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| Floating Scopes |
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| tggzzz:
--- Quote from: David Hess on June 25, 2017, 03:11:58 am ---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? --- End quote --- 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:
--- Quote from: David Hess on June 25, 2017, 03:11:58 am ---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. --- End quote --- Three paragraphs that should give anybody reading this enough warning that the practice is dangerous. Period. --- Quote from: David Hess on June 25, 2017, 03:11:58 am --- --- Quote from: alm on June 21, 2017, 10:20:59 pm ---If you read documentation from scope manufacturers back then, they also recommended against it. --- End quote --- 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. --- End quote --- 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. --- Quote from: David Hess on June 25, 2017, 03:11:58 am --- --- Quote from: tggzzz on June 22, 2017, 10:06:06 am --- --- 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. --- End quote --- You can both bite me. --- End quote --- Only if you think that the OP's practice of highside mains measurement by floating his scope is acceptable. --- Quote from: David Hess on June 25, 2017, 03:11:58 am ---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? --- End quote --- 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. :P https://www.eevblog.com/product/hvp70/ |
| nctnico:
--- Quote from: mtdoc on June 25, 2017, 04:19:44 am ---An informed discussion that lays out the risks (and any merits) can only be beneficial. --- End quote --- 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:
--- Quote from: David Hess on June 25, 2017, 03:11:58 am --- --- Quote from: alm on June 21, 2017, 10:20:59 pm ---If you read documentation from scope manufacturers back then, they also recommended against it. --- End quote --- 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. --- End quote --- 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 |
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