What are the traps to using an isolation transformer in scoping?

[ShreveCC]

I am not completely new to electronics, however with age you forget those small details that turn into traps you forget or forgot about.

I understand part of the- "why" an isolation transformer is used in measuring with a scope on mains; however what is or are the traps of plugging my scope into the isolation transformer for measuring my project?

In other words: My project is not plugged into the isolation transformer, however my scope is plugged into the isolation transformer. What makes the differences or traps? 

[Simon]

Isolating the scope is good to make sure you can connect the earth to say the neutral but it does mean that the otherwise earthed case and probes are now free to float to whatever you connect them to. So say you connect the ground probe to live and not neutral you now have a whole live scope and when you touch it you could be electrocuted. Of course working on other high potentials is as dangerous.

[johansen]

the capacitance of the oscope to ground, and the capacitance of the isolation transformer to ground, will affect the circuitry in sometimes unpredictable ways.

[richard.cs]

There are very few situations when it's a good idea to use the isolation transformer on the scope. It's usually equally effective and much safer to use the isolation transformer on the project.

Floating the scope means the scope chassis will float to the potential of whatever it's ground lead is clipped to. That won't damage the scope but may well damage you next time you touch it to make an adjustment. It's acceptable if you know the voltages of that node are low (yet still present, else you wouldn't bother). An example might be when you want the reference to be a conductor at/near mains earth (mains neutral maybe, or the +ve of a DC telecoms supply) but there's a danger of some of the current in that conductor returning via the scope and mains earth causing damage.

[Jeroen3]

You can't use any of the external interfaces of the scope (usb/vga/gpib/trig-out) to devices that are not on the same floating ground.

[ShreveCC]

Thank you for your response-  This question all started from a comment that was made by Dave Jones (Ep. #441) that actual put my brain into pondering this question I couldn't off hand deduct to an answer:

Is it bad or throw off any measurements by having this as my regular setup?-

I normally have my scope, frequency generator, and bench DMM plugged into my BK Precision 1604A isolation transformer, instead of the device I am testing.  Does it interfere or throw of measurements when your troubleshooting?  I can't think of any reasons if any cases; however, Dave also has a lot more experience than I have. 

  ....okay I everyone has found a few answers that slap the forehead.

[ShreveCC]

Glad I never used this new lab setup without thinking....again..... 

I normally don't work with main attached circuits, but if I did without thinking of why I got an isolation transformer in the first place.....
 

[Cliff Matthews]

Isolate EUT, but use an isolation transformer with good regulation (IE a good margin of available current). Otherwise, you live with a higher risk a fire hazard.. (as per anecdote recently posted: https://www.eevblog.com/forum/beginners/diy-isolation-transformer-question/msg806377/#msg806377)

[T3sl4co1l]

Isolation transformers aren't usually very good, anyway (aside from galvanic -- shock -- protection).

For example, you can't use an isolation transformer (on the EUT or scope) to measure a high side switching circuit: the common mode signal (the switching itself) forces switching noise throughout all the cables in the system, ruining your measurement.

The switching current flows from the EUT, through its supply cable, across the isolation transformer's winding capacitance, up the scope's cable, and along the probe cable and ground clip.  The entire loop has a complex reactive response, with a main LC component due to the loop inductance resonating with the isolation capacitance.  The voltage drop produced along cables might not be a big deal, but the voltage drop across even a very small length of uncoupled ground, at the probe, is enough to ruin your measurement.

The magnitude of current flow (approximately Vsw(pp) / sqrt(L_loop / C_iso)) may also cause problems for the switch itself.

This is mainly a high frequency problem, but it still occurs at low frequencies, if the measurement is sensitive enough.  Low capacitance isolation transformers are required for medical applications, to minimize ground leakage current to the patent (and to isolate other systems from possible defib transients!).

Tim

[dacman]

Isolation transformers I use are not ground isolated, only Hot and Neutral through a transformer.  This would not isolate the ground of a grounded scope.

I like to use a scope with an A-B (or differential) feature if I need or want to make a measurement that is not common to ground.  I also sometimes use a Fluke 196B (battery powered).

[Neganur]

It's not an isolation transformer when it's not isolating ground, is it?

[vk6zgo]

It's not an isolation transformer when it's not isolating ground, is it?

It is,if neither side of the secondary is connected to Mains ground,& the "ground" is just a direct connection between the input ground connector & the out put socket ground pin.
That is how a classic isolation transformer is connected.

Some other transformers connect one leg of the secondary to Mains ground,so there is no isolation in that case.