Floating Scopes

[Someone]

Is it any more dangerous to float my Rigol 1052 and take sensible precautions such as:

Using a shrouded probe with no ground clip attached (I even have probes with shrouded BNC connector at the scope end)
Use a scope probe 'clip' so you don't need to hold the probe on the test point when powering the DUT - even solder a short wire to the test point and clip your probe to that if needs be.
Use a separate lead from floating scope ground to croc clip on floating hot ground

Than it is to use my Fluke 79 with the negative terminal clipped to hot ground while I poke around measuring voltages on the SMPS primary side with the positive probe?

Both meter and scope have no, or very few, exposed metal parts at floating potential that could be accidentally touched.

Except that the meter has been designed for this use and contains appropriate protection for faults, and more isolation from the touchable surfaces. None of that is in the scope.

[dicky96]

Interesting points.  I haven't really ever looked into differential probes as my preferred method so far has been to use an isolation transformer whenever I can.

I didn't fully describe the 1980s isolated workbench environment - yes wooden benches, rubber mats etc.  Also we had strict working practices like you did not wander over and physically touch (or even distract) another technician  while he was working and you hooked up your floating scope and set the Volts/seconds before you clipped it to the DUT and powered up.  You looked at your waveforms and you didn't touch the scope while the DUT was powered.  If the settings were wrong you powered down and changed them.

With an isolation transformer on the DUT which honestly I do prefer to use guys, as some have already mentioned, danger still lurks if you don't understand that clipping your grounded scope to hot ground will no longer cause sparks or blow up the DUT/Scope but it also means you can get a nasty shock if you inadvertently touch hot live.  So in this case the grounded scope defeats some of the protection of using  the isolation transformer.  But using an isolation transformer with a grounded scope at least now it will not hurt you if you accidentally touch hot ground.  I do get this guys.

Reading the various views and replies one thing does seem to be common.  Understanding what you are doing seems to be the key.

By the way, regardless of understanding the risks andd taking the precautions I have still had a few zaps in the past 35 years but could count them on the fingers of one hand, and due to the working practices we were taught they have not been the really dangerous 'across the body' type of shock.  I can tell from experience though that a 220V 50Hz shock when I accidentally caught hold of a live linear transformer primary was bloody painful, felt like a deep throbbing and put two little black puncture holes in the heel of my thumb from the transformer terminals, and coming into contact with the collector of a running line output transistor on a CRT colour TV hurts - you can actually here it in your ears 15.625KHz. 

Both were from the DUT, I have never had a shock from test gear either floating or otherwise, and neither are something you want to experience again which increases your caution and attention level further.  So with all the precautions of whatever method you use I fully accept that working on live floating equipment is not 'safe', the best you can do is make it 'safer' and minimise the risks to an acceptable level at which you can work.

Rich

[nctnico]

I honestly don't think the way we were taught 'back in the day' was irresponsible or dangerous - I think we were just properly taught to understand exactly what we were doing, the risks involved and how to minimise them.  This included using shrouded probes, a separate ground connection to the chassis (not on the scope probe) and clipping the scope probe to component lead you wanted to test, then powering up the DUT.  Oh and there was nothing grounded on the wooden bench, plus it had a rubber surface mat and you stood on a rubber mat.

So you already know there is a lot more to it than just cutting the earth wire! Good, because many people think an isolation transformer is enough and happily ground the DUT again using their oscilloscope's ground lead.

The modern day equivalent to setting up an entire isolated work environment is a CAT rated differential probe. Much cheaper and much easier.

.... And the differential probe is the cheapest chinese found from ebay where the isolation can be anything a common semiconductor junction included.

This is a rather dumb remark. Note that I wrote 'CAT rated'. There are only a handfull manufacturers of differential probes and lots of rebrands (including those from Keysight, Tektronix, Lecroy, EEVblog, etc). A good differential probe starts from around $200 but sometimes good deals can be found.

[tautech]

Rich
Many think when they have a scope they can look at anything.....and with using a few old-school unsafe tricks you definitely can.
There is a fault in this philosophy in that the scope only displays what it's connected to in an unsafe manner or not.
Scope users/owners need think a little differently and realise just owning a scope and a few passive probes is not the total answer to measure things safely and quickly......purchase of a scope is NOT where it ends, there are other tools needed to use a scope to anything like its potential.

Current, differential, active and HV probes are at some time needed by most for what seems to the inexperienced...........simple measurements. Like many tools, the key piece (scope) is only the start of a # of accessories to be able to fully utilise it.

I've got all except active probes and have had them for years, they all have repaid me with time savings and safety.

Stay safe guys and gals.

[PA4TIM]

Tautech +1
I am in the repairbusiness too (but no consumer stuff) I most use a 500MHz FET probe, a high voltage 25 MHz diffprobe an AC+DC current probe and a Fluxgate currenprobe. Also have a low voltage diffprobe, an AC current probe, a bunch passive probes (1x, 10x, 100x) and 2 HV probes (over 10kV)

Safety first !
I never float my scopes. I have a Tek isolation guard (6901A) that I use for the DUT and behind it a homebuild very fast electronic adjustable " fuse"
Besides that I have a variac combined with an isolation transformer and a switchable series lightbulb. On my desk I have a big AC current meter and an emergency stop switch (with build in 10A resetable fuse)

I'm just in the middle of a review and teardown of an isolated Siglent portable scope.

[tautech]

I'm just in the middle of a review and teardown of an isolated Siglent portable scope.

Of which I've pushed and pushed for the front end to be incorporated into a good basic benchtop DSO.

Manufacturers just don't get it do they ?     

Maybe they're waiting till I push off this life so they don't have to call it their Tautech model. 

[Shock]

I think I may have mentioned this in another thread previously. But lets take the Rigol DS1000Z series manual as an example:

Ground The Instrument.
The instrument is grounded through the Protective Earth lead of the power cord. To avoid electric shock, it is essential to connect the earth terminal of power cord to the Protective Earth terminal before any inputs or outputs.

DS1000Z  series digital oscilloscopes can make measurements in Measurement Category I.
WARNING This oscilloscope can only be used for measurements within its specified measurement categories.

Measurement category I is for measurements performed on circuits not directly connected to MAINS. Examples are measurements on circuits not derived from MAINS, and specially protected (internal) MAINS derived circuits. In the latter case,
transient stresses are variable; for that reason, the transient withstand capability of
the equipment is made known to the user.

These go a bit further than the "don't put the plastic bag on your head" type warnings.
There are a few oscilloscopes around are designed for direct mains voltages, HV and channel isolation. However the majority of modern oscilloscopes are not, so you have use them in a safe manner and within the conditions they were designed for to achieve your desired measurement and preserve the integrity of the instrument.

Tektronix Floating Measurements Selection Guide.

Fortunately, there are solutions available which are not only safe but much more accurate than the extremely dangerous and unacceptable procedure of floating your oscilloscope. These fall into general categories: differential probes, isolated input oscilloscopes, isolation transformers and isolation amplifiers.

As with all equipment using it unintended way can cause unpredictable results.
Tektronix Floating an Oscilloscope: A Definition

“Floating” a ground-referenced oscilloscope is the technique of defeating the oscilloscope’s protective grounding system disconnecting “signal common” from earth, by either defeating the grounding system or using an isolation transformer. This technique allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This technique is dangerous, not only from the standpoint of elevated voltages 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 may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation. Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This potential inaccuracy results from the total capacitance of the oscilloscope chassis being directly connected to the circuit-under-test at the point where the ground lead is connected

So there it is, you could argue that you can increase the safety margin, but bypassing the protective ground/earth on oscilloscopes is inherently unsafe and avoidable to start with.

[HighVoltage]

Tektronix made the handheld THS720P and similar model scopes with two channels.
Totally isolated from ground and from each other !
These scopes are pretty cheap these days, sometimes cheaper than a good differential probe.
Perfect for this kind of repair.

[serggio]

Apart from dangerous use, regular scopes in "floating mode" can be very sensitive to noises.
This scopes and many probes too was not developed for using without ground connection.
If you doing measuring small differential signals, it can become problem. Your signal on scope screen can be affected by noise.

[Electro Detective]

Whatever methods used to prod safely, I'll take my chances using a floating (if required) or fully earth/grounded isolating transformer powering the test gear,
rather than have the full force of the power station going through the DUT and oscilloscope in the event of ANY fault.

FWIW  a GFCI/RCD or better still an RCBO, WILL WORK on the secondary side of a fully earth/grounded isolating transformer (or 'medical' transformer) so you've got some safety factor as well as isolation from the mains

If some people don't get that, or don't want to, then by all means ground yourself good and proper 100%

and good luck praying the GFCI/RCD and MCBs both work when SHTF, even when using clever probe$


oh, have you tested your GFCI/RCD lately ? once a month? will it still trip as it should? Are you sure?
The TEST button worked, and that's good enough ?  LOL, good for you   

Please 

[madires]

The problem with floating the DSO is that the "floated" ground is exposed via the BNC connectors, probe's ground clips and other connectors like USB and what have you. Any tiny mistake connecting any of those to ground/earth by some means can kill the DSO or you. And you still have the problem that all channels share the same ground. Another issue could be the Y caps in the DSO's SMPSU. They are connected to ground (same as the probe's ground) and L/N.

Best current practice: power the DUT by an isolation transformer and use differential probes

[David Hess]

The transformer issue is a tough one.  Off-line powered instruments rely on the 50/60 Hz or switching power transformer for isolation from neutral and hot (1) and floating the instrument by breaking the ground connection increases the potential across the transformer.  Some old instruments specified a peak float voltage and incorporated a special transformer to handle floating operation.

During floating operation, any exposed ground connection including the BNC connectors can become a hazard.  Instruments designed with this in mind have special construction which tries to prevent exposure and include special BNC connectors and probes.  Fluke uses or used plastic BNC connectors much like the shielded banana jacks now common on multimeters with probes to match; none of the BNC coaxial ground connection is exposed.

Even if you do float the oscilloscope safely using an external isolation transformer, considerable common mode capacitance still exists and all of the probe grounds are still tied together.  Oscilloscopes designed with isolated inputs largely avoid these problems although their inputs are still unbalanced by common mode capacitance through the environment and the isolated power supply.

If you read documentation from scope manufacturers back then, they also recommended against it.

It depends on the time period and manufacturer.  Some recommended floating the oscilloscope by disconnecting the ground and even specified the maximum floating voltage with little or no warning about the dangers.  Suitable high voltage differential probes were very rare (2) and isolated probes and inputs did not exist leaving A-B measurements with their limitations, floating the device under test which is not always practical, and floating the oscilloscope.

Unlike in a formal course where participants have some understanding of the circuitry and the risks involved, on a public forum where all and any can read and deduce rightly or wrongly the procedures and risks involved it is nothing but irresponsible to discuss the merits of floating a scope. Sure some do it but it should never be promoted in a venue such as this as we have no idea of the skill level and understanding of those reading.

IMNSHO the only way that statement could have been improved is if it had been posted in a larger font.

You can both bite me.

Do you really think it better to not discuss the merits and hazards and instead rely on an argument from authority or silence?  How do you expect people to gain understanding without discussion?  Nancy Reagan's "just say no" campaign was as much a success as abstinence only education.

If your neutral/earth are tied together you need to revise your wiring.

In the US, split phase 240 volts AC comes down from the power pole and neutral is bonded to earth ground at the main distribution panel.  One utility transformer supplies multiple homes leading to multiple grounds but it seems to work well enough despite multiple distributed earth grounds; this might even be preferable in areas with thunderstorms.

(1) Some already already isolated instruments include a second isolation barrier.  Oscilloscopes with separate isolated inputs are the obvious example but some multimeters include a separate isolated supply for their input circuitry.  This provides a higher isolation rating than a common or even special 50/60 Hz or switching power transformer will support and with a lower common mode capacitance.

For instance the isolated outputs of the Tektronix TM500 series power supplies are limited by the isolated windings on the 50/60 Hz power transformer in the mainframe to a floating voltage of about 50 volts.  But the multimeters which use the same TM500 mainframes have their own internal isolated power supply allowing their common inputs to operate at up to 1000 volts.

(2) Tektronix had some suitable differential amplifiers starting in 1960 but the first suitable x10 probes which could be used with them in a high voltage application were introduced in 1962.  They introduced a good differential amplifier in 1967.

[mtdoc]

Unlike in a formal course where participants have some understanding of the circuitry and the risks involved, on a public forum where all and any can read and deduce rightly or wrongly the procedures and risks involved it is nothing but irresponsible to discuss the merits of floating a scope. Sure some do it but it should never be promoted in a venue such as this as we have no idea of the skill level and understanding of those reading.

IMNSHO the only way that statement could have been improved is if it had been posted in a larger font.

You can both bite me.

Do you really think it better to not discuss the merits and hazards and instead rely on an argument from authority or silence?  How do you expect people to gain understanding without discussion?  Nancy Reagan's "just say no" campaign was as much a success as abstinence only education.

   My feeling exactly!  Stifling discussion achieves nothing.  An informed discussion that lays out the risks (and any merits) can only be beneficial.   This is an electronics discussion forum with many experienced, informed and articulate engineers and hobbyists.  This is exactly where this discussion should occur.   There are countless places on the web where incomplete or wrong information about this and related topics is available.  Refusing to discuss it here and provide correct information does not make anyone more safe.

Bravo to those who have provided good information here. I always learn more by reading these threads.

[BravoV]

Bravo to those who have provided good information here. I always learn more by reading these threads.

+1 

[Electro Detective]

and a BIG thanks for nothing to the  "JUST DON'T DO IT" brigade     

when all that a responsible technician, advanced hobbyist or EE requires is some simple 'how to and how not to' information,
so they can decide if their equipment and skill set is up to the task (or not) for a particular probe and measurement scenario,
taking ALL possible safety precautions, 
including 'think before you probe'      

Otherwise most uninformed/misinformed/Darwin Dumbass wannabees will try something NQR anyway, and BANG !        



 

[tggzzz]

Do you really think it better to not discuss the merits and hazards and instead rely on an argument from authority or silence?  How do you expect people to gain understanding without discussion? 

I agree that a fully argued and authoritative argument that is understood by the reader is valuable and ideal.

Unfortunately on a forum such as this it is very unusual to find such arguments - and where they occur they are often buried in a morass of other posts. An example was the first post in this thread saying, in effect, that "I was  taught to do this safely", which omitted all the other essential workshop practices that significantly reduced the dangers. The extra practices were mentioned 25 posts later; it would be easy for someone reading this thread to miss it.

We cannot assess the competence and understanding of readers. If we are with such a person then we can spot any mistakes they might be about to make that, we wouldn't have thought they might make (the perversity of the inexperienced). For example, as already mentioned: https://groups.io/g/TekScopes/topic/541a_isolation/950255?p=,,,20,0,0,0::Relevance,,541a+isolation,20,2,0,950255 I'm sure we have all seen things like that; I have - from both sides!

Given those real-world "non-idealities" and the potential consequences of making a mistake, and that there are good alternatives, there should be "don't do it" statements.

I've watched people die in accidents; dealing with the reactions of the other onlookers isn't fun.

[tautech]

The transformer issue is a tough one.  Off-line powered instruments rely on the 50/60 Hz or switching power transformer for isolation from neutral and hot (1) and floating the instrument by breaking the ground connection increases the potential across the transformer.  Some old instruments specified a peak float voltage and incorporated a special transformer to handle floating operation.

During floating operation, any exposed ground connection including the BNC connectors can become a hazard.  Instruments designed with this in mind have special construction which tries to prevent exposure and include special BNC connectors and probes.  Fluke uses or used plastic BNC connectors much like the shielded banana jacks now common on multimeters with probes to match; none of the BNC coaxial ground connection is exposed.

Even if you do float the oscilloscope safely using an external isolation transformer, considerable common mode capacitance still exists and all of the probe grounds are still tied together.  Oscilloscopes designed with isolated inputs largely avoid these problems although their inputs are still unbalanced by common mode capacitance through the environment and the isolated power supply.

Three paragraphs that should give anybody reading this enough warning that the practice is dangerous. Period.

If you read documentation from scope manufacturers back then, they also recommended against it.

It depends on the time period and manufacturer.  Some recommended floating the oscilloscope by disconnecting the ground and even specified the maximum floating voltage with little or no warning about the dangers.  Suitable high voltage differential probes were very rare (2) and isolated probes and inputs did not exist leaving A-B measurements with their limitations, floating the device under test which is not always practical, and floating the oscilloscope.

The year is 2017 and for those reading this now and in the future simple cheap differential probes will become more freely available as the most attainable safe method of avoiding floating scopes and putting themselves at risk.

Unlike in a formal course where participants have some understanding of the circuitry and the risks involved, on a public forum where all and any can read and deduce rightly or wrongly the procedures and risks involved it is nothing but irresponsible to discuss the merits of floating a scope. Sure some do it but it should never be promoted in a venue such as this as we have no idea of the skill level and understanding of those reading.

IMNSHO the only way that statement could have been improved is if it had been posted in a larger font.

You can both bite me.

Only if you think that the OP's practice of highside mains measurement by floating his scope is acceptable.

Do you really think it better to not discuss the merits and hazards and instead rely on an argument from authority or silence?  How do you expect people to gain understanding without discussion? 

As mentioned highside mains probing using floating techniques is this day and age is foolhardy but when could other floating measurements be acceptable ?
Voltage threshold that when floating a scope might be deemed unsafe ?
How can one judge what might be safe or not ?
Can the hobbyist plan to always have a buddy on watch ?

Can answers to the above questions truly be set in stone for all to follow and not expect any to push limits higher to the point where they put themselves in danger ?
Is it not best for the common good and especially for those with little experience to just call the practice of floating a scope.....dumb ?



To you and others out there......name learning institutions where the procedure of floating a mains powered and normally grounded scope is taught ?

If it's not formally taught these days, who are we to endorse it ?

Hell, the price of a 25 MHz DP-25 is ~1/2 the price of an entry level DSO and it will save lives.
Our esteemed leader Dave even recognises the need for differential probes for common use and has launched an EEVblog model. He doesn't need to live with forum readers learning about floating scopes and their possible demise from a simple mistake and neither do I. 

https://www.eevblog.com/product/hvp70/

[nctnico]

An informed discussion that lays out the risks (and any merits) can only be beneficial.

The problem with using isolation transformers and floating equipment is that it goes way beyond that. It can only happen 'safely' in an environment (workbench+room) which is setup for it, with proper training and someone else around in case things do go wrong. A home tinkerer very likely has none of these available. There are probably standards for it addressing the myriad of issues involved which go way beyond of what can be discussed in a (noisy) forum thread. All in all the best advise still is: don't do it. Argueing otherwise is like telling someone how to run in front of a car which comes at you at high speed. A stunt(wo)men with the right training can do that but for a normal person it is not recommended to say the least. There are limits to what you can learn from an internet forum.

Perhaps it is better to discuss methods which avoid having to connect defective equipment to mains. For example: Often you can test a lot by powering the control circuitry and rectified mains from a low voltage (<50V) bench power supply. That way you can probe around everywhere.

[alm]

If you read documentation from scope manufacturers back then, they also recommended against it.

It depends on the time period and manufacturer.  Some recommended floating the oscilloscope by disconnecting the ground and even specified the maximum floating voltage with little or no warning about the dangers.  Suitable high voltage differential probes were very rare (2) and isolated probes and inputs did not exist leaving A-B measurements with their limitations, floating the device under test which is not always practical, and floating the oscilloscope.

[,,,]

(2) Tektronix had some suitable differential amplifiers starting in 1960 but the first suitable x10 probes which could be used with them in a high voltage application were introduced in 1962.  They introduced a good differential amplifier in 1967.

Back then in this thread referred to 1980s. By then differential amplifiers with excellent CMRR and bandwidth were available (Tek 7A13, 7A22, plus plugins from the 500 and 5000 series), as were high voltage probes (including the P6015 if you wanted to go crazy). Also 200-series battery powered isolated scopes (e.g. Tek 200 series that could float up to 700V on battery power) and A6902 isolated probes.

I attached a snippet from the Tektronix 1982 catalog to see what they wrote back then about floating scopes. I find it high unlikely that the major scope manufacturers, Tektronix and HP (who together accounted for about 93% of the scope sales, at least in the US), were endorsing floating scopes in the 1980s.

[WackyGerman]

Seriously never float scopes , better look for suitable safe equipment like isolated scopes , Fluke , Siglent , Tek , Metrix , Rohde & Schwarz and other manufacturers sell those . Otherwise you will blow up your equipment or get an electric shock !!!!! Safety first

[SeanB]

+1 for the Wacky German, support Dave and get his new differential probe set, which gets rid of many of the pitfalls of floating either the DUT or the scope. You can of course use an isolation transformer with the probe set, as a belt and braces approach, and this also gives extra protection in case something in the DUT goes F00F during your test work, you have less fuses and semiconductors to replace as the fault current is limited by the transformer inductance.

I have a ferroresonant transformer which can also serve as an isolation transformer ( not all ferroresonant units are isolated, and many have an earthed side of the mains, typically the designated neutral of the output side, connected to the PE conductor and the case as well as a continuation of the incoming protective conductor) and this can then be fed to the Variac to provide variable voltage. As the ferroresonant unit is 2kVA, and the variac is similar in rating, this provides very little in the form of overcurrent protection, aside from the 10A fuses they each have in them. I use care in using them, being aware of the danger associated with voltages that are above the SELV level of 50V peak. Much better to probe with a lower voltage applied to the control side to find faults, and no mains applied, you lose a lot less of the output of the foundry that way.

[Shock]

and a BIG thanks for nothing to the  "JUST DON'T DO IT" brigade     

when all that a responsible technician, advanced hobbyist or EE requires is some simple 'how to and how not to' information,
so they can decide if their equipment and skill set is up to the task (or not) for a particular probe and measurement scenario,
taking ALL possible safety precautions, 
including 'think before you probe'      

Otherwise most uninformed/misinformed/Darwin Dumbass wannabees will try something NQR anyway, and BANG !       

Unfortunately explaining how and why over and over seems to add fuel to some who think the ground/earth lifting is a "hack" or "work around". "Don't do it, it's dangerous" is a perfectly adequate response considering the amount of people repeatedly chiming in saying it's acceptable practice or just be careful.

[Fungus]

If I'm an outsider reading a thread that argues this both ways I'll be trying to figure out:

* What problem is solved by floating a 'scope?
and
* When might I want to do it?

The answers are "none" and "never" so I don't think those lines of thought should be encouraged by saying, "It's perfectly OK so long as ${LONG_LIST_OF_RULES}".

The only correct answers are:
a) Get a differential probe
or
b) Use AC coupling.

If you're doing (b) and there's a DC offset then tough. Oscilloscopes are for looking at the shapes of waves, you can see ripple just fine even without the correct DC offset. Connect your multimeter in parallel if you want that number.

[mtdoc]

I've seen very few people on this forum, in this or the multiple past threads on this topic,  say that floating a scope is ever ok.  The vast majority of posters always say "don't do it".  That is as it should be.  The more important thing IMO, is the explanation and discussion about why it is not ok to do it.  There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.   "Teach a man to fish, etc..''    That is what this forum provides: experienced engineers and hobbyists who can go beyond the glib "don't do it" or " it's ok as long as you.." type responses seen so many other places on the web.   It's only through discussion that this teaching/learning happens.

The use and miss-use of isolation transformers is another closely related and perhaps more complicated topic.  The multiple discussions around that have also provided great learning opportunities.

[tautech]

I've seen very few people on this forum, in this or the multiple past threads on this topic,  say that floating a scope is ever ok.  The vast majority of posters always say "don't do it".  That is as it should be.  The more important thing IMO, is the explanation and discussion about why it is not ok to do it.  There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.

I touched on *WHY* with the 8 questions in reply #41, answer all those with something that covers ALL floating usages and we'll have something to move forward with.

If you can.