Cable joints and terminations

Abstract: A cable joint or termination has a plastics insulating layer formed by a pair of pre-formed polyethelene parts 7 and 8 which are a close fit around the casing 3 and the cable insulations 1C, 2C. The parts are bonded by ring moulds 9, 10 and 11. This is quicker and cheaper than previous techniques of injection moulding to a sleeve and the cable at the end regions of the casing, and prevents occlusion caused by cable outgassing.

Description: This invention relates to cable joints and terminations particularly joints for underwater use. BACKGROUND OF THE INVENTION The existing practice for underwater cable joints and terminations is to provide an electrically insulating layer over a cable joint which provides electrical insulation continuity from the insulation of one cable to the insulation of the other cable to be joined to it. This is done by moulding using an injection moulding technique. Conventionally polyethylene moulding is used, comprising a polyethylene moulding sleeve which is positioned centrally over the joint. The cable core i.e. the cable insulation, and the polyethylene sleeve ends are then heated in a moulding unit ready for molten polyethylene to be injected, which then amalgamates the sleeve and the core ends together. The insulating layer can also provide the main or only barrier against water penetration into the joint or termination. One form of cable joint provided with a plastics outer layer is described in specification No GB-A-2162656 This existing technique is time consuming and we have found that the heat produced by the moulding process can have a deleterious effect not only on the joint but also on the cable, particularly where an adhesive, such as an EVA adhesive is used to bond component parts of the cable to prevent relative movement between those parts, or to act as a water blocking material. It is an object of the present invention to overcome these disadvantages. SUMMARY OF THE INVENTION According to the present invention there is provided an underwater telecommunication cable joint or termination, comprising a first cable having an outer tubular plastics insulation, a pressure-resistant tubular metal casing provided with an outer plastics electrically-insulating layer covering the metal casing and part of the end portion of the cable extending into the casing, characterised in that said electrically-insulating plastics layer consists of a first pre-formed part comprising a tubular housing portion and a tubular end portion of smaller diameter than the housing portion, that the tubular end portion is bonded to the cable insulation by rings moulds molten to a depth less than the depth of the cable insulation such that during the hot moulding process hot moulding material is prevented from contacting component parts of the cable other than the cable insulation, and that the shape of the first preformed part conforms with the outer shape of the metal casing and the cable extending into the casing, such that the tubular housing part fits accurately around the metal casing and the tubular end portion fits accurately around the cable insulation, respectively, so that no distortion due to external pressure occurs, the first pre-formed plastics part being supported against external hydraulic pressure by the casing and the cable over substantially all its internal surface area. Conveniently, where a cable joint is concerned, a second pre-formed part similar to the first one can be fitted to the other side of the joint and bonded to the second cable insulation by a hot moulding process in such manner as to prevent hot moulding material contacting component parts of the cable other than the cable insulation, and bonded also to the first pre-formed part. Preferably a tubular end portion is a sliding fit over the cable insulation and is bonded thereto by a ring-shaped moulded region which penetrates the cable insulation by an amount less than the thickness of the cable insulation. Preferably also, where two pre-formed parts are fitted to opposite ends of a joint, they are joined together by a ring-shaped moulded region. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention can be clearly understood reference will now be made to the accompanying drawing which shows schematically and partially in cross section a submarine cable joint having a plastics electrically-insulating layer according to an embodiment of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawing there is shown a deep sea underwater telecommunications cable 1 joined to a similar deep sea telecommunications cable 2 by means of a joint 3. The joint 3 comprises a steel sea case 3A to which the strength member wires IA and 2A of the respective cables, are clamped in opposite ends of the sea case 3A. These clamps are represented schematically by blocks 4 and 5 respectively, but each may be like the one described in our co-pending British Application No. 2224757A. The joint internally and the casing could be similar to that disclosed in our co-pending application 2229545A. Within the casing the optical fibres of the two cables will be spliced as represented at 6, the optical fibres of the respective cables being represented by reference numeral 1B and 2B. There will also be an electrical continuity in the joint and may be represented by the sea case itself or by an electrical conductor within the sea case extending between the conductive portions of the two cables. Normally the central strength member wires 1 and 2 will themselves provide electrical conductivity, together with a tubular electrical conductor within the wires or a tubular electrical conductor closely surrounding and in intimate contact with the wires, neither of which is shown in the drawings but a typical example of which is shown in our British Patent 1550588, and this cable shows both an electrical conductor within the strength member wires and surrounding and in intimate contact with the strength member wires. Each submarine cable 1 and 2 has an electrically insulating layer IC and 2C of polyethylene. Prior to completing an electrically insulating layer over the joint housing, the insulating layers are shaved to bring them to an accurate outer diameter. A pair of plastics pre-manufactured i.e. pre-formed housing parts 7 and 8 of polyethylene are slid towards each other to cover the sea case and provide electrical insulating integrity and watertightness between the layer 2C and the layer IC. Each housing part 7 and 8 has a larger diameter housing portion 7A and 8A which encircles the sea case and a tubular end portion 7B and 8B. Portions 7B and 8B are a sliding fit on the shaved cable insulation and when the parts are in position around the sea case, the end portions 7B and 8B are bonded to the respective cable insulation IC and 2C by respective ring moulds represented schematically by 9 and 10 where the polyethylene is molten to a depth in the insulation which is less than the thickness of the insulation so the molten moulding material does not come into contact with the underlying component parts of the cable. Similarly a ring mould is made to amalgamate the facing ends of the housing portions 7A and 8A and this is represented by reference numeral 11. It is emphasised that the pre-formed parts 7 and 8 fit accurately around the sea casing and the cable insulation so that when subjected to the pressures existing on the sea bed, there is no distortion of the premoulded parts because the sea casing and the cable insulation support the parts against such pressure substantially over the whole internal surface area of the two parts. Because the moulding technique used to bond the parts 7 and 8 to the cable insulation does not heat up the cable parts nor the joint to any appreciable extent, then any parts of the cables or the joint which are susceptible to heat are thus protected, and in particular where an EVA adhesive is used to bond the cable insulation IC and 2C to the respective strength member wires IA and 2A, this adhesive is not affected by the ring moulding technique. We have found that whereas previous techniques cause moulding material to impregnate component parts of the cable causing out gassing and inclusions of gas in the moulding, thereby sometimes producing defective insulating parts, with the arrangement described above, no outgassing occurs and good insulation properties are maintained throughout the joint from one cable insulation to the other. Furthermore the process has proved to be much quicker than previous techniques. Although a cable joint is shown a similar technique would be applicable to a cable termination. The ring mould 11, being located at the centre of the sea case 3, reduces the chance of the housing portions flexing and disturbing the moulding during the ring moulding operation because the sea casing accurately locates the housing parts. The tubular end portions 7B and 8B are coextensive with the cable insulation IC and 2C over an axial length of about three inches, more than three times the diameter of the cable insulation. In the embodiment described the pre-formed parts are the only means of rendering the joint watertight at ocean depths, as well as providing electrical insulation up to 11,000 volts. The thickness of the re-formed parts is not less than 10mm throughout to achieve this.

Claims: An underwater telecommunication cable joint (3) or termination comprising a first cable (1) having an outer tubular plastics insulation (1c), a pressure-resistant tubular metal casing (3A) provided with an outer plastics electrically-insulating layer (1G) covering the metal casing (3A) and part of the end portion of the cable (1) extending into the casing, characterised in that said electrically-insulating plastics layer consists of a first pre-formed part (7) comprising a tubular housing portion (7A) and a tubular end portion (7B) of smaller diameter than the housing portion, that the tubular end portion (7B) is bonded to the cable insulation by ring moulds (9) molten to a depth less than the depth of the cable insulation such that during the hot moulding process hot moulding material is prevented from contacting component parts of the cable other than the cable insulation, and that the shape of the first preformed part (7) conforms with the outer shape of the metal casing and the cable extending into the casing, such that the tubular housing part (7B) fits accurately around the metal casing and the tubular end portion fits accurately around the cable insulation, respectively, so that no distortion due to external pressure occurs, the first pre-formed plastics part (7) being supported against external hydraulic pressure by the casing and the cable over substantially all its internal surface area. A cable joint as claimed in claim 1, further characterized in that it comprises a second cable (2) and a second pre-formed plastics part (8) similar to the first one and accurately fitting to the other side of the metal casing (3A) and being bonded to the tubular outer insulation of the second cable extending into the casing at the side opposite to the first cable (1), such that the tubular end portion (8B) of the second pre-formed part (8) is bonded to the cable insulation (2C) of the second cable (2) by ring moulds (10) molten to a depth less than the depth of the cable insulation such that during the hot moulding process hot moulding material is prevented from contacting component parts of the cable other than the cable insulation, and that the first and second pre-formed housing parts (7,8) are bonded together by a ring mould at the facing ends of the tubular housing portions (7A,8A) so as to form a continuous plastics layer. A joint or termination as claimed in claim 1 or 2, further characterised in that each said tubular end portion (7B, 8B) has an inner diameter so as to slidingly fit over the cable insulation.

Inventors: WEBBER GRAHAM, WRIGHT STUART EDWARD, WEBBER, GRAHAM, WRIGHT, STUART EDWARD
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1992
Main classification: G02B6
IPC codes: H02G15, G02B6