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