Gary, I changed the subject line, as the discussion is drifting from the original topic of verification of hi-pot testers.
There is often confusion over the purpose and meaning of the results of hi-pot testing. Most hi-pot equipment today is designed to perform two safety critical tests, i.e. "electric strength" (per 5.3) and "resistance of protective earthing conductors" (per 2.5.11). The discussion below applies only to the electric strength tests. The hi-pot test merely helps identify possible changes in the components and construction used. A unit that passes hi-pot can easily develop a primary to secondary fault the first time it is powered on in the field. A unit that fails hi-pot might last a lifetime in the field at normal line voltages without a primary to secondary fault. A few random hi-pot failures have little meaning. However, if the fails all occur at the same point, e.g. xfmr bobbin, the component construction may have changed in a manner to decrease the creepage and clearance. In other words, the hi-pot test is not a true measure of good vs. bad units, but a measure of "process" stability. Hi-pots are not intended to measure leakage current, which is specified at intended operating voltages, not at the hi-pot levels of 1000-3000Vrms. In general, the measured leakage current will be proportional to the applied voltage. Therefore, hi-pot testers must be set to accept a current of at least: I(measured leakage) x [V(hi-pot)/V(operating)] , e.g. 3.5mA x 3000/230 = 45.65mA Otherwise, all units may fail the test. The actual current limit set on the hi-pot should be high enough to avoid nuisance failures related to leakage current, and low enough not to overlook a true breakdown in insulation. For example, we once made a product "A". When its follow-on version "B" started production, we had a high fall-out rate at hi-pot. It turned out that the "B" model had a higher leakage current, but no one thought to adjust the trip current high enough to take this into account. Some insulation breakdowns under hi-pot are non-continuous arcs, which may pass a normal hi-pot test as the rms current may be under the set trip current. This is why I would recommend a hi-pot with an arc detect circuit option. Regards, George --------------------- Forwarded by George Alspaugh/Lex/Lexmark on 09/27/2000 11:59 AM --------------------------- gary.mcinturff%[email protected] on 09/27/2000 11:07:22 AM Please respond to gary.mcinturff%[email protected] To: teccomco%[email protected], rpickard%[email protected], paul_j_smith%[email protected] cc: emc-pstc%[email protected] (bcc: George Alspaugh/Lex/Lexmark) Subject: RE: Verifying functionality of the equipment for Production Safet y Testing Mike, How does the maximum leakage or touch current for the product fit into all of this? ITE has a 3.5 mA maximum requirement which is measured during the evaluation. This measurement should be looking at the system parasitic as well as designed capacitance in the system. The hi-pot tester shouldn't be able to generate more leakage current than the system has designed into it. It is only a gross check of the primary to ground insulation system and pretty much of a digital thing, it either holds or doesn't Gary ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
