John, MOV's can be expected to become warm, hot even, but not to overheat during normal operation. MOV’s are designed to absorb short pulses or bursts of energy (mains-borne spikes and surges) having very short durations and very low duty cycles. If the supply which they are used to protect approaches the conduction voltage of the MOV it will continuously conduct and dissipate way more energy than it is designed and rated to do so, and thus become extremely hot. And yes, sometimes they do ignite! (Or can ignite material adjacent to them.) Here’s what happens: http://www.youtube.com/watch?v=qV-YboDKRJM&NR=1&feature=endscreen http://www.youtube.com/watch?v=yVrapSv5jXE MOV incorporating thermal fuse protection to limit excessive over-voltage damage: http://www.youtube.com/watch?v=a8ukncqUNps&feature=endscreen&NR=1 Designers using these devices know this stuff (or should do). For whatever reason, it seems the MOV’s used in the Bose base units are likely to be beginning to conduct at the normal operating voltage of the equipment. Enjoy the videos. T
----- Original Message ----- From: John Woodgate Sent: 12/10/12 11:20 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] interesting recall by CPSC In message <20121210091236.127...@gmx.com>, dated Mon, 10 Dec 2012, Anthony Thomson <ton...@europe.com> writes: > According to sources, a MOV in the base unit can overheat and could >lead to a fire. Well, it shouldn't lead to a fire. They can be expected to overheat. -- OOO - Own Opinions Only. See www.jmwa.demon.co.uk The longer it takes to make a point, the more obtuse it proves to be. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.! ieee-pses.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...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.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...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
