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I uploaded a video to YouTube specifically to help me explain my question. Please watch it if you can.

Otherwise, here is my text-based description:

  1. I have an oscilloscope, function generator and multimeter.
  2. The function generator has an RCA cable outputting a simple 100Hz sine wave at 5V amplitude.
  3. The oscilloscope probe is on 10x and configured properly.
  4. When I connect the probe ground (earth mains ground) to ground of RCA and touch probe to center of RCA, I get exactly what I expected; a 100Hz ~5V signal on the scope. Cool.
  5. If I remove the probe/earth ground from the RCA cable and only probe the tip of the RCA directly, my scope shows 100 volts AC.
  6. I tested the AC voltage between the scope probe ground and the function generator RCA cable with my multimeter to confirm and I get 48 volts AC whether I touch the center pin or 'ground' of the RCA cable.

Why is there such a high voltage between earth ground and the signal generator's output? Does this mean that if I were grounded (like by grabbing the house's ground rod) and touched the RCA cable I would get shocked with 48-100 volts AC?

What am I missing here?

My electronics knowledge is... intermediate I suppose? I just don't understand why I would be getting such high voltage between a tiny output signal from this RCA cable.

I really feel like touching that cable while being grounded should not electrocute me lol, but 48 to 100 volts AC? I'm not brave enough to play games. I really need to understand this.

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  • \$\begingroup\$ It appears that one instrument is not earth grounded properly. Check that both instruments are earth grounded via the mains plug. Also check that your mains outlets are functioning properly (earth ground pin is functional). \$\endgroup\$
    – qrk
    Commented Dec 10, 2021 at 1:53
  • \$\begingroup\$ That is 10M probe measuring stray e field, same as you touching only the probe tip and not gnd. The sig gen must be floating gnd (DC internal only) \$\endgroup\$ Commented Dec 10, 2021 at 2:09
  • \$\begingroup\$ Which devices (make, model) they are and how are they connected to mains? It sounds like one of those devices is not properly earthed like they should be. \$\endgroup\$
    – Justme
    Commented Dec 10, 2021 at 5:14
  • \$\begingroup\$ Sounds like the usual Y capacitor leakage. The 3rd time this month! Just about any item with a switchmode supply that is not earthed will do this. Audio gear, video gear, Apple macbook, you name it. \$\endgroup\$
    – Kartman
    Commented Dec 10, 2021 at 5:48
  • \$\begingroup\$ @qrk, yeah the sig-gen is just a two-prong with no earth ground. Justme, the sig-gen is not earthed, it is a cheap model from China that companies slap their logo on and sell. Mine is "FeelTech" but the same model can be known by many names. Kartman, thank you, I will have to look into this Y capacitor leakage, I haven't heard of that before. \$\endgroup\$
    – Ryan
    Commented Dec 12, 2021 at 18:42

1 Answer 1

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When I connect probe ground (earth mains ground) to ground of RCA and touch probe to center of RCA, I get exactly what I expected; a 100hz ~5v signal on the scope. Cool.

That's standard practice.

If I remove the probe/earth ground from the RCA cable and only probe the tip of the RCA directly, my scope shows 100 volts AC!

This is not.

Two connections (probe tip to test point and ground clip to signal reference) are a must in carrying out a measurement.

You have made only one connection with the probe tip which picks up stray signals / voltages.

The problem may not have surfaced had both the oscilloscope and the function generator been connected to protective earth. That is borne out by the 48 V multimeter reading between the oscilloscope and function generator BNC connector bodies.

It appears that the function generator mains input does not have the third pin for protective earth. The 2-pin wall plug seems to confirm that.

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  • \$\begingroup\$ Thank you for the reply! So I do understand that you need two points to measure voltage, but when I probe without the ground I am essentially measuring between that floating sig-gen output and earth ground, right? Those are my two points. I just don't understand how if I ground out the signal output, then disconnect the ground it immediately goes back to the same 48V AC every time without fail. How could it be so consistent like that? And for a stray voltage it seems pretty high and consistent. Is that normal for it to behave like that? Thank you again in advance, I appreciate your help! \$\endgroup\$
    – Ryan
    Commented Dec 12, 2021 at 18:37

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