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| tggzzz:
--- Quote from: tautech on June 22, 2017, 08:01:41 am ---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. --- End quote --- IMNSHO the only way that statement could have been improved is if it had been posted in a larger font. --- Quote ---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. --- End quote --- Excellent. |
| dicky96:
--- Quote from: tautech on June 22, 2017, 08:01:41 am --- --- Quote from: dicky96 on June 21, 2017, 09:38:30 pm ---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 --- End quote --- 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. --- End quote --- 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. --- Quote ---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). --- End quote --- 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:
--- Quote from: JPortici on June 22, 2017, 09:42:03 am --- --- Quote from: Electro Detective on June 21, 2017, 11:17:55 pm ---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. :-- :-- :-- --- End quote --- maaaaaybe because it's hard and expensive to have both high speed, lots of memory an isolation? --- End quote --- It's 2017, not 1987.. technology has gone up, prices are dirt cheap, sweat shoppe labor even cheaper |
| alm:
--- Quote from: dicky96 on June 22, 2017, 10:27:21 am ---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. [...] --- End quote --- So did the guy in this story (written by a retired Tektronix sales engineer). Did not save his life either. --- Quote from: dicky96 on June 22, 2017, 10:27:21 am ---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. --- End quote --- 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. --- Quote from: dicky96 on June 22, 2017, 10:27:21 am ---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? --- End quote --- 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. --- Quote from: dicky96 on June 22, 2017, 10:27:21 am ---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. --- End quote --- 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:
--- Quote from: dicky96 on June 22, 2017, 10:27:21 am --- --- Quote from: tautech on June 22, 2017, 08:01:41 am --- --- Quote from: dicky96 on June 21, 2017, 09:38:30 pm ---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 --- End quote --- 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. --- End quote --- 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. --- Quote ---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). --- End quote --- 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. --- End quote --- 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. :scared: 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. |
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