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Jon Francis <[email protected]> wrote (in <5BC8A2628BC2EB4081CA71B2 [email protected]>) about '61000-3-2 Power Calculation', on Fri, 28 May 2004: >http://www.ieee-pses.org/symposium > http://www.emc2004.org/ >-------------------------------------------------- > >Some questions initiated by a customer. I've asked these questions of the >relevant committee, Which committee? I know of no such questions, either to the UK or the IEC committee. >but thought they may be of interest here too. Any comments / >suggestions appreciated. > >These questions concern the checking of measured power (power measured during a >test) against specified power (the power specified by the manufacturer). Power >is then used to calculate class D current harmonic limits which are defined in >terms of mA/W. > >Question 1. >Why have the manufacturer specify the power in this way at all? The manufacturer is the only person qualified to specify it. It can be specified in the EMC technical data for the product, not in the sales specification. >Clearly the >manufacturer will have an estimate of average and peak power in order to rate >their equipment, but 61000-3-2 requires power to be measured continuously in a >very specific way. Ordinary power meters will not do the job unless the >equipment is always steady-state, yet the standard polices the manufacturer's >measurements by checking them during a compliance test. The implication is that >relatively sophisticated equipment is required just to measure power before >submitting the product to a test lab and that seems like overkill. If the >measured power is not the same as the specified power (see Q2), the standard >just says "the measured power shall be used to establish the limits". So why >worry anyway? >The reality of course is that manufacturers submit product and the test lab can >pre-test or just use the measured power as the specified power in the test >report. Yes, that's part of the story. But consider also market surveillance. Testing for that involves verification of the power specified by the manufacturer as well as the harmonic emissions. Clearly, it would be stupid to allow, in the case that the manufacturer specifies '70 W', whereas the measured value is 80 W, that nevertheless the product power must be regarded as below the lower bound of the Class D limits and thus no measurement of harmonics should be carried out. >I think that there is no need for manufacturers to specify power in this >specific way. The maximum measured power could be used to calculate limits, and >reported at the end of the test. If the measured power alters the classification >of the equipment (i.e. not Class D) then that should be reported. Comments? See above. > >Question 2. >If there are good reasons for comparing specified and measured power then the >standard can be interpreted in a few different ways. In clause 6.2.2 it says: >"the manufacturer may specify any value which is within 10% of the actual >measured value" >AND >"The value of power ... measured according to the terms of this clause, shall >not be less than 90 % nor greater than 110 % of the value for power specified by >the manufacturer in the test report" > >In other words, the standard says both that >a) the specified value shall be within 10% actual value >AND >b) the measured value shall be within 10% of the specified value > >So you could interpret this three ways: >a) as a) above >b) as b) above >C) apply both a) and b) > >At close to 10%, tests a) and b) may produce different answers. >From the manufacturer's point of view they will measure the power first and then >state or specify that power when submitting to a test lab. (Method a)) >A test lab will then measure again and check against the specified value. >(Method b)) > >Strictly speaking I think we should apply tests a) AND b) and that's easy enough >to do, but it just seems like overkill for a fairly arbitrary 10% check. >Any comments please? > Consider again market surveillance. In that case, measurement b) is on another sample of the same product. So the full description of the situation is that: a) the specified value of the type-test sample shall be within 10% of the actual power (allowed to avoid the need to state, e.g. '85.7 W'). b) The actual power of ANY sample shall be within 10% of the specified value. 10% isn't arbitrary; it is considered a reasonable allowance for variations between samples, and is also specified in some safety standards in the same context. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
