Doug, I have received a reply from TC 66 that they will consider the clause F.3.2 problem at their next meeting in October. They have acknowledge that the usage of the term “clamping voltage” is imprecise. They would be receptive to any further inputs on this matter. I used the IEC system to send the message below to the chair and secretary about F.3.2. <SNIP> A question has come up in the IEEE Product Safety Engineering Society about the requirements of clause F.3.2. As I understand it the hipot test is done at “0.9 times the clamping voltage of the voltage limiter” and "not less than twice the working voltage." The second requirement is clear for AC of 230V rms equipment a test voltage of at least 460 V rms with a 650 V peak must be used. The first requirement, 0.9xclamping voltage breaks the laws of physics for MOV voltage limiters. The two specific voltages at high and low currents for an MOV clamping/limiting characteristic are: IEC 61643-331, ed. 1.0 (2003-05) clamping voltage peak voltage across the MOV measured under conditions of a specified peak pulse current and specified waveform (Normally an 8/20 impulse is used) IEC 61643-331, ed. 1.0 (2003-05) nominal varistor voltage voltage across the MOV measured at a specified pulsed current, of specific duration NOTE The MOV manufacturer specifies the current. Otherwise, 1 mA is normally used. The pulse duration should be less than 40 ms, unless otherwise specified. In general, nominal value ±10 % is specified by the manufacturer.
Let me explain, for a catalogue AC 230 V rms MOV the nominal voltage might be 473 V and the 8/20 clamping voltage 710 V. From these figures you can see that applying 0.9xclamping voltage = 0.9x710 = 630 V would cause substantial current flow in the MOV because it is far above the 1 mA 473 V nominal voltage! Whoever wrote this did not use the correct terminology. To avoid breaking the laws of physics the statement might better read “0.9 times the nominal voltage of the voltage limiter” To just satisfy both requirements the MOV would then require a 650 V/0.9 = 720 V nominal voltage. Of course, there are some more costly ways round this by using a series GDT MOV combination, but then term clamping voltage is incorrect for switching type combinations. I request you discuss this with your members. Regards Michael J Maytum SC 37B Chair <SNIP> Regards Mick - ---------------------------------------------------------------- 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]>
