GB2263021A

GB2263021A - Filled insulated boxes for electrical parts

Info

Publication number: GB2263021A
Application number: GB9226295A
Authority: GB
Inventors: Michael Frank Frampton; Julian Gordon Head
Original assignee: Prysmian Cables and Systems Ltd
Current assignee: Prysmian Cables and Systems Ltd
Priority date: 1991-12-20
Filing date: 1992-12-17
Publication date: 1993-07-07
Anticipated expiration: 2012-12-17
Also published as: GB9127083D0; GB9226295D0; GB2263021B

Abstract

An electrical part shown as a cable joint is positioned in a box spaced from the wall thereof and the box is at least partially filled with an insulating filling to cover the electrical part. The insulating filling comprises a flexible resin, such as a polybutadiene, filled with particulate resilient material, such as particulate rubber. In an embodiment, the joint 13 is supported by cradles 32 within the box 24. A lid 30 allows the filling composition to be poured in to cover the joint to the line 36. The lower portion 26 of the box and sleeve 14 are filled with foam prior to pouring the composition. <IMAGE>

Description

FILLED INSULATING BOXES This invention relates to insulating at least one electrical part wherein said part(s) is positioned in a chamber defined by wall means of a box so as to be spaced from said box wall means and said box is at least partially filled with an insulating filling to cover said said.part(s). The invention is particularly although not exclusively applicable to insulating electrical cable joints using such a technique.

Conventionally when this technique is applied to cable joints, the insulating filling is a bituminous compound. However, the use of such an insulating filling is disadvantageous in the case of where the joint is buried and where the water table is high because water penetrates the box and displaces the bituminous compound filling resulting in the formation of voids which lead to electrical failure due to low resistance paths to earth being provided via the voids.

In order to overcome the above problem the present invention proposes the use of a flexible resin filled with particulate resilient material as the insulating filling.

The resin is not displaced by water and is flexible in order to prevent cracking - and the consequent formation of voids - during curing of the resin or during mechanical, electrical or thermal tests which could lead to electrical failure under test.

The cost of such resins is relatively expensive and it is desirable to use the resin in combination with a filler. However, the use of a rigid particulate- material, such as sand, as a filler has proved unsatisfactory producing cracking during curing of the resin or under tests as aforesaid, and in accordance with the present invention the filler is a particulate resilient material.

The invention includes a method of insulating at least one electrical part comprising positioning said part(s) in a chamber defined by wall means of a box so as to be spaced from said box wall means and at least partially filling said box with an insulating filling to cover said part(s), wherein said insulating filling comprises a flexible resin filled with particulate resilient material.

The invention also includes a device comprising a box having wall means defining a chamber, at least one electrical part positioned in said chamber so as to be spaced from said wall means, and insulating filling at least partially filling said box and covering said electrical part(s), wherein said insulating filling comprises a flexible resin filled with particulate resilient material.

Preferably, said particulate resilient material is rubber, and from a cost aspect the particulate resilient material may advantageously be formed from scrap rubber.

Preferably the resin is a polybutadiene since such resins are less prone to hydrolysis than other otherwise suitable resins such as polyurethane resins.

The amount of said rubber present in the insulating filling may be about 20% by weight.

Whilst the invention is applicable to the insulation of any electrical part(s) in a box as aforesaid, it is particularly applicable to the insulation of an electrical cable joint.

Typically the cable joint will include an electrically conductive sleeve extending over two endto-end connected electrical conductors of respective electrical cables between and electrically connected to metal sheaths of such cables. In such a case, the sleeve may be spaced from said wall means by a plurality of spacer means, each comprising a cast resin member. Furthermore, foamed material may be disposed between each of said cast resin members and said walls means and/or said sleeve to fill any voids therebetween at the regions of contact therebetween.

In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described, with reference to the accompanying drawings in which the single figure is a schematic side view partly in axial cross-section of a cable joint disposed in a cable box.

In the figure, two single core cables 10, 12 rated at 66KV or above are jointed. The joint, generally referenced 13, includes an electrically conductive sleeve 14 extending over two end-to-end connected electrical conductor cores (not shown) of the cables between and electrically connected to the metal sheaths 16, 18 of the cables 10, 12. The sleeve 14, which comprises two sleeve portions 14a, 14b connected circumferentially together end-to-end at 15, provides electrical continuity between the sheaths and is connected to the sheaths at connections 20, 22.

The details of the connection within the sleeve are conventional and will not be described.

The joint is positioned within a box 24, formed from a lower portion 26 and an upper portion 28 which mate together at a horizontal plane 29. The upper portion 28 is provided with a filling opening closable by a removable lid 30. Typically the box is made of a fibre glass reinforced resin. The cables 10, 12 enter the box at respective ends thereof through openings formed by cooperating recesses in the end walls of the upper and lower box portions, with a suitable seal (not shown) being provided between the protective coverings of the cables over the metal sheaths and the box openings.

The joint 13 is positioned in the chamber defined by the box and spaced from the walls of the box by means of spacers. As illustrated, two spacers, each of which comprises a cradle shaped member 32, are utilized, engaging from underneath the sleeve 14, one at each end of the joint.

After the joint has been positioned as aforesaid, the box is filled with an insulating filling comprising a flexible resin filled with particulate resilient material to cover the sleeve 14. To this end, in the preferred embodiment, a polybutadiene resin is mixed with a hardener therefor and particulate rubber, and poured into the box through the filling opening in the box upper portion to a level above the sleeve as indicated by dotted line 34.

As will be appreciated the greater the amount of particulate rubber in the mixture the more the cost savings. However, it has been found that if there is more than 30% rubber by weight in the mixture, the mixture becomes too viscous for easy pouring resulting possibly in bubbles forming in the filling and also voids in regions into which the mixture does not flow.

It has also been determined that the inclusion of more than 30% rubber by weight appreciably reduces the elongation at break figure for the cured filling. In the preferred embodiment the amount of particulate rubber in the mixture is about 20% by weight.

It has also been found that the particle size of the rubber in the mixture is important. Reducing the particle size enables the percentage of rubber by weight in the mixture to be increased but also increases the viscosity of the mixture for a given percentage by weight. However, too high a particle size adversely effects the mechanical and electrical strengths of the cured filling. It has been found that particles of mesh size 30 provide best results although satisfactory results are achieved using larger particles of mesh size 20 and smaller particles of mesh size 40. Particles of mesh size 30 used in the preferred embodiment typically have 99% by weight particles which can pass through a 30 mesh number sieve (500 Fm); 30 to 55% by weight particles which cannot pass through a 36 mesh number sieve (425 Fm); 30 to 40% by weight particles which cannot pass through a 60 mesh number sieve (250 jay); 10 to 25% by weight particles which cannot pass through a 100 mesh number sieve (150 jan) and 0 to 15% by weight particles which can pass through a 100 mesh number sieve.

The resin used in the preferred embodiment is a hydroxyfunctional polybutadiene. More specifically it is a hydroxyfunctional polybutadiene, in which the average number of -OH groups per chain (or functionality) is 2.4 to 2.6 and the average molecular weight is about 3,000, modified with a short chain diol and aromatic oil. Less significant ingredients are a small amount of mineral filler, a water scavenger, an amine catalyst and antioxidants. The hardener (or crosslinker) used is a diphenylmethane diisocyanate based hardener containing oligomeric material. The hydroxyfunctional polybutadiene retains, in spite of the presence of -OH groups, many of the characteristics of the polybutadiene chain.

These include outstanding resistance to moisture both during cure and subsequent long term ageing. The other major advantage is retention of flexibility down to very low temperatures (approx. - 60or).

The spacer members 32 are preferably cast in the same material as the insulating filling and any voids between the spacer members 32 and the box lower portion 26 and/or the sleeve 14 are filled with foamed material prior to the above mixture being poured into the box.

The resulting arrangement performs well under tests and also has the advantage that the insulating filling may be readily cut away providing access to the joint for examination thereof and remedial work thereon should this become necessary.

Claims

CLAIMS:

1. A method of insulating at least one electrical part comprising positioning said part(s) in a chamber defined by wall means of a box so as to be spaced from said box wall means and at least partially filling said box with an insulating filling to cover said part(s), wherein said insulating filling comprises a flexible resin filled with particulate resilient material.

2. A device comprising a box having wall means defining a chamber, at least one electrical part positioned in said chamber so as to be spaced from said wall means, and insulating filling at least partially filling said box and covering said electrical part(s), wherein said insulating filling comprises a flexible resin filled with particulate resilient material.

3. A method as claimed in claim 1 or a device as claimed in claim 2, wherein said particulate resilient material is rubber.

4. A method as claimed in claim 3, wherein said particulate rubber is of mesh size 30.

5. A method as claimed in claim 3, wherein said particulate rubber has a mesh size of 40 to 20.

6. A method or device as claimed in claim 3, 4 or 5, wherein the amount of said rubber present in said insulating filling is about 20% by weight.

7. A method or device as claimed in claim 3, 4 or 5, wherein the amount of said rubber present in said insulating filling is not more than 30% by weight.

8. A method or device as claimed in any one of claims 3 to 7, wherein said resin is a polybutadiene.

9. A method or device as claimed in claim 8, wherein said resin is a hydroxyfunctional polybutadiene.

10. A method or device as claimed in claim 9, wherein the filling includes a diphenylmethane diisocynanate based hardener for the hydroxyfunctional polybutadiene.

11. A method or device as claimed in any one of the preceding claims wherein said at least one part comprises an electrical cable joint.

12. A method or device as claimed in claim 11, wherein said cable joint includes an electrically conductive sleeve extending over two end-to-end connected electrical conductors of respective electrical cables between and electrically connected to metal sheaths of such cables.

13. A method or device as claimed in claim 12, wherein said sleeve is spaced from said wall means by a plurality of spacer means, each comprising a cast resin member.

14. A method or device as claimed in claim 13, wherein foamed material is disposed between each of said cast resin members and said walls means and/or said sleeve to fill any voids therebetween at the regions of contact therebetween.

15. A kit for use in a method as claimed in claim 1 or any of claims 3 to 14 comprising a flexible resin, a hardener therefor, and a resilient filler.

16. A method or device substantially as herein described with reference to the accompanying drawing.