Gregory, You should recommend thermal overtemp cutouts in each power lead (if AC mains) that open upon overtemp rather than closing upon overtemp, with reference to IEC 60950-1, Clause 4.3.7 (Heating elements in earthed equipment).
Shorting the mains is probably going to annoy someone, if not your safety certification engineer. We once did have a small dc secondary heater circuit with an overtemp device that closed upon overtemp. Doing so shorted out the dc supply, thereby opening a 7 A fuse in front of the overtemp device. I don't remember exactly the reason for this (maybe we couldn't find a Recognized dc thermostat that opened), but the design was OK'ed by our NRTL engineer. Regards, Don Gies, N.C.E Senior Product Compliance Engineer Lucent Technologies Holmdel, NJ 07733 USA From: Gregory H. McClure [mailto:[email protected]] Sent: Thursday, October 05, 2006 12:25 PM To: [email protected] Subject: shorting the mains prohibited - Help Gentlemen, I need your collective memory. I have an engineer that wishes to design a protection mechanism to apply a short circuit across the mains in order to open a protection device upstream to stop an over-heating fault. The protection device would be in the product, we are not talking about depending on the protection in the service panel. I will not let them go there. I remember somewhere in the past that one of the standards, or perhaps a country deviation, specifically forbid shorting the mains as a means of protection but I cannot find it. I think it is from the era when we were using IEC 380 or 435 and UL 478 but I am not sure. Can someone out there point me to the standard and clause? or perhaps the deviation or an OSM decision? I am looking for all of the arguments against this practice I can pull together because I do not feel it is sound. It is one thing to crowbar the output of a power supply to protect an expensive logic board from a power supply over-voltage failure. It is quite another to short the mains input. Many thanks, Gregory H. McClure Lexmark Product Safety 859 232 3240 office 859 232 6882 fax Confidentiality Notice: This e-mail message, including any attachment, is for the sole use of the intended recipient(s) and may contain confidential information. Any unauthorized review, use, disclosure or distribution is strictly prohibited. If you are not the intended recipient, please contact the sender, by e-mail, and destroy all copies of the original message. - 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 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 ______________________________________________________________________
