Hi Greg:
The issue is not that of shorting the mains. Rather, the issue is the deliberate operation of a fuse by creating a short-circuit. As a general rule, a fuse operates in the event of a circuit fault. A fuse is not intended to operate under normal operating conditions. In anticipating this overtemperature condition, the circuit is essentially considering such a situation to be a normal condition, not a fault condition. Kinda like a paper jam in a copier. A circuit design should not deliberately create a fault so as to operate a fuse. In doing so, the fuse is now being called upon to operate under a normal operating condition because the introduction of a short-circuit in this case is taken as a normal condition by the circuit. In addition, the same fuse is called upon to operate in the event of a fault. The problem now becomes one of selecting a single fuse value (rating) that will operate both under the deliberate fault and under any other unanticipated fault. Since the circuit can detect the overtemperature condition, then a circuit can easily be designed to shut off power through the use of a triac or similar device. This presumes the system can be re-set, and the overtemperature condition is not a permanent failure. Better still, use a thermal cutout instead of the fuse. A thermal cutout is a true safeguard and is intended for this sort of situation. Best regards, Rich From: gmccl...@lexmark.com List-Post: emc-pstc@listserv.ieee.org Date: Thursday, October 5, 2006 6:22 pm Subject: shorting the mains prohibited - Help To: emc-p...@ieee.org > 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 emc-p...@ieee.org > > 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 emcp...@ptcnh.net > Mike Cantwell mcantw...@ieee.org > > For policy questions, send mail to: > > Jim Bacher: j.bac...@ieee.org > David Heald: emc-p...@daveheald.com > > 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 emc-p...@ieee.org 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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com 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 ______________________________________________________________________
