Technical Information Note No.1

Technical Information Notes
Technical Application Notes
Technical Papers

Frequently Asked Questions About EMC Testing

1. What is pre-compliance testing and what can it offer my company?

The role of pre-compliance testing is to provide an early indication of whether equipment is likely to meet the requirements when full compliance testing is carried out. Whilst not fully conforming to the requirements of the relevant standard, it should still be carried out in a manner which will give meaningful results. However, there is no specific definition of pre-compliance testing other than ‘that which is not compliance testing’ and companies should be aware that a test which is deficient in a relatively minor detail and one which bears little resemblance to the compliant form may both be offered as pre-compliance testing. There are 3 main ways in which pre-compliance testing may differ from compliance testing:

Each deviation from the standard produces an increase in the measurement uncertainty of the test and the risk to the company, both of which are often not quantified.

2. What is the advantage of using a laboratory which is accredited by UKAS?

The United Kingdom Accreditation Service (UKAS) is the body responsible for accrediting laboratories in the UK. A laboratory applying for accreditation must demonstrate that it operates a comprehensive quality policy and that it possesses both adequate test equipment and technical ability in order to be able to carry out the tests in the prescribed manner.

Accredited laboratories are subject to regular scrutiny by UKAS and many manufacturers see this as offering them a significant degree of confidence both in the testing carried out and in making their Declarations of Conformity.

3. How do I decide which standards are relevant to my equipment?

Euro-Norme (EN) standards published in the Official Journal (OJ) of the European Community can be used to demonstrate compliance with the essential requirements of the EMC Directive. For most apparatus there are two types of standard which may be used:

Product standards relate to specific types of apparatus e.g. household appliances, lighting or information technology equipment. For apparatus where no product standard exists, the generic standards may be applied. These are based on intended operating environment and consequently may apply to a wide range of apparatus.

The European Commission issues lists of standards which have been published in the OJ. A consolidated list is the subject of another Technical Information Note in this series.

4. How can I test my product so that it meets the EMC requirements in more than one environment?

Generic standards are defined for two operating environments:

and for each environment, there are both emission and immunity standards available.

Some apparatus is intended to be used in both environments and in this case careful selection of the emission and immunity standards should ensure that the requirements for both environments are met. In general, if a piece of apparatus meets the emission requirements of the residential, commercial and light industrial environment and the immunity requirements of industrial environment then the requirements of both environments should have been satisfied. Specific advice, however, should be sought in each case.

5. What is the significance of the measurement uncertainty?

All EMC tests have an uncertainty, or tolerance, associated with them. The uncertainty budget is derived from a number of contributions such as test equipment calibration and test repeatability.

The measurement uncertainty is often most noticeable during emission testing whereby a ‘grey area’ is formed around the limit line. Where emissions are measured within this area, the equipment under test cannot be said to have passed or failed the test. It is a UKAS policy that in such cases a statement is given as to whether the equipment is more likely to have passed or failed the test based upon the position of the result within the uncertainty band.

6. If my equipment fails a test and is modified, is it necessary to repeat all the tests?

This is a question of balancing cost against risk. To be 100% confident, all the testing should be carried out with the equipment under test in a final, production state, however there may be cases where the perceived risk to the validity of other test results as a consequence of a modification is small. An accredited test laboratory will always document the modification state in which each test was performed.

In cases where significant modifications are made during testing, a manufacturer would be advised to integrate the modifications into a new unit and re-submit it for testing.

7. Can I relate the test results from one product to a range of similar products?

In the main the answer is yes, depending upon how similar the products are. There are two mechanisms for doing this, one is through Extended Justification, ie a technical argument presented as part of the Technical Documentation (TD) and the second is by the use of standards. There is more information on the second mechanism contained in the May 2007 issue of the European Guide. The principle of each is to define and demonstrate compliance for the worst case model in the range and then draw up a technical argument that the EMC performance of ‘lesser’ models should not be worse.

8. My equipment is too large to be moved to a laboratory, so how can it be tested?

For equipment which cannot be taken to a laboratory for testing, a range of ‘on-site’ or ‘in-situ’ tests can be performed using test equipment taken by the laboratory to the manufacturer's premises. There are no harmonised standards currently available for on-site testing and test results must be included within the TD.

Alternatively, manufacturers may consider testing smaller sub-systems and including the results within the TD. We recommend you seek competent advice whenever TD including an Extended Justification is being considered.

9. Do I only need to test one sample of my equipment?

Some EMC test standards require the testing of multiple samples, usually for emissions, others do not. However, manufacturers should be aware that a batch of test samples will produce a spread of results which will not be identified from a single test sample. In cases where a single type test is performed, adequate margin should be allowed below the specified limits or manufacturing tolerances.

Additional Information

For more information about this topic and how York EMC Services Ltd can help you further, please contact us.


The above information is provided in good faith and no warranty is given as to its accuracy. The application of technical information depends upon the specific case in question and it is recommended that competent technical advice is obtained and source documents consulted before acting on the information in this document. York EMC Services Ltd accepts no responsibility for any consequences arising from the use of this information.

Last Updated: 2008-Aug-22