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1: Introduction: EMC system requirements
2: The EMC Directive's requirements for systems + installations
3: EMC Systems Management
4: EM Interference sources, victims and coupling
5: Grounding, earthing and bonding
6: Cabinets, cubicles and chambers
9: Lightning and surge protection
10: In situ EMC testing
Bonus A: Systems EMC procedures
Bonus B: Determining EMC performance criteria
EMC for Dummies -- About this Guide
Electromagnetic compatibility (EMC) is defined as "the ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbance to anything in that environment". Historically, EMC has been concerned principally with ensuring the proper operation of collections of electrical and electronic apparatus. Since interference is a function of separation distance, equipment used in close proximity to other equipment had to be "compatible" with its neighbors. Putting together a "system" out of several often disparate items of "apparatus" meant that these items were naturally close to each other, and their EMC was necessary in order for the system to work successfully. Hence the discipline of EMC grew up in those industry sectors where systems integration was the norm- notably in the military, where the majority of electrical and electronics equipment is used on "platforms", i.e. ships, aircraft and land vehicles; in their civilian equivalents, aerospace, rail, automotive and marine transport; and in the process control industry. The consumer, IT and professional equipment sectors largely escaped this discipline, because their individual products could assume a large enough separation distance that EMC could be regarded as a luxury rather than a necessity.
Exceptions to this state of affairs developed in industry sectors whose products created an identifiable threat to third-party radio reception: so e.g. products using spark ignition, electric motors, power switching devices, fluorescent luminaires and of course digital devices began to fall under suspicion, and eventually were regulated in their radio frequency emissions, in a more-or-less piecemeal fashion. But system builders were unaware of this, since it affected only the equipment itself, not the systems that were built from it- unless those systems included radio receivers. Commercial systems that faced issues of safety integrity had often to meet requirements for immunity from various phenomena, such as radio frequency fields, electrostatic discharge, and various types of conducted transients. But these were contractual requirements, agreed between the equipment suppliers, and the system designers and operators; they were instigated as a result of operational experience, not because of legislation.
This somewhat ad hoc framework for EMC has changed enormously as a result of the EMC Directive. The consequence of the piecemeal process of regulation of electrical and electronic products, was that different countries within Europe took different approaches to that regulation. These discrepancies led to a particular instance of a "technical barrier to trade". The European Commission, having identified this barrier in the mid-80s, put forward a Directive whose intent was to harmonize EMC regulations throughout the EU and therefore encourage the free movement of apparatus, while at the same time protecting the electromagnetic environment.
The EMC Directive was clearly intended from the start to apply to apparatus, that is, individual product items. But its scope was drawn more widely than that: apparatus is defined as "all electrical and electronic appliances together with equipment and installations containing electrical and/or electronic components", therefore catching not just apparatus as it’s commonly understood, but also whole systems and installations. In the early days of interpretation there was some concern that large installations such as nuclear power plants or telephone exchanges would have to carry the CE Mark, but such excess zeal was quickly dampened by the definition of the "excluded installation" in the regulations of 1992, which aligned with the first round of Commission Guidance published later. This meant that installations comprised of items put together at a given place to fulfill a specific objective but not supplied as a single functional unit, were excluded from the scope of the regulations.
Nevertheless, many classes of system do still remain within the Directive's scope, and this now presents system designers and installers with a set of legal requirements, and consequent technical requirements, with which they are wholly unfamiliar. This arises because the system is now regarded "as a whole" and its interaction with its external environment is regulated. As explained earlier, historically this was hardly considered, whereas the "internal" EMC was an operational necessity.
A significant issue in this context is the insistence by the European Commission that products (including those intended for use in systems) should come with "instructions for use" which have the purpose of ensuring that no EMC problem is encountered when the products are put into service. With a few rare exceptions, this concept is totally new to system builders, and to their equipment suppliers as well. Installation instructions have traditionally only been concerned with functional requirements and knowledge of EMC issues is not only rare among installation engineers but also among the applications engineers of equipment suppliers, who are normally responsible for advising their customers on product use. Although the Controllers have made the superficially reasonable assumption that EMC instructions are capable of being supplied and implemented, the reality is rather different from this, at least at the time of writing.
Fortunately, the techniques needed to deal with internal EMC also to some extent affect the external interactions, so that system builders are able in part to kill two birds with one stone. But the overlap is by no means exact, and there are a number of areas where external EMC puts demands on design, procurement and installation of apparatus which have no real precedent. And even in the realm of internal, operational EMC, for many designers and installers this has traditionally been dealt with by totally ad hoc methods: an operational problem, traced to internal interference, would be solved on the spot by application of a range of "fixes" such as one-off filters or shielded cable. In by no means all cases have lessons learned from this one problem been applied in subsequent designs, and equally rare is the abstraction of common principles of interference control which form the basis of design techniques to be applied across the board.
This guide has been written to address the resulting problem, of designers and installers being suddenly faced with responsibilities for which they have neither an in- house solution nor the expertise to handle.
It’s presented in two parts: the first is relatively non-technical, and looks at the need for EMC in the context of systems, followed with a discussion of the somewhat complex way in which the EMC Directive applies to systems and installations. A further section then covers the management aspects of systems EMC, including control and test plans, and the important aspect of procurement specifications.
The second part covers the technical aspects of systems EMC, looking at the various established methods which can be applied to ensure compatibility, and putting these in the context of the new responsibilities facing system builders. These techniques include ground/earthing and bonding, the use of shielded and unshielded enclosures, choice and routing of cables, filtering and transient suppression. The assumption is made that the system builder is faced with a number of electrical and electronic modules which have to be interconnected in a way that is compatible both within the system and with the external environment, and does not have any say or indeed interest in the way that the modules themselves are designed and built. The question of lightning protection sits uneasily with the context of the EMC Directive, but it’s of prime concern to the operators of systems and installations, so we have included a short section covering it. There is an initial section on the fundamental issues of interference sources and coupling paths between source and victim, and a final section looks at the question of how you might prove the compatibility of a system through testing. Most larger systems can only be tested in situ, once they have been installed, and such testing has its own problems and solutions, which are inadequately dealt with in most of the existing test standards, since these were written around laboratory testing of apparatus.
This is not an academic guide. Where theory is necessary, it’s presented with as few trappings as possible. Its intended readership is those engineers who are faced with the practical problem of implementing EMC requirements in their day-to-day work, who need a guide to the techniques that they can use straight away, and who don’t have the time for dealing with the complex mathematics that undoubtedly underlies much rigorous electromagnetic theory. At the same time, though, a list of prescriptive actions by itself is less than helpful, without the understanding needed to adapt each technique to a specific set of circumstances. Apt quote: "insight is considered more valuable than the recipe for tackling a symptom". So we hope that the treatment herein, while acknowledging the value of recipes and indeed providing a few of them in suitably pre-digested form at the back, will be seen as a foundation on which to create your own variations as each case dictates.
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Updated: Tuesday, 2012-11-06 1:43 PST