I did a huge mistake, but all went fine so far - Page 1
Forum section: electronics / general

author: mzdenkov
author_rank: Regular Contributor
timestamp: October 07, 2018, 01:37:07 pm
content: Greetings.Well i was measuring some stuff with my scope and shorted High voltage to the ground.Scope survived but(probe did too), 2A fuse on the board that had to be serviced didn't burn as well.Ill attach the schematic of the board that i was working on.Shorted Gnd_P to the mains earth gnd.Should i be worried?Scope is: Rigol DS1052EI think now is the time to put 230V-230V transformer that i have in the box to use it for galvanic isolation.Cheers.

author: Zero999
author_rank: Super Contributor
timestamp: October 07, 2018, 02:01:23 pm
content: The standard advice is if you make that sort of nube mistake, then you don't have adequate experience to be using an oscilloscope with mains powered equipment. Yes, an isolation transformer will prevent that from happening, but it's still easy to receive a fatal shock, as when the scope's chassis is connected to the circuit, the rest of the circuit becomes live, with respect to earth and any RCD protection is lost.I think it's highly likely the bridge rectifier, GBL06, will have been damaged, but it shouldn't be hard to find a replacement part. If you unsolder either the + or - connection, you should be able to check whether it's good or not, using the diode test function on a multimeter.https://www.vishay.com/docs/88609/gbl005.pdf

author: mzdenkov
author_rank: Regular Contributor
timestamp: October 07, 2018, 02:15:04 pm
content: Well, the PS survived, but the Led Back-light was dead before the try of repairing the TV. That was too expensive repair for me to do.So i gave up on that.When i worked with mains i always worked in galvanic isolated area, and now i forgot that because i did that repair at my home.Perhaps if you know, what is the max short current for oscilloscope probe?-and by watching the teardown of this particular oscilloscope, i came to conclusion that its GND can stand quite of theshort current to the mains gnd, before the pcb blows up or the cable from oscilloscopes shield to the gnd blows too.

author: Brumby
author_rank: Unknown
timestamp: October 08, 2018, 03:09:17 am
content: Quote from: mzdenkov on October 07, 2018, 02:15:04 pmIf I were to answer that question, my answer would be: "Anything that is not zero.". Just asking that question causes me to be anxious.If you are wondering about the point at which damage can occur, then there is no general answer. Each probe and each scope may have quite different construction that translates into very different answers.This is another example where the difference between signal earth and protective earth diverges into two very different outcomes for a given scenario. The scope has a signal earth - and you are talking about using it (albeit inadvertently) in a manner that has more in common with a protective earth.Bottom line ... Do not ever wonder aboutmaximumshort circuit currents flowing through a scope that might cause damage - just consider thatANYshort circuit currents flowing through a scope willbe a bad thingblow it up. IMHO.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2018, 05:27:20 am
content: The last time I was doing this, the probe I was using had a micro-grabber for the ground attachment and it turned white hot in front of me.

author: Berni
author_rank: Super Contributor
timestamp: October 08, 2018, 05:54:10 am
content: That must have been a pretty exiting bang when it happened.Its a good idea to get yourself a 1:1 mains isolation transformer if you work on stuff plugged into the wall. But do NOT use the transformer to float your scope off the ground, you really have to know what you are doing there since that can give you a very dangerous shock.Instead use the transformer to isolate your DUT from the mains as this has quite a few advantages. You can clip your scope ground anywhere since its floating, or when you don't have anything clipped you can touch any single point of a circuit without getting a shock, but you should NOT actually use this as a feature tho, just potential insurance if you happen to touch something by mistake. If you do get shocked by it then the current will not flow to ground. So as long as you use the one hand rule you will only get currents inside your hand and not travel going up your arm to the more important parts like the heart. Another nice benefit is that the transformer has a limited short circuit current. The low impedance of mains wiring going to wall sockets will happily deliver >200A into a short circuit (Until the breaker pops), but a few 100s VA transformer will have a short circuit current somewhere in the range of 5 to 50A depending on the size and design of it. This means that shorting the other side just produces a bit of crackling sparks and a loud hum of a very unhappy transformer (That is now quickly becoming hot, so make sure you quickly disconnect it or use a small 5A circuit breaker to protect it).I keep a small 220V 250VA 1:1 transformer around for any work i do on mains stuff. The size of it conveniently limits the current to around 16A so it doesn't pop the standard 16A wall outlet beakers while still providing enough juice to run things up to about about 400W (Not continuously of course) before the output voltages falls of a cliff.

author: tautech
author_rank: Super Contributor
timestamp: October 08, 2018, 06:40:13 am
content: Quote from: mzdenkov on October 07, 2018, 02:15:04 pmUntil something fuses, in your case it was the TV.Just check probe/scope grounding has not been damaged by way of a continuity check from the probe clip to the mains ground pin on the plug.

author: 2N3055
author_rank: Super Contributor
timestamp: October 08, 2018, 09:55:45 am
content: If you do this kind of work on a regular basis, get at least one of these:https://www.batterfly.com/shop/micsig-dp10013?filter_name=micsigIt's a differential probe in which neither of the sides is grounded. So you can measure safely voltage ACROSS any component, floating or not.It is proper way to measure those kinds of circuits.. Floating DUT is very dangerous if you don't know exactly what are you doing, or if a circuit you are testing is broken..Which it probably is, because you are trying to repair it. If there is a grounding error or leak, whole frame can get to high voltage, any you will get hurt..

author: Electro Detective
author_rank: Super Contributor
timestamp: October 08, 2018, 11:04:38 am
content: Oscilloscope manufacturers have been screwing multitudes of noobs and overconfident lab rats with this same routine for decades.ALL oscilloscope inputs should be isolated from the mains, and channel to channel, and external trigger inputs, end of storyIf that toplogy works for multimeters (and high end snob CRO/DSOs), why not oscilloscopes for the average home/student/repair tech punters?It's DANGEROUS to flog a device that has it's single ended ground referenced to an earth linked neutral,which in reality that so called 'neutral' is a grounded LIVE conductor with the same potential to go BANG as the Active linePlease spare us the 'extra cost of isolated inputs' BS, that's been flogged to death by apologists for years.Mass production always sorts that out anyway, manufacturers and their reps are either DISHONEST and or have s*** for brains for continuing this practice for decades.You would think with the release of cheap mass produced DSOs they would have done it by now... instead they flog expensive addons that can cost as much as the DSO itself,or force desperate users to stuff about with half baked isolation transformers.To the noobs reading this: WATCH YOUR BACK with standard CRO and DSOs, they are not 'floating' multimeters with a displayA good probe short BANG can send welding temperature metal shrapel into your eyes before you can react.You won't read about that in any instruction manual, it might affect 'sales' and 'commissions'It's not an 'electronics learning experience' ordeal you need...

author: 2N3055
author_rank: Super Contributor
timestamp: October 08, 2018, 12:38:59 pm
content: In Europe , both mains conductors are treated the same, because plug is not polarised, and there is not telling how you will rotate the plug when connecting.Because of that, protective ground is mandatory on sockets and on all equipment that have chance of contact voltage.Inside switcher there is a well known good ground, connected to the same ground as your scope...Problem is that beginners don't understand scope inputs are not floating, but implicitly grounded, and that CANNOT make differential (between two points) measurements. All you can do with standard probes is measurement of one point in regards to ground. For exactly that purpose, you have differential probes, so you can measure potential difference (voltage) between any arbitrary points in circuit.Isolated scopes are great. But not necessary (if you have differential probes), and they have they own limits, because of special design. They are something to buy if you need it for everyday work.But yes, I agree, i checked in few scope manuals, they mention it that scope is grounded, they warn you to NOT FLOAT the scope, but none of them say, in big red bold words :" Please make note that scope channels are all connected together, and with mains ground with that negative terminal (crocodile). If you connect that crocodile to anything with any potential (voltage) in regards to GROUND, it can KILL your scope, your device or YOU..."

author: German_EE
author_rank: Super Contributor
timestamp: October 08, 2018, 06:44:55 pm
content: The secret of expensive mistakes is to learn from them. Making the same expensive mistake more than once could be a career limiting experience.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 08, 2018, 10:41:20 pm
content: Quote from: mzdenkov on October 07, 2018, 01:37:07 pmGround your stuff before your stuff grounds youAn isolation transformer is a MUST in working with line operated switchers.In my study days at university, they had a lab sign saying (High Voltage Lab):Your first error is your last error.Play safe !

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 09, 2018, 12:13:07 am
content: An isolation transformer or other isolated supply can make for a false sense of safety when a scope probe makes it have a connection to ground again.I also wonder about scopes that are CAT II rated (implying that they're safe to use on plug in mains devices) yet have ground referenced inputs like most scopes do. Do you think the IEC standards should be updated so that any device that is rated CAT II or higher be required to have floating inputs able to safely handle being connected to such a circuit (of the marked voltage and CAT rating) in any way?

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 10, 2018, 05:12:03 am
content: Really old tube scopes don't even have ground pins.Though if you use one, still isolate the DUT.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2018, 02:41:14 pm
content: Quote from: Electro Detective on October 08, 2018, 11:04:38 amIn the past, most people using Oscilloscopes already had a fair degree of knowledge about the Mains supply that appears at the wall socket .(GPO in Oz speak)(In fact, most people in the general population knew that you shouldn't mess with it!)"Noobs" never got within a "bulls roar" of an Oscilloscope, without the mentoring of an experienced person.After all, such an instrument was, in those days, anything from a years wages, to a new car, to a house, in terms of monetary value.As they became used to working on equipment using an analog multimeter, they would become aware of the need for caution around Mains circuitry.Most of the stuff they would be dealing with returned the metalwork to the Mains PE.If the meter probe slipped, they could short the Active to chassis and Blam!When they finally got to use 'scopes, they knew you couldn't put the ground clip on the wrong side of the Mains.With linear power supplies, there was little reason to probe the primary side of the power transformer, apart from simply confirming the presence of Mains, which could be done without the ground clip connected to anything.( the PE of the 'scope provided the return path)Another way was to use two probes & channels in "a+b"mode, with the ground clip either disconnected altogether, or connected to a known, verified, ground point.Some (very few) Oscilloscopes were made with "floating" inputs---- my old BWD, for instance.There are two ways of making such a scope, if it is intended to be powered from the Mains.The first way is to connect both sides of the BNC connector to the input circuit via capacitors, with a normal "guts" with its common side connected to PE.Unfortunately, that yields an "ac only" instrument.The other way is to "float" all the internal circuitry.The downside to this, is that, if you accidentally put the probe "ground clip" onto the Mains Active, all the "common" side of that circuitry will be at 230v RMS w.r.t earth.All good, but that then places a requirement upon all of the rotary controls to have shafts insulated to that voltage or greater.All the switches will have the same requirement.Transformerless radios had the same problem, to an even greater extent, because one side of the circuitry wasalwaysconnected directly to one side of the Mains.It was pure luck, which one!Of course, my 'scope examples are more to do with conventional analog instruments, because they need such things as rotary controls.The controls on DSOs may be a bit easier to isolate ( opto-isolators, etc).If a 'scope has a metal case, that would normally be connected to protective earth in case some of the internal isolation fails over time.For effective shielding, this may require capacitors between the internal "common" or the internal shielding & the case.There we have some more potential points of failure.It is a lot more complex than it seems at first glance.Of course, anybody can get caught out, I have even been caught.I always used an isolation transformer, but this day, someone had borrowed it, & I was quite gingerly, working without it.Just for that second, I forgot, connected the ground clip & probe tip up to both sides of the Mains in a SMPS, walked over to the GPO, plugged it in & switched it on.Bang! Nice flash, the RCD operated, & dropped off the supply to my workshop."Welding temperature shrapnel" ? ----Nah! Bit of a burn on the clip, not affecting its functionality.(I have an oxy set, & have encountered welding temperature shrapnel when cutting sheet metal with some sort of grot on it)You will note that I placed the scope probe connections while the DUT was unplugged & the GPO switch turned OFF.I then took a few steps to plug in & turn on the power.This is exactly what I did normally.Once you have connected the probe, there's no need to hover over it, so any flash will tend not to be in your face.If it is hard to clip the probe hook so it stays clipped, requiring you to hold the probe in position, check which side of the Mains is Active, using. DMM, beforehand.I would urge you to try to clip it so it is hands free, though, if you at all can.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 10, 2018, 03:07:12 pm
content: Hi,did you see Daves review of a scope with 1kV isolated inputs ?maybe that one is four you.

author: dmills
author_rank: Super Contributor
timestamp: October 10, 2018, 04:25:59 pm
content: Personally I have no need for an isolated scope, in truth very few users really do.If you need it there are suitable tools out there, but why offer an expensive, slightly tricky feature on a scope that for 99% of uses has no real benefit?Given a choice between isolation (more hardware!) and say a license for well just about anything like more bandwidth or jitter measurement, or FFT or whatever, give me the license any day.Regards, Dan.

author: Electro Detective
author_rank: Super Contributor
timestamp: October 10, 2018, 10:13:35 pm
content: I doubt electronics retail sales staff or online floggers are going to mention electrical safety and no-no interfacing watchits to the purchaser of a DSO or CRO.Once they get it on their bench, toss the user manual aside, and start probing circuits and power supplies to see the squiggly lines on the screen,it's mostly LUCK (or monitoring audio signals) if they don't do the BANG thing sooner or later.The lucky ones will have a fast functioning RCD/GFCI on their bench that kicks in at 15 to 20ma, as well as the main premises 30ma RCD/GFCI to bail them outSome here are familiar with the full deal on using oscilloscopes wisely and safely, but the "MainsPE challenged to understand"average unboxing punter is not.They are in 'floating multimeter' modeThere may be an electrical wiring or polarity issue in their 6x6 lab or mancave,for the more clued types, the shiny new OS origin 'GoodLuck isolating transformer' may have the chassis and or secondary output tied to earth/ground,and or internal loose connections (nah, can't happen)or even faulty or damaged elcheapo oscilloscope probes.I've seen enough oscilloscopes trashed by their unwary users, and plastique DSOs aren't immune to magic smoke n spark vending either,so I'm sticking with my previous comment above.

author: dmills
author_rank: Super Contributor
timestamp: October 11, 2018, 12:09:42 pm
content: The real issue there is people over estimating their own skills and understanding of the tools, nothing the tools can (or should) do about that!I don't know when mains became fair game, but when I was in school it was VERY much in the "Until you understand why we are telling you not to fuck with this, don't fuck with this" area.I mean it is not like there are not a whole raft of electronical things to play with that don't involve that level of feeding the fuckup fairy.The use of isolation transformers is also IMHO dubious in the hands of a beginner, there are still plenty of ways to screw up with those things (Mainly because the correct thing to do with the output side earth terminal is a subtle thing and highly dependent on what you are doing), and while I own one it seldom gets much use.Regards, Dan.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 11, 2018, 03:44:46 pm
content: Quote from: dmills on October 11, 2018, 12:09:42 pmI use an older one that has no ground pins anyway. To connect 3 pins I use a grounded to non-grounded death-dapter.

author: mzdenkov
author_rank: Regular Contributor
timestamp: November 11, 2018, 09:30:19 pm
content: Quote from: Wolfgang on October 10, 2018, 03:07:12 pmPerhaps that one is for me.For other things mentioned here. Mostly in schools and in college all Power Supplies are isolated from Mains Earth Ground. So it's easy to forget the most crucial things + No-one mentions this when doing measurements, well not in college lessons that we had.