EP0290343B1 - Assembly of a sheath and a screw connector - Google Patents

Description

The present invention relates to an assembly of a cable shielding sheath for protection against electromagnetic radiation, in particular in the range of myriametric and kilometer waves and of a screw connection.

A particular problem to be solved is that of the hardening to electromagnetic interference (EMI) of very low frequency (VLF) and low frequency (LF) reception or emission installations as well as acoustic detection installations, or any other system operating in these frequency ranges.

We also seek to attenuate the radiation of cables conveying energy which are often sources of parasite V L F and L F (for example the harmonic of rank 170 of 60 Hertz can disturb the omega reception at 10.2 Khz).

In a previous embodiment, hardening, that is to say insensitivity to electromagnetic interference operating at low and very low frequencies, consists in shielding the cables with metal steel tubes in the straight part of their path. In the bent parts of the wiring, a flexible austenitic stainless steel pipe is used.

The multiplicity of specific connecting members, between these various products, implies the production of more or less complex adaptation pieces individually.

If the very thick steel tube is an effective shield at these frequencies, the flexible austenic stainless steel tube is of little interest by the very fact that the magnetic permeability of these stainless steels is close to 1.

Document FR-A-2 520 548 describes a concentric multilayer shielding sheath containing a screen with a high coefficient of magnetic permeability formed by the alternating superposition of layers formed of homogeneous spindles of non-ferromagnetic conductive elements and layers formed of ferromagnetic elements .

Patent FR 2 532 463 is also known, which describes a cable protection sheath against electromagnetic disturbances and corrosion, having a characteristic combination of ferromagnetic layers with low magnetizing power, such as mild steel, and with high magnetizing power, such as silicon steel, inserted in a conductive filler material itself comprised between two layers of conductive plastic. This sheath is not suitable for low frequency electromagnetic radiation.

It is known that the shielding efficiency in the frequencies from 10 to 100 kHz is significantly improved when using a screen with a high coefficient of magnetic permeability. The H component of the electromagnetic wave is more active at low frequencies than at high frequencies.

Optimal shielding efficiency is obtained if the electric field and the magnetic field of the wave are attenuated on the one hand.

If one masters the shielding technology of the electric field, that of the magnetic field has not followed the evolution from raw materials.

Patent FR 2 461 342 describes, however, a cable with high immunity against electromagnetic radiation. The cable is coated with a flexible coating which mainly comprises a composite magnetic material comprised between two flexible conductive shields. However, this coating is mechanically linked to the conductor cable and does not have the possibility of radial elasticity for its adaptation to cables of even slightly different dimensions.

The present invention overcomes the drawbacks mentioned above by proposing a sheath having a certain flexibility, both axial and radial, allowing its adaptation to various types and configurations of cables, which offers effective protection of these cables against electromagnetic radiation in the range of myriametric and kilometer waves, and which can be fitted with standard fittings.

The invention therefore relates to an assembly according to claim 1.

Preferably, the screen is made up of two superimposed bands of metal wound in contiguous turns, placed between two tinned copper braids.

The shielding sheath makes it possible to harden an installation in a homogeneous manner. It has a certain flexibility which allows the same product to be used over the entire length of the wiring (straight or curved part), and can be fitted with fittings provided with a flexible contact at the junction of the crimping body and the part threaded connection which facilitate the implementation of the product.

The use of fittings adapted to each type of socket or cable gland enhances the overall efficiency of hardening, improving electromagnetic immunity by limiting the risk of electromagnetic leakage.

The superimposition of metallic layers with high magnetic permeability in contact with metallic braids ensures effective galvanic continuity.

• la figure 1 est une vue en écorché de la gaine.
• la figure 2 représenté schématiquement un raccord d'assemblage.

Additional advantages will appear on the following reading of embodiments given by way of example and without limitation with reference to the drawing in which:

• Figure 1 is a cutaway view of the sheath.
• Figure 2 schematically shown an assembly fitting.

The shielding sheath is composed of concentric layers from the inside to the outside, defined as follows, around an electric cable 20.

The first layer 1 is a 2 mm diameter stainless steel wire wound in a helix with a pitch of 10 mm. The purpose of this wire is to ensure mechanical resistance of the product to radial forces.

The second layer 2 is an insulating film which has the function of avoiding direct contact with the stainless steel wire and the elastomer and which may be necessary in the production.

Then there is a third layer 3 of elastomer which is a self-extinguishing silicone gum and exhibiting no release of halogen or toxic gas.

The fourth layer 4 is a very thin insulating film which separates the upper layer of copper from the elastomer. This film facilitates the stripping of the sheath.

The screen with a high coefficient of permeability follows a fifth layer 5 which is a tinned copper braid with a high recovery rate.

There are then two layers 6 and 7 of mumetal about 25 mm wide and a tenth of a mm thick wound in contiguous turns, for example at an angle of 60 ° with the longitudinal axis of the sheath. The relative magnetic permeability of the metal must be at least 50,000.

The purpose of the strip 7 of mumetal identical to the strip 6, is to ensure recovery of the contiguous part of the preceding strip.

The metal can be replaced by an iron-nickel-boron alloy with an amorphous structure, possibly containing phosphorus like that known under the trade name "MET GLAS", which then retains a high magnetic permeability even after possible mechanical deformation.

The screen is covered with a braid 8 of tinned copper with a high recovery rate, itself covered with a layer 9 identical to layer 2, and with a layer 10 of elastomer identical to layer 3.

Finally, a coating 11 must resist mechanical and chemical attack, in particular from the military environment.

When assembling the ducts, conventional screw fittings can however be crimped by magnetoforming. The use of an austenitic stainless steel being to be prohibited, and to ensure the galvanic continuity between the magnetic and electrical shields with the ground planes, one uses a martinsitic or ferritic stainless material to combine a good resistance to corrosion and a quality magnetic permeability.

FIG. 2 shows a specific fitting 21 adapted to the sheath according to the invention, comprising a crimping zone 22 separated from a threaded ring 23, by a flexible contact 24 made for example of phosphorous bronze or copper, to ensure galvanic continuity between the crimping zone 22 and the ring 23. A steel ring 25 ensures the mechanical retention of the parts 22 and 23. Finally, the threaded ring allows mechanical connection to a cable gland 26.

Claims (9)

1. The assembly of concentric multi-layer screens for the shielding of electric cables (20), in particular for protection against low-frequency electrical radiation, with the distinction that the screens include, between two insulating layers (2-4 and 9-10), a high-magnetic-permeability screen (5-8) composed of at least two tapes of iron-nickel alloy of crystalline structure or iron-nickel-borium alloy of amorphous structure, placed between two metal braids (5, 8) with low electrical resistance and a closely-woven structure. This assembly uses screwed connectors made of martinsitic or ferritic stainless material, additionally crimped by magneto-forming, and fitted with a flexible electrical contact (24) at the junction of the crimping zone and the threaded part of the connector.
2. Assembly in accordance with claim 1, with the distinction that it includes, between two insulating layers (2-4 and 9-10), a high-magnetic-permeability screen (5-8) composed of at least one tape of mumetal (6) placed between two metal braids (5, 8) with low electrical resistance and a closely-woven structure.
3. Assembly in accordance with claim 2, with the distinction that the screen is composed of two tapes (6, 7) of superimposed mumetal wound in adjacent turns and placed between two metal braids (5, 8).
4. Assembly in accordance with claim 3, with the distinction that each of the mumetal tapes are approximately 25 mm in width and about one tenth of a millimetre in thickness.
5. Assembly in accordance with one of the claims numbered 1 to 4, with the distinction that the metal braids (5, 8) are in tinned copper.
6. Assembly in accordance with claim 1, with the distinction that the internal insulating layer (2-4) is composed, from inside to outside, of an insulating film (2), a layer of silicone elastomer (3), and a further insulating film (4).
7. Assembly in accordance with claim 1, with the distinction that the external layer (9-10) is composed, from inside to outside, of an insulating film (9), a layer of silicone elastomer (10).
8. Assembly in accordance with one of the claims numbered 1 to 7, with the distinction that it also includes an internal stainless steel wire (1) wound as a helix.
9. Assembly in accordance with one of the claims numbered 1 to 8, with the distinction that it also includes an external covering (11) for mechanical and chemical protection, composed of a plastic material.

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