Minnesota Mining & Manufacturing Company v Tyco Electronics Pty Limited
[2001] FCA 1359
At a glance
Source factsCourt
Federal Court of Australia
Decision date
2001-07-01
Before
Sackville J
Source
Original judgment source is linked above.
Judgment (38 paragraphs)
BACKGROUND TO THE INVENTION 20 An electrical connector is defined in a technical dictionary to mean "a coupling device employed to connect conductors of one circuit or transmission element with those of another circuit or transmission element." (IEEE, Standard Dictionary of Electrical and Electrician Terms (2nd ed 1978)). In simpler terms, it is a device which contains a metal element designed to connect two wires together so as to enable an electric current to pass from one wire to another. Electrical connectors are used in industries where it is necessary to join two wires. Many devices which are or include electrical connectors have distinctive names such as terminals, plugs, sockets, jacks and printed circuit boards. 21 The quality of the connection is affected by a number of factors, including corrosion, aging and the manner in which the connection is made (for example, twisting of wires together is generally less effective than soldering). Once the connection is made, the wires must remain in place in order to maintain the connection. It is therefore necessary to incorporate into the electrical connector a means for retaining the wires in place and to prevent withdrawal from the passageway in which the wire is located, for example in a radial direction. This feature is known as wire retention. After the insertion of the wire into the connector, the function of the retainer is to secure the wire so that the electrical connection is maintained and, if the connector is a multiwire connector with a cover, to secure the other wires in place when the cover is removed and the connector is the subject of maintenance. A means of wire retention is also needed to retain a wire in the connector during the assembly process. 22 The connector may also be affected by physical stress on the connection point (that is, the contact element). The greater the physical stress at the connection point, the greater the loss in consistency of the electrical connection. In the telecommunications industry, "strain relief" generally refers to a feature designed to minimise force transmitted to the connection point. More specifically, it refers to a feature incorporated in a connector which is designed to minimise the effect of tensile forces applied to the conductor in a longitudinal direction away from the connection point. Such a force occurs when a wire or conductor is strained or pulled axially or laterally. The strain relief feature is designed to move the point of tension created by that force away from the contact element to the point where the feature is located. The means of affording wire retention does not necessarily afford strain relief. Indeed, 3M says that one inventive feature of its invention is that it provides a means both for wire retention (by a slot of a particular shape into which the wire is inserted) and strain relief (by means of tongues which grip the conductor and deflect towards the connection or contact point). 23 According to Mr Weir, a number of forms of strain relief can be incorporated into a connector. These include the following approaches: · The wire is enclosed in an immobilising long passageway extending axially back from the contact element. The length of the passageway should ensure that any flexure of the wire outside the corridor occurs at a remote distance from the contact point. The longer the passageway the greater the strain relief. · The wire is threaded along a tortuous path towards the contact element. Such a path ensures that the wire experiences strong frictional contact with the walls or posts of the enclosing passageway. This frictional contact resists any axial tension applied externally to the wire and prevents such tension from being transmitted along the wire to the contact element. The more tortuous the path the greater the strain relief. · The wire is gripped, squeezed or pressed by contact forces acting perpendicular to its axis and at a location along the wire some way removed from the point of electrical contact. The gripping force is usually imparted by a surface or surfaces acting on the wire insulator and leads to strong frictional contact between the insulator and the gripping surface. This frictional contact resists any axial tension applied externally to the wire, and prevents such tension from being transmitted along the wire conductor to the electrical contact element. The stronger the gripping force, the greater the strain relief. · An axially compressed insulation tip is used, whereby the wire insulation is compressed axially by contact forces acting along the wire close to the point of electrical contact. The axial force is usually imparted by frictional contact from surfaces acting on the wire insulator, when those surfaces are displaced axially towards the electrical contact element. The axial compression so imparted will oppose any axial tension applied externally to the wire, and therefore reduce the extent to which the electrical contact element will experience tensile forces. The stronger the axial compressive force, the greater the strain relief. Mr Weir's view, which I accept, was that the connector claimed in the Patent displays both gripped wire and axially compressed insulation tip strain relief. The connector does not utilise long passages or tortuous path strain relief. 24 In the course of the hearing, the claimed invention was quite often referred to as having a "self-locking action". Mr Weir said that the concept of a self-locking action effected by means of an oblique member hinged at one end to a supporting body and placed into compression when it presses against a counteracting surface at the other end, is universally known and is exemplified by door stops, deck chairs and sailing cleats. He also said that it was not correct to suggest (as Mr Douglas had) that the claimed invention exclusively relies on a self-locking action. Rather, strain relief is achieved (aside from any gripping effect) by the axially compressed insulation tip, which overcomes the slight initial movement in the conductor that would be characteristic of a purely self-locking action. In re-examination he said that there is a self-locking action on the tongues as they start to do their job, but that the axially compressed insulation tip phenomenon reduces the extent to which harmful forces are transmitted to the joint with the electrical contact. 25 I do not think that anything of substance turns on the use by witnesses and indeed counsel of the expression "self-locking mechanism" in relation to the claimed invention. I accept Mr Weir's analysis, but I did not understand other witnesses (except perhaps Mr Douglas) to challenge his analysis. Rather, they adopted the expression "self-locking" as convenient shorthand to refer to the functioning of the claimed invention. 26 The design of connectors has to take into account the requirements of particular industries. In the electrical industry standard wires are typically used. Standard wires are not themselves insulated, but are twisted together. The bundles of twisted wires are twisted together and placed in an insulation sheet. Because the wires are flexible they are less sensitive to physical stress than solid wires. Wires used at the priority date in the electricity industry were generally relatively large. Wires were placed into a connective element and pressed into the wall of the housing, so that they were locked into position. Typically strain relief was achieved by routing the wires around an S-bend. Since a force travels only in a straight line, the S-bend minimised the tension that reached the contact element. Sometimes clamps were used for additional strain relief. 27 Wires used in the telecommunications industry are generally solid copper wires. Each single copper wire, or conductor, is insulated with material such as paper, pulp or plastic. Usually wires are placed in a configuration of bundles of 10 or 25 pairs of wires. The wire gauge (that is, the diameter of the solid copper wire without insulation) used in the telecommunications industry in Australia prior to 1989 ranged from 0.32 mm to 1.2 mm, although most commonly conductors were between 0.32 and 0.64 mm in diameter. 28 Cables in the telecommunications industry consist of multiple pairs of conductors. Generally speaking, as Mr Douglas explained, telecommunications cables are built up, commencing with 10-pair conductor groups which are then arranged in 50 or 100 pair bundles. The bundles may be combined to make even larger bundles. The physical mass of large size cables is such that they can only be handled in relatively short lengths. Paired conductors may therefore have many joints along their length. 29 A distinction is drawn in the telecommunications industry between internal and external plant. "Internal plant" refers to equipment within telephone exchange buildings that is concerned with telephone circuit switching. "External plant" commences from the point at which cables are attached to what is known as the Main Distribution Frame ("MDF") in the telephone exchange building and extends to the entire telecommunications network outside that building, including customers' premises. Conductors used in the external plant are generally subject to greater stress than those used in the internal plant. 30 Cables are divided into two types: main cables and distribution cables. The main cable goes from the exchange to the first distribution pillar in the street. From that point, the cables are part of the distribution network. From the MDF, large size main cables (each containing up to 3,500 pairs of wires) are carried in protective conduits along the streets to the distribution pillars. At the distribution pillars, the large size main cables are connected to smaller branching distribution cables. At each distribution pillar the cable size is progressively reduced to a size appropriate for that type of premise. At business premises, the cable entering the premises may contain 50 pairs or 100 pairs or more, depending on the size of the building. 31 Depending on the pair count, the length of cable between connections in the telecommunications industry can vary from about 100 to 2000 metres. If cables have to be restored, for example because of construction activity, they may have to be cut and reconnected. A telecommunications network therefore generates a need for many connectors. In 1989, the typical industry standard required a connection to last from 20 to 25 years. The quality of connection accordingly had to be maintained throughout this period. 32 The Patent involves the use of what is known as insulation displacement contact technology ("IDC technology"). Prior to the introduction of that technology in the late 1970s or perhaps somewhat earlier, the usual techniques for connecting individual or standard wires to a connector required the insulation to be stripped from the end of the wire (or conductor). The exposed conductor end was then crimped or soldered to the connector. Crimping required metal to be compressed around the conductor end to make the connection, while soldering required the wires to be twisted together and soldered, thereby enabling the current to pass between the two wires. Other techniques such as the use of "screw type" connectors and "twist and sleeve" connectors were also employed but these, too, required the insulation to be stripped from the conductor. 33 IDC technology is designed to provide a means whereby: (i) individual wires, or strands of wire, can be inserted into a housing so as to make reliable contact with the contact element, thereby ensuring that a complete electrical circuit is created; (ii) the insulation surrounding the wire, or strands of wire, can easily be penetrated or displaced without breaking the wire, thereby achieving good electrical contact between the contact element and the wire (that is, the electrical conductor); and (iii) the wire is held firmly in place with some form of strain relief, thereby ensuring a reliable long term electrical connection even when tension or other force is applied to the conductor. 34 IDC technology involves the use of a metal contact element, with a U-formed slot (usually known as a "U-contact element"). The insulated wire is placed over the U-contact element and then pressed into it. The U-slot is narrower than the diameter of the copper conductor contained inside the insulated wire. When the insulated wire is pressed into the U-contact element, the protruding legs of the U-slot pierce the insulation surrounding the conductor. This splicing, as it is known, enables the electrical connection between the U-contact element and the conductor to be made. The U-contact element does not pierce the conductor itself. 35 In order for a reliable connection to be made, some force must be applied between the conductor and the U-contact element. This force is supplied in two ways: (i) the deformation of the conductor; and (ii) the deflection, within the limits of its elasticity, of the U-contact element. When the conductor is pressed into the slot of the U-contact element, the conductor is forced to deform in order to "press fit" within the contact area. "Press fit" refers to the conductor and the contact element fitting together in a manner that prevents air gaining access to the contact area. If air does pass between the contact element and conductor corrosion may occur. 36 As the conductor deforms to press fit into the contact area, the U-contact element deflects sideways away from the conductor but within its elastic limits. (All materials have a capacity for deformation when force is applied. When the force is removed the material returns nearly to its previous form. This property is known as elasticity.) The elastic limits of the U-contact element depend on the nature of the material used to make the element. While the U-contact element is deformed, it applies a constant force towards the deforming element, namely the conductor. 37 Because of the interrelationship between the conductor and the contact element, and the need for the U-contact element always to deform within the limits of its elasticity, it is important in designing a connector to understand the physical properties of both the conductor and the metal contact element. If, for example, the element is too rigid it may cut the wire; if it is too soft it may deflect beyond its elastic limits and may not create a sufficient force on the conductor to maintain a reliable connection.