Purchasing EMC-compliant equipment

Many final apparatus contain complex electrical and/or electronic items which have been purchased from other suppliers, e.g.:

++ Finished products may contain bought-in sub-assemblies such as computer boards, or complete units such as power supplies, PLCs, computers, motor drives, panel meters, instrumentation and control modules, etc. (some of which may be finished products in their own fight).

++ Finished systems and installations are usually constructed from bought-in finished products, and systems, such as computers, telecommunications gear, instrumentation and control equipment, machinery, etc.

Compliance of the final apparatus depends upon the EMC performance of the bought- in items. Liability for non-compliance cannot easily be passed on to the supplier of a non-compliant item. Even where this may be possible, contingent losses such as product recall costs or harm to brand-image may well prove impossible to recover from suppliers or their insurers.

Where a final apparatus is found to be non-compliant by reason of the non- compliance of an incorporated item, enforcement agencies are likely to take action against both the final manufacturer and the supplier of the item.

The correct way to ensure that incorporated items don’t compromise the compliance of the final apparatus is not to rely on CE marking, but instead to ensure that their EMC and safety engineering performance is adequate. The recommendations below are straightforward for engineers to adopt, being similar to the process they go through to ensure that functional performance is adequate. These recommendations make it quite easy to achieve due diligence for the final apparatus whilst also minimizing development and manufacturing costs and timescales and reducing business risk.

Even so, some companies may not feel that they have the resources to follow these recommendations. As long as the sum total of the hazards and risks of their products to users are low in the global sense (i.e. only a few people exposed to low levels of hazards) they may be able to demonstrate due diligence without going through all of them, but in such cases it’s recommended that their local enforcement agency should be asked to endorse their approach.

Determining the EMC specifications for an incorporated item

Electromagnetic stresses on the final apparatus

To begin with it’s necessary to decide which EMC and safety standards the final apparatus needs to comply with, taking into account its physical and electromagnetic environments.

This may not be as simple as choosing harmonized standards from a list, because harmonized EMC standards may not adequately cover the actual electromagnetic environment, and harmonized safety standards may not adequately cover all the essential health and safety requirements for the physical environment. Other standards may have to be employed, and/or unique specifications written, to ensure that the final apparatus meets the essential requirements of all the relevant Directives.

If, say, an operator is expected to use a walkie-talkie radio handset whilst controlling a machine, the generic industrial EMC immunity standard (EN 50082-2)

will not be tough enough to cover the level of exposure to VHF or UHF fields expected for the control panel. Neither of the early generic immunity standards include tests for the AC supply surges caused by distant lightning, or by the small dips and dropouts normally experienced on AC supplies, so additional EMC standards to cover these environmental conditions may need to be applied.

Foreseeable extremes and misuse

For the EMC of non-critical functions it’s enough to consider the normal operating environment of the apparatus. But for the EMC of all critical functions it’s necessary to consider all reasonably foreseeable situations, even if they have a low probability.

This includes the probability that an operator or visitor will use a mobile radio device (e.g. cellphone or walkie-talkie) in areas where their use is banned.

Where electromagnetic interference can cause a safety hazard or increase risk, such possibilities are covered by safety Directives and not by the EMC Directive, so they should figure in all risk analyses under these Directives. An example here is the possibility of interference with a PLC controlling an industrial robot, causing it to "go wild" and operate outside of its programmed range. It’s known that some robot manufacturers don’t consider this safety risk when creating the technical documentation required by the MSD, despite only guarding for the robot’s programmed range, and despite deaths known to have occurred in Japan due to this very problem.

Physical and electromagnetic stresses on an incorporated item

Having determined the physical and electromagnetic stresses for the final apparatus, the specifications for the items to be purchased may be derived.

Sometimes incorporated items are protected from the external environments to some degree (e.g. a shielded metal enclosure can protect against physical impacts and reduce field strengths), but sometimes they are exposed to higher stresses (e.g. an item mounted near to a variable-speed motor drive may suffer locally intense electromagnetic fields). The resulting engineering specification for the purchased item will ideally be a list of harmonized EMC standards, but may have to include modifications to them, (e.g. field strength increased to 30V/m in the VHF band to cope with 4 watt VHF walkie-talkies no closer than 0.5 meters). Other standards may also need to be added, e.g. surge testing to EN 61000-4-5 and/or EN 61000-4-12 at a defined level, and/or relevant IEC or ISO safety standards that are not harmonized.

IEC 61000-2-5, which is developed from it, will be found to be very useful by people who are not EMC experts, in helping to assess electromagnetic environments. EMC site surveys are recommended where safety-critical issues are concerned, although these may fail to detect electromagnetic events with a low probability of occurrence, or changes to the environment after the survey is carried out, so some estimation is generally required for some phenomena.

Functional performance requirements

To complete the engineering specification for the EMC of a purchased item, the functions that the item performs (or that depend upon its correct operation) are analyzed for their criticality). Safety functions are allowed no significant degradation of performance over the whole range of electromagnetic stresses, including those caused by misuse, overload, failure of another item, and low-probability environmental extremes.

Where the degradation of a function may cause significant financial loss (such as loss of production), or embarrassment to a project (such as a satellite launch being delayed) it may be decided to treat it as if it were safety-critical. Less critical functions may be allowed temporary degradations of performance during transient stresses.

Monitoring, reporting, and alarm functions often fall into this category, as long as they automatically recover after the event.

The use of a product is important when deciding criticality of functions. Some DC power supplies actually switch their output off whilst they are experiencing transient overvoltages, whereas others will ride through such transients without significant deviation of their output voltage. Both of them may legally claim that they meet the relevant generic EMC immunity standard, since these allow any amount of temporary degradation of performance during transient tests -- and don’t mandate that this degradation is specified by the manufacturer to the user, as long as it’s what he may "reasonably expect". Where the power supply is feeding lamp and indicator circuits it may be acceptable (although annoying) for it to respond incorrectly during a transient, as long as it recovers afterwards. But where the power supply is feeding a circuit involved in critical functions (e.g. a PLC, relays, contactors, pneumatic solenoids, etc., controlling machine operations) it’s obviously important to choose a power supply which rides- through the transient, especially as some premises have been logged as experiencing several hundred transients on their mains supplies every day.

Emissions may be too high

Harmonized emissions standards allow emissions to occur, and these may be too high in situations where sensitive apparatus is nearby. This is especially important in some scientific and medical situations, usually where sensitive measurements are involved.

How many machinery manufacturers who, when asked to install a waste crushing and packaging machine to a hospital, would automatically ask what there was on the other side of the wall that their contactors and motor drives might interfere with (such as a life support ward using sensitive physiological monitoring apparatus)? Although continuous RF emissions from 150kHz to 1GHz are covered by harmonized emissions standards, emissions outside this range are not, yet may need to be controlled to prevent interference problems which would upset the user (or third parties) and prevent compliance with the protection requirements of the EMC Directive. Transient emissions are another example of a phenomenon which is generally not much controlled by harmonized standards, yet is known to cause problems in real life.

Emissions can add up

The total of the electromagnetic emissions from a number of incorporated items will exceed their individual emissions. In some cases this will result in a busier emitted spectrum without any increase in emitted levels, but in other cases the emissions from the various units will be so close together in the spectrum that they will measure as higher emissions levels.

Increases in emitted levels are most likely to occur when a number of similar items are incorporated into the final apparatus. For identical items whose internal electronic operations are not synchronized together (such as motor drives) ten of them may be crudely assumed to increase the total emissions by 10dB (square root of ten). When items employing digital processing or switch-mode power convertors have their respective internal electronic operations synchronized by a "master clock", ten of them may be crudely expected to give an emissions level increased by 20dB. Where a number of items are incorporated in one enclosure with a single power lead, the emissions standard that applies to the final product may be exactly the same as the emissions standards that apply to the incorporated items -- one of the reasons why the CE + CE approach to EMC compliance is unreliable. These crude rules of thumb help determine the emissions specifications for an individual incorporated unit to be calculated on the basis of the number of those identical units which are to be fitted in a single enclosure.

Specifying the item

Once all the above considerations are complete, it’s possible to write a complete engineering specification for the EMC performance of an incorporated item. This should include all the electromagnetic stresses it’s to withstand, the amount of functional performance degradation allowed during the application of those stresses, and the amount of electromagnetic emissions it must not exceed.

In many cases this specification will be able merely to list harmonized EMC standards to describe the stresses and electromagnetic emissions. As long as critical functions are not involved the specifications for functional performance degradation may not be onerous for the item supplier.

The specification should be sent to the favored item suppliers for their replies, pointing out that actual independent evidence of conformity with the specification will be preferred. Sales people will readily supply an EU Declaration of Conformity, but this is not evidence, so some education of suppliers' sales people is to be expected.

Negotiating with suppliers

Suppliers may not be able to meet the specification, or may not be able to provide all the evidence that is required. Negotiations may ensue, leading to the acceptance of a reduced specification or reduced amount of evidence. It may also prove possible to alter the design of the apparatus to accommodate the specifications of standard items.

All engineering is compromise, and the great advantage of following these recommendations is that the designer of the final apparatus will be working with known compromises rather than invisible or unexpected ones. Murphy's Law (to which all other physical laws are subservient) guarantees that an unknown engineering compromise will cause the worst possible problems at the worst possible moment, so these recommendations may be thought of as an anti-Murphy defense.

It’s almost always commercially best to use items with adequate EMC and safety performance, rather than to purchase items which are (or may be) inadequate and deal with the resulting issues. Material costs may increase, but since it costs less to deal with problems at earlier stages of integration the final apparatus should benefit from least overall cost, and improved financial margins.

A final requirement is to make sure that the agreed safety and EMC specifications (and the agreed requirements for evidence that they have been achieved) are written into the purchasing contract accepted by the supplier of each item.

Suppliers who follow the prevailing culture of high specifications, low cost, and CE marking, without being able to provide acceptable evidence of actual performance, know that in the final analysis the law is "buyer beware". So it will be appreciated that following these recommendations tends to limit the number of suppliers to those who have shown that they can actually satisfy their customers' real engineering needs.

Checking suppliers' evidence of EMC performance

The real EMC performance of an item is unknown until evidence of engineering performance and quality control has been seen, and checked to be satisfactory (ideally by comparison with its purchasing specification determined as described above). Not many suppliers yet provide as a matter of course the functional performance specifications achieved by their items during EMC immunity tests, so it may be necessary to pursue this vital data. Items for which the necessary evidence is not available (for whatever reason) should not be purchased, unless it’s intended to put the final product through further EMC and safety compliance tests, and unless contingency costs and timescales have been allowed for remedial work and re-testing.

If potential suppliers claim design confidentiality issues as a reason for not providing evidence, insist on a trusted third party report which confirms that the item meets all the engineering specifications for EMC without revealing any (supposed)

secrets. Such reports need not be expensive or difficult for a supplier to obtain, if he actually has the evidence he claims.

Checking Declarations of Conformity

A suppliers' Declaration of Conformity (D of C) cannot in general be considered to be actual evidence, although it may be possible for small companies making low volume apparatus with no safety implications to rely on them (check with the local EMC enforcement officers). Even so, Declarations of Conformity are useful as a guide to the intended use of the item and the competence of its supplier. Things to look for in a D of C include whether it lists the EMC standards required by the engineering specification for the item. It may prove difficult to judge whether items are suitable if they list different standards. Some standards, such as EN 61800-3 for the EMC of motor drives and EN 61131-2 for the EMC of PLCs, cannot be applied to the final apparatus and so may be of little help.

It’s also worth checking whether the D of C actually covers the item concerned (and not something else), and is clearly signed and dated by the supplier’s Technical Director or equivalent. Dates which are only a few days old, for items which have been on the market for many months, must be suspect.

Also check for any inappropriate or unreasonable warnings, limitations to use, or attempts at disclaimers, such as "Don’t use this product if it causes interference" or "May stop working when interfered with", neither of which are unknown. Products not intended for safety-critical application (such as ordinary PLCs) should make this clear, and this may appear on their D of C, but don’t rely on it being so obvious. Legally, manufacturers are only required to state such restrictions in their "instructions for the user". An example of the sort of Declaration of Conformity which is commonplace and quite legitimate.

Problems to watch for concerning standards

It’s impossible to discuss the full range of EMC standards here. There is often a lot of confusion over the generic EMC standards -- with suppliers choosing those that make it easier for their CE marking, rather than providing the engineering performance that their customers actually need.

Remember that it’s the function and user environment of the final apparatus that governs which standards apply to it, rather than the technology it incorporates. This can lead to a number of problems with the standards applied to incorporated items, some of which are described below. E.g., a washing machine which incorporates a microprocessor has to use EN 55014 (the EMC emissions standard for household appliances), and cannot use EN 55022 (the EMC emissions standard for information technology). Note that functional safety may require tests to additional EMC phenomena to be considered, and/or alterations to the range or levels of the test standards that have been applied for EMC Directive compliance, as discussed elsewhere in this guide.

------ Example “declaration of conformity” ----

Declaration of Conformity

Manufacturer/importer name

Address certifies that the following apparatus conforms with the protection requirements of Council Directive 89/336/EEC on the approximation of the laws of the Member States relating to electromagnetic compatibility: Name of product (description of variants)

EMC standards applied: EN 50081 part 1 : 1992 (Electromagnetic compatibility- generic emission standard part 1: residential, commercial and light industry) and EN 50082 part 1 : 1992 (Electromagnetic compatibility- generic immunity standard --- part 1: residential, commercial and light industry)

Signed: Date of Issue: (signatory)

XX/XX/XX

-----

Problems to watch for concerning the generic EMC standards

There are two sets of two generic EMC standards, covering emissions and immunity for two different environment classifications, making four generic EMC standards in all:

++ EN 50081-1: the tightest emissions standard, for residential, commercial and light industrial environments. This is equivalent to EN 55022 Class B, VDE 0891 Class B, CISPR 22 Class B, and broadly similar to EN 55014-1, EN 55011 Class B, and FCC Part 15 Class B.

++ EN 50081-2: a more relaxed emissions standard for (heavy) industrial environments. This is broadly similar to EN 55011 Group 1 Class A, and EN 55022 Class A.

++ EN 50082-1" immunity for residential, commercial, and light industrial. The use of issue 2:1997 is preferred over the original 1992 issue, especially as it completely supersedes the 1992 issue on 1st July 2001.

++ EN 50082-2: the toughest immunity standard, for (heavy) industry environments.

The best items for general uncontrolled use, or where the user's environment may not be very well defined, are those that meet the toughest standards for emissions and immunity: EN 50081-1 and EN 50082-2. The best items will also meet IEC 61000-4-5 for surges at level 2 (light industrial) or 3 (heavy industrial), and the mains dips and dropout tests of IEC 61000-4-11. Although surge, dip, and dropout tests are not included in the early generics we know that they do occur in real life. Standardizing on such items makes the selection of items and their use in custom engineering projects much easier.

Items declared using EN 50081-2 are sometimes sold for incorporation in apparatus intended to be used in light industrial and commercial environments -- but their emissions are too high for these environments and their use would necessitate additional EMC work and probably EMC testing of the final apparatus, for due diligence to be achieved. Similarly, items declared using EN 50082-1 are often sold for incorporation in (heavy) industrial environments, where their immunity will be too low without additional EMC work and probably some testing of the final apparatus, for due diligence.

Despite the practice being impossible to justify, some items are declared using EN 50081-2 and EN 50082-1, the easiest of all the four generics -- but this means they are too noisy for residential, commercial, and light industrial environments and not immune enough for heavy industrial environments, so they cannot be used anywhere without significant additional EMC work, plus (probably) some testing of the final apparatus, for due diligence.

The implications of EN 55022 Class A

Items which may be classed as information technology or telecommunications equipment (ITE), e.g. computers, modems, printers, VDUs, keyboards, etc., are allowed to apply the Class A EMC emissions limits in the product-specific EMC standard EN 55022 for use in the commercial and light industrial environments -- though not for domestic use.

But almost all other EMC emissions standards require tighter limits for commercial and light industrial environments (usually equivalent to EN 55022 Class B). So when an item which meets EN 55022 Class A is incorporated into final apparatus that is not allowed to declare compliance using EN 55022 because it’s not ITE, such items can cause excessive emissions and lead to non-compliance with the relevant EMC emissions standard.

The reverse situation is also true, as described above: immunity is inadequate for use in the heavy industrial environment. This is a common problem when integrating computers and peripheral devices into industrial control systems or other more stringent applications.

The implications of EN 55011

Items declared using EN 55011 are known as "ISM" (industrial, scientific and medical) equipment: which means they may generate and/or use electromagnetic energy to achieve their main function. Examples include dielectric heaters such as wood gluers, plastic sealers and welders; induction heaters; RF stabilized electric welders; spark erosion machines; magnetic stirrers; and diathermy equipment (both medical and cosmetic). EN 55011 allows very high, and even unlimited levels of EMC emissions at specified frequencies (the so-called "free radiation frequencies" of the standard- the most important are 13.56MHz, 27.12MHz, 40.68MHz, 2.45GHz, 5.8GHz and 24.125GHz), which could cause considerable immunity problems for other equipment, and even health hazards for their operators. As the standard says, "special measures to achieve compatibility may be necessary where other equipment satisfying immunity requirements is placed close to ISM equipment". When incorporated in final apparatus that cannot utilize EN 55011, ISM items can cause excessive emissions which lead to non-compliance, and may require significant additional EMC work and probably some testing of the final apparatus for due diligence to be achieved. (But note that use of EN 55011 does not necessarily mean high emissions- other apparatus which does not generate high levels at the free radiation frequencies may also declare compliance against this standard.)

Checking assembly and installation instructions

For an item to actually achieve the EMC performance that its test and other evidence implies, it’s necessary to assemble or install it fully in accordance with its supplier's detailed instructions. This is very important for EMC, which can easily be compromised, for example by the use of the wrong type of cable, or the incorrect use of a "pigtail" on the screen of a cable. It’s also very important for safety, which can easily be compromised by inadequate mounting or ventilation.

Suppliers who cannot (or do not) provide detailed assembly and installation instructions relevant to safety and EMC should be treated with suspicion: it may be possible to assume that an item which is devoid of such instructions does not need any special installation precautions, but it would be wise to seek specific assurances. If the supplier cannot provide them, their products are best avoided.

A big problem for many one-off engineering projects is that assembly and installation staff don’t usually follow suppliers' detailed instructions, preferring to use what they have considered to be "best practices" (often unchanged for the last twenty years). Many modern EMC installation best-practices run counter to typical electrical practices, as can be seen by a review of IEC 1000-5-2.

Suppliers' instructions should be checked for inappropriate or vague limitations or instructions, e.g.:

++ "Don’t use this product if it causes interference."

++ "If interference occurs, fit filter and/or fit product in shielded box."

++ "This product may require manual reset after transient interference."

Assembly and installation instructions should also be checked to see if they specify expensive or exotic cables or connectors, additional filters or shielding, or unusual environmental conditions. These can significantly affect the overall project cost and timescales, a good reason for carefully reading an item's assembly and installation manuals before making the decision to purchase it, rather than after as is more common.

Checking test results and certificates

With a little experience, suppliers' test reports can be more revealing than they may have realized. Full test results from an accredited test laboratory make the most convincing evidence. "Accredited" means that their measurement integrity, understanding of the standard, quality systems, and independence have all been checked and approved by a government-appointed accreditation body, giving a useful degree of confidence in their testing and results.

EMC tests are notoriously inaccurate, with even world-class laboratories experiencing differences in measurement on the same test item of 6dB (that is, + 100% or-50%). A similar issue concerns the interpretation of safety standards, with one laboratory safety manager usually managing to achieve several points of difference from another on any harmonized safety standard. Accreditation helps to reduce differences in interpretation, though it does not necessarily mean a test is more "accurate". When we say "accuracy", we are really referring to a known and declared value of measurement uncertainty. Accredited labs are required to know their uncertainty (sic), but don’t have to give it in their test reports except in certain circumstances -- although many do.

Test laboratories can only be accredited for specified test standards -- so although we tend to say "Accredited Laboratory" what we really mean is "Laboratory which is accredited for the test standards concerned". So don't take for granted the logo of the accrediting body on the test laboratory letterhead: check whether the laboratory actually is accredited for all the tests covered by its report. An accredited lab's report should identify explicitly any test results which it reports that are not covered by its scope of accreditation.

Full test results should include: the exact identification of the model (and version) tested; detailed sketches or photographs of the test set-ups; lists of the test equipment used and their calibration dates; whether the item passed or failed the test; and the signature or identification of the test engineer. EMC reports should include emissions graphs and result tables showing the margin under the limit lines, and the functional performance criteria for the immunity tests.

Checking test set-ups

Proper EMC test reports will include sketches or even photographs of the test set-ups employed, and descriptions of how the tests were conducted. These should be checked for the following:

++ Do they agree with the supplier's detailed EMC installation instructions? Watch out especially for the use in the tests of special types of cables or connectors, or ferrite clamps or additional earth bonds, which are not mentioned in the installation instructions.

++ Do the test set-ups relate to how you intend to use the item? Check especially for a lack of some of the external cables (cables usually create the biggest EMC problems, so leaving them off usually gives better EMC results).

++ Were the emissions consciously maximized, and immunity consciously minimized, by the test procedures, methods, and set-ups? In all EMC test reports, make sure that there are no comments along the lines of "the product was modified and passed the test ..... ". It’s not uncommon for the supplier's engineers to apply remedial measures to products during testing, which an aware test engineer will fully record in the test report. What can then happen is that these remedial measures get "forgotten" when the items are manufactured.

Checking EMC Technical Construction Files (TCFs)

Where a product has been declared compliant with EMC Directive by using a TCF rather than harmonized standards, it will be valuable to read this - or at least, the Competent Body's report -- along the above lines. It’s not uncommon to find a number of comments in TCFs where the assessor has decided not to declare the product non- compliant, but nevertheless has serious concerns. Such warnings are often along the lines of: “The supplier should make clear to the customer certain specific installation requirements and limitations to use ...”

Suppliers' quality control

The fact that a supplier has had an example of an item tested for EMC performance using acceptable standards, and it has passed, of itself proves nothing about the EMC or safety performance of any of the other units of the same type and/or model. For that, a quality control system is necessary.

Even where a supplier has an EN ISO 9000 quality system in place, by itself this is no guarantee that the standard items supplied to the manufacturer of the final apparatus have any EMC or safety performance. All it means is that the company is audited against its quality manual, so it’s important to discover what their quality manual says about maintaining the specified EMC performance in production. To control the EMC performance of standard products a supplier must have controls over design changes and production concessions, unit build standard, repairs, refurbishment, and upgrades, at least as far as all EMC issues are concerned.

Even with all these controls in place a number of elements are still uncontrolled- especially the performance of the components that they buy in- and this makes it necessary for suppliers to have a sample-based testing policy for EMC. The better the suppliers' controls over its design, purchasing, production, and repair, the lower need be its rate of sample testing.

Companies with a "supplier approval" procedure will find it quite easy to add the necessary additional requirements to ensure that the EMC evidence provided by the supplier stands some chance of being representative of the items actually purchased.

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