Electricity: conductors and insulators – Conduits – cables or conductors – Combined
Patent
1986-04-15
1987-10-06
Askin, Laramie E.
Electricity: conductors and insulators
Conduits, cables or conductors
Combined
174 88C, H02G 15184, H02G 15103
Patent
active
046984585
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a joint for cables having an extruded insulation and provided with a sleeve, and in particular, a sleeve of the monolithic type which is fitted upon and tightened over the joined ends of the cables.
The known joints of such type have the following structure.
At the extremities of the cables, the extruded insulation and the outer semiconductive screens are stripped, in a staggered manner from the conductors.
The conductors, disposed end-to-end, are joined together, for example, through welding, or by means of a compression clamp or the like, and over the joining zone of the conductors, there is disposed a metallic adaptor.
The metallic adaptor is a cylindrical body, having its outer diameter equal to the outer diameter of the insulation of the cables.
The ends of the adaptor are in contact with the insulation of the cables, and the adaptor is provided with an inner cavity inside which the fastening between the conductors is enclosed.
A sleeve of the monolithic type, i.e. a sleeve comprising at least one insulating layer and a semiconductive field modifier which is embedded into the insulating layer and faces the sleeve cavity, is fitted upon and tightened over the joined ends of the cables.
If each cable has a semiconductive layer on its outer surface, then even the monolithic sleeve will have a semiconductive layer on its outer surface. The configuration of the monolithic sleeve is such as to bring the outer, semiconductive layer of the sleeve into contact with the outer, semiconductive layers of the two cables.
The known joints described above has been found to be relatively satisfactory for the connections of medium voltage cables and low voltage cables. However, although proposed, they have not, up to now, found any practical application in instances of high voltage cables.
The reason for this is that at high voltages the joints of the type under consideration are inevitably subject to being perforated after they have been made to operate for a certain amount of time.
It has been noted that what causes these perforations is the infiltration of air and the origination of small interspaces which, with the passage of time, become formed between the surface of insulating material of the monolithic sleeve cavity and the outer surface of the insulation of the cables.
Such infiltration of air, as well as the formation of the above-mentioned interspaces, takes place mainly due to the reasons set forth hereinafter.
The cables with extruded insulation are manufactured by means of a continuous extrusion of the semiconductive layers and of the insulating layer around a conductor.
The extruding operation is followed by the cross-linking of the extruded material and, successive to this, an immediate cooling of the cable is carried out.
Owing to the above-described operations, inside the extruded materials of the cables, and particularly, in the extruded insulation of the cables, internal tensions arise, both longitudinally as well as circumferentially, and are caused by the difference between the thermal expansibility of the metallic material of the conductors and that of the extruded cross-linked materials.
As a matter of fact, the thermal expansion coefficient of any metallic material (which, as an example, is 24.times.10.sup.-6 for aluminum) is less than the thermal expansion coefficient of any plastic material (which, as an example, is 5.times.10.sup.-2 for a cross-linked polyethylene) out of which the insulation of the cable is formed.
This difference between the thermal expansion coefficient allows for a greater shrinking to be expected on the part of the insulating material following a sudden cooling which the cable undergoes after its vulcanization as compared to the shrinking of the conductor.
However, since there exists a close contact between the conductor and the insulation of a cable, the friction between these is so high that the maximum shrinking possible for the insulating material of the cable is limited to that of the conductor.
As a consequence of this, internal ten
REFERENCES:
patent: 3804972 (1974-04-01), Gommans et al.
patent: 4034151 (1977-07-01), Silva et al.
Lanfranconi Gianmario
Parmigiani Bruno
Askin Laramie E.
Societa Cavi Pirelli S.p.A.
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