Conducted emissions testing may be carried out on an Open Area Test Site (OATS) intended for testing radiated emissions, but it is not necessary to use an OATS and conducted emissions can be tested relatively easily, with high accuracy, in the comfort of your own building.
Editor’s note: This is a continuation of “Guide to Testing Conducted Emissions Part 1” which ran in the July 2011 issue of In Compliance Magazine.
Authors note: Since this guide was first written, the standard it covers has been updated and some details may have changed. Always use the relevant version of the test standard.
The Test Site
Conducted emissions testing may be carried out on an Open Area Test Site (OATS) intended for testing radiated emissions, but it is not necessary to use an OATS and conducted emissions can be tested relatively easily, with high accuracy, in the comfort of your own building.
The test site requirements for conducted emissions are very relaxed compared with the problems of radiated testing, and a simple arrangement of metal plates can be sufficient if the ambient noise of the site is low enough in the frequency range to be measured. Ambient noises can be separated out from the emissions of the EUT by first making a measurement with the EUT switched off – using the peak (PK) detector to save time – then again with the EUT switched on to create a list of ‘suspect frequencies’ that are known to be caused by the EUT and not by the ambients. Where it is thought that an EUT emission might be lying on top of an ambient, check the reading by “zooming in” the frequency span of the measuring instrument and switching the EUT off and on again. (Figure 1)
Figure 1: A typical compliance measurement procedure
to deal with ambients, for each mains conductor
Dealing with numbers of ambients can take a lot of time, especially where they change during a measurement. Where noisy ambients are a problem (either conducted via the site’s mains supply or radiated) a low-cost screened room with a filtered mains supply can be used. There is no need for any RF absorber (cones or ferrite) in the room to control the room resonances.
Full Compliance Testing
Virtually all CISPR-based test standards specify limits on conducted emissions of the AC (mains) supply, measured from 150 kHz to 30 MHz. The three most commonly referenced standards EN 55011, EN 55014-1 and EN 55022 (based on CISPR 11, CISPR 14-1 and CISPR 22, respectively) all set such limits and their methods are largely common, although there are detailed differences. EN 55013 and EN 55015 (based on CISPR 13 and CISPR 15 for broadcast receivers and lighting equipment, respectively) also require a similar test, although EN 55015 extends the measurement range down to 9 kHz for some apparatus.
To appreciate the constraints on fully compliant conducted emissions tests, it helps to be familiar with the ‘test equivalent circuit’ shown in Figure 2. This shows that in the mains port test you are measuring a combination of DM and CM sources on each line (L or N) with respect to the ground reference plane (GRP), which is connected to the EUT’s ‘earth’ connections if it has any.
The factors outside the EUT that control the coupling, and hence the measured value, are:
- stray capacitance from EUT to GRP
- RF impedance of the mains cable
- RF impedance of the LISN
The equivalent circuit shows that stray capacitance between the EUT and the GRP is an important part of the coupling path. The standard test set-up for table-top EUTs in a screened room (as shown by Figure 9 of [1]) and regularizes stray capacitance by insisting on a fixed separation distance between the two; 400mm is the norm, with at least 800mm clearance from all other conducting surfaces. A fully compliant test requires great care in achieving these distances. All test houses have a 800mm high wooden table on which the EUT can be spaced 400mm away from a vertical GRP (or a wall of a screened room). An alternative that is allowed in some standards is a 400mm separation from the bottom of the EUT to a horizontal GRP (the floor of a screened room).
Figure 2: The ‘test equivalent circuit’ for a conducted
emissions test on an equipment’s single phase AC mains supply
The third important aspect is the impedance introduced by the mains cable, which can have a significant effect above 15 MHz and so must be controlled. Laying it on the GRP will introduce excess stray capacitance. Coiling extra length will introduce more inductance. Keeping it off the GRP, bundling it if necessary in the way prescribed by the standard, controls both these factors and minimizes the variations introduced by the cable. However, nobody bundles cables exactly the same way, making cable bundling rather hit-and-miss. It is preferable to use a standard unbundled 1m length of cable for these tests, whatever the length that will be supplied with the final product.
All the necessary test set-up details for table-top and other styles of EUT (e.g. floor standing equipment) will be found in the relevant sections of the appropriate test standard. [2] contains some useful detail on performing full compliance conducted emissions tests, especially with regard to the control of test instrumentation (Figure 4).
Figure 3: Example of a conducted emissions test setup
in a screened room (derived from Figure 9 of [2])
Figure 4: Example of a spreadsheet used to calculate actual conducted emission in dBμV
On-site Testing of Systems and Installations
This was discussed in general and from the point of view of radiated emissions testing in Section 1.11 of [3]. Chapter 10 of [4] is also a useful reference.
Which ground reference to use is a crucial factor when testing on-site. At an EMC test site it is defined by the ground reference plane (GRP, see earlier), but in-situ referencing has to make do with what exists and what can practically be achieved. CISPR 16-2 recommends the following:
The existing ground at the place of installation should be used as reference ground. This should be selected by taking high frequency (RF) criteria into consideration. Generally, this is accomplished by connecting the EUT via wide straps, with a length-to-width ratio not exceeding 3, to structural conductive parts of buildings that are connected to earth ground. These include metallic water pipes, central heating pipes, lightning wires to earth ground, concrete reinforcing steel and steel beams.
In general, the safety and neutral conductors of the power installation are not suitable as reference ground as these may carry extraneous disturbance voltages and can have undefined RF impedances.
If no suitable reference ground is available in the surroundings of the test object or at the place of measurement, sufficiently large conductive structures such as metal foils, metal sheets or wire meshes set up in the proximity can be used as reference ground for measurement.
“Sufficiently large” probably means that the added metal foils, sheets or meshes should underlie the whole of the EUT and spread beyond it for at least half its height, so as to maximize its stray capacitance. But it all depends on what you are trying to achieve – if you are trying to test a product which will be manufactured in volume and sold into other environments, maximizing stray capacitance with metal sheets corresponds more closely to the proper test set-up (see above) and represents a worst-case set-up.
However, if you are measuring the conducted emissions from a custom-made item of apparatus when it is installed at its permanent site, it is then more reasonable to determine whether this single apparatus is compliant as installed by using only the existing bonded metal structures and not add to them (for example, by following the Technical Construction File route for the compliance of a large system).
Both LISN and voltage probe tests require a reference, which would typically be the boundary of the system where the power supply connects to it – often the terminals of a power outlet or a supply transformer dedicated to the system. Transducer reference connections must be bonded using a very short, wide strap to the chosen reference point – lengths of green-and-yellow wire are not adequate.
The layout of the mains cable should be as close as possible to normal operation during the test and excess cable or coils of cable should be avoided. Whatever the mains cable layout is, it should be fixed for the duration of the tests and drawn (or photographed) for the test report.
Almost no test standards provide adequate guidance for in-situ testing of conducted emissions on the AC (mains) supply, so site-specific test plans have to be developed. Many decisions will have to be taken by EMC engineers on the spot. Some basic practices (which also apply to conducted tests in the laboratory) also apply here:
Take an ambient scan with the EUT switched off. Create a list of ambients. With the EUT switched on and operating, take a peak detector sweep with a reasonably fast scan speed, taking into account the EUT’s cycle time, to create a list of significant emission frequencies. Subtract known ambients from this list, leaving a list of ‘suspects’.
Test the suspect frequencies individually using the quasi-peak and average detectors as required to make the comparison with the limits in the relevant standard, modifying the EUT’s operation to maximize the emissions if this is relevant.
It is a good idea to recheck the ambients from time-to-time during a test to make sure that new ambient sources (such as someone using an electric drill nearby) aren’t being mistaken for EUT emissions.
This procedure is repeated for all the mains phases at each location to be measured.
Loading, Filtering and Isolation
When being tested for conducted emissions, the EUT should be operated in its normal manner. Some equipment may require the use of resistive loads to replace auxiliary equipment that it would be impractical to bring to the OATS or other test site.
If you are testing on a site that suffers from high levels of electrical noise in its mains power supply, it may be possible to use filters to help reduce the noise levels. There are a number of issues that will need to be taken into account to suppress the interfering frequencies effectively. Suitable filtering techniques are described in Chapter 8 of [4] and Part 4 of [5].
Mains isolation transformers can sometimes be used to help reduce the electrical noise at an emissions test site by breaking ground current paths. The lower their leakage and the higher their isolation the better (in other words the lower their low primary-to-secondary capacitance).
If working on exposed live equipment while performing emissions tests (e.g., when trying to modify an EUT to make it pass the test), an isolating transformer can help reduce electric shock hazards. As before, high-isolation types are the best, also choose transformers that are rated for the likely surge levels (at least 6 kV, using the IEC 61000-4-5 test method) to help ensure safety.
Measurement Uncertainty
All measurements suffer from inaccuracies, and EMC measurements are no exception. Accredited test labs in the UK are required to calculate the measurement uncertainty for their conducted emissions tests and make the result available to customers. The method described by LAB 34 (from the United Kingdom Accreditation Service, UKAS) is suitable for calculating measurement uncertainty. A typical measurement uncertainty for a full compliance conducted emissions test to EN 55022 or EN 55011 would be ±2.5 dB.
In the UK it has been the custom for accredited test laboratories to draw lines on either side of the limit line to which the test is being made. The upper line represents the limit line plus the measurement uncertainty, and the lower line the limit line minus the measurement uncertainty. Then, in a test report for a full compliance emissions test, if the emissions fell between the upper and lower limit lines, the report would state “Pass not proven”.
Only if the emissions were above the upper limit line would the report state “Fail” and only if they were below the lower limit line would the report state “Pass”.
It is very easy to make erroneous emissions measurements, and the process of calculating the measurement uncertainty helps to ensure good quality results (Figure 5).
Figure 5: Example of reporting measurement uncertainty
Important Safety Note:
Always take all safety precautions when working with hazardous voltages, such as 230 V or 400 V (3-phase) electricity. If you are not quite certain about all of these precautions – obtain and follow the guidance of an electrical “health and safety at work” expert. When constructing equipment that employs hazardous voltages, always fully apply the latest versions of the relevant parts of EN 61010-1, at least.
Low-cost and/or Non-compliant Testing
Testing using alternative methods from those in EN 55022 or EN 55011 cannot give any confidence that “full-compliance” tests for conducted RF emissions would be passed. But such non-compliant tests may be valuable for improving the performance and reliability of a product, and its ability to be used in close proximity to other equipment.
Many equipment rental companies have stocks of the calibrated test gear needed to do conducted RF emissions tests properly, and will rent them out for daily, weekly, or monthly periods. So the easiest way to perform these tests with reasonable accuracy and lowest cost is often to hire the equipment and do the tests yourself.
A comprehensive discussion of low-cost and ‘pre-compliance’ testing methods for conducted emissions can be found in [6]. But always remember that saving money on test labs by doing testing oneself requires skill and attention to detail. RF testing is difficult enough to do accurately even on a purpose-built EMC testing site. So the more money it is desired to save, the greater will be the skill and attention to detail required.
Correlating Alternative Test Methods
When an alternative conducted RF emissions test method is used for design, development or troubleshooting after a test failure, repeatability is very important (even though correlation with EN 55022 or EN 55011 may not be). All such tests will need to follow a procedure that has been carefully worked out to help ensure that adequate repeatability is achieved.
When alternative methods are used as part of a QA program or to check variants, upgrades or small modifications, a ‘golden product’ is recommended to act as some sort of ‘calibration’ for the test equipment and test method. Golden product techniques allow low-cost EMC test gear and faster test methods to be used with much more confidence. Refer to Section 1.9 of [3] for a detailed description of how to use the golden product correlation method.
If alternative methods are used to gain sufficient confidence for declaring compliance to the EMCD, the golden product method is very strongly recommended. Without a golden product or some similar basis for correlating a full compliance test with the alternative method actually used, the alternative method can only give any confidence at all by using severely reduced emissions limits, and this can result in very expensive products.
The closer a test method is to using the proper test transducers and methodology in the relevant standards, the more likely it is that a good correlation will be achieved. So-called “pre-compliance” testing should always use the correct test equipment and methods, with the deviations from the full compliance tests not being sufficient to cause significant measurement errors.
Buying Second-hand Test Gear
Some rental companies sell off their rental equipment after a few years, and second-hand test gear is also available from a number of other sources. An unexpired calibration certificate on a second-hand purchase is well worth having, if only because it makes the possibility of expensive repairs to achieve your first calibration less likely.
When buying second-hand immunity test gear, it is very important indeed to check that it is capable of testing the versions of the standards that you need to use. Some of the test gear is only available second-hand because it is not capable of performing compliant tests to the latest versions of the relevant immunity standards. Such equipment should cost less than compliant test gear, and may still be useful for preliminary investigations, QA testing, etc.
EN 55022 and EN 55011 and Compliance with the EMC Directive
This handbook is concerned with testing conducted emissions on the AC (mains) supply lead, to the typical domestic/commercial/industrial EN standards over the frequency range of 150 kHz to 30 MHz. Some people will need to measure below 150 kHz or above 30 MHz – for example when measuring equipment to some automotive or military standards.
The radio-frequency (RF) emissions standard for information technology and telecommunications equipment, and business machines is the venerable CISPR 22 [7], which has been adopted in the European Union (EU) as EN 55022 [1] and listed under the Electromagnetic Compatibility Directive (EMCD) [8]. Although EN 55022 is a product family standard in its own right, it’s test methods are often called up as a basic test method by other emissions standards (generic, product, and product-family) listed under the EMCD, such as the generic emissions standard for residential, commercial and light industrial environments: EN 50081-1 (soon to be made obsolete by EN 61000-6-3).
CISPR 11 [9] is another RF emissions standard, originally developed for industrial, scientific and medical (ISM) equipment that uses RF energy to perform its intended function. It has been adopted in the EU as EN 55011 [10], with some modifications from the original CISPR document, and listed under the EMC Directive. It too has an extra duty as a basic test method for generic, product and product-family emissions standards, such as the generic emissions standard for the industrial environment: EN 50081-2 (soon to be made obsolete by EN 61000-6-4).
When a product-family standard like EN 55022 or EN 55011 is used as a basic test method by other standards, only the actual test methodology and equipment specified in the basic standard is used. The emissions limits and other aspects relevant to the type of product the basic standard was originally written for are not employed.
When complying with the conformity assessment part of the EMCD, you can either follow the “standards route” (Article 10.1 of [8]) or the Technical Construction File (TCF) Route (Article 10.2 of [8]). When EN 55022 and EN 55011 are used for their specified types of equipment, they should be listed on the equipment’s EMC Declaration of Conformity (DoC). But when they are used as basic test methods they should not be so listed – only the relevant generic, product or product-family harmonized EMC standards (that in turn call up EN 55022 or EN 55011) should be listed.
When using the TCF route, it is possible to use CISPR 22, EN 55022, CISPR 11 or EN 55011 directly, in which case they should be listed on the equipment’s EMC DoC. In such cases the product manufacturer should assess the electromagnetic environment of the equipment and ensure that it is designed and/or tested so as to comply with the EMC Directive’s essential ‘Protection Requirements’ (Article 4 of [8]).
Figure 6: Relationship between EN 55022 and the EMC Directive (EMCD)
There may be significant financial or compliance benefits in performing conducted RF emissions tests which go beyond simple compliance with the minimum conformity assessment requirements when following the Self-Declaration route under the EMC Directive. This is especially true where sensitive electrical or electronic equipment (e.g., radio or TV receivers, scientific instrumentation, etc.) could be used nearby. The emissions limits in EN 55022 are chosen to protect radio and TV receivers whose antennas are at least 10 meters away from the equipment being tested. Even then, the limits are not low enough to guarantee protection. In the case of EN 55011, this ‘protection distance’ is 30 m.
Many items of equipment are operated closer than 10 (or 30) meters to radio or TV receivers. In such cases, simply complying with the emissions limits in EN 55022 or EN 55011 may not ensure conformity with the EMCD’s Protection Requirements.
Close proximity to sensitive electrical or electronic equipment is specifically not covered by any of the generic, product or product-family immunity standards listed under the EMC Directive. This means that it is up to the manufacturer to assess the electromagnetic (EM) environment that their product will be used in, and test it accordingly to comply with the EMC Directive’s Protection Requirements. How to deal with this issue is described in the later section, “When EN 55022 (or EN 55011) are insufficient in real life”.
Compliance with the EMC Protection Requirements is a legal requirement that applies in addition to the requirement to follow one of the conformity assessment routes (Self-Declaration, Article 10.1 or TCF, Article 10.2). Products that pass tests to the relevant emissions standard listed under the EMCD, but nevertheless cause interference in normal use because their emissions are too high for their intended real-life EM environment, do not comply with the EMC Directive’s Protection Requirements and are therefore illegally CE marked.
Applying emissions tests which go beyond the minimum requirements of the EMC Directive’s listed standards (e.g. by extending the tested frequency ranges and/or applying lower limits) can also be a way to improve the functional performance of equipment, increase customer satisfaction and reduce exposure to product liability claims.
The second edition of the EMC Directive, 2004/108/EC [10], replaces [8] on the 20th July 2007. Equipment already being supplied in conformity with 89/336/EEC was allowed to be supplied until 20th July 2009, by which date it too must comply with [10] if it is to continue to be supplied in the EU. Whereas [8] requires the involvement of a Competent Body with all TCFs, [10] effectively allows the TCF route to be used with the optional involvement of a Notified Body (the new term for Competent Bodies).
Under 2004/108/EC, all ‘fixed installations’ must comply with the EMC Directive’s Essential Requirements and have documentation that shows how this has been achieved. Equipment manufactured specifically for use at a named ‘fixed installation’ may not have to comply with any EMC requirements at all when it is supplied, but testing to EN 61000-4-27 at specified levels could be one of the EMC specifications imposed on the supplier by the purchaser to help ensure that a particular ‘fixed installation’ complies with the Essential Requirements of [10].
Figure 7: Relationship between EN 55022 and the second edition of the EMC Directive (2004/108/EC)
This series of handbooks is concerned with testing to the EN standards for typical domestic, commercial, light industrial and industrial environments. But other kinds of immunity tests may be required by the EMC standards for automotive, aerospace, rail, marine and military environments. To improve reliability and/or safety, some of these industries have developed their own test standards based on their own particular kinds of EM environments.
This handbook describes, in basic terms, how to apply EN 55022:1994, and describes conducted emissions testing in a manner that will also be of use for CISPR 22, EN 55011 and CISPR 11. Details peculiar to EN 55011 testing are not gone into here, and there are a number of modifications to these standards in preparation at the time of writing (especially CISPR 22 and EN 55022) which will also not be described here. It is always best to use the latest version of the test standard, except where regulatory requirements for the EU (or elsewhere) specify the version or edition to be used. Since many national tests for RF emissions in countries outside the EU are based on CISPR standards, this handbook may also be of use where non-EU EMC specifications apply.
Where an electronic product could interfere with equipment performing a safety-related or legal metrology function, or requires high reliability or is mission-critical, mere compliance with the EMCD is often insufficient for ensuring that it has been designed correctly. Additional and/or tougher emissions requirements may need to be applied. Refer to the IEE’s guide [11] and the on-line article [12] for more on this.
What To Do When New Versions of Test Standards Are Issued
It is clearly impractical for manufacturers to rush to test labs to retest all of their types of equipment on the very day a new version of a test standard is issued, so each new version of a CISPR emissions standard includes a date on which it supersedes its previous version. This is the “date of withdrawal” (DOW), and it provides a transition period during which manufacturers can choose between using the old or the new versions of the standard. After the DOW only the new version should be used. The DOW is preserved in the EN versions of the IEC standards.
Where a generic or product EMC standard uses an emissions standard such as EN 55022 as a basic test method, it will specify either a dated reference (e.g., “EN 55022:1998”) or an undated reference (e.g., “EN 55022”). If it specifies a dated reference, then this is the version of the basic test method standard that should be used. If it specifies an undated reference, then the latest published version of the standard should be used. The generic and product standards also have DOWs, so there is always a transition period before the new version must be used.
But the European Commission (EC) has ruled that, where compliance with an EU Directive is concerned, only the DOW dates that are published in the Official Journal of the EU (OJ) have any relevance and not any DOW dates put into standards by their committees. These will often be the same dates, but not always. So it is always best to use the DOW dates published on the Commission’s homepage for EMC Directive standards: http://ec.europa.eu/enterprise/policies/european-standards/harmonised-standards/electromagnetic-compatibility/index_en.htm, instead of the DOW dates published in the standards themselves.
Usually it makes best commercial sense to test new equipment to the latest version of a standard, retesting older equipment when they are due for retesting anyway as a result of a design change or upgrade (as long as this happens before the DOW). Some equipment is sold for such short periods of time that they may never need to be retested to any new versions of standards (Figure 8).
Figure 8: What to do when new versions of standards used as basic test methods are issued
References
- EN 55022:1994, “Limits and methods of measurement of radio disturbance characteristics of information technology equipment” (Note: Amendment A1:1995 and Amendment A2:1997 both apply.)
- “Making radiated and conducted compliance measurements with EMI receivers” Application Note 1302, Agilent Technologies (previously Hewlett Packard).
- “EMC Testing Part 1 – Radiated Emissions”, Tim Williams and Keith Armstrong, EMC & Compliance Journal Feb 2001, http://www.compliance-club.com/KeithArmstrongPortfolio.
- “EMC for systems and installations” Tim Williams and Keith Armstrong, Newnes, January 2000, ISBN 0-7506-4167-3 (available from RS and Farnell).
- “EMC for Systems and Installations”, Keith Armstrong, EMC & Compliance Journal, A series of six articles during 2000, http://www.compliance-club.com/KeithArmstrongPortfolio
- “EMC Testing Part 2 – Conducted Emissions”, Tim Williams and Keith Armstrong, EMC & Compliance Journal April 2001, http://www.compliance-club.com/KeithArmstrongPortfolio.
- CISPR 22:1993, “Limits and methods of measurement of radio disturbance characteristics of information technology equipment” (Note: Amendment A1:1995 and Amendment A2:1996 both apply.)
- European Union Directive 89/336/EEC (as amended) on Electromagnetic Compatibility. The Directive’s official EU homepage includes a downloadable version of the EMC Directive; a table of all the EN standards listed under the Directive; a guidance document on how to apply the Directive; lists of appointed EMC Competent Bodies; and progress on the 2nd Edition EMC Directive; all at: http://europa.eu.int/comm/enterprise/electr_equipment/emc/index.htm.
- CISPR 11:1997, “Industrial, scientific and medical (ISM) radiofrequency equipment — Radio disturbance characteristics — Limits and methods of measurement” (Note: Amendment A1:1999 applies and Amendment A2:2002 is available for use now and must be applied from 1st October 2005.)
- EN 55011:1998, “Industrial, scientific and medical (ISM) radiofrequency equipment — Radio disturbance characteristics — Limits and methods of measurement” (Note: Amendment A1:1999 applies and Amendment A2:2002 is available for use now and must be applied from 1st October 2005.)
- The IEE’s 2000 guide: “EMC & Functional Safety”, can be downloaded as a ‘Core’ document plus nine ‘Industry Annexes’ from http://www.iee.org/Policy/Areas/Emc/index.cfm. It is recommended that everyone downloads the Core document and at least reads its first few pages. Complying with this IEE guide could reduce exposure to liability claims.
- “EMC-related Functional Safety – An Update”, Keith Armstrong, EMC & Compliance Journal, Issue 44, January 2003, pp 24-30, at: http://www.compliance-club.com/KeithArmstrongPortfolio.
EN and CISPR standards may be purchased from the British Standards Institution (BSI) at: orders@bsi-global.com. To enquire about a product or service call BSI Customer Services on +44 (0)20 8996 9001 or e-mail them at cservices@bsi-global.com. CISPR standards may be purchased with a credit card from the on-line bookstore at http://www.iec.ch, and many of them can
be delivered by email within the hour.
Acknowledgements
Some of this material was previously published in 2001-2002 in the EMC Compliance Journal’s series “EMC Testing”: http://www.compliance-club.com or http://www.compliance-club.com/KeithArmstrongPortfolio. Many thanks are due to Tim Williams of Elmac Services: http://www.elmac.co.uk; timw@elmac.co.uk, my co-author for that series.
