FIELD OF THE INVENTION

The present Invention relates to connectors for electrical conductors, and to shear bolts for use In such connectors.

BACKGROUND ART

Electrical power distribution cables typically comprise three insulated conductors, the conductive part of which typically consists of copper or aluminium as a solid rod or as braided strands. The conductors are available in a range of sizes to suit the Intended level of power to be transmitted through them. The conductors need to be connected to electrical equipment or to other conductors, and a range of connector designs are known.

Tte connectors come In two main forms. One form has a lug on one end allowing it to be connected to equipment (e.g. via a bolt) and a bore at its other end for receiving the conductor. The conductor is held in place within the bore using shear bolts driven into transverse threaded holes that extend into the bore from one side; the shear bolts fracture at a specific torque, leaving their threaded shank behind and securing the conductor. Other connectors have two bores, at opposing ends of an elongate body, allowing two conductors to be joined end-to-end.

EP-A-1,284,031 shows a lug-type connector and addresses the need to provide for varying thicknesses of conductor. Thinner conductors are accommodated by way of an insert that fits within the bore. SUMMARY OF THE INVENTION

The insert that is disclosed in EP-A-1,284,031 Is an unsatisfactory way of dealing with varying sizes of conductor. It is an additional part, which imposes an additional cost and which must be tracked, stocked, and reliably bundled with the connector. When a bundle is used for a large-diameter conductor, tine Insert is wasted.

It is also unsatisfactory to simply provide a longer shear bort, one that will be able to project into the longitudinal bore and grip against even a narrow cable. One problem is that, fixed to the connector body and projecting into the bore, such a bolt will be mechanically less secure. A more serious problem is that when used with larger-diameter conductors, the bolt shank left behind after the head shears off will project out of the connector body and provide a number of projecting edges that, under high voltage, will be a source of discharge that leads over time to degradation of the surrounding Insulation. Some have tried to solve this by providing a shear bolt with multiple frangible points along the bolt shank, but these are complicated and expensive to Implement, and do not reliably shear at the "correcT location. If the bolts shear In the*wrong" location, they are dlfflailt to remove and replace.

We have therefore sought to provide a connector that can accommodate various conductor sizes without needing an insert, which can use a shear bolt with a simple design, and which does not leave sections of bolt protruding from the outer envelope of the connector.

The present invention therefore provides a connector assembly for electrical conductors, comprising a connector portion and at least one shear bolt, the connector comprising a body with a longitudinal internal bore for receiving an electrical conductor, and a transverse threaded hole extending through the body into the longitudinal Internal bore, the shear bolt comprising a head, a threaded bolt portion, and a frangible section therebetween, the threaded bolt portion being sized to engage with the transverse threaded hole, wherein the diameter of the transverse threaded hole is at least as large as the diameter of the longitudinal internal bore.

The transverse threaded hole that is at least as large as the diameter of the longitudinal Internal bore (preferably substantially equal internal diameters) allows the shank of the shear head bolt to engage with the connector body through the complete depth of the body. Thus, there Is no need for a length of shank that is able to project into the bore, and a relatively short shank will be sufficient for all conductor sizes, even with a single fracture point This prevents the shank from having to extend outside the connector envelope when used with larger conductors.

This does result In a (potentially) shorter shank length over which the mechanical load must be carried. One way of coping with this is for the threads of the threaded bolt portion and the transverse threaded hole to be formed as buttress threads. A buttress, or sawtooth, thread has a load bearing thread face perpendicular to the screw axis, and better resists axial loads. It is therefore possible to transfer a greater force along the length of the thread before the mechanical strength of the thread fillets are compromised. A sawtooth thread also has good friction properties, akin to those of a square thread.

The frangible section of the shear bolt is preferably in the form of a tapered section, tapering toward the threaded portion. This provides a reliable fracture point adjacent the threaded portion, leaving the minimum amount of material behind.

Two transverse threaded holes can be provided in the body, both extending into the longitudinal internal bore, to provide greater security through redundancy. In that case, the assembly will generally indude a total of at least two shear bolts. The two transverse threaded holes are preferably non-parallel and misaligned along the horizontal axis (I.e. offset from the longitudinal centre (which may coincide with the axis of the bore) of the connector in opposite directions), ideally at an angle of no more than 90° when viewed along the longitudinal internal bore, most preferably at 45° to each other. This provides the most secure retention of the conductor against slippage.

The connector assembly can include a second longitudinal internal bore for receiving a second electrical conductor. The second bore is ideally of the same design as the first This then creates a double-ended connector suitable for end-to-end jointing of conductors. An elongate body is convenient, with one longitudinal internal bore extending into the body from a first end thereof, and the second longitudinai internal bore extending into the body from a second end thereof. Alternatively, the longitudinal internal bore can extend Into the elongate body from a first end thereof, and the second end of the body can be formed with a mounting lug. This then provides a connector that is suitable for connecting the conductor to an item of equipment Other physical forms for the connector are of course possible.

We also prefer that the threaded bolt portion has an extent no greater than the depth of the transverse threaded hole from an external face of the body to the longitudinal bore, thus ensuring that the threaded portion that remains after shearing will not project beyond the connector regardless of the size of the conductor. The present invention also relates to the shear bolt Itself, especially one that is adapted for use in the above-defined connector assembly. Such a shear bolt can comprise a head, a threaded bolt portion, and a frangible section therebetween, the threaded bolt portion having a diameter that is greater than its threaded length. As noted above, the thread of the threaded portion is preferably a buttress thread, and the frangible section is preferably tapered toward the threaded portion.

In addition, the present invention relates to the connector. This may comprise a body with a longitudinal Internal bore for receiving an electrical conductor, and a transverse threaded hole extending through the body into the longitudinal internal bore, the diameter of the transverse threaded hole being at least as large as the diameter of the longitudinal Internal bore. Other preferred aspects of the connector are as described above.

Finally, the Invention also relates to a kit of parts comprising a connector and at least two shear bolts as defined above;

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present Invention will now be described by way of example, with reference to the accompanying figures in which;

Figure 1 shows a first embodiment of the present Invention, in a longitudinal sectional view;

Figure 2 shows a view of the first embodiment from above; Rgure 3 shows a section on on figure 1;

Rgure 4 shows a second embodiment of the present invention, in a longitudinal sectional view;

Figure 5 shows a view of the second embodiment from above; Figure 6 shows a view from one side of a shear bolt for use in the first and second embodiments;

Rgure 7 shows a view of the shear bolt of figure 6 from above;

Rgure 8 shows the thread pattern of the bolt of figure 6;

Rgure 9 shows a sectional view of an embodiment of the Invention in the process of being used;

Rgure 10 shows the first embodiment, partiy assembled; and

Rgure 11 shows the second embodiment; partiy assembled.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Rgure 1 shows a connector 10 according to a first embodiment. The connector 10 comprises at a first end an elongate body 12 of circular cross section. A longitudinal internal bore 14 extends from the first end of the connector through the elongate body to provide a body of substantially constant cross sectional area. The internal bore 14 is sized to receive an electrical conductor of any one of a range of standard dimensions, A second end of the connector is comprised of a mounting lug 20 connected to the elongate body 12 by a chamfer. The mounting lug has a lug hole 22 for mounting the connector to an item of electrical equipment (not shown).

The connector 10 is provided with at least one transverse hole for accommodating a shear bolt. In the present embodiment a first transverse threaded hole 16 and a second transverse threaded hole 18 longitudinally adjacent the first transverse threaded hole 16 extend through the body 12 and into the longitudinal bore 14, The first 16 and second 18 transverse threaded holes are offset from the longitudinal centre of the connector 10 in opposing directions and are offset from one another to allow sufficient space for the shear bolt heads. The first 16 and second 18 transverse threaded holes are offset from one another at an acute angle of no more than 90° when viewed along the longitudinal Internal bore as shown In figures 2 and 3.

The diameters of the transverse threaded holes 16, 18 are at least as large as the diameter of the longitudinal bore 14, so that a portion of the shear bolt Inserted through a transverse threaded hole can pass fully into the longitudinal bore if desired. In the present embodiment the diameter of the transverse threaded holes 16, 18 are at least as large as the diameter of the longitudinal Internal bore 14.

Figure 3 shows a first embodiment of a shear bolt for use with the various embodiments of connector disclosed herein. A first shear bolt 24 Is sized for insertion Into the first transverse threaded hole 16 and a second shear bolt is sized for cooperation with tiie second transverse threaded hole 18. The first 16 and second 18 transverse threaded holes are offset at a 45° angle to one another.

Shear bolts 24, 26 are substantially the same, comprising a bolt head 28 at a first end, a threaded bolt portion 30 at a second opposite end and a frangible portion 32 therebetween. As shown in figure 7, In the present embodiment the bolt head 28 Is of hexagonal shaped cross section, in alternative embodiments, the bolt head may be of any desired shape as required to cooperate with a spanner or socket tool, such as a square head bolt or an alien socket head form. The threaded bolt portion 30 Is sized to engage with the transverse threaded hole into which it Is Intended for insertion.

The frangible portion 32 in the form of a tapered section tapers toward the threaded bolt portion 30 to provide an area of minimum cross sectional area immediately adjacent the threaded bolt portion 30.This area of minimum cross sectional area provides a single reliable fracture point 34 adjacent the threaded bolt portion 30. In use, torque is applied to bolt head 28 using standard manual or automated means. Once tile Shear bolt reaches a preferred depth, torque that is no longer required for installation causes tiie bolt head 28 to shear off at fracture point 34, leaving only the threaded bolt portion. 30 within the connector. The shear bolt Is thus left with no drive means or means for removal.

Figure 4 shows a second embodiment of the connector In which connector 40 comprises a first longitudinal internal bore 44 extending from a first end of the connector into connector body 42 and a second longitudinal internal bore 46 extending into connector body 42 from a second end of the connector. The second longitudinal internal bore 46 can receive a second electrical conductor to permit end to end connection of two discrete electrical cables. In the present embodiment, each longitudinal bore is equal In both depth and diameter. However, in alternative embodiments each longitudinal bore may vary in depth and diameter. figure 5 shows the first longitudinal Internal bore 44 having first 48 and second 50 transverse threaded holes and the second longitudinal internal bore 46 having first 52 and second 54 transverse threaded holes arranged in the same manner as those of the first embodiment. A pair of transverse threaded holes are provided in relation to each longitudinal internal bore so that two shear head bolts may be used to retain each electrical conductor inserted in the connector. In use, the conductor is subject to vibration that would otherwise result in the conductor coming loose from the connector. By providing two shear bolts offset from one another around the circumference of the longitudinal bore the conductor Is retained within the bore at two pressure points offset from one another that provides Improved retention. This arrangement also maintains electrical contact between the conductor and the connector should one shear bolt fail or become loosened.

Figure 8 shows the thread of the threaded bolt portion 30 as a buttress thread 56. A buttress, or sawtooth, thread has a load bearing thread face 58 perpendicular to the screw axis 60, as indicated In figure 6. The perpendicularity of the thread face 58 allows the shear bolt to better resist axial loads. It Is therefore possible to transfer a greater force along the length of the thread before the mechanical strength of the thread fillets are compromised, thus resulting in an improved resistance to thread stripping and pull out

Figure 9 shows a connector 10, 40 according to the first or second embodiment in use. A shear bolt 66 according to a second embodiment has a frangible section 68 comprised of ah elongate section 70 and a waist section 72. The waist section 72 provides a reliable fracture point in the form of a section of reduced constant cross sectional area, the fracture point Is adjacent threaded bolt portion 74 as per the first embodiment

The buttress thread of the threaded bolt portion 74 cooperates with the buttress thread 64 of the connector 10, 40 so that the convex bottom face 76 of the shear head bolt 66 is urged against the electrical conductor 62 when torque is applied to the shear bolt head 78. The threaded bolt portion 74 retains the electrical connector 62 in place against the surface of the longitudinal bore 14, 44, 46 of the connector 10, 40.

Where the electrical conductor is a large diameter cable 80, as shown in figure 10, the threaded bolt portion 30 of the sheaf head bolt 24, 26 engages with the transverse threaded hole 16, 18 of the connector to a depth whereby the convex bottom face 36 of the shear head bolt 24, 26 applies pressure to urge the electrical conductor 82 against the surface of the internal bore 44. Once sufficient pressure is applied to the electrical conductor 80, the torque applied to the shear bolt head 28 will cause the bolt 24, 26 to shear at the fracture point 34 of frangible portion 32 adjacent the threaded bolt portion 30. The threaded bolt portion 30 has an extent which is no greater than the thickness of the wall section of the connector, between Its external face 86 and the longitudinal bore 14. This, once engaged within the transverse threaded hole 16 the entirety of the threaded portion that remains will be within the transverse threaded hole 16 such that no section of bolt protrudes from the connector. Although a large diameter cable 62 Is shown in figure 9, the shear head bolt 66 is dimensioned so that the convex bottom face 76 can contact the internal surface of the longitudinal internal bore 14, 44, 46 and the shear bolt head 78 remain proud of the connector 10, 40 when the shear head bolt 66 is fully inserted Into connector 10, 40. Very small diameter electrical conductors can therefore be accommodated within the connector, Where the electrical conductor is a narrow cable 82, as shown in figure 11, the threaded bolt portion 30 of the shear head bolt 24, 26 can engage with the transverse threaded hole 16, 18 of the connector to a depth whereby the threaded bolt portion 30 passes the central axis of the longitudinal internal bore 44. The convex bottom face 36 of the shear head bolt 24, 26 applies pressure to retain the electrical conductor 82 against the bottommost surface 84 of the Internal bore 44. Once sufficient pressure is applied to the electrical conductor 82> the torque applied to the shear bolt head 28 will cause the bolt to shear at the fracture point 34 of frangible portion 32.

It will of course be understood that many variations may be made to the above- described embodiment without departing from the scope of the present invention.