EMI Shielding for Cables
An electric cable can both radiate and receive or—conduct—electromagnetic interference. Thus, EMI shielding for cables is often a necessity when it comes to protecting electrical and electronic systems from interference. This article explains EMI shielded cables and their use in both power and signal cables.
Table of Contents
What is a Shielded Cable?
Cable shielding is a common (and primary) way to protect power and signal cables from EMI. Before we can explore what shielding cables from interference means, let’s first understand what a shielded cable exactly is and where to use it. Also, why it’s important that cables are protected from interference, whether conducted and radiated.
Shielded Cable Meaning
A shielded cable is used to mean a cable that offers protection from electromagnetic interference. It typically consists of an inner conductor (or conductors) surrounded by an EMI shield, which can be braided wires or foil, and an outer layer of insulation.
The EMI cable shielding jacket is typically made of a conductive material, usually copper or aluminum. This acts as a barrier against interference, while the outer insulation helps to protect the cable from environmental hazards such as moisture, heat, and chemical exposure.
Why is Wire Shielding Used?
Here is why shielding is used for cable protection: An electrical conductor, based on the law of induction, will act as an antenna or receiver. That means it possesses the ability to pick up or radiate electromagnetic signals.
A power or signal cable, therefore, has the potential of receiving electromagnetic radiation from nearby sources and carrying it to the circuitry, as mentioned earlier. When acting like an antenna, it sends out EMI waves from itself and other pieces of equipment.
An EMI cable shielding wrap helps prevent this effect by blocking the radiation of energy between the inner conductor and its surroundings. The shield will also act as a path of low resistance to ground and reduce EM noise.
Shielded Vs. Unshielded Cable
The difference between a shielded vs. unshielded cable is that the latter does not have any protection from EMI radiation. As a result, it is much more susceptible to interference from external sources.
In comparison, a shielded cable will absorb or reflect any incoming electromagnetic signals and protect the inner conductor from unwanted noise. This helps ensure that the signals that travel along the cable don’t suffer from interference or distortion.
When used in conditions where EMI levels are high, unshielded cables must be placed inside shielding conduits, used in shielded enclosures, or have other measures taken to protect them from external sources of interference.
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What is Shielded Cable Used for?
Wire shielding for cables is necessary where sensitive signals or electric power need to be transmitted without interference. This includes data transfer and power transmission in industrial systems, home automated devices, aircraft and other high-end applications.
Shielded Signal Wire
Most often, an EMI shield is used for signal cables. These are cables that may be used to transmit sensitive digital data, such as those used in computer systems, telecommunications equipment, and other electronic devices. In these applications, data accuracy is paramount. Examples include the following:
- Shielded Ethernet cable
- EMI Shielded USB cable
- Shielded audio cable
The shield prevents distortion of these signals due to outside interference, ensuring they are transmitted accurately. That’s because a shielded signal wire will not act as a receiver and pick up unwanted noise from other sources, such as radio waves, or act as an antenna and send EMI.
EMI Shielded Power Cables
EMI shielding for power cables is also sometimes a necessity in certain applications. For example, in situations where power lines carry higher voltages, these cables may be shielded to prevent disruptive or hazardous occurrences such as electric shocks.
Motor cables, for example, can cause electrical interference to the motor control circuits and sensors if not properly shielded. EMI shielded power cables are also used in applications such as medical imaging equipment, industrial robots, and other sensitive electronic systems.
The shielding of high voltage wires helps ensure the dielectric field inside the cable insulation does not leak to the outside, thereby reducing the risk of electric shock. The shielded power cable may use copper or aluminum as the protective material, sometimes also polymer.
EMI Cable Shielding Techniques
EMI cable shielding techniques typically involve either braided material (mesh) or metal foil. These two materials offer different protection levels, and suit different types of applications. Most often, both braid and foil are used in combination for the most effective protection.
Cable Shielding Mesh
Braided shields are made from copper and aluminum wires and cover the entire circumference of the cable, providing 360-degree protection against EMI radiation. Due to its nature, this type of EMI shielding for cable protection can usually only offer between 70% and 95% coverage.
This is often enough, although some situations call for a more comprehensive protection. In these cases, combination of braid and foil shields can be used for maximum efficiency. Note that one of the major benefits of using braided cable shielding wrap is that it allows for flexibility and cable termination.
Cable Shielding Tape
Cable shielding tape or foil is a thin aluminum foil or conductive fabric that is wrapped around the cable core. This type of EMI shielding for cable protection is best when cable must flex a lot. In such conditions, the foil is wound spirally around the cable core, allowing for flexibility while also offering shielding.
The foil wire shielding wrap is usually very thin, but can still break if subjected to too much stress. A combination of foil and mesh is thus often needed for most flexible cables. Also, when a cable consists of several wires, these may be shielded with foil and then encased in a braided shield (or both mesh and foil).
How to Ground Shielded Cable
Grounding a shielded cable is essential for it to be effective. A proper ground connection ensures that any EMI interference the cable picks up is diverted to ground and away from the sensitive circuits it connects to.
The common practice when it comes to shielded cable grounding is to only ground one end of the cable. However, in cases where the shield is part of the circuit’s signal path, it is important to ground both ends of the cable. This is often the case in coaxial cables.
The ground connection should be made as close to the cable entry point as possible. This will help ensure that the entire length of the cable is properly grounded for maximum effectiveness. The ground connection can be made using grounding clamps.
Conclusion
EMI Shielding for cables is usually a requirement when it comes to protecting sensitive applications and high-end electronics. Depending on the situation at hand, this can be done using braided mesh, metalized foil, or a combination of bot. In most cases, using braid alongside foil offers better results: it provides the best of both worlds.