Ken, Thanks for your reply, but I still don't see the reasoning. (for others, see Ken's reply attached below)
I assume that your product has: a. The proper spacing/insulation from primary to secondary based upon the working voltages of both. b. The proper spacing/insulation from primary to Earth based upon the working voltage of the primary (assuming Earth has no working voltage). c. I assume that your "Earth" ground is actually tied to the real Earth via a ground conductor in the line cord or some other means. This means that your Earth ground already has "double" protection from the primary. It would have spacings/insulation equal to single insulation, plus it is Earthed. So, it would take a double fault (breakdown of primary to Earth insulation and disconnection of ground conductor) for Earth to become hazardous. It also means that your secondary has "double" protection from the primary. It would take the breakdown of two layers of insulation or breakdown of insulation and a ground disconnection for the secondaries to become hazardous. With these assumptions, why would your secondary need "primary" type spacings to Earth? My understanding of the UL 1012 paragraph you quote below is that, circuits are considered two at a time. You are considering the spacings between your secondary (which has its own "secondary" working voltage) and Earth (which has no working voltage if it is tied to Earth). So, in this case, the highest of the circuit voltages is your secondary voltage and your spacings and component ratings should be made accordingly. It seems to me that the engineer you are working with interprets the paragraph to mean that you consider all of the circuits in a product, find the highest working voltage from all of them. Then space all of the circuits (including ground) away from each other using the highest working voltage. Along this reasoning, you would need "primary" type spacings not just between secondary and ground, but between a 12V secondary and a 5V secondary ... I don't buy it. I'd get a second opinion from another UL engineer. Not to be a pain, but just to understand the reasoning. Either that, or I'm just not familiar with UL 1012. Maybe it has completely different isolation requirements than EN 60950 (similar to UL 1950) and EN 61010-1 (similar to UL 3101-1) which I am familiar with. Happy Holidays! Standard disclaimer about my employer, relatives, friends, pets ... not being responsible for my views. Chris > -----Original Message----- > From: Matsuda, Ken [SMTP:[email protected]] > Sent: Tuesday, December 19, 2000 4:22 PM > To: 'Maxwell, Chris'; 'Ed Eszlari'; Matsuda, Ken; [email protected] > Subject: RE: Spacing Issue > > > Chris, interesting discussion I had today with UL. Per UL 1012, Clause > 28A.1.7 "With respect to evaluating spacings, spacings between uninsulated > parts of different circuits shall be based on the highest of the circuit > voltages" Per Clause, primary spacings between the traces of earth > ground > and secondary are required. But what really got me was there was no > reference in terms of rating components in this case, such as the proposed > cap. Interesting dilemma > > > > -----Original Message----- > From: Maxwell, Chris [mailto:[email protected]] > Sent: Tuesday, December 19, 2000 12:54 PM > To: 'Ed Eszlari'; [email protected]; [email protected] > Subject: RE: Spacings Issue > > > I don't get this one? And I'm not ashamed to ask the questions that > confuse > me. Maybe some of the more safety oriented people can shed some light on > this subject. (See Ken's original email at the bottom.) > > Capacitors between secondary circuits and chassis ground needing to meet > High Voltage isolation requirements? > > I can only think of a few of scenarios where high voltage safety testing > of > secondary caps might apply. > > One would be if your "secondary" is still carrying lethal voltages with > enough energy to be considered "hazardous". > > The other would be if your primary to secondary isolation doesn't include > double insulation or equivalent. > > Another case would be TNV circuits. But TNV circuits aren't > "secondaries". > They are their own class of potentially lethal circuits that I consider > much > like a primary. > > There may be other instances, such as high voltage measuring circuits ... > > In my experience, our secondaries are considered "SELV" (if their ground > is > floating) or "SELV-E" (if they reference Earth ground) because they carry > non-lethal voltages, have low energy and they are isolated from primaries > and TNV with double insulation. We have all sorts of capacitors that > reference these SELV secondaries to chassis ground. We never once think > about their high voltage withstand and/or isolation. These products have > been safety tested at a third party lab with no problems. So what gives? > > Maybe Ken has a secondary that violates one of these principles (either > voltage, energy or isolation)? Maybe that is why his agencies are > considering short circuit testing? > > I especially wonder why they would short primary to secondary. I could > only > see a reason for this if there was no double (or, in some cases, > re-inforced) insulation from primary to secondary. My understanding is > that > shorting double insulation constitutes a double fault test. The safety > standards that I am familiar with require only single fault testing. > > Is there something that I'm missing here? > > Happy Holidays! > > Chris Maxwell, Design Engineer > GN Nettest Optical Division > 6 Rhoads Drive, Building 4 > Utica, NY 13502 > PH: 315-797-4449 > FAX: 315-797-8024 > EMAIL: [email protected] > > > > > > > -----Original Message----- > > From: Ed Eszlari [SMTP:[email protected]] > > Sent: Tuesday, December 19, 2000 9:34 AM > > To: [email protected]; [email protected] > > Subject: Re: Spacings Issue > > > > > > > > Hello Ken, > > > > First of all the "floating secondary ground" (if not connected directly > to > > earth) should be treated as part of the secondary circuit which in many > > cases will require reinforced insulation to the primary circuit > according > > to the working voltage measured. Next, in most applications, the > > secondary circuitry can be cap coupled to earth ground and does not > > require basic insulation spacings for a primary circuit. I have seen > > equipment which incorporates TNV circuitry that does require > supplemental > > insulation spacing at the primary working voltage from the TNV to earth > > due to Nordic Country deviations. I believe this is required due to poor > > earthing in these countries, and the thought is that if the safety earth > > was removed, reinforced insulation will remain between the primary > > circuitry and the TNV (basic + supplemental = reinforced). This is the > > information I had received in the past, but maybe some others in the > group > > can provide you with more information. > > > > Ed > > > > >From: "Matsuda, Ken" > > >Reply-To: "Matsuda, Ken" > > >To: [email protected] > > >Subject: Spacings Issue > > >Date: Mon, 18 Dec 2000 18:37:22 -0500 > > > > > > > > > > > >Hello Group, > > > > > >A circuit uses a floating ground in the secondary, and caps are used > for > > >protection from the secondary outputs to earth ground. The question is, > > > >under spacings requirements, would the secondary have to meet the high > > >voltage requirements for spacings for primaries due to this earth > ground. > > > > >A few agencies have expressed desires to short the primary to secondary > > and > > >require the the secondary to meet primary voltage spacings to this > earth > > >ground trace. > > > > > >Any help would be greatly appreciated. And thanks again for all your > > >opinions > > > > > > > > > > > >Thanks, > > > > > >Ken Matsuda > > > > > >------------------------------------------- > > >This message is from the IEEE EMC Society Product Safety > > >Technical Committee emc-pstc discussion list. > > > > > >To cancel your subscription, send mail to: > > > [email protected] > > >with the single line: > > > unsubscribe emc-pstc > > > > > >For help, send mail to the list administrators: > > > Jim Bacher: [email protected] > > > Michael Garretson: [email protected] > > > > > >For policy questions, send mail to: > > > Richard Nute: [email protected] > > > > > > > > > > _____ > > > > Get your FREE download of MSN Explorer at <http://explorer.msn.com> > > > > > > ------------------------------------------- This message is from the > IEEE > > EMC Society Product Safety Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: [email protected] with the > > single line: unsubscribe emc-pstc For help, send mail to the list > > administrators: Jim Bacher: [email protected] Michael > Garretson: > > [email protected] For policy questions, send mail to: Richard > Nute: > > [email protected] > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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