 40 PSI) by non-uniformly loading the electrodes 32,34 (e.g., by employing the loading as shown in Figures 10-12) having the desired spring rate, then the desired low device resistance, reduced switching current, and low residual current are provided without any re-conduction. This, however, is at the expense of increases in the let-through current, due to the relatively higher spring force needed to obtain the desired package resistance over the smaller area of contact, and increases in erosion of the current-limiting material 38 at the areas of relatively higher pressure.

[0037] Accordingly, there is a desired optimum between pressure distribution and spring rate in order to minimize the total let-through current. In addition, package cost versus performance is another factor. The optimum combination of spring materials and pressure distribution on the current-limiting material that results in the desired relatively low residual current, without re-conduction, may only be slightly better in performance than a relatively lower cost, longer life, alternative design.

[0038] Referring to Figures 10-12, another current-limiting device 50 is shown including two exemplary copper electrodes 52,54, two exemplary "money-clip" springs 56,58, a suitable current-limiting polymer material 60, and suitable insulators in the form of the exemplary red glass polyester 62,64, respectively. The current-limiting material 60 has a first side 66 and a second side 68 (Figure 10). The first and second electrodes 52,54 are structured for carrying current through the current-limiting material 60. Each of the electrodes 52,54 has a first side 70 and a second side 72. The second side 72 of the first electrode 52 engages the first side 66 of the current-limiting material 60. The first side 70 of the second electrode 54 engages the second side 68 of the current-limiting material 60.

[0039] Each of the insulators 62,64 has a first side 74 and a second side 76. The second side 76 of the first insulator 62 engages the first side 70 of the first electrode 52. The first side 74 of the second insulator 64 engages the second side 72 of the second electrode 54.

[0040] The first and second clips 56,58 engage different portions of the first side 74 of the first insulator 62 and the second side 76 of the second insulator 64, in order to compress the first insulator 62, the first electrode 52, the current-limiting material 60, the second electrode 54 and the second insulator 64.

[0041] The first and second clips 56,58 have opposing clip spring clamping arm members 78,80. The first arm 78 of the first clip 56 engages a first portion 82 of the first side 74 of the first insulator 62 and the second arm 80 of the first clip 56 engages a corresponding first portion 82 of the second side 76 of the second insulator 64. The first arm 78 of the second clip 58 engages a second portion 84 of the first side 74 of the first insulator 62 and the second arm 80 of the second clip 58 engages a corresponding second portion 84 of the second side 76 of the second insulator 64. The first and second arms 78,80 of the clips 56,58 have predetermined spring rates of about 100 lbs./in. to 800 lbs./in. with a preferred range between about 150 lbs./in. to 400 lbs./in.

[0042] The first side 74 of the first insulator 62 and the second side 76 of the second insulator 64 preferably include a third portion 86 which is not engaged by the clips 56,58, in order to provide a non-uniform pressure distribution between the first insulator 62, the first electrode 52, the current-limiting material 60, the second electrode 54 and the second insulator 64. Alternatively, the third portion 86 is not required and the force distribution suitably varies along the length of the arms 78,80.

[0043] The exemplary electrodes 52,54 are solely electrically connected by the current-limiting material 60. External electrical connections to the electrodes 52,54 are preferably provided by exemplary electrical conductors 88,90 (shown in Figure 11), respectively, which are suitably electrically connected (e.g., welded, brazed) to the electrodes 52,54 or which, alternatively, are made part of such electrodes.

[0044] Referring to Figures 13-16, another current-limiting device 100 is shown including two exemplary copper electrodes 102,104, two exemplary "money-clip" springs 106,108, a suitable current-limiting polymer material 110, suitable insulators in the form of the exemplary red glass polyester 112,114, respectively, and a shunt 116 between the electrodes 102,104. The current-limiting device 100 is similar to the current-limiting device 50 of Figures 10-12, except that the shunt 116 is employed. In Figures 13-16, the electrodes 102,104 are electrically connected by the current-limiting material 110 and by the shunt 116.

[0045] The shunt 116 is a conductor which is electrically connected to the electrodes 102,104. The shunt 116 may be made from iron wire of about 0.1 in. diameter. Depending on the desired resistance and energy absorption, the wire diameter, length and material may be suitably selected. Preferably, each of the ends 118,120 (as shown in Figure 13) of the shunt 116 is cinched to a corresponding one of the electrodes 102,104. The shunt 116 preferably includes one or more serpentine portions 122 (as best shown in Figure 14) which are parallel to the arms 124,126 of the clips 106,108. The serpentine portion 122 is proximate the arms 124 of the clips 106,108 (as shown in Figures 14 and 15), and the serpentine portion 128 is proximate the arms 126 of the clips 106,108. Preferably, the serpentine portion 122 is parallel to the arms 124, and the serpentine portion 128 is parallel to the arms 126.

[0046] A wide variety of different types of springs may be employed to provide the desired force and spring rate in a given dimension.

[0047] Although exemplary package and current-limiting polymer material sizes, shapes and electrode/current-limiting polymer connections have been disclosed herein, a wide range of such sizes, shapes and connections may be employed within the spirit of the present invention.

[0048] Figure 17 shows a conductive polymer current-limiting resistance device 150, including a plurality (e.g., three) of the conductive polymer current-limiting devices 50 of Figures 10-12, which devices are connected electrically in series with three power lines between a three-phase load 152 and a three-phase circuit breaker 154, with a three-phase power source shown as 156. As the current-limiting polymer material 60 (Figure 10) in one of the devices 50 undergoes a sharp increase in resistivity due to a large influx of current in one of the phases of the power circuit, its temperature rises above its switching temperature, T_s, at which a rapid increase in its resistance takes place to transform it to a high resistance state.

[0049] The current-limiting devices 50 and 100 of respective Figures 10-12 and 13-16 are relatively easy to manufacture, and readily facilitate the venting of gas from those devices.

[0050] The exemplary current-limiting device 50 of Figures 10-12 allows for quick and easy assembly/disassembly compared to known prior devices. The relatively low spring force increases the life of the device and lowers the assembly costs. The device 50 has a relatively compact design and employs a thermoset polymer (long life) compared to known prior art thermoplastic devices. Furthermore, the device 50 has a low switching current (i.e., about 4.5 kA) compared to known prior devices (about 6-8 kA).

[0051] In addition to the foregoing advantages and benefits, the current-limiting device 100 of Figures 13-16 provides a relatively longer life (less polymer erosion), and an increased safety factor (the shunt is a backup in case polymer resistance gets too high). Furthermore, the device 100 can absorb relatively larger inductive faults.

[0052] While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art, that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only, and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.

REFERENCE NUMERICAL LIST

[0053] 

10

low voltage current-limiting resistance device

12

metal or plastic case

14

metal bifold springs

16

polyethylene terephthalate (Mylar) sheets

18

copper electrodes

20

conductive current-limiting polymer composition

22

vents

24

plot with a shunt

26

plot without a shunt

28

point

30

current-limiting device

32

electrode

34

electrode

36

force

37

force

38

current-limiting material

40

first portion of the current-limiting material

42

second portion of the current-limiting material

44

relatively flat portion of the current plot

50

current-limiting device

52

copper electrode

54

copper electrode

56

suitable clamping structure, "money-clip" spring

58

suitable clamping structure

60

current-limiting polymer material

62

insulator, red glass polyester

64

insulator, red glass polyester

66

first side

68

second side

70

first side

72

second side

74

first side

76

second side

78

clip spring clamping arm member

80

clip spring clamping arm member

82

first portion

84

second portion

86

third portion

88

electrical conductor

90

electrical conductor

100

current-limiting device

102

copper electrode

104

copper electrode

106

"money-clip" spring

108

"money-clip" spring

110

current-limiting polymer material

112

insulator, red glass polyester

114

insulator, red glass polyester

116

shunt

118

end

120

end

122

serpentine portion

124

arm

126

arm

128

serpentine portion

150

conductive polymer current-limiting resistance device

152

three-phase load

154

three-phase circuit breaker

156

three-phase power source

Claims

1. A current-limiting device comprising:

a current-limiting material having first and second sides;

first and second electrodes structured for carrying current through said current-limiting material, each of said electrodes having first and second sides, with the second side of said first electrode engaging the first side of said current-limiting material, and with the first side of said second electrode engaging the second side of said current-limiting material;

first and second insulators, each of said insulators having first and second sides, with the second side of said first insulator engaging the first side of said first electrode, and with the first side of said second insulator engaging the second side of said second electrode; and

first and second clips engaging different portions of the first side of said first insulator and the second side of said second insulator, and compressing said first insulator, said first electrode, said current-limiting material, said second electrode and said second insulator.

2. The current-limiting device of Claim 1, wherein each of said first and second clips includes first and second arms, with the first arm of said first clip engaging a first portion of the first side of said first insulator and the second arm of said first clip engaging a corresponding first portion of the second side of said second insulator, and with the first arm of said second clip engaging a second portion of the first side of said first insulator and the second arm of said second clip engaging a corresponding second portion of the second side of said second insulator.

3. The current-limiting device of Claim 2, wherein the first and second arms of said first and second clips have a predetermined spring rate.

4. The current-limiting device of Claim 3, wherein said predetermined spring rate is about 100 to about 7000 pounds per inch.

5. The current-limiting device of Claim 4, wherein said predetermined spring rate is about 100 to about 700 pounds per inch.

6. The current-limiting device of Claim 5, wherein said predetermined spring rate is about 300 pounds per inch.

7. The current-limiting device of Claim 1, wherein said first and second electrodes are solely electrically connected by said current-limiting material.

8. The current-limiting device of Claim 1, wherein said first and second electrodes include a shunt operatively associated therewith; and wherein said first and second electrodes are electrically connected by said current-limiting material and by said shunt.

9. The current-limiting device of Claim 8, wherein said shunt is a conductor which is electrically connected to said first and second electrodes.

10. The current-limiting device of Claim 9, wherein said conductor includes a pair of ends each of which is cinched to one of said first and second electrodes.

11. The current-limiting device of Claim 9, wherein said conductor includes at least one serpentine portion.

12. The current-limiting device of Claim 11, wherein said at least one serpentine portion is parallel to the arms of said first and second clips.

13. The current-limiting device of Claim 9, wherein said conductor includes first and second serpentine portions, with the first serpentine portion being proximate the first arms of said first and second clips, and with the second serpentine portion being proximate the second arms of said first and second clips.

14. The current-limiting device of Claim 13, wherein the first serpentine portion is parallel to the first arms of said first and second clips, and the second serpentine portion is parallel to the second arms of said first and second clips.

15. The current-limiting device of Claim 1, wherein said first and second electrodes are made of copper.

16. The current-limiting device of Claim 1, wherein said first and second electrodes are made of a conductive metal or alloy which is plated in order to reduce oxidation.

17. The current-limiting device of Claim 1, wherein said first and second electrodes are made of copper and are plated with a plating material selected from the group comprising silver, nickel, gold, and platinum.

18. The current-limiting device of Claim 1, wherein said first and second clips have a spring rate of about 333 to about 714 pounds per inch and are cooperatively structured with said current-limiting material and said first and second electrodes for limiting a maximum residual let-through current to about 475 amperes to about 750 amperes.

19. The current-limiting device of Claim 1, wherein said first and second clips have a spring rate of at least about 300 pounds per inch and are cooperatively structured with said current-limiting material and said first and second electrodes for minimizing re-conduction through said current-limiting material.

20. The current-limiting device of Claim 1, wherein said current-limiting material, said first and second electrodes, and said first and second clips are selected to provide a predetermined residual let-through current through said current-limiting material, and a predetermined spring rate.

21. The current-limiting device of Claim 1, wherein said first and second clips have a spring rate of at least about 300 pounds per inch and are cooperatively structured with said current-limiting material and said first and second electrodes for eliminating re-conduction through said current-limiting material.

22. The current-limiting device of Claim 1, wherein said first and second insulators are made of polyester.

23. The current-limiting device of Claim 1, wherein said current-limiting material is a molded thermoset material.

24. The current-limiting device of Claim 23, wherein said molded thermoset material is a carbon black filled thermosetting resin.

25. The current-limiting device of Claim 1, wherein said current-limiting material is a molded thermoplastic current-limiting polymer.

Drawing