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Good EMC design techniques: EM mitigation and zoning (Part 7)

25 February 2011

Keith Armstrong continues his discussion on ‘EM zoning’ techniques for installations

In the last part of this mini-series [1] I briefly introduced issues of ‘earthing’ and ‘grounding’ in a system or installation, pointing out that because all currents (including ‘strays’) always flow in closed loops, the only purpose of connecting the common bonding network (CBN) to the mass of the earth, is electrical safety.

For controlling EMI, the CBN (which is often called something like the earthing/grounding network or structure) must be treated as the path through which stray currents flow from one item of equipment to another. Single-point earthing/grounding constructions try to force all these stray current loops to flow via the main earthing terminal, creating very large loops that act as efficient antennas – meaning that the stray currents will often find an easier route by flowing through the air as capacitive and/or inductive fields or (at higher frequencies) fully-fledged radio waves, causing EMC problems – sometimes functional ones too.

Since all currents naturally prefer to flow in smaller loops that create weaker fields and weaker radio waves, all we have to do to control EMC in a system or installation is to provide paths that the stray currents prefer to take – paths that create smaller current loops. The background to all this is described in much more detail, without using any difficult mathematics, in [2].

In an installation we can do this by combining existing metalwork with additional bonding conductors to create a ‘meshed’ or gridded CBN, known as a MESH-CBN, and recommended by [3].  By routing all electrical conductors in very close proximity to elements of the MESH-CBN (eg cable trays, ducts or conduit), and by connecting the metal chassis/frames/enclosures of each item of equipment directly to elements of the MESH-CBN, we provide very much smaller loops for stray currents to flow in, which they then naturally take.

Figure 1 is an overview of bonding metalwork to create the MESH-CBN. It was originally drawn by Alain Charoy, a French EMC consultant with great experience of systems and installations (http://www.aemc.fr/).

Many systems and installations have significantly improved their EMC (and sometimes their functionality) by bonding existing metalwork as shown in Figure 1. The bonding materials used are low-cost but it is not often a very quick job to do, and so is not inexpensive – although it is often the least expensive way of improving the EMC of an existing system or installation. But it is much better to create the MESH-CBN at the time the system or installation is constructed, because then it hardly costs any more at all.

Cable trays and ducts are the most important elements to bond, at all joints and to every item of equipment, because they are the best path for achieving the smallest loops for stray currents. Types of tray and duct and how to bond their joints and cabinet connection will be discussed in a later part of this series.

Concrete reinforcing bars are also a valuable resource for a MESH-CBN, as they are for lightning protection. But of course they have to be bonded before the concrete is poured, and the bonding method has to reliably withstand the forces that occur during that process. The lightning protection standard IEC 62305 describes appropriate rebar bonding methods using specified clamps or 30mm seam-welds.

Figure 2 is copied from [3] and is intended to show that the mesh size depends on the frequencies to be controlled. We can say that a mesh provides a low-impedance path for stray currents up to a maximum frequency, fMAX, of 50/D MHz, where D is the largest diagonal dimension of the individual meshes in metres.

Although EMC performance is improved up to 50/D MHz, it is not very good at 50/D and is much better at 5/D MHz, and very good indeed at frequencies below 0.5/D MHz.

But at frequencies of 100/D and higher, the meshes can resonate and provide no benefit at all, so it is important to be sure that the mesh size is small enough to help provide control of the highest frequencies of current that could flow in them. Obviously, seam-welded sheet metal has a D of zero and so works well to very high frequencies, but sheet metal is not always practicable.

We usually rely on cable screens/shields, filters in the equipment, and shielded equipment cabinets to take care of the very highest frequencies, so the MESH-CBN usually only needs to deal with frequencies below about 50MHz (D no greater than 1 metre), and only up to 10MHz for lightning (D no greater than 5 metres).
Reference [4] expands on this brief article, and also discusses 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]  “The Physical Basis of EMC”, Keith Armstrong, Nutwood/Armstrong October 2010, available from www.theemcacademy.org/books.asp
[3] IEC 61000-5-2:1997 ““Electromagnetic Compatibility (EMC) – Part 5: Installation and Mitigation Guidelines - Section 2: Earthing and cabling”
[4] “Good EMC Engineering Practices for Fixed Installation”, Keith Armstrong, available from www.reo.co.uk/knowledgebase

Readers are advised to access the digital edition from the PSB homepage to view the diagrams that accompany this article


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