EP1261000A1

EP1261000A1 - Electric citrcuit protection device having electrical parts ultrasonically joined using a brazing alloy

Info

Publication number: EP1261000A1
Application number: EP02078046A
Authority: EP
Inventors: Charles Allen Newland
Original assignee: Siemens Energy and Automation Inc
Current assignee: Siemens Energy and Automation Inc
Priority date: 1997-09-30
Filing date: 1998-09-21
Publication date: 2002-11-27
Anticipated expiration: 2018-09-21
Also published as: EP1261000B1; ES2199463T3; EP1019930B1; DE69819791D1; DE69814432T2; DE69819791T2; DE69814432D1; EP1019930A1; WO1999017321A1; ES2211852T3

Abstract

An electric conductor assembly comprising: an electrically conductive contact having a i constituent comprising a non-alloying material and further comprising silver; and an electrically conductive contact carrier comprising a predominance by weight of nonferrous material, mechanically joined together at a particular location by the application of ultrasonic energy, brazing material being disposed between the contact and the carrier at the location where they are joined. The brazing material comprises at least two materials, with elemental copper being the largest single constituent of the brazing material, and the contact comprises serrations in the surface joined to the brazing material, said serrations being filled with excess silver from the contact, the brazing material forming a layer overlapping the serrations.

Description

Field Of The Invention

This invention relates generally to the ultrasonic joining of electrically conductive materials. More specifically, it relates to the realization that by selecting certain brazing materials, ultrasonic welding can be used to join two diverse electrically conductive materials, which it is believed, have been previously considered incompatible for joining by ultrasonic welding to create a joint that can withstand electric current faults, such as occur in circuit breakers.

Background Of The Invention

Electric circuit protection devices, such as circuit breakers, for example, may be used to interrupt current flow relatively quickly to a circuit that is being protected by the protection device upon the occurrence of an overload, such as a fault. This has been referred to in the industry as tripping the breaker. Once the condition that gave rise to the trip has been corrected, the circuit breaker may be reset, such as by appropriately manipulating an operating handle in the case of a manually resettable circuit breaker.
When a circuit breaker trips because of a dead short fault, it is believed that relatively large magnitudes of electric current may flow through the interruptable current path of a circuit breaker, so as to initiate the trip. However, the circuit breaker integrity must be maintained until it finally completes the trip. Moreover, because it is resettable, a circuit breaker must be capable of maintaining its integrity over its specified life, during which the circuit breaker may be subjected to multiple instances of tripping and resetting.
The internal construction of a circuit breaker comprises various individual electric parts. Some of these parts are joined together by welding or brazing. A known method for joining certain parts comprises high temperature welding. An example of high temperature welding is resistance welding wherein pressure is applied to the parts at a location where they are to be joined, and welding current is passed through the location to create temperatures sufficiently high to cause a certain degree of localized material melting and flow migration between the parts so that upon termination of the welding current, the molten mass solidifies to create the joint. It is believed that the thermal effects of resistance welding may act on the parts in a manner that undesirably affects one or more physical properties of at least one of the parts being joined. One example of such a side effect comprises some annealing of all or a portion of a part.
Another known method for joining certain parts comprises ultrasonic welding wherein pressure is applied to the parts at a location where they are to be joined, such as by clamping them in a suitable fixture in an ultrasonic welder. Ultrasonic energy is then applied to that location to create a certain upsetting and flow of material between the parts which ceases upon termination of the application of the ultrasonic energy, thereby creating the joint. It is believed that one advantage of ultrasonic welding is the elimination or at least attenuation of annealing of the parts being joined.
In any particular application, it is believed that the choice of using either resistance welding or ultrasonic welding depends on the composition of the parts being joined. For example, in a circuit breaker application where an electric contact is to be joined to another electric part, such as a terminal or a movable contact arm or blade, if the contact comprises the combination of a refractory element, such as tungsten or molybdenum, and an electric conductor, such as silver or copper, and the other part comprises, either predominantly or exclusively, a non-ferrous electric conductor, such as copper, the disposition of an attachment agent on a face of the contact that is to be placed in intimate surface-to-surface contact with the non-ferrous conductor is generally believed to be suitable for high-temperature welding. The attachment agent should be compatible with the contact's conductor material; for example, being fine silver when the contact conductor comprises silver. It is believed, however, that such materials are inappropriate or at least not optimally suited for ultrasonic welding. It is also believed that refractory-based materials are at least generally not optimal candidates for the alloying that is necessary to create an acceptable joint by ultrasonic welding.

Summary Of The Invention

The present inventions relating to a novel method and a brazing material selection that provide for materials that previously have been believed to be inappropriate for satisfactorily joining parts by ultrasonic welding.
Although the principles described herein are not necessarily limited to the joining of parts of an electric circuit protection device such as a circuit breaker, it is believed that these principles may provide significant advantages when used in a circuit breaker, especially one where one of the parts to be joined predominantly comprises a refractory material. It is believed that one advantage is that improved integrity, and hence useful life, of a circuit breaker is attainable, enabling a tripped breaker to be reset multiple times after having been subjected to relatively large magnitude fault currents that have caused multiple tripping events.
One aspect of the invention relates to a method of mechanically joining electrically conductive parts comprising, providing first and second electrically conductive parts that are to be joined to establish electric conductivity between them wherein a constituent of the first part comprises a non-alloying material and the second part comprises a predominance by weight of non-ferrous material, disposing brazing material between the parts at a location where they are to be joined, and joining the parts at that location by the application of ultrasonic energy, wherein the brazing material is selected to comprise at least two materials, one of the two materials being elemental copper, and the elemental copper being the largest single constituent of the brazing material by weight.
Another aspect of the invention relates to an electric circuit protection device comprising an interruptable current path for interrupting current flow to a load including trip means for tripping the interruptable current path upon occurrence of a load fault, the interruptable current path comprising first and second electrically conductive parts that have been joined by ultrasonic welding to establish electric conductivity between them, the first part comprising non-alloying material, the second part comprising non-ferrous material, and a brazing material that has been acted upon by the ultrasonic welding to join the parts, the brazing material being selected to comprise at least two materials, one of the two materials being elemental copper, and the elemental copper being the largest single constituent of the brazing material.
Still another aspect of the invention relates to an electric conductor assembly comprising an electrically conductive contact and an electrically conductive contact carrier joined together by ultrasonic welding to establish electric conductivity between them, one of the contact and the carrier comprising non-alloying material, the other of the contact and the carrier comprising non-ferrous material, and a brazing material that has been acted upon by the ultrasonic welding to join the contact and the carrier, the brazing material being selected to comprise at least two materials, one of the two materials being elemental copper, and the elemental copper being the largest single constituent of the brazing material.
More specific aspects related to the aforementioned general aspects comprise: the elemental copper of the brazing material being at least 50% by weight of the brazing material, more particularly being an alloy consisting of substantially 80 parts elemental copper by weight, substantially 15 parts elemental silver by weight, and substantially 5 parts elemental phosphorus by weight; the non-alloying material comprising a refractory material, more particularly the refractory material comprising at least 35% by weight of the contact, and more particularly being from the group consisting of silver tungsten, silver tungsten carbide, copper tungsten, copper tungsten carbide, and silver molybdenum; the non-ferrous material being copper; and the brazing material comprising silver as another elemental material, the contact being infiltrated with silver.
Products resulting from the claimed inventions also embody the inventive principles.
The foregoing, along with additional features, and other advantages and benefits of the inventions, will be seen in the following description and claims which are accompanied by drawings and disclose preferred embodiments of the inventions according to the best mode contemplated at this time for carrying out the inventions.

Brief Description Of The Drawings

Figure 1 is an elevation view, schematic in nature, illustrating a stage in the inventive method.
Figure 2 is an elevation view, schematic in nature, illustrating a further stage in the inventive method.
Figure 3 is an elevation view, schematic in nature, of parts that have been joined by the inventive method.
Figure 4 is an elevation view of a portion of a circuit breaker mechanism containing a contact arm assembly that includes a refractory-based contact joined to a two-piece contact arm in accordance with the present invention.
Figure 5 is a view in the direction of arrows 5-5 in Figure 4.
Figure 6 is a view in the direction of arrows 6-6 in Figure 5 of one of the two contact arm pieces by itself.
Figure 7 is a view substantially in the direction of arrows 7-7 in Figure 4.
Figure 8 is a top view of Figure 4.
Figure 9 is an enlarged photomicrograph of a cut-away cross section through an actual joint between a contact and a contact arm created in accordance with the present invention.
Figure 10 is an enlarged photomicrograph of a cut-away cross section through another actual joint between another contact and another contact arm created in accordance with the present invention.
Figure 11 is an enlarged view, generally within the area 11 of Figure 10, that has been enhanced in brightness and contrast to show a particular feature.

Description Of The Inventions

One application of the inventions is presented by the following specific example of the joining of one part of a circuit protection device to another, the example showing the joining of a contact to a contact arm of a circuit breaker, which may include insulated case and molded case circuit breakers that are well known. Figures 1 to 3 show a contact, designated by the reference numeral 10, and a contact arm, designated by the reference numeral 12. An exemplary contact 10 is a part having, for example, a silver-infiltrated refractory composition, silver-infiltrated tungsten or silver-infiltrated molybdenum. An exemplary contact arm 12 is a part of substantially elemental copper.
A brazing alloy designated by the reference numeral 14 for use in joining the two parts 10, 12 comprises at least two materials, one of the two materials being elemental copper, with the elemental copper being the largest constituent of the brazing material. One example of a suitable brazing material is an alloy consisting of 80 parts elemental copper, 15 parts elemental silver, and 5 parts element phosphorus, all by weight.
It may be advantageous for the brazing alloy to be applied to a face of the silver-infiltrated refractory contact in an inert atmospheric oven (the oven not being shown in the drawing). After this joining of parts 10 and 14 (Figure 1), they are allowed to cool. The face of contact 10 that contains the brazing alloy is then disposed in surface-to-surface contact with the copper contact arm 12 in an ultrasonic welder 16. In Figure 2, the parts are held clamped in the welder and suitable pressure and ultrasonic energy are applied to the clamped region to create a certain upsetting and cross-flow of materials between the intimately contacting surfaces. The application of ultrasonic energy is then ended or terminated to stop the cross-flow. The joined parts are finally removed from the welder to yield the finished assembly shown in Figure 3.
The joint that has been created is believed to be capable of withstanding relatively high electric current densities that occur in a circuit breaker upon occurrence of a circuit fault that causes the breaker to trip. Moreover, it is believed that the joint has been created with little or essentially no degradation in the physical properties, such as hardness, of the joined materials. Moreover, the beneficial use of a refractory based contact has been advantageously retained.
While a specific composition for the brazing alloy has been given in the example just described, it is believed that satisfactory results may be obtained with somewhat different compositions. It is believed that one consideration for the brazing alloy is that it comprise at least two elemental materials and that the major constituent be an abundance of elemental copper over any other constituent material.
Ultrasonic welding machines for performing the joining that is the subject of the present invention are commercially available. They can be selected and set to meet specified performance criteria to accomplish part joining in accordance with relevant specifications, such as current carrying capacity, trip time, etc. Examples of suitable ultrasonic welders are: Sonobond Model No. MH-1545, available from Sonobond Ultrasonics Company, 200 East Rosedale Ave., Westchester, Pennsylvania 19380; and ATE Ultraweld 20 System available from American Technology Equipment, Inc., 25 Controls Drive, Shelton, Connecticut 06484.
Figures 4 to 8 illustrate a portion of a circuit breaker mechanism including an actual contact 10 joined to a contact arm 12. Contact arm 12 forms a carrier for the contact by an illustrative two-piece construction, comprising a first contact arm piece 12A and a second contact arm piece 12B. Contact 10 is joined to the distal end of contact arm 12, creating a contact arm assembly 13. The two pieces 12A, 12B are essentially mirror images of each other. As shown by Figure 6, confronting portions of each piece 12A, 12B proximate the distal end of contact arm 12 comprises respective zones 16 where they are joined directly together. Such joining may be accomplished by ultrasonic welding or brazing.
Proximate to zones 16, the joined pieces 12A, 12B form a bifurcation. Proximate the distal end of this bifurcation which is proximate zones 16, each piece 12A, 12B has an abutment 18 whose free end is adapted to abut, but at the minimum at least closely confront, the free end of the opposite abutment 18. At the proximate end of contact arm 12, each piece 12A, 12B has a lobe 20 that, in the completed circuit breaker, defines an axis 22 about which contact arm assembly 13 executes swinging motion.
Further portions of the mechanism shown in Figures 4, 7 and 8 comprise a load terminal 24, a flexible connector, or braid 26 and a bi-metal 28. Load terminal 24 is adapted to be mounted on a casing (not shown) of a circuit breaker, for example by a fastener, such as a headed screw, whose shank is passed a hole in the casing and threaded into an extruded hole 30 in the load terminal. The end of terminal 24 designated 24A leads to a load circuit (not shown). The end designated 24B provides a cantilever mounting for bi-metal 28.
Bi-metal 28 comprises a nominally flat strip having a relatively higher expansion side 28A and a relatively lower expansion side 28B. The distal end of bi-metal 28 and one end of braid 26 are joined together, such as by brazing. The opposite end of braid 26 is joined, by brazing for example, to contact arm 12 distally proximal to a lobe 20 of one of the two pieces 12A, 12B, the one piece being 12A in the example depicted.
When a finished circuit breaker containing the mechanism just described is in its "on" position, contact 10 has direct contact with another contact that is connected to a line terminal (not shown). Hence a complete circuit exists from that line terminal and its associated contact, through contact 10, contact arm 12, braid 26, and bi-metal 28, to load terminal 24. One type of fault condition that should cause the circuit breaker to trip is due to thermal energy input to bi-metal 28 sufficient to warp the bi-metal to an extent that causes operation of a trip mechanism (not shown). As a result, contact arm assembly 13 swings in the sense of arrow 29 in Figure 4 to separate contact 10 from the line terminal contact that it had been engaging. This breaks the continuity through the circuit breaker between the line terminal and load terminal 24, causing the circuit breaker to operate to "tripped" condition.
Figures 4 and 6 show adjoining flat rectangular surface areas 32 of contact arm pieces 12A and 12B. Contact 10 has a rectangular surface area 34 of slightly larger overall area than the combined surface areas 32. Joining of surface area 34 to the surface areas 32 is advantageously accomplished by the present approach. The occurrence of a fault that should trip the circuit breaker may create relatively large current densities through the joint between contact 10 and contact arm 12. The present approach is believed to aid in better maintaining the integrity of the joint under such high stress conditions. This is important where small areas are involved. Although general principles of the invention are not intended to necessarily be limited to particular interface areas, it is believed that interface areas less than about 1/4" X 5/16" are especially well-suited for successful joining, at least in the case of joining a contact to a contact arm in an electric circuit protection device like a circuit breaker. "Interface area" is understood to include the area where the actual joining takes place. Specific examples of interface areas that have been used in practice of the inventive principles are 9/16" X 5/32" and 1/8" X 9/32". By gathering a contact in an ultrasonic welder, it is believed that the thickness of the contact may be any of a number of different thicknesses typically used for contacts. Examples of typical thicknesses may range up to about 3/16".
Figures 9 and 10 show two examples of actual joints created by use of the joining procedure. The contact arm comprises predominantly copper, with small amounts of iron and silicon. An example is Cu at least about 97-99%, Si 1% or less, and Fe 2% or less. The brazing alloy consists of substantially 80 parts elemental copper by weight, substantially 15 parts elemental silver by weight, and substantially 5 parts elemental phosphorus by weight. The contact is 50% Ag and 50% WC by weight. The accompanying patent drawings of Figures 9 and 10 contain 50X photomicrographs. In each of Figures 9 and 10, it can be seen that brazing alloy 14 was joined to contact 10 before the ultrasonic welding of the contact to contact arm 12. Excess silver fills troughs of serrations in the contact. The brazing alloy appears as a layer overlapping the serrations, and the region of ultrasonic bonding is so labeled. It is believed that there is essentially complete bonding across the joint. Figure 10 shows evidence of "swirling" at the interface, which is believed to indicate relatively good quality. Figure 11 is believed to show the swirling in more detail.

It is contemplated that the inventions are suitable for joining an electric conductor that is primarily copper to the following refractory-based materials: Silver-Tungsten; Silver Tungsten Carbide; Copper Tungsten; Copper Tungsten Carbide; and Silver Molybdenum. Of course, the relative percentages of the contact constituents may vary. Specific examples are given by the following table wherein hardness, density, and electrical conductivity data are representative. In the table, IACS refers to International Annealed Copper Standard.

Class	Composition % by weight	Hardness (Rockwell)	Density gm/cc	Conductivity %IACS
Silver Tungsten	50Ag50W	B65	13.2	62
	40Ag60W	B75	14.0	55
	35Ag65W	B85	14.5	51
	25Ag75W	B90	15.5	45
	45Ag50W5C	B50	10.6	40
Silver Tungsten Carbide	65Ag35WC	B55	11.5	55
	60Ag40WC	B65	11.7	50
	50Ag50WC	B80	12.2	47
	40Ag60WC	B95	12.7	43
	35Ag65WC	B100	12.9	34
Copper Tungsten	50Cu50W	B65	11.9	50
	40Cu60W	B80	12.8	47
	30Cu70W	B90	13.9	46
	25Cu75W	B95	14.5	44
	20Cu80W	B100	15.2	40
Copper Tungsten Carbide	50Cu50WC	B95	11.0	45
Silver Molybdenum	50Ag50MO	B75	10.1	52
	45Ag55MO	B80	10.1	48
	40Ag60MO	B85	10.1	45
	35Ag65MO	B87	10.0	42
	30Ag70MO	B90	10.0	39

The following numbered clauses form part of the present description, and are not to be taken as claims, in accordance with the European Patent Office's Legal Board of Appeal decision J15/88.
I. An electric circuit protection device comprising: an interruptible current path for interrupting current flow to a load including trip means for tripping the interruptible current path upon occurrence of a load fault; the interruptible current path comprising first and second electrically conductive parts that have been joined by ultrasonic welding to establish electric conductivity between them; the first part comprising non-alloying material; the second part comprising non-ferrous material; and a brazing material that has been acted upon by the ultrasonic welding to join the parts, the brazing material being selected to comprise at least two materials, one of the two materials being elemental copper, and the elemental copper being the largest single constituent of the brazing material.
II. An electric circuit protection device as set forth in clause I wherein the elemental copper of the brazing material is at least about 50% by weight of the brazing material.
III. An electric circuit protection device as set forth in clause I wherein the brazing material is an alloy consisting of substantially 80 parts elemental copper by weight, substantially 15 parts elemental silver by weight, and substantially 5 parts elemental phosphorus by weight.
IV. An electric circuit protection device as set forth in clause I wherein the first part comprises a refractory material as the non-alloying material.
V. An electric circuit protection device as set forth in clause IV wherein the refractory material comprises at least about 35% by weight of the first part.
VI. An electric circuit protection device as set forth in clause IV wherein the refractory material is from the group consisting of silver tungsten, silver tungsten carbide, copper tungsten, copper tungsten carbide, and silver molybdenum.
VII. An electric circuit protection device as set forth in clause I wherein the second part comprises copper as the non-ferrous material.
VIII. An electric circuit protection device as set forth in clause I wherein the first part comprises a refractory material as the non-alloying material, and the second part comprises copper as the non-ferrous material.
IX. An electric circuit protection device as set forth in clause I wherein the brazing material comprises silver as another elemental material, and the first part is infiltrated with silver.
X. An electric conductor assembly comprising: an electrically conductive contact and an electrically conductive contact carrier joined together by ultrasonic welding to establish electric conductivity between them; one of the contact and the carrier comprising non-alloying material; the other of the contact and the carrier comprising non-ferrous material; and a brazing material that has been acted upon by the ultrasonic welding to join the contact and the carrier, the brazing material being selected to comprise at least two materials, one of the two materials being elemental copper, and the elemental copper being the largest single constituent of the brazing material.
XI. An electric conductor assembly as set forth in clause X wherein the contact comprises non-alloying material and the carrier comprises non-ferrous material.
XII. An electric conductor assembly as set forth in clause XI wherein the elemental copper of the brazing material is at least about 50% by weight of the brazing material.
XIII. An electric conductor assembly as set forth in clause XII wherein the brazing material is an alloy consisting of substantially 80 parts elemental copper by weight, substantially 15 parts elemental silver by weight, and substantially 5 parts elemental phosphorus by weight.
XIV. An electric conductor assembly as set forth in clause XI wherein tile contact comprises a refractory material as the non-alloying material.
XV. An electric conductor assembly as set forth in clause XIV wherein the refractory material comprises at least about 35% by weight of the contact.
XVI. An electric conductor assembly as set forth in clause XIV wherein the refractory material is from the group consisting of silver tungsten, silver tungsten carbide, copper tungsten, copper tungsten carbide, and silver molybdenum.
XVII. An electric conductor assembly as set forth in clause XI wherein the carrier comprises copper as the non-ferrous material.
XVIII. An electric conductor assembly as set forth in clause XI wherein the contact comprises a refractory material as the non-alloying material, and the carrier comprises copper as the non-ferrous material.
XIX. An electric conductor assembly as set forth in clause XI wherein the brazing material comprises silver as another elemental material, and the contact is infiltrated with silver.
XX. A method of mechanically joining electrically conductive parts comprising: providing first and second electrically conductive parts that are to be joined to establish electric conductivity between them wherein a constituent of the first part comprises non-alloying material and the second part comprises a predominance by weight of non-ferrous material; disposing brazing material between the parts at a location where they are to be joined, and joining the parts at that location by the application of ultrasonic energy, wherein the brazing material is selected to comprise at least two materials, one of the two materials being elemental copper, and the elemental copper being the largest single constituent of the brazing material by weight.
XXI. A method as set forth in clause XX wherein the elemental copper of the brazing material is selected to be at least about 50% by weight of the brazing material.
XXII. A method as set forth in clause XXI wherein the brazing material is selected to be an alloy consisting of substantially 80 parts elemental copper by weight, substantially 15 parts elemental silver by weight, and substantially 5 parts elemental phosphorus by weight.
XXIII. A method as set forth in clause XX wherein the first part is selected to comprise a refractory material as the non-alloying material.
XXIV. A method as set forth in clause XXIII wherein the refractory material is selected to comprise at least about 35% by weight of the first part.
XXV. A method as set forth in clause XXIII wherein the refractory material is selected from the group consisting of silver tungsten, silver tungsten carbide, copper tungsten, copper tungsten carbide, and silver molybdenum.
XVI. A method as set forth in clause XX wherein the second part is selected to comprise copper as the non-ferrous material.
XVII. A method as set forth in clause XX wherein the first part is selected to comprise a refractory material as the non-alloying material, and the second part is selected to comprise copper as the non-ferrous material.
XVIII. A method as set forth in clause XX wherein the brazing material is joined to the first part before being disposed between the first and second parts.
XXIX. A method as set forth in clause XVIII wherein the brazing material is selected to comprise silver as another elemental material, and the first part is infiltrated with silver to facilitate flow of material between the parts during joining.
While the present inventions have been described with reference to the embodiments as currently contemplated, it should be understood that the invention is not intended to be limited to the described and preferred embodiments. Accordingly, the claimed inventions are intended to encompass various modifications and arrangements that are within the scope of the claims.

Claims

An electric conductor assembly (10, 12, 14) comprising:

an electrically conductive contact (10) having a constituent comprising a non-alloying material and further comprising silver; and

an electrically conductive contact carrier (12) comprising a predominance by weight of non-ferrous material,

mechanically joined together at a particular location by the application of ultrasonic energy, brazing material (14) being disposed between the contact (10) and the carrier (12) at the location where they are joined;
characterised in that

the brazing material comprises at least two materials, with elemental copper being the largest single constituent of the brazing material, and in that

the contact comprises serrations in the surface joined to the brazing material, said serrations being filled with excess silver from the contact, the brazing material forming a layer overlapping the serrations.
An assembly according to claim 1 in which the brazing material comprises at least 50% elemental copper by weight.
An assembly according to claim 2 in which the brazing material (14) consists of substantially 80 parts elemental copper by weight, substantially 15 parts elemental silver by weight, and substantially 5 parts elemental phosphorus by weight.
An electric conductor assembly according to any of claims 1-3, wherein the non-alloying material comprises a refractory material.
An electric conductor assembly according to claim 4, wherein the refractory material is selected from the group consisting of silver tungsten, silver tungsten carbide, copper tungsten, copper tungsten carbide and silver molybdenum.
An electric conductor assembly according to claim 4 or 5, wherein the refractory material makes up at least about 35% by weight of the contact (10).
An electric conductor assembly according to any one of claims 1 to 5, wherein the contact (10) comprises the non-alloying material and the carrier (12) comprises copper as the non-ferrous material.
An electric circuit protection device comprising an interruptible current path for interrupting current flow to a load including trip means for tripping the interruptible current path upon occurrence of a load fault, characterised in that the current flow path comprises an electric conductor assembly (10, 12, 14) according to any one of claims 1 to 7.