I think I have told this story before but you asked so now you have to read it.
Back around 1975 I was minding my own business when I got a letter from the FCC stating that the police department's car radios could not transmit when in the vicinity of a bank in Fallon Nevada. This was for a class A computing device - bank teller and back room automation. I don't remember the whole letter and but they obviously wanted to know what we were going to do about it. There was urgency to the letter but not a specific date or a direct turn off the equipment requirements. We had a very specific signal to look at from the letter. I reviewed our test reports and the signal was not found, certainly not on the suspect list or final measurements. We had a 3 dB of margin requirement in our design process. We were measuring on our own 10 meter site that was registered with the FCC.( It really didn't take much to register in those days, site attenuation, physical description of the site etc.). EMC design and control was in its infancy for digital devices and this system had 6 to 10 cable interconnected devices - a real rat's nest. So I jumped on a plane to the Fallon. with the analyzer, antenna, and close field probes. We set up the antenna near one of the teller platforms and started looking - nothing, switched to close field probes - nothing. Next station - nothing. It was nearing midnight so we started to shut down the equipment preparing to come back the next day. All of a sudden as I was starting disconnect the antennas a huge spike appeared at the suspect frequency. I asked my cohort what he was doing and he responded that he was shutting things down. I had him turn them back on and very quickly the spike disappeared. Equipment off - large spike, equipment on no spike. Came back the next day and called the police radio room again and had her hold the phone to their receiver. As soon as I turned off the suspect equipment I could hear the tone coming from the receiver via the telephone. I asked them to check their logs to see if they could identify any particular time when things seemed to go hay! wire. All of the complaints were after the bank closed for the day and had turned the equipment off. A little investigation found that a video display was changed from a +12 Vac only unit to one that required + and -12Vdc. The switch being used to power the old display only had a single pole and perfectly acceptable for the old single voltage design. From the designers aspect things were working just fine. Turn off the power and the display went blank. Undetected by them was that the video amplifier was being driven into oscillation when the plus voltage was removed. The solution was simply, we could get dual pole switches that fit the unit and could be replaced in the field. We determined the number of field units, built up power switches and cables and instructed the field service guys to make the change. I responded back to the FCC that the problem was found, what the root cause of the signal was, and our field updates and projected timeline for complete retrofit, along with a note that any new complaints would be retrofit immediately. I don't really remember whether we got a response back from the FCC saying the accepted our resolution - I think we might have - but other than that we never heard from them again. The short of it was - actual field complaint sent to the FCC, they tracked us down, we responded with the root analysis and repair information within two weeks and life went back to normal. This and the one other incident does bring up the test modes issue though. The requirements at the time were for operating, scrolling H's to all peripherals, printers, disc drives etc. all variable control set to maximum etc. at least as a minimum The above is an example of a mode one would never consider - the equipment turned off. I also had a problem with a vendor who was selling us a monitor and I kept pointing out a signal 30 dB over the limit. He insisted it wasn’t there, I insisted it was. Again I jumped on a plane with the offending monitor with me. Went to the suppliers test facility and started running tests. Again it was a single frequency of concern. He set everything up using a set of support equipment he had and turned everything on. He kind of smirked when the offending frequency wasn't there. I walked out to the monitor and adjusted the contrast form the required maximum to a more user preferred level. The smirk was quickly replaced with a whole different look as a plus 30 dB signal sat right where I told him it would be. The point is one has to try to standardize the test setup and modes as much as possible - but there are a huge number of permutations that can apply and the highly unlikely will undoubtedly bite you in a sensitive part every so often. -----Original Message----- From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Wednesday, October 14, 2015 10:04 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] SV: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where? Throwing in my two cents: Keep in mind that Measurement Uncertainty goes both ways. For instance, EMC labs will typically have a Measurement Uncertainty between ±4 - ±5db for Radiated Emission. That is "Plus or Minus", not just minus. Since no EMC lab is perfect it is very likely that a product can pass by 4db at one lab and fail by 4db at another lab and still be ok.?? I've told this story before, but it is worth repeating. Years ago in a life far far away, my employer at that time (not my current employer) did a buy/sell of a computer system built by another company in another land. Let's call is land Korea. By contract, this computer was to meet all safety and EMC requirements. Our EMC lab was not allowed to test this buy/sell product for ignorance is bliss (according to our corporate lawyers). One day, the good people of Sweden decided to verify the compliance of this system; to which they discovered that it failed Radiated Emissions by 2db. They sent us the test report which said that though it failed the limit, it was within their measurement uncertainty, so they could not declare the product as Non-Compliant. No action was taken or had to be taken. All parties were happy. Don't get me wrong. I've been doing this a long time and a huge supporter of margin and performing production audits. As an EMC engineer, we are responsible to our employer to produce a compliant and reliable product without over-burdening the design or the company we work for. It is a narrow path we walk. Sometimes decisions are left up to us and we must use our knowledge and familiarity with our own products to decide what margin we must have and what areas we can get away with less margin. We all do it. It's part of our jobs. Measurement Uncertainty makes the mathematicians happy and it gives us EMC Engineers something to go to our boss with to help support the argument for margin. But for me, we always design for 8db margin, and allow 5db margin during production audits. Any signal within 5db must be verified on several units and found to be stable. It would be irresponsible to go to production with test results with little margin on a single golden unit. We don't keep golden units. We are confident that we can pull any unit off the production line and it will pass all tests with good margin. It is the only way I can sleep at night and I am blessed to work for a company who supports the decisions our department makes. CISPR11 section 12 refers to the old 80/80 rule. If you do the statistical assessment, for instance, one unit can fail by 2db and be ok as long as you test 5 more units and they pass by 5db. One unit can fail by 4db if the next 7 units pass by 5db. Interesting, isn't it? How does this fit within the EU CE/DoC scheme where a manufacturer declares that every instrument they put on the market is compliant? So "compliant" doesn't necessarily mean "100% pass the limits", does it? I used to work for a man (again, in another life) who asked if he was going 55mph in a 55mph zone, if he was breaking the law? Of course the answer is, no. But the point he was making is would not another lab have to fail a product by more than their Measurement Uncertainty to be able to claim for sure that the product "Failed the Limit", and then, wouldn't the lab have to test between 3 and 12 units to perform a true Statistical Assessment to determine if the manufacturer is "Compliant" or not with the EMC Directive? Very Interesting. For this reason, I don't think many companies run into trouble with non-compliant instruments unless they fail by a fairly large margin or their products are causing very noticeable interference. Do you agree? I know it is difficult to share stories on this topic because this information can be considered confidential but I would love to hear if minor non-compliances are ever even found and when they are found what actions, if any, are taken and what penalties are imposed. We only seem to hear about the Big Non-compliances and even those stories are very rare in the area of EMC. Does anyone have first-hand experience dealing with EMC failures in the field? If you fail by 1db, are you dragged through the mud, fined, banned, prosecuted, black helicopters circle your house, masked men drag you out of bed in the middle of the night? Or is action taken only on severe non-compliances? What's the likely scenario? The Other Brian -----Original Message----- From: Pearson, John [mailto:john.pear...@polycom.com] Sent: Tuesday, October 13, 2015 1:34 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] SV: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where? Hello If you are selling into the EU your DoC declaring to the harmonized std (assuming you are taking this route) states that you are confirming that each and every item of product placed upon the market is compliant to the limit and not just the test sample. Does that not mean you will either have to test every item prior to shipment and sale, or apply some level of margin to feel comfortable in making this declaration? Of course testing every item is the total solution but is likely cost prohibitive unless you sell very high value items in single figures a year. Even if the stds makers have applied margin in writing the spec the DoC process does not recognize this and requires the above. -----Original Message----- From: Brian O'Connell [mailto:oconne...@tamuracorp.com] Sent: 13 October 2015 18:22 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] SV: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where? Bingo. Safety and EMC standards have 'built-in' margins per committee members that cared to converse with this plebian. A supplier's margin is in internal policy, or is per your customer's spec, or is per empirical numbers from the end-use installation. Measurement Uncertainty is not necessarily a 'margin', but does have statistical relevance. Control of manufacturing process and product construction cannot be reasonably specified in EMC standards, unless you want to write EMC standards to the content and format of safety standards. Have fun with that. Indeed, per Mr. Crane, why assume anything? Brian Sr Burrito and Ale Test Engineer -----Original Message----- From: Crane, Lauren [mailto:lauren.cr...@kla-tencor.com] Sent: Tuesday, October 13, 2015 9:44 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] SV: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where? All the issues being raised regarding possible variability must be known to the members of various standards committees. Does anyone know that the issues are *not* taken into account when the committees set test levels? If standards are followed, including any instructions regarding EUT sampling and measurement uncertainty, why assume additional margins must be applied? Regards, Lauren Crane KLA-Tencor - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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