Probe Oscilloscope Ground Blow up issues USB - Page 1
Forum section: electronics / beginners

author: rajiv85
author_rank: Contributor
timestamp: July 10, 2017, 09:37:19 am
content: Ok guys I am new to electronics and took special attention to this grounding problem which can result in blowing up the instrument or injuring myself.I watched all the videos available on utube on this.However am still not clear about a range of issues. And in my view things are still not CLEARLY explained. It is a lot about what not to do.. but not a lot about what to do FOR SURE. Too many approximate 'you might' you could.. maybes.. all these leave me with a lot of confusion.1. I ask myself: why does not oscilloscope not be designed in a way that.. it is itself completely protected. Let us say.. I use some kind of capacitor or small internal battery to pass a small fixed current to a small fixed value resistor that will give me a fixed voltage drop. Let us say this value is 0.1 volts. I then use this as my reference and digitally substract that value from my measurements. My idea is to create a reference called ground without needing an actual ground within the oscilloscope. >> The device itself designed in such way that there is no connection to ground.Why is something like this not done?2. I still do not understand what is the absolute proper way to do the measurements without the risk of blowing the instruments.For example: one possibility is to use math function and use same ground==> please note that the DAVE video does not explain this.. which goes to the point that I mention that they all tell you to avoid the ghost.. but dont explain to you how not to create it in the first place.3. Please see attached image. What if my scope is rated at 300 V. Does this still pose a problem? How do I know what is the voltage limit of a probe? ( As I clearly see that my scope has 400 V limit).4. What about USB oscilloscopes? I am connecting the scope via USB, not the test equipment. So?5. Are there issues with the function generator? I apply a function generator to my circuit.. now am really confused. Where do I put this probe to avoid any risk of blowing things up? Where do I put the ground.Let us say I want to measure the voltage out accross a resistor.6. I dont understand this whole isolate your test equipment thing. I see one case for an actual test equipment machine. One for an arduino circuit ( still dont know how he connected that isolation ring/coil.. as the cables mysteriously dissapear at the back( utube video).7. Why do some people tell you: not to isolate your oscilloscope.. and some tell you to isolate it..??! If there is no proper consensus among experts on this.. as a beginner it is confusing.8. If I connect two separate DC power supplies.. for DC biasing in transistor circuits.. what are the risks?

author: Electro Detective
author_rank: Super Contributor
timestamp: July 10, 2017, 09:46:09 am
content: This gentleman has amust watchvideo that should answer many of your questions and concerns

author: rajiv85
author_rank: Contributor
timestamp: July 10, 2017, 10:02:47 am
content: Thnx. But if you read my question well you will see that I did check all the videos on this matter.

author: madires
author_rank: Super Contributor
timestamp: July 10, 2017, 01:27:51 pm
content: Quote from: rajiv85 on July 10, 2017, 09:37:19 amI assume you've meant earth/PE by ground. You can get scopes with floating inputs (like a DMM) or differential probes.Quote from: rajiv85 on July 10, 2017, 09:37:19 amDifferential probes. But based on what you like to measure or see differential probes can screw up the signal. It's a tradeoff.Quote from: rajiv85 on July 10, 2017, 09:37:19 amYes. One problem is the short created by the scope's (also probe's) ground which is connected to earth/PE via the mains socket. And the other one is that any metal part of the scope connected to ground and accessible by you is hot, 230V AC in your case.Quote from: rajiv85 on July 10, 2017, 09:37:19 amWould create a short via the PC which got a connection to earth/PE. Same problem, just larger loop.Quote from: rajiv85 on July 10, 2017, 09:37:19 amIs the output of your function gen floating or referenced to earth/PE?Quote from: rajiv85 on July 10, 2017, 09:37:19 amSorry, I don't know which video you're talking about.Quote from: rajiv85 on July 10, 2017, 09:37:19 amIf you're a beginner don't float your scope. If you're an expert you shoudn't either, but you'll know the risks and how to deal with them.Quote from: rajiv85 on July 10, 2017, 09:37:19 amThe only important thing is to connect the ground clips to the same potential when using multiple channels/probes. Otherwise you could create a short via the ground clips. This assumes the PSUs are isolated and low voltage.

author: bson
author_rank: Unknown
timestamp: July 10, 2017, 09:45:53 pm
content: When you attach the ground clip at that point, this is what you get. You're shorting to ground, because the ground clip is, well, grounded.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 11, 2017, 10:06:01 am
content: Quote from: rajiv85 on July 10, 2017, 10:02:47 amMy apologies, though I did read your post thoroughly, and figured that video link was a good starting point to answer most of your concernsThere are many more videos that explain it too.In short, most standard oscilloscopes are not like the standard two probe battery powered multimeters that are isolated/floatingwith no reference to the earth/ground of your house electricity (usually connected to Neutral, aka AC Line 2)Buying an expensive oscilloscope with separate isolated inputs is similar in use to a multimeterA differential probe is a cheaper alternative and works with most standard oscilloscopesIt cost me a trashed oscilloscope to learn this stuff 20 years ago when there was no internet/youtube/google , or properly documented tech warnings,so please be carefulThose crocodile clips can bite HARD if you let them

author: Codebird
author_rank: Regular Contributor
timestamp: July 11, 2017, 10:22:02 am
content: We've all done this. There's a video on youtube of me tripping the house RCD three times trying to take a simple measurement.I later learned that the 'isolation' transformer I'd purchased off of eBay was actually an autotransformer.

author: dmills
author_rank: Super Contributor
timestamp: July 11, 2017, 10:37:05 am
content: It is somewhat strange to me how often this seems to come up around here.I have **never** in a 25 year career across many different industries, needed to float a scope, and frankly don't think it would occur to me as being the way to do this.If I saw one of the guys in out workshop here doing this, QUESTIONS would be asked, and there would probably be swearing, because you basically never need to do it (And I have a perfectly good high voltage differential probe for that once in a blue moon thing).Scope ground clips are connected to the scope chassis ground (And this to each other and to mains the safety bonding conductor), in much the same way that the output on most signal generators is ground referenced (You can blow them up the same way), these are not multimeters (Battery powered, entirely inside a closed plastic box).Ways to make such measurements (one the once in a blue moon you need to):Differential probe (of suitable CAT rating).Isolating transformer to float the device under test, NOT the scope, note that the right thing to do with the safety bonding connection on the output side of the transformer is highly situation dependent and I would always use an RCD on the OUTPUT of the transformer.Math mode (With two high voltage probes, of suitable CAT rating, and with the ground clips treated appropriately. This may or may not be workable depending on the details of how your circuit under test is referenced).Battery powered scope with all plastic case (Fluke Scopemeter or the like), no good for anything fast and differential, as you end up adding way too much cap to the signal you have connected to the 'ground' clip, but useful for things like having a look at power harmonic currents.In no case should such work be done on your own, and almost the only place you should need to work live with a scope is when fixing CRT tellies or switchmode supplies, which is frankly not beginner territory IMHO.A beginner should NOT be working with anything that needs this sort of thing, learn your trade with small bench power supplies or batteries (NOT Rechargeable ones they can source proper current), not the mains, you will smoke test less expensive things.73 Dan.

author: tautech
author_rank: Super Contributor
timestamp: July 11, 2017, 10:45:29 am
content: Dan, it comes from a greater awareness of advanced scope use andsomeknowledge of the traps that come with it.A recent thread that should be liked here:https://www.eevblog.com/forum/testgear/floating-scopes/

author: dmills
author_rank: Super Contributor
timestamp: July 11, 2017, 11:43:53 am
content: That's probably fair, but these techniques are very much specialist and should be something only rolled out when you really need them and fully understand the risks, I get the feeling that some people think they are very much more commonplace then they really are.As I say, I have never felt the need to float a scope, and I am not exactly a stranger to those instruments.It is difficult and expensive to make a fully isolated multi channel scope with say 2GHz of bandwidth and reasonably fast triggering that extends down to DC, it can be done of course, but not cheaply without compromising all sorts of other things. At the lower frequency end, Fluke make kit that is designed for this sort of thing, but most people do not need that often enough to make owning it worthwhile.Given a choice between isolation, and say another channel, or more bandwidth or better triggering options, for a bench scope the isolation can go as far as I am concerned, the other stuff is far more useful in every day use.Regards, Dan.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2017, 01:56:37 pm
content: Quote from: rajiv85 on July 10, 2017, 09:37:19 amThis feature exists in the form of an offset control which adds or subtracts a DC reference from the input. On analog oscilloscopes the vertical position control operates as a limited offset control but this is less common on DSOs which may have separate position and offset controls. Some older oscilloscopes took this capability to an extreme allowing 100s of volts of offset adjustment. For instance a 7A13 vertical amplifier can have a 1mV/div sensitivity anywhere within +/-10 volts of ground through its offset capability; it is like having a vertical position range of 20,000 divisions. At 10mV/div this extends to +/-100 volts. At 100mV/div this extends to +/-400 volts which is the maximum rated voltage for its inputs.Two things prevent an offset control from being an effective solution for isolated measurements; the difference between the oscilloscope ground and the ground for the device under test may not be stable enough and a short ground connection is still needed for good high frequency performance.QuoteThe proper and safe way is to use a differential probe, make a differential measurement using two channels, or use an isolated probe or input.QuoteThe 400 volt limit is for the signal input and has nothing to do with the ground connection. The ground connection effectively has a 0 volt limit because it is grounded and it is only meant to be connected to ground.Oscilloscopes with isolated inputs remove the restriction on ground for each input and for them, each signal ground may be connected as required within the specified voltage rating which is usually 400 volts or more.QuoteUSB oscilloscopes are even more dangerous because they can have multiple ground connections other than the signal inputs. One will normally be through the power supply and the other will be through the USB connection.QuoteAlmost all function and signal generators have the same limitation as far as ground. The exceptions should have documentation saying so and advising of their limitations.Unlike most oscilloscopes, bench multimeters all have floating inputs or at least they better have. Power supplies intended for design and development usually have floating outputs.QuoteIf using a standard oscilloscope, then the resistor needs to be floating. You might think that having one side of the resistor sharing a ground with the oscilloscope would be enough but this is actually not the case because then connecting the oscilloscope ground will likely cause a ground loop. A low current ground loop is not dangerous but will often corrupt the measurement.QuoteIf necessary, I almost always isolate the device under test instead of the test instrument.QuoteSometimes do to limitations in available equipment, isolating the test instrument is the only way to make a measurement. It is rare but sometimes a measurement cannot be made or made easily without an isolated oscilloscope although in this case an oscilloscope with isolated inputs should always be acceptable but that just leads back not having the proper equipment. Some people are willing to accept the hazards of isolating a test instrument from ground for greater convenience.QuoteIf the power supplies do not have floating outputs, then you risk creating a ground loop which may be serious.Power supplies which have floating outputs still have 10s to 100s of picofarads of capacitance between their outputs and ground so they are only floating at DC. Usually their isolation is only rated to a relatively low voltage like 40 volts from ground.

author: rstofer
author_rank: Super Contributor
timestamp: July 11, 2017, 02:03:46 pm
content: Before you decide to connect the probe ground clip to some portion of a circuit, maybe it would be a good idea to measure that point to an actual earthed ground with a DMM. An easy place to get at is the scope BNC connectors.Know your circuit! Should the place you are attaching the ground clip actually be grounded? For the vast majority of hobby electronics, this simply isn't an issue; you know where ground is. It's the oddball stuff that will get you; SMPS, TVs, etc.Using the Math function is a decent way to make differential measurements. Differential probes are even better.In my little corner of the sandbox, I work on 12V and lower, usually much lower, like 3.3V. All of my projects are fault current limited by the USB connection, wall wort or batteries. I like it here!

author: dmills
author_rank: Super Contributor
timestamp: July 11, 2017, 02:16:36 pm
content: Yea, but that 2.5W USB power limit is somewhat limiting, and there are times when nothing but hundreds of amps of three phase will do.It all comes down in the end to knowing your tools and understanding their limitations and the trade offs involved in working around those limitations. Some of those workarounds raise the risk of killing yourself and people take various different views of what level of risk is acceptable, which is why you see little consensus.What does seem to confuse is that when people talk about this stuff they act as if it is a commonplace thing to need to do, and it (for most engineering, most of the time) just is not. I have made it 25 years into an engineering career without ever having done it (Yes, I know how to, but have never felt the need), and that has included some fairly heavy power engineering upon occasion.Regards, Dan.

author: rstofer
author_rank: Super Contributor
timestamp: July 11, 2017, 04:14:03 pm
content: Quote from: dmills on July 11, 2017, 02:16:36 pmI made an entire career out of high power and industrial power systems. In my retirement, I'll just keep it light!I agree, I can't recall ever wanting to scope something where it required a floating input. Of course, I can probably count on two fingers the number of times I scoped an industrial power device. But if it was necessary, the company would have bought the proper probes.Really, all floating the scope does is show that you don't have the right tools.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2017, 04:57:52 pm
content: Quote from: dmills on July 11, 2017, 02:16:36 pmAnother thing which does not help is the terms. A single ended input or output has a ground return orcommonwhich may or may not be connected to earth ground. Most differential inputs or outputs do also including differential probes. (1) In US power distribution, the common is called neutral. On the Tektronix PS503A power supplies that I like, the floating outputs include an explicitly labeled common and the non-isolated +5 volt output is explicitly shown connected to ground. On most of my bench multimeters, the isolated input's common is labeled LO or LOW.(1) It is a bad idea to connect a battery powered differential probe to an isolated oscilloscope input and reputable manufacturers warn against doing this. Doing so allows the isolated input to charge to the common mode input voltage which may be considerable and the differential probe does not protect the user from this without a connection to earth ground which the isolated input does not provide.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 12, 2017, 12:47:06 am
content: Perhaps ElectroBOOM would like to treat his loyal viewers to a Youtube feastDemonstrating all the possible Oscilloscope Ground Blow Up issuesForum posts can only go so far it seems

author: Scottjd
author_rank: Frequent Contributor
timestamp: July 12, 2017, 01:30:30 am
content: Gentlemen and Ladies,I stopped reading this thread at post 10. Why? Because as someone mentioned it is amazing how often this comes up. Yes, their are different methods and some people learned with different ways before 2015.But I've read the last thread just like this one and I noticed a pattern. The person asking the question still may be confused while everyone else debates on the topic with large EE terms that a newby to the hobby may not understand and that person is no better. If anything that person is still confused and now even more confused and probably afraid to ask questions on these different words and terminologies being thrown around and debated. Probably will feel intimidated like they are asking a stupid question. And just because a person is asking the question doesn't mean they are going to do it, maybe it's more so they know what not to do. Just as the title of Dave's video, "How not to blow up your oscilloscope" that you saved up so much money for.So while everyone chimes in and runs off with this great debate once again the main person who asked the question hasn't learned anything.Observation; Have you noticed the people asking the question rarely chime in again after a certain point. I thought this forum was to teach and educate as well as debate, but let's not forget the educate part first and make sure the main question that started the topic is closed and this person isn't confused anymore. Debating the topic with even more terms a new person doesn't understand doesn't help, in fact they may feel intimidated to ask any questions.**If you are new and reading this post, please please ask any question. It could save your life, so no question is a dumb question.I've given this a lot of thought the last few days. Think back to when you first got your first scope as a hobbiest. Of course you wanted to hook it up to something and play with the little knobbys and controls, push buttons and see what they do. Most people do learn by using somesomething and are visual. I'm one of those people. When I got my first scope someone told me I "NEED" to watch Dave's video, same one on post #2. And I did, and I was still confused. In fact I still ended up closing that ground loop with my scope and making the same mistake the video was trying to tell me not to make. My scope was ok for other reasons that saved it, but that's off topic and for another thread.After looking back now I realize that I was told I need a DMM, and a scope, and a bench power supply, and a good soldering iron, and blah blah blah if I'm getting into the hobby. Great, spent money, I had it. What I was missing is the understanding of the terminology being used. In fact I just watched the first 6 minuets of it again and had a completely different understanding today from years ago. I think what I was missing is the understanding of the terminology like "floating path / mains earth referenced / closed loop circuit" and other terms and phrases used. I think that might be the big part missing and "trap for newcomers" as he says. Buying the scope, wanting to see it to understand it since most people are visual, wanting to use your new scope you saved up for. But maybe we need to break it down in Laymans terms a little when answering the questions and and make sure they understand the answers to the question they asked before debating a topic that will just confuse most newcomers more?Then maybe this topic won't also come up so much, as if they search for the answer they might find the thread that explains it and doesn't debate it?Thank you,Scott

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2017, 01:56:05 am
content: Scott, you make some valid points and if I may I'll make one that might help clear some misunderstandings.Most of this stems from the basic knowledge that we can't just clip the ground lead of a scope probe just anywhere.Of course the reason is that it's mains ground referenced and has the potential to short the signal/circuit out.But let's make this simpler, the commonly used term of a probegroundlead in some way hides its real function.......it is the connection that provides areferencefor the signal and need be firmly thought of as such, BUT it's also mains ground referenced and this determines where it can be connected to.We use the term probe ground lead to remind us that it is mains ground referenced, a term I personally abhor, preferring to know it as the probereferencelead, knowing that for ALL but a very few scopes (isolated, floating etc) it is indeed mains ground referenced too.You may've seen in posts I make that I try to take care now I use the reference/ground lead terminology in some hope that others can see in a similar way that has given me good guidance over the years.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 12, 2017, 03:55:00 am
content: Perhaps the poster/s of these questions don't chime in again because:1. They are in hospital on life support and can't type2. They have become one with the Earth(via the shortest path to it)3. Surfing Ebay for another scope with isolated inputs,or / and clicking 'Buy It Now' for the EEVblog differential probe4. Disappointed/intimidated? they did not get a fast fix one-liner comment like 'disconnect those croc clips before cluelessly attempting to probe your house mains voltage' (see 5.)5. Asking the same question at another forum6. Combination of any of the above, in no particular order...

author: Scottjd
author_rank: Frequent Contributor
timestamp: July 12, 2017, 05:45:52 am
content: I appreciate the reference you make in your posts. This wasn't to point at people but in hopes of maybe coming up with ideas I missed in how to help new people get the correct answers better.I know this can be hard since most is learned from experience, but I thought I would try to approach the topic. I know some of my experience is from magic smoke escaping, some hard lessons learned.Their was another thread that was 4 pages long, and 3.5 of it was debating on best practices. The answers and suggestions were all over the place. I don't think it really had an answer, or it had multiple answers? If I had to pick the best one I would say it was the post that mentioned each individual situation was different with to many variables to consider. The internet is already full of bad advice, and good advice also and people have to weigh what they believe to be the correct answer these days. I was told before buying a scope it was a digital multimeter on steroids, well that's not exactly true as a multimeter isn't references to ground so you can't probe the same way with a scope as you can with a DMM.I just keep thinking and wondering if more can be explained for something that is life threatening then "Don't do it" with some reason behind it. That might be the last lesson ever learned the hard way.I thought maybe if I posted about it that some people might take a little more time explaining a little more, or asking questions to the person who is asking the original question to get more facts on what they are trying to measure and concider those variables?Scott

author: alm
author_rank: Super Contributor
timestamp: July 12, 2017, 06:34:42 am
content: I think it is partly an interaction. If the original poster sticks around and asks follow up questions, then this naturally steers the discussion in that direction. If they do not, then threads can wander and end up at some only tangentially related topic. This is not necessarily bad, and hard to avoid on a lightly moderated forum like this.How are the people who respond supposed to know what the person asking the question does and does not understand without that person asking for explanation? There is no way to tell someone's experience, especially if they are new to the forum. They might even have an EE degree but just never worked with power electronics until now.About the confusion because of conflicting opinions, that is again the result of an open forum. People's tolerance for risk, environment, experience and frame of reference all affect their attitude towards this topic. You can not very well ban certain opinions from the beginners forum.In my opinion there is no way to explain how to safely float a scope (assuming this is possible in the first place) on an open forum. For many people, explaining why it is dangerous is more effective than just saying 'do not do this'. And then you get into specific examples and alternatives with 'big EE words'.

author: tautech
author_rank: Super Contributor
timestamp: July 12, 2017, 07:35:30 am
content: I see detailed explanations of the techniques used , the when to do, when to not, what is sort of safe, what isn't, is a bit like knot tying instruction.......show somebody how to tie a Hangman's noose and they're just likely to use it.Therefore IMO we're better to point out what's dangerous and universally frowned upon today now that we have reasonably priced tools to allow us to use scopes in pretty much anyway we like.I said this before, just because you have bought the most universal piece of test equipment....a scope, doesn't mean the spending stops there.For any sort of serious work especially in power electronics you will need current, differential, high voltage and active probes in order to get decent results without undue risk. At entry level this means as much investment again as has been paid for a scope, like it or lump it.Or you can write a Will, that cost nothing.

author: madires
author_rank: Super Contributor
timestamp: July 12, 2017, 09:57:40 am
content: Since we have this topic over and over again, what about creating a "how to not blow up your scope or kill yourself" thread for dummies?

author: Electro Detective
author_rank: Super Contributor
timestamp: July 12, 2017, 11:01:14 pm
content: Quote from: madires on July 12, 2017, 09:57:40 amIt won't ever be complete or thorough enough to hit home,unless Youtube electronics acrobatElectroBOOMdemonstrates how it's doneIn the meantime, hospitals, eye surgeons, funeral parlors and Ebay will still cater to dummies

author: Scottjd
author_rank: Frequent Contributor
timestamp: July 12, 2017, 11:44:49 pm
content: Quote from: alm on July 12, 2017, 06:34:42 amI would never want to explain or teach how to float a scope. Maybe how to use an iscolation transformer if it was for the proper device under test like an old radio from the 60's. Or point them to the tool they need to purchase for the job and some references on how to use that tool properly and hope they get it.Scott

author: rajiv85
author_rank: Contributor
timestamp: July 13, 2017, 01:13:47 am
content: Thank you a lot guys for all the inputs.But I still need to say it is my own thinking that is doing it for me now.. not the actual explanations. For some reason it is all missing the crucial points.So let me sum up what I definitely think I understand. Please correct me if am wrong.After thinking about it for myself for some days. My feeling is that:1. The only thing you have to do is make sure there is ONLY 1 ground point in the entire circuit.2. Have only one probe having the ground. Remove the ground probes from all the other probes.3. Put the ground probe at the unique negative terminal and keep it there and never move it.4. When you want to measure the voltage between 2 points: use the math function with 2 probes between those points>> if they are far away. Exception to this is where the component to be measured is close to the negative terminal then the ground and the probe of one probe can be used. If this component is further away you cannot take the ground and move it further.. keep the ground there and use math function with 2 probes ( the second probe should have no ground connected to it--should be removed for safety.)In my view if this rule is respected.. and I need to be honest I realised this from my own thinking: then there is no need for all these: isolation transformers ( including confusing modification to those equipments--like having to disconnect ground connections inside those equipments, floating and all these really confusing hour long explanations. )Is this correct? Will be so grateful to get feedbacks.Thank you.

author: Electro Fan
author_rank: Super Contributor
timestamp: July 13, 2017, 02:57:32 am
content: Quote from: Scottjd on July 12, 2017, 01:30:30 amWhat Scottjd said:

author: Kinvert
author_rank: Newbie
timestamp: July 13, 2017, 03:25:12 am
content: I can vouch for that. We teach kids STEM and it's really important to not do this.Not only will the person stop responding to the thread but they'll potentially avoid asking other questions they may have in the future.Scottjd said it well.

author: Electro Fan
author_rank: Super Contributor
timestamp: July 13, 2017, 03:36:17 am
content: Quote from: tautech on July 12, 2017, 01:56:05 amTautech, your post was helpful, like always.I'm late to this thread but I think it's a good thread on at least 2 levels.1. Per my comment on Scottdj's post, I think he is right on the mark about how sometimes an OP's question can turn into a more complex debate rather than an opportunity for some understandable education. (I think the OP on this thread is looking for some feedback above on his current understandings.)2. On the subject of "ground", fwiw, I have found it to be among the most difficult to grasp for something that seems to be so fundamental, not to mention so important from a safety standpoint. I have read various threads here on ground and I have watched Dave's video about 4 times and the only thing I'm sure of is that I don't fully understand it. It's just a guess but I would be surprised if I really understand half of it. When you combine the fact that it's hard to know what you don't know with the fact that one wrong probe or alligator clip could lead to something bad or worse, this subject seems like it could use several of Dave's videos plus one very comprehensive but very clear sticky post. With clear terminology defined from the gound (ha ha) up, some good illustrations, and perhaps a video or a few videos to explain, demonstrate, and summarize "ground" perhaps the key concepts could be made more accessible for beginners and other enthusiasts.EF

author: rajiv85
author_rank: Contributor
timestamp: July 13, 2017, 10:08:23 am
content: Can someone please comment on my last post?

author: Electro Detective
author_rank: Super Contributor
timestamp: July 13, 2017, 10:14:50 am
content: Quote from: rajiv85 on July 13, 2017, 10:08:23 amhttp://en.wikipedia.org/wiki/Russian_roulette

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 11:55:51 am
content: Quote from: rajiv85 on July 13, 2017, 01:13:47 amIf the device under test is floating (e.g. battery powered), then you can pick pretty much any single point as ground and clip a ground lead to it. You can also clip the ground lead to other parts that are directly connected to that point (i.e. measure very low resistance with a DMM with the device under test powered off). For this reason, it is often convenient to pick the negative or positive power rail, since that is easily available across the device. If the device under test is connected to earth ground, take for example an ATX desktop computer, then you should only clip the ground lead to parts that are already connected to earth ground (you can measure this with a DMM). Connecting even a single ground lead to the +12 V wire in a computer would short the power supply.Quote from: rajiv85 on July 13, 2017, 01:13:47 amThis will reduce the risk of damaging something by shorting it through the ground leads. However, for good signal integrity, you will need to use ground leads on all probes. So this is a trade-off between signal integrity and risk of screwing up. It may not be a terrible way to start and gain some confidence.Quote from: rajiv85 on July 13, 2017, 01:13:47 amAgain, working this way reduces the risk of screwing up, but for optimal signal integrity you usually want to pick a ground point close to the point you are measuring. But again, may not be a terrible way to start.Quote from: rajiv85 on July 13, 2017, 01:13:47 amThat sounds fine.Quote from: rajiv85 on July 13, 2017, 01:13:47 amI added some caveats because a blanket X is always safe or you should never do Y can be problematic without at least a very basic understanding of the limitations of those absolute statements. If you follow the above rules, you would be able to work on floating devices (e.g. battery powered clock), work on low voltage devices that are connected to ground (desktop PC) with the limitation that you can only clip your ground lead to certain parts, and not work on things like mains-powered switching power supplies.

author: rajiv85
author_rank: Contributor
timestamp: July 13, 2017, 12:17:44 pm
content: Thank you Alm for excellent response.So I want to ask. From your experience, if you can give more variations to the configuration I mentioned? If possible point by point, all the different possibilities.For example,1. The configuations I mentioned.( Improvements including the hooking up ground probes from the other probes--in the same spot as the first ground probe-- for fidelity.)2. The possibility of using any ground point if the device under test is floated or battery powdered.Are there more configurations and variations? Specially when we extend to high voltage etc?I am specially interested to know under which conditions ( ideally an exhaustive list) is it possible to connect a ground + probe at one component without risk of blowing anything? I understand so far: differential probes. Is there more or is that it?

author: tronde
author_rank: Frequent Contributor
timestamp: July 13, 2017, 04:00:10 pm
content: rajiv85:Do you live in Austria as indicated by your profile, or in the US as indicated by your drawing?If you live in Europe I can try to make a description that is relevant to your lab.

author: rajiv85
author_rank: Contributor
timestamp: July 13, 2017, 07:56:36 pm
content: I am in Austria. Really appreciate!!

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2017, 09:03:30 pm
content: Its really quite simple:Nearly all*mains powered oscilloscopes have a direct low resistance connection from the probe ground clip right back to the Earth pin of their mains lead. They are only intended to be operated from a three pin mains socket with an effective Earth connection..If you connect more than one node#of the circuit under test to Earth, you'll short out some part of it which will certainly give incorrect results, and if one or more of those Earth connections is via your probe ground or test leads, if a high enough current is available it is likely to blow up your scope and/or any other grounded test gear you have connected..All instruments with ground referenced outputs or inputs must have mains supplies connected as directly as possible to the same earth point. Otherwise a fault elsewhere in the building can cause current to flow through their input or output ground connections and damage them.Ideally all test equipment should be fed from the same mains outlet wallplate. If that cant be done then they should either be on the same mains circuit or a heavy bond wire should link the Earths of all bench mounted mains sockets or extension leads used for the bench.When you are working with low voltage circuits and the PSU or battery is capable of providing more than 100mA, if you aren't sure, try a low wattage test lamp (appropriate to the supply voltage) between the probe clip and the point you intend to connect it to. If it glows at all, *STOP*, and choose another node for your ground clip.Its unsafe to use the probe ground when testing a non-isolated mains powered circuit, because you don't know where in the supply system the Neutral to Earth connection is+and if there is a high load current flowing through any part of the neutral conductor between you and that cross-connection (which may be right back at the distribution transformer), there can be a significant voltage between Neutral and local Earth, with a low enough impedance to supply hundreds of Amps if shorted to Earth. Because the currents in the Neutral conductor between you and the distribution transformer are outside your control, its *NEVER* safe to connect the ground clip to Neutral, even if you don't measure a significant voltage between them right now. You risk blowing your scope when the aircon or immersion heater cuts in or the lift starts in the building across the street.Old CRT scopes with metal cases and chassis were more robust - there was enough metal from the BNC input socket back to the Earth pin of the power in connector to carry a heavy current, so if you goofed, the odds were fairly good that the scope would survive and you'd only have to replace a melted probe. Some modern scopes use a PCB track for the BNC ground and its likely to melt before the probe lead does. If you blow a ground track on a board inside the scope, its almost always game over as a voltage will be applied between the ground pins of chips before and after the break, so the odds are the ADCs and logic board will be blown as well.* except ones with isolated input channels, or if you are using anisolateddifferential probe.# a single node is all points in a circuit directly connected by wires or PCB tracks (or other solid metal connections) that have insignificant voltage drop across them. If its got any components (except reliably mated connectors) between any two points they aren't a single node.+ You may *think* you know where the Neutral to Earth connection is, possibly right at the service entrypoint, but it only takes a single loose connection for one much further away to dominate.

author: tautech
author_rank: Super Contributor
timestamp: July 13, 2017, 10:12:05 pm
content: Quote from: Ian.M on July 13, 2017, 09:03:30 pmExcept as Tek found out decades ago with the TDS510 and 520 (general recall) that mains earth connectivity to the input BNC's MUST be robust enough to trip breakers or blow (normal) fuses in the mains supply.To say that modern scopes manufacturers haven't learnt from this is an unfounded generalisation.

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2017, 10:43:50 pm
content: I'll grant you that and have edited above to "Some modern scopes use a ..."However as it isn't a major selling point, there is no easy way of checking the input BNC grounding integrity before purchase - you can only go by the manufacturers reputation or by teardown photos.

author: tronde
author_rank: Frequent Contributor
timestamp: July 13, 2017, 11:07:04 pm
content: Quote from: rajiv85 on July 13, 2017, 07:56:36 pmI assume you have this kind of wall socket? Do you know if you have a separate earth wire (PE) usually green/yellow in the socket? I know you can find some older houses with only two wires in wall sockets in some European countries. If you don't know about the earth wiring, I will assume you have three wires because this has been standard for many years.I ask about details now, so we get it as close to what you have as possible.I will come back tomorrow evening with a beginning. I will go forward when I understand that you have understood it too. Maybe the beginning will be known to you, but then it can be useful for someone else.

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 11:22:23 pm
content: Let me post some schematics of some of the possible configurations. Obviously this list is not exhaustive.First is a fully floating device. This could be something powered by batteries, or something that is mains powered but double isolated (equipment with two-prong mains plugs). There is no connection to earth, so you can clip the ground lead anywhere. I labeled the common terminal as 0 V and the other terminals +5 V and -5 V relative to this. This is a somewhat arbitrary choice. I could also have marked the bottom terminal of BT2 as 0V, then the common terminal would be +5 V, and the top of BT1 would be +10 V.Now you clip the ground lead of your first probe to the common terminal between BT1 and BT2. Now that rail is connected to earth ground. So connecting a second ground clip to the top of BT1 would short BT1, and could cause damage. You could connect a second ground lead to anything connected to the common terminal, for example the point between D1/D2.In this example, there are long wires between D1 and U1. If you wanted to measure the output of U1 with a second probe, then leaving off the ground lead or connecting it near BT1/BT2 would degrade your signal, so you would want to clip it near pin 1 of U1. You could verify that this is connected to the common terminal by measuring resistance (with the batteries removed) or measuring voltage (should read close to 0 V). Alternatively, you could clip your ground lead to the top of BT1, and clip another ground lead to pin 3 of U1. It depends on what you find more convenient as reference.Now look at a very similar circuit, but with the common terminal connected to earth ground. This applies to a standard desktop computer, and many other devices with a three-prong plug (but necessarily all). In a desktop computer, the metal case and the 0 V (black) wires from the power supply are all connected to earth ground. Note that this is very similar to the first circuit after you clipped on your first ground lead. You can only connect probe ground leads, even the first one, to points connected to the common terminal marked as 0 V, like the point between D1/D2, U1 pin 1 or the point between D3/D4.Consider another modified version of the first circuit. I added a +1000 V power supply, which might for example be an inverter for a CCFL light (would not actually be a battery, obviously), also powered by batteries. The circuit still has no connection to earth ground, so you could clip your first ground lead anywhere. However, if you clipped your ground lead to the top terminal of BT1, pretty much all the rest of the circuit would be at approximately -1000 V relative to earth ground. This would make it really easy to shock yourself by touching for example an output connector tied to the common terminal between BT2/BT3. So this is not a smart choice to clip a ground lead, even though it would not damage anything unless you clipped a second ground lead somewhere else. In addition (this is getting a bit more advanced), the CCFL inverter may not be happy with the capacitance that the ground lead might represent.The last circuit is a mains-connected transformer. The primary (left) side of the transformer is connected from hot to neutral. There may or may not be any earth connection, but neither of the legs of the transformer is at earth ground potential. Note that the 3 VAC at the neutral terminal is not in anyway accurate or stable, so a lot of current could still flow if you shorted it to earth ground. In this example, there would be no place to clip the ground lead. You could use the CH1-CH2 math trick and leave off all ground leads. And obviously this circuit is also a shock hazard, so you should not be touching the probes and circuit with the power applied. The same does not apply to the secondary (right) side of the transformer, which could either be floating or connected to ground.I also attached the KiCad project with the schematics for anyone who wants to demonstrate that I am wrong, stupid and ugly.

author: tautech
author_rank: Super Contributor
timestamp: July 13, 2017, 11:26:24 pm
content: Quote from: Ian.M on July 13, 2017, 10:43:50 pmI'd strongly suspect there's an industry or electrical standard now for mains earth to BNC connectivity robustness as if it fails the BNC inputs are then floating and potentially lethal.With the Teks I mentioned a mainboard replacement was part of the general product recall.Tek to their credit had an online SN# search to allow subsequent owners to check it their units had been recalled and repaired. If your unit was before a certain SN# then it had to go to a service centre and get fixed.One I had was luckily after the specified SN#.

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2017, 12:28:41 am
content: Quote from: tautech on July 13, 2017, 11:26:24 pmThe fact that you as a distributor cant immediately refer me to the applicable standard for test equipment grounding integrity, leaves us the users with little certainty.As the O.P. asked about USB scopes, I should also note that a USB scope, connected directly to a PC can usually be considered as a complete system electrically equivalent to a 'mains powered scope' as I discussed above, but with *really* *shitty* ground integrity so the odds of blowing up the scope pod or PC if you make a mistake connecting the probe ground are much higher. The only exception is if the USB interface is fully isolated. Then you have to consider whether or not the scope pod was designed to float safely - and trusting the isolation of an external PSU is foolhardy unless its designed for the purpose and has a proprietary connector so it cant easily be substituted by one with inferior isolation ratings.

author: rajiv85
author_rank: Contributor
timestamp: July 14, 2017, 02:04:36 pm
content: @tronde: Yes for sure such plugs are actually 3 pin too. The earth is on the side. Although it is 2 pin when you look at it.. it is actually 3 pins.

author: rajiv85
author_rank: Contributor
timestamp: July 14, 2017, 02:19:50 pm
content: @alm: Many thanks! Very well illustrated and explained. I am studying it carefully. You definitely broadened my understanding with those examples and may it be helpful to others trying to learn these. I think it is the most important thing to understand properly before going forward with oscilloscopes.I have a question. How can I know where the ground of a circuit is? Let us say I attach a function generator or a DC power supply to the inputs of my circuit. I actually tried the math function without the grounds and got very weird signals. When I put the ground probes then I get the correct signal. But what if there is a hidden ground somewhere in some device at high voltage and I accidentally connect a new ground probe somewhere. So how to make sure I avoid such a situation?

author: tronde
author_rank: Frequent Contributor
timestamp: July 14, 2017, 09:47:33 pm
content: This is meant as a reply to the initial post made by rajiv85.I will start with some childish-looking drawings so we can make up some internal pictures about what is going on. The wall sockets indicated is meant to look like the "schuko" sockets in his house.I will write several posts, but this is the beginning. If somebody feels this is to simple for them, it's OK. I will try to reach most beginners.This is NOT meant as description of how to wire your house. It is meant to tell why the wiring in the house can make difficulties for us when we use mains connected testgear. Because of this, I have deliberately omitted fuses and RCB's (residual current breakers, [gruond fault protection]).rajiv85 says he lives in Austria, so I will use the wiring system that is standard there in my examples. We have many different wiring systems in use around the world, and each person must make sure how his house or lab is wired. In Europe we will at least find systems known as TN-C-S, TN-C, TN-S, IT and TT. In other parts of the world you can find something completely different. Ask google or wikipedia for more details.You must also be aware that several systems can be used in the same country. Most of central Europe is TN-C-S, but you will find a lot of IT in hospitals feeding life supporting equipment. IT is also used a lot in some industries. This is because a single ground fault will not trip a fuse and cut power to critical equipment if the wiring is IT. TN-xxx wil trip the fuse at once.The wiring system I will describe here is known asTN-C-S.TNmeans Terra-Neutral. Terra is the same as the earth we walk around on. The meaning of this is that the neutral conductor is connected together with the physical earth (soil) conductor.-Ctells us that the neutral conductor and the PE (protective earth) conductor is the same wire from the power company's transformer and in to the first distribution panel or fuse panel in the house. This particular wire is known asPENand it is usually blue with green and yellow markings on it.-Stells us that the PE wire is SEPARATED from the neutral wire inside the house, but all neutral wires and all PE-wires are connected together in one connection where they continue to the power company as the PEN-wire. We have one physical connection to earth at the transformer and one in the house.The TN-C-S system is a three phase system where we get 230V between the neutral connection of the transformer and one of the phases. Between two phases we will get 400V. In the first drawing I have indicated the three phase transformer and the three phase wires. Since we are dealing with domestic mains power, I will only use ONE of the phases and neutral. No need to complicate in the beginning. The neutral wire is usually blue and the phase wires are brown, white (or grey) and black. The colour coding may be different in differnt countries, and also because of the age of the wiring. This is not important for us. I have choosen brown for the live wire because that is the colour most used in wall sockets.I know many people get confused about AC voltage. To make it a litle bit easier, we can imagine that the voltage we get from the power company is a large 230V battery instead. Then we can say that the live wire is +230V and the neutral wire is 0V. Batteries are usually marked withplusandminus, but it is much easier for us if we just decide that minus is 0V. That will be used as a reference value in later examples.The main reason for all our trouble is beacuse many test instruments have a metal enclosure that is connected to PE and that all input or output connectors have a shield or screen that is connected to te same enclosure and therefore to the PE wire of the mains supply.I know rajiv85 asked about USB-scopes, but I will cover the normal scope first, because we will have fewer wires to worry about.The last figure is meant to be an oscilloscope. We can see that the enclosure is connected to PE, and that the screen of the input connector is connected to the same PE. We can also see that the PE is connected together with the neutral wire in the fuse panel of the house.I have deliberately marked the isolation barrier between the mains supply voltages and the internal voltages of the oscilloscope. We will come back to why in another post, but take notice of it now.We will face the same problems with other kind of test equipment if they share PE and input/output connectors.More will follow tomorrow or Sunday I hope.

author: rajiv85
author_rank: Contributor
timestamp: July 14, 2017, 09:53:18 pm
content: @Tronde: Thank you so much!! I will be studying these carefully. Hope these can also help others in the future.

author: alm
author_rank: Super Contributor
timestamp: July 15, 2017, 04:56:19 am
content: Quote from: rajiv85 on July 14, 2017, 02:19:50 pmIf the device under test is fully floating, then the CH1-CH2 may give incorrect results, and you would have to ground some part of the circuit. Once you have done that, you can use the CH1-CH2 between any two points in the same circuit. For example, in the first schematic in my previous post, you could clip CH1 to the -5 V rail, CH2 to the +5 V rail and ground to the 0 V rail. In the mains example, you should not need a ground connection, since it is already referenced to ground.How do you know where the ground is? Well, if the device has no connected to earth ground, then it is unlikely to be grounded. So battery powered or double isolated (two prong mains plug) devices would not likely to have any parts connected to ground. Note that cables to other grounded equipment, like a USB or analog audio cable, can also provide ground.Where is that ground connected to the circuit? You can guess and measure (with the device powered off, measure resistance). Often it will be one of the power rails. You can sometimes also determine by visual inspection where the green/yellow wire from the chassis or mains leads connects to the printed circuit board. If there is a metal case, then it will also likely be connected to earth ground.

author: rajiv85
author_rank: Contributor
timestamp: July 15, 2017, 09:59:10 am
content: Thank you Alm, another question, that I always wondered: why are metal cases grounded? It seems to me like the most dangerous thing to do. Why do such a dangerous thing? Why are equipments even made with metal cases that are grounded? Why not plastic coat them or make them non conductive?

author: alm
author_rank: Super Contributor
timestamp: July 15, 2017, 11:57:26 am
content: Because the earth ground is pretty much always at a safe potential to touch. This means that if one of the mains wires inside the equipment gets loose and shorts to the case, the case can never end up at some dangerous voltage. The short to case will cause a high current to flow, blowing the fuse or tripping the breaker or GFCI without endangering the user. A metal case that is not connected to ground could end up at some dangerous voltage after a defect. That is why metal cases are usually grounded. This is the so calledIEC protection class I. If the case was not connected to earth, it would be class 0 equipment, which can not be sold anymore in most western 230V countries.The alternative is indeed a plastic case that is not connected to ground. Now the designer has to pay special attention to make sure the user can not come into contact with the mains wiring, since there is no metal case that will blow a fuse in case of a loose wire. This is so called double insulated (class II) equipment. Double insulated because it requires more sturdy insulation and larger clearances to reduce the risk of the user coming into contact with dangerous voltages. The low voltage parts of class II appliances will usually be isolated from ground.The basic principle is that a double faults, two faults, have to occur before the appliance can be a danger for the user. In the case of class I equipment, the ground connection would have to become broken and something would have to short to the case for the user to get shocked. For class II equipment, two layers of insulation (or something considered equivalent like reinforced insulation) would have to become compromised for the user to get shocked. Of course all of this goes out of the window as soon as you open the case.

author: tronde
author_rank: Frequent Contributor
timestamp: July 15, 2017, 10:13:15 pm
content: What did we learn from my previous post?The most important thing we learnt, is thatthe neutral wire and the PE wire is connected togethersomwhere in the house. This is usually in the fuse panel.The second most important thing we learnt is thatthe metal enclosure of the scope, and the shield / gruond part of the signal input connector is connected together with the PE wire.Now it is time for some measurements. Then we need a probe. As we can see, the probe is connected to the BNC-input connector. We can also see that the ground clip of the probe is connected to the shield of the BNC connector.So, what does this mean for us?This means that the tiny little innocent lookingground clip of the probe is connected to the neutral wire.The probe's ground clip is connected to a wire that ends up in the same hole in the wall socket as the neutral wire. The very same wall socket that is able to continously push out three and a half kilowatt of power if it is protected by a 16A fuse. 3500W is in fact avaiable to us from that tiny little alligator clip if we goof.Now it's time to switch on our brain, isn't it?We have a device that we will test with the scope. It can be anything, but let's assume it is a cheap 230V LED lamp. It does not have any PE wire because it is made to be double insulated. We don't know exactly which wire is meant to be neutral and which is meant to be live, but we think we can connect the ground clip to left connector in the figure below.Now we will face a problem with the Schuko mains socket. The Schuko socket is not polarised, so we have no idea about which hole is netutral and which is live. We take a chance and connct the wire from the LED-lamp to the uppermost hole in the figure.Well, that was wrong...As we can see from the figure, we managed to short the live wire to neutral via the probe's ground clip. Not very smart.Lession learnt: Don't connect the ground clip to the live wire.Then you think - It must be OK to connect the ground clip to the neutral wire because PE and neutral is connected together and should therefore be the same?Maybe not...More about this next time.

author: tronde
author_rank: Frequent Contributor
timestamp: July 17, 2017, 01:02:13 am
content: In the previous post we learnt not to connect the ground clip to LIVE.So, how about connecting it to neutral?This is a simple drawing showing us how the lab bench is wired from the fuse panel. It is on a branch that is shared with other things as well. Among them is a water heater. We can see that the water heater is connected to the wiring AFTER the lab bench. The wiring is made of 2,5mm2wire, and the distance from the fuse panel to the bench is about 15 meter. The water heater is a 2kW type, so it will draw about 9A from the wall socket. All wires will have some resistance, and 15 meter of this wire will have close to 0,1 ohm in the live wire and the same in both the neutral and PE wires. Because of this resistance we will have a voltage drop at the lab bench's wall socket so we will measure about 228V there.Why care about the resistance in the wires? They are there, and it sometimes makes sense to be aware of them. I used them to calculate the expected voltage between PE and neutral. I have rounded the values. We don't need exact values.Before we decide to connect the ground clip to neutral we measure the voltage between the enclosure of the scope and the neutral wire we intend to connect the ground clip to. We find that the voltage is about 1V, so we think this is OK. It is almost the same as nothing, so PE and neutral should be the same because of this?As we can see from the figure we have about 1V between PE and neutral and we have a current of 9A in the circuit. The scope and the device we intend to test consume almost no energy, so I have omitted the current for them.Since we think everything is OK, we decide to connect the ground clip to the netral wire on the device.Shit!Just as we connected the clip, we got a spark.What happened???Since the ground clip connects to PE via the scope, and PE and neutral is connected together in the fuse panel, the return path from the water heater is split between the neutral wire in the wall socket and the PE wire from the scope.The resistance in both the neutral wire and the PE wire is the same value since they are equally long. The ground wire in the probe and the PE-wire in the power wire between the scope and the wall socket will add something to the total resistance in the PE current path, but not very much compared to the total. The result is that we will have about half the current from the water heater in each wire. That is 4,5 ampere, so nothing strange about the spark.As we can see, we need to understand the wiring hiding in the wall too. If this water heater had been controlled by a thermostat or disconnected we would have measured 0V between PE and neutral when it was off, and no return current had mixed with the measurement. Spend some time and consider poentially dangerous situations caused by thermostat (or automatically) controlled equipment that can influence on our safety.So, you think - let's make sure we have no return currents. I do really advice against that. The neutral wire is part of the power supply, and unexpected things can happen. We must try to find something that is safe to use.Things to discuss in later posts will be about floating the scope, using an isolation transformer, and isolated differential probes. Maybe something else too.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 17, 2017, 01:41:13 am
content: Good show TrondeLet's hope some of it sinks through to the deathwish prodders that think their single ended oscilloscopes can be used like a multimeteror bypassing the earth/ground wire on the oscilloscope 'gets it done'or the Ebay click turkeys that buy a secret earth ground bonded 'isolation transformer'and 'transform' their test gear/DUT set up into a DIY fireworks display

author: tronde
author_rank: Frequent Contributor
timestamp: July 17, 2017, 09:29:41 pm
content: I did some testing of the ground connection on an old probe and a 1,5m long mains cord of the same type as we usually connect to the power supply input of the scope. The combined resistance in those two was slightly more than 0,1 ohm. My guess in the previous post was that it was almost negligible. 0,1 ohm is not much, but it does increase the total value by 100%, so in this exact application it is a significant change.If we use 0,1 ohm as the combined value for the the probe and the power cord, we will get at total resistance from the ground clip down to the fuse panel of some 0,2 ohm. The 9A current from the water heater is split between this 0,2ohm and the 0,1 ohm in the neutral wire. That gives us a current in the ground clip of some 3A, and the current in the neutral wire will be around 6A.This is not an important difference for what we need to understand; namely that the ground clip and the probe will be part of the return path. I just make this comment if somebody think I got it wrong in the previous post.___________________________________________What we have done until now, is to find out that we can't connect the ground clip to any of the mains wires. This makes it difficult for us if we need to measure anything connected to the mains with an oscilloscope.Now, you might ask: Can't we just remove the ground clip from the probe? The ground-input of the scope is connected to both neutral and PE, so why do we need a ground clip?Yes, we can infact remove the ground clip. On most probes it is meant to be removed, and the contact is usually covered so we can't touch ground by accident. The problem by doing so, is that the ground or common contact of the scope's input will be connected to a very long wire going from our lab bench in to the fuse panel and back to the workbench. In our example this is 30 meter. This will act as a very good antenna picking up a lot of noise. In addition the neutral wire from the lab bench will carry currents that is not from the device we test. This will add alot more noise. This wil more or less render our measurements useless, so wemustfind another solution.What all this is about, is how we can use our scope without damaging it, or ourselves, or the device we intend to test.Mains voltages have a big damage potential. We all know it can kill given the right circumstances.But - it is also very powerful. I mentioned that we can get 3500W from the wall socket if protected by a 16A fuse. Normal fuses have a rather large overload capacity, and many fuses are rated for 45% overload in one hour. This gives us 23A and 230V x 23A = 5300W in one hour before the fuse will trip.Think about all kinds of power tools we can buy and connct to this socket. Think about how much manual labour these tools can save us from. Then we understand that 3500W or even worse 5300W is a lot of power. If we release all this power in less than a second as we do if we short something, the damage can be really large. That is the reason we need tounderstandwhat can go wrong, andwhy.I will wait some days before I post more.If rajiv85 is still around, it would be nice to know if some of this makes sense to him.

author: rajiv85
author_rank: Contributor
timestamp: July 17, 2017, 10:48:18 pm
content: @tronde: thank you a ton. I am actually saving all your detailed explanations in my notebook. I will study them in detail today. What you are explaining is extremely useful.

author: tronde
author_rank: Frequent Contributor
timestamp: July 17, 2017, 11:41:09 pm
content: Quote from: rajiv85 on July 17, 2017, 10:48:18 pmFine,Don't hesitate to ask.When I know you are here reading, I will wait some time now with the rest so you can mange to "digest" this first. Do you have some kind of test equipment? If you have something, please tell. Don't worry if you don't have a scope.You said you are a beginner. How much beginner are you? Are you familiar with ohm's law and Kirchhoffs laws about how currents and voltages are distributed in a circuit?

author: rajiv85
author_rank: Contributor
timestamp: July 23, 2017, 08:29:21 pm
content: Thank you so much Tronde. I studied the drawings you posted. In fact as said I have an e-notebook and I saved the illustrations, specially the last ones . Really really good!!!!! I hope many others follow this and find answers. I will update my first post to point to forum readers to check in thread 2/3 for your explanations.

author: tronde
author_rank: Frequent Contributor
timestamp: July 30, 2017, 08:11:37 pm
content: I have weighted pro and con on this for a while, but I landed on that I will write something on wiring calculations. What I write is aimed on beginners, and I feel that they will benefit from some basic understanding about what is hiding in the wall before we look closer on what is going on on the lab bench.In a previous post, I used 0.1 ohm as resistance in the wires. This post will be about why it is 0.1 ohm.First we must know something about the material we use in the wiring. In normal European house wiring, we use copper. Copper is a rather good conductor. This means it has low resistance to electrical current. The price is also sensible.The physical property that tells us about how good any material will conduct electricity is known as"conductivity".Usually we want to know how muchresistance(the property we measure in OHM) a given wire have instead of how muchconductanceit has. Lucky as we are, it does exist a property for that as well. It is known as"resistivity".Resistivity tells us how good a matrial is to hinder the flow of electric current.You can read more about them here, and you will find a table listing several different materials.https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityI stole this table from wiki just in case they change it.Here we can see that theresistivityfor copper is 1.68 x 10-8ohm x meter if the area is 1m2. The strange littlepis known as "rho" and is a Greek letter. I use to remember it as a tadpole (baby-frog).I am usually a great fan of the SI-system, but when it comes to wiring calculations we end up with some strange values. The reason is because the SI-system uses meter as a unit. We will never buy electrical wires with an area (cross section) of something in the meter-region. We buy them in some square-milimeters instead.One square meter is 1000mm x 1000mm = 1000000mm2. We can write this as 1 x106mm2instead.If we multiply 1.68 x 10-8with 1 x106we get 1.68 x 10-2.A more beginner-friendly way of writing 1.68 x 10-2is 0.0168.The meaning of this number-tweaking is to give us a more user friendly value to use in our calculations than what is listed in the wiki-table. We have now transformed the area from square meters to square millimeters as we have in our wires.As a side note, I can tell that many tables aimed at electricians will list resistivity based on an area measured in square millimeters.If you don't fancy the math used in the transformation, don't worry. Just remember that resistivity for copper should be something like0.0168. Another material used in wiring is aluminium. If we transform that into something useful we get 0.0282.You will find slightly different values listed in different tables. One thing to look for, is the temperature used. Resistivity will increase with temperature. The different values will not differ that much, and if the temperature is around 20 degrees C, you will do nicely regardless of the exact value. Just make sure it begins with 0.01 for copper and 0.02 for aluminium if you use an area measured in square millimeters.So, what use can we make of this 0.0168? What does it tell us?It tells us that if we have a wire made of copper that is one meter long, and with an area of one square millimeter, it will have a resistance of 0.0168 ohm. That is, it wil be the same as a 0.0168 ohm resistor.It does also tell us that if we have a two meter long wire made of copper with anarea of 1 square millimeter it will have a resistance of 2 x 0.0168 ohm = 0.0336 ohm.ORif the one meter long copper wire has an area oftwosquare millimeters, it will have a resistance of 0.0168 ohm / 2 = 0.0084 ohm.We can transform this knowledge into a formula.I can't get any useful greek symbols from my keyboard on this forum, so I will write it in plain text.Resistance of the wire = ( lenght of wire[measured in meter]x rho ) / area of wire[measured im square millimeters]In the example, I used a lenght of each wire of 15m and the area was 2.5mm2If we use these values, we will find that the resistance of one wire is:Rwire = (15 x 0.0168) / 2.5Rwire= 0.1008 ohm.If a math teacher reads this, he will freak out becuase I give the answer with 4 decimals.The freak out is because 4 decimals indicates a much better precision in the answer than what is possible when we use 0 decimals in the lenght and 1 decimal in the area.I will happily leave to any math teacher to write something useful about it.I know that beginners like to use as many decimals as possible, and by doing so it seems like they get some confidence in that they used the right formula. I will keep on using many decimals because of this, but now I have told that I know it is actually wrong to do so, and why.<peace, please>Tomorrow I hope to do more calculations on the example from 17.07

author: tronde
author_rank: Frequent Contributor
timestamp: August 01, 2017, 09:41:25 pm
content: Last time we found that the reistance in each wire from the fuse panel and up to the lab bench was 0.1008 ohm. We will use0.1 ohmin the calculations.First, we need to make a drawing of how the wiring is made.The following drawings are based on previous drawings, so I have omitted some info for clarity.When we do calculations, we must know exactly which resistor we use in the calculations. As you can see, I have given each resistor an unique name. We can use whatever we want, but it makes sense to start withRand use something unique (and descreptive) for the rest.I have named the resitor in the live wire forRL. This is meant to mean something like R-LIVE.Next, I have named the resitor in the neutral fire forRN. This is meant to mean something like R-NEUTRAL.Then I have named the resisor in the PE-wire forRPE. This is meant to mean something like R-PE-wire.Last we have the resistor in the power cord between the scope and the wall socket. I name this forRPEC. This is meant to mean something like R-PE-wire-cord.In this drawing, I have indicated the resistance in each wire, and I have also added a resistance in the power cord. Last time, I assumed the power cord had negligible resistance, but as I showed in the comments it did actually have some. Therefore I have added it here.As we can see, I have also added know values. We have already calculated the resistance in teh Live, Neutral and PE-wires to be 0.1 ohm, so I add the value close to each resistor in the drawing. Teh supply voltage of 23oV is also indicated where it belongs.This drawing shows how everything is connected before we clip the probe clip to the neutral wire.Next drawing will show what it is like when we do so.As we can see, I have added a resistance in the probe wire. I name itRPROBE. I have also added the water heater (RHEAT) and indicated that the current flowing through it is 9A. We usually indicate currents withI.When we do calculations, we try to "transform" as many items as possible into resistance. A water heater is basically a resistor so nothing strange there.The reason we try to use resistance is because we can draw a schematics with only resistors, and we will automtically (in our head) treat the item as a resistor. To do so is OK as long as the main function of the item is mostly similar to a resistor. When we deal with low frequency mains we can often do so, but sometimes we can not. It is not possible to give a perfect description of when we can, and when we can not transform into resistance for pure beginners.This transformation to resistors is also the reason why we often use a battery as power source in our drawings. We try to make it as simple as possible, and if we can use resistors in our calculations, we can also use a battery instead of for instance AC power as we actually have in this setting.If you find a drawing somewhere that draws a motor as a resistor, it is because it is OK in that particular setting, while it can be wery wrong in another setting. This can be confusing for beginners, but after some time you will know and understand more. Just don't think people are crazy because something is drawn like a resistor in a drawing. It has a (good) reason.These drawings shows us in a close to life-like style how things are connected together. Quite often it is difficult to use such a drawing for calculations. Therefore, we will transform this last drawing into a normal schematic-style drawing.The previous drawing transforms into this.Don't just believe it because I say so. Spend some time, and follow each wire on both of them and make sure you can identify them on both drawings. This is infact important for every circuit you will work on. Try to make some real schematics out of other things you can find around you.As you can see, I have also marked each resistor with the information we already know. All of them have got a name, and some of them have got the value as well. This is very important, because it shows us clearly what we know, and what we don't know about each item.Now, you might ask - where is the scope and the device I test on?In an earlier post I said the scope and the device uses so little energy compared to everything else in this circuit, so we can omitt them in th ecalculations. That is still true. The goal for these calculations is to find out what goes wrong when we connect the probe clip to the neutral wire, and to calculate the short circuit current.We don't know yet the value of RPEC (resistance in the power cord) and RPROBE (resistance between the ground clip and the power cord).The power cord is a 1.5m long cord with an area of 0.75mm2.When we know this, we can calculate the resistance just as we did for the other wires.RPEC = (0.0168 x 1.5) / 0.75RPEC = 0.0336 ohmThis is the same as 33.6 milli ohm, so not much.But, just for the sake of calculations, we will use that value. I said earlier that the combined value of the PE-wire in the power cord and the ground wire in the probe was about 0,1 ohm. to make it very easy for us, I deciede that RPROBE is 0.0664 ohm.When we add these two last values, it looks like thisI have addedINandIPto the drawing. They are meant to indicate the current in the neutral wire (IN) and the current in the ground wire of the probe (IP).We will calculate them later on.Enough for today.

author: rajiv85
author_rank: Contributor
timestamp: August 05, 2017, 08:02:04 pm
content: WOW..good stuff Tronde. I would highly suggest you consider making a small electronics tutorial book on that subject. Even if it is only 25 pages you can sell it on Amazon because it addresses a specific topic in good detail that other books or tutorials do not cover.

author: tronde
author_rank: Frequent Contributor
timestamp: August 05, 2017, 08:04:24 pm
content: This post has been edited. The reason is the original post was based on the logical flaw that the total current in the circuit would not change when we connected the ground clip. If you read the origial text, all calculations was correct for a current of 9A. The comment from Vtile in reply #60 is based on the original text.In the last post we saw that the current flowing from the water heater split into two paths. One path is the neutral wire, and one path is via the ground clip, and from there into the PE wire.We have some options when we choose how to calculate the value of these two currents. Which method you choose depends on several things, among them which methodyouprefer, and which data are available for the items that are part of the circuit.When we do calculations we can almost always discuss which method is "best". If we don't try to cheat the laws of physics we will always get the same answer. Sometimes we will maybe use slightly more time on the calculations, but that is not relevant for us now.If we look at the last drawing, we can see that we have only one resistor in the neutral wire, and that there are three resistors in the path through the PE wire. The first we will do now, is to transform those three resitors in PE into one resistor.They are connected in series, so we can just add the values together.I mentioned earlier that we must pay attention to how we name all our parts. Since we now will transform these resistors into a new "logical" resitor, we will give it an unique name. We will name itRPRP = RPROBE + RPEC + RPERP = 0.0664 ohm + 0.0336 ohm + 0.1 ohmRP = 0.2 ohmThis gives us this circuit to calculateI have indicated that we don't know the value of I in this circuit. Before we connected the ground clip, it was 9A, but because we changed the circuit when we connected the clip, the total current will also change.The first thing we must do, is to find the value of this new current.If we look at the previous drawing we can see that the resistance in RHEAT (the water heater) is important to know if we want to know the current in the circuit.Almost all kind of heaters are made from a material that does not change its resistance when the temperature changes. This is the same as it is stable when the power or current changes. We will use this knowledge when we calculate RHEAT.The "secret trick" is to calcultate the resistance based on the current we know the value of, namely the current we hadbeforewe connected the ground clip. We can do this, because the resistance is stable regardless of the current.This is the circuit without the ground clip, and I have added all known values.To find RHEAT, we must find the voltage across it.Kirchhoffs voltage law (KVL)https://en.wikipedia.org/wiki/Kirchhoff's_circuit_lawstells us that the sum of all voltage drops should be the same as the source voltage.Since we know both the resistance in RL and RN as well as the current, we can easily find the voltage across each of them.Usually we will specify the values so we know we get it right.URL= RL x IURL= 0.1 X 9URL= 0.9VURN= RN X IURN= 0.1 X 9URN= 0.9VWhen we add these two values to the drawing we get this.I have also indicated the voltage across the resistors. This can be done in several ways. Most textbooks wil use an arrow. Some will use an arrow pointing down, I am used to an arrow point up. I guess some people can spend a lot of time and words on discussing pros and cons. What is most important, is that you stick to only one standard. Else you will confuse yourself. The way I see it, is that the arrow point to the higher potential. Potential is the same as voltage.The way I see it, is that the arrow point to the higher potential. Potential is the same as voltage.We can see from the figure that we can use Kirchhoffs voltage law to calculate the voltage across RHEAT.URHEAT= U - URL- URNURHEAT= 230 - 0.9 -0.9URHEAT= 228,2VThe resistance of RHEAT is determined by the voltage across it, and the current flowing through it.R = U / IRHEAT = URHEAT/ IRHEATRHEAT = 228.2 / 9RHEAT = 25.356 ohm_______________________________________________Since we now know the value of the resistance in RHEAT, we can continue with the circuit "modified" by the ground clip.Because we transformed RPROBE, RPEC and RPE into one logcal resistor (RP), we have only two diferent resistors to care about in that part of the circuit.Now, we face a problem:How can we calculate the value of the new logical R based on RN and RP?If we look in textbooks and formula books we often see these two formulas.I snipped this from wikipediahttps://en.wikipedia.org/wiki/Series_and_parallel_circuitsI am a strong believer in trying to understand the secret hiding in the formula instead of just looking it up.There is also another "secret" with formulas. Many formulas does not give new knowledge. They exist just because it makes our calculations easier. This was most important in the older days before we got pocket calculators, but are not as important today. The second formula from wiki is aspecial version only valid for two resistors. It is of the "time savings" type. Useful, but no new knowledge.Before we continue, we will spend some time on the first formula. What does it tell us?When we calculated the resistance of a wire, I mentioned "conductance" and "resistance".Conductance is a value for how easy it for electrical current to flow. We measureconductanceinsiemens, and we use to say thatG = S.Resistanceis a value for how diffilcult it is for electrical current to flow. This is what we measure inohm, and we use to say thatR = ohm.We can see conductance and resistance as opposites.Think about a road with a lot of cars. If we have only one lane, the flow of cars will be limited. This is because the "conductance" of the road is low. We can also see it as the road has high "resistance".If we have the same number of cars, but increases the number of lanes to two lanes, the flow of cars increases.One lane will still have the same "conductance", but since we have added one lane, that lane will also have the same value for "conductance".The new two-lane road will have a total "conductance" described as this:Gtotal= G1 + G2Back to the first formula:It says something about 1/Rtotal= 1/R1 + 1/R2What this formula actually does, is that it calculates the conductance of all resistors connected in parallel.1/Rtotalis the same as if we wrote Gtotal =1/R1 is the same as if we wrote G11/R2 is the same as if we wrote G2So, how do we tell the calculator how to do this?Amost all calculators, except the most simple types will have one button that does the "one divided-by-something" function.Look for a button marked1/xorX-1To make sure it is the right button, you can easily test it.Enter 100, and hit the button. The answer will be 0.01. Hit the button once more, and the ansver will be 100.And now comes something important for beginners:If we look at the first formula, we see that it calculates the conductance. We are more interested in the resistance. This means we must hit the1/X- button once more after we push the=button, else we will get the wrong answer.Now we can continue with our calculations.We know that the new R must be calculated form the values ofRNandRPIf we use the formula we will write it as this:1/R = 1/ RN + 1/RP1/R = 1/0.1 + 1/0.2When we use a calculator we will enter this0.1[1/X -button]+ 0.2[1/X -button]=[1/X -button]=If we did it right, we will get0.066666667as the answer.This is the same as the combined value of RN and RP is0.0667 ohmWe name this restor just R.We will make a new drawing to help us.Since we now know the value of all of the resistors, and the total voltage, we can calculate the value of the current.We will name the total resitance RTOTAL.RTOTAL = RL + RHEAT + RRTOTAL = 0.1 + 25.335 + 0.0667RTOTAL = 25.502 ohmNext ting now, is to calculate the current. We remember that the current must change from the initial 9A because we changed the circuit when we connected the ground clip.I = U / RI = U / RTOTALI = U / 25.502I = 9.019AThe total current in the circuit changed from 9A to 9.019A because of the ground clip.Does it make sense? Yes, the ground clip made it slightly easier for the curent to flow because it added a second wire in parallel to the neutral wire form the workbench / water heater.Since we now know the value of this logical resitor (R) and the current flowing through it, we can calculate the voltage across it.We will use ohm's law.U = R x IUR= R x IRUR= 0.0667 x 9.019 =0.602This means that the voltage across the resistor is0.602VOK,now we know the voltage across the logical resistor R. We transformed RN and RP into this resistor only because we needed to know the voltage across RN and RP.We know the voltage now, so let's draw a new drawing where we show where it belongs.If we look at this last drawing, we can easily see that we know the value of RN and the voltage across it. We can also see that we know the value of RP and the voltage across it as well.We can also see that we want to know the value of the ciurentINand the currentIP.Back to ohm's law:U = R x II = U / RWhen we want to calculate the current IN, we must use the correct values for the resistor that determines the value of IN. That resistor is RNIN= URN/ RNIN= 0.602 / 0.1IN= 6.02AThen we will calculate IPIP= URP/ RPIP= 0.602 / 0.2IP= 3,01ANow, we can check our calculations.We know that the current flowing from the water heater into the combination of RN and RP is 9.019A.Kirchhoffs current law (KCL) tells us that the sum of all currents flowing into a node is equal to the currents flowing out of that nodehttps://en.wikipedia.org/wiki/Kirchhoff's_circuit_lawsSince 3.01A + 6.02A = 9.03A, we can assume that we got it right. I mentioned something about precision and number of decimals in a previous post. I have already done the calculations with more precision than what we can account for, so we must assume this value to be correct.It does always make sense to do some simple checks like this. Sometimes we enter the wrong numbers, or forget a decimal point or something else that will give us the wrong answer.This is what we know about the circuit at the moment

author: Vtile
author_rank: Super Contributor
timestamp: August 06, 2017, 12:06:24 am
content: Oh. superb post tronde. For conductance .. it used to be marked as mho or upside down omega (resistance symbol) before the Siemens unit came standard. As the 1/x operation is called inverse the old 'mho' made sense. Inverse is one of the most powerfull tools in calculator btw.

author: tronde
author_rank: Frequent Contributor
timestamp: August 06, 2017, 11:51:24 pm
content: Since I changed and added more to the previous post (#59), this wil be a complete re-write. Most of the original information is moved into the previous post.Time to round off this part of the "how not to kill your scope" tutorial.Since we ended up with a lot of calculations, we can spend some time on formulas.I said I was a strong believer in trying to understand the secret hiding in the formula instead of just looking it up.When a beginner start to learn about electricity, Ohm's law is among the first law he will meet. Quite often it will be in this form:Then the beginner tend to say "oooh so many formulas to learn".First ting to be aware of, is that this is actually not only Ohm's law.Ohms law isU = R x I, but we can see something likeP = U x Ias well.What is thisPabout? P is about power, and the formula that reads P = U x I is known as Watt's law. Watt was the guy with the steam engine.My claim is that this "wheel" is actually only two formulas that gives us new knowledge. The formulas that gives us the knowledge, is those we must pay attention to. The rest is just a collection of "make calculation easier" formulas as I mentioned earlier.Ohm's law is marked green, and Watt's law is yellow.Let's see how we can turn these two formulas into twelve.#1 to #6 are easy. That is just shuffeling the formulas around.The "secret" is hidden in what we do to get #7 and #10.First #7:We start with P = U x I.Then we know that we can write U as R x I.R x I is #1.# 8 and #9 are just modifications of #7.Then we can look at #10.We start with P = U x I once more.We know that we can write I as U/R.U/R is #2.#11 and #12 are just modifications of #10.If we understand this, we can spend the time un understanding how to use the knowledge instead of remembering a lot of formulas.A sidenote about the "wheel".Differnt countries and languages will use different letters for voltage. In this wheel the letterUis used for voltage. You can also find thatEorVis used.Sometimes you will findWused instead ofPfor power for the same reason.So, why care about watt in this tutorial?We have spent a lot of time calculating current. Sometimes we don't know the current consumption of something, but we know how many watt it consumes and the voltage. Then we can calculate the current.___________________________________________________________The last drawing I have prepared is a complete circuit, including the scope and the device we test. Click on the first thumbnail (complete.jpg) to see it.I have indicated both as resistors, and it is possible to give them some values if you want to calculate the circuit again.You can for instance say that the device is a 20W / 230V led driver. How many ohms will that transform into?I have made the ground clip into a switch that can be on or off, and it is possible to see how the circuit will change. I have also indicated that the RHEAT is in fact conected to some wires. I indicated in the first post that the heater was in another room, so it is obvious it must have some wires attached. It is possible to give these wires some resistance. Try to calclulate the circuit if they are, say 10m long and have an area of 2.5mm2.What will happen if we don't have the water heater mixing itself into the circuit?What will the result be if we just connect the ground clip to the neutral wire of the device and we power it up?If you have some kind of ground fault protection this will trip and cut the power to the lab bench if the current in the PE wire is above 30mA (if in Europe).Suggestions, please.Spend some time playing with the complete circuit and do some calculations using other values than I did. It is important to be familiar with this basic stuff, and I advise you to use pen and paper instead of some of the fancy simulators until you feel really familair with "decoding" such circuits as we have here.Next will be about floating the scope I think.

author: tronde
author_rank: Frequent Contributor
timestamp: August 07, 2017, 09:45:57 pm
content: I post this just so those who follow will get a notice. I have added some important info to post #59 and done a complete rewrite of #61 because of this.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 07, 2017, 10:35:24 pm
content: Anyway, I think we've discussed before as to why scopes are designed like this whilst multimeters never have been. Likely reason seems to be that in the old days they were always metal cased, and being valve based had some impressive voltages inside. So, earthing the case and inputs was the safer option, even though it creates the risk of a shock or shortTOthe case or probe outers from a live terminal, which would not otherwise exist.Nowadays with plastic cases, 12v internal supplies and readily available optoisolators etc there is really no reason for them to have earthed input circuitry, but it is something which has persisted.Thinking about this a little more, the earthing on scope probes most likely violates electrical standards, which require that any earth connection should be robust enough to blow the supply fuse should a short occur.

author: tronde
author_rank: Frequent Contributor
timestamp: August 14, 2017, 12:06:12 am
content: Since nobody felt they would answer my questions, I will do it.What will happen if we don't have the water heater mixing itself into the circuit?What will the result be if we just connect the ground clip to the neutral wire of the device and we power it up?First thing we must do, is to make a drawing of this new situation.or like this.Since the water heater is disconnecected, the 9A return current from it will not interfere with our measurements. Can we now consider it safe to connect the ground clip to the neutral wire?I said the device is a LED driver marked 20W / 230W. We can transform this in to resistance, and do a complete calculation as we did in the previous post, but it does not make any sense for what we want to find out. The transformation will depend upon a lot of things to be correct, and the resistance we calculate will most likely be wrong in a real-life situation because the device is an electronic circuit.We are not interested in the most accurate value of the currents. What we are interested in, is to find out if we do something stupid or dangerous. For us, this will be to find the approximate value of any current and/or voltage we, or our equipment will be exposed to.I have also included the scope into this circuit becuase it will have much larger impact on the total than when we had the 9A current mixed in. The scope is marked 25W / 230V.If you really want to do complete calculations you can use formula 11 from the previous post anf calculate RSCOPEto be 2116 ohm and RDEVICEto be 2645 ohm.Instead, I will suggest a more simpler calculation only using the current in each item.First we will calculate the current in the device. We will use Watt's law.P = U x II = P / UFor the device this gives us:IDEVICE= PDEVICE/ UDEVICEIDEVICE= 20 / 230IDEVICE=0.087Aor87mAFor the scope this gives us:ISCOPE= PSCOPE/ USCOPEISCOPE= 25 / 230ISCOPE=0.109Aor109mANew drawingWe can see that the current from the scope and the device will combine into a new current of 196mA.To calculate IN and IP we need to know the combined value of RN and RP. We have already calculated it to be 0.0667 ohm. Remember that the wiring has not changed, so we can use all relevant values calculated earlier.Since we know the resistance of R, and he current, we can calculate the voltage across it. Remember that the current has changed, so we must use the "new current" in this calculation.U = R x IUR= R x IRUR= 0.0667 x 0.196AUR=0.0131VThen we can calculate each of the currents.U = R x II = U / RIN= URN/ RNIN= 0.0131 / 0.1IN=0.131Aor131mAIP= URP/ RPIN= 0.0131 / 0.2IN=0.066Aor66mAIf we add 66mA and 131mA we get 197mA. This is 1mA too much, but it is because of rounding numbers.Since the resistance in RP is twice the resistance of RN,we can just say that the current in RN must be twice the current in RP.This is the new situation when we connect the ground clip to the neutral wire.So, what does this tell us?First, we can see that we still will have a current flowing through the ground wire in the probe. It has a much lower value than when the water heater was mixed in, but the probe is not meant to carry any current, so it is still wrong to connect the ground clip to neutral.* * * * * * *I did also mention something about ground fault protection and 30mA. If the lab bench is protected by any kind of a RCD (residual current device) it will trip and cut power to the bench because of the 66mA current flowing in the PE wire.Many people think they will be safe if they have a RCD, but that is a very dangerous assumption. RCDs are not meant as a device that will allow you to do silly things with electricity. It is meant to be a last resort if something goes wrong with the insulation in the complete system so you will not get a dangerous current flowing through your body.In Europe, they will most likely trip if there is a leakage current larger than 30mA. This is not the same as any current below 30mA can be considered safe. Most people wil survive an electric current of no more than 30mA, but some will not.Since we are measuring on electronic devices, we must also be aware of something important regarding RCDs.The normal type used in Europe is known as "type A"."Type A" will not trip reliably if there is more than a 6mA DC-current component in the leakage current.If we have a DC-component, we must use a "type B" RCD. They are expensive and not common. If you intend to use a "Type B" in an installation where you already have a "Type A" you must check the datasheet for both types. Sometimes they will influence on each other and don't trip as expected.Some reading about RCDshttps://en.wikipedia.org/wiki/Residual-current_deviceBecause of size limitations, I will post two attachments about RCDs from ABB and Schneider Electric in the next post. Worth reading.The first attachment in this post is a modified complete schematics for this post.

author: tronde
author_rank: Frequent Contributor
timestamp: August 14, 2017, 12:11:13 am
content: Attachments to previous post about RCDs from ABB and Schneider Electric

author: rajiv85
author_rank: Contributor
timestamp: August 22, 2017, 12:33:26 pm
content: Great contributions Tronde.. will take some time to assimilate all of it.. I am having some exams this month. But as soon as I am done I will come back to study what you explained to make sure I got it.Thank you!! Perhaps such posts should be pinned as it can be useful to many other people.

author: swgertsch
author_rank: Contributor
timestamp: August 29, 2017, 06:29:40 pm
content: As a newbe what we need are some clear ABSOLUTES (guidelines) to so we do hurt ourselves or damage any test equipment. I accept the fact that the process of learning electronics involves the death of many a component; a certain amount of sacrifice on the alter of learning is required. These "sacrifices" help cement our understanding of what is going on, how things can go bad, and most importantly how to trouble shoot and repair electronic devices.With respect to oscilloscopes here is one thing I have learned: the simple definition of an oscilloscope being a time based graphing voltmeter causes problems for newbes. This simplistic definition causes a newbe to equate oscilloscope probes to DMM-Voltage measurement mode probes and are therefore are likely to attach the ground clip with little regard to how oscilloscopes actually work.With respect to using any tool it is important to understand how the tool works in order to use it safely and effectively.(I do not understand why oscilloscopes do not have better protection (fast blow fuses, etc.) on probe ground leads and between non-isolated channel BNC connectors. I suspect protection is not there because it degrades measurement accuracy.Right now I restrict myself to using a single probe with the ground clip always attached to the circuit's GND and using power sources with a floating ground voltages sources: <24VAC, <30VDC.

author: tronde
author_rank: Frequent Contributor
timestamp: August 29, 2017, 07:42:10 pm
content: Quote from: swgertsch on August 29, 2017, 06:29:40 pmThis is a very wise approach. Accept that you have some limitations in your knowledge, and progress step by step as you understand more.I can see that a list of what you call "absolutes" would be nice to have, but the problem is that no lab bench is similar. I have some more subjects to cover, and I will write more about them within not so long time.My best and honest advice to any beginner, is to spenda lotof time calculating what is known as "combination circuits".http://www.physicsclassroom.com/class/circuits/Lesson-4/Combination-CircuitsUse pen and paper, not a simulator. When you do this, you get familiar with how the currents and voltages distribute in a circuit, and it will be more easy to see and understand what can go wrong when you connect any kind of test equipment to a circuit.I know many beginners find this boring, but this most basic stuffisimportant. It is like learning to swim or ride a bicycle. Difficult in the beginning, but you will never forget it when you have learnt it.The problem with test instruments, is that theyinteractwith the circuit you test on. That is why it is so importat to "see" how the circuit changes when we connect something external to it. To see this, we must also know a lot about the instrument we connect to the circuit.

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2017, 08:08:42 pm
content: Quote from: tronde on August 29, 2017, 07:42:10 pmNot so much really.We keep using the termprobe ground leadand in my very strong opinion this is where misunderstandings start and grow.Instead if we all think REFERENCE lead as the signal is referenced to the probe lead. That the probe lead is atmains ground potentialtoo is VERY IMPORTANT.Just these 2 things need clear understanding:Probe lead is for signal reference AND is at mains ground potential.Mentioned previously in the first page of this thread:Quote from: tautech on July 12, 2017, 01:56:05 amSure everybody, keep using the term probe ground lead at your peril, one day in a moment of inattention you'll clip it where it shouldn't be connected and either blow your scope, probes or the DUT.Thinkreference..........andtied to mains ground.

author: tronde
author_rank: Frequent Contributor
timestamp: August 29, 2017, 08:35:51 pm
content: The "problem" with ground as a term is based on history. Ground as a term was used in radio long before it became a thing for safety.Ground is widely used for reference level, but if you work with mains, you wil see the term "protective earth" or "earth". Connecting things to the soil for mains safety was not implemented before around 1920/1930, and by then the term "ground" was already used for the reference level.The problem is not that we call the clip for ground clip, but that we call protective earth for ground.

author: alank2
author_rank: Super Contributor
timestamp: August 30, 2017, 12:50:58 am
content: Is the problem ALWAYS that the ground lead allows a high current situation when connected to something that is not at the same potential it is?For example, if you have a USB scope that says it can handle mains level voltages and you can connect the ground clip safely, could you then measure them without a differential probe?

author: Brumby
author_rank: Unknown
timestamp: August 30, 2017, 01:55:21 am
content: Quote from: alank2 on August 30, 2017, 12:50:58 amThat is the principle "destruction" issue. It is most applicable to situations where there is a low impedance path in the rest of the circuit formed - which results in high current flow.There are other equipment survival issues as well. Ones that may not be quite as dramatic. Just imagine a circuit that is earth/ground referenced and taking a point in that circuit which is at a different potential ... and then grounding it by using the scope reference/ground clip. There is the possibility that this could cause stress on the circuit that might result in damage. Such damage could be the invisible demise of a component or letting the magic smoke out. Even if there were no damage, the circuit operation would almost certainly be affected and the measurement corrupted.QuoteWith those two caveats very firmly enforced, then, yes. BUT, in the real world, there are real risks which would make it a less than wise approach. For starters, there's no way I would (personally) run mains level voltages through a USB scope - unless it was on a computer that I was not worried about frying.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 30, 2017, 06:47:19 am
content: All oscilloscopes (except a few portable units) are basically 'Cat zero' safety rating. The have no fuses, MOVs or other input protection, and putting the probe outer onto a high current connection is going to ruin your day.Why are things like this? Basically the reason is historical. They never have had input safety devices, so I guess the manufacturers don't see any need for them. Although when I think about it, even the old multimeters like the AVO8 had a mechanical trip that would usually save your bacon if you made a silly mistake.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2017, 02:52:39 pm
content: Quote from: IanMacdonald on August 30, 2017, 06:47:19 amIt is historically this way because of its difficulty. Making an oscilloscope with ground isolated inputs was somewhere between difficult to impossible and most applications did not require them. It is so difficult to do with good performance that high voltage differential probes are easy in comparison. The situation is only different today because high bandwidth digital isolation is available and this was starting to be used in oscilloscopes for this purpose starting in the late 1980s.In rough order of development, solutions for the isolation problem were:1. Floating the oscilloscope.2. High voltage differential probes up to about 50 MHz.3. Analog isolated probes up to about 20 MHz.4. Digital isolation of the digitizer.Oh, and another method I have used in the past is to convert the voltage to a current in the circuit and use a current probe; this commonly works up to about 50 MHz however it is feasible up to 1 GHz in some AC only applications.For a multimeter, making a ground isolated input is trivial because the bandwidth is very limited.

author: Mr. Scram
author_rank: Super Contributor
timestamp: August 30, 2017, 06:21:07 pm
content: Quote from: bson on July 10, 2017, 09:45:53 pmSo how would you get the voltage drop across that resistor?

author: tronde
author_rank: Frequent Contributor
timestamp: August 30, 2017, 06:51:13 pm
content: Quote from: Mr. Scram on August 30, 2017, 06:21:07 pm

author: Mr. Scram
author_rank: Super Contributor
timestamp: August 30, 2017, 06:55:31 pm
content: Quote from: tronde on August 30, 2017, 06:51:13 pmSo putting the ground/neutral lead behind the load, is what you're saying? Wouldn't you also get the load resistance included?

author: tronde
author_rank: Frequent Contributor
timestamp: August 30, 2017, 07:09:26 pm
content: My drawing is showing exactly the same as the one you posted, only slightly re-arranged.If you move the shunt into the neutral wire, and connect the cround clip to the end of it most close to the source (lowest voltage potential), you will not get a short. But, as you can see from some of my previous posts, you can end up with an unwanted current in the ground clip / wire.The best choice to measure the voltage drop across the shunt is to use a differential probe or use two probes without ground clips and the math function in the scope.Measuring mains with a normal scopeischallenging, and it is easy to forget a tiny detail that will ruin everything.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2017, 08:48:43 am
content: Quote from: dmills on July 11, 2017, 10:37:05 amAll good but I don't see why you'd use an RCD on the secondary side of an isolation transformer? It won't do anything! If one side of the secondary is earthed, there will be no leakage and the RCD won't trip.A better solution would be an insulation monitoring circuit. This could be made with a neon lamp and some high voltage resistors, so the lamp glows, if one side of the transformer is earthed, thus warning you the other side is live, with respect to earth.