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EMC, or electromagnetic compatibility, is becoming more important, especially in modern buildings. The performance of complex IT equipment – and its increasingly elaborate and sensitive integrated cabling systems – in such buildings can be compromised by electromagnetic interference. Although cable support systems are basically passive, and are not subject directly to the EMC Directive, the type and method of containment does have an impact on the EMC performance of an installation.

There is considerable confusion surrounding the subject, with suppliers and European standards propagating conflicting advice. The European standard EN 50174-2 sets out the requirements for the planning, implementation and operation of data cabling networks, and is intended for those who procure, install and operate them. The standard includes a section about EMC but, in the opinion of several leading EMC experts in the UK, 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 fall into the trap of making 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); and an electromagnetic coupling channel that allows the disturbance to propagate from the source to the victim. By reducing the effectiveness of this coupling channel, you reduce the disturbing effect of the source and protect the victim from interference that could reduce the quality of the transmitted signal.

Modern IT equipment is designed to operate with some interference present, and its software is designed to recognise interference and ignore it. If a signal is sent to a computer and it appears corrupted, the computer will ask for the signal to be sent again. This could happen many times in the everyday operation of 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 of the system and cannot be tolerated.

It is well known that surrounding the victim with metal reduces the effect of 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 connected to earth, preferably every 15 to 20 m.

This is why cable management can play an important role in the protection of a cabling system from electromagnetic interference, and why the continuity of the tray or trunking system 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 any significant protection from electromagnetic interference.

Theoretically, the cross section of the containment system should have an impact on protective capability. 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 has commissioned a series of independent tests to compare the EMC performance of different types and configurations of containment.

Engineers at the AEMC Mesures laboratory in France placed a Category 5e 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 created in the cell against the induced power in the data cable. The cable was then placed in a number of different cable containment systems and the engineers noted the power in the cable and compared it 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.

Then the engineers compared the power induced in the Cablofil-protected cable 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. Closed trunking cut the 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 fitted with a steel cover.

In another set of tests performed independently by Cetim Laboratory, a power cable was placed alongside a Category 6 data cable. Once again, the resultant disturbing power was measured. More than 100 different configurations were tested, and engineers measured and analysed the effects of earthing the containment, changing the cable separation and using dividers for several different cable management products.

First, 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, if the containment system is not properly earthed, it will offer virtually no protection. Changing the separation of power and data cables has a measurable effect. Measurements were taken with separations of 0, 100, 200 and 300 mm and, as expected, much less power is induced in the data cable as the distance 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 inside to the same extent as fully enclosed trunking – even if the cover was made of wire mesh. A divider was shown to be effective as a protective screen, but only if it was properly connected to earth.

In independent tests several years ago, it was shown that Category 6 cables can be placed in standard cable basket with no need for a lining to rest the cables on. 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 responsible for EN 50174-2, and it has accepted that the standard should be amended.

The results of these tests – along with information on mechanical resistance, corrosion, continuity, ventilation of cables and fire resistance – are now available in Cablofil’s recently published Technical Guide.