Cable Shielding

Introduction

Electromagnetic interference (EMI) is caused by an electromagnetic field or wave that can be radiated, conducted, induced, or a combination of these. EMI is a serious problem encountered in industries of various sectors as it can alter the operation and damage equipment, devices, or appliances, with some of them being more susceptible to generating and conducting this type of disturbance, such as large motors, generators, transformers, control relays, supply and control lines in close proximity.

Unshielded cables are commonly used in signalling and instrumentation circuits. They are highly recommended for discrete and digital signal communication in controlled environments with low electromagnetic interference levels. Often preferred for their flexibility and low cost, these cables can also be significant contributors to the distribution of external interference, acting as transmitters, carrying noise to other equipment, or acting as an antenna that radiates this noise, and as receivers, receiving interference from other components. To contain the effects of EMI, shielded cables are generally used because, in this way, the noise problem in the mentioned cases is solved, and more excellent reliability in the transmitted data is achieved.

Figure 1 — Unshielded Cable

Types of Shielding

• Electromagnetic Shielding

Used to eliminate electromagnetic noise, it consists of twisting pairs of wires with a balanced and uniform twist rate formed by a series of adjacent circuits. Any magnetic field passing through this pair will tend to be eliminated by these circuits because the currents induced by magnetic fields into the circuits of each wire will be in opposite directions. Twist rates between 50 and 75 mm are the most recommended for electromagnetic attenuation efficiency and production speed, contributing to lower industrial costs.

• Electrostatic Shielding

Proven through an experiment known as Faraday's Cage, electrostatic shielding occurs when a conductor of electricity is electrified, and electric charges are distributed uniformly on its surface until they reach electrostatic equilibrium. One of its properties is that the electric field inside it is null.

Types of Cable Shielding

Shielding for cables consists of wrapping a set of conductors in a metallic shield to reduce the effects of electromagnetic interference or electrical noise, act as an energy reflector (an effect caused by electrostatic shielding) or a grounding point, preventing EMI from reaching the conductors and propagating through them, ensuring safer communication.

Cables can have different levels of shielding, each offering different efficiency. To know which type of shielding is most suitable for each installation, it is necessary to consider the conditions of the installation site, cost, and other aspects required by the project, such as the diameter, weight, and flexibility of the cables.

There are two most commonly used types of shielding, namely aluminum foil shielding and mesh shielding, and a combination of both may also exist.

• Aluminum Foil Shielding

It is made with a thin layer of aluminum, usually combined with a polyester film, to add resistance and rigidity to the material. It offers 100% coverage for the conductor, having less weight, less thickness, and low cost. However, it becomes more challenging to terminate appropriately due to its physical aspects; thus, a drain wire is usually provided to facilitate the termination and grounding of the shield. It is more effective for attenuating high-frequency noise.

Figure 2 — Cable with Aluminum Foil Shielding

• Mesh Shielding

It consists of a mesh of twisted bare or tinned copper wires, one clockwise and the other counterclockwise. It provides a low-resistance path to the ground and is easier to terminate and connect than aluminum foil shielding. It has superior structural integrity while maintaining good flexibility and service life; however, it does not provide 100% coverage of the cable surface area, allowing only 70% to 95% coverage. It is ideal for minimizing low-frequency interference due to its greater thickness and better conductivity than aluminum foil shielding, but it adds size and cost to the cable.

Figure 3 — Cable with Mesh Shielding

• Combination of Aluminum Foil and Mesh Shielding

It stems from the combination of the two shields mentioned above. It has maximum efficiency across the frequency spectrum and combines the advantages of 100% conductor coverage, low resistance, and ease of shielding termination.

Figure 4 — Cable with Combined Shielding

Conclusion

In summary, to have effective shielding, it is essential to consider the following points:

1. Ensure that the shielding is appropriately sized for the application. Only aluminum foil may be sufficient for installation in environments with moderate noise. Still, in boisterous environments, it is advisable to adopt a cable with mesh shielding or a combination of aluminum foil/mesh.
2. Use a cable suitable for the application. In some cases, there is a need for a more flexible cable than in others, becoming an essential factor for both the cable and shielding lifespan.
3. Ensure that the equipment to which the cable is connected is properly grounded. Always check the connection between the equipment and the ground, as a low-resistance path to the ground is necessary for eliminating these noises.
4. Ensure that the connector provides shielding efficiency equivalent to the cable.
5. Ground the shielding at only one point. Thus, the emergence of potential differences between shielding grounding points and the circulation of current through them is avoided.