Raymond - Please see the below.
Regards, Peter L. Tarver, PE ptar...@ieee.org > From: Raymond Li > Sent: Thursday, November 06, 2003 9:18 AM > > Peter, > I am pleased to give you greater details below as > required. > > > What we don't know (or I've lost track of): > > DK2) what mains connected part(s) > are involved in the > dielectric breaking down (transformer winding > terminations, > leads or bodies of filtering, surge suppression or other > components and how they are mounted) > > According to Rich's suggestion, the PCB mounted > AC receptacle contributes the breakdown. From > the signs of breakdown, there were two places of > breakdown between the mains copper tracks and > earth copper track of the receptacle where have > less than 5 mm spacing. Also per Rich Nute's other posts, it was stated that the electrical spacings in IEC60950 are conservative, inferring breakdown might not occur, even at significantly smaller creepages for the same applied potential. I agree with this statement and must suggest that the spacings must be much closer to 2.5mm (for Basic Insulation, up to 250Vac) than your earlier e-mails led me to believe. This is no doubt a problem with my interpretation of your problem statement. > DK4) if the supply uses an > appliance inlet, has a > nondetachable power supply cord or if it a direct plug-in > type and how the mains circuit is brought to the power > conversion element(s) of the primary circuit > > The power is brought in via a detachable power cordset. > > As explained by Rich, the class 1 AC receptacle > is designed for the hi-pot of 1,500 Vac. It > cannot withstand 3,000 Vac together with > associated pcb tracks. Probably why the designer > has to design it in that way. Such design gives > challenge at final stage testing in production > and incoming QC. I think you may have misread or misinterpreted Rich's statement. Rich did not state the receptacle was at the probable root of the breakdown, but rather related to Basic Insulation used in Class I Equipment for which this style appliance inlet is used. I know from direct experience that an IEC320 C14 appliance inlet (assuming this is what's used) can withstand 3000Vac >from both poles to ground. If the problem location is at the appliance inlet, it must be due to the particular implementation of the inlet, or at the board it's mounted to. To verify the veracity of my memory, I went to my lab and took two manufacturer's C14 appliance inlets and applied 5kVac from both poles to the earthing terminal for 7 mins. each, with the instrument sensitivity adjusted to its maximum (eg, minimum current flow tripping the indicating circuit). Both showed no signs of breakdown, except for a misapplied lead on one of the EUTs (which was corrected as soon as DB was noted as the test potential approached 4.8kV; reapplying the leads more carefully and retesting proved very successful). No typographical error: 5kVac for 7 mins. I note that IEC60320-1, ยง15.3, only requires 2kVac from both poles to the earthing terminal. This suggests to me that, if the appliance inlet was responsible for the DB, the implementation of the C14 design would have to be responsible. In the sake of fairness, it should be noted that both of my EUTs had solder loops and were not intended for surface or through-hole mounting to a PWB. For the sake of our mutual edification, it would be interesting to see how the appliance inlet in Raymond's customer's power supply might perform, desoldered from the board and tested to determine its ultimate breakdown potential outside the power supply. This would be a good exercise for Raymond to also provide his customer with the best possible advice. If you do this, Raymond, I'm certain the group would appreciate knowing the test results. Regards, Peter L. Tarver, PE ptar...@ieee.org This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
