To isolate test equipment eg. oscilloscope using UPS

[Sal Ammoniac]

I've noticed that several people around here use one of these things between their oscilloscope plug and the wall socket. I don't think it's safe at all.

[maginnovision]

For isolation I use a battery powered scope which has two channels isolated from each other. For you I'd recommend diff probes. Assuming your department has ANY budget the cost shouldn't be an issue. If it's just you I'm sure you can afford them with such a fancy title.

[nctnico]

I agree about the differential probes. By far the safest way to measure high voltages.

[PlainName]

I don't think it's safe at all

Very unsafe, I'd say.

I have a special IEC lead for when I need to leave ground disconnected. On that, the ground lead is pulled back through the cable clamp of the plug so it is waving in the air outside the plug as a visual clue that things are not quite right (in addition to the hazard warning labels, etc).

[tggzzz]

I don't think it's safe at all.

Disconnecting the protective mains earth (PME) is indeed very dangerous. It kills scopes and people.

See, amongst many examples, https://www.eevblog.com/forum/testgear/ground-of-oscilloscope-always-connected-to-earth/msg1967150/#msg1967150

[madires]

Somewhere in this forum is an image of a rare case why you would float a scope. IIRC, it was a scope to monitor an RF transmitter powered by high voltage. To protect the engineers the scope is behind an acrylic glass and the few controls needed are extended by plastic rods to provide sufficient distance to the high voltage.

[tggzzz]

Somewhere in this forum is an image of a rare case why you would float a scope. IIRC, it was a scope to monitor an RF transmitter powered by high voltage. To protect the engineers the scope is behind an acrylic glass and the few controls needed are extended by plastic rods to provide sufficient distance to the high voltage.

The scope below was sold on fleabay, with no indication of why it was floated. It remains a good illustration of what is necessary to float a scope "properly" - and hence why you shouldn't

Fundamentally the right way to achieve the measurement is almost always to use the right probe (isolating and/or HT and/or differential) or an special-purpose scope with isolated inputs.

[Jeroen3]

Floating the scope only works if there is no Y capacitor inside, but a real foiled transformer. When there is a Y-cap, you'll spark your board to death.

*learned from experience* 

[macboy]

Let's imagine that one can actually "float" the scope, by whatever means: isolation transformer, a disconnected UPS, or whatever. One must remember that all BNC shells are now not at safe ground potential, but at whatever potential they are connected to in circuit. Remember too that they are all connected together; you can't just arbitrarily probe four signals by connecting the four ground clips to four separate points.

Here is another huge consideration: once you connect any/all of the "floating" ground clips to an arbitrary point in the circuit, all of the probe cables' ground leads and shields, as well as the chassis of the scope are (obviously) connected to that node. That's a big antenna, and a big conductor to provide capacitance to actual ground (i.e. to most other conductive things in the area). In other words, you will inject a large amount of RF into that node, and you will load that node with lots of pF to ground, much more than any proper differential probe would do. Even a correct probing technique can load a circuit to the point that the measurement can't be taken as 'truth' without consideration of the loading. If anyone thinks that the act of connecting a "floating" ground clip to an arbitrary node will not significantly affect the circuit, he is a fool.

[tggzzz]

Let's imagine that one can actually "float" the scope, by whatever means: isolation transformer, a disconnected UPS, or whatever. One must remember that all BNC shells are now not at safe ground potential, but at whatever potential they are connected to in circuit. Remember too that they are all connected together; you can't just arbitrarily probe four signals by connecting the four ground clips to four separate points.

Here is another huge consideration: once you connect any/all of the "floating" ground clips to an arbitrary point in the circuit, all of the probe cables' ground leads and shields, as well as the chassis of the scope are (obviously) connected to that node. That's a big antenna, and a big conductor to provide capacitance to actual ground (i.e. to most other conductive things in the area). In other words, you will inject a large amount of RF into that node, and you will load that node with lots of pF to ground, much more than any proper differential probe would do. Even a correct probing technique can load a circuit to the point that the measurement can't be taken as 'truth' without consideration of the loading. If anyone thinks that the act of connecting a "floating" ground clip to an arbitrary node will not significantly affect the circuit, he is a fool.

And in some cases (e.g. with FETs) such extra capacitance can damage the UUT.

[AllTheGearNoIdea]

I would not really consider using anything but an aproved isolation transformer. These are not very expensive and have a well proven useage case.  There is nothing wrong with being inventive but I am very very lazy.

[tggzzz]

I would not really consider using anything but an aproved isolation transformer. These are not very expensive and have a well proven useage case.  There is nothing wrong with being inventive but I am very very lazy.

Thinking you are safe because you are using an isolation transformer is as accurate as thinking you are safe because you have a network firewall.

Botjh firewalls and isolation transformers have many many interesting and subtle traps for the unwary. The Dunning-Krueger syndrome is a real issue.