The present invention relates to electrical floor heating cables and in particular to cables generating acceptably small electromagnetic fields. In general the magnetic field is best reduced/eliminated by a concentric shield, as in coaxial cables, but this is not a practical solution with heating cables. The field generation problem can also be somewhat reduced by using two-conductor cables instead of single-conductor cables because the fields generated from the two conductors tend to eliminate each other. The fields from two-conductor cables having a resistance conductor and a return conductor should - however - be further reduced.
The object of the present invention is to reduce the field generating properties of electrical floor heating cable having a first, near end and a second, far end and comprising within an outer protective sheath a cable core consisting of
The main features of the invention appears from the accompanying claims. The problem of eliminating or greatly reducing generation of electromagnetic fields which could be harmful to human beings occupying rooms heated with electrical floor heating cables has been solved by letting the return conductor means consist of at least two separate conductors which are distributed in the cable core. The electromagnetic field is reduced to about 1% of the field experienced from single conductor heating cables, and to some 10% of the fields from normal two conductor cables.
The prior art document WO 92 08036 A discloses an electrical heating cable comprising within an outer protective sheath a cable core consisting of: a central resitance conductor, a concentrical sheath of insulation material return conductor means arranged to be interconnected with the resistance conductor in a far end of the cable and consisting of at least two separate conductors.
The present invention is characterised in that the return conductors are embedded in a second insulation sheath arranged over said first sheath.
Above mentioned and other features and objects of the present invention will clearly appear from the following detailed description of embodiments of the invention taken in conjunction with the drawings, where
In Figure 1 to 4 a resistance conductor 1 is provided with a first concentrial sheath 2 of insulation material such as crosslinked polyethylene. All embodiments have an outer protective sheath 4 of PVC or other suitable material and an earth screen 7 in contact with earth means 6 or 20. The earth screen could be a conventional Al/PE tape in contact with Cu wires 6 or 20. In the near end of the cable the resistance conductor can be substituted with a cold end copper conductor. Return conductors 5;10,11;15,16,17 are embedded in a second common insulation sheath 3 arranged over the resistance conductor sheath 2. The sheath 3 could be a PVC sheath.
In Figure 1 the return conductor means is divided into three separate conductors 5a, 5b and 5c which are distributed in the cable core. In Figure 2 the return conductors means is constituted by two conductors 10 and 11 arranged 180° apart whereas in Figure 3 there are three conductors 15, 16 and 17 arranged 120° apart. In Figure 4 the return conductors 5 are arranged 90° apart.
In Figures 1-3 an earth wire 6 is arranged in a slot in the second common insulation sheath 3, whereas in Figure 4 the earth conductor 20 is arranged as a group of conductors between the sheath 3 and the earth screen 7 and covering 10 - 25 % of the sheath surface.
All elements of the cable are preferably arranged in parallel with the cable axis, thereby avoiding stranding operations.