GB2229313A - Screened electric conductors having metal braid embedded in semi conductive plastics - Google Patents

Abstract

An insulated electrical conductor shielded against external electromagnetic fields by a metal embedded in an electrically partially-conductive layer containing a conductive particulate filler. The plastics layer may be a viscous fluid and its outer surface may be hardened by ultra-violet crosslinking if required. The filler may be carbon-black, graphite, a metal, or combinations of these materials. The plastics material fills the voids between strands.

Description

Screened Electric Conductors This invention relates to screened electric conductors, that is to insulated electric conductors having a shield against external electromagnetic fields.

In order to minimise the interfering effect of external electromagnetic fields on signals being transmitted along an insulated wire, metallic barriers, which are electrically conductive and optionally magnetic, are placed concentrically around the insulation in the form of a braid or tape. The more effective form of screening is provided by a taped metal screen, but air gaps are produced when the wire is flexed, and these air gaps give rise to interference. Metal braids give less effective electromagnetic screening but are much more flexible. However, the areas between the strands of the braid still allow the penetration of interference from external electromagnetic sources.

Accordingly, the invention provides an insulated electrical conductor shielded against external electromagnetic fields by a metal braid embedded in an electrically partially-conductive plastics layer containing a conductive particulate filler. The plastics layer effectively fills the gaps between the strands of the braid, to complement the screening action of the metal braid.

The plastics layer is preferably a viscous fluid, so that it is capable of maintaining its configuration during flexure of the conductor. It it did not maintain its configuration, the relative movement between the insulation, acting as a dielectric, and the partially-conductive plastics layer would give rise to an electrical charge, which would produce microphony.

In some applications, the whole of the plastics layer may be relatively viscous, but it is preferred to harden an outer layer of the plastics layer in order to present a non-tacky, non-adherent external surface. An outer layer or skin of the plastics layer may be hardened by ultra-violet (U-V) crosslinking, and this may be achieved by the use of a U-V-absorptive conductive filler, e.g.

carbon black, so that U-V irradiation of the plastics layer crosslinks only an outer skin.

However, with some materials, microphony may be avoided even though the plastics layer is fully crosslinked, in which case the crosslinking could be achieved by U-V irradiation using a non-U-V-absorptive filler, or by electron beam or thermal irradiation using a black-U-V-absorptive filler.

Suitable plastics materials for the plastics layer are "RIM" polyurethane and low molecular weight silicone resins.

Suitable conductive particulate fillers are carbon black, graphite, metal, and combinations of these materials.

Claims (4)

1. An insulated electrical conductor shielded against external electromagnetic fields by a metal braid embedded in an electrically partially-conductive plastics layer containing a conductive particulate filler.

2. A shielded conductor according to Claim 1, in which the plastics layer is a viscous fluid.

3. A shielded conductor according to Claim 2, in which an outer layer of the plastics layer Is hardened in order to present a non-tacky external surface.

4. A shielded conductor according to any preceding claim, in which the particulate filler comprises carbon black, graphite or metal.