Link boxes are widely used where it is desired temporarily to interrupt the supply of electric current in a cable or cables, typically a three-phase AC mains supply. Such boxes are commonly installed in underground chambers in roadways or other paved areas. In the simplest case, the link box is interposed between two parts of a cable and serves to either permit or interrupt the flow of current along the cable. In such a case, the link box contains two sets of terminals, for the individual cores of the two parts of the cable, and a set of mechanical links are used to connect the corresponding terminals to complete the current path. Physical removal of the link interrupts the flow of current. More complex arrangements are also possible in which a number of cables are connected to the link box and a variety of possible connections can be achieved by the use of appropriate links.

Originally, link boxes were connected to the supply cable in situ. By this is meant that a break was made in the cable, the ends of the cable cores were exposed and connected directly into the link box. After all the connections had been made, the link box was filled with a resin which surrounded and insulated the connections. Such an assembly procedure suffers from the disadvantage that it is carried out in an environment in which there may well be electrically live components, and therefore there is an associated safety risk. Also, the link box assembly cannot easily be tested.

For these reasons, it is known to use so-called"pre-tailed"link boxes. Such link boxes are pre-assembled with short lengths of cable which are, in use, connected into the mains cable using straightforward mechanical connectors. Being factory- assembled, pre-tailed link boxes can be subject to stringent quality control and their installation carries much reduced safety risk. Pre-tailed link boxes too are filled with resin, but a disadvantage of known forms of pre-tailed link box is that they comprise a number of discrete chambers, typically three such chambers. The"tails"are

connected to terminals in the two outer chambers and are connected to spigots in the middle chamber, the spigots being used for connection of the links. Because the box is sub-divided into separate chambers, the insulating resin is present not as a unitary body but in several parts. There are therefore interfaces within the body of the link box which can have an adverse influence on the electrical properties of the box.

There has now been devised an improved form of link box, which overcomes or substantially mitigates the above-mentioned or other disadvantages of the prior art.

According to the invention, there is provided a pre-tailed link box comprising a housing within which one end of each of at least two cable tails is terminated, terminals to which the cable tails are connected being connected to spigots so as to form spigot assemblies across pairs of which removable links can be fitted so as to create connections between the cable tails, wherein the link box is formed as a unitary chamber.

The link box according to the invention is advantageous primarily in that since it is formed as a unitary chamber, when it is filled with insulating resin the resin forms a single block which surrounds and insulates the electrical connections with no interfaces or other discontinuities which might reduce or impair the electrical performance of the link box.

The housing of the link box is preferably formed in two halves, each of which is most preferably moulded in plastics material, eg by injection moulding, or vacuum forming.

The halves preferably cooperate to define openings for each cable tail and an enlarged central portion within which the appropriate electrical connections are accommodated.

The upper half of the housing is most preferably formed with one or more wells within which the spigots to which the cable tails are connected are accessibly mounted.

Most preferably, separate and discrete wells are provided for each pair of spigots that, in use, are to be connected by links. Preferably, the dividing walls between

wells are hollow and can be filled with resin during the resin casting process, effectively forming phase barriers.

The link box according to the invention may be manufactured particularly conveniently in an inverted position. The cables are first terminated onto the spigot assemblies, preferably using a suitable jig, then mounted into the inverted upper half.

The lower half is then fitted on top and the housing is filled with resin. A support, fitted below the upper half, preferably serves to limit the extent of the resin and is provided with recesses or the like to receive the tips of the spigots so that they project from the resin block once the resin has cured.

Thus, according to another aspect of the invention, there is provided a method for the assembly of a link box having at least two cable tails and comprising an enclosure formed in upper and lower halves, which method comprises the steps of a) positioning the upper half in an inverted condition upon a support which extends into link-receiving spaces in the upper half ; b) connecting exposed ends of the cable tails to terminal assemblies having upstanding spigots; c) positioning said cable tails upon the inverted upper half such that the tips of said spigots are received within recesses in said support; d) positioning the lower half upon the upper half and connecting the upper and lower halves so as to form the enclosure; e) filing the enclosure with a resin; f) causing or allowing the resin to cure; and g) removing the enclosure from the support.

The support preferably is formed so as to close any openings in the upper half and so limits the upper extent of the resin block in the completed link box. Where the link box is formed with one or more wells in its upper surface, the support thus determines the depth of the wells. In such a case, the support preferably is formed with recesses or slots to receive the tips of the spigots. This ensures that the tips of the spigots are accessible in the finished link box and also facilitates accurate and precise positioning of the spigots during manufacture.

To facilitate filling of the link box with resin, the lower half is most preferably provided with one or more fill openings. The resin used may be any suitable resin, including those conventionally used for the filling of link boxes and other electrically insulating applications.

The link box may also contain a terminal for combined neutral and earth (CNE) conductors, which are commonly multi-stranded conductors that surround the live conductors of a three-phase cable, between those conductors and the outer insulation of the cable. In some cases, for example with a four-core cable, it may be preferable to solidly connect one or more phases by passing a busbar through the link box rather than by providing a link and connecting arrangement for each individual phase.

The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is an exploded view of a link box according to the invention; Figure 2 is an exploded view of a link spigot assembly forming part of the link box of Figure 1; and Figure 3 is an exploded view of a CNE take-off assembly forming part of the link box of Figure 1.

Referring first to Figure 1, a link box according to the invention comprises a number of major components. A lower shell half 6 is moulded in plastics material and cooperates with an upper shell half 4 to form an enclosure within which one end of each of two three-phase cable"tails"5 is received. The shell halves 4,6 are each generally hemi-cylindrical at their ends, the hemi-cylindrical end portions of the two halves 4,6 (when joined together) defining openings which closely surround the cable tails 5.

Each of the cable tails 5 comprises three solid cores 51,52,53 which are respectively provided with colour-coded red, yellow and blue insulation. The cores 51,52,53 are of segmental cross-section, subtending angles of approximately 120°, so that the cores 51,52,53 fit together to form a circular cable. Multi-stranded CNE conductors

54 surround the insulated cores 51,52,53 and are covered by an outer insulating sheath 55.

At the end of each cable tail 5 which is received within the link box, the solid cores 51,52,53 are stripped of insulation so that they can be connected to spigot assemblies 25, one of which is shown in more detail in Figure 2. The spigot assembly 25 comprises a terminal spigot 10 which is fastened by means of a bolt 7 and interposed washer 8 and O-ring 9 to a terminal connector body 11. The connector body 11 is generally of inverted U-shape, the downwardly depending limbs being formed with juxtaposed grooves 11 a which serve to receive and retain an insert 12. The insert 12 and connector body 11 together define a socket into which the exposed end of the respective core 51,52,53 can be inserted. The insert 12 has a threaded bore 12a in which a shear head bolt 13 is engaged.

In a first stage of assembly of the link box, the exposed end of each core 51,52,53 is inserted into the socket of each spigot assembly 25 and the shear head bolt 13 tightened until its head shears off. The bolt 13 thus clamps the end of the core 51,52,53 in the spigot assembly 25. Spigot assemblies 25 are fitted in this manner to each of the three cores 51,52,53 of each of the two cable tails 5 (ie six spigot assemblies 25 in all). Fitting of the spigot assemblies 25 to the ends of the cores 51,52,53 may be facilitated by the use of an appropriate jig.

Referring once again to Figure 1, the upper shell half 4 is formed with three wells 41,42,43 which extend approximately one-half the depth of the upper shell half 4 and have hollow walls. To assemble the link box, the upper shell half 4 is inverted and mounted upon a jig (not shown). The jig extends into each of the three wells 41,42,43 and so defines the boundary of the resin which is filled into the link box as described below. The assembly of cable tails 5 is then positioned (also inverted) upon the inverted upper shell half 4. The cable cores 51,52,53 are arranged so that the cores of corresponding colour are aligned with the same well 41,42,43. The upper surface of the upper shell half 4 is embossed, at the sides of the wells 41,42,43 with the letters R, Y and B to indicate red, yellow and blue. The spigots 10 extend downwardly through the bases of the wells 41, 42,43, the tips of the spigots 10 being received within recesses in the upper surfaces of those parts of the support

that extend into the wells 41,42,43. This means that when the link box is filled with resin the tips of the spigots 10 will project from the resin into the wells 41,42,43.

The exposed strands of the CNE conductors 54 of each cable tail 5 are then drawn together and connected to a CNE take-off assembly 30, shown in more detail in Figure 3. The CNE take-off assembly 30 comprises a connector body 14 of U- shape, the limbs of which are formed (in a similar manner to the limbs of the connector body 11 forming part of the spigot assembly 25) with grooves 14a in which an insert 15 is received. The connector body 14 and insert 15 together define a socket into which the ends of the CNE conductors 54 can be inserted. The insert 15 has a threaded bore in which an elongated fixing stud 20 is engaged, together with a washer 16 and nut 17.

The ends of the CNE conductors 54 of the two cable tails 5 are inserted into the socket in the CNE take-off assembly 30 and clamped in place by tightening of the stud 20. The stud 20 has a shearable head 20a which breaks off. The projecting end of the stud 20 is then passed through a hole 4b in the side of the upper half 4 and an upstanding spigot 18 is captivated on it (externally of the upper half 4) by a nut 19. The lower shell half 6 is then placed over the inverted upper shell half 4 so that flanges 6a, 4a on the two components abut. The two shell halves 4,6 are then joined at the flanges 4a, 6a by foam tape and sprung steel clips (not shown).

The completed enclosure defined by the shell halves 4,6 is then filled with an insulating resin by injection of the resin through fill holes 61,62 provided in the base of the lower shell half 6. After curing of the resin the entire interior volume of the link box is filled with a unitary block of insulating material which surrounds the cores 51,52,53, the spigot assemblies 20 and the CNE conductors 54 and CNE take-off assembly 30. The link box is then ready for use and can be packaged together with a lid 1 and a set of link assemblies 2. The lid 1 is cast in iron (or any other suitable heavy material) and is dimensioned to fit over the outer periphery of the wells 41,42,43. The lid 1 has a considerable weight to ensure that it remains securely in place during use and is not lifted, eg by water in the event that the chamber within which the link box is fitted becoming flooded. The link assemblies 2 comprise a pair of elongate plates, the bottom edges of which are flared outwards, the plates being

resiliently biased together by clips 2a. Colour-coded link gaskets 3, corresponding to the embossed letters R, Y, B, are fitted over the projecting tips of the spigots 10 in the bases of the wells 41,42,43.

The link box is installed and used in the manner of a conventional link box. A typical installation will be in an underground chamber set into a roadway or other paved area. The free ends of the cable tails 5 will be connected to three-phase mains supply cables using a standard cable-to-cable joint. The CNE conductors 54 of the two cable tails 5 are connected via the neutral take-off assembly 30 embedded in the resin interior of the link box. The other three cable cores 51,52,53 are connected only if link assemblies 2 are fitted in the respective wells 41,42,43 to form connections between the protruding tips of the spigot assemblies 20.

In normal use, the lid 1 is positioned over the wells 41,42,43. If it is desired to interrupt the flow of current along any particular core (red, yellow or blue) the lid 1 is removed and the corresponding link assembly 2 is removed from the appropriate well 41,42,43. To restore the flow of current, the link assembly 2 is replaced.