Insulated conductors and conductive layers become indispensable in the production of signal and data cables. Both are necessary since insulation only provides mechanical and environmental protection; it is not effective against electromagnetic energy. Proper electromagnetic interference shielding not only reduces electromagnetic radiation and lowers the impact of electrical noise on signals, but it also protects people and machinery and prevents any crosstalk between two cables that are in close proximity.
Types of Shields for Cables and Wires
Many manufacturers offer different types of shielding for electrical wires and cables. These products have been designed for the purpose of reducing or eliminating the impact of electrical noise on electrical circuits. Shielding from electrical noise mainly is possible via aluminum foil or braiding it with individual strands of aluminum or copper.
The main constituent of a foil shield is aluminum foil that has been laminated to polypropylene or polyester film. This sort of film backing imparts extra mechanical strength and durability to the aluminum, thereby preventing any breakage or tearing of the foil when cables are stressed or flexed. The inclusion of a drain wire helps simplify ground terminations and provides a path for electrostatic energy to get discharged continuously.
Foil shields are usually more flexible and lightweight than their braided counterparts, but their low mechanical strength decreases their lifespan. Foil shields provide 100 percent cable coverage and are highly effective against radio-frequency noise.
Wrapping a wire in a spiral fashion around an inner cable core creates spiral/serve shields. This kind of shielding provides 97 percent coverage but still manages to maintain a high termination ease and flexibility.
As the name suggests, a braided shield includes alternating weaves of aluminum or copper strands. One layer is woven in a clockwise manner, while the other goes anti-clockwise. These shields offer less DC resistance than foil shields and impart structural toughness to the cable. The flexibility of these shields is also high, and they last longer. However, they can never cover the whole cable; only 60 to 95 percent. Thus, they are not recommended against high-frequency noise signals. A braided shield is heavier, resulting in larger cable diameters. Some of them are more difficult to terminate as well.
French Braid Shields
A French braid shield is nothing more than a double spiral tying two spirals together through a single weave. This design makes the shield a lot more flexible than a regular spiral. This variety lasts longer as well compared to regular braided shields. Another major benefit of French braid shields is the reduced triboelectric and microphonic noise.
•Foil Braid: It is a common practice to use foil shielding together with an outer braid. This combination provides greater protection against interference and noise. The mechanical strength of this shield is higher as well.
•Tri-Shield: When a foil braid is combined with an extra layer of outer foil, a tri-shield is formed. The name comes from the design of this shield, which goes foil-braid-foil. The second layer of foil adds an extra barrier to any stray noise signals and enhances the reliability of the shield.
•Quad Shield: Tri-shielding with an additional outer braid – that is what’s known as quad shielding. The second layer of braid provides additional protection. The design of this shield is foil-braid-foil-braid.
When foil or braided shields are used, they isolate the conductors from the stray electromagnetic fields, thereby decreasing the chances of reduced performance, interference, and crosstalk between conductors. Shielding is also effective in preventing any electrostatic pickup of noise signals that have been capacitively coupled from the adjacent conductors via insulation.
Cable and Wire Shielding Variations
Shielding of communication cables is a common practice to avoid any EMI effects on the data transmitted via cables. Moreover, the communication cables are paired and shielding is done individually so that there is no risk of any coupling and crosstalk. Some applications require servo cables, and for these, double or even triple shielding might be a necessity. There are some applications, however, that do not need any shielded cables. For instance, if a cable is used in a cabinet or away from other electrical noise sources, there is no need for shielding as it is already protected against the effects of EMI noise.
Take Your Environment into Consideration
Always ensure that the cable you are using has sufficient shielding as per the requirements of the application. If you’re working in an environment with moderate electrical noise, a foil alone will suffice. However, if the noise is more, consider using braids or foil-braid combinations.
If the application involves flexing the wires and cables, a spirally wrapped shield should ideally be used instead of a braid. However, it is best if you do not resort to foil-only shielding on flex cables as the continuous movement might tear the foil. Thus, the application will determine which cable or wire you choose.
Proper grounding is an absolute necessity. Earth grounds are the best, so use them wherever you can. Make sure you check the connection between the equipment and the ground point. Remember, the lower the resistance to ground, the lower the noise. It’s best to ground cables and wires at one end. This does away with the problem of noise-inducing ground loops.
Proper electromagnetic interference shielding has become a necessity in modern industries that use a lot of cables and wires. The effects of EMI are dangerous, and shielding helps decrease the vulnerability of equipment as well as health to potentially dangerous electrical disturbances.