Hi Ken and the group, The higher level for the power lines is because EFT only occurs on power lines and couples to I/O lines. The capacitive clamp is meant to simulate the coupling mechanism, but is, in my experience, nowhere near what really happens. The clamp also has an inductive component and combined with its dimensions is why is is (unintenionally) directive which in turn sends more energy in the wrong direction.
Still not sure what is meant by 6 dB insertion loss of the capacitive clamp. I would like to see how the lab thinks that is to be defined/measured. The generator impedance is 50 Ohms and is fed into the clamp directly. The CDN couples the generator onto powerlines dirctly through capacitors so the source impedance is not 150 Ohms. The load of course is not anything like 150 Ohms most of the time for this broad band stress (bandwidth about a few hundred MHz). Doug Ken Javor wrote: > I haven't watched these tests performed, nor performed them, so I am not in > any way an expert. But I see a flaw in the logic. When any of the > requirements implemented with a CDN call out a test potential, it is cited > as an open circuit potential with a 150 Ohm source impedance. Since good rf > measurement practice (at least at high frequencies) would be to calibrate > the CDN-injected potential into 150 Ohms, the matched potential would indeed > be precisely 6 dB down from the open-circuit limit. This is borne out by > the fact that the AE side of the CDN is supposed to be an impedance much > higher than 150 Ohms, so it wouldn't load the shunt-injected signal. > > So if the capacitive clamp truly has a 6 dB insertion loss, then it should > inject precisely the same value as the CDN, when both devices are fed with > the same potential, and loaded by 150 Ohms. > > My guess (and it is just that) is that the rationale for a higher test level > on the ac lines vs. the signal lines is that the ac lines are generally > longer and are more efficient pickups. > > >>From: "Jim Eichner" <[email protected]> >>Date: Fri, 12 Aug 2005 11:38:29 -0700 >>To: "EMC-PSTC - Forum" <[email protected]> >>Subject: Capacitive Clamp insertion loss >> >> >>>A lab we're using has an interpretation of EFT testing that never >>>occurred to me. I'm interested in other people's understanding and >>>opinions. Please feel free to point me to the archives if this has >>>been discussed before. >>> >>>The generic immunity standard EN61000-6-1 calls out EFT on different >>>types of ports, and gives the applied test level. For signal ports >>>it's a 500V test using the clamp and for AC ports it's 1000V using a >>>CDN. If the clamp has a 6dB insertion loss, as our lab claims, then >>>the 500V spec in the standard results in application of 250V to the >>>EUT cable. Presumably the authors of the standard know that and >>>accounted for it. The CDN directly couples without 6dB of loss so >>>presumably the authors said 1000V with the intent of the EUT cable >>>actually seeing 1000V. >>> >>>So what do you do for an AC port that draws more current than the CDN >>>available is rated for? >>> >>>The lab's answer is to use the cap clamp, but because of the 6dB >>>insertion loss they doubled the generator setting to 2kV to obtain the >>>1kV test that the standard calls for. Opinions? Comments? >>> >>>In 61000-4-4 it says that if the line current is higher than the >>>specified current capability of the CDN you should use a 33nF coupling >>>cap per figure 10, in which case the EUT would be powered directly >>>from the grid and the EFT generator's connection to the EUT is via the >>>coupling cap and appropriate grounding provisions (another question is >>>what that looks like). I may end up insisting that the lab use this >>>approach, but in the meantime I wanted to get the forum's opinion on >>>the cap. clamp method discussed above. >>> >>>Thanks, >>> >>>Jim Eichner, P.Eng. >>>Compliance Engineering Manager >>>Xantrex Technology Inc. >>>e-mail: [email protected] >>>web: www.xantrex.com >>>Any opinions expressed are those of my invisible friend. >>>Confidentiality Notice: This email message, including any attachments, >>>is for the sole use of the intended recipient(s) and may contain >>>confidential and privileged information. Any unauthorized review, use, >>>disclosure or distribution is prohibited. If you are not the intended >>>recipient, please contact the sender by reply e-mail and destroy all >>>copies of the original message. >>> >> >>---------------------------------------------------------------- >>This message is from the IEEE Product Safety Engineering Society >>emc-pstc discussion list. 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Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: 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: Scott Douglas [email protected] Mike Cantwell [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
