Rich, Your colleague may be correct in his theory, but the only 'real-life' example I am aware of relates to deterioration of a connector containing EMI suppression capacitors. The capacitors were hi-pot tested as components, were tested again as part of the connector by a second manufacturer, when incorporated into a cable assembly by a third manufacturer and finally when incorporated in the end-product: always using a D.C. voltage. Problems were identified when the dielectric broke down at the much lower normal working voltage of the connector and the connector overheated as a result (the capacitors were located in a power supply line). Subsequent tests in the lab showed that the problem resulted from the excessive hi-pot testing. Another issue I would foresee of using only a DC voltage in production is that the dv/dt can be much higher than using an AC test voltage (some of which can be set to start the ramp-up at zero volts) if the test engineer decides to use a ramp time that is too short. Another 'issue' with DC Hi-pot testers is that they usually have a high output resistance and so it is necessary to measure the applied voltage directly rather than by using a third winding on the EHT transformer as can be the case with 500 VA + AC testers (otherwise the indicated voltage is higher than the actual applied voltage). In general therefore, it seems wise to use the minimum voltage setting permitted in the standard and to select a ramp time that is long enough not to over-stress the insulation and a dwell time that is as short as permitted in the standard. If you're going to use a DC voltage, make sure that voltage indicated on any meter associated with the hi-pot tester is the actual voltage applied. Some of the more expensive hi-pot testers also have an 'ionisation' detection function that signals the existence of r.f. currents that precede breakdown, i.e, a form of partial discharge testing. Regards, Richard Hughes Safety Answers Limited.
From: ri...@sdd.hp.com [mailto:ri...@sdd.hp.com] Sent: Friday, May 23, 2003 10:01 PM To: rsto...@lucent.com Cc: emc-p...@majordomo.ieee.org Subject: Re: Safety testing after equipment repair Hi Richard: > is the below information true > for both the AC and DC hipot methods? > Some companies have contractors,subcontractors, > incoming and final hipot... > so it does and can occur at least 4 times, > before its shipped to a customer. The theory says that the onset of the breakdown process starts with partial discharge in voids within solid insulation. Partial discharges occur more rapidly with change of voltage. Therefore dc voltages have a lower deteriorating effect than ac voltages. I have a colleague who, for this reason, insists on dc voltage for production-line tests. Best regards, Rich 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 <http://www.ieeecommunities.org/emc-pstc> 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
