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Oscilloscopes typically have multiple input channels which have a common ground (which also happens to be earth referenced). The problem is that the probes can be connected to ground through a huge alligator clip, which is very impractical and likely to cause short-circuits with other signals. For this reason, connecting each probe to ground, which is arguably the best solution, is often undesirable.

However, I find that in many cases, connecting just a single probe to GND and let the others connect to the signal only works just as well. In some cases, not connecting any probe to GND and just letting the earth reference do its job works well enough.

In which cases is it fine to leave the probes unconnected to GND ?

In the scope of this question I assume only a single device is measured (or multiple devices which already have their grounds connected)

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  • \$\begingroup\$ For measuring multiple devices at once. (RF transmitter, RF reciever). Connecting multiple grounds to the same circuit can introduce ground loops. \$\endgroup\$ – JWRM22 Feb 7 '17 at 14:21
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    \$\begingroup\$ If you are looking for general waveforms or voltage levels, a single ground point is usually OK. Using the ground at the end of the probe is better when scoping low voltage and/or high frequency signals to avoid capturing noise. In many cases actually even the typical short ground lead with the alligator clip you referenced is too long. For high frequency or low SNR sampling, people will often create a short ground lead, like described here: electronics.stackexchange.com/questions/221292/… \$\endgroup\$ – AngeloQ Feb 7 '17 at 14:21
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    \$\begingroup\$ Read the Bob Pease article: ti.com/ww/en/bobpease/assets/… \$\endgroup\$ – Peter Smith Feb 7 '17 at 14:34
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I would have thought the answer is obvious: When you need accurate low noise results.

Half the time you just need a quick look to see if a signal is there or not, there is normally no need for a ground connection in that situation.
But if you actually want to measure anything you need a ground connection. The more sensitive the measurement (both in terms of amplitude and frequency) the better the ground needs to be. In this situation closer to the point of interest and shorter wires/connections is generally better.

And for future reference, those jaws are called a Crocodile clips in English.

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    \$\begingroup\$ Or "alligator clips" in American. \$\endgroup\$ – Wossname Feb 7 '17 at 14:21
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    \$\begingroup\$ and sometimes that croc-clip isn't good enough due to the ground loop & you need to make use of the Tip's gnd via some wound wire to further reduce the ground loop & equally use a VERY close GND w.r.t. the signal of interest \$\endgroup\$ – JonRB Feb 7 '17 at 14:35

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