-
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 -
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'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.
-
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? -
Quote
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.
-
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 ?!!
-
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. -
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.
-
I've googled this and yes it had been discussed but I still don't understand why this is a big no no..............
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.
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
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. -
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 -
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.QuoteIn 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. -
I've googled this and yes it had been discussed but I still don't understand why this is a big no no..............
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.
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
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.QuoteJust 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 -
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
-
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)
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.
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. -
But they can be contacted with............and that's the safety issue.I've googled this and yes it had been discussed but I still don't understand why this is a big no no..............
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.
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
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.QuoteJust 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.
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. -
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.
-
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. -
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"!
-
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.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.
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 differential probe is the cheapest chinese found from ebay where the isolation can be anything a common semiconductor junction included.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.
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
-
..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?
-
..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 ?
-
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.
-
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