Hi All, This exchange prompts me to mention a recurring theme that began over 30 years ago, when I transitioned from MIL-SPEC hipot testing, where hipot leakage was specified, to UL hipot where leakage was not specified. At that time I was incredulous that UL did not specify what seemed to me critical factor, but it was explained that with a 500VA hipot, breakdown would be obvious.
As hipots got smaller, UL said that less than 500 VA could be used if there was a voltmeter at the output to assure voltage was applied for the full duration, and a reliable means to indicate breakdown. In the early days that means was often a current-sensing relay to detect some level of excessive leakage. Hipots got progressively smarter, using microprocessors and circuits to distinguish non-linear changes in leakage, but also circuits to distinguish between capacitive leakage (typically from mains capacitors in switching supplies) and arc-over currents. Some units have capacitive leakage of over 20mA, so a hipot with lower current capabilities will indicate "failure". Testing with a more powerful AC hipot, or with a DC hipot, might allow the same unit to pass. My concern is that UL (and the similar agencies) still does not specify a maximum leakage, so any effort to use resistors for calibrating a hipot appears to me to be setting a leakage standard that may have more to do with the hipot current capability than its arc-over breakdown detection capabilities. Resistors are convenient, no doubt, but I suggest that the true measure of the hipot's ability to detect breakdown is either a) a controlled spark gap or b) an avalance current detector which looks for an exponential rate of rise of current rather than just the level of current. This topic continues to be a source of debate within UL as recently as last week. Comments? Mike Harris/Teccom -----Original Message----- From: Ron Pickard <[email protected]> To: [email protected] <[email protected]> Cc: [email protected] <[email protected]> List-Post: [email protected] Date: Tuesday, September 26, 2000 12:11 PM Subject: Re: Verifying functionality of the equipment for Production Safety Testing > > >Hi Paul, > >First, I would caution you from holding the hipot test lead while the tester is in operation. It >could certainly be hazardous to your health, and all that. I wonder if this is one of those "don't >try this at home" cases. > >Anyway, in another life while undergoing an initial BABT 340 quality system audit, the auditor asked >us "How do we know that the tester is working properly, and how do you know the tester will give a >failure indication when expected?" Well, these were questions we had not anticipated and were >unprepared with a quick answer. To meet the auditor's challenge, however, we fashioned a setup that >verified the complete tester, including cables. This verification setup included 2 resistors (one >for hipot and the other for ground continuity). The values for each resistor were selected so that >the testers would indicate a failure at just over the current trip point of each tester. To verify >the setup each day, the testers were tested using these resistors (a failure indication was a pass >and a pass indication was a failure). The resistors and the test equipment were all placed on a >routine calibration cycle. This satisfied BABT and other safety agencies that performed the >facility/product audits. > >I hope this helps. > >Best regards, > >Ron Pickard >[email protected] > > > >------------------------------------------- >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] > > ------------------------------------------- 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]
