GB2219146A

GB2219146A - Improvements in explosion-proof cable glands

Info

Publication number: GB2219146A
Application number: GB8808738A
Authority: GB
Inventors: Alan Charles Durward Wales
Original assignee: CMP
Current assignee: CMP
Priority date: 1988-04-13
Filing date: 1988-04-13
Publication date: 1989-11-29
Anticipated expiration: 2008-04-13
Also published as: GB8808738D0; GB2219146B

Abstract

An explosion-proof cable gland assembly comprises a body (14) threadedly received in a hole (10) in the wall (12) of a housing. An outer sleeve (16) is threadedly received on the body to clamp an exposed armour wire (44) between the two, preferably using a wedge (53) chosen from a set of wedges of different dimensions. A compression ring (18) clamps a sealing ring (36) to a cable passing through the gland. The number of threads of the body engaged in the wall and those engaging the outer sleeve with the body is at least five in both cases. While the inside of the housing is in communication with the threads the risk of explosion is avoided by the tortuous paths through the threads. <IMAGE>

Description

IMPROVEMENTS IN CABLE GLANDS The present 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.

In potentially explosive environments the' junction between and explosion-proof enclosure and a cable entering the enclosure must be designed to prevent the transmission of any potential sources of ignition across the enclosure wall. This can be done by providing an effective isolation between the explosive environment on one side of the enclosure and any potential sources of ignition on the other side of the enclosure.

Cable glands are known in which first and second explosion-proof seals are compressed on to a cable passing through the gland. The first seal is situated to one side of an exposed earthing armour wire clamped to the cable gland and engaging the outer sheath of the cable. The second seal is situated to the other side of the armour wire and engages an exposed inner sheath of the cable.

Each of the first and second seals requires a pair of relatively rotatable parts which can be screwed together, each pair compressing a sealing ring on to the cable. Thus, the cable gland must come in at least three main parts which can be,screwed together and at least two associated seal rings.

Each separate item adds to the cost of manufacture of the cable gland and also creates more steps which the fitter must take when the gland is assembled. In order to minimise the time spent assembling such cable glands the number of turns of the thread required to tighten the pair of adjacent parts has been restricted to two or three complete revolutions.

It is an object of the present invention to provide a cable gland assembly for explosive environments that is both simpler to manufacture and install.

According to the present invention there is provided an explosion-proof cable gland assembly for a cable passing through, for example, an aperture in a wall, the assembly comprising; a gland body, through which the cable passes, having a first thread toward one end of the body which is arranged to engage at least five complete turns of a complementary thread associated with the aperture; an outer sleeve threadedly mounted on the body and adapted to co-operate with the body to clamp an armour wire portion of the cable therebetween; and an annular explosion-proof compression seal arranged toward the end of the outer sleeve remote from the body, which seal is arranged to engage sealingly the cable passing through the gland; wherein the outer sleeve is threadedly engaged with at least five turns of the thread on the gland body and wherein the threaded engagement of the outer sleeve with the gland body communicates with the aperture through the gland body.

The present invention is thus able to dispense with a sealing ring between the armour wire and the aperture since this seal is replaced by the tortuous path between the threads on the gland body and the outer sleeve, respectively. This tortuous path acts in conjunction with the similar tortuous path created between the threads on the gland body and wall defining the aperture, respectively.

In the event of a combustion of gases on the inside of the housing, the pressure created by the rapidly expanding gases is reduced across the two tortuous paths as opposed to one in the known cable gland.

Consequently, not only does a cable gland assembly according to the invention dispense with the step of assembling a separate seal, but it is also found that the two tortuous paths act in concert with one another to relieve the pressure on each thread.

It has been found empirically that five complete turns provide a sufficiently tortuous path to drop the pressure of the expanding gases when an explosion occurs and also to cool those gases such that they do not represent a potential source of ignition of the atmosphere on the other side of the 'gland.

Eliminating the second explosion-proof seal will also mean that the gland assembly is shorter. This will enhance the resistance of the assembly to impact loads.

Preferably, the body is formed with a radially extending flange adjacent the first thread, the' flange having a channel in which is received an O-ring which forms a seal with the outer surface of the housing.

It is desirable that the O-ring is made of nylon as this material has certain significant advantages. As well as being resilient, nylon has a tendency to absorb moisture over time which will cause the O-ring to expand slightly. As a result, it has been found that the quality of the seal provided by thee nylon 0ring "matures" after it is first installed.

Preferably, the body and the outer sleeve are formed with complementary axially extending tapered surfaces which co-operate to clamp the armour wire therebetween when the outer sleeve is threadedly drawn on to the gland body.

Preferably, one of the tapered surfaces is formed on a separate insert which is selected from a set of inserts, each of which has a different internal radial spacing with respect to the other co-operating tapered surface. It is desirable that the insert be formed with an annular seat at one end which engages a complementary surface in the gland to prevent axial movement away from clamping engagement with the armour wire as the gland is assembled. If is preferred that the insert constitutes the tapered surface of the outer sleeve.

The invention also extends to a cable gland comprising a gland body through which a cable passes and an outer sleeve mounted on the body, the body and the sleeve having complementary axially extending tapered surfaces which co-operate to clamp the armour wire there between when the outer sleeve is drawn on to the gland body wherein one of the tapered surfaces is formed on a separate insert which is selected from a set of inserts, each of which has a different internal radial spacing with respect to the other co-operating tapered surface. It is desirable that the insert be formed with an annular seat at one end which engages a complementary surface in the gland to prevent axial movement away from clamping engagement with the armour wire as the gland is assembled. It is preferred that the insert constitutes the tapered surface of the outer sleeve.

In this way, the same cable gland assembly can be used for cables having armour wire of different thicknesses.

This is especially useful when cable gland assemblies are to be used in similar applications but in connection with cables adhering to differing national standards. It will be appreciated by a skilled person that, for instance, some continental European countries use a current-limited earthing system in electrical installations. In this case, the earthing armour wire can be significantly thinner than is the case in, for example, British applications. By using the inserts, essentially the same gland can-be used regardless of the armour wire thickness.

Preferably, the armour wire extending out of the outer sheath is formed with a radially splayed dog-leg shape before the co-operating clamping surfaces of the body and the outer sleeve.

The present invention can be put into practice in several ways, some of which will now be described by way of example with reference to the accompanying drawings in which; Figure 1 is a partially longitudinally sectioned view of a first embodiment of a cable gland assembly according to the invention; Figure 2 is a partially longitudinally sectioned view of a second embodiment of a cable gland assembly according to the invention; and Figure 3 is a partially longitudinally sectioned view of a third embodiment of a cable gland assembly according to the invention.

Figure 1 shows a cable gland assembly received in a threaded aperture 10 of a wall 12 of a housing for electrical equipment or other plant. The cable gland assembly comprises a main gland body 14, an outer sleeve 16 and a compression ring 18. A cable 20 extends through the middle of the assembly and the aperture 10 into the housing.

At one end, the gland body 14 is formed with a first externally threaded sleeve portion 22 which engages a complimentary internal thread in the aperture. Toward the other end, the gland body 14 is formed with a second externally threaded sleeve portion 24. An external frusto-conical surface 25 extends axially from the second sleeve portion 24 away from the first sleeve portion 22.

Between the threads, the gland body is formed with a externally hexagonally shaped flange 26 for engagement by a spanner. The face of the flange 26 adjacent the first thread 22 is formed with an annular channel 28 which accommodates a nylon O-ring 29 which sealingly engages an adjacent face of the wall 12.

The outer sleeve 16 is screwed onto the second external thread, having a complementary internal thread for the purpose. The outer sleeve 16 is also formed with a radially inwardly facing frusto-conical surface 30 which is complementary to the frusto-conical surface 25 on the gland body 14. A cylindrical portion 31 extends from the end of the surface 30 away from the gland body 14 and terminates in an external thread 32. Radially opposite the external thread 32, the sleeve 16 is formed with an internal annular tapered ledge 34. A resilient compression seal ring 36 is located on the ledge 34. The compression ring 18 is received on the external thread 32, and its end which is remote from the gland body 14 is radially inwardly swaged to form an annular lip which engages a compression bush 40.

The axially inner face of the bush 40 is tapered toward the gland body and engages the seal ring 36.

When the gland body 14 is received in the aperture 10, it creates a tortuous path between the threads for any gases inside the housing. Thus, in the event of an explosion, the rapidly expanding gases undergo a substantial pressure drop and cooling as, small quantities of the gas travel along the flamepath between the threads. The gases reachinr an explosive atmosphere on the other side of the wall 12 are thus no longer a potential source of ignition. It has been found that to produce an adequate pressure drop and cooling effect, the minimum number of full threads engaged between the housing and the gland should be at least 5. The provision of the nylon O-ring 29 creates a barrier to moisture entering the housing through the threads.

As discussed earlier, it has been found that nylon is a particularly advantageous material for the O-ring 29 as it tends to expand slightly over time by absorbing a small quantity of moisture from the atmosphere.

Before the cable 20 is inserted in the cable gland, it is prepared by removing an outer sheath 42 to reveal a sheath of armour wire 44 beneath it and an inner sheath 46 housing the transmission cables 48 beneath that. To assemble the remainder of the cable gland, the exposed armour wire 44 of the cable is laid on the outwardly facing frusto-conical surface 25. The outer sleeve 16 is then tightened onto the gland body and the inwardly facing frusto-conical surface 30 thus moves towards the armour wire 44 which is consequently clamped between the two surfaces to provide an earthing connection.

The compression ring 18 is tightened onto the end of the outer sleeve 16. In so doing, the compression bush 40 is wound axially along the inside of the outer sleeve 16. The tapered surface on the bush forces the sealing ring 36 radially inwardly into sealing engagement with the outer sheath 42 of the cable to create a flameproof seal between them.

As with the thread associated with the aperture 10 in the wall, the threads engaging the outer sleeve 16 on the gland body 14 also form a tortuous flamepath through which a minimal amount of expanding gas escapes. As before, the gas suffers a substantial pressure drop and is cooled so that it does not present a potential source of ignition to an explosive atmosphere on the other side of the housing. For this, the engagement must also consist of at least five fully engaged threads.

It has been found that, because virtually the entire length of the cable 20 within the cable gland is exposed to the pressure of any expanding gases in the housing, the effects of pressure piling on the inside of the cable gland have to be considered. Firstly, the peaking of pressure will tend to distort and even damage permanently the exposed armour wire 44 by forcing it radially outwardly as the sudden pressure bears on the exposed wire between the outer sheath 42 and the clamping frusto-conical surfaces. In addition to relieving the pressure by providing the two tortuous paths to atmosphere, the effects of this pressure piling have been found to be substantially alleviated by forming a dog-leg kink in the armour wire.

Figure 2 illustrates a second embodiment of a cable gland assembly similar to that in Figure 1 and like numerals have been used to denote like parts. The assembly is different in that the flameproof seal at the end of the outer sleeve 16 remote from the gland bsdy incorporates a star washer 50 which rests against a ledge 51 in the outer sleeve. The sealing ring 36 is urged axially toward the gland body 14 and radially inwardly by the compression bush as before. The star washer thus provides some support for the seal ring in the space between the internal surface of the cable gland assembly and the outer sheath 42 of the cable 20.

Figure 3 illustrates a third embodiment of a gland assembly according to the present invention,.and again, like numerals have been used denote like parts in Figure 1. The outer sleeve 16 has been modified in that the radially inwardly facing tapered surface 52 which co-operates with the frusto-conical surface 30 is formed on a separate insert 53 which rests on an annular ledge 54 facing the gland body 14. As before the tapered surfaces co-operate as the outer sleeve 16 is screwed onto the gland body 14 to clamp the armour wire 44.

The insert 53 is selected from a group of inserts of different thickness which can be used to accommodate cables having armour wire of different thicknesses. In a particular application, the spacing between the tapered surfaces can thus be adjusted by choosing the appropriate insert to allow the necessary five full turns to be completed in the engagement of the gland body 14 and the outer sleeve 16. It will be apparent to the skilled person that the use of an excessive number of engaged threads will require an unnecessary waste of time in assembling the cable gland. Thus, the adjustable space between the tapered surfaces also allows the number of threads engaged to be limited to that required to produce a sufficiently tortuous path in applications where the armour wire is excessively thin, for example, in telecommunications cables.

In each of the embodiments described the wall thickness of the outer sleeve 16 is designed to withstand the effects of bursting forces during gas explosion, the heating and bursting effects of electrical explosions during an earth fault on the cable armouring, and also the mechanical impact of falling objects on the cable glands during its service life. The compression seal provided by the seal ring 36 is designed typically to accommodate the worst explosion reference pressure of acetylene at 146 psi (10.07 bar), with a safety factor of 1.5 with an explosive time of 22 milli seconds within an enclosure having a volume in excess of 2,000 cm3.

Claims

1. An explosion-proof cable gland assembly for a cable passing through, for example, an aperture in a wall, the assembly comprising a gland body, through which the cable passes, having a first thread toward one end of the body which thread is arranged to engage art least five complete turns of a complementary thread associated with the aperture; an outer sleeve threadedly mounted on the body and adapted to cooperate with the body to clamp an armour wire portion of the cable there between; and an annular explosionproof compression seal arranged toward one end of the outer sleeve remote from the body, which seal is arranged to engage sealingly the cable passing through the gland; wherein the outer sleeve is threadedly engaged with at least five turns of the thread on the gland body, and wherein the threaded engagement of the outer sleeve with the gland body communicates with the aperture through the gland body.

2. An assembly as claimed in claim 1, wherein the body is formed with a radially extending flange adjacent the first thread, the flange forming a seal with the outer surface of the wall.

3. An assembly as claimed in claim 2, wherein an o- ring seal is located between the flange and the wall surface.

4. An assembly as claimed in claim 3, wherein the o- ring is made from nylon.

5. An assembly as claimed in any preceding claim, wherein the body and the outer sleeve are formed with complementary axially extending tapered surfaces which co-operate to clamp the armour wire therebetween.

6. An assembly as claimed in claim 5, wherein one of the tapered surfaces is formed on a separate.insert.

7. An assembly as claimed in claim 6, wherein the insert is formed with an annular seat at one end which engages a complementary surface in the gland.

8. An assembly as claimed in claim 6 or 7, wherein the insert constitutes the tapered surface of the outer sleeve.

9. An assembly as claimed in any of claims 1 to 8, wherein the exposed armour wire is kinked between the cable and the clamping engagement between the cooperating body and outersleeve.

10. A cable gland comprising a gland body through which a cable passes and an outer sleeve mounted on the body, the body and the sleeve having complementary axially extending tapered surfaces which co-operate to clamp an armour wire sheath of the cable there between when the outer sleeve is drawn on to the gland body, wherein one of the tapered surfaces is formed on a separate insert which is selected from a set of inserts, each of which inserts has a different internal spacing with respect to the other co-operating radial spacing.

11. A cable gland as claimed in claim 9, wherein each insert is formed with an annular seat at one end which engages a complementary surface in the gland to urge the insert into clamping engagement with the armour wire.

12. A cable gland as claimed in claim 9 or 1,0; wherein the insert constitutes the tapered surface of the outer sleeve.

13. A cable gland as claimed in any of claims 10 to 12, wherein the exposed armour wire is kinked between the cable and the clamping engagement between the cable and the co-operating body and outer sleeve.

14. An explosion-proof cable gland assembly substantially as specifically describe herein with reference to Figure 1, 2 or 3 of the accompanying drawings.