A change was made in 3rd Ed. for routine mains hipot tests while clamping devices are still in the circuit; specifically clause F.3.2 (Ed. 3).
The standard states the test can be carried out at 0.9 times the clamping voltage of the device and "not less than twice the working voltage." Edition 2 said "not less than that of the working voltage." My application is 230Vac and I initially selected 275Vrms MOVs. Plugging in the F.3.2 equations, I get a hipot failure. These devices have a very soft voltage knee and as a result of the dynamic resistance, the knee very time dependent. The MOV supplier states the "maximum" clamping voltage using the 8/20 mS surge is 710V and the DC clamping voltage with a 1 mA current source is 473V. I would assume the 473V is very similar to the peak of the 275 Vrms 50/60 Hz waveform, plus some headroom for component tolerance (~18%). If you try running the numbers to re-select a different MOV value, it is very possible to get all tied up in knots. So, which "clamping voltage" are we to use? My thought is to use 2 x 230 = 460V and select the next higher MOV from the catalog. Opinions, rulings, decrees from on high? -- Thanks, -doug Douglas E Powell [email protected] 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]>
