Still unsure about measuring Mains Voltage with an oscilloscope - Page 1
Forum section: electronics / beginners

author: VanillaIceCream
author_rank: Contributor
timestamp: March 02, 2021, 10:34:36 pm
content: Hi, I wanna measure mains voltage (230V, 50Hz) with my battery powered, non grounded oscilloscope with a x100 high voltage probe rated for 2000V DC/ACpk.Ive already watched dozens of videos and read dozens articles about how to do it but couldnt find the answers for my problems.1. Its best to connect the phase wire to the positive lead of the voltage probe so that you dont get an electric shock incase you somehow touch the bnc ground connector on the oscilloscope, right?2. Is it a problem if I measure the mains voltage on an extension cord while some device like a light bulb or pc for example is already connected to it and running?Would appreciate some help, thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2021, 10:50:16 pm
content: The *ONLY* advice you'll get here without providing the exact make and model of your handheld scope and x100 probe is:*DON'T*!With *most* cheaper handheld scopes and ordinary x100 probes you'd be one moment of inattention or slip of the hand + a loose supply side Neutral connection away from a serious shock and possible electrocution. Like jaywalking on the freeway, its not a good idea . . . .*If* you provide the make/model no.s and it turns out your scope and probe are both CAT II 600V or better rated, with a fully shrouded BNC plug, (or other touch-safe shrouded connector system), we may give different advice.

author: cliffyk
author_rank: Frequent Contributor
timestamp: March 02, 2021, 10:58:08 pm
content: Don't touch anything metallic and shiny and you'll be OK. If you fell you might do so "accidentally" then forget about doing it at all...

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2021, 11:06:03 pm
content: Quote from: cliffyk on March 02, 2021, 10:58:08 pmUnfortunately if the scope isn't at least Cat I rated, your hand could be only a millimetre or two away from metal inside the case that isn't isolated from the BNC shell. Add a little dirt and a sweaty hand and it could easily arc over through the case seam to your hand and on to ground.

author: VanillaIceCream
author_rank: Contributor
timestamp: March 02, 2021, 11:23:01 pm
content: Quote from: cliffyk on March 02, 2021, 10:58:08 pmThanks but Is it a problem if I measure the mains voltage on an extension cord while some device is already connected to it and running or not?

author: cliffyk
author_rank: Frequent Contributor
timestamp: March 02, 2021, 11:31:37 pm
content: Quote from: VanillaIceCream on March 02, 2021, 11:23:01 pmOf course not--why would it?To be frank, that you ask this question indicates to me you need to gain more knowledge before mucking about as you seem inclined to do.There are many, many human endeavors in which not having sufficient knowledge and being careless (by definition "without care") that can get you killed--don't be ignorant or careless and you'll be OK....

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 03, 2021, 12:06:16 am
content: You're asking the wrong questions, there is no simple universal rule set of how to probe something safely with a scooe or what is safe to probe. You MUST understand the circuit you are probing, the circuit of the oscilloscope probe and input, and how the two will interact. Oscilloscopes and probes are not, for lack of better term, "idiot proof" like a well designed modern multimeter.Mains pretty much looks the same anywhere you go, and there's nothing you can do to change it if it's especially ugly, so there's little point in viewing it. It's a sine wave with the peaks clipped off to some degree by all the rectified loads that only pull power on the peaks. If you insist on seeing it the easy and hazard free way is to find a low voltage power transformer (any old "wall wart" with AC output will be ideal) and probe the output of that. And again, understand how doing it that way makes it safer for you and the scope.

author: cliffyk
author_rank: Frequent Contributor
timestamp: March 03, 2021, 12:49:20 am
content: Quote from: BrokenYugo on March 03, 2021, 12:06:16 am^^^ A GREAT answer/advice--well done!!!

author: wizard69
author_rank: Super Contributor
timestamp: March 03, 2021, 01:03:17 am
content: DON'T!!!Seriously the questions you are asking tells me you are not ready yet. Spend some time leaning about AC systems, probing techniques and general safety around mains wiring. Frankly the web is a terrible place to learn about safety related issues when it comes to high energy systems. The problem is no immediate feedback in the conversation.

author: MikeK
author_rank: Super Contributor
timestamp: March 03, 2021, 02:59:14 am
content: Watch EEVBlog #279:EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2021, 03:02:43 am
content: If you are in any way unsure, the best advice isdon't do it. There is no second chance if you screw this up and wind up dead. And what are you expecting to see anyway? Touch your finger to the tip of the probe and you'll see the mains voltage waveform due to your body acting as an antenna and picking it up out of the air.

author: golden_labels
author_rank: Super Contributor
timestamp: March 03, 2021, 03:03:12 am
content: VanillaIceCream:WHY? If curiosity is what drives you, a collection ofmains waveformshas already been posted. If not that, there still may be some other option.Out of possible options, you have chosen the most dangerous: a floating scope. You don’t even have to make any mistake to get zapped, because in that setup connectors for other channels may be live. So… don’t. And the proer answer is: use a differential probe rated for high voltages.

author: Brumby
author_rank: Unknown
timestamp: March 03, 2021, 08:06:24 am
content: Quote from: wizard69 on March 03, 2021, 01:03:17 am+1

author: RoGeorge
author_rank: Super Contributor
timestamp: March 03, 2021, 08:50:23 am
content: Quote from: VanillaIceCream on March 02, 2021, 10:34:36 pmAs long as you keep the oscilloscope isolated from the mains and from the ground, e.g. put it on a wooden plank or some other good electrical insulator. Do not connect other wires to the oscilloscope or to the power supply/battery that feeds the oscilloscope. Keep them floated. Do not attach other USB cables or LAN cables or GPIB cables to the oscilloscope, just the measuring probe.As long as the oscilloscope is floating, it's like a DMM, you can connect it in any 2 points, but don't touch any metallic parts and operate it only when disconnected from the mains, just to be sure.Do not connect more than one probe, keep in mind the channels are usually not isolated, they have the same ground which is also the chassis of the oscilloscope.To answer your questions1. Yes, don't put the live wire to the oscilloscope's chassis.2. No problem. A battery powered and electrically floating oscilloscope is like a DMM as long as you connect only one probe/channel. Don't touch any metallic parts of the oscilloscope.

author: Ian.M
author_rank: Super Contributor
timestamp: March 03, 2021, 09:39:46 am
content: .... except any handheld multimeter that didn't come out of a cereal packet will be designed to attempt to protect the user holding it from electrical mishaps inside it, with deeply overlapped case seams, no exposed metal parts that aren't fully insulated from the interior, and if its CAT rating isn't fake and is appropriate to the mains work you are doing with it, you can be pretty confident that it will contain any mishap and you wont get a shock off the meter body.Cheaper handheld scopes don't have any of that because they are*NOT* *DESIGNED* *FOR* *MAINS*, so don't use them on mains circuits! If it and its probes have a CAT rating and its a reputable brand, (so not fake ratings), RTFM, then use it with cautious confidence.Also, testing the mains voltage on a power strip is somewhat *MORE* *DANGEROUS* if there is another load on the same power strip. The number of connections the Neutral goes through is increased, they are inherently less reliable than the connections in a properly installed fixed circuit feeding a wall outlet, and if any of them are bad or intermittent the other load will pull Neutral up to the full mains Line voltage. Keep a three indicator light socket tester plugged into the power strip for the duration of any experiment, and if the lights indicate a fault at any time, *STOP* and immediately unplug the power strip from the wall.N.B. Fully qualified competent engineers who *THOUGHT* they knew what they were doing have been killed by using floating scopes on mains circuits. Please try *NOT* to join the club . . .

author: tggzzz
author_rank: Super Contributor
timestamp: March 03, 2021, 03:33:43 pm
content: Quote from: Ian.M on March 03, 2021, 09:39:46 amJust to emphasise that, fromhttps://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465Example fatality fromhttps://groups.io/g/TekScopes/topic/7632418#10795withmy emphasisQuote

author: Siwastaja
author_rank: Super Contributor
timestamp: March 03, 2021, 03:42:36 pm
content: Quote from: tggzzz on March 03, 2021, 03:33:43 pmSound like a lot of idiot-proof precautions were taken. It would be really interesting to hear what finally happened, what did he miss.Such practices get more dangerous the more time you play around with them. Eventually, you get distracted and start to "autopilot", by doing things you normally do in a safe environment. For example, touch that one metal knob by habit even though you assured yourself 30 minutes ago you wouldn't do itthis time.I have ran some tests like that but prepare well and keep them very short and hands-free. Scope is problematic because you need to operate all those knobs. If the story is true, having prepared clearly wasn't enough.

author: tggzzz
author_rank: Super Contributor
timestamp: March 03, 2021, 04:03:16 pm
content: Quote from: Siwastaja on March 03, 2021, 03:42:36 pmThat anecdote was written by the engineer that investigated the incident as part of his professional duties. It is much more credible than urban legend.Yes, of course it is an anecdote - but one that is a concrete example of sound proven theory and practice. It is much more credible than urban legend.It is worth repeating anecdotes, because some people - particularly those that distrust "theory" and "experts" - tend to place more reliance on personal stories than impersonal theory. Strange, but true, and worth understanding.(BTW, the correct reference appears to behttps://groups.io/g/TekScopes/message/10754I don't know what changed)

author: Siwastaja
author_rank: Super Contributor
timestamp: March 03, 2021, 04:38:56 pm
content: Note that GFCIs (a.k.a. RCDs) have greatly reduced such fatal incidents. They are lifesavers, literally.Still, don't let your guard down. If nothing else, there's a possibility this particular circuit is not protected by GFCI for any number of reasons, or you manage to get yourself between live and neutral in which case the GFCI can't do anything to save you.

author: Matje
author_rank: Regular Contributor
timestamp: March 03, 2021, 07:54:05 pm
content: Quote from: VanillaIceCream on March 02, 2021, 10:34:36 pmAs others already said, don't, at least not the way you described.What may actually work and would be safe would be the following:Take your extension cord, put it on your desk in front of the scope.Take some length of wire, I'd guess a foot or two. Remove the isolation from the last centimeter of it. Take some sticky tape and fix the isolated end of the wire to the cord. Tightly wrap the wire around the cord as often as it will go (making a kind of coil), fix the other end with tape too.Now plug the extension into the wall socket, then clip your x10 probe tip to the unisolated end of the wrapping wire.Bingo, you should see the mains voltage capacitively coupled into the wrapping wire, just with a much smaller amplitude and with no risk of electrocution. You can try if plugging some light fixture or so into the extension changes anything, although I think it won't.Beware, I didn't actually try that one will see anything on the scope, the voltage coupled in may be too low after all. If you try it you could report back so there is something besides "don't!" we could tell people who want to see the mains voltage form.

author: Ian.M
author_rank: Super Contributor
timestamp: March 03, 2021, 08:25:25 pm
content: A further argument against direct mains connection of a probe and scope that don't both have an appropriate (and not faked) CAT rating is fast high voltage transients on the mains. Transients exceeding 1KV riding on top of the mains waveform are not unusual, and if you are in a multi-story building with a lift or heavy duty aircon, or live near an industrial estate, factory or farm, larger transients may well be common. If you live near heavy industry or a welding shop, the transients are likely to be even bigger. Even 1KV transients have a significant risk of blowing the input of a non-CAT rated scope. Leave it connected long enough and damage becomes statistically certain. How lucky are you feeling today?Quote from: Matje on March 03, 2021, 07:54:05 pmUnfortunately capacitively coupling the probe wont show you anything useful except the frequency, as obviously the amplitude is uncalibrated, and higher frequency components will pass through the coupling capacitance more easily so it will make the waveform look more distorted than it actually is. You might as well:Quote from: james_s on March 03, 2021, 03:02:43 amwhich will show you much the same and is far easier and less risky!

author: John B
author_rank: Frequent Contributor
timestamp: March 03, 2021, 08:38:23 pm
content: The big problem I've found is not being unaware of the risk, it's just human error in the execution. One time I was doing some repetitive test on a mains circuit - turn on, test, turn off. At one point I was thinking about something in the circuit and went to go grab a handful of live parts. Thankfully the spidey senses kicked in at the last instant. It just happens. So best to ensure you have test gear that removes that possibility in the first place, like a differential probe.

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2021, 08:43:10 pm
content: Quote from: John B on March 03, 2021, 08:38:23 pmMore than once I've had several things plugged into a power strip and unplugged the wrong thing. Mistakes can happen to anyone.

author: tooki
author_rank: Super Contributor
timestamp: March 04, 2021, 06:18:19 am
content: What is it about so many beginners that causes so many of them to want to scope the mains? I feel like we get this question every few months...Now one of them just needs to make a video about scoping mains and soldering to produce the ultimate “low skill and I’m gonna prove it!” tutorial...

author: Gyro
author_rank: Super Contributor
timestamp: March 04, 2021, 09:57:30 am
content: Quote from: tooki on March 04, 2021, 06:18:19 amThat's because we do!I think beginners are buying scopes too early these days, maybe their first purchase, together with, or before, a DMM and don't have anything to look at. I suggest buying the simplest possible function generator kit, eg:https://www.ebay.co.uk/itm/High-Precision-Function-Signal-Generator-DIY-Kit-Sine-Triangle-Square-Output-UK/133614956705which serves the dual purpose of learning to solder something and providing something to probe.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 04, 2021, 10:51:36 am
content: Most beginners play around with Arduino or similar these days and have plenty of interesting signals to look at with a scope, including UART, SPI, PWM...

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 04, 2021, 10:55:07 am
content: In addition to the "please don't" sentiment: forget about "neutral" and "hot". While it may mean something for your mains connection, it won't in other parts of the world. In addition, you're putting your confidence in an entire chain of executions, all of which may be faulty (ie grid company, electrician, cable supplier, distribution box, …Do yourself a favour, think of both wires as 'hot' and 'hot'.

author: Microdoser
author_rank: Frequent Contributor
timestamp: March 04, 2021, 11:53:23 am
content: Personally, I have a healthy respect for any power source over about 30V or 0.1A stemming from having worked decades ago with huge amperage 650V DC rails/cables near 25KV AC lines.My advice would be that if you are unsure about mains voltage, don't use it until you are sure.It just takes the slightest improper use to cause danger to either your devices or your health. As has been said, a slip with the probes could give you a big jolt or blow your scope, improper grounding could give you a big jolt or blow your scope.Basically, like the guy who asked the price of a Rolls Royce and was told "If you need to ask, you can't afford it":If you are unsure then you simply shouldn't until youaresure of what you are doing.Besides that, I don't see much reason to connect your scope to mains voltage anyway.Use a multimeter instead in AC 500V mode to check for mains voltage presence, it's what it is for. Use your scope to probe signal paths because that is what it is for, don't fall victim to the golden hammer fallacy.http://en.wikipedia.org/wiki/Law_of_the_instrument"If the only tool you have is a hammer, you view everything as a nail"

author: tggzzz
author_rank: Super Contributor
timestamp: March 04, 2021, 12:25:46 pm
content: Quote from: Microdoser on March 04, 2021, 11:53:23 amSnipped good advice, but the problem with those statements is the Dunning Krueger Syndrome. At least the OP had the sense to ask!

author: Gyro
author_rank: Super Contributor
timestamp: March 04, 2021, 12:30:48 pm
content: Aaargh! I'm going to seriously flip the next time somebody throws "Dunning Krueger Syndrome" into a conversation!:

author: tggzzz
author_rank: Super Contributor
timestamp: March 04, 2021, 01:18:17 pm
content: Quote from: Gyro on March 04, 2021, 12:30:48 pm"The Dunning Kruger syndrome is the reason we need a police force". Discuss....sits back and waits for reaction...

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2021, 01:33:28 pm
content: Quote from: Gyro on March 04, 2021, 12:30:48 pmYes, the Dunning Kruger meme is the '20s equivalent ofGodwin's Lawfrom the 90's. Nobody makes hyperbolic comparisons to a certain single slug mustached former German leader any more because its the death of rational conversation. Similarly, anyone who cites Dunning Kruger in other than an academic context has (IMHO) no further interest in the discussion!

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2021, 01:59:34 pm
content: Quote from: Ice-Tea on March 04, 2021, 10:55:07 amYes. Lets list the (english language) mains supply conductorsLine(L) - one of the phase conductors.Neutral (N)- the return conductor for single phase circuits, and star wired three phase circuits.GroundorProtective Earth(PE) - the grounding conductor for chassis or metal casings.*BOTH*LineandNeutralare*Live*conductors, they carry operating current and are hazardous to touch. Significant leakage from either to PE will trip a properly installed RCD (GFCI). Neutral is *USUALLY* near ground potential but you cannot and must not count on that. e.g. in many European countries anything using a Shuko plug is *NOT* reliably polarized i.e. L and N can be swapped depending on the orientation of the plug.SupplyGroundakaPEis generally regarded as safe to touch. However if you have a poorly maintained (by the utility)TN-C-Ssupply and either export PE outside the equipotential bonded area (usually the interior of the house or apartment) or import a true Earth ground from outside the equipotential bonded area, without bonding it, you can encounter dangerous voltages between PE and true ground.Edit: See Siwastaja's and my comments below on the misleading and confusing use of the word 'Ground'!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 04, 2021, 03:15:19 pm
content: It's a safety disaster we use such similar words as LINE and LIVE to convey partially the same, but in safety sense, totally different concepts! They are very easy to mix up not only because of the one-letter difference, but also because line IS live; but the opposite (live = line) is true only 50% of the time which makes the mixup so dangerous.Repeat after me:NEUTRAL IS LIVENEUTRAL IS DANGEROUSYOU DON'T KNOW IF YOU HAVE A NEUTRAL OR LINE EVEN IF YOU THINK YOU DOTHEY ARE BOTH LIVENEUTRAL IS DANGEROUSAlso, avoid using term "ground" when talking about PE. Too high of a risk of mixup because "ground" is widely used as a "reference" in electronics. This is totally different function from PE. Just say PE. Make people google it if they don't know what it is. Or better, don't let anyone who doesn't know what PE is, do anything regarding electric installations. I'm always "on guard" when someone says "ground" because it's an alarm bell they may not know what they are talking about (although sometimes it is just an innocent colloquialism a real professional may use).

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2021, 03:46:02 pm
content: Unfortunately the whole of (allegedly) 'english' speaking 'Westpondia' uses 'Ground' for PE, (its all the way through the US NEC electrical code), so as members of an international forum, we are stuck distinguishing between true ground to Earth, mains supply ground (PE), chassis or signal ground, and local common misnamed 'ground', all of which may be at dangerously different potentials, on a case by case basis.

author: Gyro
author_rank: Super Contributor
timestamp: March 04, 2021, 04:29:11 pm
content: You'resolast year! In the UK it's now CPC (Circuit Protective Conductor) according to IEE Wiring Regs.

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2021, 04:50:59 pm
content: Quote from: Gyro on March 04, 2021, 04:29:11 pmYes, probably because you can no longer trust the supply (utility) company to provide you with an Earth referenced protective grounding conductor, as during the course of buried cable repair work and replacement in areas originally on TN-S supplies, their contractors have failed to maintain the integrity of the grounding conductor, and have done partial and piecemeal conversion to TN-C-S, with the bond/split between N+PE and PEN aka CNE totally uninspectable as its buried in a splice somewhere, and often failed to notify affected property holders. Its now causing major issues with the safe installation of domestic electric vehicle charging points (and anything else that exports the CPC (formerly PE) conductor outside the equipotential zone). Simply leaning against your EV while plugging/unplugging its charging cord is now a non-negligible electrocution risk!

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 04, 2021, 05:17:33 pm
content: Quote from: Ice-Tea on March 04, 2021, 10:55:07 amI live in the United States, where we've had polarized mains sockets more or less forever, I make a habit of testing them.I'd say about 1 in 4, regardless of general construction quality, have either the hot/neutral reversed or some madman who thinks he's smarter than he is has installed a grounded 3 wire socket on a old 2 wire circuit with neutral and "ground" shorted together, or maybe left it floating. This can lead to problems beyond the obvious, like the mystery of the intermittently tingly shower at my parents house. Turns out the washing machine was plugged into one of those bootleg ground sockets, and the grounding screw on the machine connected to the floating (don't ask) plumbing, so a trickle of the return current of this high power appliance far from the breaker panel was chosing to go through the occupant and down the drain.So yeah, neutral is kinda live (because copper isn't a superconductor) and not always where you think it will be.