Floated scope: how to verify if no damage has been done? - Page 1
Forum section: electronics / testgear

author: analias
author_rank: Contributor
timestamp: July 03, 2018, 02:07:22 pm
content: In a shared environment I discovered a 1980s Tek scope that was plugged into the mains via a cable marked "earth disconnected". In other words, somebody had floated the scope - and then left it as a boobytrap for the next poor sod.I have two questions...Firstly, what internal damage might have occurred? I can think of:transformer insulation. A Tek note indicates that can result in future electric shock, even when it is no longer floatedmains filter X/Y caps (think RIFA!) and potentially smoothing caps. That could result in fire and smoke damageother components near the SMPSare there any other types of damage?Secondly, what is (a) necessary and (b) sufficient to find any damage, and preferably determine none has occurred? I can think of:visual inspection of the capacitors and "nearby" component. Certainly necessary, probably sufficient given that the risk is merely firePAT testing the mains input and transformer; if there are multiple "classes" of test, which is appropriate? Presumably necessary, but I don't know that it is sufficient given that electric shock is a riskare there any other types of test?Unfortunately we know neither the purpose for which it was floated, nor the "floating" voltage. Hence that cannot be used to help assess any damage done to the scope.Please note I'mnot interestedin a discussion as to whether or not "floating a scope" is safe/normal/dangerous, since that's been done to death (ho ho) in other threads, e.g.https://www.eevblog.com/forum/testgear/floating-scopes/msg1238945/#msg1238945

author: The Doktor
author_rank: Regular Contributor
timestamp: July 03, 2018, 02:37:21 pm
content: Plug it into a GFCI protected outlet. If it don't trip, it's fine.

author: analias
author_rank: Contributor
timestamp: July 03, 2018, 11:19:13 pm
content: Quote from: The Doktor on July 03, 2018, 02:37:21 pmWhich failure modes/mechanisms does that apply to, and why?I rather doubt it would check a dodgy RIFA capacitor!

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2018, 11:21:20 pm
content: Better advice may come if you tell us the model #.

author: james_s
author_rank: Super Contributor
timestamp: July 03, 2018, 11:37:21 pm
content: Why would you expect it to be damaged? Floating it isn't really advisable technique but it shouldn't damage the scope. It's not a bad idea to test any equipment for electrical leakage but other than that just use it.

author: CopperCone
author_rank: Super Contributor
timestamp: July 04, 2018, 12:24:20 am
content: Maybe they mean if you don't have the filter the transformer can be damaged by fast transients a bit? You are floating the capacitors in the filter, so you basically have small series inductance infront of the transformer primary. Not sure what they can do.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 07:58:41 am
content: Quote from: tautech on July 03, 2018, 11:21:20 pmProbably not, since I'm after information about the damage that can be done to a generic scope. While this forum is better than many, there is a tendency to descend into minutiae and miss the big picture.Having said that, it is probably relevant to mention that it is an SMPS PSU rather than a linear PSU, from the 24xxA series.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 08:12:04 am
content: Quote from: james_s on July 03, 2018, 11:37:21 pmThe manufacturer states that scopes in general can be damaged by floating. Basically the "floating" voltage is added to the voltage applied to the transformer. Look at a generic scope's circuit diagram, and consider what the voltage differences are if the scope's "ground" is at, to pick a large value, 1kV relative to the mains ground.Excerpt from Tektronix' "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes Application Note"https://info.tek.com/www-fundamentals-of-floating-measurements-and-isolated-input-oscilloscopes_ty.htmlQuote from: TektronixIn addition it is conceivable that other components may be stressed, but that is likely to be scope specific.QuoteDo you have any specific tests in mind?What qualifications do you have to make such a statement?

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 08:14:54 am
content: Quote from: analias on July 03, 2018, 02:07:22 pmProbably none. The usual justification for floating a 'scope isn't to expose it to much higher voltages than normal, it's just so that they can put the ground clip wherever they like.Quote from: analias on July 03, 2018, 02:07:22 pmSwitch it on and leave it for a day. If it appears to work OK then it probablyisOK.Quote from: analias on July 03, 2018, 02:07:22 pmI don't think that you can*garantee*that, not without putting in more work that the 'scope is worth.This scope is just as likely to be dangerous through age as it is to be dangerous through being floated.Quote from: analias on July 03, 2018, 02:07:22 pmYou should be doing that anyway if it's from the 1980s: Visually inspect all the caps and check the voltages at all the test points.Some people would be replacing all the caps anyway, simply because of age.If you want to replace the X/Y caps out of paranoia then fair enough. It's cheap/easy to do.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 08:15:30 am
content: Quote from: CopperCone on July 04, 2018, 12:24:20 amThe principal reason for the X/Y filter caps is to prevent EMI escapingfromthe scope's SMPS back into the mains lead, not to protect the SMPS.QuoteCan you explain that in more detail?

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 08:30:12 am
content: Quote from: Fungus on July 04, 2018, 08:14:54 amIndeed, but who knows why this scope was floated? Perhaps they were testing an off-line SMPS.QuoteIt is a shared environment with relatively inexperienced people. I don't care too much about the scope, but some people are (or ought to be) concerned about thenextuser of the scope.I'm certainly not in that location to babysit it for that kind of time. I can't guarantee anybody else is either.QuoteWhile no guarantees are possible, it it prudent todemonstrablytakeappropriateprecautions. I'm interested in what those precautions should be.While the mains caps could be problematic and cause smoke/fire, that is unlikely to be lethal and can probably be dealt with even by non-expert users.I see no reason why a Tektronix scope in good cosmetic condition is likely to be a shock hazard, unless it has been overstressed.QuoteIt isn't my scope and I have neither the time nor inclination. There are very few people in that environment with the skill to replace caps; I can't presume their inclination.

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 08:35:35 am
content: Quote from: analias on July 04, 2018, 08:12:04 amThis is all true in theory, but unlikely in practice. If the previous users were playing with high voltages they'd be more likely to kill it outright than to leave it in that state.If you're as risk-averse as seem to be implying then you shouldn't be usinganytest gear with an undocumented history, and probably no 1980s test gear at all.PS: If you want to be 99% sure that the above hasn't happened to your transformer then the only way is to pull it out of the power supply and attach it to an insulation tester.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 08:45:24 am
content: Quote from: Fungus on July 04, 2018, 08:35:35 amActually most people think that I do things which indicate I'm not adverse tocalculatedrisks. However whatIdoto myselfis my business, whereas I would get rather annoyed ifsomeone elsedid itto me.QuoteI don't know whether that is possible, nor whether it would be sufficient. It is almost certainly "not economic".Which in-situ tests, e.g. via the mains lead, might be sufficient?If the scope is damaged by such a test, it is preferable to a user being damaged by the scope.

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 08:51:48 am
content: Quote from: analias on July 04, 2018, 08:30:12 amThe only sensible tests would be:* Inspect caps, check voltages at test points.* Switch it on for a day or so and see if anything bad happens (put it in a place with a concrete floor and nothing around it).While the mains caps could be problematic and cause smoke/fire, that is unlikely to be lethal and can probably be dealt with even by non-expert users.Quote from: analias on July 04, 2018, 08:30:12 amEven if it's been overstressed it's unlikely to a shock hazard. All the external metal parts are grounded.Quote from: analias on July 04, 2018, 08:30:12 amIn that case it's a simply a game of chance.But... if it powers on OK and survives the first week then it's a game of chance with a high probability of winning, IMHO.

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 08:56:13 am
content: Quote from: analias on July 04, 2018, 08:45:24 amThis is the real point being made (I think).Quote from: analias on July 04, 2018, 08:45:24 ama) Open it up, make sure that no critters made any nests inside and that no caps are obviously bulging/leaking.b) Power it on for a day, sniff it before/during/after. If the smell gets worse then be suspicious of it.c) Check voltages at test points towards the end of test (b).If you have the manual then it might have a list of general diagnostic tests.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 09:01:04 am
content: Quote from: Fungus on July 04, 2018, 08:51:48 amHey, I said that too. We are in agreementQuoteExcept when they aren'tMaybe some other idiot "that knows better" will float it again.QuoteYeah but...What happens if there is a "problem" and the police or insurance company investigates? A documentary chain of "reasonable precautions" would be helpful ammunition.Such "reasonable precautions", preferably with justifications, are what I'm after.

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 09:03:00 am
content: Quote from: analias on July 04, 2018, 09:01:04 amIf that's a genuine worry then don't use it.Quote from: analias on July 04, 2018, 09:01:04 amPut a big sticker on it saying "Do not float!", that's yourresponsibilitydealt with.As for physical safety: You can't protect yourself from these people, they can float a new device just as easily as an old one.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: July 04, 2018, 09:14:20 am
content: Quote from: analias on July 04, 2018, 08:12:04 amPlug it in, turn it on, float the earth, and then with the tongue if it tastes salty it's alright.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 09:32:03 am
content: Quote from: Fungus on July 04, 2018, 09:03:00 amThere was (and is) such a signThe issue is what to do now that it has been floated

author: tautech
author_rank: Super Contributor
timestamp: July 04, 2018, 10:19:57 am
content: Quote from: analias on July 04, 2018, 07:58:41 amFrom that era floating scopes was a common occurrence by those that knew how and manufacturers allowed for this now frowned upon usage in their designs.Other than insulation tests on the SMPS transformer (removed of course) it's certainly a good question you ask.Personally, if the scope operates correctly I'd reconnect the mains Gnd and just use it ......after a burn in for piece of minds sake.

author: Gyro
author_rank: Super Contributor
timestamp: July 04, 2018, 12:48:32 pm
content: Quote from: tautech on July 04, 2018, 10:19:57 amGet it PAT tested, that will reveal any abnormal leakages. In a commercial environment it should be done anyway, especially if its mains lead has been messed with. That's theproperthing to do.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 01:11:37 pm
content: Quote from: tautech on July 04, 2018, 10:19:57 amPerhaps for valve scopes, but I'm unconvinced that was true for 1980s SMPS/transistorised scopes.Even in the 70s I knew that floating a scope was To Be Avoided.QuoteUnfortunately it isn't just "personal".It is a shared environment, and "ingenious fools"arepresent => it is difficult to make it foolproof.Regrettably I think that Electro Detective's reply 86 inhttps://www.eevblog.com/forum/testgear/floating-scopes/is the appropriate attitude, including ..."I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodderThere are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'which usually means a good chance for an earth/ground related BANG! "

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 02:37:03 pm
content: Quote from: Gyro on July 04, 2018, 12:48:32 pmThat's certainly my gut feeling. I think it is probably necessary to PAT test, but are there other beneficial tests?The Tektronix statement I noted earlier, plus high voltages rapidly spreading in the event of a fault, raises concerns.

author: mtdoc
author_rank: Super Contributor
timestamp: July 04, 2018, 03:06:06 pm
content: I get the concern here but with the many thousands of used analog scopes out there, many bought and sold repeatedly (via eBay and other venues), cycled through several users - surely many have been floated or subject to worse abuse. Many are now sitting on shared benches, hacker spaces, schools, etc and used regularly by novices. Is this really such danger once the obvious defects revealed by inspection and a trial burn in has been done?

author: ebastler
author_rank: Super Contributor
timestamp: July 04, 2018, 03:22:11 pm
content: This whole discussion seems a bit obsessive to me. Why are you so particularly concerned with the specific potential effect of the scope having been floated? In any scope with a 30 year history, who knows what kind of abuse, bad line voltage, or just degradation of critical components it may have suffered? If you encounter any scope that old in a multi-user environment, where you feel repsonsible for safety of all users (a classroom setting maybe?), wouldn't you have to decomission it?@analias, I was wondering -- in your user name, what does the "ias" stand for?Sorry, couldn't resist...

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 04:48:19 pm
content: Quote from: mtdoc on July 04, 2018, 03:06:06 pmThat's a reasonable question.The issues are related to"am I my brother's keeper" if I (or others) know there is something that might harm them. To put more of an emotional twist on it, replace "brother" with "child""plausible deniability" in the event the worst happensI don't have a neat answer to those, unfortunately.Neither do I know what I would do if someone's knowing negligence killed one of my children. Sh1t happens and has to be accepted, but knowingly putting others at risk is different.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 04:57:57 pm
content: Quote from: ebastler on July 04, 2018, 03:22:11 pmSee my previous response.QuoteReasonable questions.I'm not responsible, other than in the unsatisfactory "safety is everybody's responsibility" mantra - which isn't taken too seriously in that environment. Ownership is unclear.It may be that decommissioning is the thing to do, but I would like to avoid that if possible. The floating probably hasn't done it any damage, but nobody has indicated the "floating" voltage so we simply don't know.

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 05:13:36 pm
content: Quote from: analias on July 04, 2018, 04:48:19 pmNeither does anybody here.I think the "floating" thing is a red herring. Almost all the risk of a floating device is to the users, not the device.Using this device is really no different to usinganyold, undocumented piece of test gear. If the chassis is grounded then there's almost zero risk of electric shock from it.Childrenmightpoke wires through the holes in the case, yes, and an old CRO has higher voltages inside it than a modern DSO but that'snothingto do with it having been floated (or otherwise) in the past. I wouldn't place any real emphasis on that. This should be viewed for what it is - an old CRO. The biggest risk is from dried up capacitors and cobwebs inside it.

author: Fungus
author_rank: Super Contributor
timestamp: July 04, 2018, 05:17:59 pm
content: Quote from: analias on July 04, 2018, 04:57:57 pmIt may not have been floated for "high voltage".A lot of people floated them simply to avoid accidentally shorting things with the ground clip while working.

author: Simon
author_rank: Unknown
timestamp: July 04, 2018, 05:21:19 pm
content: Erm someone floated it and made it clear it was floating by labelling the socket. As such it could be dangerous to persons but there is no reason it is damaged.

author: mtdoc
author_rank: Super Contributor
timestamp: July 04, 2018, 05:29:02 pm
content: Quote from: Fungus on July 04, 2018, 05:13:36 pmI agree. Visual inspection,check chassis grounding, then power it up and watch it for a while. If all is ok, then spend efforts on proper safety instructions for novice users and some relevant signage perhaps. That’s were the safety payoff is, not in worrying about theoretical but extremely unlikely events related to past use history.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2018, 05:37:33 pm
content: I don't understand why people obsess over ridiculous specific hypothetical things like this. Floating the scope won't damage it, it's risky to do because the whole scope is live and you could get a shock while the scope is floating, not because it could damage the scope itself. This reminds me a bit about the thread where the guy was so paranoid about the possibility of a lithium ion battery catching fire that he was talking about incorporating a pyrotechnic ejection charge. It borders on mental illness.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 07:28:15 pm
content: Quote from: james_s on July 04, 2018, 05:37:33 pmI accept thatyoudon't understand it. However, your assertion is false; see my reply #7 to your earlier assertion.Do you have any response to the questions I posed at the end reply #7?QuoteSuch analogies generate more heat than light, and aren't worth discussing.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 07:38:56 pm
content: Quote from: Simon on July 04, 2018, 05:21:19 pmThe immediate risk is to anyone using a floating scope; there's no disagreement there (in this thread at least!).However, I indicated in response #7 how the scope can suffer latent damage.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 07:46:50 pm
content: Quote from: mtdoc on July 04, 2018, 05:29:02 pmUnfortunately there was signage, which was ignored.Unfortunately currently instruction is patchy, and instructing novices is frowned upon - but this may be a stimulus to get that changed.Neither instructions nor signs will help with people thatthinkeither it isn't dangerous or they know how to do it safelySince resolution of those real and valid people-related issues cannot usefully be discussed here, I chose (and choose) to concentrate on a specific technical issue.

author: tautech
author_rank: Super Contributor
timestamp: July 04, 2018, 08:33:06 pm
content: Quote from: analias on July 04, 2018, 01:11:37 pmQuite so, ED's reply is the 'modern' philosophy in today's litigation mad society where practices like floating scopes could never be taught for fear of culpability. It was by necessity back then as the vast bulk of scopes didn't have differential plugins nor were differential probes as widely available as they are now.What I would offer as comment is a Tek 24** wasn't a cheap scope back then and any that would set it up as a dedicated floating scope would've known the risks to both themselves and instrument.The likely extreme voltages it ever saw in floating mode would most likely be just rectified mains of which it could handle with comparative ease.I get that if this scope is to be shared, culpability again rises it's ugly head where really any use of a scope and in fact any test gear exposes the operator to risk.If this is too much to bear along with usage of a scope with uncertain history.....then buy a new scope !

author: rstofer
author_rank: Super Contributor
timestamp: July 04, 2018, 08:52:55 pm
content: In the end, I think future failure modes are unknown and can't possibly be known. How do I know my brand new scope won't self-destruct? It's a Rigol, after all... I have no idea what was done to my Tek 485 in the years before I bought it. Floated? Not floated? Over-ranged? I don't know. Nor do I care, I just plugged it in and have used it for the last 14 years. Used it yesterday and it works fine!If the risk is perceived to be high, junk the scope. Personally, I would replace the cord to ensure that the chassis and BNC connectors are grounded and put it back in service. I can't possibly predict some inter-winding anomaly and I doubt that I would worry so long as all exposed non-current carrying metallic surfaces were grounded. Even if the scope went into hand grenade mode, I doubt the damage would be all that extensive. A few pops, a few sizzles and that would be about it.But in a group environment I wouldn't be so cavalier - I would let somebody else buy a different scope. I wouldn't want to be involved with the purchase either. Not my circus, not my monkeys!

author: 17_29bis
author_rank: Regular Contributor
timestamp: July 04, 2018, 09:52:18 pm
content: I have been reading this topic and silently wondering how far this thoughtful investigation about possible damages because the ground was disconnected would go. I am trying to understand the magnitude of this problem and somehow all what I see or can explain as a former el. engineer is that it is mostly a liability related issue (versus common sense) than anything else.Just think for a second - in recent past many countries (like Soviet Union for example) did not have 3 pole plugs at all and it did not prevent them from creating, debugging, manufacturing, using different electronic devices to allow people to go to space or to the bottom of the ocean. I don't want to switch the topic and start a discussion whether or not my example covers all bases, but I think you got my point.And when I think about liability issues in North America (let me be politically correct, since not all countries in North America are the same) I think that cases like Liebeck v. McDonald's Restaurants, when a 79-year-old woman who suffered third-degree burns in her pelvic region when she accidentally spilled hot coffee in her lap after purchasing it from a McDonald's restaurant was awarded $640,000 ( at first the amount was 2.8 million but later was reduced to $640,000). And this list is huge. And it even includes some urban legends like "Winnebago driver case" when the driver set the cruise control at 70 and went into the back to make coffee and later sued Winnebago for not advising him in the owner's manual that he couldn't actually do this ;-(So is it really a technical problem being discussed here or something else?PS: edited 10 minutes later - even my MacBook pro has two adapters - 2 and 3 pronged plugs (N.A. style) And whatever you connect through USB (a scope, a logic analyzer etc ) to your laptop will be floating too in case of 2 pronged plug. What is a big deal here?

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 09:58:36 pm
content: Quote from: tautech on July 04, 2018, 08:33:06 pmAch. I don't think ED's statement supports the contention that the fear of culpability is a central issue. His "conversion" seems to be based on a realisation of the reality that there are subtle dangers that cannot be quickly and easily taught to newbies.That floating was done in the past is, as you say, because the equipment wasn't (widely) available.QuoteISTR an anecdote on TekScopes about someone that realised it was dangerous, took precautions - and still died.QuoteThat could add 350V (240*sqrt(2)) across some components.QuoteThe "it might hurt someone else" is the key consideration, as opposed to the Darwin Award.If someone is prepared to float an old scope, there's nothing preventing them floating a new scope - but that's outside the scope of this thread.Ways of increasing confidence in lack-of-danger is the topic of this thread.

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 10:03:53 pm
content: Quote from: rstofer on July 04, 2018, 08:52:55 pmThose were my initial thoughts, until I did some research and came across the Tek statement. That raised doubts that myinaction might result in a newbie being hurt.QuoteQuite; that's part of the dilemma

author: analias
author_rank: Contributor
timestamp: July 04, 2018, 10:08:57 pm
content: Quote from: 17_29bis on July 04, 2018, 09:52:18 pmI've tried to frame it as a technical problem which can, hopefully, be resolved technically.There are indeed non-technical issues that also need to be addressed. The technical issue may be a good way to force other people to deal with the non-technical issues - but that's a different argument.

author: tautech
author_rank: Super Contributor
timestamp: July 04, 2018, 10:36:24 pm
content: Quote from: analias on July 04, 2018, 09:58:36 pmYes but if any of us on the forum suggest floating scopes, aren't we in some way culpable ?The dangers aren't subtle, they're deadly !QuoteQuote from: analias on July 04, 2018, 09:58:36 pmAs per above and user error cannot be overlooked.Drive a car at highway speeds on back roads without a seatbelt and the result can be similar.QuoteQuote from: analias on July 04, 2018, 09:58:36 pmExactly but it's an offset that the whole scope is subjected to not so much a stress on individual componentry across the whole scope. The PSU is where the greatest stress will be.QuoteQuote from: analias on July 04, 2018, 09:58:36 pmThe safest move is to restore the mains Gnd, period. That assumes the supply ground is sound and in perfect order.Quote from: analias on July 04, 2018, 10:03:53 pmAnd actually how old is that Tek statement ?You can be quite sure their documentation has been updated to make users fully aware of the risks of floating scopes so not to expose themselves to today's litigation mad world.Many Tek documents that were once on their website are now gone without trace unless you can find them stashed on another website.

author: mtdoc
author_rank: Super Contributor
timestamp: July 04, 2018, 11:00:01 pm
content: Quote from: analias on July 04, 2018, 10:08:57 pmThere is no technical resolution. As stated, there are basic checks that can be done on any old piece of test equipment. Beyond that, common sense measures like GFCI outlets and basic safety rules is all you can do. If that doesn’t satisfy your safety and litigation fears, then removing yourself from the environment (or position of responsibility) is likely the only way to resolve your concerns.

author: EEVblog
author_rank: Unknown
timestamp: July 05, 2018, 01:16:27 am
content: MODERATOR:User analias has been banned for operating multiple accounts.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2018, 01:19:48 am
content: I just now noticed what the OP's username is. I wonder why?