Using shielded cables

The EMC directive¹, in force from 20th April 2016, determines the conditions that equipment and systems must fulfill when they are to be placed in a shared physical space, and where parts of them radiate electromagnetic waves, and as such it is the first reference that one must consult in order to identify which cables are suitable for an installation.

In addition, given the mechanical characteristics of these cables, during the installation process it is very important to respect the minimum bend radius, as well as the tensile forces on the wire, to ensure that the shielding is not damaged, which would negatively affect the cable's subsequent performance. Furthermore, to make the shielding efficient, there should be continuity along the whole length of the shielding, and it should be earthed at one end only, to avoid the current circulating along it which as a result may couple at the same time.

Finally, it must be kept in mind that an unsuitable earthing system may be the original source of interference waves. It must be ensured that the state of earth impedance at the time of the installations' launch be maintained throughout the service life of the shielded cables.

EMI can be radiated (>30MHz) or conducted (<30MHz). Radiated interference is, for example, what is produced by electric equipment generating or transforming energy, where the electromagnetic interference travels through the air, entering the victim circuit by means of coupling. Conducted interference enters the victim circuit through shared connections, or through cabling or metal parts.

In addition, there can be different types with regard to their source: couplings, which are reactive (<30Mhz), will at the same time be either capacitive (high voltages) if they come from electrical fields, or inductive (large currents) if they come from magnetic fields and finally, radiation if they come from electromagnetic fields.

For these interferences to be produced, three elements have to be present: emitter, receiver, and conductive medium. Therefore, to eliminate the interferences we have three options: remove or reduce the waves, reduce the efficiency of the conductive medium, or protect the receiver.

Although the best option is to remove the emitting source, this is not always possible given that it can be difficult to identify the offending source, or we may be dealing with active signals intrinsic to the system itself, and therefore necessary. In such cases, action must be taken on the other two factors.

Shielded cables

One of the most frequently employed options is to use shielded cables to supply sensitive equipment. Shielded cables are ones designed with incorporated metallic elements that act like a Faraday cage, protecting against electromagnetic interference or electrical noise in both directions: from the cable to its surroundings, reducing wave emissions, as well as from the surroundings to the cable, reducing interference. As such, the aim of cable shielding is to stop electromagnetic flow exiting and keep it from entering. In this way, it works to stop interference emissions just as much as reception.

Usually, the shielding effect is achieved by means of wire braids, concentric wires, or metal layers, each with different shielding properties. Also, shielding can be multiple or simple. Shielding can be applied around the whole of the conductor or partially, between multiple conductors, to avoid crosstalk, which is the effect produced when some of the signals present in one of the circuits, called the disturbing circuit, appears in the other, called the disturbed circuit.

The shielding's efficiency depends on the material that it is made from, its thickness, the type of electromagnetic interference to which it is subjected, its frequency, the distance from the source of interference, continuity of the shielding, and earthing system.

Choosing the type of shielding will depend on the function it is going to serve. For low frequencies, originating from electrical fields, the most effective type is braid shielding, while for high frequencies (higher than 100kHz), coming from electromagnetic fields, tape shielding is the most effective. Braid shields have openings that keep their coverage below 100%, which facilitates heat dissipation from the conductor and increases flexibility, although this also reduces the efficiency of the applied shield given that it allows the passage of electromagnetic waves, which is more crucial for the shielding of magnetic fields than electrical fields.

¹Directive 2014/30/EU of the European Parliament and of the Council of 26th February 2014 on the harmonisation of the laws of the member states relating to electromagnetic compatability

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