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I have seen a video on the web where a person is testing the galvanic isolation of several phone chargers. I am talking about this kind of phone charger.

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They are plugged into the wall with no connection to earth, and are supposed to deliver 5 V between the VCC and GND pin of the USB port. I have measured the voltage between the GND pin and the earth of some of my own phone chargers. The results are disparate: 1 VAC, 8 VAC, 60 VAC and 80 VAC.

I am a bit confused with the process:

  • How come the (AC) voltage between earth and GND is a means to qualify the galvanic isolation of a phone charger? Shouldn't it be between GND and neutral? The earth is not connected in the circuit.
  • When the galvanic isolation is not so great, is it GND or earth that oscillates? If this is GND, does it mean that VCC and GND are oscillating but the difference is always 5 V? If this is earth, what does it have to do with the charger?
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    \$\begingroup\$ that is not a charger ... it is a power supply ... the charger circuit is inside the phone \$\endgroup\$
    – jsotola
    May 6, 2021 at 6:52
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    \$\begingroup\$ The neutral is connected to earth somewhere in your electrical system (at least it is supposed to be) so technically it really does not matter much if you measure something in reference to neutral or earth. \$\endgroup\$
    – Justme
    May 6, 2021 at 7:45
  • \$\begingroup\$ @jsotola It's a device used to charge one's phone; a phone charger. Virtually everyone calls it that, and everyone understands it. Pedantry about the battery charging circuit proper residing inside the phone helps no-one. \$\endgroup\$
    – marcelm
    May 6, 2021 at 12:07

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How come the (AC) voltage between earth and GND is a means to qualify the galvanic isolation of a phone charger? Shouldn't it be between GND and neutral? The earth is not connected in the circuit.

On the mains side, neutral and earth will be connected at some point (and maybe several points). So measuring between power supply GND and earth is much the same as measuring between power supply GND and neutral (give or take a few volts).

When the galvanic isolation is not so great, is it GND or earth that oscillates? If this is GND, does it mean that VCC and GND are oscillating but the difference is always 5 V? If this is earth, what does it have to do with the charger?

By general consensus, the Earth is at 0V. You can only measure voltage between two points, and it's helpful to have something that you can declare to be 0V as a reference for everything else. So if the GND terminal of the power supply shows an AC voltage, then that's relative to the Earth's 0V.

You're seeing it because most switch mode power supplies have a class Y capacitor between the AC input and the DC output. You're seeing a tiny current leaking through that. The capacitor reduces electrical noise from the power supply.

Because your plug is reversible, you may see different results if you turn it around and plug it in the opposite way up.

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  • \$\begingroup\$ Thank you for your answer. So neutral and earth are connected somewhere, and it's not anormal to read a small voltage difference between them. And as far as I understand, all the grounds of our electric devices (household appliances, smartphone, PC) should be at 0 V relative to earth. Am I right ? \$\endgroup\$
    – AladdinSane
    May 6, 2021 at 8:21
  • \$\begingroup\$ @AladdinSane It's pretty much universal that earth and neutral are connected at the supply transformer. They may also be connected elsewhere. In theory, if wires had zero resistance and grounding rods worked perfectly, then both your system earth and neutral would be at zero volts. But reality is more complicated, and as soon as you pass a current through a wire, you get a different voltage at each end. So you may see a few volts between earth and neutral, and even between the earth at your sockets and a metal rod hammered into the soil outside. \$\endgroup\$
    – Simon B
    May 6, 2021 at 10:27

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