Cable duct with detector wire

#This invention concerns a process for applying at least one electrically conducting detector wire in a plastic cable duct designed for laying in the ground, in which the wire is embedded and entirely surrounded in the duct wall, running along a neutral bending line parallel to the axis of the cable duct.

#In order to be able to detect a plastic cable duct at any required moment, it is necessary for the duct to have a metallic conductor running along it. For nonmetallic fiber optic cables laid inside plastic ducts, without a separate metal core, it is necessary to include at least one electrically conducting detector wire for this purpose. EP-A-0 159 307 describes a process of the type mentioned in the preamble, in which the detector wire is drawn through gas flames in order to heat it before being applied to the duct wall. The hot wire is pressed against the duct wall, which melts in its vicinity, so that as a result the wire becomes embedded in the wall. The aim is for the duct wall material to surround the wire which is pressed into it, and unite with the wire so intimately that the wire is completely embedded and enclosed on all sides, without any pores or cracks leading to the outside. It is important for it to be enclosed in this way, since it is necessary to ensure that over the years or decades the detector wire does not become interrupted at points along it as a result of moisture-induced corrosion. There is also the danger that with the known process, if the process parameters are not maintained exactly, in particular the preheating temperature of the wire, the latter will either become only slightly embedded in the duct wall, so it may become exposed by rough handling during installation, or it will sink too deeply into the duct wall, so that in certain cases it will not meet the contact elements of the cable connector when one is fitted. Finally, only detector wire which can be sufficiently strongly heated can be used.

#EP-A-0 159 307 describes a further process, in which by means of a special extruder nozzle, the detector wire is led out of the extruder directly into the duct wall as it is being extruded, before the duct leaves the extruder. In addition to requiring a special extruder nozzle, this has the disadvantage that the exact depth of the wire inside the duct wall is not absolutely controlled, since during the process of introducing the wire the duct wall is still very soft and easily deformable.

#The invention is described below with the aid of several embodiment examples, showing:

#FIG. 1 a simplified overall view of an apparatus according to the invention for applying a detector wire in a cable duct and more precisely in the wall of the duct;

#FIG. 2 a sectional view of the arrangement in FIG. 1 along line II--II;

#By means of the apparatus 10 shown in FIG. 1, described in greater detail below, cable ducts 12, in particular for fiber optic cables can be provided with at least one and preferably two detector wires 14 during their manufacture, thus ensuring that said wires 14 are reliably embedded in the duct wall 16 and enclosed on all sides, so that they are protected against corrosion.

#In FIG. 1, a duct emerging from an extruder (not shown in the diagram) is transported from left to right (arrow A) and then brought into its transport form, in particular coiled, where the coil axis is parallel to the plane of the drawing. Before reaching the device 10, the cable duct 12 runs through a cooling and gauging device (also not shown).

#In accordance with the invention, a preferably V-shaped receiving groove 18, as for example shown in FIGS. 5 and 7, is fashioned in the duct wall 16, and the detector wire 14 is led into said groove 18 in the area of the base of the groove, after which the groove 18 is filled with a separate softened filler material 20.