An MOV (or similar) needs to be removed if its clampling voltage is less than the hipot test voltage.
Jim Hulbert From: Ralph McDiarmid <[email protected]> Sent: Thursday, March 12, 2026 6:02 PM To: [email protected] Subject: Re: [PSES] Dielectric test, electric appliances This message is from an EXTERNAL EMAIL system. Use caution and think before opening attachments, clicking links, or responding. Traditionally, dielectric strength (hipot) tests were done only to verify clearance. Now, the industry seems to think there is figure of merit associated with leakage current in solid insulation during that test. There are other tests which can be performed to verify the integrity of solid insulation. As Rich points out, the pass/fail criterion of a hipot test is breakdown/flashover of the air (a spark) which should trip a properly designed tester. If not removed or otherwise isolated from the hipot test, a MOV or a GDT will likely trip the tester by their excessive leakage current. Y1 and Y2 capacitors should not cause a trip and can be left in circuit during the test unless their reactance presents a difficulty. Most standards will permit a d.c. test voltage in lieu of an a.c. test. Ralph From: Richard Nute <[email protected]<mailto:[email protected]>> Sent: March 12, 2026 2:43 PM To: [email protected]<mailto:[email protected]> Subject: Re: [PSES] Dielectric test, electric appliances Hi Brian: The hi-pot (dielectric withstand) test is to verify that the power line solid insulations and air insulations (clearances) to ground will withstand (without breakdown) the transient voltages coming in on the power lines. The hi-pot tester (ac) must provide sufficient current to the Y-caps to achieve the required test voltage. You can calculate the test current from the leakage current, as the test current is simply a voltage multiple of the leakage current voltage. (You can use a dc hi-pot tester, at the peak of the ac test voltage, but there still will be current due to the rate of voltage rise charging the Y-caps.) Duration of the hi-pot test usually is one minute, but need only be a few seconds as ionization of air only takes a few seconds. I assume your solid insulation has more than adequate dielectric strength, so air insulation (clearance) is more likely to fail. Y-caps and MOVs are rated to withstand the hi-pot test. They have to withstand the transient voltages in service! They do not need to be removed during the hi-pot test. Pass criterion is very simple: No breakdown! If a breakdown occurs, you can hear it! And, a good hi-pot tester will shut down and sound an alarm. The hi-pot test tests the entire primary insulation between mains and ground. From power input to the isolation to grounded secondary (if any). Good luck! Rich From: Brian Gregory <[email protected]<mailto:[email protected]>> Sent: Thursday, March 12, 2026 12:54 PM To: [email protected]<mailto:[email protected]> Subject: [PSES] Dielectric test, electric appliances Hello safety colleagues, We are looking at some changes for End of Line dielectric test for a 240-V rated residential appliance - not medical! - for N. American residential applications. Interestingly, the appropriate UL standards detail test voltages and duration, are a bit vague about legal removal of solid state components "that are capable of being damaged" and absolutely silent on the level of allowable leakage current. I have looked at 3 different safety standards for north american products, and not one specifies a failure limit. This includes the main BESS/PV inverter standard, UL 1741, which is typically very precise. Past experience at an NRTL involved setting the HiPot to 5 or perhaps 10 mA for the test, but I can't remember what mandated the successful test level. My friendly AI bot a few weeks ago suggests 3.5 or 5 mA as allowable for residential appliances, but not what requirement(s) there may be apart from perhaps 60335. UL 60335 is not cited in any of the product safety standards that cover our current products. That same bot is now telling me 100 mA is allowed during dielectric withstand testing. So, if we can find a HiPot tester with 100mA or more capacity, we can speed up our production line by not having to remove Y-caps and/or MOVs, or go to the 1-second test levels, or both, and still get a base level check that a board's insulation system wasn't compromised during production or assembly. This brings up some interesting questions as we evaluate bringing up a new production line. - is there an actual requirement for the mA draw, or is it just what one can find in a hipot device? The testers we've bought & rented stop at 10 or 20 mA. - are there stated mA limits for dielectric breakdown detection? If the bot is right and 60335 is correct, what's the specific clause? - is there anything besides board level defects the dielectric test is supposed to catch? - could one include calculations for mA draw from a device's Y-caps and/or MOVs in an allowable EoL test plan, in case the inspector doesn't trust some chinese-made hipot with 200 mA capacity and asks us to justify a given limit? thanks all, stay safe. Colorado Brian ________________________________ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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