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AMAZON multi-meters discounts AMAZON oscilloscope discounts EMC does not happen by itself, whether we are looking at a product, a system or an installation. As with any other aspect of the process, it must be managed. This section looks at the tools that project managers need to ensure that EMC is properly accounted for throughout the project. The level of detail and activity to which EMC control is taken will depend on the degree to which EMC affects the outcome of the project. A space system, e.g., will have many more critical aspects than a building management system and will justify more effort in the planning. This section therefore presents the control activity in its fully-developed form; some areas can be reduced or omitted as circumstances dictate. The EMC control plan A schematic form of the structure of a system development plan with respect to EMC is given. The control plan has to fit into this development structure; its initial release creates a rubric for the EMC aspects of the programmed including basic design guidelines, while subsequent updates incorporate refinements as the program progresses. The EMC control board It’s essential to have a management structure defined for the EMC activities. This is achieved by establishing an EMC control board or committee, whose purpose is the effective execution, on schedule, of the EMC control plan. Typically, it will be chaired by the prime contractor's project manager, or a deputy with specific EMC expertise. A large part of the control board's activity will be concerned with sourcing equipment to the correct specifications, so the board must have procurement authority. Given that normal practice in system development is to proceed via a series of review stages, progress to the next stage being allowed only on successful review of the previous one, the control board must also have authority in the review process. Finally, it must have the ability and the resources to define and test the various module- or equipment-level requirements that will ensure total system-level compatibility. ===== ----- EMC deliverables in the system development programmed Programmed milestones: Project inception: establish EMC control board Requirements definition review Preliminary design review Submit first EMC control plan Critical design review EMC compliance test -- EMC deliverables: EMC performance specifications: System interface requirements Known operational environment(s) Define system level EMC requirements: Define module level emissions and immunity requirements and procurement specification (choose standards to apply) Define power system compatibility requirements; Complete EMI environmental assessment for radiated and conducted requirements; Define power distribution and grounding/earthing/isolation architecture; Assign cable design engineering function and responsibility for module and system interconnections: define wiring layout and shielding considerations; Preliminary test for power line conducted emissions to define extra filtering needs Identify specific EMC threats and victims; Review engineering model test data ; Verify module-level and equipment-level requirements; Define and resolve outstanding EMC issues; System tests (if necessary) for system-level EMC assurance; Pre-shipment or in situ conformance tests ==== Identifying EMC issues A checklist for the EMC issues to be addressed is a good starting point for any control plan. Various sources of such checklists exist. Our own checklist for managers is given. The following bullet-point list is derived, regarding IT installation planning within buildings:
-----The environment A crucial initial stage in the EMC control of any systems project is to assess the electromagnetic environment that will be enjoyed (or suffered!) by the system. This will in turn define the degree of rigor that will be needed by the system's components in dealing with various phenomena, in the levels of stimulus or limits of emissions that are applied, and in the amount of degradation of functional performance allowed for each function for each disturbing phenomenon. The various product and generic standards that are available to some extent present such a picture of the environment, but only in very broad terms. For a closer match for any given project, a more specific review of the intended environment is needed. For most commercial and industrial systems projects, this review can use as its basis the international standard IEC 61000-2-5, "Classification of electromagnetic environments". The advice given in this standard is expanded. Other application environments -- e.g. military, space, automotive, railway - emphasize different EM phenomena and most have explicit documents detailing these. Using 1/2 as a guide, the environment for a given installation can be tabulated, and this will provide a clearer picture of the overall requirements that will be needed for individual apparatus and for the installation practices that are used. In some cases, the requirements may be relaxed over those inherent in the generic or product standards, in others they will be more onerous. === The content of the control plan: EMC programmed management System level performance and design Sub-system performance and verification EMC analysis System level verification Content: Procurement responsibilities, reporting structure, control of design changes Planning and scheduling the control programmed, including resources, co-ordination and review procedures Definition of the intended environment Definition of critical circuits and modules System and sub-system design issues: Controlled ground/earthing scheme, structural bonding, wiring layout, shielding and termination practices, corrosion control etc. Allocate EMC performance at the equipment level, via specific standards or tailored requirements Integrate the test results from equipment level EMC tests, to analyze and determine the effects of acceptable degradations Prediction of intra-system compatibility based on known sub-system characteristics, together with solutions for predicted or actual interference problems System-level test plan, including rationale for selection of critical sub-systems for safety margin demonstration, performance criteria and operating configuration for in situ compliance tests ==== -----System interface requirements The system components interface both with each other and with the greater environment and must exhibit electromagnetic compatibility in each case. Since interference is propagated through the interfaces (including the enclosure as an interface, in the same manner as cable connections), identifying these and listing the environmental phenomena that may apply to each is a necessary step towards specifying the compatibility levels and tests that will be necessary. -----Module level emissions and immunity Once the interface requirements have been defined based on the environment, then the compatibility requirements for each module can be listed, and hence the test compliance specification for each module. These can be separated into emissions control and immunity control. Bear in mind that RF emissions control is intended to protect victim radio receivers, while RF immunity is intended to guard against aggressive radio transmitters: there is a large difference in amplitude (microvolts per meter versus volts per meter) between these. In many cases the requirements are covered by compliance with the genetic or product standards, but in some cases there will be extra or more onerous needs. This would be the case if e.g. portable radio transmitters and /or RF production equipment (such as dielectric welders) are known to be used in close proximity to the apparatus, a situation which is specifically excluded from consideration in most harmonized immunity standards. -----Power system compatibility The mains power supply is a fundamental interface to many if not all functional modules in a system, and compatibility with disturbances that exist on it- and control of disturbance generation- is necessary for all such modules. IEC 61000-2-5 lists a number of interference phenomena that are associated with propagation via the power supply. Many of these are low frequency phenomena, some of which (such as frequency and voltage variations within operational limits) may well be regarded as functional rather than EMC specifications. The power supply connection also acts as a conduit for high frequency disturbances which are much harder to predict and deal with other than by rigorous filtering at the module supply interface. The basic characteristics of disturbances on the supply may be similar throughout the system. But if the supply is broken into a number of different segments within the system, and if each segment has some degree of filtering or suppression isolating it from others, then the supply interface EMC requirements may well be different for different clusters of components. -----ground/earthing, bonding and isolation An important tool in EMC control for any system is a controlled equipotential bonding scheme. In general, using any part of the mechanical structure as a functional electrical current path is to be avoided. All return paths should be carried in their relevant cable harnesses. Where functional connection via the metalwork is necessary for weight or topological reasons, the current path through the structure must be established and controlled, and the impedances across significant paths (and across the whole of the applicable frequency spectrum) have to be analyzed. Whilst this is possible for some closed systems such as aircraft and spacecraft, it normally isn't for distributed systems in buildings and industrial plant. Use of the structure for interference (as opposed to functional) ground/earthing is often necessary, and this brings in considerations of electrical bonding and corrosion control. Bonding is needed for the management of interference current paths and to minimize voltage differentials across the structure both for interference and safety purposes. -----Cabling issues System components are interconnected at their interfaces, and these interconnections are typically made by electrical cables. The effect of the cable engineering on EMC issues has historically often been neglected, but it should form an important part of the overall EMC control plan. Good cabling design can enhance the barrier between the system and external interference, while poor design can worsen it. All aspects of cable design are relevant: routing, segregation, allocation of signal classes, choice of signals in each bundle, choice of conduit, not to mention the selection and termination of the cable itself. The decision as to whether to use shielded or unshielded cable should be made at this stage, once the characteristics of each interface have been detailed as already discussed. Other considerations may dictate a prior choice of cabling: e.g., structured cabling in a building services system may already be in place, so that the choice of cable layout, segregation and shielding may not be available. In this case, the control plan is driven backwards, towards specification of the interfaces based on the known limitations of the cabling, rather than vice versa. Identifying and sourcing critical parts Systems are composed of interconnected modules. There are two aspects to the criticality of a module in this context: functional criticality, and EMC criticality. That is, how important is a module to the operation of the system, and how important is it to the EMC of the system. To take a couple of near trivial examples: the emergency stop switch is absolutely critical to both the safety and the functioning of a process control system, but because it’s entirely electromechanical it has no EMC importance whatever. Vice versa, we can visualize an electronically controlled or fluorescent lighting unit which has no effect on the daytime running of the system and perhaps only marginal effect at night, yet which creates unacceptable interference perhaps both inter- and intra-system when it’s switched on. ==== Functional criticality Functional category: Safety critical -- Mission critical -- Non critical Description: EMI problems could result in loss of life, property or the system EMI problems could result in injury, damage to the system, loss of or delay to operation, or performance degradation which unacceptably reduces operational effectiveness EMI problems could result only in annoyance, nuisance or minor discomfort, or acceptable and temporary loss of performance ==== EMC criticality EMC category: EMC benign EMC relevant EMC critical Description: Won’t cause or suffer from interference; has no effect on the EMC performance of the system (e.g. passive switches, indicators) Won’t of itself cause or suffer from interference, but may have an effect on the EMC of related items; e.g. cables, connectors and enclosures Active electronic equipment, other interference sources such as motors and lighting equipment ==== Categories of functional criticality can be divided up. These will determine the weight that is given to the electromagnetic immunity of each item (). EMC criticality is a classification of whether and to what extent EMC issues need to be considered for this item. Partly, it’s a measure of the "electromagnetically benign" nature of the part (). But there are some items which though they are not "electromagnetically active" still need consideration from the EMC viewpoint, since they affect the compatibility aspects of other equipment. Cables are a good example of this. The interplay between functional criticality and the EMC phenomena which may degrade various functions is discussed in more detail. Control of procurement The non-benign system components which are bought in must be subjected to some overview at the purchasing stage. Change control system It’s in the nature of purchasing that parts become obsolete or unobtainable and have to be replaced by other parts. To control the impact this has on EMC, there should be a system which protects the compliance status of bought-in parts. This can typically be a review of the aspects described (declaration of conformity, assembly instructions etc.) which is triggered whenever a change in specification occurs. Obviously this is not necessary for EMC-benign parts, and therefore the parts list for each system must identify which parts are relevant or critical, so that the change control system is only invoked when necessary. This should be no more than is already done within a company's manufacturing system for other purposes: in such cases the extension to EMC aspects is only administrative. Controlling assembly / installation Maintaining EMC requires observance of a number of specific assembly and installation practices. These may include:
The EMC control plan should show how this is to be achieved. It would be usual to invoke in-house company practices, provided these were adequate, though in many system companies that have had no previous EMC experience these will have to be imported or built from scratch. Even if there is an existing body of in-house practice, this won’t always be enough to ensure a fully compliant result, and may in some cases work against it. Two further possibilities exist:
In either case, the control plan must provide for a flexible and informed response to such situations, and a means of ensuring that changes to the build state which could affect EMC aspects are reviewed. Next: EMC test plan Prev: Enforcement and future of the EMC Directive (coming soon) |
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Updated: Tuesday, 2019-07-09 19:20 PST