To protect cable circuits in steel or plastic sheaths from electromagnetic influence (EMI), a tape shield is laid over the wrapping. In case aluminium or copper sheath, the sheath takes shield functions due to the less electrical resistance: the first because of the high electrical conduction, the second – relatively large thickness.
A shield can be made of soft copper or aluminium tape. It is laid on the core spirally or longwise. The last method is more processable because it isn’t connected with rotational motion. In spiral laying it is overlapping by width 10…15%. Longwise shield can be smooth or ridged. Ridged shield is more resistive to multiple curving, though it takes additional 10…15% of material as to smooth. The flute high is 0.8…1.0 mm.
Aluminium shield is wide used. Soft aluminium foil paper (0.10…0.15 mm thickness, more rarely up to 0.20 mm) is a material for it. The basic design data of aluminium foil is shown in table below. UTS is minimum 40 MPa (kgf/mm2), elongation percent is minimum 3%. The foil is made with width 10….500 mm and 5 mm intervals.
To prevent the connection of steel and aluminium in steel cover cable, a separation sheathing (paper or plastic tapes) is laid.
Cross shield made of alumopolyethylene stripe has much more high physical and mechanical properties. It is constitute an aluminium foil, covered with polyethylene film (thickness 0.03±0.01) on one or both sides. Double-coated tape is laid on wrapping with metal-side inwards and polyethylene – outboard.
Shield of double- or tree-layered alumopolyethylene type is used with polyethylene cladding, The shield is welded from the inside while the overlapping process of polyethylene cladding. When a shield core comes through the extruding head (200 C° in it ), polyethylene layer of alumopolyethylene tape heats and welds with a sheath, formed in the head. This way a monolithic construction of aluminium sheath, covered with metal pad from the inside, appears. This type of sheath is called aluminium polyethylene sheath.
For example, an aluminium-polyethylene double coated tape (aluminum foil thickness 0.10 and 0.15 mm) is used in Russia. Tape width is 1.1…1.2 from core perimeter.
Aluminium shield, which is one unit with the sheath, is characterized as a one with higher repeated bending resistibility, if to compare with a single shield. Also it increases sheath moisture resistance, because it works as a barrier between cable inside and vapor diffusion of wet.