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Good EMC design techniques: EM mitigation and zoning (Part 10)24 August 2011Keith Armstrong continues his series on 'EM Zoning' techniques for installationsThis part of the series [1] deals solely with shielding techniques at EM Zone boundaries. Figures 1 and 2 apply where the highest frequency to be shielded is quite low; say, less than 30MHz.
In typical industrial cabinet wiring, cable shields are terminated by pigtails routed in the plastic trunking all the way to a copper busbar called the “star earth”, “single point earth” or simply “cabinet earthing bar”. These pigtails can easily be 1 metre in length, and may even be 3 or 4 metres.
In such cabinet assemblies, it is possible that a cable type specified by its supplier as having a shielding effectiveness better than 40dB up to 1GHz (1,000 MHz) is reduced by the effect of the length of the pigtail to being no better than 20dB up to 1MHz.
In Figure 1 the EM Zone boundary lies along a DIN Rail terminal block mounted on the backplate of an unshielded industrial cabinet. Replacing the traditional long pigtails from the cable shields with P-clips electrically bonded directly to the earthed backplate can increase the lowest effective frequency of the shielding at this EM Zone boundary from about 1MHz or less (with long pigtails), to 30MHz or more (with the P-clips).
It is difficult to be precise about the dBs and frequencies of the shielding, because of crosstalk between the pigtails that are routed together in the same trunking, and because of the proximity of all the Zone 1 conductors and components to those in Zone 2.
EMC test laboratory measurements of EMC emissions and immunity, on the unshielded cabinet shown in Figure 1, showed that – providing everything else was done correctly – the use of P-clip shield termination allowed it to pass the radiated emissions tests and helped declare it compliant with the EMC Directive.
The P-clips used in Figure 1 were tin-plated steel pipe-clips from a hydraulic component supplier. Figure 1 also shows a comparable shield-terminating P-clip from an EMC component supplier (Kitagawa).
The people who wired the cabinet shown in Figure 3 drilled and tapped each P-clip’s screw fixing hole in the backplate, and said that the overall assembly time was about the same as using traditional long pigtails to a common earthing bar. And, because the plastic trunkings were not as full (no pigtails in them), cable routing was easier and quicker.
This is the sort of thing that I really get a kick out of – significantly improving the EMC of a cabinet by using standard parts, whilst also saving time and cost (or at least breaking even on them).
Figure 2 shows shielded cables outside of a cabinet, following the route of a bonding ring conductor (BRC) in Zone 2 (in this sketch) and then passing through to EM Zone 1 with their shields bonded to the BRC at the Zone boundary, as required by good EMC installation practice. 360°-shield-bonding cable glands are used, installed in one wall of the cable tray (= BRC), although they could just as easily and effectively be installed in the base of the tray.
The shielding effectiveness might be as good as up to 20dB up to 30MHz, providing that very good segregation/separation is applied between all of the cables and components in Zone 2, and all of the ones in Zone 1.
Meshing the common bonding networks (CBNs) in Zones 1 and/or 2 should improve the shielding effectiveness at frequencies up to that at which the mesh width equals one-tenth of that wavelength (assuming a square mesh), providing that all cables are routed very close to their meshed CBNs, and all items of equipment RF-bonded directly to the meshed CBN, in both Zones (wavelength in metres equals 300 divided by the frequency in MHz).
Figure 3 shows a method of crossing an EM Zone boundary in an explosive atmosphere, where the EM zone boundary is also the boundary around the area in which explosions could occur. It uses a shielded version of an explosion-proof “cable transit”, from MCT Brattberg in Germany.
Where rooms containing (or keeping out) explosive atmospheres are lined with seam-welded metal plates – as they almost always are – they can make very good shielded rooms at frequencies up to 100MHz or more.
The 360°-bonding of the cable’s overall shields to the metal wall of the room, by the cable transits shown in Figure 3, means that the overall shielding effectiveness is governed by the shielding effectiveness of the metal room itself. Attention to the shielding of all the other details of the room (e.g. windows, doors, etc.) can quite easily increase this upper frequency to 1GHz, or more.
The next article in this series on EM Zoning will continue this latter theme of using shielded rooms as EM Zone Boundaries. If you can’t wait, find out more from [2].
References:
[1] Previous PSB columns in this series are archived at: www.psbonthenet.net/company.aspx?CompanyID=12242
[2] “Good EMC Engineering Practices for Fixed Installation”, Keith Armstrong, available from www.reo.co.uk/knowledgebase
Online readers please note that the figures will be available in the digital online version of PSB September, due to be published approximately mid-September
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