GB2180104A - Cable glands - Google Patents

Abstract

A cable gland assembly for a high voltage cable 12 comprising a metal gland body 14, and insulating bush 30 connected to the gland body 14 and a locking assembly 32, 34 clamping the bush to the wall 10 of an enclosure. The cable 12 is stripped back and the screening 12C is clamped to the gland body 14 by a clamp sleeve 16. The stripped part cable is covered by an insulating sleeve 36 which is radially spaced from the gland body 14 and the insulating bush 30. The gland body has an earth connection 26, which, when disconnected from earth can be used as a terminal for testing by applying a voltage between the earth connection 26 and the cable core 12A and between the earth connection 26 and the cable outer sheath 12D. <IMAGE>

Description

SPECIFICATION Cable glands This invention relates to cable gland assemblies which are arranged to connect a high voltage cable e.g. 6.6 kv to a wall, for example of an enclosure within which is mounted electrical equipment being served by the cable passing through an aperture in the wall. The Applicants are leading manufacturers ofthistype ofcablegland and these glands are manufactured in different designs depending upon the voltage to be carried bythe cable, the number of cores the cable may have, the necessary insulating requirements of a particular cable, and other structural characteristics ofthe cable such as reinforcing insulating and other layers ofthecable.

By virtue of the introduction of new insulating materials, for example of thetype involving crosslinked polymers, which serve e.g. for insulating layers immediately outside a metallic cable core and also forthe outercablesheath, it has been found possible to reducethethickness ofthe insulating layers and, as a result, the radial dimensions of the cables, and the glands cooperating therewith, have been capable of being reduced.

Partlyforthe above reasons, and also partly because of a more modern need to test cables in service, rather than just allowing severe faults two develop with possible damage of an irretrievable nature to the cable and/or gland, a demand has arisen to provideforin situ testing ofthecable and gland assemblies.

In many ofthe constructions manufactured both by the Applicant, and by other manufacturers, glands of this type are provided with earthing lugs to which earthing protective devices and/orearthing leads may be connected, andthesetags, in many applications ofthe gland, butt up againstthe enclosure wall to which the gland is attached. As a rule, such an enclosure wall is itself earthed and, as a consequence, it is not possible, for testing purposes, to isolate the earthing tag by disconnecting its earth lead or other earthing protecting devices, in orderto apply a voltage to that part of the gland to which the earthing tag is either connected orof which it forms an integral part.

An object of the present invention therefore is to provide a gland assembly which will enable in situ testing of the gland,togetherwith the cable located thereinto be carried out without difficulty. Another object of the present invention is to provide for methods of in situ gland and cable testing which have not hitherto been achievable.

According to the invention, there is provided a cable gland assemblyforsecuring a cableto an enclosure wall having an apertureforthe cable, comprising: a generally sleeve-like metallic gland body including meansforsecuring a cable passing therethrough against axial movement with respect to the gland body,the cable having a stripped-back section in which any external sheathing and metallic screening has been removed; an insulating sleeve surrounding the stripped-back section from the beginning of the stripped-back section and through the aperture in the wall to a predetermined distance; an insulating bush located between the gland body and the wall,the insulating sleeve being radially spaced from both the gland body and the bush; and an earth connection integral with or connected to the gland body which when disconnected from earth or from earthing protective devices is insulated from the wall by the bush thereby permitting text voltages to be applied between the earth connection and other parts of the assembly. Preferably, the bush is threadably connected to the gland body. Preferably, the screening ofthe cable is clamped to the gland body by a metallic screen clamp sleeve.

The insulating bush may be clamped to the enclosure wall by means ofa plastics flanged cylindrical bush whose flange abuts the inside of the wall and whose cylindrical portion is threadably connected to the insulating bush. Alternatively, the insulating bush may be clamped to the enclosure by means of a non-inductive metallic entry sleeve which threadably engages the bush, extends through the aperture and is secured by a lock nut on the inside of the wall.

Preferably, the meansforsecuring the cable to the gland body comprises a locking nut engaging sealing ring to force it into contact with the outer sheath ofthe cable. Preferably, there are no metallic parts in the axial region between the engagement of the insulating bush with the gland body and with the wall. Preferably, there is a continuous gap from the screening ofthe cable to the opposite side ofthewall.

According to a further aspect of the present invention a method oftesting a gland assembly includes the application of a testvoltagefirstly between the earthing connection and metallic cable cores ofthecableand secondly between the earthing connection and an external sheath of the cable. Preferablythe voltage applied in either test is of the order of 2.5 times the rated voltage of the cable and such testing is desirably carried out at high relative humidities e.g. of the order of 80% and at relative high temperatures of the order of 22-28C.

Whilst different forms oftesting might be required for different assemblies, the invention envisages particularly, but not exclusively, the capability of applying two voltage tests of an in situ type to a cable gland assembly which has been mounted into or onto an aperture in an enclosure wall with a cable located in and secured to the cable gland assembly and passing therethrough and through the aperture in the enclosure wall and for example connected to electrical equipment within the enclosure.These two tests, which are envisaged, are each carried out by isolating the earthing tag of the cable assembly by disconnecting any earthing leads or earthing protective devices. The first test is carried out by applying a test voltage between the metallic cable cores within the enclosure and the earthing tag, and the secondtestapplies a similar voltage between the earthing tag ena the outer cable sheath externally of the enclosure wall.

Tests of this type would normally be at a voltage which is 2.5 times the rated voltage of the assembly and thetestwould be applied for 1 minute. The tests, for exam ple for testi ng glands capable of test voltages of up to 25 KVwould, of course, require a gland assembly of atype which has in itselfa maximum leakagecurrent, even ifthe gland assembly itself is structurally sound, and the cable passing through it has no fault. Itis envisaged, for example with a test voltage of up to 25 KV, that the maximum leakage current for an entirely correct assembly would be no more than 0.1 mA.The Applicants have tested glands according to the present invention at relatively high temperatures of 23 and high relative humidities of 81% and the leakage current for an entirely satisfactory assembly,-where a DC voltage has been applied between the enclosure and an isolated integral earth lug has resulted in the following testfigures:: 15 KV (negative) applied for 1 minute leakage current0.02 mA 17.5 KV (negative) applied for 1 minute leakage current0.02 mA 20 KV (negative) applied for 1 minute leakage current0.03 mA 22.5KV (negative) applied for 1 minute leakage current 0.05 mA 25 KV (negative) applied for 1 minute leakage current 0.06 mA These figures illustrate that the criterion considered generally satisfactory of a maximum leakage current of 0.1 mA has been entirely satisfied by the tests carried out on prototypes in accordance with the present invention.

According to a preferred embodiment of the present invention a cable gland intended for securing a cable to a wall, having an aperture forthe cable, comprises a metallic gland body intended to be located externally of and spaced from one side ofthewall,the gland body being generally of sleeve-like form and having means for securing a cable passing therethrough against axial movement with respect to the gland body, an insulating bush having, at one end thereof, first connecting means for connecting the bush to the end ofthe gland body intended to be closest to the wall, and second connecting means, at the other end of the bush for connecting ittothewall,there being no metallic parts disposed in an axial region between said first and second connecting means, the gland body having connected to it, or integral with it, an earthing connection which, with no earthing lead or other earthing device connected thereto, is entirely insulated electricallyfrom the second connecting means, thereby to permittesting bythe application of testvoltages between the earthing connection and parts of a cable connected bythe gland to thewall.

According to another preferred embodiment of the present invention a cable gland assembly comprises a cable gland mounted on an enclosure wall with a portion of a cable gland located in, on, or around an entry aperture in the wall, a cable extending through and secured to a metallic body of the cable gland, the cable extending through the enclosure wall and into the enclosure, the cable having a stripped back section within which any external sheathing, metallic screening or the like has been removed and at least part of this section being surrounded byan insulating sleeve which extendsthroughthe aperture in the enclosure wall andtoa predetermined distance into the enclosure, the gland including an insulating bush which insulatesthe metallic body ofthe gland from the enclosure wall, and the arrangement being such that the insulating sleeve is radially spaced from an internal bore of the metallic body, and also is spaced from the interior bore ofthe insulating bush, the metallic body having connected to it, or integral therewith, an earthing connection, which, when disconnected from earth or earthing protective devices, is insulated from the enclosure wall by the insulating bush to permittestvoltagesto be applied from the earthing connection to other parts ofthe assembly.

Preferably, the axial length of the insulating bush on its interior surface, over which axial length there is no other metallic part, is such as to be capable of withstanding a predetermined test voltage applied to an earthing tag ofthe assembly.Preferably, the insulating sleeve, surrounding the stripped back section ofthe cable, extends inwardly beyond the enclosure wall by a similar or greater length thereby similarlyto withstand a predetermined test voltage.

The invention may be carried into practice in a number of ways but one specific embodiment will now be described byway of example with reference to the accompanying drawing which illustrates, in a diagrammatic manner, a cable gland assembly according to the invention in which a cable gland is shown having a cable extending therethrough and into an enclosure, awall of which is shown with thegland mounted thereon.

Referring to the drawing the assembly shown therein illustrates diagrammatically a wall 10 e.g. of an enclosure for electrical equipment to which the cable of the assembly is secured. The wall 10 includes a threaded aperture 1 0Ato which a cable gland is secured in a manner two be described. The cable 12 in the assembly shown includes a metallic cable core 1 2A of multistrand type, the core being surrounded by a semi-conducting elastomeric screen or sheath 1 2B. This is surrounded by a metallic screen 1 2Cformed of a number ofwires, although in other constructions a braided form of screen may be appropriate. The metallic screen 1 2C is in turn surrounded by an elastomeric or plastics outer cable sheath 1 2D.In the assembly shown the outer sheath 12D has been cut back to a shoulder 1 2E to reveal an axial length of the elastomeric screen 128 which is arranged to extend well within the enclosure, as illustrated. The metallic wires of the sheath 1 2C are cut back to the length 1 2F shown in the drawing and these are splayed out and engaged on a tapered external surface 1 4A of a cable gland body generally indicated at 14. The cable gland also includes a metallic screen clamp sleeve 16 one end 1 6A of which is internally threaded to engage an externallythreaded section 14C ofthecable gland body 14and when these threaded regions are drawn up, a clamping ring 18,which surrounds the metallic screen wires 1 2C is drawn axiallyto the right in the drawing firmly to secure the screen wires 1 2C against the seat 1 4A thereby axially to locate the cable firmly within the gland assembly.

The assembly also includes a conventional weatherproofseal,20forexample an elastomeric ring,which is forced into engagement with the external surface ofthe sheath 12D by means of a clamping ring 22 and a lock nut 24, these parts being conventional.

The cable gland body 14 has a radial flange 14D to the right of its externallyscrewtheaded portion 14C, and at one point of the periphery of the flange 14D an integral earthing lug 14F is provided which can be connected, as diagrammatically shown to earth via an earthing lead 26 either to earth orto earthing protective devices. As shown, in the drawing the earthing lead is connected to the earthing tag by a nut and bolt28.

To the rightoftheflange 14D, and the earthing tag 14F,the cable gland body 14 has a tubular extension 14G oflesserexternal diameter than the part having the external thread 14C, but also having an external thread 14H.

Adapted to cooperate with the external thread 14H is an internally threaded end 30A of an insulating bush generally indicated at30 and which, when assembled abuts againstthe right-hand end oftheflange 14D.The insulating bush 30 at its other, right-hand, end has a further internally threaded portion 308 which is adapted to cooperate with a non-inductive metallic entry sleeve 32 which is externally threaded over its whole length to cooperate, firstly, with the internally threaded end 30B of the insulating bush 30, secondly to cooperate with the threaded aperture 1 OA of the enclosure 10, and thirdly to receive an internallythreaded metallic lock nut 34 by which the whole assembly is secured firmly to the enclosure wall 10.

Alternatively, the sleeve 32 and nut 34 may be replaced by a single plastics flanged cylindrical bush which threadably engages the insulating bush 30.

It is to be particularly noted that the insulating bush 30, between its threaded ends 30A and 30B, has a generally sleeve-like portion 30D, the interior bore 30E of which is ofthe same diameters the internal bores ofthe cable gland body 14 and the entry sleeve 32so that these three parts together have a continuous internal surface of the same diameter.

In the bared back section ofthe cable which extends from the root of the splayed out metallic screen wired 1 2C to within the enclosure itself, the semi-conducting elastomeric screen 12B is surrounded by a shrink-applied insulating tube or sleeve 36. The axial length of this tube 36 indicated at '1 ' to the right ofthe lock nut 1 OA and the right hand end ofthe non-inductive sleeve 32 is determined so that the withstand voltage creepage path along the surface of this shrink applied tube is adequateto withstand the desired withstand voltage during testing which will be applied to the assembly. As discussed above it is generally envisaged that in the apparatus and methods ofthe present invention this maximum leakage currentwill be no more than 0.1 mA and the length '1' is determined accordingly.The same criteria apply to the length of the sleeve section 30D of the insulating bush 30 in order to provide an adequate axial length of insulating material to withstand the same voltage between the enclosure wall and the gland body thereby ensuring no greater leakage current (in an entirely satisfactory assembly) of more than 0.1 mA.

It is to be particularly noted that the arrangement is such that the external surface of the shrink-applied tu be 36 is radially spaced from the bores ofthe cable gland body 14, the insulating bush 30 and the metallic entry member 32, thereby to provide an annularairgapwithin the entire assemblyfrom within the enclosure upto the root of the metallic screen wires. It is also to be noted pa rticularly that in the region ofthe sleeve portion 30D ofthe insulating bush 30 there are no parts of a metallic nature, so providing an entirely uninterrupted axial section of insulating material between the gland body 14 and the enclosure wall 10 with its non-inductive metallic entry member 32.

This construction therefore entirely insulates the gland body 14 and its earthing lug 14from the enclosure 10,and enables the application of the two tests outlined above to this assembly i.e. thefirsttestcan applya negative test voltage between the earthing lug 14 (when any earthing leads or earthing protective devices have been disconnected therefrom) and the metallic cable core 12A. The second test applies a similarvoltage between the earthing tag 1 4F and the external surface ofthe sheath 1 2D ofthe cable. The test voltage normally applied would be ofthe order of 2.5 times the rated voltage of the cable and such tests would be applied for period of 1 minute and these are found to detect and predict gradual earth leakage conditions which startto occurdueto cable deterioration in service. Routine testing ofthis kind can therefore preventexplosionsin hazardous areas or damageto personnel, or plant, particularly if the leakage current dueto such faultstends to exceed 1 OmA.

Claims (15)

1. Acable gland assemblyforsecuring a cableto an enclosure wall having an apertureforthecabie, comprising: a generally sleeve-like metallic gland body including meansforsecuring a cable passing therethrough against axial movement with respect to the gland body, the cable having a stripped-back section in which any external sheathing and metallic screening has been removed; an insulating sleeve surrounding the stripped-back section from the beginning ofthe stripped-back section and through the aperture in the wall to a predetermined distance; an insulating bush located between the gland body and the wall,the insulating sleeve being radially spaced from both the gland body and the bush; and an earth connection integral with or connected to the gland body which when disconnected from earth orfrom earthing protective devices is insulated from the wall by the bush thereby permitting test voltages to be applied between the earth connection and other parts of the assembly.

2. An assembly as claimed in Claim 1 in which the bush is threadably connected to the gland body.

3. An assembly as claimed in Claim 1 or Claim 2 in which the screening ofthe cable is clamped to thegland body by a metallic screen clamp sleeve.

4. An assembly as claimed in any preceding claim inwhichthe insulating bush is clamped to the enclosure wall by means of a plastics flanged cylindrical bush whose flange abutsthe inside ofthewall and whose cylindrical portion is threadably connected to the insulating bush.

5. An assembly as claimed in any of Claims 1 to 3 in which the insulating bush is clamped to the enclosure by means of a non-inductive metallic entry sleeve which threadably engages the bush, extends through the aperture and is secured by a lock nuton the inside ofthewall.

6. An assembly as claimed in any preceding claim inwhichthe meansforsecuring the cabletothegland body comprises a locking nut engaging a sealing ring to force it into contact with the outer sheath of the cable.

7. An assembly as claimed in any preceding claim in which there are no metallic parts in the axial region between the engagement of the insulating bush with the gland body and with the wall.

8. An assembly as claimed in any preceding claim in which there is a continuous gap from the screening of the cable to the opposite side ofthe wall.

9. A cable gland assembly constructed and arranged substantially as herein specifically described with reference to and as shown in the accompanying drawing.

10. Amethod oftesting a cable gland assembly as claimed in any preceding claim,the method comprising disconnecting the earth connection and applying a test voltage firstly between the earth connection and the cable cores and secondly between the earth connection and the cable outer sheath.

11. A method as claimed in Claim 10 in which the test voltage applied is ofthe order of 2.5 times the rated voltage of the cable.

12. A method as claimed in Claim 10 to Claim 11 carried out at a relative humidity of at least80%.

13. A method as claimed in any of Claims 10 to 12 carried out at a temperature between 22 and 28"C.

14. Amethod as claimed in any preceding claim in whichthevoltage is applied for at least 1 minute.

15. A method oftesting a cable gland assembly substantially as herein specifically described with reference to the accompanying drawings.