JohnWoodgate's Column

Cambrian Convictions

Further information has come to light about the recent prosecutions for violation of Directives. Unfortunately, there is no documentary support for this information, because a report on the case has not been released by the DTI.  It has, however, been stated by reliable sources that the prosecutions involved violations of the Low Voltage as well as the EMC Directive, and that the defendants had not been entirely co-operative when first approached by the authorities regarding remedial measures.

Some eyebrows were raised over my remark about consultants ‘close’ to the DTI. Government departments are naturally concerned about ‘divided interest’, and may well feel it inappropriate for experts to be involved in both ‘gamekeeper’ and ‘poacher’ rôles. Consultants obviously have to take this into account.

The BSI committee concerned was motivated to consider whether the UK should propose changes to CISPR 14-1 (on which EN55104-1 is based) in order to prevent any future allegations of the EN  being inadequate under Article 8 of the Directive.

Basic EMC standards

New editions of several of the IEC/EN 61000-4 series appeared during the year, together with some amendments. Some of the changes are very significant, but require careful study of the new editions to appreciate. In particular, IEC/EN 61000-4-2 has provisions to prevent over-testing for ESD of (safety) class 2 equipment. The only way to get up to date with these changes is to buy the standards.

People ask on the IEEE mailgroup, and on Usenet, what standards say about certain things. This is not only very risky - the information is bound to be paraphrased and compressed and thus may be misleading or incomplete - but what is more, it probably costs your company at least the price of the standard to get that information, in terms of your time spent.

You can find details of all the changes of edition and amendments of  the standards FREE at www.iec.ch The new web site is much improved over the former one.

IEC 61000-3-2: full revision to go ahead

There was a proposal to abandon or postpone the full revision of IEC/EN 61000-3-2 in favour of amending the existing standard, but the proposal has not been accepted. However, it IS agreed that some amendments to the existing standard can be progressed, as a deviation from the normal IEC rules on maintenance of standards. It now appears that the full revision cannot be published before 2005. What is still far from clear, despite the tens of thousands of pounds already spent on the project, is whether the new standard will have limits substantially different from the present one, and whether they will be more or less stringent. There is STILL denial of the ‘flat-topping’ effects of the pulsed current demands of SMPS on the supply. I measured 3.75% total harmonic distortion on my supply here during the day and 2.4% in the early hours of the morning. If that difference is not due to TV sets and home PCs, it must be aliens! Part of the problem is that this effect is much more significant in Europe than in USA, due to big differences in the configurations of the distribution systems.

IEC 61000-3-12: limits for supply harmonic currents for equipment up to 75 A/phase

This standard will apply to 230 V and 240 V 50 Hz equipment rated above 16 A/phase, and to certain very rare equipment rated below 16A/phase. It is still proving controversial at the Working Group level; there are severe problems with repeatability of measurements due to the non-availability of high-current supplies with very low voltage distortion.  There has also been some difficulty at the CENELEC level due to requirements to consult with, and achieve agreement with, the electricity supplier (the one who owns the cables, not necessarily the one you buy the juice from), over the connection of high-power loads. It is believed that this difficulty can be resolved by a suitable choice of words. There is no doubt that high-power loads can’t just be connected anywhere there happens to be a mains supply.

IEC 61000-3-11: problems with certain ‘difficult’ loads

I can’t be very specific about this, because the subject is commercially sensitive, but certain products apparently can’t meet the flicker requirements of IEC61000-3-11 unless connected to a supply of impracticably low impedance.  But there is some discrepancy between the print-out of one of those delightful dumbed-down EMC test boxes, that seem to be programmed only to print ‘FAIL’ in big letters, [^_*] and calculations based on the equations in IEC 61000-3-2.  Further data has been sought from the manufacturer of the product and investigations will continue. Watch this space.

Fusion of IEC 60065 and IEC 60950

IEC TC108 has now set up its ‘Hazard-based Standard Development Team’  to undertake the BIG task of replacing IEC 60065 and IEC 60950 with one unified ‘Safety of multimedia equipment’ standard. There is a lead team and several sub-teams, studying specific subjects, such as electric shock, fire, chemical hazards etc. ‘Hazard-based’  implies a structured approach, where a chain is identified:

Energy source à Transfer mechanism à Victim

This should make it easier to agree on requirements expressed in terms of performance, rather than construction or design, which tend not only to restrict new developments but also make it difficult to cover all possibilities. For example, it is more direct to say ,’This insulation shall withstand 4.5 kV for 60 s’,  rather than, ‘This insulation shall be at least 0.4 mm thick, unless it is made of PEP, in which case it shall be at least 0.1 mm thick provided it is not subject to mechanical deformation or abrasion.’  (For extra credit, consider what thickness of PEP is required if there IS mechanical deformation, but only a little!)

This standard is unlikely to be published before 2006, and a long transition period for the EN is already envisaged, during which EN60065 and EN 60950 would still be acceptable. But, if HBSDT does a really good job, everyone will want to use the new standard as soon as possible.

J. M. Woodgate B.Sc.(Eng.), C.Eng. , MIEE, MAES, FInstSCE

Email: kernel@jmwa.demon.co.uk

Web: http://www.jmwa.demon.co.uk/

ã J.M.Woodgate 2002