How to Kill Yourself with an Isolation Transformer - Page 1
Forum section: electronics / beginners

author: calzap
author_rank: Frequent Contributor
timestamp: April 14, 2024, 08:29:37 pm
content: Isolation transformers can provide a degree of safety when working with high voltage equipment (device under test or DUT). Here, I’m defining high voltage as a voltage as one that can give a harmful shock via dry skin contact. However, the safety provided by isolation transformers can be easily defeated. Worse, isolation transformers can nullify the benefits of better protection … a GFCI/RCD-protected power source.It’s easy to re-reference an isolation transformer output to ground via test equipment … with potentially lethal results. For example, connect one side of the transformer output to the DUT and connect the ground lead of a mains-powered oscilloscope to a part of the DUT that has low resistance to where the transformer output is attached. Attach the other output of the transformer to a part of the DUT that has a high resistance to where the first output is attached. Pay your life insurance, and touch this second part of the DUT with one hand and something grounded with the other.The diagram and pictures below illustrate the scenario. In the pictures, there is a light bulb instead of a body. I do not recommend doing the setup in the pictures unless you are very familiar with working with mains power. In the pictures, you are the light bulb. In the first picture, the DUT, a plug cord in this case, is connected to the 120 VAC output of an isolation transformer. Your left hand (the white lead of the bulb socket) is touching something conductive connected to one side of the isolation transformer. Your right hand (black lead) is touching a part that is grounded … the red clips, green and then black wire go to the ground pin of the orange plug in the receptacle. What could go wrong … you’re isolated … right? Wrong in this case! The other side of the transformer secondary is also grounded via the oscilloscope.The isolation transformer in the picture was plugged into a GFCI-protected socket. The GFCI didn’t trip because there was no ground fault that it could detect … it was isolated from the fault by the transformer. The GFCI receptacle on the isolation transformer secondary provides little extra protection. Something would have to go seriously wrong inside the transformer for it to trip. Its ground socket is not connected to anything.The scope was plugged into a the same GFCI-protected receptacle as the transformer. So why didn’t that trip the GFCI? It didn’t because current was passing through the scope ground chassis to the ground line of the cord and pin. From there, it passed into the ground contact of the receptacle and into mains wiring ground. A GFCI monitors hot and neutral currents and trips if they aren’t the same; it does not monitor current in ground wires directly.In the second picture, the DUT is plugged directly into an outlet protected by a GFCI breaker. The bulb is not lit. Why? Because the GFCI tripped. No shock to the body here!If the DUT was plugged directly into a functioning GFCI/RCD, a real body might feel a brief tingle before power was removed. I experienced this when a garage door installer put a lag screw into a mains cable. I’ve seen GFCI receptacles fail live. Rare, but it happens. That’s why they have test buttons. When’s the last time you tested yours? I thought so … you’re like me.But yeah, a DUT plugged directly into a functioning mains-powered GFCI is shock-safer with regard to grounding than using an isolation transformer. All my shop receptacles are GFCI-protected.So, will GFCI protect test equipment against a ground fault? With regard to starting a fire or shocking the operator … yes. However, GFCI’s operate very quickly, but not instantly. It might be possible for a ground fault to cook some semiconductors before power is removed. Battery powered test gear is best for high voltage work. However, some battery powered instruments are not designed for high voltage use, especially some oscilloscopes. For mains-powered oscilloscopes, differential probes or probe isolators are recommended. And plug any mains-powered test gear into GFCI/RCD-protected receptacles.Now, all you have to do to get shocked is touch two hands to high voltage parts of a DUT.Mike

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 14, 2024, 08:46:55 pm
content: Maybe the best way to avoid all these problems is to simply isolate the test equipment.Most test equipment anyhow is isolated, apart from the ground reference they have for the signals.The only thing that one would lose with isolating the test equipment, is maybe the power supplyfault detection.The load is almost always higher power than the test equipment. To have the RCD where the energy anda mistake could kill you, is most probably the better thing to do. In the end we have isolated test equipmentand they are called multimeters.Or use isolated probes. Not the Differential ones, with the 10MΩs. Isolated.

author: Benta
author_rank: Super Contributor
timestamp: April 14, 2024, 09:08:33 pm
content: I think there's a Darwin award lurking here.Why on earth (pun intended) would you want to touch the high voltage secondary end of an isolation transformer when the other end is grounded?I mean, if you have a 500 V battery, you don't touch both ends at once (unless you're a moron).

author: Benta
author_rank: Super Contributor
timestamp: April 14, 2024, 09:09:36 pm
content: Quote from: TrickyNekro on April 14, 2024, 08:46:55 pmNEVER!That will get you killed.

author: CaptDon
author_rank: Super Contributor
timestamp: April 14, 2024, 09:14:43 pm
content: I think most experienced technicians were aware of this type of fatal flaw just as we / they are aware that even with an isolation transformer if you happen to be working on a Carver PM-2.0T that incorporates both a + and - 130VDC power rail with a kilowatt of power available you can still be killed and D.C. doesn't like to let go. I guess the real problem comes through the rumor mill that connecting your unit under test to an isolation transformer protects you in every imaginable situation. It seems many noobs get their information 3rd or 4th hand from folks who weren't very electrically savvy in the first place. It is amazing on Iceland folks working with 240vac may tend to take chances working a hot circuit but when faced with 240vdc which exists in some areas they take insane amounts of precautions!!! A 240vdc arc flash will sustain forever and the plasma ball can get huge when copper vapor is involved. An incredibly bigger fireball than a 240vac arc flash.

author: tggzzz
author_rank: Super Contributor
timestamp: April 14, 2024, 10:22:25 pm
content: Quote from: TrickyNekro on April 14, 2024, 08:46:55 pmNo. Never Never Never.Read andunderstandthe safety references and anecdote athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Consider why someone did this...QuoteUsing an HV differential probe is a much better starting point.

author: coppercone2
author_rank: Super Contributor
timestamp: April 14, 2024, 10:24:50 pm
content: man I renember playing blue shift (half life security guard episode) a long time ago thinking it was a sick joke where it tells you that protecting the black mesa facility equipment is first or second priority objective and personal safety is tertiary."Low Priority : Personal Safety"I guess its true. That 200$ clapped out ebay oscilloscope is that important.what happened to it being cool to destroy malevolent machines, like captain kirk?Like even at work near the 200k oscilloscope, its just a box.... no one really cares about it. literally like a flashlight will get more attention because it does useful immediate work for humans. Everyone knows its marked up as hell because its cutting edge and it bearly does anything better then the other thing and its only good for advertising that the tests are done 'diligently'. May be useful for precision on extreme manufacturing scales to give people better 'projections' for making decisions that involve millions of units being made with theory based design changes to meet deadlines. If you need it, its considered cheap for you. if you don't need it, its just breaking some expensive luxury good. luxury. aka that thing you might play with if you have free time. not necessary.get a cheaper scope that you can afford to break ....... its common sense stuff

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 14, 2024, 10:58:49 pm
content: Quote from: tggzzz on April 14, 2024, 10:22:25 pmI do get why someone did this. Every metal of the oscilloscope will be referenced to whatever point one is probing with the ground lead. But you would be essentially "protected" by the same RCD as the load.And you can definitely get some very very sensitive RCDs, that you can use extra for your specific test bench. There are also 6mA RCDs.If you feel you need even more protection, then in comparison to the year old tek scope there, you can remote control most scopes through ethernet nowadays.The 1500V isolation should be enough for most mains powered applications, one would guess. Probe with the power down, control remotely.And you probably should be wearing gloves or similar if you have anything in your load that can dump some serious energy on you. No other protection will save you if you are probing capacitor banks.And there are isolated scopes also, those seperate the ground at the inputs. But cost a pretty penny.So unless you are working with motors, which tend to leak some residual current and you anyhow should be very careful and use differential / isolated probes, well you can get away with a lot.And you are going to need differential / isolated probes, if you are trying to probe different parts at different potentials anyhow. ( for example you are probing the driver of a Motor ).Edit: As a side note, if you use ethernet, take extra care that the cable is unshielded.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 14, 2024, 11:36:38 pm
content: Quote from: tggzzz on April 14, 2024, 10:22:25 pmDepends on what you are probing. If you are probing sensors along side with power points then the ~60 - 30uA could potentially throw your measurements off, or your system big time.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 14, 2024, 11:41:21 pm
content: Quote from: Benta on April 14, 2024, 09:09:36 pmThen why is Tektronix offering battery powered oscilloscopes for "floating oscilloscope measurements"?I do get that every exposed oscilloscope metal part will be out to kill you, but this is actually practiced.Edit: No Seriously, unless I am missing something big here... They say in their webpage that using isolation transformersis dangerous and can kill you and all the nice stuff, but then tell you that using a TDS3000 series oscilloscope is ok, just usewith caution... just a bit of caution, it´s all ok, it uses batteries and everyone is happy... just a bit of caution...Damned if I ever seen some legal speech and marketing speech in my life combined...And again, unless I am missing something big here, I can not really take all this seriously. Use caution with our exposed metalbut dare you not buy our certified product, you will die a horrible death... I mean come on.Either understand what you are doing and use generally caution, because a mistake could seriously cost your life, or this is all marketing for handheld oscilloscopes.I do seriously stand to be corrected here.Edit No.2: And I do understand that isolation transformes leak themselves and in some scenarios that could again be dangerous or messy for the readings. As always understand what you are doingand use much more than needed caution. It is the "marketing double speak" that drives me a bit nuts here.

author: coppercone2
author_rank: Super Contributor
timestamp: April 14, 2024, 11:49:02 pm
content: that is a qualified oscilloscopebut its still not as good as active probes. because channels are common ground. differential active probes = full isolation on every channel.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 15, 2024, 12:02:43 am
content: Quote from: coppercone2 on April 14, 2024, 11:49:02 pmDifferential probes are NOT isolated probes. They might be good enough for some application probes, but they are not isolated.Try to probe different differential sensors with them and you´ll see ( or won´t see ). Depends on the system. But they are definitely NOT isolated.Maybe there are some models that are, but normally they have a 5MΩ path to oscilloscope ground.

author: coppercone2
author_rank: Super Contributor
timestamp: April 15, 2024, 12:17:01 am
content: that is pretty good isolation. You probobly wont blow up anything with 28 microamps at line voltage

author: johansen
author_rank: Super Contributor
timestamp: April 15, 2024, 12:22:13 am
content: How many of you all drive home from work slightly tired?Quote from: calzap on April 14, 2024, 08:29:37 pmno shit sherlock.all you have to do is nod off at the wheel.happens pretty often.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 15, 2024, 12:24:57 am
content: Quote from: TrickyNekro on April 14, 2024, 11:41:21 pmA battery powered oscilloscope is designed for that use. That's a different story than floating a scope designed to be grounded.Also, many battery powered oscilloscopes are designed for automotive applications, and they are intended to be grounded to the vehicle chassis. They aren't necessarily intended to be floated at a high voltage.

author: coppercone2
author_rank: Super Contributor
timestamp: April 15, 2024, 12:30:39 am
content: Quote from: johansen on April 15, 2024, 12:22:13 amthese people should join the army it pays better then trying to protect a scope if you don't mind the riskthis way the government pays for a new scope and you don't have rent. risk management. put the risk else where

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 15, 2024, 01:20:26 am
content: Quote from: ejeffrey on April 15, 2024, 12:24:57 amThen I´ll go ahead and mention all the above sources:Here is the Tektronix blog / post / website:https://www.tek.com/en/documents/technical-brief/floating-oscilloscope-measurements-and-operator-protectionHere is the teardown by Dave:There is absolutely nothing special about that scope, as far as I can tell, in comparison to any other low / mid range scope in the market today, apart that it can use a battery.Exposed metal back and front referenced to the ground of the BNCs.Somehow it´s ok to float that scope and not others. Again, I do get that most isolation transformers are leaky and that might be a concern.But, in practice, I absolutely do not see how that differs from floating any other oscilloscope, or equipment in that case, with a low to no leak isolation transformer ( or use two, at which point just buy probes ).Sure, don´t touch exposed metal when connected, that much is clear. The rest is legal speech.About the automotive case:Most of the time, you need the isolation because you need to probe different signals and buses that are either differential, like the CAN bus, or because of the long wiring of the car and different loading conditions mightsit in a bit different ground point. This is another use case scenario and has to do with measurement and not with safety ( unless I guess you are probing the battery of an EV ).The major concern is not to touch any exposed metal when the instrument is floated and connected to a high potential. And if the potential is that high you might also not want to touch the intrument in general, at whichpoint you´ll have to remote control it. And you might have to remote control it even wirelessly via an accesspoint if the common mode is that high. And at which point you might already need a high voltage probe.But the question of the OP was about household mains, as far as I understood it. Not 11kV lines...It is much much much safer, for someone who wants to experiment with these relatively low voltages to have a very sensitive RCD with the load and have someone around to kill the power and call for help,if anything goes wrong, than anything else ( plus some rated gloves ). And that´s because, touching any naked metal will trip the RCD and in case you touch two wires, you will not let the transform keep dumpingits energy on you. Edit: This is assuming you won´t touch two wire at the absolute exact same moment and that there is a path to earth through your body.Isolating the load is EXACTLY the reason why so many people got injured or killed by trying to make Lichtenberg Figures on wood!!!!Just screaming you´ll die, if you do it, is for the most part unproductive, offers no reasoning and no solution other than throwning money at a problem, while still not teaching you where exactly the problem lies.And you better understand what you are doing, because one day the time will come, when you´ll have to play dirty to make something work.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 15, 2024, 02:10:15 am
content: Quote from: coppercone2 on April 15, 2024, 12:17:01 amBlow up no, but it could potentially mess up measurements of sensitive signals. Talking from experience.And you´ll get it relatively fast when you see ac on your readings. But if it´s between signals, it will give youa very good head scratcher...

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2024, 09:54:36 am
content: Quote from: TrickyNekro on April 14, 2024, 11:41:21 pmThey aren't.They are selling scopes that are completely designed from the ground up (pun intended) to be operated when floated, and those scopes happen to be battery powered."Little" things like the BNC socket locks being made from plastic, not metal.

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2024, 09:58:52 am
content: Quote from: TrickyNekro on April 14, 2024, 11:36:38 pmYou appear to have missed the phrase I haveemphasised.There is a reason there are many different types of probe: there are many difference measurement use cases which have different requirements and tradeoffs.Your use case is too vague for it to be possible to comment.

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2024, 10:04:14 am
content: Quote from: TrickyNekro on April 15, 2024, 12:02:43 amWhy are you fixated on floating a scope and not using HV differential probes?Your contentions are strawman arguments: nobody has suggested that one (of many) type of probe is suitable for every use case.In that vein, I suggest that the probe-earth capacitance of a floated scope will cause problems when "probing differential sensors".

author: xvr
author_rank: Frequent Contributor
timestamp: April 15, 2024, 11:37:06 am
content: Isolation transformer is not a safety device. Output voltage from it the same deadly as input. It is not a silver bullet, there is ONLY usage area for them is debugging DUT that directly Mains connected (without any internal isolation).Debugging such devices always is a dangerous business, with transformer or without. Engineer that works with such device should have appropriate qualification and experience. Isolation transformer will not help him to be more safe, it only allow not to fry up test equipment, but instead it could help him to fry up himselfIMHO beginners should not deal with Mains connected devices at all (with Isolation Transformer or without - doesn't matter).Unfortunately we can't stop him. As I can see in this conference they sometimes try to build some derivation of electric chairAn this is much worse than any Isolation Transformer

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 15, 2024, 12:26:29 pm
content: Quote from: tggzzz on April 15, 2024, 10:04:14 amDunno, that was just an example and a side discussion.I don´t have a problem against diff probs. One should use whatever is appropriate. But sometimes, when one only has anisolation transformer, then an RCD is cheaper than a diff prob. I would have thought.Anyhow, the question is not whether diff probs are a problem. The question is whether or not it is ok to float equipment.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 15, 2024, 12:29:39 pm
content: Quote from: tggzzz on April 15, 2024, 09:54:36 amI pointed exactly to the sources I was referring to in a post above and the reasoning of my comments.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 15, 2024, 12:44:26 pm
content: I've yet to have a need for an isolation transformer at home. As for safety, I guess the only 100% sure safe way is pay someone who is competent to do the work.

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2024, 12:52:27 pm
content: Quote from: joeqsmith on April 15, 2024, 12:44:26 pmI've occasionally wondered about getting one - and haven't found a use case. I have my bench powered through a simple domestic 30mA RCD.If I wanted to invest money in protections, I'd go for a different RCD with a lower trip current. Mind you, that would probably pop every time I plugged in some old TE

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 15, 2024, 01:00:53 pm
content: What about the idea of adding a second GFCI, and then not mounting it in the regular way, but only hook up the earth wire though the sense circuit. That way, whenever there is a current though the earth wire, it will turn off your desk and all equipment on it.

author: CaptDon
author_rank: Super Contributor
timestamp: April 15, 2024, 01:04:06 pm
content: Coppercone mentions blowing up $200K scopes as trivial? We are in an era now where differential probes are selling for $5K plus and the last scope we bought for differential logic measurements in the 2GHz+ range was $400K. Blowing them up is not trivial if you expect to remain in business. They are often damaged beyond repair when damage occurs. But, sadly, where I was working there was very little inspection of test setups by senior engineers prior to the circuit being energized and we had some degree'd H1B engineers running the testing whom I swear got their degrees from a Cracker Jack box and that is because the company hired 'degrees' and not proven talent!! I have seen a lot of gear get destroyed by idiots who I would have guessed knew better. We have one person who claims to have been a professor and that person was responsible for a lot of damage. They seemed clueless about the very job they were hired to do! I walked in a couple of times and immediately hit the master lab kill switch just glancing at their testing methods!! They had not a clue what 1700uf charged to 1400vdc could do to them! everything exposed, nothing isolated!!They were trying to determine the actual capacity at 1400vdc. They were going to measure the voltage decay time across a 1K 1 watt resistor to determine total uF capacitance! Glad I walked in!! May have saved a life!! Some of the 'Senior Engineers' were 40 years my junior with no more experience than a college degree. When I would point out flaws in their methods the reply was always "You're old school man".

author: Silenos
author_rank: Regular Contributor
timestamp: April 15, 2024, 01:12:57 pm
content: Uuum what about having both DUT and scope powered by the same mains phase, and their ground/neutral also the same? Why messing voltage levels with isolation transformers?

author: CaptDon
author_rank: Super Contributor
timestamp: April 15, 2024, 01:13:15 pm
content: Those who say "I have never found a need for an isolation transformer" have apparently not done much work on the hot side of SMPS devices!!! So much high power audio gear now have 'kilowatt plus' SMPS units for the + and - 100vdc and higher power rails and those SMPS's seem to be easily broken in cheaply made gear. You would be an idiot to probe around the hot side of a SMPS without connecting it through an isolation transformer. More than once I've seen idiots put the ground lead of a scope on a 'hot chassis' television where neither the scope or the T.V. was isolated. It didn't end well!!! Lots of smoke and a destroyed customer television!

author: joeqsmith
author_rank: Super Contributor
timestamp: April 15, 2024, 01:21:41 pm
content: Quote from: tggzzz on April 15, 2024, 12:52:27 pmI don't have anything outside of the circuit breaker in panel or the tiny one in the line strip. When it comes to protecting the equipment and safety, education trumps anything else. Complacency is never a good thing.

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2024, 01:27:17 pm
content: Quote from: CaptDon on April 15, 2024, 01:13:15 pmThat sounds like a strawman argument.The issue is whether to use an isolation transformer or an HV differential probe.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 15, 2024, 01:34:35 pm
content: Quote from: CaptDon on April 15, 2024, 01:13:15 pmInteresting. I've worked on several SMPS over the years. Not so much for repairs but more for failure analysis (repair is just a side effect). Larger ones were in the >kW and PFC stages in the >400V. I've never though about needing an isolation transformer to do this work.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 15, 2024, 02:24:20 pm
content: Quote from: joeqsmith on April 15, 2024, 12:44:26 pmNormally,I amthe "someone who is competent to do the work".For some 10 years, I spent much of my time working on Picture Monitors & at times Domestic TVs.The standard SMPS in those days used BJTs, & from time to time these would fail, which sometimes took out other components.One side of the circuitry prior to the "choke" (actually to all intents and purposes a transformer), so it was quite easy to get caught out using a normal CRO probe if you just run the DUT "bareback".On the output side of the "choke", nothing was connected to either side of the Mains, so a normal CRO probe was quite safe to use.We did, from time to time, have to look the early stages of the SMPS, which is where the isolation transformer came in.You can get caught out, as I did one day, when someone else needed the isolation transformer, & I set to looking at the isolated parts of a SMPS then moving back to its input.Powering the DUT down at the power point, I hung the probe between the test point & "common".Turning the DUT on there was a "zap" & the bench RCD dropped out.Damage amounted to a small burn on the ground clip!Much of the troubleshooting with the SMPS could be done unpowered, using a Fluke 77, so it wasn't normally a major drama.Most people who have been around for a while in the Electronics repair side of things have copped a "bite" of 120v or 230v & are still around, but such incidents have been a vanishingly small percentage of the time they have spent on such things.It seems to be the trend to present working on power supplies as of the same degree of peril as crocodile wrestling,but really, if people use a modicum of care, it is as safe as any other activity.The other things I spent a lof time on were Radio & TV Broadcasting transmitters, which ran off 3 phase supplies, with HTs of around 10kv DC.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 15, 2024, 02:55:50 pm
content: Quote from: vk6zgo on April 15, 2024, 02:24:20 pmThat has been my experience.Normally when I am doing any powered up testing (inside the supply) I am still using a DVM. If I need to use a scope, I am using a diff probe designed for it. At home, this never really comes up or I would invest in a good diff probe just for this purpose.

author: madires
author_rank: Super Contributor
timestamp: April 15, 2024, 02:59:22 pm
content: Quote from: Doctorandus_P on April 15, 2024, 01:00:53 pmI see two major issues. No power supply for the sense circuit (usually some special RCD/GFCI IC). And the built-in test function won't work.

author: madires
author_rank: Super Contributor
timestamp: April 15, 2024, 03:22:40 pm
content: Yay! Another isolation transformer bashing thread.The actual problem is to understand how to use an isolation transformer properly and when to use one. GFCIs/RCDs also come with a few issues, e.g. different trigger types, regular testing, or trigger currents (30mA can kill you). No matter if it's an isolation transformer or a GFCI/RCD, you can do stupid things with both of them and harm yourself. And please don't rely on just a single safety measure!

author: jonpaul
author_rank: Super Contributor
timestamp: April 15, 2024, 04:00:47 pm
content: Have used isolation trsf for 50 yrs on amny SMPS, HV, EBU and never had any issues.Like any tool they can be misued as you schem shows.Learn how to work safely and dont blame incorrect use of the transformer.Enjoy!@J

author: Jeroen3
author_rank: Super Contributor
timestamp: April 15, 2024, 04:13:26 pm
content: Quote from: madires on April 15, 2024, 02:59:22 pmElectrical code specifies, that when you are using ungrounded networks you must use isolation monitoring.I see isolation transformers purely as a means to reduce short circuit capacity and slightly clean-up the waveform.They can be usable in test scenarios, but they should never be used with the intent of floating for scopes.If you intend to measure with a ground referenced tool on a DUT powered from a transformer, it should already be grounded, not through the scope lead, before power-up. Because, ground must always be able to carry the fault current!Do not forget that an accidental short, blown varistor, diode or capacitor can easily take out an eye. Always use caution.

author: coppercone2
author_rank: Super Contributor
timestamp: April 15, 2024, 04:37:35 pm
content: Quote from: CaptDon on April 15, 2024, 01:04:06 pmif someone goes out and buys a 400k scope and its so critical that its more important then employee safety their business plan sucks. no its not trivial it might cost your job or transfer but better then a defibrillator and for the business it often IS trivial. some people feel alot of empathy for a company (oddly), but they should just worry about their job, because this is still a cost that is small to a company doing this work. It won't tank it, its not being some folksy blue collar protecting everyones job hero. no one should be made to feel like that over safety, that is too much pressure. getting cred for protecting dangerous ass company assets is silly. and if you really care for the company regardless if your fired, then the safety crack down that comes after a workplace incident is guaranteed to be worse financially then replacing basically any capital equipment.then you get a goverment funded safety research study into the "x field" that gets so out of control it ends up hurting the GDP of a country lol

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2024, 04:50:15 pm
content: Quote from: madires on April 15, 2024, 03:22:40 pmNot at all. Strawman arguments.Simply a discussion about appropriate tools.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2024, 04:53:54 pm
content: Quote from: calzap on April 14, 2024, 08:29:37 pmI wish we could all stop talking about isolation transformers as a magical safety blanket. Or, as in this case, setting up the strawman argument that isolation transformers are a magical safety blanket and then high-handedly repudiating that argument, one that nobody is actually making.Isolation transformers were introduced for test and repair purposes because radios and TVs of the time were designed with large metal chassis that could be live at mains potential. Many of them would be live when the device was on and the cord was plugged in one way, but if the cord was plugged the other way they would be live when turned off. Working inside a TV where most of it was at mains potential was a pretty big hazard and isolation transformers helped. They still work for this purpose, but it isn't as common a use case anymore.A few years ago I posted how I might use an isolation transformer to test an SMPS and got the same set of usual responses that it is 'wrong', 'dangerous' and even illegal.https://www.eevblog.com/forum/testgear/probing-smps-with-oscilloscope/msg3152256/#msg3152256Yes, the DUT (and even the chassis) may be live and shock you if you touch them when doing this. If you don't have an isolation transformer, you also may get zapped if you touch your DUT. Your magical RCD may not even save you if you use two hands--or if the DUT has its own isolation or a large capacitor. And if you have a probing accident, the hundreds-to-thousands of amps of fault current may do substantial damage.Isolation transformers allow you to arbitrarily select the point in a circuit that is going to be at ground potential. This can be useful and in some cases will yield better results than any other method short of the very expensive Tek IsoVu or similar setups. They also limit fault current and effectively make everything connected to them a CAT 1 environment so you can safely use your fragile bench meters and oscilloscopes. They don't allow you to randomly fondle, grope and poke your DUT with your sweaty fingers nor do they fully protect you from probing accidents or dropped tools--although limiting the fault current can be a big help when those things happen.

author: m k
author_rank: Super Contributor
timestamp: April 15, 2024, 05:06:07 pm
content: Quote from: joeqsmith on April 15, 2024, 02:55:50 pmMe too.If primary side, or CRT higher, CRO was needed it was connected before DUT was on.With CRTs, even if isolated, the tendency was towards learning the previous.Other tools were stuff like plastic screwdriver, regular one's handle side or cold spray.Hot air situations were nasty, you never knew which side you really are.For unknowns I had a variac.Few times it was actually needed.Isolation transformer was a law for repair shops, so every one at least had had one.I was lazy, I had N marked plugs, finally it didn't go well, test equipment vice.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 15, 2024, 06:15:48 pm
content: Quote from: TrickyNekro on April 15, 2024, 01:20:26 amYou obviously didn't read the link you posted. The TDS3000 series is 100% absolutely not intended to be used floating. The link your provided has a screenshot of the dialog that pops up whenever it it on batter power which says "To prevent electrical shock when operating from battery power, always connect a ground wire between the ground terminal on the rear panel and earth ground."That oscilloscope is meant to be used in automotive or field use where wall power isn't easily available, but should still be grounded.Scroll down further on the same page for an example in the THS700 -- an oscilloscope built from the ground up for isolation.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 15, 2024, 06:26:54 pm
content: When repairing SMPS in consumer equipment I power it from a UPS which is not plugged into the AC supply. I keep the scope grounded. Doesn't obviate the need to be careful.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2024, 07:33:25 pm
content: Quote from: ejeffrey on April 15, 2024, 06:15:48 pmActually they say you can use it floating if you are "sure" that the circuit you are testing has no voltages greater than 30VRMSto ground. The channel grounds are all common, of course.I don't know when that document was published, but they don't mention the TPS2000 series which is the only Tek lunchbox-style scope that is actually properly isolated both from ground and between all channels.

author: Jwillis
author_rank: Super Contributor
timestamp: April 16, 2024, 12:42:21 am
content: Quote from: bdunham7 on April 15, 2024, 07:33:25 pmThat not what it says.Operating Information 2–4 TDS3000 Series Service Manual"WARNING. To avoid electric shock, always connect the rear-panel groundterminal to earth ground when operating the instrument from battery power.For safe operation, the oscilloscope chassis must remain at earth ground potentialeven when operating from battery power. Without a connection between thechassis and earth ground, you can receive a shock from exposed metal on thechassis if you connect an input to a hazardous voltage (>30 V RMS, >42 Vpk ).Protection against this condition is provided when you attach the Tektronix-sup-plied grounding wire from the terminal on the rear panel to earth ground. If youuse a different grounding wire, it must be at least 18 gauge.When you start to use the battery pack, an on-screen message reminds you toconnect the grounding wire between the ground terminal on the rear panel andearth ground."It does not indicate that you can float the instrument.People need to get in the habit of reading and Fully understanding the manuals of the instruments they are using. If you refuse to fully understand the safety precautions and /or defeat the safety features of such instruments, you need to leave the electronics lab and find a different profession or hobby.

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2024, 01:30:42 am
content: Quote from: Jwillis on April 16, 2024, 12:42:21 amThe document that was linked previously and is being referred to here specifically refers to floating the scope at voltages less than 30VRMS. It also states that the warning applies only when voltages greater than that are present. Read Table 1 and then the section on battery powered scopes. The manual excerpt you quoted isn't quite as explicit, but it still indicates that you can float the TDS3000 series on battery power as long as the voltage limitations are observed.https://www.tek.com/en/documents/technical-brief/floating-oscilloscope-measurements-and-operator-protection

author: watchmaker
author_rank: Unknown
timestamp: April 16, 2024, 02:15:56 am
content: I listened to those who put the utility of the CFCI over an isolation transformer. And I grew up with isolation transformers.One hand rule still applies. GFCIs did not exist in the 1960s. Polarized plugs are now standard. Differential probes are available.I was troubleshooting a device under power and shorted a power transistor collector to ground. The device was two prong power into a transformer.GFCI tripped immediately and saved not only me but the device.I am a reformed believer.I do not know the use of iso xformers in vintage equipment today. But I can see that they can invoke a false sense of security and I do not see the sense in circumventing the GFCI as I think some have suggested.

author: madires
author_rank: Super Contributor
timestamp: April 16, 2024, 08:50:29 am
content: If you rely on a GFCI/RCD for your workbench make sure to get a type B or B+ to cover most fault current situations.

author: pdenisowski
author_rank: Super Contributor
timestamp: April 16, 2024, 01:24:15 pm
content: Quote from: TrickyNekro on April 14, 2024, 11:41:21 pmIf you mean the THS3000, it was discontinued some time ago. We (R&S) do still manufacture and sell a battery powered, isolated oscilloscope (the RTH) - up to 600 V at CAT IV and 1000 V at CAT IIIhttps://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscopes/rs-scope-rider-handheld-oscilloscope_63493-156160.htmlQuote from: TrickyNekro on April 14, 2024, 11:41:21 pmThe Tek app note on floating and isolated measurements is, in my opinion, very well written. I have often been asked to do a video on floating and isolated measurements, and if I ever were to do one, it would start with a huge warning / disclaimer as well (in red, all caps, bold, and underlined).I knew someone who used to joke that the reason he studied electrical engineering instead of civil engineering is that people wouldn't die if he made a mistake. Floating / isolated scope measurements is a good example of where that (tongue-in-cheek) statement clearly isn't always true.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 16, 2024, 03:22:12 pm
content: Quote from: TrickyNekro on April 14, 2024, 10:58:49 pmEthernet twisted pairs are isolated at both ends with high frequency isolating transformers. So, unshielded twisted pair is more safe then shielded with grounding connectors. Basically, if you don't have a shield - you don't have a problem.

author: madires
author_rank: Super Contributor
timestamp: April 16, 2024, 03:52:03 pm
content: In TP terminology 'shielded' refers to a single pair and 'screened' to the whole cable with multiple pairs.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 16, 2024, 05:13:23 pm
content: WARNING: Don't put your dick inside bulb sockets, you might die!!Same as you don't die while driving, your gas oven doesn't blow your home up, you don't suffocate with plastic bags...People learn their skills so they don't die using isolation tranformers.Five people might die using isolation transformers every year, but hundreds from electric shock and thousands due car crashes.Accidents do always happen, same as the 1849813th time you cut chicken, but that day the knife slipped.Let's ban chicken and knifes! And cars, electricity...Calm down your overthinking!

author: tggzzz
author_rank: Super Contributor
timestamp: April 16, 2024, 06:35:19 pm
content: Quite right.Also: stop wearing seatbelts, and stop telling inexperienced (little) people not to run into the road without looking.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 16, 2024, 07:29:10 pm
content: Don't run downstairs in darkness with a knife in your mouth, hands in the pockets!

author: tggzzz
author_rank: Super Contributor
timestamp: April 16, 2024, 08:04:13 pm
content: Quote from: DavidAlfa on April 16, 2024, 07:29:10 pmSome risks areworthtaking.Is that risk (or the subject of this thread) one of those?

author: bdunham7
author_rank: Super Contributor
timestamp: April 16, 2024, 09:35:55 pm
content: Quote from: watchmaker on April 16, 2024, 02:15:56 amHmmm. Was this power transistor on the primary or secondary side of that transformer? Assuming secondary--since a BJT on mains would be rather unusual--it seems to me that your GFCI likely tripped due to a surge and not an actual ground fault. (No input fuse??) In that case, while the GFCI tripping may have been fortuitous, your bacon and perhaps the devicewas actually saved by the isolation transformer.Stepdown transformers are typically isolating as well and have the same protective characteristics--breaking ground reference, limiting surge voltage and limiting fault current. Typically if you short something on the mains side, you'll get quite a spark and physical damage to your probes even with a GFCI. No GFCI or MCB is fast enough to protect any normal-sized semiconductor from direct mains fault currents. The bond wires would simply be vaporized.

author: watchmaker
author_rank: Unknown
timestamp: April 16, 2024, 10:40:09 pm
content: I have to digest this. Thanks. I am sure I will have followup questions.This was a power transistor regulating the sweep voltage in a BK 501A. It was on the secondary side and I knocked it into the frame (fucking essential tremors; what a joke). It was the GFCI that tripped, not breakers. Fuse on unit was not blownIt sounds like I do not understand what happened.As far as risk; most of us are not risk averse. We would not be doing what we do. But we DO value risk mitigation. I refuse to work with "Pennsylvania Mountain Men" who use a 24 inch bar with no chaps or hat and wear sandals while using a chainsaw. But I have no problem approaching a 28 inch oak that needs to be made horizontal; always with two others (one to get help, the other to take care of injuries).It is pretty easy to tell a pro from a wannabe. The pro wants to come home with all their fingers. The wannabe wants to prove he is fearless.It this offends anyone, too bad. At 71, I have to earned the right to speak my mind.

author: tggzzz
author_rank: Super Contributor
timestamp: April 16, 2024, 11:17:28 pm
content: If that offends anybody, it says lots about them and nothing about youIt is very difficult to accurately assess all possible risk/failure mechanisms. That has to be factored into plans and procedures. Unfortunately some of the reasons behind some of the more obscure procedures is.easily omitted and forgotten.Too many people think (I use that word loosely) that if they can't recognise/understand a risk, then it doesn't exist.Beginners think about how things work. Professionals think about how things fail.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 17, 2024, 12:57:09 pm
content: Quote from: Vovk_Z on April 16, 2024, 03:22:12 pmI do think that´s what I saidBasically it´s a reference to NOT using CAT 7 or similar cables where the outside "sleeve" of the connectors is metalconnected to each other and usually grounded, or directed to ground through a snubber.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 17, 2024, 01:04:00 pm
content: Quote from: pdenisowski on April 16, 2024, 01:24:15 pmIndeed. Generally HV can be an expensive hobby... one way or the other...

author: ironcurtain
author_rank: Regular Contributor
timestamp: June 20, 2024, 05:47:47 am
content: The one corner case: what about folks doing EMC testing with a LISN for AC? What do you think is the sensible approach to isolate mains from DUT to measure EMC characteristics?I have good filtering in place for the lab (two inline filters plus a workbench mounted 20db + 60db filter bank where all the test equipment is plugged). Recently got an isolation transformer for AC LISN use primarily. Say you want to do EMC testing of a connected ATX PSU, what would be your suggested approach?I think this is interesting for hamradio folks too. I was recently testing an old HF transceiver that got noise coming in through a benchtop PSU, the harmonics of the switcher peppered pretty much the entire HF spectrum. Because of the cycling/spacing it was obvious, and I changed the PSU and was able to do proper tests and measure the noise floor, but someone else (or myself on a rainy day!) could easily forget and think the DUT is busted.

author: thm_w
author_rank: Super Contributor
timestamp: June 21, 2024, 12:37:17 am
content: Quote from: wasedadoc on April 15, 2024, 06:26:54 pmInteresting but no different than using an isolation transformer. I don't see any advantage to it.With isolation transformer you can add a variac to ramp up the voltage.

author: m k
author_rank: Super Contributor
timestamp: June 23, 2024, 10:01:28 am
content: With SMPS variac can be a pain.First nothing happens and then *something* happens.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 23, 2024, 03:05:27 pm
content: Quote from: thm_w on June 21, 2024, 12:37:17 amThe advantage for me is that I have the UPS but don't have an isolating transformer.

author: johansen
author_rank: Super Contributor
timestamp: June 25, 2024, 01:14:59 am
content: Ground the lisn to your test equipment ground.. as in actual ground.Kinda defeats the point of an isolation transformer. Suppose you need an isolation transforner to make your dc - or + your ground point for measurements. Emc measurements in that case may be useless because the equipment is not functioning as it would in the wild

author: wobbly
author_rank: Regular Contributor
timestamp: July 09, 2024, 05:18:12 pm
content: I'd like to ask if someone could review the safety advice given in this video posted on this exact topic just today (1 hour ago at time of writing)...I'm not saying it's right or wrong, I'm just trying to learn. I like this guy's channel, his methodical approach has been useful on many occasions.Mains power makes me very nervous which is why I want to learn more about the safety aspects. I'd value some of your opinions on this video.Many thanks.

author: madires
author_rank: Super Contributor
timestamp: July 10, 2024, 01:12:58 pm
content: When Rich grounds the output of the isloation transformer via his lab PSU (lab PSUs usually have isolated outputs) and/or scope (probe's ground clip) the output isn't floating anymore. While probing with the scope he needs to be careful (one hand in his pocket). A better approach would be to keep the DUT floating and to use differential probes.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 11, 2024, 06:22:12 am
content: Can anyone explain why these discussions are still active?HV differential probes are easily available today - no need to ground your DUT with the scope. Measurements with HV differential probe are much more trustworthy than without because of the symmetrical input of defined impedance. So if available, I´d always use this probe for everything >100V.Isolation transformers can be an experts tool for risk mitigation but should never be seen as something that makes lethal voltages safe to touch. And at least in countries with serious mains voltage you also should never see a RCD as a personal safety device. It´s intended to switch off defect appliances, and also not to make live wires touchable. Especially if you are working with larger SMPS then the charge in the main buffer capacitor alone can really badly harm you.If you want to work with these devices:- Earth all equipment that has an PE-connection- Fix your test leads before power up so that you don't need to take your hands into the danger zone- Solder a small neon to the mains cap - just as a warning indicator- Use high impedance probes without any ground wiring leading to the DUT- Use an isolation transformer to prevent ground loops, as an EMI-filter or whatever. But never as a single point of operator insulation!What also many people miss in my opinion: The isolation transformer only isolates very low frequencies like DC and mains power. Its impedance for SMPS-like frequencies is much lower. That means, if you insulate your DUT with such a transformer, and connect scope GND to SMPS primary ground then you not only have a very serious safety risk, then you also have an additional ground loop that´s not present in normal operation. And this also spoils your measurement results. Things get even much worse if you connect scope GND not only to SMS primary ground but even e.g. to some point in the snubber network.Sorry if I repeat things anyone in this field should know. But experience shows many people believe it´s worth to measure with a risky setup. And in 99.9% of the cases it´s just pain no gain.

author: tggzzz
author_rank: Super Contributor
timestamp: July 11, 2024, 08:58:36 am
content: Quote from: Phil1977 on July 11, 2024, 06:22:12 amFirstly because there is no shortage of new inexperienced people. They, just like children, will make old mistakes. While everybody was young and inexperienced at one time, the sane ones try to avoid making old mistakes.Secondly because there is no shortage of people confidently stating the equivalent of "I walk into the road without looking and haven't come to any harm, so that means you can too".There's an English proverb "A little knowledge is a dangerous thing". I expect there is the equivalent in most languages. especially on social media and doubly so on yooootooob.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 11, 2024, 09:38:16 am
content: It´s of no shame to have little experience - but many of the stories and social media contributions demonstrate that it would be of any necessity or advantage to measure that way. And that´s what is indeed annoying, that people show off by doing something stupid while they are claiming to spread knowledge.

author: magic
author_rank: Super Contributor
timestamp: July 11, 2024, 10:11:07 am
content: Quote from: Phil1977 on July 11, 2024, 06:22:12 amThis only works if source impedances are matched, or at least close enough.Example: connect the negative to a hot ground and the positive to a 10kΩ voltage divider. Positive rise time for common mode signals increases by 100ns assuming 5pF probe input capacitance, negative remains unaffected.

author: m k
author_rank: Super Contributor
timestamp: July 11, 2024, 11:28:08 am
content: At certain age mom doesn't have an authority anymore.One scientist has always wondered how pre-teen boys stay alive.Internet time is not new anymore, so young adults are net natives.Maybe it takes few generations.Maybe arrogance and immortality of young adult mind prevents learning from books forever.Maybe knowledge of practices just don't materialize without practical experience.No practice no skill, it's a given.But a skill of don't do.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 11, 2024, 11:46:56 am
content: Quote from: magic on July 11, 2024, 10:11:07 amBut it´s still better than just bridging a huge unknown impedance to the negative side as you do by connecting a scope-GND isolated by a mains transformer. There´s no probe on the world that does not influence the circuit where its connected. But you can keep the disturbance low by using the right tools.