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Good EMC design techniques: EM mitigation and zoning (Part 8)04 May 2011Keith Armstrong continues his series on ‘EM Zoning’ techniques for installationsIf a site is not designed using EM Zoning it can be very difficult indeed to control electromagnetic interference (EMI) on the site – and may even be totally impracticable. Previous articles in this series introduced EM Zoning, and are available from [1].
Figure 1 shows a simple EM Zoning scheme that uses ‘geographical separation’. All the conductors and equipment associated with each EM Zone are kept physically inside that zone, and there is plenty of fresh air between zones (metres – preferably tens of metres).
By ‘conductors’, I mean anything that can conduct electricity, whatever its purpose, whether electrical/electronic or not; eg hydraulic/pneumatic pipes, motor shafts, even water (if it is not distilled and therefore has low-conductivity).
Space is one of the best EM mitigation methods, and – if you have enough of it going spare – it also has the lowest-cost.
The HV portion of the switchyard plus the HV equipment and all their cables makes one zone. The MV portion of the switchyard plus the MV equipment and part of the LV switchroom, and all their cables, makes a separate zone. The ‘noisy’ LV equipment and its cables makes a third zone, and the ‘sensitive’ LV equipment and its cables makes a fourth zone.
In the simple scheme illustrated in Figure 1 the only conductive connections between the zones (other than their lightning earth grids, which could be common) are the isolating transformers: HV to MV, HV to MV, MV to ‘noisy’ and MV to ‘quiet’ LV. With plenty of space between the zones, the EM isolation achieved between them depends mainly on the EM specifications of their isolating transformers.
Mains transformers are generally good at attenuating differential-mode noise above a few hundred kHz, but they don’t attenuate common-mode noise above a few hundreds of kHz, because of their stray primary to secondary capacitance. Different winding methods or internal shields can be used to improve common-mode attenuation, but where high isolation is required mains transformers might need to be combined with surge suppressers and/or filters.
Where this simple method can’t be used, it is still a good idea to design using it as far as possible, so as to reduce the need for the costly EM mitigation that will be required for each of the conductors (whatever their function) that must cross from one zone to another.
To deal with conductors that must cross an EM Zone Boundary, we need an ‘RF Reference’ – a conductor that surrounds an EM zone and has a low impedance (less than 1 ohm) up to the highest frequency that we need to control. Every conductor that crosses it - ie that passes from one EM zone to another - must be ‘RF-bonded’ to this conductor, which, at its most basic, is the Bonding Ring Conductor (BRC).
A BRC should generally have a conductivity equivalent to at least 50mm2 of copper, although more is better (and may also be needed to be able to handle the maximum fault currents).
In the case of Figure 1, BRCs would pass underneath the mains isolating transformers, with the transformers’ primary circuits and their transformer connections lying on one side (in one EM zone) and their secondary circuits and their transformer connections lying on its other side (in a different EM zone). The transformer cores and any cases would connect directly to the BRC.
Figure 2 shows an example of one floor of a factory or other type of site, with a BRC all around the inside of its outer walls, creating an EM zone 1 (within the EM zone 0 that we designate for the ‘outside world’). The main earthing bars for this floor of our factory form part of the BRC, and – where practicable – they should all be close together on one side of the building.
Each item of equipment in zone 1 has its frame/chassis/enclosure connected to the BRC by a short (less than 2m long) conductor equivalent 50mm2 of copper (more, if needed for fault currents). Figure 3 shows a real-life example. Note that the BRC and all bonds to it are additional to the protective earthing conductors that are provided with the mains power conductors, as required by the IEE Wiring Regulations.
Where an item of equipment is more than 2m from the BRC, two conductors should be used in parallel to connect it to the BRC, and they should be spaced as far apart from each other as is possible without making their lengths any longer than necessary.
Where equipment has to be more than 4m from a BRC, it is best to add a conductor that links from one part of the BRC to another, to which the equipment can be connected as described. This ‘BRC cross-linking conductor’ should be the same type as the BRC that runs around the walls.
The BRC is also connected to all the building’s structural steelwork - for example, steel girders in the walls, as shown in Figure 2.
A BRC like this provides a low impedance up to 10kHz or more (depending on the degree of cross-bonding in the walls and roof) – a very much higher frequency than can be controlled by using the traditional single-point earthing (‘star earthing’) technique. This BRC is capable of controlling the frequencies at which the conducted effects of lightning have their maximum power.
For lightning protection to IEC/EN 62305 (in force in UK since 2008), all the structural metalwork in the walls and roof should be cross-bonded to provide a mesh that is no more than 5m on a side, to help provide a shield from lightning electromagnetic pulse (LEMP).
With such a cross-bonded external structure bonded every 5m around the BRC, the BRC will provide a low impedance ‘RF Reference’ up to about 10MHz.
Figure 2 shows that all of the conductors entering an EM zone are ‘RF-bonded’ to the BRC at the point where they cross it. It also shows another EM zone (numbered 2) contained within EM zone 1. This has its own BRC that is closely cross-meshed to help control the higher frequencies as required by the equipment in that zone. Conductors (whatever their purpose) that come from zone 0 into zone 2 are RF-bonded to the BRCs for both zone 1 and zone 2.
Later articles in this series will describe the use of various EM mitigation techniques when RF bonding a conductor to a BRC, and extending BRCs to create meshed areas and volumes to control higher RF frequencies and provide shielding. 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 at: www.psbonthenet.net/company.aspx?CompanyID=12242.
[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 from www.reo.co.uk/knowledgebase
Note to readers: the figures can be viewed via the digital issue which is accessible from the home page of the PSB website
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