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Good EMC design techniques: EM mitigation and zoning (Part 5)

10 November 2010

Keith Armstrong goes into more practical detail on segregation techniques, known as ‘EM Zoning’

In the last four editions [1] I introduced the concept of EM Zoning, and briefly described some methods for achieving it. Zoning is an essential EMC engineering technique if filtering, shielding (screening), galvanic isolation, surge and transient suppression are to work as intended at frequencies above about 100kHz, whether we are constructing a cabinet, a system, or an installation of any size – even up to the size of a national network.

Filters, shields, surge arrestors, galvanic isolation, ‘earthing/grounding’ and other electromagnetic interference (EMI) mitigation measures are applied at the boundaries of EM Zones. Previous articles in this series have discussed how to choose the right parts and assemble them using “RF-bonding” to give their best performance [1].

As Part 1 of this series showed, we can take an electrical/electronic cabinet itself as an EMC Zone. Figure 1 shows how we need to apply the various mitigation techniques at the EM Zone boundary – the walls of the cabinet.
In my work, I am often called out to fix EMI problems, and many of them stem from a failure to properly apply EM Zoning. What is not appreciated enough is that all electrical power and electronic signals (analogue, digital, radio, whatever) are really propagating electromagnetic (EM) waves that do not stay in the cables.

This is obvious because a close-field probe, which can be as simple as a loop made from a paper clip will – when connected to an oscilloscope or spectrum analyser – pick up some of the power or signal in the cable, outside its insulation. 

In fact, power, signals and noise follow the path of least impedance, and the higher the frequency the higher the impedance of wires and cables and the higher the proportion of power, signal or noise that flows through the air into some other conductor, whether an item of metalwork or another cable [2]. We use this phenomenon for radio communications and to watch broadcast TV, but it is a nuisance when it happens at other times, causing EMI.

So, not observing the basic “EM Hygiene” sketched in Figure 1 allows power, signals and noises to ‘leak’ where they are not wanted and cause EMI. Even where we don’t care about passing EMC tests and complying with the EMC Directive, we have to care about making products that are unreliable in operation because they suffer too much EMI from their operating environment (e.g. other equipment running from the same mains supply). And we have to care whether our product causes other equipment to suffer from excessive EMI. When we ignore such things, it hits us where it really hurts – in our bank balance. If EMI hasn’t caused significant financial losses yet, don’t worry, it will.

At radio frequencies, the ‘leakage’ is very strong. EMC test chambers are well-shielded rooms and radios won’t work in them. But I’ve seen an FM radio in a shielded room get up to full volume when a thin wire just a few inches long was poked through an air vent that was six feet away. And I’ve often seen expensive well-shielded cabinets fail EMC tests completely because a mouse cable had been passed through the wall without being shielded and/or filtered, with the shield or filter RF-bonded to the wall as shown in Figure 1.

The designers said “It can’t be the mouse cable because it’s not carrying high-speed signals!” but the mere fact of having a conductor penetrating the metal wall without being correctly RF-bonded is enough to interconnect the EM environments inside and outside the cabinet, making it a very costly physical barrier, but no barrier to EMI.
Every conductor, whether it’s a hydraulic/pneumatic pipe, bracket, push rod, air conditioning duct, electrical/electronic cable, or drawstring in a fibre-optic cable or whatever, absolutely must be RF-bonded to the wall of the metal cabinet at the point of its penetration.

If this is not done, then there’s no point in spending more on a shielded cabinet. And if using an ordinary metal cabinet with no specific shielding performance, following Figure 1 will very significantly improve its shielding performance. I like getting something for nothing!
References:
[1] Previous PSB columns in this series are archived at: www.psbonthenet.net/company.aspx?CompanyID=12242. Please note that these archives do not include the figures, though more recent articles(with their figures) are now accessible via the digital issue archive at www.psbonthenet.net. I also plan to post my complete set of columns (with figures) on my website http://www.cherryclough.com/ by autumn 2010.
[2]  “The Physical Basis of EMC”, Keith Armstrong, Nutwood/Armstrong October 2010, available from www.theemcacademy.org/books.asp

[Please note that this article can be viewed, along with its illustrations, by going to the digital issue archive, which is accessible from the home page]


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