GB2123222A - Cable glands - Google Patents

Abstract

A cable gland comprises an earthing lug (16) which firmly engages the gland body (10) but is formed as a separate component (i.e. is not integral therewith). In particular, the lug may be screwed onto the body and retained by a thread penetrating adhesive, welding, brazing or silver solder; alternatively it may be soldered directly onto the body. <IMAGE>

Description

SPECIFICATION Cable glands This invention relates to cable glands and particularly glands which are used for the entry of an electrical cable through the wall of a housing orjunction box, or other wall to gain access to electrical equipment.

The present applicants are a leading manufacturer of glands of this type and many of their designs incorporate so called earth tags or lugs by which the cable gland can be connected to earth in order to protect the cable gland and equipment in the event of a short circuit. In many cases the applicants provide the so called earth tags as loose parts of the gland for assembly by the electrician concerned when the gland is secured to the appropriate wall.

These loose tags or lugs are provided with an aperture to fit over a suitable spigot of the gland body and the earth tag is clamped between a face of the gland body and the associated wall of enclosure by the application of force by a lock nut. Such arrangements have been the custom and practice for many years, relying simply on this form of manual clamping force for a low resistance connection, but difficulties are experienced since the force applied may not always be adequate due to careless operation by the operator concerned, and this can have serious consequences in the event of arcing occurring between the earth tag and the cable gland during a short circuit. The currents involved during these short circuits can be very considerable, and poor earth connections can result in melting of parts of the equipment and explosion.In an alternative arrangement, which the applicants have previously proposed, a portion of the body of the gland, which may for example be formed of brass, has an earth lug cast integrally with it when it is manufactured, and this form of lug naturally overcomes difficulties of poor assembly since the short circuit current can pass immediately from the gland to the earth lead without any difficulties of arcing arising due to improper assembly. Such designs ensure perfect earthing under conditions of phase-to-earth short circuit faults developing on equipments solely protected by circuit breakers. Nevertheless, in these circumstances the armour wires of the cable are called upon to carry the full short circuit current which, on a 31 MVA 415 volt system is 43 KA.It is desirable therefore that the armour-to-gland-toearth path is competent to pass this full current for a period of one second to ensure that the circuit breaker operates.

The two types of earth lug which have been proposed and which are referred to above, each have their difficulties. Reference has already been made to the difficulties of loose earth lugs which may not be assembled correctly by the operator to obtain correct electrial continuity, and the integral cast earth lugs are relatively expensive to manufacture, since the remainder of the portion of the cable gland to which the lug is integrally formed, may itself be a complex casting, and the tooling and manufacturing costs of including the cast lug are considerable.

Accordingly, the present invention has as an object the overcoming or alleviating some of the above disadvantages.

According to one aspect of the present invention, a cable gland comprises a body arranged to be mounted in a wall, the body having a bore to allow a cable to pass therethrough and an earthing lug formed as a separate component, the lug firmly engaging with the body. In this way the body can be assembled in a permanent or semi-permanent way to ensure good electrical continuity. Furthermore, the earthing lug may be attached during the manufacturing stage of the cable gland whereby, when the operator fits the cable gland to the wall, there is no room for operator error in attaching the lug to the cable gland.

The lug may have an aperture through which the body of the cable gland extends. This may provide a large area of contact between the lug and the body to ensure good electrial continuity.

The means by which the lug firmly engages with the body can vary according to requirements.

The aperture of the lug may be screw threaded, which screw thread engages with a screw thread on an external diameter portion of the gland body.

Tightening of the lug on the body to a large torque ensures that the lug may not be readily disengaged from the body.

The screw thread on the lug and body may be of different axial pitch or, alternatively, or additionally, at least one of the screw threads may be tapered in diameter. This allows the screw threads to more firmly engage each other. A thread penetrating adhesive may be loca-ted between the meshing screw threads to form a permanent engagement of the body and lug.

Preferably, the lug is welded or brazed or silver soldered or the like to the body, whether or not the lug is screwed on to the body or a hole of the lug is located on a spigot of the body. In each case a suitable slight clearance is provided between the lug and body in order to ensure that there is adequate space for the ingress of the soldering material or the like.

In arrangements where the lug is connected by soldering or the like, the solder is preferably applied to both axial and radial facing surfaces of the lug and body.

Accordingly, the present invention has as an object the overcoming or alleviating some of the above disadvantages.

The invention may be carried into practice in a number of ways but certain specific embodiments will now be described by way of example with reference to the accompanying drawings in which: Figure l is a schematic longitudinal cross section of a cable gland embodying the present invention; Figures 2 to 5 are similar views of alternative forms of cable gland each with a cable extending therethrough, and Figures 6to 13 illustrate various forms of earth lug; Figure 1 illustrates a cable gland mounted on a wall 11 having aperture 11A. An insulated top-hat sleeve section 1 OA of the gland extends through the aperture 1 1A from one side of the wall. A further insulating sleeve 10B is situated on the other side of the wall 11.

The gland also includes a connecting sleeve 10C extending within the top hat sleeve 1 OA and carrying a lock nut 10D at its inner end. This cable gland, so far as its attachment to the wall 11 is concerned, is arranged in a manner generally similar to that disclosed in our British Patent Specification 1602435.

The body 10 has a bore indicated generally at 12 through which a cable (not shown) may pass and thus pass through the wall 11. To the left, as seen in the drawing, the cable body is arranged to receive and grip the cable. The arrangement is again, so far as the gripping of a cable is concerned, generally similar to that disclosed in Specification 1602435 mentioned above.

In particular this arrangement comprises a nut 13 which is screw threaded on to a screw threaded boss portion 15 of the body 10 and arranged to grip, between itself and the body 10, armour wires of a cable. A further nut 14 is provided, which is screw threaded on to a threaded diameter of the nut 13 and arranged to grip, through the intermediary of a slipper 14A and a deformable seal 14B, the outer sheath of a cable passing through the gland. It should be understood that the arrangement for gripping a cable in the body 10 and the arrangement for mounting the body 10 in the wall are purely exemplary so far as this present invention is concerned, and may be replaced by a number of other possibilities.

As mentioned above, the body 10 is provided with a screw threaded boss 15 on to which the nut 13 is fitted and tightened to grip the cable armouring. In this embodiment of the invention an earthing lug 16, provided with a screw threaded bore 17, is mounted on the boss 15 as part of the manufacture of the gland and is screwed tight against a shoulder 18 of the body 10. The lug 16 is preferably tightened to such a torque that it cannot be readiy removed from the body 10 and thus becomes to all intents and purposes as one with the body 10 in subsequent assembly of the gland to a wall in use. In order to ensure that the lug 16 is not removable, the threads may be locked by a thread penetrating adhesive such as that sold under the trade mark LOCTITE.

In another arrangement the lug may be locked by a small area of welding, brazing, silver soldering or the like between the lug and the body 10 as indicated at 19, or it may be locked by the threads on the boss 15 and in the lug 16 having slightly different axial pitch or by one of them being tapered.

As shown in Figure 1 the lug 16 is angled to lie back along the axis of the gland and is provided with a screw-tapped hole 20 to receive a bolt 21 carrying a nut 21A by means of which an earth lead indicated typically at 22 may be attached to the lug. The bolt 21 preferably is of such length that it extends through the lug 16 to engage as indicated at 23 the nuts 13 or 14 (or their equivalent in other arrangements) to prevent them loosening under vibration or for some other reason in an assembled gland.

Figure 2 shows a somewhat similar arrangement comprising a low current cable gland mounted in a wall 30 but insulated from the wall by a top-hat sleeve 32 and a further sleeve 33. The cable gland includes a main body 34 which is internally threaded at 34A to engage one end of a connecting sleeve 34B, the other end of which carries a lock nut 36 by which the body is clamped to the wall 30. Extending through the cable gland is a cable generally indicated at 36 and which includes a number of conductors 36A, an outer sheath 36B and an internal metallic cable armouriscreen 36C. Connected to the outer end of the main body 34 is a screw threaded boss portion 38 similar to the boss portion 13 of the first embodiment, but in this case connected to a metallic connector 40 for earthing purposes.A further screw threaded nut 42 is mounted on the nut 38 and through a slipper 44 engages a weather-proof seal 46.

In this embodiment an alternative form of earth lug or earth tag 50 is incorporated, which has a main radial portion 50A extending around a spigot of the main body of the gland and located in an annular recess 34C in the body. The earth lug or tag has an axial step 50C merging into a further radial portion 50D which, by virture of the step 50C, is displaced away from the wall 30 in order that the head 52 of a bolt 52A is spaced from the wall 30 by the required insulation clearance. By means of the bolt 52A together with a washer 52B and a lock nut 52C, an earth lead 54 is secured to the earth lug or tag.

Reference is also made to Figures 6 to 10 which show various forms of earth lug which are appropriate for the configuration of Figure 2.

A low current lug is designed to have a minimum thicknes of 1.5mm with a tolerance of 0.25mm between the lug clearance hole and its mating spigot on the gland body to permit bonding solder to flow under controlled conditions and which, when cooled, will form a low resistance integral connection to withstand the temperature of its rated short circuit condition, which in this instance is 13.1 KA for one second.

The applicants know of no known formula for the electrical ratings of cable glands due to the varying sections and their associated environment, or for the attachment of current carrying lugs to the cable gland bodies and therefore the embodiments of this invention, shown in the drawings, have been produced as a result of empirical design and testing.

Figure 3 shows an embodiment very similar to that of Figure 2, but in this case it is a high current cable gland and the earth connections are rated to take up to 43KA for one second.

The form of the earth lug is generally similar to that in Figure 2 except that it is of higher duty and may be of the form shown in Figure 6 or Figures 8 or 9 read together with Figure 11. In this arrangement the lug is designed to have a minimum thickness of 6mm and the shape shown is so controlled to prevent necking currents melting the lug as the current separates around the circular section in contact with the gland body. The lug again has a clearance hole with a tolerance of 0.25mm to permit bonding solder to flow under controlled conditions.

The solder employed has a melting point in excess of 250"C which will withstand a high fault current short circuit and also maintain a low resistance connection.

Figures 4 and 5 show two alternative arrangements respectively for low and high earth currents.

The arrangement of the earthing lug or tag in each case is similar to Figures 2 and 3 respectively, but the glands in this case are not of the insulating type and therefore the gland body is arranged to be directly connected to the wall of the electrical enclosure, for example by being screw threaded into the wall. In these arrangements there is not a necessity to ensure an air insulation clearance between the bolt head and the enclosure and the clearance shown in Figures 4 and 5 is merely for assembly purposes.

It will be noted that in all the embodiments of Figures 2 to 5 the bolt is arranged with its head nearest the enclosure. This has the advantage that, particularly with the insulated arrangements, the spacing of the end of the head from the enclosure is fixed, which would not necessarily be the case if the arrangement were the other way around, i.e. with the head on the further side of the earth lug, since in that case the shank of the bolt would project towards the wall and the end of the shank can be at a varying distance from the wall depending on the thickness of the earth lead and the lug.In certain arrangements it may also be possible to arrange the crank or step in the earth lead or lug to be close to the bolt head such that the crank prevents rotation of the bolt, so simplifying assembly of the earth lead since in that case it will not be necessary to place a spanner on the bolt head to prevent its rotation.

As has been mentioned, Figures 6 to 13 show various forms of earthing lug. Figure 6 is a generally pear-shaped arrangement for low or high earth currents. Figure 7 illustrates a low current arrangement in which the lug is of reduced width adjacent to the step or crank, this being suitable for low current shapes. Figure 8 shows an arrangement similar to Figure 6, but with a slightly altered connecting end.

Figure 9 illustrates a low or high current shape, useful for restricted accommodation.

Figures 10 and 11 show the types of lug illustrated in Figures 2 and 3 respectively in which the lug has a plain hole to be assembled on a spigot of the cable body and to be bonded thereto. In contrast in the embodiments of Figures 12 and 13, which in each case illustrate high current lugs, the lugs have threaded bores to engage threaded spigots on the cable gland body, again appropriate bonding taking place at the thread surfaces and/or between the mating radial faces of the lug and cable gland body.

Claims (10)

1. A cable gland comprising a body arranged to be mounted in a wall, the body having a bore to allow a cable to pass therethrough, and an earthing lug formed as a separate component, the lug firmly engaging with the body.

2. A cable gland as claimed in Claim 1 in which the lug has an aperture through which the body extends.

3. A cable gland as claimed in Claim 2 in which the aperture is screw threaded, which screw thread engages with a screw thread on an external diameter portion of the gland body.

4. A cable gland as claimed in Claim 3 in which the lug is screwed on to the gland body by cooperation of the screw threads and tightened to a large torque.

5. A cable gland as claimed in Claim 3 or 4 in which the screw threads on the lug and body are of different axial pitch.

6. A cable gland as claimed in Claim 3,4 or 5 in which at least one of the screw threads is tapered in diameter.

7. A cable gland as claimed in any of Claims 3 to 6 in which a thread penetrating adhesive is located between the meshing screw threads.

8. A cable gland as claimed in any preceding claim in which the lug is welded or brazed or silver soldered to the body.

9. A cable gland as claimed in any preceding claim in which the thickness of the lug is at least 6mm.

10. A cable gland substantially as herein described with reference to, and as shown in the accompanying drawings.