Floating Scopes

[dicky96]

I've googled this and yes it had been discussed but I still don't understand why this is a big no no

I do a fair amount of repair work on plasma/LCD/TVs and also some switch mode PSU repair

 I understand the whole reason of using an isolation transformer on the DUT but currently I don't have my isolation transformer here where I am working in Gran Cararia (it's heavy and still in UK) and I need to connect scope ground lead to hot ground and probe the SMPS 'half bridge' controller on a Plasma TV I am trying to repair.  I have an analog scope in my workshop here and a Rigol 1052

Yes I know if I disconnect the mains ground the scope ground then floats at up towards whatever potential is on the probe gound - but the Rigol internally has a SMPS so the probe circuitry and 'ground' isolates from the mains live/neutral there, and anyway it is made from plastic apart from the BNC ground and a couple connectors on the rear.

Everywhere you google says this is really dangerous, yet in the early 80's when I did my City&Guilds in TV repair we were taught by our instructors to float the scope when working on TVs. Unless you were lucky enough to work in a repair shop with isolation transformers which were not that commonplace.

Also no TVs had an earth connection then anyway, they were all captive leads with two conductors.

In the Valve sets and earlier transistor sets the chassis was connected to neutral (but you always taught to check the mains plug wiring first just in case the owner had it the wrong way round  )

And If the TV was one of those 'new fangled' 1970s designs that used a full wave bridge rectifier followed by a 100Hz Thyristor 'chopper' circuit to regulate the supply voltage by chopping a chunk out of each mains half cycle, then the whole chassis was connected to the bridge rectifier negative terminal and floated at what they called 'half mains voltage' - which actually meant 'full mains voltage for half of the time'

Depending which half cycle of the mains was coming out of the bridge rectifier negative at that instant the chassis was either at Neutral or Live potential!!  This is exactly the same as what is called 'hot ground' these days

So your floating scope (of which you were taught to be careful not to touch the metalwork) was only at the same potential as the entire TV chassis you were working on anyway - which you were equally careful not to touch! So between floating scope and floating chassis I honestly don't understand the extra danger

So what I don't get is why it is more dangerous and a huge NO NO to disconnect the ground on a modern plastic scope in 2017 to do what was normal practice with a metal chassis scope back in the day???

Surely if you understand and respect what you are doing this is as safe (or even more so due to almost no exposed metal on a Rigol) for you, your test equipment and the DUT as it was in the 80s??

By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves.  Doesn't that say something about the perceived 'danger' of floating scopes?

On googling, some folks on this forum say there is no reason to ever ever float a scope - I would ask those posters if they ever worked on SMPS primary side controllers?

Rich


Rich

[grumpydoc]

Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO).

[savagemadman]

I've been wondering about this myself. My scope doesn't even have a grounded plug on it! The manual says to just ground to the device under test.

[alm]

If you read documentation from scope manufacturers back then, they also recommended against it. And yes, back then people were killed by their floating scope. Tektronix made a special isolation transformer that would ground the instrument as soon as its ground floated to far away from earth. As for isolation transformers being heavy, differential probes are much lighter and can do the majority of the measurements you would float a scope for. As are scopes with isolated channels.

I've been wondering about this myself. My scope doesn't even have a grounded plug on it! The manual says to just ground to the device under test.

That must either be a special kind of isolated scopes or a very crappy/old one (WW2 era). Tektronix made a couple of battery-powered scopes that were isolated. Kind of hard to guarantee an earth connection in a battery powered scope. But they took special care in the mechanical design and used special probe connectors.

By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves.  Doesn't that say something about the perceived 'danger' of floating scopes?

If one in sixteen would kill themselves over the course of a repair course of a couple of months (approximately 25% change to die each year), there would be pretty much no repairman over 30 years old. I am sure the chance to die is much lower than 25% per year. But still, does TV repair really pay enough to take such unnecessary risks? Just get the proper tools, especially if you are doing it professionally.

Do you also rip seat belts and airbags out of your car, because people did without them in in the seventies?

[savagemadman]

That must either be a special kind of isolated scopes or a very crappy/old one (WW2 era). Tektronix made a couple of battery-powered scopes that were isolated. Kind of hard to guarantee an earth connection in a battery powered scope. But they took special care in the mechanical design and used special probe connectors.

Its a ~17 year old portable Fluke, probably made by Phillips. AC or battery powered. The AC has no ground connection. As far as I know the newer units do not have a ground either. I'm assuming it's a similar situation to the Tektronix models you mention. The probes look like normal BNC to me though, just shrouded.

[Electro Detective]

Why the cheapass scope manufacturers did not MASS produce ALL scopes with isolated inputs, deserves an epic scale public tar and feathering of the lot of them. 

That's the real problem, not the user who wants to prod freely into a circuit like a decent floating multimeter   

In 2017, they are STILL doing the same BS and flogging 2 and 4 channel PLASTIC scopes that are mains earth grounded on the BNC connectors

It's ABSURD to say the least. 


Can you blame any struggling tech for floating a scope in desperation to get the job done, whilst being overly cautious (I hope!) to not zap something/someone ?   


Seriously, how much more can isolated inputs cost to produce at the manufacturing stage?       

$10 more to cover parts and cheap teenage labor ?!!   

[DaJMasta]

If you want the safety of having a grounded, shielded chassis on your scope and the flexibility to attach to ground referenced  GPIB, USB, Ethernet, etc. ports... I'd imagine properly isolating a scope costs quite a bit more than $10 just in parts... and if you're not working on high voltage or your working on other ground referenced devices, there is no issue with having a ground referenced scope.  The issue is that the chassis being earthed is counted on in the design and the safety features of the scope, so it was never designed for a floating operation, even if the metal chassis behemoths of yesteryear were (and I don't necessarily think that's the case).

Even if you can be fairly safe with it in practice, can you really afford the risk?  We have the tools and the understanding to be substantially safer than the process you're describing, so why risk it?  Sure if you know what you're doing you can make do without proper isolation, but even people with tons of experience and who know what they're doing can mistakenly reference their unit once in a thousand repairs or mistakenly probe something that they shouldn't.  Isolation is recommended and "required" in workplaces and courses now because it is safer and because it results in fewer injuries.  Of course it was possible before, but just accepting that when the safer alternative is there.... it's negligent - not something you want to be towards students and not something a business can really afford to be.

[alm]

Isolating multiple 10 GB/s+ signals may not be that cheap. Add also the extra costs of (fragile) insulated BNC jacks and isolated probes (proper Multicontact IsoProbe probes are more expensive than your average cheap Chinese 10x probe). Plus isolation might negatively affect common mode noise performance. I imagine a large portion of their customers are doing low voltage work on the bench, and have access to an isolation transformer when necessary. The amount of component level repair is decreasing. And there are plenty of isolated scopes available. Many of them portable. So it is not like you do not have a choice.

[tautech]

I've googled this and yes it had been discussed but I still don't understand why this is a big no no..............


So what I don't get is why it is more dangerous and a huge NO NO to disconnect the ground on a modern plastic scope in 2017 to do what was normal practice with a metal chassis scope back in the day???

Surely if you understand and respect what you are doing this is as safe (or even more so due to almost no exposed metal on a Rigol) for you, your test equipment and the DUT as it was in the 80s??

By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves.  Doesn't that say something about the perceived 'danger' of floating scopes?

On googling, some folks on this forum say there is no reason to ever ever float a scope - I would ask those posters if they ever worked on SMPS primary side controllers?

Rich

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.

In short......DON'T FLOAT SCOPES.
There are tools available today at quite reasonable pricing that there is no need for this dangerous practice.

Just yesterday I sold a differential probe to a very experienced repair tech that had floated scopes for years knowing all the time the risks involved and has now given that game away in favour of safer practices.
I suggest the OP does too.

[JPortici]

Why the cheapass scope manufacturers did not MASS produce ALL scopes with isolated inputs, deserves an epic scale public tar and feathering of the lot of them. 

maaaaaybe because it's hard and expensive to have both high speed, lots of memory an isolation?
yokogawa is doing this in its new automotive scopes (from acquisition chip to mainboard through optical fibers) but samplerate is 100MS/s or so
tek does 1GS/s, 2.5 kpts with four different acquisition boards each with the asics and everything, then connects to the mainboard through an isolated bus

others, i think RS does it in a portable scope but i don't know how it's actually implemented

[tggzzz]

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.

In short......DON'T FLOAT SCOPES.
There are tools available today at quite reasonable pricing that there is no need for this dangerous practice.
Just yesterday I sold a differential probe to a very experienced repair tech that had floated scopes for years knowing all the time the risks involved and has now given that game away in favour of safer practices.

Excellent.

[dicky96]

I've googled this and yes it had been discussed but I still don't understand why this is a big no no..............


So what I don't get is why it is more dangerous and a huge NO NO to disconnect the ground on a modern plastic scope in 2017 to do what was normal practice with a metal chassis scope back in the day???

Surely if you understand and respect what you are doing this is as safe (or even more so due to almost no exposed metal on a Rigol) for you, your test equipment and the DUT as it was in the 80s??

By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves.  Doesn't that say something about the perceived 'danger' of floating scopes?

On googling, some folks on this forum say there is no reason to ever ever float a scope - I would ask those posters if they ever worked on SMPS primary side controllers?

Rich

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.

In short......DON'T FLOAT SCOPES.

As I said, I do have a 2KVA 220V isolation transformer but until I decide if it is worth fixing a lot of stuff here then it is in the UK with my same rated variac because it's bloody heavy, and yes I really miss my isolation transformer when I have a fault like this that is proving difficult to diagnose (no obvious shorts, faulty caps, diode junctions etc)

I accept that is a very sensible thing you say TauTech about this being a public forum of varying skills or the lack thereof.   

It is true - I was trained in a formal environment full time for 12 months to repair TVs (got three distinctions in course work, theory and practical) and an average 99% score over six exams so I guess I was good at it).  I then did two more years part time training at 'night school'  and worked in TV and then industrial electronics repair for many years.

Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO).

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.  This made it almost impossible to come into contact with a grounded object while simultaneously accidentally coming into contact with a floating hot ground chassis.

In fact, an isolation transformer makes things safer, but it does not make things safe when working on high voltage mains powered circuits.  I firmly believe that having the understanding of how to work as safely as possible on floating circuits using a floating scope in fact makes it safer to use an isolation transformer because you are not just relying on the isolation to keep you safe - you actually understand what the isolation transformer is doing, what hazardous situations it protects you from and what hazards it does not protect you from. 

Therefore dismissing old practices as being irrelevant, irresponsible, or no longer acceptable (actually do you think it is strange that I actually valued my life as much in the 70s and 80s as I do now??)  seems unreasonable.

Would you not agree having a good knowledge and practical experience of working on floating mains powered equipment actually makes it even safer to then work with an isolation transformer, rather than not giving folks the 'old knowledge' because it is 'unacceptable and irresponsible'  and just let them blindly rely on the isolation transformer instead? 

Surely in this case more knowledge keeps you more safe?


The only real reason I posted this thread in the first place is because when you google this topic all you get is 'it is too dangerous to do'.

I wondered then, if there were any additional considerations in floating modern 'digital' scopes like the Rigol compared to the old Techtronix and similar we were using? 

Actually the only extra risk I can think of is that the chassis ground of the DUT is grounded while your primary side SMPS circuit and floating scope are not - so you would need to be extra careful not to come into contact with both as you now have a grounded item on your bench as well as floating circuitry, which we did not have in the 70s/80s. 



OK let me rephrase the question.

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.

Let's all acknowledge the fact that your DMM is always going to be floating when working on SMPS primary and similar circuits yet no one seems to think that is a such a dangerous situation, but it is when you float the scope to take the same measurements/waveforms.




Funny enough by the way, as someone mentioned this, my first scope I owned as a hobbyist was an early 60's or possibly late 50's 'portable' scope that was mostly plastic construction had about half a dozen valves in it and had a captive 2 wire power cord and no ground connection. So you just connected the probe clip going to the chassis of whatever you were working on.  I bought this from the local 6th form college physics lab when they sold off some of their old equipment to students for next to nothing in 1975.   So A level science students had been using those.  I had it for years by the way, and it did fine until I got something better in the mid 80s when my employer was writing off old test equipment and selling to staff for some nominal fee like £1 per item.
 

Rich

[Electro Detective]

Why the cheapass scope manufacturers did not MASS produce ALL scopes with isolated inputs, deserves an epic scale public tar and feathering of the lot of them. 

maaaaaybe because it's hard and expensive to have both high speed, lots of memory an isolation?

It's 2017, not 1987.. technology has gone up, prices are dirt cheap,
sweat shoppe labor even cheaper

 

[alm]

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. [...]

So did the guy in this story (written by a retired Tektronix sales engineer). Did not save his life either.

In fact, an isolation transformer makes things safer, but it does not make things safe when working on high voltage mains powered circuits.  I firmly believe that having the understanding of how to work as safely as possible on floating circuits using a floating scope in fact makes it safer to use an isolation transformer because you are not just relying on the isolation to keep you safe - you actually understand what the isolation transformer is doing, what hazardous situations it protects you from and what hazards it does not protect you from.

No argument that engaging your brain and understanding what you are doing improves safety. Yes, clipping a grounded scope to a neutral/phase wire with an isolation transformer will mostly defeat any protection. The optimal strategy is to use an isolation on the DUT to protect you from touching something, and an isolated scope or differential probe (or poor man's CH1-CH2 differential probe) to prevent the scope from causing a short.

I wondered then, if there were any additional considerations in floating modern 'digital' scopes like the Rigol compared to the old Techtronix and similar we were using?

Not really. Obvious issues are still the connectors (including connectors on the back). Obviously a scope hooked up to a computer or printer (not very common in the eighties) introduces extra risk. As do any logic channels that only have skimpy isolation rated for 40 V or so. But I would still not rely on that plastic front panel to provide sufficient isolation. Never mind that your signal integrity may suck because those caps to ground in the SMPS for shunting noise are now useless.

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.

That DMM has no exposed metal connected to the DUT at all, and has been tested so even in the case of a transient, it will not expose the user to danger due to a much more solid case with any external connections (apart from the insulated banana jacks) optically isolated through IR LEDs. That scope has not been designed to the same standards, never mind all of the exposed metal. In addition to differential probes, there are scopes with fully isolated inputs. Brands like Owon, GW-Instek, Siglent and Micsig make portable scopes with isolated channels (no personal experience, but some have gotten decent reviews). These should provide a similar safety to a DMM of the same quality.

[tautech]

I've googled this and yes it had been discussed but I still don't understand why this is a big no no..............


So what I don't get is why it is more dangerous and a huge NO NO to disconnect the ground on a modern plastic scope in 2017 to do what was normal practice with a metal chassis scope back in the day???

Surely if you understand and respect what you are doing this is as safe (or even more so due to almost no exposed metal on a Rigol) for you, your test equipment and the DUT as it was in the 80s??

By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves.  Doesn't that say something about the perceived 'danger' of floating scopes?

On googling, some folks on this forum say there is no reason to ever ever float a scope - I would ask those posters if they ever worked on SMPS primary side controllers?

Rich

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.

In short......DON'T FLOAT SCOPES.

As I said, I do have a 2KVA 220V isolation transformer but until I decide if it is worth fixing a lot of stuff here then it is in the UK with my same rated variac because it's bloody heavy, and yes I really miss my isolation transformer when I have a fault like this that is proving difficult to diagnose (no obvious shorts, faulty caps, diode junctions etc)

I accept that is a very sensible thing you say TauTech about this being a public forum of varying skills or the lack thereof.   

It is true - I was trained in a formal environment full time for 12 months to repair TVs (got three distinctions in course work, theory and practical) and an average 99% score over six exams so I guess I was good at it).  I then did two more years part time training at 'night school'  and worked in TV and then industrial electronics repair for many years.

Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO).

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.  This made it almost impossible to come into contact with a grounded object while simultaneously accidentally coming into contact with a floating hot ground chassis.

In fact, an isolation transformer makes things safer, but it does not make things safe when working on high voltage mains powered circuits.  I firmly believe that having the understanding of how to work as safely as possible on floating circuits using a floating scope in fact makes it safer to use an isolation transformer because you are not just relying on the isolation to keep you safe - you actually understand what the isolation transformer is doing, what hazardous situations it protects you from and what hazards it does not protect you from. 

Therefore dismissing old practices as being irrelevant, irresponsible, or no longer acceptable (actually do you think it is strange that I actually valued my life as much in the 70s and 80s as I do now??)  seems unreasonable.

Would you not agree having a good knowledge and practical experience of working on floating mains powered equipment actually makes it even safer to then work with an isolation transformer, rather than not giving folks the 'old knowledge' because it is 'unacceptable and irresponsible'  and just let them blindly rely on the isolation transformer instead? 

Surely in this case more knowledge keeps you more safe?


The only real reason I posted this thread in the first place is because when you google this topic all you get is 'it is too dangerous to do'.

I wondered then, if there were any additional considerations in floating modern 'digital' scopes like the Rigol compared to the old Techtronix and similar we were using? 

Actually the only extra risk I can think of is that the chassis ground of the DUT is grounded while your primary side SMPS circuit and floating scope are not - so you would need to be extra careful not to come into contact with both as you now have a grounded item on your bench as well as floating circuitry, which we did not have in the 70s/80s. 



OK let me rephrase the question.

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.

Let's all acknowledge the fact that your DMM is always going to be floating when working on SMPS primary and similar circuits yet no one seems to think that is a such a dangerous situation, but it is when you float the scope to take the same measurements/waveforms.

But they can be contacted with............and that's the safety issue.
Elevated voltages INSIDE the DUT are one risk, OUTSIDE the DUT are another. 

I get that you are somewhat experienced but not all are and I am truly perplexed with your ongoing discussion trying to normalise this unsafe practice. 

As for using modern DSO's in a floated environment, consider this.....years ago when there was little in the way of affordable differential probes, scopes were designed that they might be floated and appropriate interchannel and PSU isolation clearances provided...........today ? 

If you value your safety and your instrument, get a differential probe. Period.


BTW I have floated scopes....... when there weren't the tools we have today and in cases where isolation transformers could not have been used for the DUT. (High power mains phase control)

Again get a differential probe. DP-25 is one of the best/cheapest options.

[Fungus]

Is it any more dangerous to float my Rigol 1052 and take sensible precautions ...
Than it is to use my Fluke 79 with the negative terminal clipped to hot ground

Yes!

With the fluke you normally make momentary contacts and you're looking at what you're doing. Even if you clip the leads onto the DUT, there's only one COM connection on the Fluke and you're very unlikely to touch the metal of the other input jacks with your fingers.

Not so the 'scope! When one BNC of the 'scope is connected to something hot, all the BNCs are connected to something hot. The BNCs are exposed metal which is easy to touch if (for example) you decide you need another probe.

Plus you might have multiple probes connected to the device at any time and that would create hot ground clips lying around the bench every time you connect to the DUT.

[Vtile]

Sounds dodgy to float the scope by removing the "ground pin", I personally wouldn't trust the scopes internal design to keep it "floating" as the neutral is in most electrical distributing systems still ground connected somewhere in the building or in the power grid.

Floating the scope with safety isolation transformer (the english term might be slightly different, the point is that your typical galvanically isolated transformer is not build this kind of use in mind [even if that reads in the front plate of chinese cheap no-name unit], it is just not electrically and mechanically strong enough.) is another disqussion. Not recommended these days anywhere, technically possible though.

I read one rather amusing story of one case where old tek were powered with internal combustion engine generator, lift up to insulation stand several meters above ground (obviously with the generator) and were connected to 20kV power distribution line to measure some part of an prototype thyristor reactor. The scope were then used with long insulation pole.  Obviously something that hobbyist (or even majority of professionals) should not try out.

Personally I don't like the mystification part involved the HV work, in the long run it leaves too much room for false information. One military explosive specialist once told that explosives are safe to handle as long as you respect them, but when you get arrogant you soon will be dead. This also apply to high voltages.

[wasyoungonce]

It's 2017, not 1987.. technology has gone up, prices are dirt cheap, sweat shoppe labor even cheaper

We used ground busters in late 70's and early 80's in Airforce.....yes got a few "kicks"!

[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.

[Vtile]

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.

There is also not much discussion of wallwart type PSUs or insulation properties of a random laboratory PSUs. Also potential high voltages (ie. In case of high current short at next room) on mains ground lead is hardly mentioned.. There is traps here and there.

[Electro Detective]

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.

There is also not much discussion of wallwart type PSUs or insulation properties of a random laboratory PSUs. Also potential high voltages (ie. In case of high current short at next room) on mains ground lead is hardly mentioned.. There is traps here and there.

..and the cheap or overpriced differential probe is still connected to the BNC socket, 

which is connected to the mains neutral earth/ground utility power, that has lots of energy available for unlimited free fireworks   

An isolation transformer is a must, it can only supply so much rated current/voltage and isolates you from the incoming line,

and always a better bet than only relying (PRAYING) that some overpriced sealed piece of plastic sweat shoppe crafted 'differential' probe
won't suddently become 'indifferent' and die, arc or bridge internally,
or after a drop or bump some loose internal silly tiny self tapping screw or solder blob gets the party going early         

Please   

[Fungus]

..and the cheap or overpriced differential probe is still connected to the BNC socket, 

which is connected to the mains neutral earth/ground utility power, that has lots of energy available for unlimited free fireworks   

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

PS: Your 'scope has a transformer in it, too. I guess all 'scopes are "floating" to you, right?

[Electro Detective]

..and the cheap or overpriced differential probe is still connected to the BNC socket, 

which is connected to the mains neutral earth/ground utility power, that has lots of energy available for unlimited free fireworks   

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

PS: Your 'scope has a transformer in it, too. I guess all 'scopes are "floating" to you, right?

I don't know what planet you're from but here in Australia ALL neutral buss bars are linked to the earth buss bar in the switchboard.

Some oscilloscopes do have switchmode power supplies last time I checked (and millions of other scope owners)

Anything else? 

Hey,     wanna talk about the latest Flook 87V ? 

[wasyoungonce]

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

I think he means Multiple Earth Neutral (MEN).  Used a lot these days.

[Electro Detective]

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

I think he means Multiple Earth Neutral (MEN).  Used a lot these days.

yes correct,    MEN relates to the TN-C-S system here

He knows what I meant, he also thinks he's a moderator here too 

Just question where the current Fluke 87V might be made/assembled and you get threatened with a ban 

after a few of them try gang up like schoolyard bullies    and cause an 'investigative' post to get locked   

[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. 

[mtdoc]

From the OP:

I've googled this and yes it had been discussed but I still don't understand why this is a big no no

And that is exactly the point. Any curious mind should want to know the why.

I've just looked back through this thread, and other than one trolling type post, what I see is everyone agreeing that it is never a good idea to float a scope. No one has endorsed it or suggested that it be endorsed or that it is currently endorsed by any institution. Many have provided good concrete explanations about why it is dangerous.  They have come at it from different angles but all reach the same conclusion. 

This is a good thing and how learning occurs

[tronde]

There are many things to be said about how to measure live circuits safely.

First of all: This is an internationl formum. Therefore we must consider the implications of that fact. From what I can see, it is one most important factor that has not been discussed in this thread, namely the mains power distribution system. It has only been briefely mentioned and from some comments I dare to say some should read up on it.

Within Europe you will find mostly some kind of the TN-system with protective earth (PE) and neutral connected together at least one place in the building. You will also find the IT-system with only local floating PE and no ground on the source transformer. You can also find the TT-system with grounded source transformer and local floating PE. You can find homes with some rooms defined as "dry" and no PE in the wall outlet. In the US you can find the split phase system with grounded neutral and as I understand some other systems as well. I know that other systems also exist, and is in use in different countries. It can also be that the distribution system used in an industial estate or office can differ from what is normal in the country.

This meaning of this is that what you will find in your lab or home is not necessarily the same as what the other person will find in his lab or home.

If we fail to understand this, we can end up by giving some very wrong and dangerous advices.

It is said that you should use an isolated differnetial probe to be safe. Soemtimes this is wrong. It is never wrong to use the probe, but the claim of being safe is sometimes wrong.

If you are connected to a TN-system AND have polarised plugs and wall sockets you will know which wire is neutral and therefore close to the ground potential. If you test a product with a live chassis, the chassis will most likely be at the same potential as PE and you can touch that and your scope safely.

Edit: Be aware that the wall socket known as "French" or "Type E" or CCE 7/5 is polarised by design, but the wiring of it is implemented differently in the countries that use it. This means that you need to test which wire is "live" and which is "neutral". Even if you live in a country that uses polarised sockets that is expected to be wired in a certan way, you should test the polarity to be sure.

BUT if you don't have polarised plugs and sockets you will not know which wire is live or neutral, neither on your scope or product. If you manage to connect the chassis so it is on the live wire, you are in deep shit if you touch that and your scope even if you have an isolated differential probe. The probe will save your scope, yes, but not you. We also have the possibility of touching other parts of the circuit than the chassis. If we touch something that is "live" and the  scope, no differential probe will save you.

We must remember that the normal Schuko plug is used in many countries regardless of the distribution system and it is not polarised.


Next thing is about isolating transformer for the device you test. In previous discussions on this forum some has said that an isolating transforer is a thing of the past and that is is better to rely on a Residual Current Breaker (RCB) that will trip if you get a shock.

First of all: a RCB is not intended to be a safety device to protect you from doing silly things. Its main function is to be a last resort if everyting else fails.

Then again we must understand that what you have can be something completely different from what is available to others. I understand that RCBs are differnet around the world, but in Europe we will most likely find a 30mA RCB known as "type A". We do also have a less known "type B".  There is one most important difference between "type A" and "type B". Type A will not trip reliably if there is a steady DC component in the earth current. Since we quite often test equipent with rectifiers we will most likely not be protected by the RCB used in most of Europe as of today. If you think a "type B" is something you should have, you must consult the data sheet for that particular type first. If connected in the wrong place, it can block the function of other RCBs in the installation.


I know that the reason for the suggestion of avoiding an isolation transformer is that you can still get a shock if you manage to connect youself in the return loop of the probe. This is true, but as we have seen a RCB can give a very false impression of safety too.

The least unsafe will probably be to use a proper isolating transformer (one with a grounded static shield between primary and secondary) for the device you test and an isolating differential probe. If you manage to touch both "live" and "neutral", no transformer or RCB will save you.




It seems like most people with a background in electronics are scared about mains voltage, and that it is somethiong really dangerous. Yes, it can be dangerous. Yes it can kill, but we are still talking about 240 volts and not kilovolts. Reinforced clearance distance is usually 4mm and creepage is 5mm and normal insulation is half of those values. It is not so that the voltage will jump out of the wall and bite your nose. What puzzles me is that people are afraid of 240V but consider 12V as safe. That is really ignorant. A charged car battery can cause a lot of danger too. Evaporating wires and metal splattering around for instance.


What all this boils down to is that we really must switch on our brain before we start to work on any circuit. If the available energy is large enough a low voltage circuit can be really damaging to both you and your test equipment. If you don't fully understand what can go wrong, you should stay away from testing on that product regardless of the voltage.


You will lnever be "safe" when you work on any kind of exposed enrgized circuit. You will only have different grades of "unsafe".


Edit: added info regarding the "French" polarised socket.

[Fungus]

I vote to leave that^ as the final post in this thread. It should now be locked.

[nctnico]

There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.

The danger in that is that you are going to explain how to do something which is dangerous 'safely' and very likely important bits are left out or interpreted wrong. Besides that taking measurements using floating mains takes acquiring discipline through training & mentoring which cannot be replaced by a few posts on a forum.

[David Hess]

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

If you can put up with the high common mode capacitance, floating the oscilloscope provides higher common mode rejection and lower noise than using a high voltage differential probe but see below for better options.

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

c) Float the device under test although this has many of the same problems as floating the oscilloscope.  I have occasionally floated the DUT and used a differential amplifier to get higher common mode rejection ratio and lower noise than a high voltage differential amplifier will provide.
d) Use an oscilloscope which has isolated inputs.
e) Use a probe isolation amplifier.

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.

The 7A13 and 7A22 only have excellent common mode rejection when special adjustable attenuating probes are used and calibrated or the input attenuators are switched out and x1 probes are used.  (1) Modern high voltage differential probes never have excellent common mode rejection but they are good enough for many applications including usually off-line power supply design.

High voltage attenuating probes including the 7A13 and 7A22 when attenuation is used have another problem; they have high noise in direct proportion to their input attenuation.  An oscilloscope with floating inputs or the A6902 probe isolation amplifier solves this problem.

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.

Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of "low voltage".

(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

[alm]

Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of "low voltage".

Probably limited by what the relevant standards considered safe to touch at that time. For example, all Tektronix products would have passed UL testing.

(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

A scope with isolated inputs will work if you can tolerate the low impedance from the reference lead to chassis. Tektronix specifies the capacitance to chassis for the THS720 as 55 pF typical. That is a lot higher than a differential amplifier with attenuating probes, or a differential probe.

[David Hess]

Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of "low voltage".

Probably limited by what the relevant standards considered safe to touch at that time. For example, all Tektronix products would have passed UL testing.

Right about the time multimeters were practically required to use shielded banana jacks, the oscilloscope probe manufacturers were required to downgrade the voltage rating of their miniature and subminiature probe tips because of clearance and creepage requirements.

(1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low.  The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs.

A scope with isolated inputs will work if you can tolerate the low impedance from the reference lead to chassis. Tektronix specifies the capacitance to chassis for the THS720 as 55 pF typical. That is a lot higher than a differential amplifier with attenuating probes, or a differential probe.

That is definitely a problem but the same applications which will have problems dealing with high common mode capacitance are usually unbalanced enough to degrade the common mode rejection of a differential probe.

[mtdoc]

There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.

The danger in that is that you are going to explain how to do something which is dangerous 'safely' and very likely important bits are left out or interpreted wrong. Besides that taking measurements using floating mains takes acquiring discipline through training & mentoring which cannot be replaced by a few posts on a forum.

But that is not what is occurring. The consensus in this thread and all the others I've seen on this forum regarding floating scopes is "don't do it" and by explaining why it is unsafe you are providing those who are skeptical of "proclamations from authority" the understanding needed to prevent them from doing it.  In addition, the ancillary discussions that arise from this topic are educational in their own right.

In the end, there is no cure for stupid. Those who are going to ignore all advice regardless - will do it - no matter what is discussed or not discussed on this forum.  But I've yet to see anything on this forum that would make that more likely - quite the contrary - I've seen lots of good discussion that should convince anyone why it is unsafe.   The reality is that there are also many people who need to understand why a practice is unsafe in order to fully internalize the advice.  Paternalistic and overly rote recitation of safety rules without context may work in some environments but IMO not on an electronics discussion forum where many people come to learn. Hopefully the excellent discussion that has ensued in this thread has caused the OP to now fully justify and internalize the advice not to float a scope.

[colorado.rob]

The world can always use more Darwin Award winners to serve as examples for others on what not to do. 

[serggio]

Wow, wow... 
How many words in this thread...
TN-C, TN-S, TN-C-S, IT, TT and other mains distribution schemes...
TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

All that need to know, that in 95% scopes (except of isolated channel scopes and portable scopes) ground conductor at your passive probe always connected to all open metal parts of your scope (chassis, USB shield, neighbor's BNCs and etc.)
End of tip on your passive probe have open metal part - ground connector, at least.

Always need remember about it when you try to connect your probe to high voltage source.

[PartialDischarge]

I'll add more fuel to the fire. I can't recommend anyone doing this, but I do it cause I know exactly what I'm doing so please don't waste your time critizing me, ok fellas?.
Just floated my Micsig tablet scope to 620V to measure the Vgs of a switching mosfet. Of course I did set the scope vert and horiz before energizing the circuit and I didn't touch the screen afterwards. Also, the scope was placed in a wooden surface and the probe cable didn't go near metallic areas.

[tautech]

I'll add more fuel to the fire. I can't recommend anyone doing this, but I do it cause I know exactly what I'm doing so please don't waste your time critizing me, ok fellas?.

Right. 

Just floated my Micsig tablet scope to 620V to measure the Vgs of a switching mosfet. Of course I did set the scope vert and horiz before energizing the circuit and I didn't touch the screen afterwards. Also, the scope was placed in a wooden surface and the probe cable didn't go near metallic areas.

BUT......by virtue of your tablet scope is not mains ground referenced you haven't floated it at all. 

[Vtile]

First of all I do want to say that high voltages, high current or high energy work with live or powered, but disconnected device under test is something that doesn't belong to hobbyist of young age or hobbyist of high age, but low experience (including lack of understanding of theory) nor with educated person without knowledge of what he or she is doing.

Someone did claim my wording about differential probes are not reasonable. It were, as the response did give more specific information of the type of "usually safe" differential probes. The CAT IV text in the faceplate of the instrument doesn't mean that it really fullfills the requirement (ie. with all the counterfeit product floating in the markets).

Now I have one extra question that also haven't discussed and also is a trap for anyone who doesn't think. I (and I assume many hobbyist that shouldn't do high energy work ) do have one of those handy USB scopes, well it is isolated and floating (with random electrical and mechanical values) when I attach it to my battery powered laptop running charger disconnected and no other equipment connected. Again the same hazard is there the metal parts of the USB scope and propably also on the laptop (through USB shielding wire) just became potentially life threatening to touch.

There is so many traps in this area of electronics and electrical work in general.

PS. Also your ESD cover/mat on the workbench is grounded through random impedance.
PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there. 

[Specmaster]

The way I look at this is that it does not matter if your scope is battery powered in other words a true portable, floated, isolated or double insulated and the same thing applies the DUT unless battery powered, there are many ways that you can still receive a fatal electric shock. Fact is that all things electrical demand respect and due care and diligence and in our game, especially if you're in the repair game, then live working is unavoidable so common sense must prevail.

In the real world, we would all love to have the very best and finest of equipment, especially if cost was not a factor but sadly it so often is so we each and everyone of us has to weigh up the risk with everything we do in life and the question of safety in the work area / bench is no different. If you don't feel confident working on live equipment, or if uncertain about the suitability of a piece of test equipment etc, then don't do it.

[JohnG]

Just floated my Micsig tablet scope to 620V to measure the Vgs of a switching mosfet. Of course I did set the scope vert and horiz before energizing the circuit and I didn't touch the screen afterwards. Also, the scope was placed in a wooden surface and the probe cable didn't go near metallic areas.

I have to ask: Were you measuring Vgs of a high side MOSFET in a half-bridge, where the common mode voltage is a giant square wave or similar? Did you get a decent waveform? Is it believable? If so, could you send me a picture of it? 100 MHz is likely too slow for me, but I have only been able to do what I want with a borrowed Tek IsoVu (which works very well for what it does).

John

[WastelandTek]

so let me get this straight

say I have a 5A21N here, I can hook a conventional probe to each input and then probe 2 points in a circuit that are BOTH above (or below) ground potential safely right?

I have the diff amp, so I don't need the diff probe, or am I missing something?

[alm]

Yes, assuming that you observe the limits of both the probes and the plugin. And be careful where, and if, you connect the ground leads.

Keep in mind that attenuating probes, e.g. 1:10 or 1:100 probes, may degrade your common mode rejection ratio (CMRR). A high CMRR relies on a close matching of the attenuation of both channels. If your probes have a 1% mismatch in attenuation ratio, your CMRR will be limited to 100:1. Tektronix made some probes with adjustable DC attenuation for that (like the P6055 and P6009), so you could adjust one probe to match the other. Obviously 1x probes will have a very good match in attenuation .

[HighVoltage]

For the young players:

When I was 16 years old, I had to learn a rule when measuring any dangerous voltage:
"Keep one hand in the pocket"

Do I follow this rule these days?
No!

But it made me aware of what I am doing every time I measure dangerous voltage, especially high voltage to 80.000 Volt.

[tronde]

TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

Yes, you can find som older buildings wired that way in Europe. But, you should not find any legally installed wall socket with a disconnected earth contact. If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket. This is not the same as floating the scope, because PE will always be connected to neutral at least one place in the building if you have any kind of a TN-system.

As I mentioned in my previous post, you can find some places using wall sockets without any earth contact. They have been used regardless of the distribution system, and you will of course not have any PE connected to your scope so it will be floating in that case.

[PartialDischarge]

I have to ask: Were you measuring Vgs of a high side MOSFET in a half-bridge, where the common mode voltage is a giant square wave or similar? Did you get a decent waveform? Is it believable?

Exactly. My scenario is pretty specific, but I'd say it is accurate and shows the Vgs gate charge 'waveform', Also the peak Vgs which was what worried me, is within calculations and specs.

[serggio]

If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket.

This is completely stupid. This is prohibited in Russian standards. If during any works at mains, phase's and PEN conductor will changed each other before branching to your socket, that is means that you'll have phase at your ground contact in socket.
PEN should be divided to PE and N at building central switchboard or floor central switchboard and PE should not be interrupted after that division.
Never make connecting ground contact to "neutral" contact directly in socket!

I see, that you very experienced at electrical safety and have knowledge about (I)T(N)-(C)-S and etc. protective grounding schematic, but still telling about evident's safety errors. 

[exe]

PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there. 

I got a shock every time I simultaneously touch the radiator and my laptop (that has metal enclosure). Why is it so?

[tronde]

If you wire with two wires, the earth contact in the socket should be connected to the neutral wire/contact (the PEN wire you mention) within the socket.

This is completely stupid. This is prohibited in Russian standards. If during any works at mains, phase's and PEN conductor will changed each other before branching to your socket, that is means that you'll have phase at your ground contact in socket.
PEN should be divided to PE and N at building central switchboard or floor central switchboard and PE should not be interrupted after that division.
Never make connecting ground contact to "neutral" contact directly in socket!





I see, that you very experienced at electrical safety and have knowledge about (I)T(N)-(C)-S and etc. protective grounding schematic, but still telling about evident's safety errors. 

You must read the post I answered to.

TN-C, for example, obsolete for now, but still present at many many countries, predominantly at old buildings, do not propose any third PE (ground) conductors in mains outlet.
All that you have - this is Phase conductor and Neutral (PEN) conductor in your mains. In this case your modern scope will be always float!

What I say, and this IS true, is that if you find TN-C in a wall socket, it will be wired with PEN connected to both the earth connector AND the neutral conector.

"serggio" gave the impression that TN-C in a wall socket would give you a floating scope. This is wrong, because the earth connector of the socket will be connected to the neutral (which will be the PEN-wire in that particular instance).

It is correct that this is bad practice (and I guess illegal in most countries by today), and that is also stated by "serggio" when he called the practice "obsoleted".

We must understand that you will find a lot of different wiring around the world, and it was in fact legal to wire TN-C in wall outlets in some European countries long time ago.

This is why I said in my first post that we must know what is present in our home or lab. You can not rely on what other people will have.

[IanMacdonald]

"Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO)."

I would disagree. In those days a lot of equipment was live chassis, and bench engineers took their own safety very seriously.  More so than today, when people have gotten complacent thanks to much safer gear. In those days it was projectors with 35kV on the anodes, or Marshall guitar amps with a 1kV supply that could deliver 250mA. Either could kill you stone dead if you weren't careful enough.

The insistence of manufacturers on earthing 'scope inputs arises, I suspect, from a misunderstanding of the purpose of a protective  earth conductor, which is to form a Faraday cage around and enclosing live connections. However, if the live connections are not inside the Faraday cage, then no protection is given. In that case, an earthed object which is liable to be touched by the operator is actually a safety hazard in its own right, and the interests of safety would be best served by eliminating that object from the testbench. 

Building site practices have clearly identified that hand tools with insulating cases and NO earth have a much better safety record than earthed metal tools. If the case has to be metal then earthing is the lesser of two evils compared to a floating case, but the least hazard is achieved by eliminating the touchable metal. 

I mean, we have DMMs with fully isolated data ports. That is to prevent hazardous situations arising though the test prods being earthed via the data cable.

I daresay that isolated scope inputs might be a little harder to design since they handle much higher frequencies than meter prods, but it would be a really big safety advantage if they were isolated from earth, and from each other.  Although, with modern opto-isolators and PSU modules I really don't think it would be that hard to provide post-preamp earth isolation.

-How many lives does it have to save, for the design effort to be worth it?

"Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of 'low voltage'. "

I recall that device, and thinking what an incredibly dangerous gadget it was. It gave the impression that the 'scope was isolated, but had the operator held a probe outer and touched a live terminal, once 50V appeared on the scope it would then have proceeded to complete the circuit and electrocute him. 

Might add that the practice of earthing 'scope inputs dates from the valve era when 'scopes themselves contained some truly eye-watering voltages. Even the preamps would have run from maybe 150v, so not earthing the input on that kind of beast would have invited having an internal fault put a dangerous voltage onto the probe. That is long-gone history though. The modern LCD scope is basically no different from a bench DMM as regards operator safety.  It would be better if the same safety principles were applied.

[serggio]

PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there. 

I got a shock every time I simultaneously touch the radiator and my laptop (that has metal enclosure). Why is it so?

Equilibrium potential. It's correct to say - no equilibrium potential. Your radiator probably have different potential from you laptop case. Your laptop have no ground connector at AC/DC power converter, but your laptop power converter can have capacitive coupling to AC (to phase), same time your radiator can be grounded thru the building construction. 
Same time your body can have resistance less that 100 kOhms (it's completely depended from many factors: external humidity, your own skin conductance and etc, etc..), so you feeling electrical shock.

That is, second thumb rule, why you should never have ground contact to neutral contact connection in wall socket at wet room (bathroom e.g.) if you have TN-C protective scheme.

[Vtile]

PPS. The water circulating radiator under your workbench where you rest your feets seeking a warmth is also propably a directly grounded.. What a nice electrocution chair there. 

I got a shock every time I simultaneously touch the radiator and my laptop (that has metal enclosure). Why is it so?

I assume your laptop is powered by a charger. There is a few possible explanations, but I think I can assume (but it should be threated as high energy fault until it is solved ) that the shock you have had so far is rather low energy discharge as you are still wondering it. It is possible to get only a mild shock from full mains voltage, that is why it should be threated as a high energy fault.

The real couse is of course almost impossible to quess without derailing this topic with more questions and answer. If you have reasonably trustworth DMM with atleast CAT II and similar level probes you can measure the voltage between the two objects. The probe placement is essential and needs to be on proper high impedance slots, typically the COM and V. In these kind of measurements DMM should start from the highest AC voltage range. You also should have a good workin position and not to touch both of the objects (except with the probe tips) while doing the measurent.

What you have come across is indicating that something is wrong in your laptop or in your homes / workplace radiator. The problem is something that should be inspected by an qualified electrician by visiting on your place and measuring (the cheaper route might be to get the laptop inspected first at local electronics repair shop and if it is OK, then the radiator). For a starters you could start a new thread to ie. repair, general chat, or a beginners section.

[tronde]

Your laptop have no ground connector at AC/DC power converter, but your laptop power converter can have capacitive coupling to AC (to phase)

It is quite normal to find a 1nF class Y1 capacitior connected from each of the mains supply phases to 0V output of the charger. This is becuase of noise reduction. You are normally allowed to have a ground leakage current of less that 0.25mA in Class-2 (double insulated [double rectangles] ) equipment. As you say, this can give a feeling of shock.

[Red Squirrel]

TBH I sometimes float my scope, if testing something that I am unsure of that is ground referenced or any time I work with mains stuff.  I actually feel safer floating it as I don't have to worry so much about where I'm placing the ground clip.  I can put it anywhere I want, and sometimes you want that anyway as you want to test between two specific points, and they may not be ground. 

Of course the proper way would be to put the device you're testing on an isolation transformer, but that's not always an option if it's a large stationary object like a ground referenced battery bank or other hard wired stationary equipment.   

Idealy, what would be a good solution is a battery operated scope.  Suppose one could run a scope off an inverter with battery pack for doing isolated tests.

[tautech]

Idealy, what would be a good solution is a battery operated scope.  Suppose one could run a scope off an inverter with battery pack for doing isolated tests.

The cheapest SAFE solution is differential measurement using 2 channels and maths or a differential probe. Period.

[tronde]

Something battery powered with bluetooth could be useful. Does USB to bluetooth adapters exist?

[serggio]

Idealy, what would be a good solution is a battery operated scope.  Suppose one could run a scope off an inverter with battery pack for doing isolated tests.

It would not be good and safe solution till user will not understand dangerous from touching open circuit parts of high voltage scheme/open test and measurement equipment parts connected to this this scheme or ever unprotected parts of leads this equipment.
Battery operated scope this is not panacea from potential danger of high voltage injury, because at this scope can be connected probes with open ground contact at the end.
Battery operated scope only suitable for floating measurement with no damage scope itself (no short circuit thru the scope).
But this is not safe at all.

Something battery powered with bluetooth could be useful. Does USB to bluetooth adapters exist?

So many solutions.... What you need exactly?

[tronde]

Something battery powered with bluetooth could be useful. Does USB to bluetooth adapters exist?

So many solutions.... What you need exactly?

Something that could transform a USB scope into wireless. A battery powered Bluetooth or WiFi scope could be really useful for mains work.

[timb]

"Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of 'low voltage'. "

I recall that device, and thinking what an incredibly dangerous gadget it was. It gave the impression that the 'scope was isolated, but had the operator held a probe outer and touched a live terminal, once 50V appeared on the scope it would then have proceeded to complete the circuit and electrocute him. 

It also monitored current across the ground connection and, if it exceeded a set amount (0.5, 3.5 or 5ma), would completely sever the connection by opening the L-N-G relay. (Not unlike a GFCI.)

They were actually pretty cool little devices.

[tautech]

The first few minutes of this recently posted vid (Pt 2) properly examines the use of an isolated channel HH scope and when there is need to use some form of channel isolation or using 2 channels and differential measurement won't work.

https://youtu.be/rNElNKeDNyg

From this thread:
https://www.eevblog.com/forum/testgear/review-siglent-shs-1602-isolated-scopemeter/

[tronde]

"Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts.  I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time.  It is suspiciously close to the common definition of the maximum of 'low voltage'. "

I recall that device, and thinking what an incredibly dangerous gadget it was. It gave the impression that the 'scope was isolated, but had the operator held a probe outer and touched a live terminal, once 50V appeared on the scope it would then have proceeded to complete the circuit and electrocute him. 

It also monitored current across the ground connection and, if it exceeded a set amount (0.5, 3.5 or 5ma), would completely sever the connection by opening the L-N-G relay. (Not unlike a GFCI.)

They were actually pretty cool little devices.

User manual and schematics here
http://w140.com/tekwiki/wiki/A6901

[tautech]

From another thread not far back that discussed safe measurement techniques:
https://www.eevblog.com/forum/beginners/dumb-oscope-question/

...Does that mean if I take a tube amp and plug it in without the ground plug, the amp is isolated from the mains (ie the hot and the neutral are isolated via the power transform and the ground is not hooked up)?

...

Seems like that could be dangerous too since the amp is then not grounded. But for a few tests maybe ok?... assuming I test to make sure the grounded components aren't carrying any potential with respect to ground. Thoughts?

In general, removing/disconnecting a ground pin is a risky plan.  There are some situations where it can help a lot with testing, but it's not something I'd do lightly. When you have gained some experience in electrical systems design you can make your own evaluation.

I suggest the following thought experiment:

• In the beginning, the chassis was grounded
• Maybe some internal circuitry - on the isolated side of the transformer - was grounded. Maybe the DC 0V (aka Ground).
• You disconnect (or 'lift') the chassis ground from the power plug
• As this point in time you're probably OK
• You want to measure something, so you connect your 'scope ground to DC 0V and measure.
• No problems yet
• You want to measure something else - maybe the voltage across the output transformer primary.
• You connect your scope probe to DC+ (could be well over 100V depending on amplifier).
• The '0V' rail is now forced down (because DC+ is connected to mains earth via your scope), so amplifier chassis is now live to negative(DC+) volts.
• You rest your hand on the chassis while poking around with the 'scope probe and get a shock.

When we consider the above scenario, we see that it takes 3 steps to shock yourself: steps 3, 8 and 10. Steps 3 and 8 were 'necessary' for your measurement plan.  Step 10 was a mistake. Maybe you're tired. Maybe someone distracted you. Maybe you dropped the probe and went to recover it.  What steps 3 and 8 did was put you one mistake away from danger.

And now I look like a safety nutter / the fun police... When you receive a significant electrical shock, several things can happen. You can suffer electrical burns at the point of contact (painful, not so bad).  You can fall off your chair (or ladder for extra fun times) and injure yourself. You can suffer an immediate cardiac event (e.g. ventricular fibrillation), collapse and possibly die.  Good thing you've got a buddy there keeping an eye on you, right?

You can even damage part of your heart and/or set up an irregular rhythm, feel OK, and then suffer a cardiac event when you go to sleep for the night, and your partner wakes up next to a dead body.  So if you do receive a significant shock (static discharge onto a door handle doesn't count) you should probably go see your doctor or local emergency department to get checked out.  It will likely take at least a couple of hours, or all bloody day depending on your triage level.  (Pro tip: tell them you had an electric shock as soon as you get in because a) it's important and b) it'll increase your triage priority level.) It may cost a lot of money (depending on public health care, insurance or lack thereof).

So that's why I wouldn't float the ground.

[Electro Detective]

I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder   

There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG!

'Isolation' devices are no guarantee of safety, accuracy or performance once hooked up with faulty or suspect wired DUTs and other test gear powered by mains electricity, nor do they have crystal ball capability

BANGS can cause injuries, blindness, or facilitate an inexperienced or mis-informed prodder's early funeral,
or more important, cost MONEY due to damage 


FWIW  I'm still a  'die hard'  isolation transformer fan     for many uses not just lab use,
but I have a good clue how the whole deal works,
check and and prep before I switch on, fail-safes galore,
and document working scenarios for future use, so no guessing games or 'impatience oopsies'   

[IanMacdonald]

Slightly oftopic as it's not a test instrument, but everything said here also applies to earthed soldering irons. (I was going to say in spades, but actually it applies to all shapes of tip :groan:)

If you use an iron with a 240v element then earthing is essential for safety, since an insulation failure could put mains on the tip. No question about that, and it's one of the reasons that direct-mains irons are a poor choice for any serious work.

Most decent bench irons run from 24v though,  and the question then arises, is earthing desirable or not?

Static discharge is necessary if working on MOSFETs, but that does not actually require a hard, zero-ohm earth.

The downside of earthing irons which don't actually require an earth for safety, is the risk of damage to equipment due to residual charge on electrolytics being dumped to earth through any sensitive component you are soldering on or near.

That, and if soldering on high current gear with an earthed iron you are in the same position, safetywise,  as using a multimeter which does not have proper HBC fuses. If a terminal is unexpectedly live, that is much the same as forgetting to take your unprotected DMM off the 10A range before measuring voltage.

[alm]

A hard connection to ground is obviously not optimal for ESD, since it would be causing a discharge with a large peak current. However, I do not agree that a hard connection to earth is obviously not required for safety or that it is similar to a DMM fuse. What is the insulation rating of the handle of a low voltage iron? The hand holding the solder and/or tweezers? Touching a node that unexpectedly has high voltage on it could easily shock the user if the iron is not grounded.

If the iron is grounded, then a GFI or fuse should trigger, protecting the user. So I would argue that a grounded iron is essential when working on circuits with non-SELV (separated extra-low voltage) voltages on them, unless you are fully isolated from the board/solder/tools, or you can insure that there are no dangerous voltages in your circuit. So basically quite similar to scopes . Only in circuits with only low voltages but a very high current capacity (e.g. battery banks) with no up converters could you argue that a tip with a larger resistance to ground is safer.

That a careless user may kill some semiconductors is not a safety issue.

[Safar]

I have one solution else.
It's "semi-stupid" but anyway it can work. I use it for low voltage (devireg bus). Direct connection earth probe connector to any pole completely kill the signal. And I had no differential probe at this time. So I simple connected one signal pin of one channel to one pole, and second channel to another pole. And not connected earth connectors of probes to DUT. And had set math channel on scope A-B. So I get 1 differential channel from 2 scope channels.

Not sure how it safe for high voltage, but I guess that it possible to avoid danger voltage on BNC connectors if use 1:10 divider at least and not to disconnect probes. And you can still use eathed scope.

Of course I understand that HV differential probe is better.

Sent via Tapatalk

[alm]

That is a perfectly safe solution (assuming suitable probes) that I am pretty sure was discussed in this topic. The main limitation is that the common mode rejection ratio is quite limited, i.e. you are likely to see a signal (for example mains frequency) superimposed on your trace. That is where a 'real' differential probe performs much better.

[Safar]

Yes, CMRR is a problem in this method. Especially that is need to accurately setup signal level on both channels to avoid  ADC overload. And useful signal can be very low after math op

[David Hess]

That is a perfectly safe solution (assuming suitable probes) that I am pretty sure was discussed in this topic. The main limitation is that the common mode rejection ratio is quite limited, i.e. you are likely to see a signal (for example mains frequency) superimposed on your trace. That is where a 'real' differential probe performs much better.

The big problem is usually the limited common mode input voltage range for a given sensitivity.  The common mode rejection ratio is less of a problem if you have an analog oscilloscope because:

1. The vertical variable controls can be used to match the DC gain on each input for maximum common mode rejection ratio.  (1) Compensation can be used to trim the AC common mode rejection ratio.
2. Subtraction in a DSO *adds* the quantization noise of each channel and reduces the number of significant bits.  Some very early DSOs did the add and invert in the analog domain before the digitizer and do not suffer from this problem as much.

(1) It is actually a little eerie to perform this adjustment.  You can *see* the noise "null out" when the variable gain control is adjusted correctly.

[tronde]

Slightly oftopic as it's not a test instrument, but everything said here also applies to earthed soldering irons. (I was going to say in spades, but actually it applies to all shapes of tip :groan:)

If you use an iron with a 240v element then earthing is essential for safety, since an insulation failure could put mains on the tip. No question about that, and it's one of the reasons that direct-mains irons are a poor choice for any serious work.

Most decent bench irons run from 24v though,  and the question then arises, is earthing desirable or not?

Static discharge is necessary if working on MOSFETs, but that does not actually require a hard, zero-ohm earth.

The downside of earthing irons which don't actually require an earth for safety, is the risk of damage to equipment due to residual charge on electrolytics being dumped to earth through any sensitive component you are soldering on or near.

That, and if soldering on high current gear with an earthed iron you are in the same position, safetywise,  as using a multimeter which does not have proper HBC fuses. If a terminal is unexpectedly live, that is much the same as forgetting to take your unprotected DMM off the 10A range before measuring voltage.

I don't consider this offtopic, because it can affect the safety on the workbench you also use for measurements.
As I wrote in reply #52 you need to be aware of the entire system. You can not just rely on a differential probe to be safe, so the grounding of the soldering iron is certainly worth looking at.

As you say, a mains supplied soldering iron will need a "hard earth" for safety, while a low voltage iron will need a "soft earth" for slow / controlled discharge of static electricity.

This is OK as long as you know what kind of earth you have. I have a Weller WD2 transformer. On that transformer you can change the resistance from the tip to earth by means of a 3.5mm plug / socket with a built in switch. No plug gives 0 ohm (hard earth). Plug only gives no  connection to earth, and a resistor installed in the plug gives you an earth resistance with that value. At first this seems to be rather smart. BUT - Weller goofed. The plug / socket is just a normal el-cheapo thing, and the plug can easily work loose. Then you will have a hard earth while you believe it is a soft earth. The plug they use is rather small, and I doubt you can find space for a resitor that will safely withstand 250V. I modified my transformer so I have a proper resistor installed inside the enclosure.

The grounding of the antistatic mat is also something to consider. With 230V mains it will normally be connected to earth by a 1Mohm resitor. If we place something with an earthed chassis on the mat it can bypass the safety resisor. A normal antistatic mat will have a rather high surface resistance, but the resistance to earth will decrease and a leakage current will increase.

[JDW]

A lot of good info in this thread, but one thing overlooked now in 2023 are newer scopes like the Rigol DHO800/900 series come floated due to having a USB-C type power plug and a power brick from Liteon that is only 2-prong at the wall socket.  Sure, they include a separate grounding wire (with banana connectors at both ends) "for safety," but the fact the scope comes this way is a recipe for unsafe practices, especially here in Japan where 3-prong wall sockets are utterly impossible to find in most homes.  Indeed, here in Japan, the only time you'll see a ground is near a wall socket in a room where there is a toilet (Japanese love their electronic bidet devices), or a refrigerator in the kitchen.  They are almost always screw terminals for connecting bare wires that hang off the appliances or bidets. Any other place, which is the most likely place you'd be using a scope, the wall sockets are 2-prong only.  But even if they were 3-prong, the design of the power adapter is such that the user isn't FORCED to use a grounded wall socket plug.  That Rigol grounding wire is "optional" in that it isn't built into the main power cord.  That is a big issue.

Tektronix offers battery powered scopes which are basically floating devices, but even Tektronix cautions you about them, properly saying not to test voltages above 30Vrms or 42Vpeak.  They also warn against the use of Isolation Transformers as being "Dangerous."

https://www.tek.com/en/documents/technical-brief/floating-oscilloscope-measurements-and-operator-protection

But even if you are using less than 30Vrms (which is basically all I test, personally), grounding the scope provides a way to avoid Common Mode Noise on your measured waveforms.

Lastly, the Rigol DHO800 documentation that came with my scope doesn't even use terms like "floating," and references to Earth Ground are few, which mean that people searching the documentation for important safety info might miss the topic altogether or think they are safe to use their new 12-bit scope in a floated condition.

[Andy Chee]

Lastly, the Rigol DHO800 documentation that came with my scope doesn't even use terms like "floating," and references to Earth Ground are few, which mean that people searching the documentation for important safety info might miss the topic altogether or think they are safe to use their new 12-bit scope in a floated condition.

I did find this warning in the manual though!

[JDW]

I did find this warning in the manual though!

I consider that single mention inadequate.  Furthermore, it makes no mention of other benefits, such as addressing common mode noise riding on measured waveforms. Dave's video about floating scopes even shows how the noise vanishes when the ground lead is connected (not on the DHO800/900, but another scope).

Not everyone reads the full documentation anyway. Therefore, putting a warning sticker on the AC power adapter itself and a second sticker on the ground wire that Rigol includes would be prudent.  In the absence of that, I suspect many, MANY people will use it without the grounding wire.  Which again is okay if you're testing under 30Vrms or under 42V peak, as Tektronix points out (which is all I test).  Even so, proper grounding can address noise issues, as I've said.

[Andy Chee]

I consider that single mention inadequate.

I'm sure someone with a bigger test instrument history than I could comment on this, but I might guess that every documentation from every manufacturer, printed in the last 25 years, has inadequate safety warnings!

Certainly older documentation could serve double purpose as a HOWTO instructional course in instrument usage and application.  Modern documentation however implies a high level of assumed knowledge.

[madires]

In case that power brick comes with an EMI supression cap between primary and secondary I wouldn't call the DSO floating.

[JDW]

In case that power brick comes with an EMI supression cap between primary and secondary I wouldn't call the DSO floating.

That kind of statement seems to suggest this: "Go ahead and remove the ground from your scopes and then test high voltage because as long as you have an EMI suppression cap between the primary and secondary, it's not floating and therefore A-OK."

By using the term "floating" we are primarily talking about "safety from electric shock." And secondarily, we are talking about common mode noise reduction on measured waveforms too.

[johansen]

My hakko iron...

I lifted the ground and installed a neon lamp and 40k resistor

So it lights up when im soldering on live circuits.

[GnomeZA]

A lot of good info in this thread, but one thing overlooked now in 2023 are newer scopes like the Rigol DHO800/900 series come floated due to having a USB-C type power plug and a power brick from Liteon that is only 2-prong at the wall socket.  Sure, they include a separate grounding wire (with banana connectors at both ends) "for safety," but the fact the scope comes this way is a recipe for unsafe practices, especially here in Japan where 3-prong wall sockets are utterly impossible to find in most homes.  Indeed, here in Japan, the only time you'll see a ground is near a wall socket in a room where there is a toilet (Japanese love their electronic bidet devices), or a refrigerator in the kitchen.  They are almost always screw terminals for connecting bare wires that hang off the appliances or bidets. Any other place, which is the most likely place you'd be using a scope, the wall sockets are 2-prong only.  But even if they were 3-prong, the design of the power adapter is such that the user isn't FORCED to use a grounded wall socket plug.  That Rigol grounding wire is "optional" in that it isn't built into the main power cord.  That is a big issue.

Tektronix offers battery powered scopes which are basically floating devices, but even Tektronix cautions you about them, properly saying not to test voltages above 30Vrms or 42Vpeak.  They also warn against the use of Isolation Transformers as being "Dangerous."

https://www.tek.com/en/documents/technical-brief/floating-oscilloscope-measurements-and-operator-protection

But even if you are using less than 30Vrms (which is basically all I test, personally), grounding the scope provides a way to avoid Common Mode Noise on your measured waveforms.

Lastly, the Rigol DHO800 documentation that came with my scope doesn't even use terms like "floating," and references to Earth Ground are few, which mean that people searching the documentation for important safety info might miss the topic altogether or think they are safe to use their new 12-bit scope in a floated condition.

The thing is, that scope still has the outside part of the BNC all tied together, and the ground clips on all channels will thus be connected in parallel to the outside of the BNC.  So if you use multiple channels, and they are at different potentials in reference to each other (with low impedance), you will be causing a short by connecting your ground clip.

ie. I use one probe and the ground connector is connected to live, and I connect the other probe's ground connector to neutral.
Or I use a single probe and I accidentally have the ground connector connected to something that is not at earth potential and I touch any BNC connector on the scope (ouch time).

I looked into this because it seemed like a win, win over a handheld scope for example.
Use the Rigol and power it from a USB-PD power bank and suddenly you have all the benefits of a handheld scope.

The power bank would remove the common mode noise problem, but the safety aspect has not changed.

Personally I realised I don't have the discipline to avoid ever touching the BNC while probing potentially lethal voltages, so floating is not for me personally.
But I certainly think it is better to make people fully aware of the situation than shout endless warnings without reasons.
Its become like a religion in that way.  Pointless posts about it is dangerous without well articulated reasons or anecdotes about how someone got the shock of their life without explaining in detail why.

[Psi]

You can float your own scope if you want to, but it can be very dangerous unless you fully understand exactly what you're doing.
So it's best to simply tell people to never do it.
Eventually, once they are experienced, they will start to realize how and why it can sometimes be done, and hopefully by then they will have the knowledge to
either
- Do the task a different way that doesn't require floating the scope
Or
- Do it safely.

It's a bad idea to try and explain to a beginner or intermediate skilled person what the risks are and how to do it safely.
To do it safely you really need to understand what you are doing at a fundamental level, not just memorize a list of do's and dont's.

It's like trying to give a newbie a list of do's and dont's to work with a microwave oven transformer.

[tggzzz]

My hakko iron...

I lifted the ground and installed a neon lamp and 40k resistor

So it lights up when im soldering on live circuits.

If you are soldering live circuits, what stops any voltage applied to the tip from causing current to flow through you? What.is the current carrying capacity of a neon bulb?

When soldering sensitive components, what stops any static electricity or induced voltage on the tip topfrom subtlety damaging the components?

In other words, floating a soldering iron is dangerous.

[GnomeZA]

You can float your own scope if you want to, but it can be very dangerous unless you fully understand exactly what you're doing.
So it's best to simply tell people to never do it.

But that is no different from a religious/cult leader telling their believers not to question the religion or cult.
In the world we live in now, with the amount of information and access we have, gatekeeping is pissing in the wind, it does absolutely nothing.
Saying on a public thread that you think the readers may not be as educated as you are <or insert superiority reasons here> is like asking for the Streisand effect to happen.
Some people on that thread WILL be smarter than you (there is always someone smarter or better than you).
By saying that you've solidified in their mind every bad thing they think about you and that they are actually discovering something useful.
I think the best option if you are unable to articulate reasons, is to stay silent, it'll do more to prevent someone from proceeding than gatekeeping.

Sorry bit of a preach but it really is annoying to have to scroll past pointless posts because they distract from the gold nuggets of actual useful information.

And someone will obviously take issue with what I said here so let me ask to that person this:
When last did you accept someone shutting down something you asked with dogma, then walked away from that thinking, now that was some good feedback and so valuable.
Never to think about it again.

[JDW]

Pointless posts about it is dangerous without well articulated reasons or anecdotes about how someone got the shock of their life without explaining in detail why.

Very well... Give us your "well articulated reasons" and "explain in detail why" your position on this topic is vastly superior to all others here.  You have thrashed most people here who are trying to help others by cautioning them, so it's time for you to defend your position about NOT cautioning them.  Otherwise, your dialog on the matter too becomes the very "pointless posts" you preached against.

More specifically, all you've really said is this:

Personally I realised I don't have the discipline to avoid ever touching the BNC while probing potentially lethal voltages, so floating is not for me personally.
But I certainly think it is better to make people fully aware of the situation than shout endless warnings without reasons.

Those two sentences aren't "well articulated reasons" or "explaining in detail" to justify your stance or show us why you have the moral or engineering high ground.

Right now, I see most people here basically telling little kids NOT to play in a busy street or they might get hurt, while you are basically trashing their parents and telling the kids, "it's okay to play in the street, so long as you play it safe."  Yes, it really is basically that.

[tggzzz]

Right now, I see most people here basically telling little kids NOT to play in a busy street or they might get hurt, while you are basically trashing their parents and telling the kids, "it's okay to play in the street, so long as you play it safe."  Yes, it really is basically that.

It is pleasing to see people picking up on the analogy of playing in the road or walking into the road without looking. It is a helpful analogy, since it points out the weakness of the stated position.

In addition to your points I will note that gnomeza is simply wrong to claim "without well articulated reasons or anecdotes". Maybe he can't understand the reasons and specific anecdote mentioned in
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

It seems he isn't as well-read or clued up as he would like to think.

What's the betting this is sufficiently easy?... Here's the reference to the experienced engineer in Sylvania lighting laboratory: https://groups.io/g/TekScopes/message/10754

[coppercone2]

amazing how much people don't want to buy floating probes LOL

[tggzzz]

Ah, it is good to see that photo I posted long ago being remembered

It does skewer some (other) people's points quite effectively and amusingly.

[madires]

In case that power brick comes with an EMI supression cap between primary and secondary I wouldn't call the DSO floating.

That kind of statement seems to suggest this: "Go ahead and remove the ground from your scopes and then test high voltage because as long as you have an EMI suppression cap between the primary and secondary, it's not floating and therefore A-OK."

Nope! You're reversing cause and effect while adding a thrid thing, which might be even dangerous based on the specific measurement setup.

[Psi]

You can float your own scope if you want to, but it can be very dangerous unless you fully understand exactly what you're doing.
So it's best to simply tell people to never do it.

But that is no different from a religious/cult leader telling their believers not to question the religion or cult.
In the world we live in now, with the amount of information and access we have, gatekeeping is pissing in the wind, it does absolutely nothing.
Saying on a public thread that you think the readers may not be as educated as you are <or insert superiority reasons here> is like asking for the Streisand effect to happen.
Some people on that thread WILL be smarter than you (there is always someone smarter or better than you).
By saying that you've solidified in their mind every bad thing they think about you and that they are actually discovering something useful.
I think the best option if you are unable to articulate reasons, is to stay silent, it'll do more to prevent someone from proceeding than gatekeeping.

Sorry bit of a preach but it really is annoying to have to scroll past pointless posts because they distract from the gold nuggets of actual useful information.

And someone will obviously take issue with what I said here so let me ask to that person this:
When last did you accept someone shutting down something you asked with dogma, then walked away from that thinking, now that was some good feedback and so valuable.
Never to think about it again.

You raise valid points. But it's also valid to choose not to explain dangerous and unnecessary things publicly, and to instead just say "don't do this".
At the end of the day everyone has to live with themselves and the effect they have on the world.
With the wide reach of the internet sometimes posting something will almost definitely lead to someone somewhere trying it and being inured.
The hard part is knowing where the line is.
eg, Does helping 9999 people make up for the fact that 1 person died or was injured while following something you posted online?

[JDW]

In case that power brick comes with an EMI supression cap between primary and secondary I wouldn't call the DSO floating.

That kind of statement seems to suggest this: "Go ahead and remove the ground from your scopes and then test high voltage because as long as you have an EMI suppression cap between the primary and secondary, it's not floating and therefore A-OK."

Nope! You're reversing cause and effect while adding a thrid thing, which might be even dangerous based on the specific measurement setup.

I honestly don't understand what you're talking about.  Let's keep it simple, shall we?  I've said nothing other than things which advocate safety as per the use of a grounded and non-floating scope, for the sake of (1) preventing inadvertent electric shock, and (2) for helping to avoid common mode noise issues on measured waveforms.  It's really that simple.