Electricity: conductors and insulators – Conduits – cables or conductors – Combined
Reexamination Certificate
2001-09-26
2003-01-14
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Combined
C174SDIG008, C174S08800C
Reexamination Certificate
active
06506975
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cable joint between two relatively high voltage electric power cables, each cable having a conductor surrounded by, successively, a core insulation, a semi-conductive screen, a wired-screen and an outer cable sheath, said cable joint comprising, successively from an innermost to an outer side:
a junction connector adapted for connecting together bared ends of said conductors;
a semi-conductive layer adapted for covering said junction connector and ends of said core insulations, but separated from the semi-conductive screens of the two cables;
an insulating layer adapted for covering said semi-conductive layer and bared ends of said core insulations;
a conductive layer adapted for covering said insulating layer;
bared ends of conductive wires of said wired-screens; and
an insulating joint-cover adapted for covering said cable joint including ends of the outer cable sheaths of the two cables.
Such a cable joint is already known in the art, e.g. from the European Patent Application EP-A2-0 272 131 “HV cables” of RAYCHEM GmbH, first filed on Dec. 12, 1986 (GB 8630335). Therein, a conductive layer is positioned on top of a stress-controlling layer so as to provide a Faraday cage around the connector. Additionally to this conductive layer forming an electromagnetic protection, the bared ends of the screen or earth wires are formed into a pigtail, brought together over the joint and interconnected by a crimp connector. The joint is further enclosed within an outermost heat shrinkable tube of insulating material forming the joint-cover recovering the outer jacket of each of the cables.
In such a known cable joint, the relatively large diameter of the screen wires arranged in pigtails and more particularly of the crimp connector interconnecting them is located over the joint connector. This provides locally a larger radius of the cable joint, i.e. a bump. As a consequence, when the heat shrinkable tube is applied onto the joint, it exerts thereon a pressure that is locally higher over the bump. The underlying layers are then over-compressed by the bump with the risk of damaging them and of also damaging the joint-cover, e.g. but cutting. The bump is even more detrimental in case of a joint containing mastic inside. Mastic is generally used in a cable-joint as insulating or filed controlling material and may for instance be applied below the insulating layer. A problem with a heat shrinkable tube is that the extra pressure caused by the bump would make the mastic to flow away.
A possible solution to avoid this problem is to locate the screen interconnection, i.e. the crimp connector, at an extremity of the joint. However, the screen wires arranged in a pigtail and running over the joint then still form a longitudinal bump. Additionally, the length of the whole joint will be increased, with all the drawbacks thereof: cost, accessibility, . . .
SUMMARY OF THE INVENTION
An object of the present invention is to provide a cable joint of the above known type but with a smooth wired-screen connection, while remaining relatively cheap and simple, and reducing as much as possible the length of the outer joint-cover.
According to the invention, this object is achieved due to the fact that the bared ends of said conductive wires are substantially equally distributed over the circumference of the underlying conductive layer, that a conductive metal plate adapted for covering the ends of said conductive wires is provided between said conductive layer and said joint-cover, and that said joint-cover is made of retracted cold shrinkable insulating material.
In this way, an asymmetrical connection is avoided because there is no longer a crimp connector lying on the joint. The diameter of the joint is maintained constant owing the distribution of the screen wires all over it. Moreover, a good electrical contact is ensured between the screen wires of the two cables via the conductive metal plate that is continuously pressed thereon by the shrinked or retracted cold shrinkable joint-cover. Furthermore, since the screen connection is located over the joint connector, the length of the joint, and more particularly of its outer joint-cover or protective sleeve remain relatively short.
It is to be noted that the joint-cover disclosed in the above-mentioned European Patent Application is a heat shrinkable joint-cover, which is thus positioned by heating. Once the heating is removed, the joint-cover remains in place but, contrarily to the present cold shrinkable joint-cover, a pressure is then no longer exerted on the underlying layers of the joint. The cold shrinkable joint-cover maintains a pressure on the underlying layers of the joint for ever and this maintains a good electric contact between the conductive metal plate and the underlying conductive wires. This permanent pressure is further cylindrically distributed in a symmetrical way so that the underlying insulating or field controlling mastic is equally spread over the joint. Another advantageous effect of this symmetrical pressure is that the mastic is prevented to flow away in any particular direction. This avoids the creation of gaps with a resulting risk of partial discharges and product failure.
It is also to be noted that the French Patent Application FR-2 551 927 “Dispositif de blindage électromagnétique de jonction” of the SOCIETE DES ETABLISSEMENTS JACQUES DUBOIS & Cie—FR, filed on Sep. 8, 1983, also discloses a cable joint. However, this other known cable joint relates to low voltage cables adapted to transmit electronic signals such as telecommunication (PTT) signals. Although this known document discloses a sleeve of braided wires interconnecting the wired-screens of the cables, the purpose of the sleeve is to ensure the continuity of the electromagnetic shielding of the cables. The known sleeve is thus not adapted to allow relatively high currents to flow therethrough and the cables are further not provided with all the above mentioned layers. These cables are thus not adapted to withstand the above-mentioned relatively high voltage.
The two known documents relate to electromagnetic shielding of cables and not to screen interconnection as it is the subject of the present invention. However, applying the knowledge of this low voltage joint to the above known high voltage joint will not lead to the present invention. Indeed, none of the known documents teaches the use of a cold shrinkable joint-cover adapted to exert a permanent pressure onto the conductive metal plate for maintaining a good electric contact between this plate and the underlying conductive wires.
The European Patent EP-B1-0 549 942 entitled “Electrical cable junction, premounted junction, assembly and manufacturing process” of EUROMOLD (BE), first filed on Dec. 17, 1991 (FR 19910015666) also discloses a conductive metal plate that is a screen tap mounted on the bared ends of the semi-conductive screens of the cables. This known screen tap is of a relatively complex design and is thereby relatively expensive. It further requires a special tool to fix the screen tap onto the semi-conductive screen of the cable. Therefore, using the teaching of such a screen tap to apply it to the present improvement of a screen connection will result in making the latter complex and expensive and it thus against the object of the present invention.
The present invention also relates to a method of forming a joint between two relatively high voltage electric power cables.
Such a method is well known in the art and is for instance disclosed in the above mentioned European Patent Application EP-A2-0 272 131. Additionally to the known problems associated to a cable joint realized according to the known method and mentioned above, it is to be noted that the method disclosed in this European Patent Application further requires the use of a special crimping tool. This crimping tool is necessary to realize the interconnection of the bared ends of the screen or earth wires that have each to be arranged in a pigtail. The use of the crimping
Nexans
Nguyen Chau N.
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