Introduction
The correct purchase management and technical control of the components necessary to design a machine or a complex system from the point of view of electromagnetic compatibility (EMC) is vital to ensure mandatory compliance with the EMC Directive, 2004/108/CE . The concept “CE marking + CE marking = CE marking” (CE+CE=CE) deals with a very common practice in the machinery and large fixed installations sector. It is based on the idea that if you buy a number of components or devices for a system, all of which are CE marked, the complete system made up of these components does not need any additional work to be CE marked. Thus, the assembly could be declared compatible with all relevant safety, low voltage and electromagnetic compatibility (EMC) directives. With the practical ”CE+CE=CE” theoretically the CE marking could automatically be applied to the complete machine. But, the most certain thing is that if the EMC tests are carried out, the complete machine does not comply and the CE marking cannot be applied. It is also evident that if any of the components incorporated into the machine is not compliant and does not have the CE marking, the complete machine will not be compliant either and cannot bear the CE marking. The CE+CE=CE practice is totally wrong in every way and is not legally accepted.
The use of increasingly sophisticated electronic technologies in machines is increasing in all areas of human activity, including those in which errors or functional failures may have implications for functional and human safety. All electronic technologies are vulnerable to errors or malfunctions caused by electromagnetic interference (EMI). More sophisticated and complex technologies tend to be more susceptible. In addition to natural sources of EMI, such as lightning and lightning, all electrical and electronic technologies are sources of EMI. The trend is to increase the frequency of processing and therefore to increase electromagnetic emissions (EM). The consequence of all this is that, if electromagnetic compatibility is not considered in the engineering designs of machines, functional failures can arise with uncontrollable consequences for the people who use them. With this there can be uncontrolled economic risks for manufacturers and service providers who use electronic technologies in their machines.
When failures in the electronic components of machines can have functional or personal safety implications, it is necessary to ensure a good level of EMC to control the safety risks and the associated economic risks. Historically the disciplines of functional safety and EMC have been developed separately. In general, safety review engineers do not have a detailed understanding of EMC and EMC or electronic design engineers do not have a thorough understanding of functional safety. There are not many safety standards that include EMC requirements in functional safety.
Economic risks can arise above all due to product liability, but also due to non-compliance with safety or EMC regulations. This may lead to the prohibition of entry into the market of dangerous products in the European Union or their withdrawal from the market. It can also negatively affect the manufacturer's brand reputation. The costs of having good EMC and security management are always less than the legal costs of any liability claim.
The EMC can affect the safety and proper functioning of the machines. A machine is made up of components that must comply with EMC, but their incorrect installation can make the complete machine non-compliant. A good management of purchase and analysis of the components then facilitates compliance with the CEM. Likewise, a good installation of the components, following good design rules, is key to ensuring EMC compliance. All of this ensures functional safety, reduces reliability risks and contributes to better machine quality.
The designer and creator of the machine is responsible for ensuring that all the components that make it up have the appropriate safety and EMC features. Achieving both objectives can mean that the technical specifications regarding EMC and safety of the components that make up the machine will surely have to be changed.
At the same time, electromagnetic mitigation measures must be foreseen in its installation. The machine designer must have the necessary knowledge of the application and full control of the design, to ensure compliance with the desired levels of safety and EMC performance. If not, it would be wise to outsource external experts.
Negotiation and commitment of component suppliers
Suppliers may not be able to meet specifications, or may not be able to provide all required EMC test documents. The acceptance of a reduced specification or a reduced number of tests can be negotiated.
It may also be possible to modify the design of the component to suit the specifications of the final machine. Engineering is always commitment and the great advantage of following a methodology is that the designer of the machine will know the agreed commitments. It's much better than working with unknown specifications.
It is almost always commercially better to use components with a good EMC design, rather than buying components that might be insufficient and then dealing with any EMC problems that show up in the machine, at the end of its build process. Component costs could increase, but by costing less to address EMC issues in the early stages of machine integration, we can benefit from lower overall cost. Experience has shown that it can cost about 10 times as much to address an EMC issue at a higher level in the integration process. If the EMC problem appears when the machine is already installed in the customer's factory, the costs can be multiplied by much larger factors (100 to 1000 times).
We must ensure that the agreed EMC technical specifications are written into the purchase contracts negotiated with the component suppliers. In the most critical components, it is recommended to agree on indemnity clauses, so that the supplier commits to indemnify the buyer if its components do not meet the required specifications.
Suppliers who offer good specifications, low cost and CE marking, but cannot provide acceptable evidence of their actual EMC performance know that, in practice, they are passing the responsibility for compliance with the EMC Directive onto the buyer. By following these recommendations the number of acceptable suppliers tends to be reduced to only those that demonstrate that they can really satisfy what their customers need at the EMC specification level.
EMC conformity check of components
The actual electromagnetic behavior of a component is unknown until it is tested. Not many vendors provide the results obtained for their components during EM emissions and immunity tests. Components for which the necessary evidence is not available should not be purchased unless you want to risk EMC problems in the final machine, unforeseen costs and longer lead times for the machine to the customer.
If potential suppliers cite trade secret issues as a reason for not providing the lab test report, we must insist on having a report confirming that the product in question is EMC compliant, without revealing any alleged secrets. These reports simply give the test results against the standards and do not need to go into the details of the component's internal design. The trade secret excuse is not acceptable and does not hold up.
A supplier's declaration of conformity for a product is not definitive proof, although it may be for products from small, low-volume appliance manufacturers where there are no major safety implications. However, declarations of conformity are useful as a guide to the intended use of the product and the professional competence of the supplier. Details to note in the statement should include the list of required standards. It may be difficult to judge if the components are suitable if only the different standards are listed. Some standards, such as EN 61800-3 (Variable speed electrical power drives) for inverter/motor units and EN 61131-2 (Programmable controllers: Equipment specifications and tests), cannot be applied to the final machine and can be of little help. These two standards have very relaxed levels. The needs, costs and risks of machine building customers are more demanding. In practice it is good to require a large confidence margin for the emissions (for example about -10dB below the requested limits for the whole machine) to take into account the unavoidable variations in your own series production and the accumulation of the emissions that often occurs in machines. But if each component only has a -2 or -3 dB margin at various frequencies, the chances of having some exceedances of the norm limit is probably 50% or more. To mitigate the situation described, the method may be to impose a minimum margin of -6 dB with respect to radiated emissions and a margin of -3 dB for conducted emissions.
It is also worth checking whether the compliance report is clearly signed and dated by the supplier's manager or his technical equivalent. Dates that are recent for components that have been on the market for many months may be suspect.
In the instructions for use you must also observe, in your report of conformity, inappropriate or unreasonable warnings, limitations of use, or attempts at disclaimers, such as "Do not use this product if it causes interference" or "may leave to function if the equipment is interfered with”. Products not intended for safety-critical applications (such as conventional controllers) should state that they are not intended for such use, although few do.
Problems to observe in terms of standards
There is often a lot of confusion about generic EMC standards. Many suppliers choose the standards that make their CE marking easier, instead of providing the features that their customers really need. Remember that it is the function and electromagnetic environment of the user of the end machine that governs the applicable standards, rather than the technology that the component incorporates. This can lead to EMC issues with the standards applied to the components. For example, industrial and commercial control panels that use microprocessors have to apply for EN 61000-6-1, and should not use the EN 55022 emissions standard, which is often thought to apply to anything that uses microprocessors. digital technology.
Some products are declared using the EN61000-6-1 and EN61000-6-4 standards, the easiest of the four generic standards to meet But this means the products are too noisy for residential, commercial and light industrial environments and are not immune enough for the most demanding industrial environments that they cannot be used anywhere without significant additional EMC improvement work.
The best products for general uncontrolled use, or where the user's environment may not be very well defined, are those that meet the most stringent standards for emissions and immunity: EN61000-6-3 and EN61000-6-2. The best products will comply with EN 61000-6-3, as it includes tests for transients ( EN 61000-4-5 ) and voltage drops ( EN 61000-4-11 ) that occur in real life. The standardization of these elements makes the selection of the products and their use in the final machines much easier.
Products declared to EN61000-6-1 are usually sold for incorporation into appliances intended for use in industrial and light commercial environments, but their emissions are too high for these environments and their use requires EMC reinforcement and probably , carry out further EMC tests on the final machine, to ensure compliance. Similarly, EN61000-6-4 declared items are often sold for incorporation into noisier industrial environments, where their immunity will be too low without further boosting.
Products that can be qualified as information technology or telecommunications (ICT), for example, computers, modems, printers, keyboards, etc., are authorized to use the class A emission limits on their specific products in the standard. EN 55022, for use in the commercial environment and light industry. But almost all other EMC emission standards require more stringent limits for commercial applications and light industrial environments (usually equivalent to EN 55022 Class B). When a component that complies with EN 55022 Class A is incorporated into the final machine where it is not allowed to declare conformity using the EN 55022 standard, this component can cause excessive emissions and cause non-compliance with the corresponding emission standard. This is a common problem in integrating computer equipment and devices into industrial control systems, or printers, keyboards, and displays in almost any machine or fixture.
The products declared according to the EN 55011 standard are ISM (Industrial, Scientific and Medical) equipment. ISM products may use electromagnetic energy to achieve their primary function. Examples include dielectric heaters such as gluing and drying machines, plastic welders and bag sealers, induction heaters, electric welders, spark eroding machines, magnetic stirrers, and diathermy equipment, whether medical, physiotherapy, or cosmetic (such as some epilator machines used in beauty salons).
The EN 55011 standard allows some categories of products to emit high values and even unlimited levels of emissions, at specific frequencies and thus can cause considerable immunity problems in other equipment and possible serious health risks for their operators. When incorporated into a final machine, where EN 55011 cannot be applied, ISM products can cause excessive emissions, leading to EMC non-compliance and EMC improvements and probably some additional testing may be necessary.
Checking the installation instructions
For a component to truly achieve good EMC results, it needs to be fully installed in accordance with the detailed instructions of its supplier. This is very important for EMC, because it can easily be compromised simply by using the wrong type of cable or connector, or by incorrectly connecting a shield to a shielded cable using a 'pigtail'. Vendors of complex electronics that cannot provide detailed installation instructions should be avoided.
A big problem for many of the custom engineering projects, turnkey, is that the installation does not usually follow the detailed instructions of the suppliers of its components and prefers to use what is considered as "best practices". Many of these practices have survived unchanged for many years or more and need to be updated with current best techniques considering systems use higher frequencies than before.
In the instructions for use of the components of the suppliers should be checked for limitations or inappropriate and unacceptable instructions, such as what is seen in real life:
• ”Do not use this product if it causes interference”.
• ”Do not use this product where it may be interfered with”.
• ”If interference occurs, add a filter and/or add a metal box”.
• ”This product may require a manual reset after a transient interference”.
• ”This product may fail when exposed to transients and surges”.
Component assembly and installation instructions should also be reviewed to see if they specify expensive cables or exotic connectors, additional filters, shields, or unusual environmental conditions. This can significantly affect the cost and timelines of the overall project. It is a good reason to carefully read the installation manuals before making the decision to buy the component and not after.
For example, the right time to discover that the cable needed to meet EMC per supplier instructions is only available on special order, has a 3 month lead time, has a 2 kilometer minimum order quantity and the costs are high, it is before ordering the component. Instead, you can go with a different vendor, whose product may cost more, but allows you to get more out of your project by not needing that more expensive specialty cable. The worst time to discover the exposed negative facts is when it has just been installed in the final product and it is discovered that it also does not work correctly.
Verification of test results and certificates
Component test reports can be demonstrative of their EMC performance. Comments in test reports such as “this part of the standard is not met…” are very revealing. Full, positive results from an accredited testing laboratory are the most convincing proof. EMF tests can be inaccurate. Even good accredited laboratories experience differences of ±4dB to ±6dB when measuring the same device due to inaccuracies in the standard and laboratory equipment and facilities.
Carrying out EMC tests correctly is not easy. In some very unlikely case, it could happen that a laboratory incorrectly denied conformity to a well-designed piece of equipment. Complete test results must include: exact identification of the model and version of the tested component, detailed drawings or photographs of the test setups, descriptions of how the test was carried out, list of test equipment used and their calibration dates and whether or not the device passed the test. The report must be signed by the laboratory test engineer. EMC reports must include graphs showing that emissions are comfortably below the limit lines of the standard, as well as functional response criteria for immunity tests. These reports must be reviewed with the following criteria:
• Do they agree with the supplier's detailed CEM installation instructions? Special care must be taken in the use of special types of cables or connectors or the use of ferrites.
• Are the test setups like the setup to be used on the final machine? Check especially the lack of some external cables. Wires often create the biggest EMC problems and leaving them out often gives the best results. This strategy is not correct and is misleading.
• Have the emission limits to be met been raised consciously by changing the procedures, methods or setup of the tests to pass them more easily? This practice is not acceptable.
• Have the limits of immunity been consciously reduced by changing the procedures, methods or assemblies of the tests to pass them more easily? This practice is also not acceptable.
In all EMC test reports, we must ensure that there are no negative comments along the lines of: “the product complies with the standards when…”. It is not uncommon for supplier engineers to add corrective measures during laboratory testing and for the test engineer to reference this in the report. Then, it can happen that these temporary corrective measures are “forgotten” and are not applied when the device goes into production. Thus, the components in production will not comply with the EMC Directive. Sometimes providers offer a certificate of tests carried out in their own test laboratory. Self-certification is completely legal if done correctly. Testing agency logos, such as VDE, SEMKO, DEMKO, NEMKO, UL, CSA, SEV, BSI, AENOR, etc., may also appear on the product or its documentation. However, there are examples of fraudulent suppliers marking their products with agency logos or the CE mark, without having that approval from the appropriate agency or laboratory. In some cases, the laboratory test report is misleadingly modified to cover a product that has not been tested. So it would always be best to confirm all suspect certificates with the CEM testing lab, especially when the item in question seems to be priced unreasonably low. The easiest thing to do is to send the purported test report to the appropriate test lab and ask them to confirm that it is 100% authentic and unaltered.
Control of the quality process in suppliers
The fact that a supplier has had a tested prototype pass EMC testing does not prove anything at all about the EM behavior of any of the other units of the same model in production.
Even when a supplier has an ISO 9000 quality system, by itself it is not a guarantee that their standard products supplied to the final machine builder have good EMC performance at all. All it means is that the company is audited according to its quality manual.
Therefore, it is important to know what your quality manual is and to know if it properly maintains the EMC features specified in production.
For example, there may be changes in EM behavior when there are changes between old and new versions, where the changes may be the type of cable and its installation, or a change in the shield or box, even if the manufacturer of the same type of cable changes. chip. The same chip reference, if the manufacturer is changed, could have problems in critical circuits due to its different internal design.
To control at the CEM level the operation of the products in manufacture, the supplier must demonstrate having controls over the design changes and their passage to production, the construction processes, the redesigns and the updates, also in what refers to all the EMC problems.
Even with all these controls, a number of devices could still be uncontrolled and this makes it necessary for suppliers to have a testing policy based on samples from production (this is required by the EMC Directive).
The better the suppliers' controls over their design in the purchasing, production and post-sales departments, the less will be the need for sample testing of components.
Companies with a “supplier approval” procedure will find it quite easy to add the additional requirements to ensure that the EMC tests provided by the supplier have any chance of being representative of the items actually purchased.
Installing the components on the machine
Although all the components to be installed in the final machine correctly comply with EMC and are CE marked, there are a minimum of good installation rules to be taken into account to ensure EMC compliance of the complete machine.
The cabinets must be joined with good electrical continuity, avoiding insulation caused by paint. The doors of the cabinets will be connected to the frame with copper braids that are as short and wide as possible in their ground connections. Cables must never be used to ground the different parts of the machine. Only flat copper braids should be used, as they have lower inductance. Never use two different ground planes, unless they connect very well at high frequency. Make sure that all the metal parts of the machine are well connected to earth to obtain the lowest voltage drop between them.
Contactors, relays, solenoid valves, etc. installed in the cabinets will need surge suppression devices in the coils, such as RC circuits, varistors or fast diodes. The signal cables must enter the cabinet separately from the power cables, each type through a different side in the cabinet.
All unshielded cables in the same circuit must be signal-return twisted pairs to avoid the antenna effect. This is valid both for power cables to avoid too many emissions and for signal cables to avoid immunity problems. In the case of three-phase power cables, they must also be twisted together and, if possible, shielded.
Connect unused lead wires in the hoses to cabinet ground at both ends for an extra shielding effect.
The length of the cables should be as short as possible to avoid additional inductances and coupling capacitances.
The cables have to be as close as possible to the cabinet ground or on the mounting plates to reduce the effects of crosstalk and radiation.
Signal and power cables must be separated with a minimum distance of 20cm. or separated with a metal sheet or conduit, grounding the separator element at different points of the route.
Digital and power cable screens must be grounded at both ends, checking the equipotentiality at both ends. If there is a potential imbalance in the two ground points, a cable with a minimum section of 10mm2 must be connected in parallel to reduce the current on the screen.
If the cable is very long, the cable screen must be connected to ground every /4, being the wavelength of the signal with the highest frequency that circulates through that cable. Cable shields with analog signals can only be connected at both ends if there is good equipotential bonding. The single-sided connection of the shield prevents capacitive (electric field) coupling from low-frequency disturbances such as 50Hz noise, but does not prevent magnetic coupling. The one-sided shield connection will be made on the electrical cabinet side.
Cables with braided screens have better shielding than cables with laminated screens. The best way to connect shielded cable screens is with connectors that can connect the screen in its 360º. The use of copper braids (“pigtails”) should be avoided because they greatly deteriorate the performance of the shields. Install all wiring in a reproducible (fixed) manner near the machine ground/ground plane, using metal trays or conduits that must be run continuously, without cutting.
Conclusions
The recommendations presented here generally require more work for designers than they may be used to, but should be viewed as a method of improving quality, reducing financial risk, and avoiding loss of good looks. It is always best to do the EMC tests of the final machine to be sure of its compliance. But if it is not possible for any reason, carry out the CEM tests, the steps to follow are:
1 . Review all the reports of the EMC tests of the components to be incorporated and specifically verify the worst case or at least one representative test of the desired application in the final machine.
two . Review all the results of the radiated and conducted emissions of all the components, considering the environment and the EM performance of the final machine.
3 . The radiated emissions of the selected components must be -6 dB or more below the limits of the standards to be applied to the final machine in the entire frequency range.
Four . The conducted emissions of the selected components must be -4 dB or more below the limits of the standards to be applied to the final machine in the entire frequency range.
5 . Check all cables, classify them and ensure the minimum acceptable distances, connect them correctly and fix them well.
6 . Ensure that the installation impedances are similar to the impedances typically used in the EMC tests of the components (impedances and cable terminations).
7 . Avoid any resonance in the cables.
8 . Define well the architecture of earths and masses of the final machine and make its connections correctly. Use good manufacturing practices, such as a single ground plane that covers all subassemblies.
9 . Prepare the technical construction file (TCF), justifying very well how CEM compliance has been ensured and why the tests could not be carried out.
