The cables or pipes could be underground cable or pipe installations or above ground installation in industrial and commercial buildings. The composite duct that houses the cables or pipes can afford proteetion against physical damage, chemical or corrosive damage, as well as enabling such cables and pipes to be neatly arranged in a manner facilitating maintenance, repair and even replacement of the cables or pipes or sections thereof.

BACKGROUND TO THE INVENTION The advantages of housing cables and pipes in ducts for the various purposes outlined above have long been recognized and venous forms of Su4* ducts have been proposed. Some of the advantages are more fully set out in my published international patent application number PCT/ZA01/00204 published under itnernational publication number WO 02/054553 A2, and the content of which is included herein by reference.

Some of the pnor art proposals are also set out in my published international patent application idenfified above ; British patents 2,276,694; 2,339,66; and 2260590 as well as German patent 1, 881, 169. All of these describe a duct ; two opposed semicircutar cross-sectioned channets the

edges of which are engaged with each other to form a duct to enclose cables or pipes therein. The channels are heM in their operative relationship with each other by means of damps or straps that encircle the resultant duct.

Little, if anything, is disclosed concerning the sealing of the lengths of channel to each other and the end on end connection of lengths thereof. international publication number WO 98/45915 describes an arrangement in which a semielroular cross-sectioned channel shaped duct unit has co- operant tongue and groove formations eXrading along the two free edges such that two identical units can have their co-operant edges inter-engaged with each other with the tongues of the formations along one edge clipping into the grooves of the formations along the other edge. This arrangement does not, however, provide any particular way in which the ends of the two units can be aligned. Also, appreciable force is required to engage and disengage tongues and grooves as they extend along the entire length of the duct units.

OBJECT OF THE INVENTION It is, accordingly, an object of this invention to provide a duct unit and assembles of duct units in which the drawbacks Mended above may be diminished. at least to some extent.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a duct unit of channel shape in cross section and adapted to cooperate with an identical duct unit to form a closed duct therewith, the duct unit having a closinig surface in the form of a rebate or groove extending along one alongitudinal edge thereof and a closing surface in the form of a co-operant lip or tongue » tending along the other longitudinal edge such that the dosing surfaces of two opposite -t units cooperate operativety to form an effective closure of

the duct along the two pairs of cooperating closing surfaces, the duct unit being characterixed in that it has at least two spaced catch formations offset radially outwards from the adjacent closing surface and spaced apart along each edge of the duct unit with diagonally opposite catch formations being configured so that one operatively receives the other with a capping action and wherein the catch formatiions further serve to locate the one duct unit axially aloi a co-operant duct unit Further features of the invention provide for the catch formations to be <BR> <BR> <BR> fanned integral with the duct unit, typicatfy by forming the duct unit as a single piece injection moulding of suitable plastics material such as an appropriate po ! ypropy ! ene material ; for the catch formations to be formed one as a channel hwng a catch or a slightly inwardly tipped configuration in which case a cooperant catch formation has a cross-sectional shape complementary to that of the channe typically a generally rod shape; and for the two ends of the duct unit to be formed as a male spigot and female socket respectif* adapted to form a complete spigot or socket together with a cooperant duct unit that forms a closed duct therewith with the male end being adapted to be accommodated within the female end of a coaxia. fly arranged similar duct unit Preferably, the male end of the channel shaped part has a peripheral flange emending at least a part way around the end of the duct unit in which case the femate end has a cooperating groove to accommodate the peripheral flange such that, in the closed position, the flange and groove serve to angularly align two coaxial interconnected duct units. The arragnement of the spigot and socket is preferably such that the internal diameter of a joint between two coaxial duct units remains constant across the joint. This arrangement enables, cables and the like to be drawn through the duct without any interference that could be caused by a small step change in internal diameter.

Also, the outer surFace of the duct unit can have both longitudinal and circumferential reinforcing ridges moulded integral therewith.

In order that the above and other features of the invention may be more fully understood one embodiment thereof will now be described with reference to the accompanying drawings.

BFMEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective illustration of one embodiment of the invention showing one par of duct units cooperating to form a closed length of dud ai a collinear pair in the open condition ; Figure 2 is an end view of two duct units in exploded relationship relative. to each other ; Figure 3 is a plan view of part of the length of a duct unit showing two pairs of diametrically opposite catch formations and the spigot and socket formations at the end thereof ; figure 4 is a delailed perspecitive view of the one type of catch rmation ; and, Figure 5 is a detailed perspective view of the other type of catch formation.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRWINGS In this embodiment of the invention a duct unit generanly indicated by numeral (1). is made as a single piece ptastics injection moutding having the and diameter. Typically the length could be of the order of

about a metre and the diameter can be as required, tpically from 50 to 300 mm although for illustration purposes, a very much shorter duct unit is illustrated.

The duct unit is formed as a sing ! e channel shaped part of semicircular shape in cross-section such that it has two longitudinal free edges (2) and (3) and two semicircular ends (4) and (5) (see Figure 1).

The one free edge (2) of the duct unit has a longitudinally extending sealing lip (6) the transverse dimension of which is directed in a generally circumferential direction and that is operatively sealingly received in a cooperant rebate (7) in the other free edge (3) (see in particular Figure æ The lips and rebates can be made to effectively dose the duct and the degree of sealing achieved can be selected according to requirements and according to the materials of manufacture as well as the strength of the catch formations described below.

In this illustrated embodiment of the invention the duct unit has two catch formations (8) and (9) at corresponding diametrically opposed positions on the outside of the edges (3) and (2) respectively. The catch formations ate symmetrically spaced apart along the length of the duct unit ; are integral with the duct unit, and our radially outside of the edges (3) and (2) The one form of catch formations (8) associated with the one edge (3) each assume the form of a channel that may either be inwardly lipped or have an integral catch member (10) serving as a closure for the catch formation. The other form of catch formations (9) that are associated with the opposite edge (2) are rod tike in general configuration to present a complementary surface to the channel shaped catch formations (8). The catch formations have corresponding lengths and thus, when inter-engaged with the co-operant catch formations of another duct unit to form a complete duct, the catch formations serve not only to securely attach the duct units together in a radiat to locate them longitudinally relative to each other.

End on end attachment of lengths of duct is provided for by forming a male spigot (10) and female stocket (11) at the two ends (4) and (5). As indicated above, the ends are preferably provided with cooperant flange (12) and groove (13) fbmiations to ensure that collinear lengths of duct remain angularly aligned.

Also, as indicated above, the injection moulded duct unit can be reinforced with both circumferential external ribs (14) longitudinally extending external ribs (15).

In use, an elongate duct assembly can easily be made up from a series of coaxiat duet units of the type described above, A collinear series of duct units can be laid along a required path and any desired cables pipes or other elongate service elements tnstaMed therein. The co-operand duct units are then clipped into positon in cooperating relationship with those laid on the required path to dose the duct.

It will be understood that numerous variations may be made to the embodiment of the invent : on described above without departing from the scope hereof. Also, it will be understood that, as described in my earlier international patent application identified above, various junction and terminal fittings can be produced to cooperate with duct units provided by this invention.