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I have a portable, battery powered oscilloscope (OWON HDS272S) and a 250 MHz 100x passive probe. Probe specs say that it is 2 kV peak-to-peak. I measure resistance on data line being almost 100 mΩ and about 0.1 Ω on ground lead. I want to diagnose and measure power inverters (internals including) for home deep well water pumps that usually are powered by 1 or 3 phase 220 V AC (on the pump side and input side.)

Is this safe enough for an oscilloscope to handle it in different situations given that I won't exceed 2 kV peak-to-peak voltage even though the resistance of the probe's ground lead is zero (lets say it is?)

A floating oscilloscope internal schematics enough protected for this or do I absolutely need a differential voltage probe to be on the safe side? Would differential probes provide more protection to me or the oscilloscope or even make any safety difference? Zero resistance on a "ground" lead is what concerns me.

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    \$\begingroup\$ Some high-voltage passive probes (I'm thinking of Fluke 80K-40) expect their low-voltage end to be GROUND-referenced. In the mentioned probe, an alligator ground clip is included. The manner in which you intend to use it would not be safe, because this ground clip would not be at a safe voltage - contact with it would bite you. I've done the kind of dangerous measurement you outline, and have been bitten - it isn't worth the risk. \$\endgroup\$
    – glen_geek
    Jan 27, 2023 at 14:41

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The answer is, as usual, "it depends".

A passive oscilloscope probe's ground lead has to be connected to the oscilloscope's ground with low resistance for it to function properly. As a result, you can't connect the ground lead to dangerously high voltages, as that would make all the exposed metal bits of the scope dangerous to touch (USB port, function generator output, calibration output, unused scope inputs).

In other words, you can't (or at least shouldn't) use this floating oscilloscope as a replacement for a proper differential probe since attaching the scope's ground to a high voltage is dangerous. On the other hand, it's safe to just measure a high voltage with respect to actual AC ground - the entire scope will be properly grounded through the probe's ground lead, after all.

If you still want to use the scope for floating measurements without a differential probe anyway, you should at least wear insulating rubber gloves to minimize the danger.

However, used differential probes regularly sell for less than $100, and I don't think it's worth risking your life to save a little bit of money.

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Zero resistance on the ground lead is normal, it's the shield of the coax.

In your case it's like a multimeter: you probe dangerous voltages with it, the entire PCB inside the multimeter case is at mains potential, but it is safe* because the manufacturer took care that no contact can happen between fingers and any conductive part inside the multimeter.

  • = with caveats, like proper shrouded banana connnectors, don't open the battery compartment, RTFM, don't use if wet, etc

So the question is: was the portable scope and probe designed for this purpose? The answer is most likely yes, because it's a multimeter with scope features, so if it is safe to probe mains with the multimeter probes, it will also be safe to do so with a scope probe. (I said that before looking at the manual...)

HOWEVER

  1. It has a USB-C connnector and other ports, hidden behind a flimsy rubber flap that doesn't look very robust against involuntary contact with fingers, especially if it's open with a USB cable plugged in.

You must check if these connectors are isolated from the BNC ground, because if they are not, then they will be at mains potential (and RIP if the thing is plugged in the USB port of a computer when that happens).

OK, I read the manual:

enter image description here

The USB connector ground is indeed connected internally to the BNC probe ground, which means if the probe ground is at mains potential, so will the USB connector ground. Great design!

If the usual multimeter probes are still attached to the unit while the BNC ground is brought to live potential, it is possible that at least one of them (or both in ammeter mode) will also be at mains potential. Or perhaps they are internally isolated. You should double check in the manual. If the USB port is not isolated from the multimeter test leads, then it means it can't be used to acquire and log voltage over USB, and that would make the device much less useful.

There are a bunch of warnings on page 5 that you should read.

  1. Now...

enter image description here

It comes with BNC test leads with an exposed metal BNC connector, which will also be live if you use it to probe mains. But I guess that's OK, because the BNC clip leads are x1 not x10 and the maximum allowed voltage in this case is around 40V.

Conclusion:

If you only use it on batteries, and put it somewhere stable so you don't have to hold it with only the tiny rubber flap of death between your fingers and the USB connector whose shield is live, then... why not.

Note that some scope probes have a connector at the back which mates to a connector at the end of the cable. These can come off, and then the exposed (live) shield will end up in your hand. This is definitely not the right type of probe for this. Likewise a fixed x10 probe would be better, because the x1 setting will zap the scope's input stage with full mains voltage and it will blow. The probe should also be designed to prevent any contact between your hand and the shield, which could be a problem, because you're not supposed to use a scope probe like that, so did they design it for this scenario? I have no idea.

So yes it can be done but it's not the best idea. I'm not going to clutch pearls, it's probably not riskier than working on a powered inverter with its guts open on your bench and killer capacitors biding their time for an opportunity to strike, but you get the idea.

As Jonathan said, a proper differential probe, even a cheap used one, would be a lot better. It should also provide better signal quality when probing differential signals, and allow plugging a laptop in the USB port if you need to save traces or take screenshots.

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