GB2249673A - Shielded conduit - Google Patents

Abstract

A conduit comprises a inner composite tube 1 an outer aluminium foil 2 and a flexible epoxy adhesive 3. The foil has a longitudinal joint overlap 5 which may be formed by pressure bonding. The foil is embossed to provide hills" and "valleys" so that at the overlap joint good electrical contact is formed. <IMAGE>

Description

Conduits This invention relates to conduits and particularly, but not exclusively, to 3600 electrically shielded composite conduits.

It is known to form electrically shielded conduits from an inner tube of plastic and an outer shield of metallic braid or tape. These conduits are usually of the semi-flexible variety with a convoluted inner plastic tube and the metal tape is wrapped helically around the tube, providing an undesirably long potential electrical leak path.

Alternatively rigid conduits are made of steel or aluminium and these are both heavy and expensive.

From one aspect the invention consists in an electrically shielded conduit comprising a nonmetallic inner tube having a covering of metallic foil adhered to the tube by adhesive.

Preferably the tube is flexible or semiflexible and the adhesive is also flexible or semiflexible so that no cracking occurs in the adhesive due to movement of the tube.

The foil may be wrapped around the length of the tube and joined with a longitudinal joint and it will be readily understood that this provides a very short electrical leak path. Conveniently the foil may overlap at the joint to provide a good electrical contact and the joint may be pressure bonded.

The foil may be provided with formations, which preferably form "hills and valleys". For example, the foil may be embossed with a pattern which forms a zig-zag in section.

This has two advantages; first the formations allow small amounts of conduit flexure without damage occurring to the foil and secondly they enable a joint to be formed in which the valleys of the upper layer engage the hills of foil. If this joint is formed under pressure the adhesive will be squeezed out between the points of electrical contact, but when it is cured it will hold that electrical contact firmly in place.

Although the invention has been described above it is to be understood that it includes any inventive combination of the features set out above or in the following description.

The invention may be performed in various ways and specific embodiments will now be described, with reference to the accompanying drawings, in which: Figure 1 is an end view of the conduit; Figure 2 is side elevation; Figure 3 is an enlarged view of section X-X in Figure 1; Figure 4 is a view of Figure 3 when the conduit is deflected in a convex manner; Figure 5 is a view of Figure 3 in a concave manner; Figure 6 is an end view of the conduit showing the joint overlap; Figure 7 is a diagrammatic view of a simulated lightning strike test; and Figure 8 is an enlarged view of the joint overlap shown in Figure 6.

Referring to the drawings the conduit comprises an inner composite tube 1 an outer aluminium foil 2 and a flexible eposy adhesive 3. The foil has a longitudinal joint overlap 5 of length L. The foil is suitably shaped 4 to allow for small amounts of conduit flexure.

A straight conduit is shown in Figure 3 and the angle OC is subtended by points a b c of the shaped surface 4. During convex flexure of the conduit the lengths a, b and c remain constant and the angle &alpha;' and h' are correspondingly changed. During concave flexure the angle Cz and U are produced whilst the lengths a, b and c remain constant.

These small amounts of movement are possible by means of selecting a flexible epoxy adhesive 3 and a thermoplastic composite inner tube 1 which will accommodate these movements.

The overlap joint L should be at least 40 times the thickness of foil, 1A and typically 80 times.

A longitudinal joint is preferred and should not rotate more than 1 revolution over the length of the conduit. This produces a minimum length joint reducing the potential for electrical interference.

In a simulated lightning strike test a typical transfer resistance of 2.2 m /m is produced when subjected to an applied current of 37KAmps, and a current of 3.9kA in the shield as shown in Figure 7.

The overlap joint 5 is pressure bonded to ensure that there is electrical contact between the inner and outer surfaces of the shaped surface over length L, as by way of example at point 6 in Figure 8.

The application of pressure during the adhesive curing process ensures that there are good electrical contact points and the adhesive 3 retains the foil 2 in position.

The conduit described is particularly suitable for use in aircraft, racing cars and other areas where a high degree of electrical safety is required in combination with extremely light weight.

The arrangement provides effective 3600 shielding without requiring the significant weight of a metallic conduit.

Claims (10)

1. An electrically shielded conduit comprising a non-metallic inner tube having a covering of metallic foil adhered to that tube by adhesive.

2. A conduit as claimed in Claim 1, wherein the tube is flexible or semi-flexible and the adhesive is flexible or semi-flexible.

3. A conduit as claimed in Claim 1 or Claim 2, wherein the foil is wrapped around the length of the tube and joined with a longitudinal joint.

4. A conduit as claimed in Claim 3, wherein the foil overlaps at the joint.

5. A conduit as claimed in Claim 4, wherein the joint is pressure bonded.

6. A conduit as claimed in any one of the preceding claims, wherein the foil is provided with formations.

7. A conduit as claimed in Claim 6, wherein the formations are in the form of hills" and "valleys".

8. A conduit as claimed in Claim 6 or Claim 7, wherein the foil is embossed with a pattern.

9. A conduit as claimed in Claim 8, wherein the pattern forms a zig-zag in section.

10. An electrically shielded conduit substantially as hereinbefore described with reference to the accompanying drawings.