I agree, insulating both DC conductors and the whole input circuit is good practice and easy to do with switching supplies. However it is clear the standard does not require it since 3.2.1.2 clearly admits to internal earthing which would violate 1.6.4 if it applied to DC products.
Some DC products are polarity independent by way of a full wave bridge on the input. This used to allow a universal AC/DC product as well but the harmonics requirements now interfere with that. Bob Johnson ITE Safety <http://www.itesafety.com> Richard Nute wrote: Hello Zhang Guoqing: While 1.6.4 refers specifically to the neutral of an AC system, the principle applies to any supply system, whether AC or DC. For the purposes of safety, a neutral conductor is presumed to be connected to earth at the source. The principle is that, for the purpose of safety, all supply conductors must be treated as "live" conductors. That is, the design of the equipment cannot use a current-carrying earthed conductor, e.g., a neutral conductor, as a PE conductor. For AC supply systems, the neutral and line conductors can be reversed, and the equipment will operate normally. If earthing is by means of the neutral conductor, and if a polarity reversal should occur in the installation, then those parts of the equipment that are connected to the neutral would then be live. For DC supply systems, if the plus and minus conductors are reversed, the equipment will not work. Like the AC supply system, those parts of the equipment connected to the DC line would be live. For these reasons, the DC supply conductors within the equipment should be insulated from earth just as they are for AC equipment. Best regards, Rich From: [email protected] [mailto:[email protected]] On Behalf Of Zhang Guoqing Sent: Sunday, March 04, 2007 8:03 PM To: [email protected] Cc: [email protected] Subject: question on IEC/EN/UL60950-1 clause 1.6.4 Hi All, I have a question- Is "1.6.4 neutral conductor" required for AC equipment or DC equipment or both? "Line/neutral" is used in a.c. system only, "Line/Mid-point" is used in both a.c. and d.c. system, is this correct? Zhang Guoqing - ---------------------------------------------------------------- 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 email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- 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
