Keep in mind that failure of the earthing can also be an undetected fault. The intent is to provide two levels of protection, two layers of insulation or one layer of insulation plus earthing. In each case, failure of one level may go undetected until failure of the second produces a hazard.
Often double insulation is preferred because earthing is sometimes intentionally bypassed by those who have old building wiring or wish to use two wire extension cords. In the US, cheater plugs for this purpose are commonly available. In this case the earthing failure may not be undetected, but is present nevertheless. Note also that in IEC 60950, earthing of SELV circuits is permitted. Certain testing like ampacity of the earthing path and conditions such as transient levels are adjusted accordingly. It is wise to treat all interconnections to other equipment as circuits which may be either earthed or floating unless you also have direct control over the destination circuit. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Wednesday, November 14, 2001 3:58 AM To: [email protected] Subject: Re: AW: Define Continuous DC Voltage I read in !emc-pstc that Rich Nute <[email protected]> wrote (in <[email protected]>) 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. 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