Protecting data cables against electromagnetic disturbances

Reducing electromagnetic disturbance — even with power cable and data cable laid quite close together, properly earthed basket tray considerably reduces electromagnetic disturbance — an effect which increases as the cables are separated more widely.
How much protection against electromagnetic interference does data cabling need? MATTHEW WAYS explains describes how independent tests confound common sense.EMC, or electromagnetic compatibility, is becoming more important, especially in modern buildings. The performance of complex equipment and increasingly elaborate and sensitive integrated cabling systems can be compromised by electromagnetic interference. Although cable-support systems are basically passive and not subject directly to the EMC Directive, the type and method of cable containment have an impact on the EMC performance of an installation. Considerable confusion surrounds the subject, with manufacturers and European standards propagating conflicting advice and guidelines. European Standard EN50174-2 sets out basic requirements for the planning, implementation and operation of data-cabling networks for those who procure, install and operate them. The standard includes a section about EMC, but several leading EMC experts in the UK believe its advice is based on an oversimplification of the physical processes involved. Part of the problem is that, as with many aspects of high-frequency data cabling, the subject is inherently complex. It is difficult to understand the speed and volume of data transmission, and it is all too easy to make assumptions that are not based on physical tests or practical experience. EMC basics For there to be interference, three elements must be present. • A source of interference (such as a power cable, switching equipment, lightning or some kind of generator). • A ‘victim’ (such as a data or control cable, a device or terminating equipment). • An electromagnetic coupling channel that allows the disturbance to propagate from the source to the victim. Reducing the effectiveness of this coupling channel reduces the disturbing effect of the source and protects the victim from interference that could reduce signal quality. Modern IT equipment is designed to operate with some interference present, and software is designed to recognise interference and ignore it. If a signal appears corrupted, a computer will ask for it to be re-sent. This could happen many times in a typical network. The only symptom is that the system operates a little more slowly and is less efficient. A more serious problem occurs in sensitive alarm or security systems, where interference can lead to a malfunction, which cannot be tolerated. It is well known that surrounding the victim with metal reduces the electromagnetic disturbance. Some of the disturbing wave is converted into an electrical signal in the metal and will have less impact as long as the metal is properly earthed, preferably every 15 to 20 m. This is why cable management can play an important role in protecting a cabling system from electromagnetic interference and why the continuity of the tray or trunking is so important. The lower the impedance of the joints in the cable-management system, the better the protection of the cables inside. Non-metallic systems or those that do not conduct electricity efficiently, such as cable matting, do not offer significant protection from electromagnetic interference. Theoretically, the cross-section of the containment system should have an impact on its protective capabilities. Many assume that solid-metal trunking protects cables more effectively than, say, ladder or cable tray. European standards say that even the shape of the slot in a cable tray makes a difference. However, there seems to be little evidence to support this, so Cablofil commissioned a series of independent tests to compare the EMC performance of different types and configurations of containment. The first set of tests Engineers at the AEMC Mesures laboratory in France placed a Cat5e data cable in a GTEM cell and subjected it to an electromagnetic field of 30 V/m to simulate electromagnetic disturbance. They plotted the frequency of the field against the induced power in the data cable. The cable was then placed in a number of different cable-containment system. The power in the cable was compared with the reference results from the unprotected cable. The first notable discovery was that wire-mesh cable tray significantly reduced the power induced in the data cable, protecting it from interference. The power induced in the Cablofil-protected cable was then compared with that in the cable surrounded by conventional perforated cable tray. There was no significant difference. This implies that both types of product have similar protection properties in practice. Closed trunking cut the resultant induced power still further compared with open cable tray. There was no significant difference, however, between the power induced in the cable in the trunking and that in the cable in Cablofil with a steel cover. The second set of tests In other independent tests by Cetim Laboratory, a power cable was placed alongside a Cat6 data cable. More than a hundred configurations were tested, and engineers measured and analysed the effects of earthing the containment, changing the cable separation and using dividers for several cable-management products. The results show that power induced in data cables is cut significantly if they are installed in a metallic cable tray or basket — even if the power and data cables are separated by 300 mm. The results also revealed that an improperly earthed metallic containment system offers virtually no protection. Changing the separation of power and data cables has an effect. Measurements were taken with separations of 0, 100, 200 and 300mm, and, as expected, much less power is induced in the data cable as the distance from the power cable increases. When comparing wire mesh and conventional sheet steel cable tray, Cetim confirmed the findings of the first laboratory. There was no significant difference in the Faraday-cage effect, irrespective of the slot pattern of the sheet-steel tray. Again, placing a cover on the Cablofil protected the cable to the same extent as fully enclosed trunking — even if the cover was of wire mesh. A divider was shown to be effective as a protective screen, but only if it was properly connected to earth. Summary In independent tests several years ago, it was shown that Cat6 cables can be placed in standard ‘cable basket’ with no need for a lining to rest the cables on. The weight of up to 200 mm depth of Cat6 cables has no detrimental effect on the ‘compressed’ cable at the bottom of the tray. Now it has been shown that the product can help reduce the impact of electromagnetic disturbances as effectively as traditional cable tray — and much more effectively than non-metallic support systems such as matting. These results have been presented to the CENELEC committee for EN50174-2, which has accepted that the standard should be amended. The results of these tests are available in Cablofil’s recently published technical guide. Matthew Way is managing director of Cablofil UK Ltd, 20-22 Manor Industrial Estate, Flint, CH6 5UY.
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