Isolation Transformer for Scope

[jaipursky]

Simple question: planning to buy an isolation Transformer to troubleshoot hot side power supplies.
I plan to buy it as per scope's  power rating.

I see some suggestions online to buy the isolation Transformer for the Device under Test. Devices and vary in power consumption so it requires a really large transformer if we go this route.

Is there any specific scenario in which DUT needs to be isolated and not the scope?

[ogden]

Is there any specific scenario in which DUT needs to be isolated and not the scope?

Sure. Scenario where operator cares for his life. Isolated scope may work till your first error when you grab scope part which is connected to live AC mains and... die. Either isolate DUT or even better - use high voltage differential probes.

[stj]

isolation transformers wont save your life, only your scope.
anybody who thinks an isolation transformer cant pass enough current to "get the job done" shouldnt be working with mains.

[ArthurDent]

"Is there any specific scenario in which DUT needs to be isolated and not the scope?"

The answer is every single scenario is one where the scope definitely should not be isolated. This question, for some unknown reason pops up on a weekly basis and if you search the forums you can find this question answered again, and again, and again... Generally the device under test shouldn't be isolated either. Search and you will find many references and videos on differential/isolated scope probes and cautions about doing any of these measurements if you don't know what you are actually doing.

[magic]

I have floated my scope sometimes... 

The really quick and dirty, cheap-ass solution is to just disconnect Protective Earth. The scope already contains a transformer which isolates its ground from mains, both line and neutral, but the ground is still connected to PE. I break this connection using a suitable adapter which goes onto the scope's mains plug

One immediate consequence is that if the scope's transformer breaks down, mains could appear on the scope's ground. So I always attach all probes before applying power and operate the scope with a PVC stick, regardless of what DUT it is "grounded" to.

Never "ground" it to any high voltage with respect to mains. Same applies to any other unearthed, mains-powered device too. I don't know how many secondary-to-primary volts those transformers can withstand and don't wish to find out. For probing in rectified mains circuits like PSU primaries, it is prudent to use the same mains phase as the scope is connected to.

Finally, if I were to deal with all the hassle and hazard more than once per year, I would probably get myself proper differential probes.

[tggzzz]

I have floated my scope sometimes... 

That is a stupid, dangerous suggestion to make to a newcomer to the forum. We have no idea of their experience nor competence.

Biting my tongue, I'll merely point out that I have walked into a busy road without looking.

The really quick and dirty, cheap-ass solution is to just disconnect Protective Earth.

For every problem there is a simple easy wrong solution.

That endangers the scope (nobody cares), the scope user (probably the scope user cares, do you care?) and the UUT - especially if the UUT is an SMPS.

Read and understand these threads:
https://www.eevblog.com/forum/testgear/floating-scopes/
https://www.eevblog.com/forum/testgear/ground-of-oscilloscope-always-connected-to-earth/

See what's "necessary" to float a scope so that the user is not endangered:
https://www.eevblog.com/forum/testgear/to-isolate-test-equipment-eg-oscilloscope-using-ups/msg2017633/#msg2017633
but note that won't "save" the UUT.

Read the professional way to do approach safety with scopes and probes in the refs at
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Example fatality from https://groups.io/g/TekScopes/topic/7632418#10795 with my emphasis

What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.

Anyone who teaches you that floating a scope by defeating the power cord ground lead is a very poor teacher, indeed. They simply do not know enough about making SAFE measurements to be in a position to teach electronics.

Excerpt from Tektronix' "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes Application Note"
https://info.tek.com/www-fundamentals-of-floating-measurements-and-isolated-input-oscilloscopes_ty.html,
with my emphasis

   
Advantages
    Although floating equipment is a method that uses existing equipment to make floating measurements and remove ground loops on lower frequency signals, it is an unsafe and dangerous practice and should never be done.

Trade-Offs
    This technique is dangerous, not only from the standpoint of elevated voltage present on the oscilloscope (a shock hazard to the operator), but also due to  cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure but can lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation.

    At higher frequencies, severing the ground may not break the ground loop as the line-powered instrument exhibits a large parasitic capacitance when floated above earth ground. The floating measurement can be corrupted by ringing. Floating oscilloscopes do not have balanced inputs. The reference side (the “ground” clip on the probe) has a significant capacitance to ground. Any source impedance the reference is connected to will be loaded during fast common-mode transitions, attenuating the signal. Worse yet, the high capacitance can damage some circuits. Connecting the oscilloscope common to the upper gate in an inverter may slow the gate-drive signal, preventing the device from turning off and destroying the input bridge. This failure is usually accompanied by a miniature fireworks display right on your bench.

    Yet another trade-off is that only one measurement may be made at a time - remember all the input references are tied to each other. Once you have floated one input references, all input references are now floating at the same level.

Fundamentally either use the (special) right probe for the job, or the (special) right scope for the job, or don't do the job.

[Electro Detective]

Fundamentally either use the (special) right probe for the job, or the (special) right scope for the job,

or don't do the job.

That about sums it up perfectly 


For those few 2019 Darwin Award Contenders that still won't buy: 

It can be done ***IF*** the isolation transformer is a TRUE isolation transformer (most times NOT, without a hack or two, or three...)
and physically isolated from the surrounding area (rubber feet, wooden box, custom leather profilactic)

Once confirmed that you have indeed a TRUE isolation transformer, properly zapped and SAFETY pass granted with an insulation tester at 250/500v,
and good solid shake-proof internals and secured cable tied wiring,

then, and only then, do you begin to learn the correct method to use it with a scope for that 'one off job' that some will do... regardless of the advice and cautions presented by experienced (and perhaps bitten) folks here,
some that probably took out a scope or two, and luckily not an eye or two, and or a heartbeat skip or two.. finding out how 'isolation' can go south during a heated experiment
and or fading patience         repair troubleshoot.


Ground/Earth cheaters are a no no especially if the scope has a metal housing and or touchable screws or metalwork, and the condition or insulation of the scope transformer is unknown or old,
and maybe an aging capacitor or two tied to metal chassis, to keep interference levels low..
and user risk levels high   

One must also confirm just what's coming out of the wall socket to begin with, and verify all power cabling to the isolation transformer and scope are in good shape and proper polarity.


The scope probe leads, clips and 10x switch and internals,
are they REALLY ok ?   
or a drama about to unfold when moved about, even though everything else 'appears' perfect ?


Even if you get it all spot on perfect, I would have PPE eye-wear, thin leather gloves, long sleeves shirt and rubber sole shoes/boots,
i.e. I don't want to come into contact with anything! 

This way chances are better you only pay out for fried electronics and not on hospital/funeral fees if or when it all goes horribly wrong.


 
I can keep going, but hopefully I've painted a fair picture of  -Murphy Isolation- 

If you're not 110 percent sure and or experienced, with someone nearby just in case, DON"T DO IT ! 


FWIW those experienced Youtubers and forum gurus posting their methods won't be there to bail you out...