Why use a MOV? This is a difficult subject for self and employer - both in terms of reliability and safety. Probably no other choice because most of my employer's stuff must operate in other than OV category 1 or 2.
For MTBF or MTTF, there are several Arrhenius-based models where the basis is avoiding thermal runaway, and there are terms for working voltage, typically labeled as 'MCOV' in the mfr's data sheet. The concept of thermal runaway is important because the ZnO MOV is a non-linear resistor with an exponential V-I curve. The design trade-off is physical size vs energy. Small physical-sized MOVs will almost always fail short because temperature increases occur in very small areas through the disc. The other design trade-off is ambient temp vs working voltage. Internal dissipation will result in much greater dtemp/dv for a incrementally higher ambient temp. Finally, the input fuse characteristics must be considered with respect to the rating of the MOV, and consider the unit's surge testing as part of safety TRFs. Note following references that have helped me in previous years. <http://en.wikipedia.org/wiki/Varistor> Ioannou, Comparative Study of Metal Oxide Varistors for Failure Mode Identification, USF Masters Thesis, Nov 2004. Birrell, Standler, 'Failures of Surge Arrestors on Low-Voltage Mains' IEEE Transactions on Power Delivery, 1993 Vol 8 No 1. Other ideas? Brian -----Original Message----- From: emc-p...@ieee.org [mailto:emc-p...@ieee.org]On Behalf Of Richard Nute Sent: Saturday, December 15, 2012 11:09 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Why use an MOV? Presumably, the MOVs in question are connected line to neutral. I would expect the designer would design his primary circuit to withstand the normally-expected 1500 V transient. Why use an MOV??? Best wishes for the holiday season, Rich - ---------------------------------------------------------------- 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>
