I read in !emc-pstc that CE-TEST <cet...@cetest.nl> wrote (in <nfbbilofgmjcamhepkbaaeglceaa.cet...@cetest.nl>) about 'EMI guard bands', on Thu, 20 Dec 2001: >It's just for a few months now that CISPR/IEC are considering >to implement measurement faults into the required margin.
No, the subject has been under study for several years. >Several reports have been circulated for voting. > >As far as my knowledge reaches, a margin for measurement errors >in your equipment is officially not required, however silly that >sounds. The subject is extremely complex; so complex, in fact that for practical purposes it may actually be uncomputable, but this concept is unacceptable to accreditation agencies so we have to continue to try to make it computable. Some errors are quite easily (if expensively) controlled, and equipment errors fall into this category - you just calibrate against national standard very frequently! Other errors are not controllable, such as the dependence of emissions in a given direction on the exact positioning of cables, and must be classified as 'uncertainty', which can be *estimated* but not *calculated*. If you could calculate it, it would not be uncertainty! There are ways of reducing uncertainty - such as repeated testing one sample and testing many samples, but these are very costly procedures. It is extremely difficult to reconcile the approach to this subject which is fundamental to metrologists and accreditation agencies with the practicality that demands justification for 'high accuracy at any cost'. Bear in mind that emission limits are justified pragmatically by the maintenance at acceptable levels of complaints of interference, not by appeal to some laws of physics. The levels adopted generally at present seem adequate for at least 10^6-1 out of 10^6 sites (Remember that even in a small country there are probably 10^8 'sites'). So an uncompensated measurement error that changes this figure to 10^6 - 1.001 does not seem worth spending USD10 000 to track down. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
