GB2178251A

GB2178251A - A pre-assembled fitting for terminating mineral-insulated cable

Info

Publication number: GB2178251A
Application number: GB08617398A
Authority: GB
Inventors: Peter Harvey
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1985-07-17
Filing date: 1986-07-16
Publication date: 1987-02-04
Also published as: GB8518008D0; GB8617398D0

Abstract

A slidable closure member 5 engages the wall of a cylindrical pot 1 to form with the pot and a wedge 4 for securing the pot to a cable, chamber 8, in which viscous or pasty insulating compound is contained. After sliding the preformed assembly over the prepared cable end, the closure member is driven into the pot and initially forces the insulating compound into the pot to form the required water-tight seal. When this operation is complete, further movement of the closure member drives the wedge to secure the pot to the cable sheath. A tool for use in connection of the fitting to a cable comprises an internally threaded tubular member 32 engaged by a screw 33 which has a passage for the cable, a washer 35 provided with an abutment 36 which protrudes through a slot in the body to indicate the extent of movement of the screw and a locking member releasably engageable with the tubular member for holding the pot inside the body. The tool with the locking member removed is positioned so that the fitting 19 is inside the tool, the locking member is inserted and the screw is operated to cause movement of the slidable member. <IMAGE>

Description

SPECIFICATION Termination of mineral-insulated cable This invention relates to a method for termination of cables insulated with compressed mineral powder contained in a metal sheath, to the terminations which result and to fittings for making them.

Such cables are vulnerable to the penetration of water into the mineral insulation from the ends of the cable and are usually protected by a metal sealing pot screwed or otherwise fixed to the end of the sheath and filled with a permanently-pasty waterproof sealing "compound". Reliable filling of the pot requires quite a high level of skill, and the time required for making such terminations will often be a significant contributor to total cable installation cost.

The present invention provides a technique which is significantly less dependent on the skill of the installer and which requires significantly less time for the operation.

The fitting in accordance with the invention comprises: a cylindrical pot; wedge means for securing the pot to the sheath of the cable; a closure member slidable in the open end of the pot and having at least one aperture for a cable conductor extending therethrough and which engages the wall of the pot to form with the pot and other components of the fitting a compound chamber; a quantity of permanently viscous or pasty insulating compound in that compound chamber; the dimensions of the parts being such that the compound will not drain from the compound chamber under gravity but will flow to fill voids in the pot upon the application of sufficient force to urge the closure member further into the pot and that still further movement of the closure member into the pot thereafter actuates the said wedge means.

The method of the invention comprises preparing the end of the cable with its conductor(s) stripped of insulation and projecting from the cut-back end of the cable sheath, assembling the prepared cable end with a fitting of the kind just defined so that the cable enters the pot from the end remote from the closure member and is located with the cutback end of the cable sheath within the pot and the conductor(s) extending through the aperture(s) of the closure member and then applying force to the closure member to drive it into the pot until compound penetrates between the cut-back end of the cable sheath and the contiguous end of the closure member to make an effective seal on the exposed end of the insulation and the wedge means is actuated to secure the pot to the cable sheath.

The pot will ordinarily, but not necessarily, be of circular cross-section.

The wedge means may be a simple tapered collet, preferably longitudinally slit over at least part of its length, co-acting with a tapered section of the pot. A preferred form is described in the specification of our U.K. patent No. 1174759 (now expired).

The pot and wedge means will normally be metallic. Brass is commonly used for coppersheathed cables and aluminium alloy for aluminium-sheathed cables but if desired (for example to improve performance under fire conditions) metal of higher melting point (e.g.

copper or stainless steel) could be used, though there will not normally be any benefit in using a metal with a melting point substantially higher than that of the cable sheath.

The closure member may be moulded or machined from a hard plastics material or (for high-temperature service or fire performance) from a ceramic material or glass. Alternatively it could be of metal with (an) insulating insert(s) of ceramic or other suitable material The closure member may be in the form of a flat disc in which case additional means will be needed for transmitting thrust to the wedge means. One suitable means consists of a separate annular plunger located inside the pot where one of its ends can bear on the end of the wedge means and its other end on the closure disc. Another suitable means comprises a metal or other hard sleeve, slidable inside the wall of the pot to bear at one of its ends on the wedge means and having the closure member mounted in its other end.A third suitable means comprises an upstanding projection, or series of projections, from the wedge means.

Alternatively, the closure member may be a moulded or machined member of substantial thickness, for which the use of such additional means may be optional. The closure member may have a central spigot which projects into the pot and through which of the apertures extend. In this case if an annular plunger is used it will form with the wall of the pot and the spigot an annular compound chamber in which the compound is protected from contamination without use of any auxiliary members and from which it can be expelled to its eventual position in the centre of the pot.

After the closure member has been driven fully home, it may be secured by crimping of the pot rim or other suitable means.

Apart from simplicity of application, the invention has the advantages that the compound is not handled and may be formulated accordingly to be aggressively sticky (and with less concern about any dermatitic hazard), and that the volume of compound can be smaller than in conventional sealing pots.

The invention includes a compression tool for and method of securing the pre-assembled fitting to the end of a cable.

The invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a largely-sectional side view of a first fitting in accordance with the invention; Figure 2 is a fully-sectional side view of a finished termination made with a fitting similar to the one shown in Figure 1; Figure 3 is a sectional view of a third fitting in acordance with the invention; Figure 4 is a sectional view of a termination made with the fitting shown in Figure 3; Figure 5 is a sectional view of a fourth fitting in accordance with the invention; Figure 6 is a sectional view of the termination made with the fitting shown in Figure 5; Figure 7 is a longitudinal cross-sectional elevation of a compression tool in accordance with the invention with a fitting assembled on a cable end and for compression; Figure 8 is a plan of the tool shown in Figure 7;; Figure 9 is a cross- section on the line A-A in Figure 8; Figures 10-15 are diagrammatic views of the assembly of the fitting; Figures 16-18 are diagrams illustrating successive steps in the method of the invention; Figure 19 illustrates an alternative method in accordance with the invention; and Figure 20 is a partial view of another fitting in accordance with the invention.

Corresponding parts are identified by the same reference numerals wherever appropritae throughout all the figures.

Referring to Figure 1, the fitting comprises a pot 1 which is of circular cross-section and of uniform wall thickness except that at the left hand end (as drawn) there is a section 2 of which the internal diameter decreases towards the end 3 of the pot. This tapered portion receives a correspondingly tapered collet 4 of the kind described in specification No.

1174759. Both these parts are made of brass.

The other end of the pot receives a closure member 5 of ceramic or other insulating material, the structure of which will be more fully described below, and which is a close sliding fit in the pot 1.

Between the collet 4 and the closure member 5 is a simple tubular brass plunger 6.

The closure member 5 includes a circular spigot 7 which projects into the bore of the plunger 6, thus defining an annular chamber 8 which is pre-filled in the factory with a pasty insulating compound. The spigot 7 may be a close sliding fit inside the plunger 6, or alternatively there may be a clearance between them, provided it is of capillary dimensions in relation to the flow properties of the insulating compound. If there is a close sliding fit, at least one passage 9 of capillary dimensions is provided in the spigot and/or the plunger to provide communication between the annular chamber 8 and an interior space 10 of the pot.

At least one bore 11 extends through the spigot 7 to provide for the passage of a cable conductor. In the design shown in this figure, three such passages are present; however, only one is visible, the others being of the same shape and disposed at equal angles round the periphery. The passage 11 includes an inclined section which is intended to be a close fit on the cable conductor and a counter-bore 12 which extends parallel to the cable axis and is designed to receive the end of an insulating sleeve 24 threaded over the projecting part of the conductor.

When the fitting is to be applied to a cable, the cable end is prepared in the usual way by stripping back a length of the metal sheath and carefully cleaning the length of the conductors so exposed of insulating material. As soon as the cable end has been prepared, the preformed assembly shown in Figure 1 is threaded over the conductors (this is easier if the three conductors are cut to slightly different lengths so that they can be inserted one at a time into their respective passages 11) and the cable sheath pushed in through the collet 4 until it abuts or nearly abuts the end of the spigot 7.

The closure member 5 is now driven into the pot 3 by any suitable tool. This initially causes the insulating compound to flow from the chamber 8 through the capillary passages 9 (or through the clearance between the spigot 7 and the plunger 6) to occupy and seal all the spaces within the plunger 6.

The pressure generated in the compound ensures that it fills effectively all the void spaces within the pot. Normally it will also cause the end of the cable sheath to move back slightly away from the end of the spigot, the extent of this movement varying as necessary to accommodate tolerances in component dimensions, volume of compound, recessing of the cable insulation within the cut-back end of the sheath, etc.

Because this movement of the sheath is normally needed, the characteristics of the collet 4 (in particular its flexibility-largely determined by the thickness of an unslotted disk at its wide end-and taper angle) need to be such that it will not be actuated to grip the cable by the pressure of the compound alone.

Figure 2 (which shows a modified fitting providing an earth conductor 13 welded to the collet 4 and extending through openings provided in the plunger 6 and closure member 5), shows a cable with conductors 14 and sheath 15 in place in the fitting and the closure member driven home. It will be seen that when the compound has been completely expelled from the chamber 8, the closure member 5 then directly engages the plunger 6 and so drives the collet 4 into the tapered part 2 of the pot, so firmly wedging the pot on the end of the sheath 15. The termination is completed by forming a circumferential or local indent 16 near the mouth of the open end of the pot in order to prevent the closure member 5 from moving outwards. This is preferably done before the axial pressure on the closure member is withdrawn.

As with known wedge-mounted pots, the termination can be secured in a conduit box or other fitting using a gland (shown in chain lines) that grips the pot, not the cable, and therefore can be of the same size for a variety of different cables.

In the fitting shown in Figure 3 the closure member 5 is a ceramic disc having four (say) passages 11 through each of which a respective conductor of a cable may pass. Each passage 11 is tapered to facilitate spreading of the conductors. The closure member is mounted in one end of a brass sleeve 12.

The end 21- of the pot is provided with an indentation 17 to assist with the insertion of the sleeve 18 (bearing the closure member 5) into the pot and improve the seal between pot and closure member.

Optionally an earth tail 22 may be welded to the brass ring 18.

In the pre-assembled state of the fitting a plastics or other disposable plug 23 is inserted through the end 3 into the pot to ensure the pasty insulating compound remains in position and to keep it clean.

In the use of this type of fitting the brass ring 18 fulfils the function of the plunger 6 in the fillings of Figures 1 and 2.

The fitting and termination shown in Figures 5 and 6 are similar to fitting and termination shown in Figures 3 and 4, except: (i) the end of the brass ring 18 is inserted into the pot and/or the end of the pot into which the brass ring is inserted, is provided with a chamfer to assist in relative location of the closure member 5 and the pot 1; and (ii) the collet 4 is provided with a flange 25 against which the brass ring 18 acts on compression. The end of the pot which is tapered also has a shoulder 26, which, if necessary will provide a surface against which the shoulder 25 of the collet will act to prevent overinsertion of the collet.

The compression tool shown in Figures 7-9, comprises a tubular body member 32, internally screwthreaded at least at one end; a screw 33 engaged with the internal screw thread of the body member, which screw is bored to allow passages of the conductors of a cable therethrough; a washer 35 having mounted on it an indicator abutment 36 which protrudes through a slot 37 in the body member 32 to indicate the extent movement achieved; and a locking member 34 releasably engageable with the body member for holding the pot inside the body member.

The compression tool, with the locking member removed, is positioned so that the fitting 19 is inside the tool and the locking member then inserted to secure it.

The screw 33 is tightened and causes the washer 35 to move down the body member 32 (such motion shown by indicator 36) and engage with the closure member 5 of the fitting and the end 3 of the pot is forced against the locking member to effect the required compression.

The assembly of the fitting is shown diagrammatically in Figures 10 to 15, the steps of which are as follows: (1) the collet 4 is placed into the pot 1 (Figures 10-11) and pushed (with suitable tool 41) into the tapered part of the pot 1 and sufficiently engaged to prevent accidental removal of the collet in the assembly operation.

(2) The plastics plug 23 (Figure 12) is inserted through the collet 4 and/or the brass plunger 6 (shown in Figure 1) is inserted as required for the particular design.

(3) Insulating compound is prepared for insertion into the pot. This may be done by cutting a measured length from a uniform strip 42 of a suitable compound (see Figure 13).

(4) The insulating compound is inserted around the plug 23.

(5) As the last factory operation closure member 5 is inserted so that it abuts against the plug 23 or engages the plunger as the case may be forming an annular chamber in which the insulating material is enclosed.

On site, the pre-assembled fitting is applied to a cable C as follows (see Figures 16-18).

(6) The cable C is stripped at one end to expose conductors D which are staggered in length to ease the threading of the pot on to the conductors (Figure 16); (7) Plastics plug 23 is removed from the pot and discarded (Figure 17); (8) Conductors D are inserted, entering the pot through the bore of the collet 4 and leaving it through respective passages 11 in the closure member 5 until the cut-back end of the cable sheath is inside the pot; (9) The assembly is now inserted into the compression tool and the assembly is compressed as described above with reference to Figures 7-9.

Alternatively, see Figure 19, the pot may be compressed using a plier type device 43.

In detail design of the fitting, care needs to be taken to ensure that the insertion of the cable cannot displace the collet into the pot.

This may be prevented by: (1) Placing a dab of glue on the collet 4 when it is inserted into the pot at assembly (the resulting adhesive bond being broken by the force applied on compression); or (2) providing the flange 25 of the collet 4 (shown in Figure 24) with at least one upstanding lug 45 which resists inward movement of the collet.

Claims

1. A fitting for termination of a cable insu lated with compressed mineral powder contained in a metal sheath comprising: a cylindrical pot; wedge means for securing the pot to the sheath of the cable; a closure member slidable in the open end of the pot and having a at least one aperture for a cable conductor extending therethrough and which engages the wall of the pot to form with the pot and other components of the fitting a compound chamber; a quantity of permanently viscous or pasty insulating compound in that compound chamber; the dimensions of the parts being such that the compound will not drain from the compound chamber under gravity but will flow to fill voids in the pot upon the application of sufficient force to urge the closure member further into the pot and that still further movement of the closure member into the pot thereafter actuates the said wedge means.

2. A method of terminating a cable insulated with compressed mineral powder contained in a metal sheath comprising preparing the end of the cable with its conductor(s) stripped of insulation and projecting from the cut-back end of the cable sheath, assembling the prepared cable end with a fitting comprising: a cylindrical pot; wedge means for securing the pot to the sheath of the cable; a closure member slidable in the open end of the pot and having at least one aperture for a cable conductor extending therethrough and which engages the wall of the pot to form an annular chamber; a quantity of permanently viscous or pasty insulating compound in that compound chamber, the cable entering the pot from the end remote from the closure member and being located with the cut-back end of the sheath within the pot and the conductor(s) extending through the aperture(s) of the closure member; and then applying force to the closure member to drive it into the pot until compound penetrates between the cut-back end of the cable sheath and the contiguous end of the closure member to make an effective seal over the exposed end of the insulation and the wedge means is thereafter actuated to secure the pot to the cable sheath.

3. A fitting as claimed in Claim 1 or a method as claimed in Claim 2 in which the closure member has a central spigot which projects into the pot and through which at least one of the apertures extends and the fitting has an annular plunge which engages with the wall of the pot and the spigot to form an annular chamber, from which the compound will flow between the plunger and the spigot upon compression of the fitting to enter the space within the plunger where it will seal the cable end.

4. A fitting or method as claimed in Claim 3 in which the wedge means is actuated by the plunger when the whole of the spigot has entered the plunger.

5. A fitting as claimed in Claim 1 or a method as claimed in Claim 2, in which the closure member is a flat disc.

6. A fitting or method as claimed in Claim 5, in which the fitting also comprises a metal or other hard sleeve, slidable inside the wall of the pot to bear at one of its ends on the wedge means and having the closure member mounted in its other end.

7. A fitting as claimed in any one of Claims 1, 5 or 6 or a method as claimed in any one of Claims 2, 5 or 6 in which the fitting also comprises a plastics plug inserted into the pot through the wedge means to ensure the insulating compount remains in position and to keep it clean.

8. A fitting or termination for terminating a cable insulated with compressed mineral powder contained in a metal sheath substantially as described with reference to either Figure 1 and/or Figure 2 of the drawings.

9. A fitting for terminating a cable insulated with compressed mineral powder contained in a metal sheath substantially as described with reference to either Figure 3 and/or Figure 4 or Figure 5 and/or Figure 6 of the drawings.

10. A fitting as claimed in any one of the preceding Claims and a method as claimed in Claim 2 or 3, in which an earth conductor is welded to the fitting.

11. A compression tool comprising: a tubular body member internally screwthreaded at least at one end; a screw engaged with the internal screw thread of the body member, which screw is bored to allow passage of the conductors of a cable therethrough; a washer having mounted on it an indicator which protrudes through a slot in the body member to indicate the extent of movement achieved; and a locking member releasably engageable with the body member for holding the pot inside the body member.

12. A method of using a compression tool as claimed in Claim 8 substantially as described with reference to Figures 7-9 of the drawings.

13. A compression tool for use with a fitting as claimed in any one of the preceding claims and for use with a method as claimed in any one of Claims 2-4, substantially as described with reference to Figures 7 to 9.

14. A method of terminating a cable insulated with compressed mineral powder contained in a metal sheath substantially as described with reference to Figure 1 and/or Figure 2 of the accompanying drawings.

15. A method of terminating a cable insulated with compressed mineral powder contained in a metal sheath substantially as described with reference to Figure 3 and/or Figure 4 or Figure 5 and/or Figure 6.

16. A cable termination made with the fitting as claimed in any one of Claims 1 or 5 to 9 or a method as claimed in any one of Claim 2, or 5 to 9 or 13.

17. A cable termination made with the fitting as claimed in Claim 3 or 4 or a method as claimed in Claim 3 or 4.