Hello Chris, The general rule is to test under single fault conditions. However, the loss of a ground connection is different from most other faults. The ground can be lost without any normal means of detection for the user. Most other faults will trip a fuse or render the product at least partially inoperable. When the ground wire is lost, the product will likely continue to be used as if there were no fault at all. The loss of the ground connection is treated differently because it is virtually undetectable under normal conditions of use. I agree that a single fault has occurred, but the user doesn't know it. The product could be used for years in this state until there is a second fault which could then energize the chassis can cause injury. The product is used as if no fault has occurred, so I would treat the next fault that happens as a single fault.
Ted Eckert American Power Conversion/MGE http://www.apc.com/ The items contained in this e-mail reflect the personal opinions of the writer and are only provided for the assistance of the reader. The writer is not speaking in an official capacity for APC-MGE or Schneider Electric. The speaker does not represent APC-MGE's or Schneider Electric's official position on any matter. Chris.Dupres@elek ta.com To 03/17/2008 09:06 [email protected] AM cc [email protected] Subject Re: Protective Earthing Terminal Construction Requirements Ted Ekert said: <If a ground connection fails, there will be no indication of a problem to the user. The product will likely continue to operate perfectly normally. A second failure, such as a loose line wire, could lead to the sudden and unexpected energizing of exposed metal. The product has now become hazardous without warning.> This makes sense of course, but (going off track a little bit) this implies that the equipment has TWO faults, and the premise at the moment seems to be 'Safety under SINGLE FAULT conditions'. I guess I'm asking for opinions, in view of the latest standards leaning or Risk Analysis etc., (e.g. IEC 60601-1 Rev 3) is.. "How many stacked risks should we take into account?" when defining 'Risk'. I mean, it's perfectly feasible to lose a ground connection on a case (1 fault), and for a live wire to come loose and touch the case (2 fault). Do we have to take ALL scenario's like that into account? In other words, ensure the design is safe under double, or even triple fault conditions? I'm reminded of a mains plug on a piece of UK equipment, where the lead was tugged, the Earth wire pulled out and ended up on the Live wire. Of course the whole case became live.This was found because a secretary felt a 'tingle' when she touched the case. I guess her plastic high heels saved her that time. One event, but two faults. So how many levels of fault should we take into account in order to fulfil the Risk Analysis requirements? Regards, Chris. Chris Duprés Compliance Engineer Elekta Limited Linac House Fleming Way Crawley West Sussex RH10 9RR www.elekta.com tel: +44 (0) 1293 654311 fax: +44 (0) 1293 654260 *******************Internet Email Confidentiality Footer******************* The contents of this e-mail message (including any attachments hereto) are confidential to and are intended to be conveyed for the use of the recipient to whom it is addressed only. If you receive this transmission in error, please notify the sender of this immediately and delete the message >from your system. Any distribution, reproduction or use of this message by someone other than recipient is not authorized and may be unlawful. Elekta Limited is a company registered in England and Wales whose registered number is 3244454 and whose registered address is Linac House, Fleming Way, Crawley, West Sussex, RH10 9RR - 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
