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Good EMC design techniques: EM mitigation and zoning (Part 9)27 June 2011Keith Armstrong continues his series on ‘EM Zoning’ techniques for installationsEM Zones provide a measure of protection, one zone from another, for electromagnetic disturbances. EM disturbances that travel through the air are called radiated disturbances, and we use shielding to mitigate their effects. EM disturbances that travel along conductors are called conducted disturbances, and we use filters, galvanic isolators and surge suppressors to mitigate them.
At the end of the last part in this little series [1], I had introduced the idea of a bonding ring conductor (BRC) around each EM Zone in a system or installation, and how the BRC provided a sort of “shielding effect” at frequencies up to around 10kz.
The BRC between EM Zones 0 and 1 (EM Zone 0 is always the outside world) is combined with meshed metalwork around the outer skin of a building to form a useful part of a lightning protection scheme (see IEC/EN 62305 Ed.2:2010, especially its Part 4). A mesh size of 5 metres, or less, converts a BRC into quite a reasonable shield for frequencies of 10MHz, or more.
The lightning protection techniques in IEC/EN 62305-4 can also be applied to the EM Zone boundaries within a building, e.g. between EM Zones 1 and 2; 2 and 3, etc., and would incorporate the BRCs around those areas.
IEC/EN 62305 (which is now mandatory in the UK, replacing BS6651) identifies “Lightning Protection Zones” or LPZs. Well, lighting is just one aspect of EMC, so I usually work on the idea that one or more of my EMZs are also LPZs.
Shielding at EMZ boundaries will be discussed in more detail in the next article in this series. In this one I want to focus of mitigating conducted disturbances. Once we have decided what kind of filter, galvanic isolator and/or surge suppressor we want to use, based on the types of EM disturbances we are expecting and how much mitigation we require, the next question is where and how to install it. Incorrect installation can negate all the effects of a filter, isolator or suppresser, no matter how expensive.
Correct installation is to use reliable metal surface-to-metal surface electrical connection techniques – which we call “Radio-Frequency (RF) bonding” – at the BRC surrounding an EM Zone. “Direct” RF bonds are used for all conductors that don’t carry power or signals, and they simply bring the conductor’s surface directly into contact with the surface of the BRC. For example, a cable’s overall shield, or a metal pipe (e.g. for hydraulic or pneumatic power) are often directly RF-bonded to the BRC by a metal saddleclamp or P-clip. Ducts for cables; chilled/heated air; smoke ventilation, etc., are often RF-bonded by a short copper braid strap from each side, but this has a limited effectiveness above a few MHz, when some way of making a surface-to-surface electrical bond become necessary.
Conductors that carry power or any type of signals cannot (of course) be directly RF-bonded, and it is to these that we apply filters, isolators and/or surge suppressers, which we call “Indirect” RF-bonding. The devices we use must be “earthed” or “grounded” to the BRC using Direct RF-bonding techniques.
A 50mm2 round or flat copper conductor BRC is an inappropriate surface for many RF-bonding techniques so we use “Bonding Plates” like those shown in Figures 1 and 2. These are effectively extended areas of the BRC, and so must connect in series with it. Cabinets suitable for use as shown in Figure 2 can be purchased as “explosive atmosphere” cabinets – already provided with fixings to connect their backplate in series with a BRC.
Where a BRC forms part of shielding and/or lightning protection mesh, it will help to connect one or more of the mesh elements to the bonding plate, where appropriate.
Figure 3 shows a cable tray crossing from one EM Zone to another. Of course it must itself be directly RF-bonded to the BRC, but we can also fit direct and indirect RF-bonding to its cables and pipes at that same location along the tray, as shown in Figure 3.
Figure 3 makes a point that is common to all three figures – no conductor, whatever its purpose, must ever cross an EM Zone boundary without being RF bonded, directly or indirectly, at the zone boundary. This is because inevitable stray capacitance will cause it to couple conducted EM disturbances from one side to the other, defeating the purpose of all the filters, isolators and surge suppressors and wasting time and money.
References [2] and [3] expand on this brief article, and also discuss many other good EMC engineering issues.
References:
[1] Previous PSB columns in this series are archived here. [2] IEC 61000-5-2:1997 ““Electromagnetic Compatibility (EMC) – Part 5: Installation and Mitigation Guidelines - Section 2: Earthing and cabling”
[3] “Good EMC Engineering Practices for Fixed Installation”, Keith Armstrong, available here.
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