Table of contents
- Cable shielding: Why is it so important?
- How does electromagnetic interference occur?
- What is shielding and what is it used for?
- Shielding cables: Is it always necessary?
- Types of shielding and their functions
- The importance of cable shielding
- Tips for selecting the right shielded cable
- Conclusion: The importance of good shielding for interference-free signal transmission
Cable shielding: Why is it so important?
To shield cables or not? In other words: should you buy shielded cables or is the unshielded version sufficient?
These questions are of great importance if cables are needed for an electrical system or are to be optimised. Unfortunately, there is no general answer to these questions, but there are answers for individual cases. In principle, the shielding of a cable has two functions: It should protect the cable from electromagnetic interference and thus optimise data transmission in the case of data cables, for example. At the same time, it should prevent electromagnetic interference from the cable affecting other cables or components. There are various solutions for cable shielding. This guide explains what these are. It also clarifies the areas in which cables should be shielded and where this is not
How does electromagnetic interference occur?
Electromagnetic interference can occur within an electrical system or affect the system from the outside. We can distinguish between different types of coupling. In this context, the term ‘coupling’ refers to electromagnetic connections.
Galvanic coupling
In the case of galvanic coupling, the source of interference and the interference sink are connected to each other by some element, for example, by a common supply line. If the supply line supplies two circuits with power and a current change occurs in one of the circuits, the resulting voltage drop also affects the second circuit and electromagnetic interference occurs. If you want to avoid galvanic coupling, countermeasures such as separating circuits whenever possible are useful. If a common supply line cannot be avoided, it should be as short as possible and have a sufficiently large conductor cross-section.
Capacitive coupling
In contrast to galvanic coupling, the source and sink of interference are not directly connected in the case of capacitive coupling. They are only in close proximity to each other. One example is two cables, one of which influences the other due to the electric field that arises. Capacitive coupling occurs primarily with high-frequency interference signals. This makes it all the more important to have the right fundamentals and measures in place to prevent capacitive coupling. The most important countermeasures for minimising the effects of capacitive coupling are shielded cabling, including cable glands, or a cable management system (CMS) that has been specially developed for the reduction of electromagnetic interference.
Inductive coupling
In the case of inductive coupling, the source and sink of interference are also not directly connected to each other. However, the interference influence here is the magnetic field instead of the electrical field. Inductive couplings are more likely to be observed at low frequencies. What is particularly interesting in connection with the topic of cable shielding is that cables can be influenced inductively or capacitively. The example of capacitive and inductive coupling shows that both electrical and magnetic interference can occur. It is also important to note that while electric fields are caused by voltage, magnetic fields are caused by current flow. As a countermeasure, cables can be shielded in both cases or shielded cables can be used. Such cables are suitable for combating capacitive coupling and the inductive variant. A few more words about the terms inductance (also: magnetic capacitance) and capacitance:
- cable inductivity is influenced by the structure of the magnetic field during the current flow and by the storage of energy in this field. If the current flow changes, a voltage is created in the cable or in an adjacent wire (inductive coupling). Parallel conductors can therefore be influenced by inductivity.
- So does cable capacitance. This describes the possible property of a cable to store electrons in an electrical field. A high capacitance can lead to an intense capacitive coupling. If there is a suspicion of such a coupling, it makes sense to measure the capacitance of the cable. Countermeasures? Capacitive coupling can be avoided with shielding, but also by increasing the distance between the cables. Twisted-pair wires are another way of preventing capacitive coupling. It is also important to remember that capacitance plays an important role in the manufacture of cables. This is due to the interaction between the inductance and resistance of the cable, whereby the quality of the signal transmitted by the cable is also determined by the insulation and the actual geometry of the cable.
Radiation coupling
The couplings presented so far are conducted. In addition, there is the non-conducted interference caused by radiation coupling. If, for example, a refrigerator and radio are connected to the power cable network and the refrigerator causes the radio to crackle, this is a conducted interference.
If, on the other hand, a smartphone's wireless data transmission interferes with a landline phone call, we speak of a non-conducted interference caused by radiation coupling. The widespread use of smartphones, Wi-Fi connections and the Internet of Things (IoT) with devices such as smartwatches is increasingly drawing attention to this issue.
What is shielding and what is it used for?
The shielding of cables helps to prevent the negative effects of electromagnetic interference. It works like a Faraday cage. However, especially in very sensitive systems, it is only one part of a safety plan that consists of several measures. For example, you can use EMC filters and plan electrical systems so that potential sources of strong interference and sensitive elements such as control cables are separated from each other by The overall concept is crucial.
Shielding effectiveness and transfer impedance
Shielding effectiveness is used as a measure to assess the quality of a cable shield. It is dimensionless, but is usually given in decibels (dB). According to the German Association for EMC Technology, 20 dB is a shielding effectiveness of minimal quality. 40 to 60 dB represents medium quality and 80 to 100 dB very good quality. Transfer impedance is another measurement for cable shielding. It was previously referred to as coupling resistance. Its unit of measurement is milliohms per metre [mΩ/m]. The following applies: a lower transfer impedance indicates a higher shielding performance. The effectiveness of shielding is specified as transfer impedance up to a frequency of 30 megahertz, and as shielding effectiveness at higher frequencies.
Shielding cables: is it always necessary?
How useful cable shielding is depends on two factors:
- The answer to this question depends on the risk of possible interference and
- the possible consequences of potential interference.
Two examples: The transmission of video files is prone to errors, which is why cable shielding often makes sense here due to the high risk of interference. There is also a high risk in riser shafts in apartment buildings or office buildings, where many cables are close together. In sensitive areas such as industrial production or in the healthcare sector, the consequences of interference can be very high. There is a risk of considerable costs or even loss of life. Here, too, it is important to protect the systems, which is why data or power cables should be shielded.
There is no general answer to the question of whether to use shielded or unshielded cables. This also applies to everyday IT applications. For example, if you connect a mouse to a PC with a cable, a shielded cable is usually unnecessary. The situation is different for external hard drives, because shielded cables can ensure stable data transmission without interference. If you want to use shielded cables where they are not necessary, it is not a serious mistake. But it is not sensible and makes the cabling more expensive. Often, shielded cables are also less flexible than unshielded ones: another argument against unnecessary use.
The different types of shielding and their functions
There are various ways to shield cables. Two of them are shielding with a braided shield and with foils. The braiding, for example, is made of copper and wraps around the cable. Foils made of materials such as copper or aluminium do the same. However, the wrapping is more complete here. In other words, the degree of coverage (also: surface coverage), which is given as a percentage, is higher. It indicates the proportion of the wire that is covered by the shielding. A higher degree of coverage means better shielding, which can be further improved by using a suitable EMC cable gland. Today, a degree of coverage of at least 75 to 80 per cent is considered useful. Shielding with a braided shield is particularly effective at low frequencies, while the foil provides good shielding against high-frequency fields. For your information: low-frequency fields have a frequency range of 0.1 hertz to 100 kilohertz. High-frequency fields range from 100 kilohertz to 300 gigahertz. Foil and braided shielding can be combined to achieve the most complete shielding possible. The decisive factor here is that, to prevent conducted interference, the shielding must be effective at 50 hertz fields. And why is this frequency particularly important? 50 hertz is the regular frequency of alternating current in Germany. This means that the electrical and magnetic fields generated by this current also have a frequency of 50 hertz.
What is the significance of shielding for cables?
To take a closer look at the various ways of shielding a cable, it is important to understand the basics of cable construction. A classic network cable is used here as an example. It consists of eight cores, two of which are twisted together. These cables are called twisted-pair cables. The twisting of the cables already provides protection against external interference, which is reinforced by the shielding. Both the individual wire pairs and the entirety of all wires can be shielded. The way in which manufacturers shield their cables can be recognised by abbreviations on the network cables.
Cable and cable shielding abbreviations
Three letters stand for common shielding variants
- F: Foil
- S: Shielding
- U: Unshielded
You can distinguish between UTP and FTP cables by the foil shielding of individual wire pairs in network cables. The individual wire pairs are protected with foil in FTP cables, but not in UTP cables.
You will find combined designations such as ‘S/UTP’ in stores. The letter before the forward slash indicates whether and how materials shield the entire cable with all wire pairs. In the case of an ‘S/UTP’ cable, this shielding is realised with a metal mesh. The ‘UTP’ in this designation indicates that the individual wire pairs have not been shielded.
Two more examples. With a U/UTP cable, neither the entire cable (U) nor the individual wire pairs (UTP) are shielded. This is therefore a completely unshielded cable. This does not mean that it is unusable. It is perfectly adequate for many everyday applications. On the other hand, there are S/FTP cables, which are excellently shielded: by a metal braid around all wire pairs, which are additionally individually protected with foil. The same principle can be used to explain other cable designations, for example F/UTP, SF/UTP (shared braiding AND foil around all wire pairs) or F/FTP.
Shielding for single-core cables
Shielding is not only useful for multi-core cables, but also for single-core cables. A single-core shielded cable is, for example, the line cable in a recording studio that connects elements such as the mixing desk and effects units.
Single-sided and double-sided shielding
A distinction is also made between single-sided and double-sided shielding for cables. This refers to cables that are grounded on one or two sides. While the single-sided grounded variant is suitable for protection against interference from electric fields, the double-sided grounding protects against magnetic fields. To shield against both types of field, including an electric field, triax cables are used with an inner, single-sided shield and an outer, double-sided shield.
Retrofitting cables: Does it make sense?
If you want to prevent interference from electromagnetic waves, buying cables with integrated shielding is the best choice. However, cables without shielding can also be retrofitted. Shielding cable conduit, for example, is used for this purpose. Suitable materials include a shielding braid made of copper (protection against magnetic influences) and polymer plastics (protection against electrostatic discharges). The trade offers, among other things, stretchable braided sleeves with a flexible copper braid. The sleeve can be bent relatively sharply without the braiding breaking.
- Situations in which cable shielding is particularly important
- Difference (advantages and disadvantages) between shielded and unshielded cables
- Advantages of using double-shielded cables
- Practical examples of cable shielding in everyday life
Factors that influence the effectiveness of cable shielding
Generally speaking, the more cables are in close proximity to each other, the stronger the shielding should be. And the more the cable is mechanically affected, for example by automated machines, the more likely it is that the cable seal will suffer. Protection against electromagnetic interference can suffer as a result.
Tips for selecting the right shielded cable for your specific needs
In principle, the two factors already mentioned should be used to determine
- the risk of possible interference
- the possible consequences of potential interference
how good the cable shielding should be. The higher the risk and the more problematic the possible consequences of interference, the better the shielding should be. The type and quality of the shielding can be seen on network cables by the letter combinations presented (e.g. F/FTP). However, when buying shielded cables, it is not just about the shielding. Rather, you should always consider the entire environment in which the cables are laid. In industrial production and robotics, for example, flexible and torsion-resistant cables are important, which is why not every type of shielding is suitable. Some are too stiff.
Steps for installing cable shielding
If you want to prevent interference from electromagnetic waves, buying cables with integrated shielding is the best choice. However, unshielded cables can also be shielded afterwards. Cable conduits, among other things, are used for this purpose. Suitable materials for such sleeves include a copper shield (protection against magnetic influences) and polymer plastics (protection against electrostatic discharges). For retrofitting shielding, the trade offers, among other things, expandable braided sleeves with a flexible copper braiding. Such a sleeve can be bent relatively sharply without the braiding breaking. But how do you apply the cable shielding correctly?
Power cable shielding: A step-by-step guide to setting up cable shielding can help you to shield a cable correctly. For good shielding in the higher frequency range, it is best to attach the shielding of wires on both sides. If you want to connect a cable shielding to a control cabinet, metal cable clamps that cover a large area of the shield are suitable for this purpose. As soon as the wire enters the cabinet, the shield should be placed on a shield rail and extended to the assembly. The following also applies when retroactively placing a cable shield: In areas such as the automation of industrial processes, possible mechanical influences on the cable should be taken into account. If a cable with a braided shield is used that is unsuitable for such applications, the degree of coverage and thus the shielding performance can be significantly reduced.
To connect the shielding of a power cable, proceed as follows:
- First, disconnect the wires
- then twist the shielding
- Ideally, the shielding should be connected to a rail for equipotential bonding
- If this is not possible, the shielding can be connected to the protective earth conductor on the device
How can you check the shielding?
Whether a single-core shielded cable or an unshielded cable: simple cable testers are available to check whether a cable is shielded or not. However, they are usually not sufficient for professional EMC. Test laboratories therefore offer commercial customers a wide range of EMC services. It is possible to measure shielding attenuation using the triaxial method. The basic principle: the cable to be tested is fitted with a plug at one end and a terminal resistor at the other. It is installed in a tube and short-circuited on the side connected to a transmitter. The transmitter supplies energy to the cable. Energy then enters the outer system (tube) from the shielding. The shielding effectiveness can be determined on the basis of this test arrangement.
Conclusion: the importance of good shielding for interference-free signal transmission
Good cable shielding is not necessary for every cabling application, but in many cases it is indispensable. This applies particularly in professional areas of traffic and telecommunications technology, automated production or complex medical technology. But even in the private sphere, it makes sense in some cases if cables are shielded. Suitable solutions are available in stores.
Despite careful editing and review of the contents, Stecker Express does not guarantee the timeliness, correctness, completeness and quality of the information provided.
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