Chris, I used to have a boss that said, "If you're going 55mph in a 55mph zone are you braking the law?"
This used to drive us EMC engineers and technicians crazy. It was our signature on the test reports and when a production audit unit would fail by a small margin then how do we answer the question, "It passed before, why does it fail now"? This is our motivation for requiring margin. Failing in the field by any concernable amount is highly unlikely if you do a good job designing and building a repeatable product and perform regular production audits. What you don't want is to have to re-tweak the product during each audit to make it pass. This will cost a company much more in engineering design changes than designing the margin in at the beginning. We also had a unit audited by SEMKO a few years ago. They found the unit 2db over the CISPR22 limit, yet passed the unit due to "normal variations in calibration tolerances". This assured us that the governing authorities out there are not out to "get you" if your product fails by a small margin. There are bigger fish to catch if you know what I mean. Brian PS: The examples I give in this email occurred while employed at a previous company. Those of you who know me and worked together in the past will have no choice but smile when you remember the "old days". ---------- From: Chris Dupres <[email protected]> Sent: Tuesday, October 27, 1998 2:15 AM To: emc-pstc <[email protected]> Subject: EMC limits. EMC Folk. I have been reading all the learned submissions of what constitutes the acceptable emission limits for EMC purposes. Most of you are very clever, very technical, and I'm in awe of all of you. But there seems to be a bit of a missed point here. EMC in Europe relates to the EMC Directive, which was born of the SIngle Market arrangements between Euro States, and which were born of the Treaty of Rome way back before my kids were born. The ultimate purpose of EMC Standards/limits is to provide a level trading platform for Euro countries, so that all conditions are equal in the market place, and that no-one can steal a lead over someone else by dropping technical standards and therefore saving costs and putting cheaper goods on the market. It follows that perhaps we should look at these limits in the same way that the packaging industry looks at filling cans and bottles, or the way car drivers treat speed limits. i.e, that the EMC emission limits are a target in absolute terms, and if you can show honest intent in achieving them, then the legislation has achieved it's aim. If I carry out an honest emissions test on a piece of equipment, and the graph is below the line by the thickness of the pen, then I believe that the spirit of the EMC Directive has been met. If this acceptance level was an absolute amount, such as money in banking, then I would allow a % for measurement error, but it isn't, it's an objective. No-ones head is going to explode if the emissions are 0.5dB over limit, and in all honesty dropping the emissions by 0.5dB can usually be achieved by moving a cable or snapping on a ferrite sleeve. Hardly enough to change the whole balance of trade in Europe is it? So, if I carry out properly conducted tests, with the equipment working normally, and it shows emissions right on the limit, then I think the EMC Directive has been followed, and the equipment can be CE marked with honesty and placed on the market. Just a tuppence worth (what's THAT in Euro's?) Chris Dupres Surrey, UK. --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
