Hi John, You could change the design to have Reinforced Insulation between mains and chassis. That would be the "easiest" solution... for the test lab/agency ;).
Earthed SELV should only require Basic Insulation to mains, as long as the earth connection can meet the requirements for protective earthing, such as ground bond test. As Ted noted, demonstrating this may be difficult. If the connection cannot meet these requirements, then you either need to find a way to isolate the SELV circuitry from the chassis during test (at the point where the earth connection does meet the requirements for protective earthing) or you need to improve the isolation between mains and the chassis such that it can pass the test, jokes aside. Would removing chassis cover(s) during test help the product pass, if possible? I would think you could argue that this is acceptable with the test agency, since you could pass the test for Basic Insulation with the cover(s) present, and the chassis earth connection must meet requirements for protective earthing. On Thu, Feb 25, 2016 at 8:37 AM, Ted Eckert <ted.eck...@microsoft.com> wrote: > Hello John, > > > > One common interpretation is that you use the test voltage for > double/reinforced insulation when testing between mains and user accessible > circuits even when the user accessible circuits are referenced to an earth > grounded chassis. > > > > Here is the rationale. Any given user accessible circuit may be referenced > to the chassis ground, but the connection to that chassis ground may still > be a very small conductor. Let’s say you have a circuit where the > connection to chassis ground is through a 5 mil (0.127 mm) trace. If there > is a fault that creates a short between line voltage and that circuit, the > small ground trace may open circuit before the fuse or circuit breaker > trips. In that case, the user circuit would lose its ground connection and > then would become energized to line voltage. > > > > Many years ago, I asked a VDE engineer if the test voltage for basic > insulation could be used when user accessible circuits were referenced to > chassis ground. That engineer indicated that he would accept the lower > voltage only if I could demonstrate that all of the ground paths in the > user accessible circuits could handle the potential fault currents. As this > was impractical, testing was still done at the voltage for reinforced > insulation. > > > > If your user accessible circuits are ground referenced, you are typically > allowed to run a single electric strength test at the voltage for > double/reinforced insulation. Since the user circuits are ground > referenced, the voltage applied between line/neutral and ground is also > applied from line/neutral to user circuits. This applies a higher voltage > than required across the basic insulation, but you have to do that anyway > since the user circuits are referenced to earth ground. > > > > Ted Eckert > > Compliance Engineer > > Microsoft Corporation > > ted.eck...@microsoft.com > > > > The opinions expressed are my own and do not necessarily reflect those of > my employer. > > > > *From:* Harrington, John [mailto:john.harring...@keithley.com] > *Sent:* Thursday, February 25, 2016 7:43 AM > *To:* EMC-PSTC@LISTSERV.IEEE.ORG > *Subject:* [PSES] Insulation testing > > > > Gentlemen, > > > > How do you demonstrate the dielectric strength between mains and user > accessible circuits when the accessible circuits are referenced to chassis? > > > > If you apply the test voltage for reinforced insulation to the mains > circuits and it breaks down to chassis because there is (as required) only > basic insulation between them one of two things happens > > > > 1. With the protective conductor attached to chassis current flows > to ground and the HiPot tester goes beep in a disagreeable way. > > > > 2. Without the protective conductor attached the voltage gets on > the accessible circuits because there is no or little insulation between > them and chassis. > > > > Your thoughts are very much appreciated. > > > > John Harrington > > > > *E*/ john.harring...@keithley.com > > *T*/ 440.498.2727 > > > > > > - > ---------------------------------------------------------------- > > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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