About Cable Shielding: In today’s world, where cables are running all over, powering our devices and enabling different functions, it’s more important than ever to ensure the protection of your cables. Hence, the a need for cable shielding to protect cables from noise interference and external hazards such as moisture. The article below will discuss all there is to know about cable shielding.
Why Do We Need Cable Shielding?
Most industrial applications expose wires and cables to electrical noise. Electrical noise is either conducted or radiated as EMI (electromagnetic interference) that could significantly affect the performance of various other equipment.
Cable insulation only protects the wire and cables environmentally (from spills and moisture) and mechanically (against abrasion and scraps). However, insulation is invisible to electromagnetic energy; therefore, it doesn’t offer any protection against electromagnetic interference.
As a result, you need cable shielding to fend off the effects of electromagnetic interference.
Cables could be the main cause of EMI transfer as the receiver and source. As the source, your cable could either radiate or conduct noise to other components as an antenna.
On the other hand, as the receiver, your cable could pick up EMI radiating from other sources. Luckily, cable shielding protects against all the causes of electromagnetic interference.
Various noise levels
| Noise Level | Noise Source | Typical Location |
| Low | Wiring found far from motors or power lines. Nearby power or control relays. | Light assembly operations, offices, storage areas, and labs. |
| Medium | Wiring near control relays and medium-sized motors. | Average manufacturing plants. |
| High | Nearby are heavy motors, electrolytic processes, induction heating, generators, control wires and power lines, transformers, and relay controls. | Heavy processing plants like foundries and steel mills. |
The main way to handle electromagnetic interference in cables is via cable shielding. A shield surrounds any inner power or signal-carrying conductors.
The shield combats EMI by picking up the noise, conducting it to the ground, or reflecting the energy. In both methods, the conductors aren’t affected by EMI. Although some energy still sips through the shielding, it’s heavily attenuated to cause any effects.
Types of Cable Shielding
Below are the four main types of shielding you can use for cables.
Foil Shielding
Foil shielding uses a thin aluminum layer that normally attaches to a carrier like polyester to increase ruggedness and strength, providing full coverage of the surrounding conductors.
However, its thin design makes it difficult to use, especially when you want to apply a connector. Normally, instead of trying to ground the whole shield, you can use a drain wire to ground and terminate the shield.
Foil Shielding
Braid Shielding
Braids are woven meshes of tinned or bare copper wires that provide a low-resistance way to ground and are easier to terminate via soldering or crimping when you want to attach a connector. Braids normally provide 70%—95% coverage, depending on the weave’s tightness.
If your cable is stationary, then 70% coverage should be sufficient. You can’t notice an increase in effectiveness even if you have a higher percentage of coverage.
Since copper offers better conductivity than aluminum, the braid is better at conducting noise, increasing the shield’s effectiveness. However, with all the benefits come the demerits of an increase in the cost and size of the cable.
Braid Shielding
Serve or Spiral Shields
A serve or spiral shield is normally made from bare silver or tin-plated copper strands, but steel and other metals can also be used for physical protection. The main benefit of serve or spiral shields is that they are easier to terminate and offer better flexibility than braids.
The shield is made by wrapping a shielding material in both directions on different layers without interweaving the wires. Serve or spiral shields can offer a shield coverage of up to 95% or more. Because of the frequencies they shield against, they are ideal for audio applications.
Tape Shields
Tape shields are made of various materials, most commonly aluminum, bronze, and copper. You apply tape shields to the cable and wrap them around the conductors you want to shield. For easier termination, you need a drain wire. You can combine aluminum and copper with a backing like polyester or mylar. You can get a shield coverage of up to 100% with tape shields.
A Detailed Comparison of Cable Shielding
| Foil | Spiral | Braid | |
| Description | Metal foil is laminated to a thermoplastic layer wrapped around a core with overlap. | The metal wire is helically wrapped around a core. | The metal wire is interwoven to form a tubular structure. It could also form a rectangular cross-section. |
| Single-End AWG Range | N/A | 36 – 52 AWG | 30 – 46 AWG |
| Core Size | 0.25” and larger | 0.004” – 0.450” | 0.012” and higher |
| Typical Coverage | 100% | 90% – 95% | 90% |
| Flex Life – Continuous Flexing | Poor | Poor | Good |
| Flexibility | Poor | Very Good | Good |
| Ease of Termination | Difficult – often, you’ll need a drain wire. | No interweaving; therefore, it’s easier to terminate using multiple strands twisted together or individual shield wires. | You can separate and terminate braided wire. However, it is difficult, and you’ll often need a drain wire. |
| Normal Frequency Range | High frequencies (over 100MHz) | Low frequencies (up to 1 MHz) | Low – mid frequencies (up to 100 MHz) |
| Advantages | Most flexible option. Less build compared to braid.You can easily manufacture with 95% shield coverage. Less weight. | Offers 100% shield coverage, enabling effectiveness at high frequencies. If you use aluminum, it offers low cost and weight. | Lowest resistance.Offers structural integrity while maintaining good flexibility. Proved cut-through and crush resistance. |
| Concerns | Most flexible option. Less build compared to braid. You can easily manufacture with 95% shield coverage. Less weight. | Inductive effect due to shield wires helically wrapped around the core.If flexed or bent, the spiral could unravel, reducing effectiveness to lower frequencies. | The greatest increase in OD.The braid pattern limits max shield coverage to 95%, limiting effectiveness in high frequencies. |
Degrees of Cable Shielding
Cables offer various degrees of shielding and different degrees of shielding effectiveness. The level of shielding you need depends on several factors;
- Cost of the cable
- The electrical environment of the cable
- Cable flexibility
- Cable diameter
- Cable weight
You can use an unshielded cable for industrial applications in controlled environments, for example, inside a conduit or metal cabinet where the cable is protected from surrounding EMI. The enclosure serves as a shield for the electronics inside.
You can use multiple shielding layers for excessively noisy environments. Often, manufacturers use both braid and foil. Individual pairs are shielded in multiconductor cables to prevent crosstalk between the pairs while the whole cable has with braid, foil, or both. Also, cables can use two layers of braid or foil.
In practice, the shield aims to ground any noise picked up. A cable’s shielding and termination should provide a low-impedance path to the ground. If a shielded cable isn’t grounded, it reduces its effectiveness. Any disturbances in the path could raise the impedance, lowering the shield’s effectiveness.
Practical Guidelines For Effective Cable Shielding
- Ensure your cable offers sufficient shielding for your application. In a not-so-noisy environment, a foil shielding should provide enough shielding. If your application exposes your cable to a very noisy environment, opting for a braid or braid–foil combination is advisable.
- Use a cable that is ideal for your application. If your application intends to expose your cable to repeated flexing, opting for a spiral shield over a braid shield is best. It’s advisable to avoid foil shields on flex cables because the constant flexing could rip the foil.
- Ensure the equipment you connect your cable to is correctly grounded. Use an earth ground whenever possible and inspect the connection between your equipment and the ground point.
- The majority of connector designs enable complete 3600 terminations of their shield. Therefore, ensure the connector offers shielding as effectively as your cable’s. For instance, many connectors are available with aluminum backshells, metal-coated plastic, or cast zinc. Avoid over-specifying and paying for more than what you need. However, you should also watch out for under-specifying and receiving poor shielding effectiveness.
- Ground your cable on one end, eliminating the potential for noise-inducing ground loops. Shielding systems are only as good as their weakest component. A quality cable is of no essence if you attach a low-quality conductor. The same applies if you connect a high-quality conductor to a poor-quality cable.
Conclusion
There you have it; all you need to know about cable shielding, from the various options and their benefits to the different degrees of cable shielding. For all your cable shielding termination needs,o contact Cloom Tech.
