What does screening mean and what are the advantages and disadvantages of different types of screening?
Foil screens are light and made of comparatively cheap material that increase the cable diameter only insignificantly. They show a low conductivity compared with other cable conductors. Primary a foil screen is applied with low disturbances. As such a screen covers the cable 100 % in case of an appropriate production they are very effective with high frequencies (GHz). In case of mechanical stress however, this construction type is prone to wear.
Foil screen cables Examples
Metal wire shields are braided around the cable by counter rotation with the help of special machines and multiple wire bundles, the so called folding. The resulting cable section shows the highest conductivity compared to all screening types and therefore, is especially appropriate for high disturbance energy. At the intersections of the wire bundles (folding) gaps occur that limit the effectiveness with high frequencies (>100 MHz). Equally high torsional stress causes restrictions for the braiding of the folding.
Metal wire shields cables Examples
Wrapping with metal wires is simply a “half braiding”. The copper wires are wrapped in one direction around the cable. In this way only a smaller section results compared with a metal braiding. As there is no overlapping, there is a higher flexibility for torsional applications. In case of high alternating bending stress however, gaps between the wires may occur more easily than the intersections of a braiding allow.
Cables with wrapping Examples
A filler as wire or strand is a conductor that is in electric contact with the screen along the cable core. First of all it is used to contact the screen more easily as it is more difficult to contact foil screens with the help of usual connection techniques and second it is used if the wrapping or braiding shall be connected to a pin contact of a plug.
Furthermore, there are still other screening types that are used less frequently: foils made of magnetic materials (μ-metal) are especially effective with strong magnetic fields. Metallised nonwoven tapes reduce the wear of foils but have a small conductivity themselves. Most of the mentioned measures can be combined. Moreover, all of them offer adjustment options with regard to construction details or production processes to the required shielding effects and mechanical stress.