Dear Rich,
Many thanks for your detail explanation that is useful to me. Raymond Li OSA Rich Nute <ri...@sdd.hp.com> Sent by: owner-emc-p...@majordomo.ieee.org 05/11/03 02:58 AM Please respond to Rich Nute <ri...@sdd.hp.com> To raymond...@omnisourceasia.com.hk cc emc-p...@ieee.org Subject Re: Class 1 AC/DC adapter Hi Raymond: > >If the dc output is connected to the PE terminal, > >then the dc output could become live in the event > >of a fault in the basic insulation between mains > >and the PE terminal. > > I think it may not be the case in practice. Even the dc output is > connected to the PE, if the basic insulation in the earth terminal should > fail, the dc output still safe. It is because the leakage current will go > to the earth path (low impedance) rather than the dc output + human body > path where has much higher impedance. Agreed. The situation is that the PE must be able to carry the fault current in the event of a failure of basic insulation. Consequently, the PE circuit from the furthest point where a failure of basic insulation may occur must be capable of carrying the fault current -- up to 25 amperes (according to the standard) until the mains circuit overcurrent device operates. If the PE is connected to earth, then the dc output will not rise to the mains voltage. This is the principle of protection in the event of a fault. My reference to "live in the event of a fault" is the principle behind the determination of what conductors must be connected to the PE terminal and must be capable of carrying 25 amperes for 1 minute. > >If the adapter is sealed, then the only way to > >test for earth continuity is to check from the > >dc output side to the appliance coupler earth > >terminal. > > There is a comment from the supplier that the secondary components are not > supposed to handle 25A current and unforeseen damage (early failure) may > happen. Any comments? The supplier is correct. However, the issue is that of testing the PE circuit after the unit is assembled. This can only be done by testing between the dc output and the PE terminal. This means that the secondary circuit path must be capable of 25-amperes for 1 minute in order to test the PE circuit. In practice, the construction you describe can indeed pass this test. The single-sided PWB earth plane has sufficient cross-sectional area to carry the 25-ampere current. The leads to the PWB from the dc output to the PWB and from the PWB to the PE terminal must be reasonably robust (e.g., 22 AWG) and short length. This prevents these conductors form overheating during the test. I've dealt with a number of these adapters and have had no problems with the 25-amp test, dc output to PE terminal. Best regards, Rich 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc