I read in !emc-pstc that Rich Nute <ri...@sdd.hp.com> wrote (in <200111140041.qaa06...@epgc196.sdd.hp.com>) about 'AW: Define Continuous DC Voltage', on Tue, 13 Nov 2001: > > > >Hi John: > > >> SELV can protect under single fault conditions. BUT, as I tried to >> explain, under some conditions, it can allow a single fault *to persist >> undetected*, until eventually a second, unrelated fault occurs which >> then results in a serious hazard. > >This is a problem of the double-insulation scheme: one >cannot know when the first insulation has failed. So, >your argument not only applies to SELV but also to >ungrounded accessible metal parts and any other double- >insulation scheme. > >If we pursue your argument, then we should outlaw double >insulation as an acceptable scheme of protection against >electric shock, independent of SELV.
No, you are 'extending the argument' until it looks unjustified and then using that as a hook for your critique. I said quite clearly that failure of double or reinforced insulation is acceptable because failure of it has an acceptably low probability. > >And, we should add a "new" criterion that failure of any >safeguard should be obvious to the operator *without* >presenting a hazard to the operator. An interesting >design problem. > >> With PELV, this does not happen: the grounding ensures that the >> protective device operates. > >This scheme requires that the path between the ungrounded >PELV pole and the grounded PELV pole be capable of carrying >the fault current until the protective device operates. In >other words, the ungrounded PELV pole must carry 25 amps for >1 minute (or appropriate criteria). Yes, 'appropriate criteria'. 25A for 1 min is an extreme criterion. Let's go to the other extreme, 150 mA for 100 ms (protection by RCD). > In turn, this means the >fault current, 25 amps, must flow from the ungrounded PELV >pole through the PELV source to the grounded PELV pole. In >my experience, there are few PELV circuits that can meet this >criterion. In the PELV circuits I have worked with, the 25 >A would cause the PELV source to open before the operation >of a protective device, and the mains voltage would appear on >the PELV ungrounded pole. Yes, because you have deliberately chosen a huge fault current, which is quite unrealistic in most cases. > >> Well, perhaps I have made it clearer now. My beef with SELV is the ban >> on grounding, whereas PELV which is grounded AND double/reinforced >> insulated is clearly safer for systems extended in space. > >Agreed. In the products I deal with, this is our construction. >However, we do not test the capability of the ungrounded PELV >pole to carry fault current. > Well, you might consider *designing* it to carry a realistic fault current, then there is probably no overwhelming need to test. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Eat mink and be dreary! ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
