Not To Hijack the thread "Using Isolation Transformers" - I have a question:

Thread Starter


Joined Sep 24, 2015
In This thread, (as the title references) an AAC poster is asking about using an isolation transformer. Seeing as how I don't have a good answer for that question but rather a question of my own, and I don't want to hijack the thread, I'm posting this question, which may link back to the other thread:

In that other thread the thread starter asked about isolating an o-scope. Among the advice given is that the unit under test should be isolated rather than the scope. I'm wondering if I were to take an isolation transformer and isolating (not grounding) either the scope OR the UUT (Unit Under Test); would that be a reasonable approach to isolation? Neither the UUT or the scope would be grounded. Advice against? For? Neutral on the subject?

Or perhaps an isolation transformer for the UUT and another isolation transformer for the scope? I have several ferroresonant isolation transformers available for use. Again, advice?

Delta Prime

Joined Nov 15, 2019
When I would conduct DO19 aerospace acceptance testing I would observe.
Oscilloscope users frequently need to take floating measurements in which neither point of the measurement is at ground potential. The signal common can be elevated to hundreds of volts from earth and, in addition, a number of these measurements must reject high common-mode signals. Digital storage oscilloscopes, necessitates the rejection of the common-mode signal, so that the instrument can display the difference signal. I myself employed the technique to circumvent grounding the oscilloscope and it must be floating dangerous yes effective yes but then again how do expert mountain climbers die? they fall off mountains.
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Joined Jan 6, 2004
Not intending to derail a thread that pretty much conveys some of my own doubts, I get lost quickly when I run across of "common", "ground", "earth" and "common-mode-signal" expressions, in just one or two paragraphs.

If you add that many years ago I could watch one of the probes' crocodile (ground) of my own scope partially vaporizing at the moment of measuring something (forgot what, long ago), the doubts are still there. And the user was not me but an technician with years of experience in RF.

Thread Starter


Joined Sep 24, 2015
I appreciate the responses from all. It was just a passing thought. But I guess @AlbertHall is right about the potential of a fault developing in the scope.

I remember back in high school - yes, they had them that long ago - in electronics shop, I had a signal generator that had a 110 plug with just two prongs and a scope also with a plug that had two prongs. When I rested my left hand on the scope and my right hand on the generator I got a shock. Mr. Graves (R.I.P) told me to turn one of the plugs over. I did. Still got a shock. So I turned the other plug over. STILL got a shock. I don't remember measuring the potential between the two of them but if they had both been grounded I wouldn't have gotten that shock. And I would be much smarter today.

As I mentioned, I have a few, maybe 5 or 6 ferroresonant transformers and don't know what to do with them. They are 1:1 so they will function as isolation transformers. I could disconnect the resonant cap and just use them as isolation trans'. Not sure how many watts they can handle though. There's no markings on them to clarify that. But as luck would have it - I'm in the process of cleaning out the garage. Have a TON of stuff to get rid of. Going to be adding a second floor to the garage. Not sure how I'll get the cars up there - but that's for another day. Actually, it's going to be a wood shop. Something I've been wanting to do for a while now. Looks like this year may be the right time to move forward with that.

Anyway, again, thanks for the responses. All comments are appreciated. As long as I walk away having learned something. And I did at that.
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Joined Jul 1, 2009
People forget how a scope (and everything) is connected to ground through it's power-cord. In DC environments you can sometimes get lucky with bad practices. Misunderstanding how a scope ties into any given circuit and how outlet ground plays into that larger connectivity (because it does become part of the circuit), can be disastrous in AC environments.

My one and only foray into said mistake was when I made the mistake of grounding both probes in an A/C circuit-- I was young and stupid, and after it ruined 1 $150.00 probe (thank God it didn't harm the scope) I learned what I'd done from a more knowledgeable individual who kindly shared with me that I could not ignore how the scope itself handled ground. To this day, I'm really, really careful that I understand exactly what's going to happen when I connect my scope.
Let's add this to the mix.

Let's also re-define isolation somewhat. Us hobbiest's want isolation for different reasons. Medical isolation transformers have a much higher voltage that can be applied , say accidentally in thousands of volts.

The scope of yesteryear had a metal case. You don't want that metal case to be live, so you don't isolate the scope.

In one ERA, TV's had a string of filaments that added up to the line voltage to eliminate the transformer. It used a two prong polarized plug. To safely work on that set, you needed to isolate the DUT.

You can be isolated, but still grounded.

Note the electrical schematic for an RFI filter.

There are capacitors in series with a ground point in the center. When you model a capacitor, there is a parallel resistance element and a series resistance element. If that center point is not connected, that potential is raised to about 1/2 the line voltage or 60V for 120 V circuits.

If the equipment is plugged into an outlet strip and something else is plugged in external to the strip, bang or a shock.

Thus moving the ground/Neutral connection to after the isolation is a good idea. It's also critical for noise reduction.

isolated, does not mean "not earthed". It really means the neutral/ground bond, ground rod connection is broken for the DUT.
The isolation transformer attenuates high frequency noise because of the transformer. it''s a good part and believes it's essential and I would agree.

I used an ISOBAR surge suppressor and a powervar/OneAC isolation xformer for 17 years on a single computer that was on 24/7 unless there was a power failure and there was no hard drive failure. This was a Macintosh which had a SCSI drive which typically have better reliability. Dust and floppy failures were it. I used it before and after that 17 year period. We depended on that system every day.

When we upgraded we made one portable, so we had a spare electronics. The portable unit did not have the $40,000 USD light source.

The isolated variac is a very useful tool for audio amplifier work.

I've used the a-b or a+b with b inverted to do pseudo-differential measurements.

I never had the luxury of an Active probe at work.

I got a good deal on the bay for a Philips PM8940 isolation amplifier. e.g. for home.