Not going to argue chapter and verse of the standards, nor make a case they should be changed – been there, done that, waste of time. However, they have departed significantly from the original intent and that leaves one asking: “Why are we even doing this?”
The original intent of conducted emission requirements was two-fold. One was protecting against ripple on the bus that could affect radio reception by direct conduction through the power supply from the mains. That (FCC) limit was originally 48 dBuV for class B. Secondarily, that limit protected against electromagnetic radiation from the mains due to rf currents, and that protected against radiated interference in a frequency range where it would be quite difficult to make radiated measurements. If you have a single device that plugs into a single branch circuit, albeit through multiple power supplies and multiple power cords, then all those cords should indeed plug into a single pair of LISNs. Now if the power supply connected to each one of those cords is totally different from each and every other power supply, there is likely no harm done, because in any single 9 kHz channel, you likely won’t get any superposition. But if the design were such that all p ower supplies were identical, then their emissions would add in quadrature (assuming they are not phase-locked) and 18 of them would yield 12.5 dB more signal in each 9 kHz bandwidth occupied by a clock harmonic than for a single power supply running off a single LISN. Another issue, prevalent at and around 150 kHz, is rectification harmonics. These will be directly proportional to total current draw, so that splitting the total current amongst many LISNs makes it much easier to meet the limit, and this isn’t justified if the equipment plugs into a single branch circuit. This isn’t a problem in the USA, because we have no BCB receivers below 530 kHz, but it affects European BCB reception in the LW band from 150 – 300 kHz. My opinion, FWIW, is that if the standards ignore the original intent, then the standards are an end in themselves with degraded contribution to the original intent to foster controlled levels of rfi. Ken Javor Phone: (256) 650-5261 ________________________________ From: "Pettit, Ghery" <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Tue, 3 Jun 2008 14:21:31 -0700 To: Ken Javor <[email protected]>, Untitled <[email protected]> Conversation: Placement of LISNs for Conducted Emissions Testing to CISPR22/FCC part 15 Subject: RE: Placement of LISNs for Conducted Emissions Testing to CISPR22/FCC part 15 One LISN per power cord is acceptable. One power cord per LISN is required for the power cord being measured. That way you know that the emissions being measured are from that cord, and not another one. This was simpler with the old design – two cords comes out of the cabinet, each to its own LISN. Now John has to contend with a bunch of cords. This is addressed in CISPR 22, article 9.5.1, which states: “The mains cable of the unit being measured shall be connected to one artificial mains network (AMN). Where the EUT is a system, which is a collection of ITE with one or more host units, and each item has its own power cable, the point of connection for the AMN is determined by the following rules: a) Each power cable that is terminated in a power supply plug of a standard design (IEC 60083 for example) shall be tested separately.” There is no question that the power cords are tested one at a time. A later paragraph in 9.5.1 calls for one or more additional AMNs for the additional power cables. Article 7.2.1 of ANSI C63.4:2003 has different text that conveys the same message. So, if your thought is to be accepted, both ANSI C63.4 and CISPR 22 (and probably other standards, as well) will have to be changed. Given the success in reducing or largely eliminating interference from ITE that the current standards have demonstrated over the past 20+ years, I doubt that will gain much traction. At least, I certainly hope not. :-) Ghery S. Pettit, NCE Convener, CISPR SC I WG3 Member, C63 SC 1 ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Ken Javor Sent: Tuesday, June 03, 2008 2:09 PM To: Untitled Subject: Re: Placement of LISNs for Conducted Emissions Testing to CISPR22/FCC part 15 First a direct response to the question posed, then a challenge to the premise upon which it is based. The second comment, if legitimate, is more important than the first. You could use one dual LISN, or one LISN per current-carrying power conductor, if you had eighteen different make-before-break switches that would allow each power cord to draw current either from the LISN power output port, or the LISN input power side. If you want to go with eighteen LISNs, I think it is technically acceptable to stack them, but you want the ground strap to maintain a lower than 5:1 length-to-width ratio, so that likely means stacking no more than three high. But here’s an interesting and likely unwelcome thought, which I invite other forum members to comment upon. The point of meeting a conducted emissions requirement is to protect radios operating below 30 MHz that might be powered >from the same branch circuit, or in the case of class A which likely applies here, operated within some distance of the equipment, but plugged into a different branch. If the equipment in your two racks operates simultaneously, it isn’t obvious to me that you are even allowed different LISNs – presumably all your rack equipment plugs into the same branch circuit, which should be represented by a single pair of LISNs. Immediate problem solved, but potentially more noise to filter, especially if power supplies operating off each cord operate at same switching frequencies. Ken Javor Phone: (256) 650-5261 ________________________________ From: "Flavin, John" <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Tue, 3 Jun 2008 16:40:10 -0400 To: <[email protected]> Conversation: Placement of LISNs for Conducted Emissions Testing to CISPR22/FCC part 15 Subject: Placement of LISNs for Conducted Emissions Testing to CISPR22/FCC part 15 Our company sells ITE systems that are housed in commerial 19" racks. The system is designed to be fault tolerant and redundant, so each rack has two AC mains cords. We do our own EMI certification testing (we're an accreditted lab), and our typical EUT consists of two of these rack, so there are 4 AC Mains cords to test, which we connect to 4 LISNs. A modified version of this system is now in the works, where the dual AC mains cords are replaced by multiple cords (with lower current per cord). The design now would have 10 AC mains cords out of one rack, and 8 from the other. This means the two rack EUT would have 18 AC Mains cords to test. In a perfect world, where cost were no object, we would have 18 LISNs, since this is the most efficient for testing -- set it up once, and test everything. Our question is how to place a relatively large number of LISNs and satisfy the standards' requirement of the 80cm spacing of the EUT and LISN. Specifically: 1) Are we allowed to place LISNs around all sides of the EUT, maintaining the 80cm spacing (i.e. have LISNs at the front face of the EUT, and run the mains cord from the back to the LISN)? 2) Are we allowed to stack LISNs on top of each other, as long as the LISN is bonded to the ground plane? Since we have to test each cord in turn, we could reduce the number of LISNs by combining a number of the cords not currently being tested through a second (or third) LISN. The downside of this is having to re-plug the cords after each cord is tested, which requires shutting the system down and restarting, which is a non-trivial task (and takes longer than it does to test one cord). John D. Flavin Teradata TCP Engineering 17095 Via del Campo San Diego, CA 92127 [email protected] V: (858) 485-3874 F: (213) 337-5432 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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