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As far as I know the best practice for doing high voltage measurements with an oscilloscope is either using HV differential probes or connecting the DUT to an isolation transformer while having the oscilloscope connected to AC mains with earth ground reference. I heard people say connecting oscilloscope and DUT to the same isolation transformer should be avoided.

I don't understand why (apart from maybe the devices influencing each other or someone forgetting to unplug the earth referenced USB cable.) I would consider it to be even safer, as there is no possibility to get a shock one-handed while an earthed oscilloscope could reference the isolated secondary potential to earth when measuring, which again allows to get a shock when touching. Even though the RCD would not trip when the housing of the oscilloscope gets connected to phase because of some electrical fault there is still no shock hazard as the whole system is not earth referenced (apart from leakage in the transformer coils.) If both phases are short circuited together the fuse of the isolation transformer would blow up.

Are there any obvious downsides that I have overseen or is this actually done in practice?

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  • \$\begingroup\$ What kind of HV measurements you are talking about? If the HV is earth referenced without the isolation transformer, if it is too high, you need the correct probes anyway to measure it. Isolating it would change nothing. Can you be more specific about the scenario you are after? Or does HV means "unisolated rectified mains at most" to you? Because, floating a scope, or desktop computer, or any other three pronged device makes no sense because they have mains inlet filter caps to the earth you just disconnected with the isolation transformer. \$\endgroup\$
    – Justme
    Commented Sep 6 at 4:39
  • \$\begingroup\$ I was thinking about 230V AC mains. With higher single phase voltages I wouldn't be able to plug in the scope to the same isolation transformer as the DUT. I see that because of filter caps/noise in gerneral it could make the measurement inaccurate but does this make user safety any worse? \$\endgroup\$
    – DreiDe
    Commented Sep 6 at 8:14

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If you float the scope via an isolation transformer, the chassis has the same potential as your DUT. For example, you want to measure the current on a utility 480-volt line. You use a 10:1 current ratio transformer. That means your oscilloscope is floating at 10 x 480 volts = 4800 on the scope chassis, waiting to teach you a lesson your survivors may not like.

If you keep your scope plugged into a proper outlet with an earth ground, your scope chassis will be at earth potential, protecting you from high utility voltages. It is safe to float the DUT and use differential probes.

Don't be a dope; don't float your scope. Here is a decent video oscope

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  • \$\begingroup\$ Why should I get a shock from the 4800V scope chassis? When the voltage is not referenced to earth because of the isolation transformer and not pulled to Earth via the alligator Clip as the scope is also behind an isolation transformer i guess nothing would happen assuming a leakage free transformer. And depending on the DUT it might have a much larger exposed Metal area at 4800V than the scope that will be much more risky if it gets referenced to earth via an alligator clip in a non isolated scope environment. \$\endgroup\$
    – DreiDe
    Commented Sep 6 at 8:31
  • \$\begingroup\$ @DreiDe, touch it, and you will find out. You will not live long enough to figure out capacitive coupling to Earth—there are lots of dead utility linemen who discovered it the hard way. The transmission lines are Delta, with no reference to the ground. The line goes BOOM when it touches earth. \$\endgroup\$
    – Dereck
    Commented Sep 6 at 14:00
  • \$\begingroup\$ @DreiDe -- Double insulating a scope can be difficult, as the ground lives on the BNC shield connector. \$\endgroup\$
    – Scott Seidman
    Commented Sep 6 at 18:20
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Even if you isolate the ground on the scope and you have the voltage high enough, you touch the scope and you get zapped (and fry the scope and you). Better to have the probes insulate. If you need galvanic isolation for noise purposes that's a different story.

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