There is an AC/DC power supply based on TOP246 (schematics is similar to this one).

220V GND isn't connected (suppose, a fault occurred). The voltage between RTN and 220V NEUT is about 60V, but the current flowing is very small (less than 10 microA).

Is it possible (by modifying the power supply unit) to remove the voltage between RTN and 220V NEUT or make it less than 10V?

  • \$\begingroup\$ It is hard to understand what you are looking for. \$\endgroup\$ Nov 10, 2017 at 8:02
  • \$\begingroup\$ @Marko Buršič, I measure voltage between RTN (connected to the metal case of the device) and 220V NEUT. 220V GND isn't connected. Please, refer to the schematics I gave the link to. \$\endgroup\$
    – Konstantin
    Nov 10, 2017 at 8:04
  • \$\begingroup\$ OK, but what about the earth/safety in your case? If you don't bother to have a ground connected, protecting you from being electrocuted in case of PSU failure, why are those 60V a problem to you? \$\endgroup\$ Nov 10, 2017 at 8:24
  • \$\begingroup\$ @Marko Buršič, Of course, the primary application is with the 220V GND in place. I'm just considering the case when the GND was accidentally detached. \$\endgroup\$
    – Konstantin
    Nov 10, 2017 at 8:38
  • 1
    \$\begingroup\$ The output is floating. If you don't want it floating, ground it. \$\endgroup\$
    – Finbarr
    Nov 23, 2017 at 10:12

1 Answer 1


The line filter has differential caps to ground , which has a known impedance vs f and causes the ground current you measured. These are intended to reduce conducted noise below allowed limits.

The equivalent impedance of Zc(f) of both 330pF should be the same as 6 MΩ above. This depends on tolerance of C, voltage and current spectrum (fundamental with harmonics). Zc=1/(2πfC) ( you do the math) enter image description here IEC/ UL specs limit all designs to 0.5 mA for safety.

got it?

Consider why you can get > 50Vac by squeezing a 10M scope probe tip from stray line capacitance to your body (dielectric antenna). Is this safe? and why?

  • \$\begingroup\$ If you want me to show you a faster understanding of impedance, go find RLC nomograph, trace the line for 330pF intersecting 50Hz and read the impedance which you measured (V/I), keep that PDF or printed charted handy. It can explain much more. \$\endgroup\$ Nov 27, 2017 at 15:40

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