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Good EMC Design Techniques — EM Mitigation and Zoning (Part 11)24 October 2011Keith Armstrong continues his series on ‘EM Zoning’ techniques for installationsThis part of this series [1] deals solely with shielding techniques at EM Zone boundaries, and in the previous issue we dealt with techniques that were probably good enough for most commercial and industrial systems and installations where the highest frequency to be shielded is quite low (say, less than 30MHz).
That column ended with an example of shielded cables exiting a metal room in an Explosive Atmosphere type of installation, using what is called a “cable transit” that provides a fire and explosion barrier – some types of which also have provision for “360° bonding” cable shields.
Such metal rooms – although designed for preventing (and protecting from) fire and explosion – might be expected to provide useful levels of shielding up to, say, 100MHz. They might provide much higher levels of shielding and/or higher frequencies, depending on the details of their construction.
It can be very beneficial to get involved with the design of such metal rooms early on, because many of the design features needed to get really good shielding effectiveness (RF attenuation) from them might only require small modifications with insignificant increase in construction time or cost.
As we saw earlier in this series on mitigation and zoning, we can use meshes to very good effect for providing the RF Reference Plane. Often, we can use the existing metalwork and add a little extra constructional work to electrically bond it to create a mesh, with the highest frequency at which it is effective depending on the mesh size. Suitable electrical bonding techniques for creating meshes were described in this column way back in January, October and November 2009.
The good news is that we can also use meshes for shielding, and the new lightning protection standard IEC/EN 62305 has a lot of detail on how to bond rebars to make meshes that protect electronics from the effects of LEMP (Lightning Electromagnetic Pulse).
Meshes shield to some degree up to 50/D MHz (where D is the diagonal of the largest mesh element in metres). However, they only very effective up to 5/D MHz, and only excellent below 0.5/D MHz.
So a 700 mm square mesh, with a diagonal of about a metre, would be a lot better than nothing up to about 50 MHz; really quite good up to 5 MHz; and very good indeed up to about 500 kHz. (Even so, at under 500 kHz, a mesh with D = 1 m would not compare with the shielding effectiveness achieved by sheet metal.)
Almost all of the energy in a lightning strike occurs at frequencies below 10 MHz, with most of it around 10kHz, so a 5 metre mesh in the outside walls and roof of a building is considered to be the minimum requirement for lightning protection according to IEC/EN 62305.
An important detail is that the degree of shielding increases as the distance from the actual mesh increases. Right at the meshed area there is very little shielding benefit from it.
Maximum effectiveness is achieved at 2D or more from the meshed area, so it is important not to locate sensitive equipment or route cables closer than 2D to a mesh that is being used as a shield at an EM Zone boundary.
Figure 1 shows a building with various meshes created from the existing (sometimes called ‘natural’) metalwork in its walls and roof, to create an EM Zone (Zone 1, in this case, although the same principles would apply if it was a Zone 2, 3, etc).
The equipment and cables in the building are shown as being located at least one mesh diagonal (D) in from each of the meshed surfaces – but of course at least 2D spacing from the surfaces is much better.
In this example a mesh in (or on) the floor slab is being used as the RF Reference for the cabinets and their interconnecting cables, which means (as earlier columns in this series have said) keeping them very close to the mesh at all times and bonding cabinet metalwork directly to it.
So far, so good, but what if the soil under the building is not providing good shielding? Now we are relying on the floor mesh to also act as a shield, and for its shielding to be effective the cabinets and cables need to be more than its D away from it (preferably at least 2D).
Of course, this spacing requirement conflicts with the requirements for using the mesh as an effective RF Reference. The only options are either to make the floor mesh very small indeed (e.g. expanded metal, ideally sheet metal, with seam-welded joints) or to have an additional mesh placed sufficiently far below the RF Reference’s mesh to act as a shield.
If we now imagine the single-floor in the figure being one floor in a multi-floor building, so that the floor of the shielded EM Zone in the figure is the ceiling of a different EM Zone, the need to provide shielding as well as an RF Reference is clear.
The next article in this little series will deal with the use of shielded rooms using sheet metal, which –apart from their higher shielding effectiveness have two more significant advantages over mesh shielding: they need no spacing from their surfaces; and their surfaces can also be used as RF References.
If you can’t wait that long, download [2] - its free!
References:
[1] Previous PSB columns in this series are archived (unfortunately without their graphics) 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
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