John, Way back when, I worked on something similar. One power cord of sufficient size could run the whole thing but the cleanup crew had a tendency unplug it to run the vacuum cleaner. So redundant cords were put into place, which if plugged into the same branch circuit, all stopped when the floor buffer over powered that branch. Now the cords had to plugged into different branches... and so on... Fault tolerant and redundant? I took that to mean that any one of the power cords could be pulled and the system would still run. If so, then a little musical chairs with the power cords should get them rearranged so the system never shuts down. If a number of these power cords are meant as parallel connections to reduce the current per cord then I would say that they are indeed just one cord and plug all of one parallel set into one LISN. Quickly you'll see that presents a challenge in that now there needs to be plug strip to accept these cords. The solution is left to the creative lab tech. Is the 80cm length for the power cord under test only, leaving the other cords as long as necessary, or it the 80cm for all the power cords. I thought it was just the one cord (or set of cords) under test. All this just to keep the building wiring from broadcasting the conducted noise from a system.
Ken Javor <[email protected]> wrote: 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]> 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. 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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - ---------------------------------------------------------------- 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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
