Shielding’ or ‘screening’ is a vital part of a cable’s construction, but why do cables need it, what types are available, and what are the benefits of each type? These are some of the key questions that we’ll answer in this blog.
Why do cables need screens?
All electrical equipment emits some form of electromagnetic signal. This signal is known as electromagnetic interference (EMI) or radio frequency interference (RFI). These signals are essentially disturbance, and can affect other pieces of electrical equipment nearby including cables.
External electromagnetic interference can lower the performance of a cable or stop it working altogether. This is most noticeable in cables carrying data where it can increase error rates, and in extreme cases cause the complete loss of data. The reverse is also true. Cables themselves emit electromagnetic interference. Even pairs within the same cable can cause interference with each other. A screen helps to keep the signal consistent, as well as reducing signal and data loss..
What types of screens are available?
Foil Screens
Foil screens can be made from either aluminium or copper and they usually have a plastic backing to provide strength. This plastic is commonly polyester or mylar and is normally identifiable from the metallic part because it is a different colour, often blue. Foil screens are an effective method of reducing electromagnetic interference, being both cheap to manufacture and easy to terminate.
Braid Screens
Braid screens can be made from different materials - usually either bare copper, tinned copper or aluminium wires laid together in a weave. Both bare and tinned copper are much more effective at reducing electromagnetic interference than aluminium.
How are different screens applied in cables?
Foil Screen
A foil screen is wrapped helically or longitudinally around a twisted pair, core bundle, or dielectric with an overlap to ensure that the coverage is over 100%. The overlap ensures that the signal loss is kept to a minimum. This type of screen is easy to terminate and earth when in contact with a drain wire, and is good at keeping out electromagnetic interference and reducing signal loss over long distances.
Braid Screen
Braid screens can vary in their construction and coverage, which can be as low as 25% and as high as 98%. The higher the braid coverage the more effective it is at keeping signal loss and interference to a minimum.
Braid screens are usually used as an overall screen around all of the cores or pairs in a data or control cable, or over the dielectric or foil screen on a coaxial cable.
Braid screens do have a couple of key drawbacks that it’s useful to be aware of. The manufacturing process is slow, so they cost more to produce. They can also be less flexible, depending on the material chosen and the density of the braid screen.
Foil & Braid Screens
Some cables have both a foil and braid screens. On high performance data cables each pair can be encased in a foil screen and then have an overall braid screen around all of the pairs. This type of screening achieves the best results for reducing both data loss and external interference. When transferring large amounts of data over longer distances it’s wise to choose a cable that has a high level of screening.
Which Cables Have Screens?
Lots of cables have a screen, but data cables, coaxial cables, and control cables are the most likely to include one:
Data Cables
In a data cable it’s critical to protect the data from signal loss, cross talk, and external interference. The higher the specification of data cable used the more likely it is to have a screen. The more critical the transfer of data is, the more important it is to have a screen. In data cables each pair can be screened individually, or there can be an overall screen, or there can be both. The individually screened pairs are most likely to be foil screened, however the overall screen can be either a foil, a braid, or both combined to give the ultimate interference-free protection.
Coaxial Cables
The screen on a coaxial cable has several important roles. Aside from reducing external interference it plays a critical role in the electrical performance of the cable. The screen on a coaxial cable is situated on top of the dielectric, and can be either metal braids or metal tapes and braids. The braid on a coaxial cable has a profound effect on the attenuation and return loss of the cable. The better the braid structure and material used, the better the attenuation and the lower the return loss. For instance, bare copper tapes and braids create better attenuation results and lower return loss than aluminium braid wires with aluminium tapes.
Control Cables
Control cable screens are like data cables.They are there to stop signal leakage signal from the cable itself, as well as reduce interference from external sources. These are made from a combination of aluminium foils and tinned copper braids around either pairs or cores.
Cost versus Performance
as always, cost does play a significant factor in decision making, however the longevity of a product should also be considered.
Collectively, we’re all adding more equipment to our networks and infrastructure that emit electromagnetic interference. Not surprisingly, they all play a part in disrupting the signals within the network. Therefore, it's important to recognise whereabouts in the installation it would be most effective to mitigate these external forces without having to terminate and re-install new cables in the future.
Conclusion
Screens come in many different guises across a variety of cables. They’re a critical part of a cable’s design and ensure that equipment functions optimally without interference, distortion, or complete loss of signal.
Need technical help & advice to choose the right cable for your project? Talk to our dedicated Webro Team for expert advice, sales@webro.com or call 0115 972 4483. Or you can get in touch.
