I read in !emc-pstc that Rich Nute <[email protected]> wrote (in <[email protected]>) about '"Safety Critical" etc - the future', on Mon, 12 Nov 2001: >> A supply transformer of a not grounded SELV is a safety critical component. >> A supply transformer of a grounded SELV is a safety related component. > >For me, whether or not the SELV output of a safety-isolating >transformer is grounded is irrelevant. Two safeguards must >be interposed between the mains and the SELV. In some >situations, the grounding of the SELV output winding can >serve as the required grounded barrier (a supplemental >safeguard to the Basic insulation, the principal safeguard).
This could be confusing. SELV is usually NOT permitted to be grounded. The term 'Protected Extra-Low Voltage' (PELV) is now used for a source which is grounded but meets all the other requirements of SELV. In many fault conditions, PELV is much safer than SELV! A fault on an SELV system can persist undetected for a very long time, until a second fault, or intervention with the system, *quite unrelated*, occurs, and a very hazardous situation then arises. The earthing of PELV ensures, in almost all cases, that the first fault is not undetected and does not persist. -- 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] 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.
