Why use Cable Screening?
In today’s world almost everyone and everything comes into contact with an increasing level of sophisticated electrical and electronic equipment which generate EMI (Electromagnetic Interference) or EMP (Electromagnetic Pulses) to the extent that Legislation now exists to ensure that wherever possible all equipment and ancillary components are compliant with EMC (Electromagnetic Compatibility) rules. The objective of EMC is to address the minimising of the sources of EMI and EMP and also the protection from Interference, through good product design.
Consequently this requirement applies to an increasing range of Cables where their application or operational environment means they are likely to be a source of interference or will need protection from interference. Fortunately in the cable industry there exist a wide range of solutions to address a broad range of frequency bandwidths from simple cross-talk to Nuclear EMP Shields. However this does require that each cable needs to be designed to meet its obligations with respect to EMC whilst retaining its Physical and Mechanical performance properties at an appropriate cost.
Solutions
The Solutions range from the Simple Aluminium or Copper Foil tape to multiple Optimised braids and the use of a variety of alternative Ferro-magnetic wraps or different metallic wires. However as is to be expected the more sophisticated the solution the higher the cost and this is were the Customer has to balance the cost against the cables performance in its chosen application.
As a general guide a basic Aluminium Foil will provide an electrostatic screening at a low cost as this can usually be applied during the Twisting or lay-up process. Perhaps next in line comes Helically Lapped Copper wires(Spiral Screen) which is ideal for individual core screening or as an overall screen were cable flexibility is important and when converting the cable into a retractable coil lead.
Moving down the Screening hierarchy we now come to the traditional (Woven) Braid wire screen, using a range of wire types from plain or tinned copper wires to Silver and complex specialised metals and cotton fillers. This form of Braid Screen offers endless combinations relating to the % coverage required e.g. 50 to100% which in turn is determined by the number and size of the individual wires required .As a rule high wire counts of fine wires makes for a more flexible cable whilst giving a high coverage factor.
Options
Other factors come into play depending upon the application and overall size of the cable, for example were very small cables for use in weight sensitive applications (aircraft) still require a high shielding factor the use of multiple specialised wires will need to be specified and costed accordingly. At the other end of the spectrum large composite cables that carry both power and signal wires may have a combination of Braiding wire sizes to give both strength and effective screening for both an interference and protection role within the cable?
From here the design moves onto a combination of multiple braid layers with additional layers of a metallised foil between each braid. These combinations will be dictated by the severity of the increasing levels of EMI from low with a single braid screen, through high and severe EMI levels up to the severest levels of EMP which could have 3 braids with layers of Moralised Tapes.
As you can see the problems of shielding cables are complex, however by involving the cable manufacturer at an early stage can usually result in the provision of a cable that will meet its EMC obligations.
