TechArchive

Original article date: December 1995

A few short items looking at the European EMC (CE Marking) legislation and how to design to meet it.

1. Six months and counting…

(July 1995) About 20 years ago, a farmer in East Anglia complained to a mobile radio manufacturer that its equipment was responsible for him losing a toe. He claimed that his hearing aid picked up voices from the radio transmitter and the shock caused him to drop a chain saw.

The possibility of accidents caused through electromagnetic interference of this type has risen dramatically, hence the European Electro Magnetic Compliance (EMC) Directive – the most complicated Directive to have come into play recently. It was enacted in October 1992 when an amending Directive provided a transitional period, in order for UK companies to test equipment and gain compliance. That time expires on 31 December 1995 and is extremely unlikely to be extended.

Generally, only post-production testing is acceptable as the means of certifying CE compliance. The biggest problem with the EMC Directive is that anything done to a product to change its electrical characteristics is likely to affect its EMC and will invalidate existing CE marking – and that can be modification as innocent as installing a plug!

Though designing a compliant product is just the beginning, it is imperative that EMC be considered at the design stage of any product. It has been estimated that EMC precautions need add only 2% to the cost of a product if they are incorporated at the design stage.

At the design stage, the engineer aiming to control EMC has several options. Initially, the possible EMC performance of any circuit technique must be carefully considered – a single-chip microprocessor will need fewer high-speed bus connections than a multi-chip solution, reducing the opportunity to radiate. Some microprocessor designs are optimised for best EMC performance. Thyristor-switching motor control circuits may cause fast changes of current and consequent radiation. Switches or relays should be fitted with contact suppressors. Transient suppressors are useful on power lines, to protect against surges.

At the PCB level, a ground plane and substantial power tracks will lower ground connection impedances and provide screening, reducing radiated noise problems. Intelligently-chosen decoupling and filtering components also contribute to noise reduction. A metallic enclosure will provide additional overall screening, while shielding (conductive) aerosols are available for plastic enclosures.

Conducted interference can be improved by filtering signal lines as they leave a PCB, using signal-line noise filters or three- terminal capacitors. Certain types of panel-mounting connector incorporate ferrite bead filters, although these tend to be expensive. Screened cables provide additional protection. A well-chosen mains inlet filter can help against conducted interference on the power connection.

2. CE marking for drives: best of luck, everyone (September 1995)

The proposed IEC standard for inverters has arrived as the latest development in the EMC Directive saga. Guess what? As an OEM or machine builder, it’s over to you…

Due to the confusion over the EMC compliance of inverters, the European Trade Associations have proposed a product specific standard for Power Drive Systems (PDSs), IEC 22G/21/CDV. It stipulates the required emission and immunity levels of PDSs and the test methods to measure the levels. Under European law, it takes precedence over all generic standards previously applicable.

According to the standard, a PDS consists of a motor and a Complete Drive Module (CDM). It does not include the equipment driven by the motor. The CDM is a Basic Drive Module (BDM) plus possible extensions, such as option cards. The BDM has two main parts, the frequency or voltage converter and control section. It is classed as a variable speed inverter.

However, this does not mean inverters must be CE Marked under the EMC Directive. In fact quite the opposite is true. The standard quotes the four EMC ‘validity fields’ and applies them to PDSs. It puts PDSs and their constituents into classes depending on functional characteristics and route to market, which determines whether or not the Directive applies.

1. Unrestricted complex component – sold ‘as built’ directly to the end user.
2. Restricted complex component – only sold to professional assemblers .
3. Installation – consists of one or more PDSs and is not intended to be placed on the market as a functional item.
4. Apparatus/system – includes one or more PDSs (or CDMs/BDMs) and has an intrinsic function to the end user.

Although components are excluded from the Directive, it states that those sold without any control over the application (unrestricted components), must have a sufficient degree of EMC. That is, if members of the public (normally referred to as end users) buy a component off the shelf, they will not have to worry about compliance when they fit it to their machine.

This is understandable as you would not expect the ‘man on the street’ to have an EMC test chamber. A good example is a sound card for a PC. You would expect the card to be compliant when fitted to your computer and not to need further testing. Therefore, according to the Directive, the responsibility for CE Marking such components under EMC lies with the manufacturer in this instance..

IEC 22G states that inverters come under the second category of components – restricted distribution. Again, they do not have an intrinsic function for the end user, but are sold to professional installers who incorporate them into a machine, apparatus or system. They are not on sale directly to the end user or the general public. The installer has a level of technical competence to install correctly and there is usually an exchange of information beforehand. Manufacturers have some control over the application, as they generally know where the inverter will be used. For this reason, installers are responsible for the EMC behaviour of the apparatus and hence the inverter. Therefore, inverters cannot be CE Marked under EMC and it falls to machine builders to ensure they are compliant..

However, OEMs are not left with the total responsibility. The standard states inverter manufacturers must provide installation guidelines to the assembler. The guidelines should describe the correct installation of an inverter into a typical system or process so that it complies with the requirements set out in the standard (defined levels of immunity and emission). They must state if any additional equipment is necessary, such as filters, and if any special measures need to be taken, like using no more than a certain length of cable..

Stephen Darnell of Mitsubishi Electric says his company has already researched this area and produced a comprehensive installation guide for its inverters. It has developed a range of high attenuation ‘footprint’ filters in conjunction with Roxburgh Electronics. All of its FR-A series inverters have been tested independently by certified test houses in a typical installation using the guide, and have been found to meet the requirements of IEC 22G/21/CDV. Further EMC hints and tips are included in the guide to assist installers with non standard requirements..

So, for machine builders and designers there is good news and bad news. The bad news is that inverters are likely to be officially deemed as components and therefore the responsibility for ensuring they comply with the EMC Directive is with the OEM. The good news is that the proposed standard, once it is approved, will ensure that inverter manufacturers make it as easy as possible by officially charging them with the job..

At the moment, the standard is in its final drafting stages and the approval procedure is moving along rapidly..

Mitsubishi Electric is on 01707 276100. The European Trade Associations are GAMBICA, ANIE, GIMELEC, SETELI and ZVEI. Information taken from the CEMEP draft guide on Recommendations for Applications of Power Drive Systems.

3 Start with some simple rules (September 1995)

In this month’s Designing To Avoid EMC Problems feature we bring you some basic electrical design guidelines, courtesy of a new publication from ERA Technology.

Consideration of EMI/EMC in equipment design begins (and often can end) at the power supply, as the influence of this extended component is felt throughout the equipment. Correct routing and configuration of the power supply can considerably decrease the necessity for remedial work elsewhere..

A linear power supply usually consists of a transformer with one or more secondary windings, which are rectified and smoothed to produce unregulated dc. This may be regulated if required, and it is used to power the equipment. The best design of power supply will vary from equipment to equipment, but the following rules should be observed:.

Keep all wiring as short, and as low inductance, as possible. This reduces any common impedance interaction and the short connections will radiate less..

Use twisted pairs (shielded if necessary) for all wires carrying equal and opposite currents. The twisting minimises the inductance of the circuit and also reduces the effective radiating area, as the magnetic effects of alternate twists tend to cancel, at distances greater than the twist pitch..

Take care that rectifier current is not routed together with load current. This will minimise the common impedance coupling between the dc and ac circuits. Capacitors with a low value of Effective Series Resistance (ESR) – or a number of capacitors in parallel – should be used in order to decrease this coupling still further..

Feed digital and analogue circuits from separate supplies, or run separate leads to each from the regulated point to the PSU, as this will have the lowest impedance. Remote sensing for the power supply regulator can put this low impedance point at some distance from the supply. Separate on-board regulation for analogue and digital circuits is often helpful. This precaution reduces the possibility of coupling between the analogue and digital circuits..

Route supply leads to low-level circuits as far away as possible from leads to high power circuits. This reduces the pick-up onto the low-level suppliers..

Mains wiring inside equipment shields should be kept as short as possible. Ideally, a switched and fused filtered inlet should be used, so that there is no unfiltered mains wiring inside the shield. This will decrease the risk of pick-up from and by the mains lead..

Use two electrostatic screens on the mains transformer, with one screen connected to 0V and the other to mains earth. This reduces the capacitive coupling from primary to secondary, by diverting interference on the primary to earth and interference on the secondary to 0V. It might be better still to wind primary and secondary on opposite sides of a toroidal transformer..

If a filtered inlet is not used, then the earth wire of the mains lead should be connected to the chassis as close as possible to the inlet point. This will reduce the amount of pick-up on and from the earth lead. It will probably not be possible to follow these guidelines in all circumstances. Indeed, it is possible that they may conflict with each other, in which case judgement and experiment will determine the relative importance which each should assume..

Direct off-line switching power supplies in general require the same precautions as linear supplies. The problem is greater here, as there are large amplitude fast edges inherent in the supply. For efficiency reasons, it is desirable that the switching times of the transistors should be no longer than a few microseconds..

Another source of conducted interference in SMPS is the interwinding capacitance of the transformer. Here again, a suitably positioned and connected screen will minimise the effect. SMPS are also sources of radiated EMI. It is necessary to use great care in the layout of the components, in order to reduce possible radiating loops and connections to a minimum.

Internal Wiring

Having designed the power supply and power wiring throughout the equipment for the best EMC performance, the next area to address is that of the internal signal wiring. The various interconnections will act as transmitting and receiving antennae with varying efficiencies. The wiring should also be designed with transmission line principles in mind, so that the energy is retained within the line, rather than being radiated. Such transmission lines must, of course, be correctly terminated, in order to avoid reflections and pulse distortion. Ordinary ribbon cable works quite well as a transmission line, with a characteristic impedance of 100-150ω, for adjacent conductors, depending on the signal arrangement. If extra shielding is found to be required, then a ground plane or wrap-over shield can be used, which reduces the characteristic impedance to 60-120ω. For improved rejection of magnetic coupling, ribbon cable incorporating twisted pairs can be used, again with a characteristic impedance of 60-120ω..

PCB layout is as important to the reduction of EMC problems as wiring layout and yet is often neglected, being left entirely to a computer aided design programme which will not take any notice of EMC requirements. In general terms, if circuits of various inherent speeds are used on the same board, then the fastest circuits should be closest to the connection to the board, with the slowest circuits around the periphery. This elementary precaution will reduce track lengths carrying fast signals to minimum length, as well as avoiding power rail coupling from high speed to low speed circuits. Analogue and digital circuits on the same board should be separated as much as it is feasible to do, using separate connectors if possible..

Designing for Electromagnetic Compatibility – a Practical Guide, is published by ERA Technology. This report extends to 147 pages and costs (UK pounds)85. Telephone 01372 374151.

4. EMC Directive: Brussels chaos or Chinese export? (October 1995)

A personal view from Simon Hunt of Astrosyn, who has spent two and a half years piloting the company’s progress along the route to compliance with the EMC Directive..

As long ago as May 1985, Community Ministers agreed on a “new approach to technical harmonisation and standards”. In essence, this sought to replace differing national laws and regulations with a single Community regime. Electro-Magnetic Compatibility is merely one of a whole raft of Technical Directives which include Toy Safety, Construction Products, Gas Appliances and Machinery – to name but a few. The Machinery Directive became law from 1 January 1995. The EMC Directive will be implemented from 1 January 1996..

Each Directive itemises what are described as “attestation” procedures, designed to assess product compliance. Passing is rewarded with the CE mark – of which more later. Among offences against the Directives, is affixing a CE mark to a product which does not meet the “essential requirements”, and affixing an imitation CE mark. Penalties resulting from prosecution could lead to fines up to (UK pounds)30,000, or imprisonment – or both. The above would seem to indicate that the CE mark is a highly desirable and valued designation. Strange to relate, however, nobody appears able to say what it means. Enquiries from official sources elicited a variety of definitions: Certificate of Excellence, Conformance Electrical and, the most likely, Communite Europeenne. It has been suggested that “Chinese Exports” might be more appropriate for a system where detailed requirements, including those relating specifically to CE marking, vary from one Directive to another, yet the CE mark itself remains common for all Directives..

Interestingly enough, the EMC Directive was originally planned to take effect from 1 October 1992. One can only speculate about the reasons for delay. Getting down to what this particular Directive is all about, some of us may admit to recalling the effects of radiated electrical noise on radio and television reception, caused by unsuppressed appliances and passing vehicles. As far as anyone knows, however, there are no recorded instances of an airliner falling out of the sky due to a rogue hearing aid..

This type of problem was solved rationally and by market pressures, without recourse to complex regulations, the creation of a vast bureaucracy, tons of paper, endless symposia, costly test operations and the threat of dire penalties. Why should not an equally commonsense approach be as effective in tackling all the phenomena associated with conducted and radiated electrical noise?.

Because some nations within the Community may be backward in such areas, it looks as if Brussels is yet again penalising the more developed and efficient countries with swingeing legislation and extra costs. For an example, one has only to look at the anomalies of the Common Agricultural Policy. Regrettably, the apparent inevitability of the EMC Directive and the confusion which surrounds it, has created a bandwagon effect. A virtual industry has been created, with a variety of experts using scaremongering tactics to peddle their services. Yet, at this stage, practical engineering terms of reference are still unclear. While acknowledging the possible need for an initiative to eradicate or, at least, minimise electrical interference, indications show that it is highly problematic to identify the level at which evaluation should start..

It is indisputable that components behave differently according to the manner in which they are combined. One has to ask, therefore, whether the required compatibility parameters should only be evaluated against a complete item of equipment. In any case, components do not become functionally operational until they are married to others, suggesting that it is interaction rather than generic performance which should be paramount..

There are many instances where a complete item of equipment or system is composed from a series of linked equipment. In such cases, and in terms of the impending legislation, would these linked modules be defined as components?.

At the other end of the scale, assuming that basic components like resistors and capacitors would be required to carry the CE mark, one wonders whether this might be extended to cover the materials of construction, such as copper wire, carbon chips and polymers!.

Let us take the example of the electric motor. Normally it would be classed as a component, in the sense that an electric motor only becomes truly functional when it is moving something. Drive components vary vastly from, say, a 6V battery to thyristor/switching control circuits, while the driven equipment could be anything from a simple fan to a space probe. All such elements will influence the performance and electro magnetic characteristics of the motor, as well as the equipment in which it is installed..

Factors like these may well point to the conclusion that the EMC evaluation process is not viable without all aspects of the final application being taken into account. This scenario could be so complicated as to render the whole procedure impossible to administer or enforce..

Astrosyn is on 01634 815175 5>

5 Is EMC affecting costs? (November 1995)

Alan Matthewman of Danielson looks at putting EMC protection in membrane keyboards and asks: are we over-engineering?.

The forthcoming emc legislation has important design implications, particularly for component parts, such as membrane panels, that may contribute to potential EMC problems with completed equipment. Among the components which are most affected are fast logic, signal conditioning and amplification circuits, plus all types of communications receivers..

Electromagnetic emissions behave in an almost identical manner to light waves. When they impact on a surface they can be reflected, absorbed, disrupted or transmitted. Thus, enclosing electronics within a metal case is perhaps one of the most effective forms of protection. In applications incorporating membrane keyboards, however, this is impractical, as the plastics typically used are mostly invisible to electromagnetic energy. Additionally, the keyboard may act as an aerial for RFI, providing a path for interference..

To combat emissions, received or transmitted, additional shielding may need to be incorporated in the membrane keyboard design. Effective shielding can be achieved in a number of ways, to meet the requirements of different keyboard designs, switch structures and end user applications..

Additionally, shielding can be constructed as a continuous solid, irregular shape or grid pattern. The exact shape and size of the pattern varying according to the characteristics of emissions at different frequencies. Regardless of the method of shielding specified, it is critical that the shielding is manufactured to very high levels of accuracy, as electromagnetic energy can be transmitted through gaps in shielding as small as 1% of the energy’s wavelength.

Furthermore, particular attention must be paid to earthing the shield, with the earthing method specification varying dependant on the attenuation required.

The principal methods of shielding are

• A solid or hatched copper ground plane etched directly onto the PCB.
• A solid or hatched silver ink layer, formed within the membrane structure.
• A special conductive but transparent sputter deposited indium tin oxide film, which can be used over display windows.
• An inserted layer of aluminium or copper foil.
• A blackened steel mesh embedded within a rigid display window.

Each of these shielding methods has different merits, depending on the requirements of the application. For example, the incorporation of a metal foil layer, although providing effective protection, can dampen tactile response and the use of indium tin oxide (ITO) can be an expensive option in many applications. ITO coated materials are used in membrane keyboard designs that incorporate display windows as it is the most cost-effective method of providing protection with the minimal reduction in the light throughput for higher specification shielding. However, the more expensive route of a mesh embedded in a rigid window may be required.

Fortunately, most display windows neither emit or are vulnerable to interference, as the LCDs tend to be clad in metal units which are already shielded. Window shielding is typically only required in areas such as military hardware or medical or weighing apparatus, all of which are highly sensitive to interference. However, many manufacturers and users in other production areas continue to specify this level of shielding.

This raises the question, are manufacturers over-engineering to meet legislation and how is this affecting cost? The important consideration is that the membrane structure should not be viewed in isolation, but rather as an integral component, to enable the most effective method of protection to be specified, both in terms of performance and cost.

Additionally, if the potential for shielding is designed into the keyboard at an early stage, even if it is not immediately required, it can then be easily introduced later, without the need for major re-engineering costs, with which many manufacturers and specifiers are now being faced.

Under the EC Directive, the onus is on manufacturers to demonstrate compliance. However, it is also up to the manufacturer to determine the level of design and testing needed. In the current economic climate, with cost control being the predominant issue, both manufacturers and users are concerned that compliance with the EMC Directive will cause a substantial rise in prices. However, with careful consideration to the design and incorporation of shielding, there is no reason why EMC protection should add greatly to costs. In fact, recent reports suggest an increase of no more than between two and five percent to end user prices.

6. Feedback (November 1995)

Peter Still of Telemecanique has written in to say that, in the article based on information submitted by Mitsubishi in September “CE marking for drives – best of luck, everyone”, contains a large number of misleading statements and assumptions. The article dealt with the implications of a proposed IEC standard for inverter requirements, which the article claimed would take precedence over all generic standards previously available.

Peter Still says that the proposed product standard for drives IEC 22G/21/CDV is identified by the …./CDV as a “Committee Draft for Voting”. If it receives a positive vote from a majority of the member countries of the IEC, it will then move to the next stage, a DIS (Draft International Standard). it is clearly, therefore, some time before these proposals can be published as a standard.

Even if published as an IEC standard, this document will not give automatic presumption of conformity to any EC Directives. Only compliance with a transposed harmonised European standard can provide presumption of conformity. The process of generating a European standard is quite lengthy, particularly where EMC is concerned, since TC110, the Technical Committee responsible for EMC issues, must ratify all proposals by other TCs.

Peter Still disagrees that the IEC standard will take precedence over all generic standards previously available. He says that harmonised European Standards EN 50 081 and EN 50 082 are the generic standards for emission and immunity, which must be applied if no product-specific European harmonised standard exists.

The four ‘validity fields’ quoted in the IEC 22G draft are not recognised in the EC Directives. However, the European Commission has published guidance on the application of the EMC Directive. This guidance defines ‘components’ as ‘not having an intrinsic function for an end user’. The level of complexity is not an issue and neither is the route to market.

Nevertheless, the Directive requires that electrical and electronic equipment shall have an adequate level of electromagnetic compatibility to function correctly, without interfering with adjacent equipment, when correctly installed in the intended environment. The manufacture of the variable speed drive must therefore supply sufficient information to enable the installation of the VSD to comply with the generic standards or other relevant harmonised standards. Description of a typical installation is not likely to be sufficient.

Machine builders (including those who modify or refurbish machines) already have the responsibility of ensuring the EMC compliance of their machines. Market forces will ensure that the most helpful suppliers will be the most successful. The major effect of this proposed standard thinks Peter Still, it is achieves publication as a harmonised European Standard, will be to relax the emission limits slightly, to the benefit of drives manufacturers.

Colin Hargis of Control Techniques has also written in to point out that IEC22G/ 21/CDV is a draft IEC standard and agrees that it has no impact until it becomes a harmonised European standard listed in the Official Standard as applicable to the EMC Directive.

In any case, says Colin Hargis, a standard does not in itself affect how a Directive is applied. Standards are used as one way of demonstrating compliance. The EMC Directive does not apply to components. If a product is a component in the context of the EMC Directive, then the Directive does not apply and therefore cannot require the application of standards.

On the other hand, if a product is sold to the end user as a “complex component”, in the same way as your example of a PC sound board, then it must comply with the Directive. This can be either through the standards route or the technical Construction File route. In this case, it does have to be CE marked for EMC.

The OEM has always and will always be responsible for the EMC of his product. Many OEMs would prefer to buy in all their active components ready compliant, since this minimises the effort in achieving compliance of the end product. Compliance of the end product still has to be proven, however.

In practice, it may be that fully compliant modules are considerably more expensive than non-compliant or partially compliant ones. It may be that if production volumes justify it, a more cost-effective solution would be to buy in simpler, non-compliant modules and add features to the product to achieve compliance of the whole. A simple example would be to use a single line filter for the whole product. This would give a lower per unit cost, but probably require more work in achieving compliance of the design. The decision needs to be made by the manufacturer of the end product and will be based on cost considerations, rather than rules and standards.

Colin Hargis believes that most drive suppliers would agree with the conclusion of the Mitsubishi article, that drives must be provided with comprehensive guidelines to help installers achieve compliance. Also, they can cost-effectively be made to conform intrinsically with the majority of the relevant standards and the manufacturer can provide declarations of such conformity, thus helping the installer to prove compliance of the end product.

Colin Hargis concludes with some “controversial” questions of his own: What measures are suppliers to take to prevent their “restricted distribution” products from falling into the hands of end users? If and when the standard for power drive systems becomes a listed harmonised standard, what of the OEM who finds that the PDS standard conflicts with the standard for his own product? Most commonly, the end product is a specialised item which has no product standard, so the generic standards would apply. In Colin Hargis’ view, since most OEMs will seek to comply with the generic standards, they will be looking for drives which comply with the generic standards. Is the PDS standard relevant? While the drives manufacturers discuss the finer points of EMC Directives and the relevant standards which may or may not apply, it all gets more confusing for the machine builder. Mr Butler of Concept EDM is one such reader who replied to the provocative article in our October issue written by Simon Hunt of Astrosyn, who argues that the scenario could become so complicated as to render the whole procedure impossible to administer or enforce.

Says Mr Butler: “This is the first commonsense article I have read on the subject of the EMC regulations. There are tons of leaflets, books and magazine articles, all no help at all to most small manufacturers. What is needed is some very basic equipment to enable simple tests to be made so that self-certification can be carried out on a sample of the manufactured item. Should it prove difficult to pass this test, then a consultation with an outside test house could be considered; not as is the situation at the moment that all small manufacturers are expected to go to a test house at a cost of (UK pounds)800 per day. Would not equipment such as the ELF Monitor (low frequency electromagnetic field detector RS 212-837) which costs (UK pounds)109 be adequate? All that is needed is the distance and reading that would satisfy the regulations.

“Can someone please tell me what are the other EC member countries doing? I wonder if their level of interest is determined by the number of Competent Bodies in each country: UK 23, Germany 11, Netherlands 5, Italy 4, France 3, Denmark 2, Belgium 1, Ireland 1?” Amen to that.

December 1995