If you pass the minimum requirements for surge in the environmental conditions specified, you have done enough. Good design practice says install the MOVs "downstream" of the EMI filter to buffer the surge with a small bit or impedance.
If you want to go beyond the regulatory compliance needs and into reliability, feel free to consider additional means of protection and higher surge levels. It makes for a more robust product but in many cases, this is cost the customer may not be wiling to pay. To my knowledge there is no requirement for protection in the case of a single point failure such as an open MOV; with the required surge tests presumably passing. Come to think of it, I don't believe I've ever seen a MOV quietly fail in the open circuit state. Failure mode is rather nasty and routinely pops the over current protection. Regards, Doug Douglas E Powell http://www.linkedin.com/in/dougp01 http://www.linkedin.com/in/dougp01 -----Original Message----- From: [email protected] Date: Sun, 16 Dec 2012 16:13:06 To: Richard Nute<[email protected]>; EMC-PSTC<[email protected]> Reply-To: [email protected] Subject: Re: [PSES] Why use an MOV? I would think the device only needs to pass the required surge testing of EN61000-4-5 as well as associated EFT/Burst and Ringwave, if required. Doug Douglas E Powell http://www.linkedin.com/in/dougp01 ------Original Message------ From: Richard Nute To: Doug Powell To: EMC-PSTC ReplyTo: Richard Nute Subject: Re: [PSES] Why use an MOV? Sent: Dec 15, 2012 5:51 PM Hi Doug: In both of these cases, what would be the energy the MOV would be required to dissipate? Thanks, Rich -----Original Message----- From: [email protected] Sent: Saturday, December 15, 2012 3:16 PM To: Richard Nute ; EMC-PSTC Subject: Re: [PSES] Why use an MOV? In the case of IEC 61010-1 Ed 3, it is possible to reduce clearance requirements utilizing Annex K. In another case, say you have a brick power supply approve for use in OV Cat II but your application has an extended environmental condition of OV Cat III. In order to use this power supply, you must reduce the Category by one step. MOVs are just one possibility for achieving this. Of course, use of MOVs also has a certain added concerns that must be addressed. Doug Douglas E Powell http://www.linkedin.com/in/dougp01 http://www.linkedin.com/in/dougp01 - ---------------------------------------------------------------- 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 <[email protected]> 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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
