The present invention relates to an electrical cable splice for electrically connecting at least two power cables and to a corresponding method for electrically connecting at least two power cables. For many applications, it is desired to form an electrically conductive connection between two or more wires of a power cable. For this purpose, it is well known in the art to connect the wires to be joined by soldering or welding, crimping or by means of a mechanical joint involving a crimp, a ring, a nut and a bolt. In particular, when reconnecting power cables in an emergency situation, however, these known techniques are too complicated and expen- sive.

Hence, there is a need in the art for a simple and cost-effective electrical cable splice that allows electrically connecting power cables safely and with sufficient mechanical stability.

This object is solved by the subject matter of the independent claims. Advantageous embodiments of the present invention are the subject matter of the dependent claims. The present invention is based on the idea that at least two power cables can be connected to each other by simply stripping bare their electrically conductive cores in a connection region and joining same together with a region of overlap having a predetermined length and securing this connection by means of a dimensionally recovering sleeve that covers the joined power cable cores at least in their region of overlap and mechanically fixes the cables to each other. By properly choosing the material of the dimensionally recovered sleeve, extraction forces of about 2.5 kN could be reached. Of course, the two power cables to be connected can also be the two parts of one broken cable. Furthermore, also a connection of one cable with two cables or any other combination of numbers of cables can be established by means of the cable splice according to the present invention. An important parameter of the electrical connection according to the present invention is the length of the region of overlap. The contact area that is generated by this overlap firstly determines the electrical performance of the connection and further— via the generated fric- tional forces— contributes to the extraction force that is needed for destroying the electrical connection. As a rough estimate, the total connection surface should equal the cross- sectional area of the wires to be connected. For wire cross sections of about 10 mm a length of overlap of about 10 cm can be sufficient.

Another important factor for determining the strength of the splice connection is the mechanical strength of the mounted sleeve covering the joined power cable core. In particular, the material has to be semicrystalline and cross-linkable. The cross-linked and crystalline regions serve as a supporting frame work in the expanded state. Thus, the material has the necessary characteristics for being heat-recoverable in order to fix the joined power cables. Alternatively, also cold shrink materials can be used, provided they exert a sufficiently high contact pressure in a radial direction for pressing together the cables to be joined. An additional hold-out will however be required to keep the sleeve in the expanded state.

In the finally mounted state the dimensionally recovered sleeve should have a wall thickness of about 4 mm. In the case of heat recoverable material, three criteria have to be considered in order to optimize the contact pressure: Firstly, the thermal expansion coefficient around and above the maximum application temperature has to be sufficiently high. For instance, for an application temperature of 90 °C, a thermal expansion coefficient of more than 300 μηι/Γη·Κ was found to be advantageous. Preferably, the thermal expansion coefficient should be in the range between 400 and 600 μηι/Γη·Κ. Optimally, it should be almost constant up to the maximum application temperature and rapidly rising above this temperature, in order to achieve a high degree of shrinkage when cooling down to the maximum application temperature after the heat recovery.

Secondly, the Young's modulus has to be sufficiently high over the whole range of the application temperature and even above this range in order to achieve - together with the heat shrinkage rate - a sufficiently high tensile strength. It could be shown that the Young's modu- lus should be above 600 N per mm ² and preferably be in the range between 900 and 1 100 N per mm ² .

Thirdly, the melting temperature of the material has to be above the maximum application temperature. Usually, for three phase power cables a temperature stability is required up to 90 °C, preferably, the material should be stable up to 1 10 °C to 120 °C. A suitable material that fulfills all the above criteria is high-density polyethylene, (HDPE). HDPE has a temperature stability that is as high as 130 °C. Of course, also other materials can be used if the application temperature that is required is lower. For instance, for a maximum temperature of 60 °C, also low-density polyethylene (LDPE), polyoxymethelene (POM) or polyamide 12 (PA 12) are suitable materials. Other materials may also be used when the cross sections of the wires are smaller.

According to an advantageous embodiment of the present invention, in at least a part of the region of overlap a contact element is arranged between the electrically conductive cores. This contact element can be formed by a flat electrically conductive sheet having a rough- ened surface on both sides. Such a contact element enhances the frictional forces and improves the electrical contact between the two cables in their region of overlap. The contact element may for instance be formed by a stamped copper beryllium sheet which has stamped protrusions extending in both directions to form sharp teeth which grip into the wires of the power cables. These teeth are able to puncture the outer surface of the power cable cores and therefore firstly improve the electrical contact and secondly enhance the frictional forces.

According to a further advantageous embodiment, the electrical cable splice further comprises an electrically conductive flexible sheath, which is arranged over the two power cables in at least a part of the region of overlap before the dimensionally recovered sleeve is mounted. Such an electrically conductive sheath has the advantage that it may serve as a mounting aid and furthermore interacts with the heat recoverable sleeve in order to electrically and mechanically optimize the connection. In particular, the conductive flexible sheath may be fabricated from woven or braided metal. It can either be tube-shaped or be formed as a rectangular piece that is wrapped around the connection. When using a contact ele- ment, the latter embodiment may also be used for mechanically securing the element on one of the power cables before attaching the second cable and then wrapping the flexible sheath around both cables.

The present invention can be used for all common power cables with massive rigid metal wires. In particular, a connection of two cables having a sector-shaped cross section can easily be performed by means of the inventive concept. However, also two cables with a round cross section or one with a round cross section and one with a sector-shaped cross section and also more than only two power cables of an arbitrary cross section can be con- nected to each other using the electrical splice technology according to the present invention.

The present invention also aims at providing an emergency kit comprising the cable splice according to the present invention for repairing broken power cables. The availability of elec- trie power is a key to a fast recovery from large-scale disaster situations since medical aid and supply of drinking water are difficult to maintain without electric power. The lines very often may have been interrupted and repair is likely to be delayed due to missing spare parts and insulation tools. A connection kit, which is simple to install and may cheaply be held on stock can cover the time from most urgent need for electricity to a final repair of the line in case of an emergency. Other emergency scenarios in which the emergency kit according to the present invention can prove useful may be power cable damage on a ship or any other isolated network. Such a simple to install emergency kit for broken low-voltage energy lines could assist aid organizations to set up fast help in affected areas.

The kit comprises only a few components without a limited shelf life. It is easy to install and does not require any special tools. A knife, in case cable insulation has to be taken off, and a heat source, like open fire or a magnesium torch, are sufficient. The installation can be performed by an instructed person having been trained on basic safety procedures, such as disaster relief workers. An electrician is deemed not to be required. However, for safety reasons some personal protective equipment is recommended to be used. The kit according to present invention comprises one piece of rigid heat shrink tube, one piece of copper mesh or copper sleeve and optionally one piece of contact band.

The advantage of this solution is that an inexpensive emergency kit can be provided that requires only small space, all parts can be nested and protected against dust by a heat shrink packaging. Moreover, the kit can be used for a wide range of cable diameters for under- ground cables as well as for overhead conductors.

Although in the following it is always assumed that the kit only provides a temporary connection between two cables, however, it could be shown that the stability is high enough to even fulfill the respective requirements for low-voltage connectors according to the relevant standards for connectors as valid at the time of testing. Hence, no particular restrictions on the duration of using the electrical cable splice according to the present invention seem necessary. The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings together with the description, serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form— individually or in different combination— solutions according to the present invention. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements.

FIG. 1 shows a schematic sectional view of an electrical cable splice according to the present invention;

FIG. 2 shows a schematic representation of the essential and optional parts necessary for an electrical cable splice according to the present invention;

FIG. 3 shows a first step when assembling the electrical cable splice;

FIG. 4 shows a second step when assembling the cable splice;

FIG. 5 shows a sectional view of the FIG. 4;

FIG. 6 shows a next step when assembling the electrical cable splice;

FIG. 7 shows a sectional view of the arrangement of FIG. 6;

FIG. 8 shows a final step of assembling the electrical cable splice;

FIG. 9 shows a sectional view of the finally mounted electrical cable splice.

Referring now to FIG. 1 , a schematic cross section of an electrical cable splice according to an advantageous embodiment of the present invention is shown. In particular, two power cables 102, 104 are connected to each other by means of the cable splice 100. Each of the power cables 102, 104 has an electrically conductive core 106, 108 and an insulating layer 1 10, 1 12 covering the respective electrically conductive electric cores. For establishing the electric connection, the cores 106, 108 have to be laid open by stripping the insulating layer in the connection region 1 14. According to the present invention, the two cable cores 106, 108 are put into contact to each other by overlying them with a predefined length of overlap, which in FIG. 1 equals to the length of the connection region 1 14. According to the present invention, the two electrically conductive cores 106, 108 can directly be connected to each other by means of a dimen- sionally recoverable sleeve which covers the joined power cable cores at least in their region of overlap.

This dimensionally recovered sleeve 1 16 may be formed from a heat shrink material. For such a heat shrink material a plastic material can be used which is semicrystalline and cross-linkable. In the finally mounted state, which is shown in FIG. 1 , a wall thickness of about 4 mm is reached. Any other suitable thickness may of course also be used.

It could be shown that an extraction force of 2.5 kN can easily be reached with the arrangement according to FIG. 1 . In order to achieve a sufficiently high degree of contact pressure between the conductors, the thermal expansion coefficient of the material should be sufficiently high for the maximum application temperature and above. In particular, values of more than 300 μη"ΐ/η"ΐ·Κ and preferably in the range between 400 and 600 μη"ΐ/η"ΐ·Κ would be advantageous. Even better would be a thermal expansion coefficient which is approximately constant up to the maximum application temperature and is rapidly decreasing above this temperature.

Furthermore, in order to achieve a sufficiently high tensile strength in combination with the thermal expansion rate, the Young's modulus should at least be 600 N/mm ² , preferably in the range between 900 and 1 100 N/mm ² . Usual temperatures occurring during the operation of common power cables are around 70 °C. Hence, a rated maximum application temperature should be above 90 °C, preferably between 1 10 and 120 °C.

An example for a suitable material for the heat recoverable sleeve is high-density polyeth- ylene (HDPE). This material has the additional advantage that it shows a thermochromatic behavior in that it is translucent above a critical temperature and is of a milky white below this temperature. Thus, also during the operation, an overheating through the contact can easily be detected by using the sleeve as a temperature indicator.

Additionally, but not necessarily, the cable splice 100 may further comprise a contact ele- ment 1 18, which is arranged between the two electrically conductive cores 106, 108 in at least a part of the connection region 1 14. Such a contact element, which is for instance formed by a metal sheet having punched-through protrusions for contacting both contact planes of the respective power cable, has the advantage that it firstly enhances the electrical contact and secondly increases the frictional forces and thereby the extraction forces of the electrical cable splice 100. Furthermore, an electrically conductive flexible sheath 120 can be provided between the two cores 106, 108 and the sleeve 1 16. Such a flexible sheath, which may for instance be a woven or braided metal mesh, for instance fabricated from copper, further optimizes the mechanical and electrical performance of the cable splice 100.

FIG. 2 shows the parts for forming the cable supplies 100 according to FIG. 1 in a pre- assembled state. Essentially, only the power cables 102, 104 and the dimensionally recoverable sleeve 1 16 are needed. Preferably, the dimensionally recoverable sleeve 1 16 is a heat recoverable sleeve and is stored in an expanded state, as depicted in FIG. 2. However, also a cold shrink material could be used. In this case, the sleeve would be stored in an expanded state supported for instance by a removable spiral, as this is well known in the art. Furthermore, also multilayer structures, for instance a double layer structure as disclosed in European patent EP 1 702 391 B1 can be used for holding together the power cables in splice according to the present invention.

Furthermore, as already mentioned, a contact element 1 18 can be arranged between the two cores 106, 108 in order to enhance the frictional forces and lower the Ohmic resistance. To this end, for instance a metal sheet can be used as the contact element 1 18, which has punched-through holes 122, which are punched from the two opposing sides in order to provide sharp edges on both surfaces of the contact element 1 18.

In order to further optimize the electrical and mechanical contact of the two power cables, additionally a conductive flexible sheath 120 can be provided. In the present embodiments the flexible sheath 120 is formed by an essentially rectangular piece of copper mesh, which is wrapped around the cores. In particular, when assembling the cable splice 100, firstly, the element 1 18 is positioned on one of the cores 106, 108 and a part of the flexible sheath 120 is wrapped once or twice around the core and the contact element. Next, the respective other core 106, 108 is positioned on the contact element 1 18 and the rest of the flexible sheath 120 is wrapped around both power cable cores. The individual steps for forming a cable splice according to the present invention will now be explained referring to FIGs. 3 to 9.

After having stripped bare the ends of the cores 106, 108 of two or more power cables 102, 104, the assembly method starts with positioning a dimensionally recoverable sleeve over one of the power cables to be connected. This step is depicted in FIG. 3.

In a next step, the contact element 1 18 is positioned on one of the power cable cores. FIG. 5 shows this step in a schematic cross-sectional representation. As may further be derived from FIG. 5, the cross section of the wires forming the cores 106, 108 is triangular. However, any other suitable cross-sectional form is of course also compatible with the principles of the present invention. In particular, circular or sectional forms may also be connected with each other by a cable splice according to the present invention. In any case, it is advantageous but not required, if the cross sections have a flattened area for establishing the contact.

Subsequently, as shown in FIGs. 6 and 7, the electrically conductive flexible sheath 120 can be attached. As already mentioned above, a small amount of layers can also be wrapped only around the contact element and one of the two cores in order to facilitate the mounting of the contact element.

The final step is illustrated in FIGs. 8 and 9. The heat recoverable sleeve 1 16 is shrunk by means of a heat source 124 and thereby electrically and mechanically fixes the contact between the two cores 106, 108. As already mentioned above, instead of a heat source also the removal of a supporting structure can lead to the sleeve 1 16 recovering its initial shape.

Preferably, the sleeve is long enough to securely contact the insulating layers 1 10, 1 12 of the first and second power cables 102, 104. If necessary, in a peripheral area an additional clamping piece (not shown in the figures) that encompasses the dimensionally recovered sleeve can be provided for further improving the mechanical stability. Using the electrical cable splice according to the present invention allows providing an emergency kit for broken low voltage energy lines that is simple to install and cheap to be kept on hold. For instance, the splicing kit for electrically connecting at least two power cables according to the method of the present invention comprises a dimensionally recoverable sleeve, optionally a contact element and/or an electrically conductive flexible sheath. Furthermore, no special tools are required for the assembly and a wide range of cable diam- eters and cross-sectional forms can be connected to each other by one and the same kit. Thus, in emergency situations a fast and reliable reconnection of broken power lines can be achieved. The present invention is in particular advantageous to be used with rigid aluminium wires and bars forming the cores of the power cables.

Reference Numerals:

Reference Numeral Description

100 cable splice

102 first power cable

104 second power cable

106 core of first power cable

108 core of second power cable

1 10 insulation layer of first power cable

1 12 insulation layer of second power cable

1 14 connection region

1 16 dimensionally recovered sleeve

1 18 contact element

120 electrically conductive flexible sheath

122 punched holes

124 heat source