Cable splicing method with reduced waste of cross-linked polymer insulation

Electrical cables having cross-linked polymer insulations are spliced by heating the insulations nearest the ends to be joined to a temperature such that the cross-linked polymers become amorphous, then expanding and axially moving the insulations away from the conductors such that they can be joined, then cooling the displaced insulations, then joining the conductors, and then reheating the insulations such that they return to generally their original shape and position around the conductors. The prior art technique of bevelling the insulators nearest the cable ends prior to conductor joining, with concurrent waste of insulating material, is made unnecessary.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improved method of splicing two 
insulated electrical cables, and more specifically, to the splicing of 
electrical cables which include a conductor wrapped with an insulation 
composed of a cross-linked polymer. 
2. Prior Techniques 
Normally when two electrical cables which include a conductor wrapped with 
an insulation of cross-linked polyethylene are to be spliced, the 
insulations nearest the ends of each of the cables are first removed, 
usually in a bevelled fashion such that the insulation which remains near 
the ends will have a decreasing cross-section in a direction towards the 
cable ends, and the inner conductors are joined. After the conductors of 
the two cables are joined, for example by welding or soldering, and after 
a conductive layer (i.e., inner conductive layer) has been applied around 
the joint (at least in circumstances where the cables are high voltage 
types), the joint area is normally insulated by wrapping the area around 
the exposed conductors (and the parts of the insulation which are located 
near the cable ends) with several layers of an unvulcanized 
polyethylene-type material containing a cross-linking agent, such as 
di-.alpha.-cumyl peroxide or di-tert. butyl peroxide. The joint area is 
then subjected to a sufficiently high pressure and temperature that the 
layers of the tape will melt to form a homogeneous, cross-linked compound. 
Subsequently, and especially when the cable is a high voltage cable, a 
conductive layer (outer semiconductive layer) is applied over the 
insulation at the joint. This latter step can in fact be done either after 
or in connection with the formation of the homogeneous insulation 
covering. 
The foregoing procedure has been used also when splicing cables which have 
cross-linked polymer insulations other than polyethylene. Normally, 
however, the wrapping tape which is used will be composed of the same 
polymer material contained in the original cable insulation. 
The noted prior art procedures, however, result in waste, insofar as the 
insulation which was originally present at the ends of cables to be 
spliced is at least partially removed and discarded. This insulation must 
then be replaced with new insulating material once the conductors of the 
cables have been joined. 
It is thus an object of the present invention to provide an improved 
splicing procedure for electrical cables which have an insulation of 
cross-linked polymer, and particularly an insulation of cross-linked 
polyethylene. 
SUMMARY OF THE INVENTION 
According to the present invention the conductors of the cables nearest the 
ends that are to be joined are first exposed, the ends of conductors are 
then joined, and the area around the exposed, joined conductors is 
provided with an insulation therearound; however, instead of having the 
conductors near the cable ends exposed by bevelling away the existing 
insulation, these ends are first heated and the insulations therearound 
are forced to both expand in a radial fashion away from the conductors and 
be displaced axially away from the cable ends. Thereafter the insulations 
are allowed to cool (such that they become fixed in their displaced 
positions), the exposed ends of the two conductors are joined, and the 
insulations are then heated again, such that the insulations are caused to 
return to generally their original shape and position around the 
respective conductors. Thus, the joint between the two cables is insulated 
with little or no need for use of any additional insulation materials. 
When the insulations are heated as noted above, the cross-linked polymer 
material which is contained therein changes from a crystalline state to an 
amorphous state, thus allowing it to radially expand and/or be axially 
displaced. When the expanded and displaced portion of the insulation is 
then allowed to cool, it will retain its new configuration due to a return 
of the polymer material to its crystalline form. When heated again, the 
insulation will likewise be made amorphous, allowing it to return to its 
original shape. Thus, the present invention is based on the realization 
that the property of a cross-linked polymer which allows it to expand and 
thereafter return to its original shape can be utilized when joining the 
ends of two cables, and thereby reduce the need for use of new insulating 
material in the joint area. 
If the cables to be spliced have an inner semiconductive layer, these 
layers can be displaced simultaneously with the insulation, and they will 
accompany the displaced insulation when it returns to its original 
position. Thus, no cavities will be formed between the inner 
semiconductive layer and the insulation at the joint. 
The noted displacement of the insulations at the ends of each cable can be 
achieved by use of a tool which includes an axial hole of substantially 
the same diameter as the diameter of the conductors of the cables. The 
tool itself may have a conical shape, with the pointed portion able to 
press in between the insulation and the conductor, or else, if the cable 
includes an inner semiconductive layer, in between the inner 
semiconductive layer and the conductor. The tool may, however, be just a 
flat plate. Forced contact with the tool will cause the insulation (or the 
insulation and the inner semiconductive layer) to be displaced in both an 
axial and radial fashion, thereby exposing the conductor of the cable so 
as to allow joining with a similarly exposed conductor. 
Further objects, advantages and features of the present invention will 
become apparent from a review of the accompanying drawings taken in 
conjunction with the following discussion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, two cables 1 and 2 which are to be joined together will 
include, respectively, main conductors 3 and 7, inner conductive layers 5 
and 9, insulation layers 4 and 8, and outer conductive layers 6 and 10. 
Neither cable, it should be appreciated, is shown with the conventional 
metal screen or sheath layers. The insulation layer on each cable will 
normally comprise a cross-linked polyethylene, and both the inner 
conductive layer (inner semiconductive layer) and the outer conductive 
layer (outer semiconductive layer) will have been applied via extrusion of 
a plastic material containing conductive materials, e.g., a plastic 
material such as a copolymerisate of ethylene and ethyl acrylate or a 
copolymerisate of ethylene and vinyl acetate, together with a conductive 
material such as carbon black. 
The displacement of the insulation near the ends of the respective cables, 
i.e., for effective joining of the inner conductors, can be achieved as 
depicted in FIG. 2. As shown therein, a conical tool 11 which has a hole 
12 therethrough which is slightly larger in diameter than the outer 
diameter of the conductor 3 to be operated upon is supported by an end 
plate 13, which is itself connected in spaced apart fashion from another 
plate 14 via threaded bolts 15 and 16 which are fixed to the end plate 13. 
The cable 1 to be treated is passed through an opening in the plate 14 and 
fixedly positioned with respect thereto, and adjustment of bolts 15 and 16 
and interaction with nuts 17 and 18 which contact the side of plate 14 
opposite the side facing end plate 13 will result in axial movement of 
plate 14 with respect to end plate 13 such that the cable can be moved 
towards and away from conical tool 11. When the insulation 4 of the cable 
1 has been heated to about 125.degree. C., i.e., that portion of the 
insulation at the end of the cable which is to be contacted by the conical 
tool 11, such that the polymer material changes from a crystalline to a 
substantially amorphous state, the cable is moved towards conical tool 11 
such that the forward edge thereof will be pressed in between the inner 
conductive layer 5 and the conductor 3 such that a heated, expanded 
portion of the layers around the conductor 3 will be formed, this expanded 
portion including inner conductive layer portion 5a, insulation layer 
portion 4a and the outer conductive layer portion 6a. The expanded portion 
of the layers around the conductor 3 will necessarily be also displaced in 
an axial direction with respect to the end of the stripped conductor of 
the cable. After the expanded and displaced insulation has cooled, it is 
removed from contact with the tool 11. Using this same technique, the 
layers around an opposing cable (such as cable 2 of FIG. 1) are expanded 
and displaced, such that the conditions as shown in FIG. 3 are achieved 
(the displaced layers in such a cable 2 being shown in FIG. 3 as portions 
9a, 8a and 10a). 
Subsequent to the foregoing, the conductors of the cables are joined to 
each other (see FIG. 4) by welding or by a jointing sleeve, thus obtaining 
a joint 20. The expanded and displaced portions of the cables are then 
again heated to about 125.degree. C. such that the polymer material of the 
insulating layers 4a, 8a will resume their amorphous states, and then the 
portions are caused to relocate against the respective conductors such 
that the original conditions of the cable conductors are approximately 
duplicated (FIG. 5). 
Sometimes it is desirable or required that a tighter connection between the 
insulations be obtained between the joined cables than is achievable by 
the foregoing procedure. One way of achieving this result is to apply an 
unvulcanized polymer containing a cross-linking agent between the 
insulations and then cross-linking the polymer. Such a procedure can be 
accomplished as shown in FIG. 6, which procedure can also enable an 
efficient connection of the inner conductive layers 5 and 9 of the cables. 
In accordance with the procedure, after the cables have been joined as in 
FIG. 5, the insulations 4 and 8, respectively, are bevelled into a conical 
shape at the ends 21 and 22 (see FIG. 6). A conductive layer 23 (inner 
semiconductive layer) is then applied over the exposed conductors of the 
two cables at joint 20, this layer 23 making contact with respective 
conductive layers 5 and 9. The layer 23 can be in the form of a wrapped 
tape of unvulcanized polyethylene material containing a cross-linking 
agent, e.g., an .alpha. -cumyl peroxide or di-tert. butyl peroxide 
material containing carbon black. The area outside of the layer 23 and 
between the bevelled insulations is then wrapped with a tape of 
unvulcanized polyethylene material containing a cross-linking agent, e.g., 
.alpha. -cumyl peroxide or di-tert. butyl peroxide, such that a tight 
wrapping 24 is obtained. The wrapping is then subjected to a pressure of 
about 5 atmospheres and a temperature of about 150.degree. C. for several 
hours such that the layers of tape will melt together to form a 
homogeneous, cross-linked compound which provides a continuous transition 
between the insulations 4 and 8 and has the same appearance the properties 
as the materials therein. The polyethylene in the conductive layer 23 is 
simultaneously cross-linked. 
It should be noted that instead of a tape, it is also possible to use, 
among other things, granules to produce the insulation 24. 
In order to apply the pressure and supply the heat on the noted wrappings, 
it is possible to use, among other things, a heatable pressure device in 
the form of a two-part tool (not shown) having a cylindrical forming 
cavity, a pressure vessel or a pressure wrapping of, for example, silicon 
rubber. In this regard, such heatable pressure devices for applying 
insulations when joining cables are well known. 
Upon the conclusion of the foregoing, an outer conductive layer is applied 
(not shown in FIG. 6) in order to connect conductive layers 6 and 10, as 
by painting the insulated joint with a colloidal aqueous solution of 
graphite. Alternatively, the layer can be formed in a manner analogous to 
the way the inner conductive layer 23 is formed, i.e., by wrapping a tape 
containing polyethylene and a conductive material (carbon black) around 
the wrapping 24 and cross-linking it with the rest of the wrappings. 
Turning now to the embodiment of the invention depicted in FIG. 7, a device 
is depicted which is capable of displacing the layers surrounding the main 
conductor 3 of a cable 1 in a fashion distinct from that depicted in FIG. 
2. In this device, the conical tool 11 of the FIG. 2 apparatus is 
eliminated, and the end plate 25 is instead provided with a hole 26 which 
is slightly larger than the diameter of either the main conductors 3 or 7 
of the cables to be joined. By pressing against the end plate 25, expanded 
and displaced (squashed) insulation portions are produced, together with 
the adjacent inner and outer conductive layers, 4b, 5b, 6b and 8b, 9b, 10b 
(FIG. 8). Once the main conductors are joined at 20, the expanded and 
displaced insulation portions are again heated, and these layers will 
return to generally their original shape such that the joined cables of 
FIG. 5 will be obtained. The connection between the layers may, if desired 
or required, be improved in the manner as described in conjunction with 
FIG. 6. 
In certain situations it may be suitable to cut off a piece of the exposed 
main conductors 3 and 7 before they are joined together to form a joint 
20. In such cases, the layers spanning the joint may be formed so as to be 
thicker than the initial layers which, in certain cases, will facilitate 
the formation of a joint of good quality between the layers. 
Once the joining of the layers is achieved as described above, the cable 
conductor may be provided with a metallic screen covering and a sheath at 
the joint, as is conventional. 
Although the invention has been discussed with respect to the use of 
cross-linked polyethylene as the insulation material at the joint, the 
invention is applicable to cables having insulations of cross-linked 
polymers other than polyethylene, e.g., copolymer of ethylene and 
propylene, copolymers of ethylene and propylene with diolefines such as 
dicyclo-pentadiene or 1.4-hexadiene, or mixtures of polyethylene with any 
of the mentioned copolymers. These materials can also be used as the 
material for wrapping around the joint. Indeed, if the insulation around 
the joint is to be the same type as the original insulation of the cables, 
which is normally desirable, the same polymer is used in the insulation 
wrappings around the joint as forms the insulation of the cable. It should 
be noted, however, that it is possible to use other polymer materials in 
the insulation wrappings around the joint than is used to form the 
original insulation of the cables. 
While there have been shown and described some preferred embodiments of the 
present invention, it should be recognized that various changes and 
modifications can be made therein and still fall within the scope of the 
appended claims.