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I have an isolation transformer (230V/230V @ 1kVA) and out of curiosity I wanted to see if the lamp of a tester screwdriver would light up.

My intuition said that since the secondary of the transformer was not ground-referenced, there shouldn't be any loop from the screwdriver to my body, earth, and finally back to transformer. But contrary to that, both terminals can light up the screwdriver!

Measuring the voltage between S1 and S2 (secondary terminals) I found 230 V, as I should. Measuring between S1/S2 and grid's ground I found 280V and 120V respectively. The same measurements are true for S1/S2 and grid's neutral.

Why is that? Is there any parasitic capacitance capable of closing the loop? Am I totally missing something here?

EDIT: Let's assume that the parasitic capacitance is responsible for this phenomenon. Connecting a light load between S1 and grid's neutral, as shown in the schematic below, should reduce this voltage difference significantly, since the capacitor has high impedance. Otherwise, isolation cannot be achieved and we are always under the false impression that touching a live ungrounded wire cannot hurt us, as there is no galvanic loop for the current.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Short answer: Yes. \$\endgroup\$ Nov 4, 2021 at 9:09

2 Answers 2

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Yes, transformers have stray capacitance between primary and secondary windings, so the AC voltage swings at the primary will couple on to the secondary. As the primary mains input is ground-referenced (the neutral is bonded to earth ground somewhere in the system), the outputs of the secondary winding will have some common mode AC voltage compared to ground and neutral.

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Why is that? Is there any parasitic capacitance capable of closing the loop?

Yes there is; between primary and secondary. You can prove this by connecting live to the primary and leaving neutral disconnected. Of course, the transformer isn't powered any more and there'll be no secondary voltage across its terminals, but, you should still illuminate your neon-bulb screwdriver when touching either secondary terminal.

Try it.

If that doesn't prove it's capacitance, or, in case it doesn't illuminate then I might have to rethink things!

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  • \$\begingroup\$ Interesting experiment! I will try that as soon as I return to my lab, I will inform you. \$\endgroup\$
    – thece
    Nov 4, 2021 at 9:21
  • \$\begingroup\$ please, check out my edit \$\endgroup\$
    – thece
    Nov 4, 2021 at 9:35
  • \$\begingroup\$ @thece I don't think you should be editing a question that has already got answers. If you wish to ask a new question as to how you can reduce this effect then make a new question or ask in comments or, just try it out. \$\endgroup\$
    – Andy aka
    Nov 4, 2021 at 9:38
  • \$\begingroup\$ It's more like a follow up in my line of thought, that a separate question. Reducing the presence of parasitic capacitance is a different topic. I am asking if connecting a light load (that could also be a human body touching one of the terminal) would reduce the voltage drop on S1 and S2. Of course, I will try it out as soon as i can. \$\endgroup\$
    – thece
    Nov 4, 2021 at 9:49
  • \$\begingroup\$ Nobody on this site likes a question that evolves in the presence of current answers. This is a Q and A site and not a forum or teach-in page. We like to see firmly fixed questions with no evolution. \$\endgroup\$
    – Andy aka
    Nov 4, 2021 at 9:52

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