GB2244388A

GB2244388A - Improvements in cable glands

Info

Publication number: GB2244388A
Application number: GB9009219A
Authority: GB
Inventors: Alan Charles Durward Wales
Original assignee: C M P
Current assignee: C M P
Priority date: 1990-04-24
Filing date: 1990-04-24
Publication date: 1991-11-27
Anticipated expiration: 2010-04-24
Also published as: GB9009219D0; GB2244388B

Abstract

A cable gland assembly for a combined neutral-and-earth electrical system twin conductor cable (10) passing through an aperture in a wall (12) comprises a gland body (30) mounted in the aperture, an earth tag (62) supported by the gland body, an earth clamp (54) electrically connected to the tag through the gland body and a clamping arrangement comprising two engageable tapered surfaces (36, 42) on the body and a gland sleeve (38) to which the neutral one of the conductors (18) is clamped for electrical connection to the earth tag. The gland body (30) thus provides a common path to earth for the neutral conductor (18) and the earth screen (22). <IMAGE>

Description

IMPROVEMENTS IN CABLE GLANDS This invention relates to cable glands. The invention is particularly applicable to a cable gland at the junction between, for example, a housing and a cable which passes through an aperture in the housing.

One form of electrical transmission system is 2 wire-based. In this system one of the wires serves as both the neutral conductor and an earth path. A typical cable for this system comprises an insulated live central conductor which is surrounded by a stranded neutral conductor. The surrounding stranded conductor acts as a current carrying (neutral) conductor and combined earth. This arrangement is insulated by a flexible water barrier sleeve covered by a braided electromagnetic emission metallic screen which prevents the inner conductors from radiating excessive interference. This braid must also be earthed to comply with current mandatory safety requirements.

It is an object of the present invention to provide a cable gland termination for the junction between a cable for a 2 wire electrical system and the housing through which the cable extends.

According to the present invention there is provided a cable gland assembly for a cable, having a first conductor, a second conductor and an outer insulating sheath, the cable passing through an aperture in a wall of, for example, a housing, the assembly comprising a gland body through which the cable passes, which gland body is arranged to be mountable in registry with the aperture on one side of the wall and is adapted for connection to an earth, clamping means for securing the cable relative to the gland body, and means for establishing electrical contact between the second conductor and the gland body.

Preferably an insulating sleeve extends over the assembly substantially from the wall to the exposed outer sheath of the cable projecting from the assembly away from the wall.

The assembly provides a combined earthing path from the second conductor to an earth point on the other side of the wall through the gland body itself.

As the gland body is now used to carry the neutral current as well, it is insulated by the outer sheath.

Preferably, the gland body is secured in the aperture by means of a locking nut which engages a complementary thread on a portion of the body projecting through the aperture. An earth tag on a washer may be held on the gland body by the lock nut.

To comply with safety requirements, this tag is preferably spaced from the wall of the housing by an insulating, e.g. polycarbonate, washer.

The second conductor is preferably clamped between a tapered surface of the gland body and a complementary surface on a gland sleeve received on the gland body.

Desirably, the gland sleeve is held in place by threaded engagement between the two.

Alternatively, the gland body may be received on an open threaded sleeve projecting through the aperture in the wall towards the assembly, which sleeve is threadedly received on the gland body. The sleeve has a flange which engages the wall on the other side from the assembly. The flange is preferably spaced by an insulating, e.g. polycarbonate, washer.

In both forms the central first conductor extends through the wall. The means by which the gland body is mounted in the wall is secured to earth. One form may be by the use of a modified lock nut or flange which is adapted to have bolted to it an earth cable.

The insulating sheath may be supplemented by a sealing washer at the wall-end of the assembly which is pinched between a flange face on the gland body and the surface of the wall surrounding the aperture. The main purpose of the washer is to prevent the ingress of dirt and/or moisture into the housing.

The outer electromagnetic interference sleeve of the cable may be secured to the earth connection constituted by the gland body by means of a clamp arrangement. This may also include a seal for the assembly. In one form, the assembly is provided with a clamping surface about the cable against which the screen is clamped by means of a seal urged into engagement with the washer by a further clamp member constituted, for example, by a compression nut which is tightened to deform the seal into sealing engagement with the cable.

The outer sheath may be a shrink-fit plastics sleeve. However, it could equally well be any other form of insulating covering providing adequate electrical insulation of the assembly.

The invention can be put into practice in various ways some of which will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view, partially in section, of a first embodiment of the invention; and Figure 2 is a side view, partially in section, of a second embodiment of the invention.

Referring firstly to Figure 1, a cable gland assembly is mounted at the junction where a cable 10 passes through the aperture in a wall 12. The cable 10 comprises a central inner conductor 14 having an insulating sleeve 16. The insulating sleeve 16 is overlaid with a multi-stranded intermediate conductor 18 which is itself covered by a flexible water impermeable barrier covering 20. On top of the covering 20 is a braided multi-stranded electromagnetic interference emission screen 22. The whole cable is covered in an outer insulating sheath 24. The central conductor 14 is connected inside a housing 26 to a piece of electrical equipment 28, which is not particularly specified in the drawing.

The central inner conductor 14 and. its insulating sleeve 16 extend through an aperture in the wall of the housing. An electrically conductive, e.g. metal, gland body 30 is received on the sleeve 16. The gland body 30 has a first externally threaded portion 31 projecting through an aperture generally coaxial with the cable 14, and a hexagonal flange portion 32 on the outer side of the wall 12 for engagement by a spanner.

On the opposite side of the flange from the first externally threaded portion 31 is a shorter second externally threaded portion 34. From this portion 34 a tapered sleeve portion 36 of the gland body 30 extends away from the wall 12. An electrically conductive, e.g. metal, gland sleeve 38 is received on the second threaded portion 34 by means of a complementary internally threaded portion 40. An internally tapered portion 32 of the sleeve 38 extends from the threaded portion 40 away from the gland body 30. The intermediate conductor 18 is clamped between the cooperating tapered faces of the portions 36 and 42 as the gland sleeve 38 is screwed onto the gland body 30 to establish electrical contact.

The gland sleeve 38 extends away from the gland body 30 in the form of a generally plain cylindrical portion and is terminated in an externally threaded portion 44 having a radially inwardly tapered axial edge.

A compression nut 46 is received on the threaded portion 44. The nut 46 has a radially inwardly extending flange 48 which urges a tapered skid washer 50 axially towards the gland sleeve 38. Between the skid washer 50 and the tapered edge of the gland sleeve 38 is a compressible neoprene sealing ring 52 received on the exposed electromagnetic interference screen 22.

As the compression nut 46 is tightened onto the gland sleeve, the sealing ring 52 is deformed sealingly to engage between the impermeable covering 20 and the screen 22, and the end of the gland sleeve 38.

To hold the cable in position relative to the cable gland, a metal star washer 54 is also received on the covering 20 and against the screen 22. The star washer comprises a plurality of outwardly extending fingers radiating from an inner ring. The radially outer extent of the fingers of the star washer 54 is greater than the inner diameter of the channel defined by the gland sleeve 38. Thus, the radially inner edge of the star washer is at once urged to bite into the covering 20 by the tightened sealing ring 52 and to provide an electrically conductive path for the screen to the gland body 30.

In an alternative form the star washer can also be used to earth the outer insulating sheath 24. This is applicable to situations in which the cable is transmitting at high frequencies. In this case the reflected signals due to any mis-matching at the cable joint are earthed through the star washer and the gland body.

A heat shrinkable sheath 55 extends over the gland assembly from the outer insulating sheath 24, projecting from the compression nut 46, to the wall-side face (or beyond) of the hexagonal flange 32.

A flat elastomeric, e.g. plastics, washer 56 spaces the flange 32 from the wall 12 to prevent the ingress of dust and/or moisture into the housing. The gland assembly is held in the wall by a nut 58 engaging the first externally threaded section 31 from inside the housing. The nut 58 is spaced from the wall 12 by a further polycarbonate spacing washer 60, which is against the wall 12, and a metallic or otherwise electrically conductive earth tag washer 62.

The tag of the washer 62 is bent away from the wall and forms a connection point for an earth cable 64. The tag is spaced from the wall 12 by about 12mm using the washer 60 to prevent flashback from the earth path to the wall. Similarly, the exposed part of the insulating sleeve 16 extends beyond the end of the first threaded portion of the gland body by about 12.5mm for up to 3 kvolt systems and by about 25mm for systems above 3 kvolts. In this way the exposed inner conductor is spaced from the earth path by an amount sufficient to inhibit capacitive discharge from the inner conductor.

Thus, the invention provides a terminating cable gland for a combined neutral-and-earth electrical system. The electrical path is constituted, in part, by the gland assembly itself which is electrically connected through the tag washer to the earth cable 64.

In this way both the electromagnetic screen 20 and the neutral intermediate conductor 18 are provided with a common path to earth. The assembly of this embodiment is designed to withstand earth faults of 10,000 A for one second on a fuse-based electrical system.

Figure 2 illustrates a modified version of the assembly of Figure 1. Like numerals have been used to denote like parts. The assembly of Figure 2 is designed for high current circuit breaker-based systems and is able to withstand up to 43,000 A for one second.

In this embodiment, the gland body 30a has an internal thread adjacent the wall 12. The gland body 30a is held in registry with the aperture in the wall 12 by means of an integral earth and connecting sleeve 66 which has a complementary external thread portion 68 which is received on the internal thread of the gland body 30a. The sleeve 66 has a radially outwardly extended flange 70 remote from the wall 12. The flange 70 has a tag portion 72 by which the earth cable can be connected to the sleeve 66. As the gland body is tightened onto the sleeve 66, the flange 70 bears against the spacing washer 60.

Again, the assembly provides an electrical earth path for both the electromagnetic screen and the earth/neutral conductor. The invention provides a secure cable termination at the junction with a wall for a 2-wire system which is insulated by the outer insulating sleeve.

Claims

CLAIMS:

1. A cable gland assembly for a cable having a pair of insulated conductors, an electromagnetic screen and an outer insulating sheath, the cable passing through an aperture in a wall of, for example, a housing, the assembly comprising a gland body through which the cable passes, which gland body is arranged to be mountable in registry with the aperture on one side of the wall and is adapted for electrical connection to an earth and to the electromagnetic screen, means for securing the cable relative to the gland body, and means for establishing electrical contact between the second conductor and the gland body.

2. An assembly as claimed in claim 1 in which an electrically insulating sleeve extends over the assembly substantially from the wall to the exposed outer sheath of the cable projecting from the assembly away from the wall.

3. An assembly as claimed in claim 1 or 2 in which the gland body is securable in the aperture by means of a nut engageable with a complementary thread on a portion of the gland body projecting through the aperture.

4. An assembly as claimed in any of claims 1 to 3, including an open threaded sleeve arranged to project through the aperture towards the assembly, on which sleeve the gland body is received, the sleeve having a radially extending flange to engage the wall defining the aperture.

5. An assembly as claimed in any of claims 1 to 4 in which an electrically conductive washer formed with an earth tag is held on the gland body by the nut or the flange of the open sleeve.

6. An assembly as claimed in claim 5 in which spacing means space the tag from the wall.

7. An assembly as claimed in claim 6 in which the spacing means are an insulating, for example polycarbonate, washer.

8. An assembly as claimed in any preceding claim in which the second conductor is clampable between relatively movable tapered surfaces of the assembly.

9. An assembly as claimed in claim 8 in which the tapered surfaces are respectively formed on a gland sleeve received on the gland body and the gland body.