Hi Ted et al,
In the case of a Class I product with Y-caps present for EMI suppression (very common) and with the loss of its earth connection (single fault), the product’s metal chassis/enclosure would then become energized (Y caps become a voltage divider to approx half of the AC mains voltage at normally earthed metal surfaces). I have witnessed this fault in products and also in branch circuits over the years and have experienced the result of this fault first-hand more than once, and I can assure you that parts so energized are quite objectionable to touch. If the exposed energized surfaces were not touched under such conditions in normal use, the fault would indeed be virtually undetectable, but the loss of ground fault would become very detectable if these surfaces were touched, but may not be reported. However, a loss of earth connection would not otherwise produce a degradation of product function making the fault noticeable to the user, which is what I believe you were referring to. Comments? Best regards, Ron Pickard RPQ Consulting 7372 West Luke Avenue Glendale, AZ 85303 +623.512-3451 tel, +623.848-9033 fax [email protected] www.rpqconsulting.com <http://www.rpqconsulting.com/> From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Monday, March 17, 2008 7:37 AM To: [email protected]; [email protected] Subject: Re: Protective Earthing Terminal Construction Requirements 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. 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