Ah, This is the problem, if you look at Current/Voltage curves of the MOV device, 0.9 of the Vc for the 8/20 uS point still has a significant amount of current passing through the device to earth. Too much for a hipot running 50/60 Hz or DC. It is an impossible test.
I would put a graphical clip of the curves in this email with areas of concern highlighted except I remember this is prohibited on the listserv. thanks, -doug On Wed, May 9, 2012 at 9:58 PM, Mick Maytum <[email protected]> wrote: > Clamping voltage is the MOV voltage under specified surge conditions. > Typically that will be an 8/20 impulse. > > IEC 61643-331, ed. 1.0 (2003-05) > clamping voltage VC > peak voltage across the MOV measured under conditions of a specified peak > pulse current (IP) and specified waveform > > Your 1 mA voltage is called the nominal voltage (not clamping voltage) > when the MOV draws a specified low-level current. > > IEC 61643-331, ed. 1.0 (2003-05) > nominal varistor voltage VN > voltage across the MOV measured at a specified pulsed current (IN), 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. > > Regards > Mick > On 10/05/2012 00:27, Doug Powell wrote: > > 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]> > > > -- 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]>
