Mick, Sorry I didn't review this email before I just sent my last. Clearly you see the same problem as I. I have to wonder if the committee in reviewing this clause only used data sheets and did not validate in the lab. Or is it possibly the committee members just missed this?
I would like to suggest the Vclamp we should use this the 8 mS / 10 mS rating. That's mS, not uS. -- Thanks, -doug Douglas E Powell [email protected] http://www.linkedin.com/in/dougp01 On Thu, May 10, 2012 at 6:58 AM, Mick Maytum <[email protected]> wrote: > Doug, > My previous message gave the IEC rulings - terms clamping voltage and > nominal varistor voltage - for two specific points on the MOV clamping > (clipping) characteristic. > > For completeness IEC 61010-1, ed. 3.0 (2010-06) defines the following: > *working voltage * > highest r.m.s. value of the a.c. or d.c. voltage across any particular > insulation which can occur when the equipment is supplied at rated voltage > NOTE 1 Transients and voltage fluctuations are not considered to be part > of the working voltage. > NOTE 2 Both open-circuit conditions and normal operating conditions are > taken into account. > > I'm now giving an opinion. > > Your (nominal) AC supply is 230V rms. The phase "not less than twice the > working voltage." means the hipot test is done with an AC value of at least > 460 V rms or 650 V pk. > > The hipot test voltage of 0.9 times the clamping voltage of the MOV is > clearly crazy, as from your figures of 710 V clamping voltage and 473 V > nominal varistor voltage, testing at 0.9x710 = 630 V would cause > substantial current in the MOV. > > One could skate round this and say that the 1 mA nominal varistor voltage > is the clamping* threshold* voltage - a new term - and use an MOV or > combination of MOVs to have a nominal voltage of 650/0.9 = 720 V. > > Sounds like a letter to the Chair and Secretary of TC 66 is needed to > resolve this matter. > > 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]> > > > - ---------------------------------------------------------------- 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]>
