Is it safe to measure mains voltage with my Rigol DHO804? - Page 1
Forum section: electronics / beginners

author: Rijsik
author_rank: Contributor
timestamp: December 21, 2023, 09:42:49 am
content: So I got my first oscilloscope yesterday. It's a Rigol DHO804 and I'm quite happy with it. Currently I am working on a buck converter design. It's coming along nicely, but I will spare you the details. After that I am planning to look into boost converters and then other switching topologies. Some of these topologies, for example flyback converters, convert mains to a lower dc voltage. One question I had is about measuring higher voltages with this oscilloscope. So I am almost certain that I can't measure higher voltages with the standard probes, will need a differential probe for that if I am correct. But even if I would have such a probe, there is still the question of grounding. The scope has a USB-C power adapter and comes with a seperate protective grounding lead. Attached a photo of the lead connected to the scope.First question: How do I hook up this grounding lead? Manual is not very descriptive unfortunately.Second question: If it would be properly grounded, you would have the right probe, and you would have the skills and knowledge to work safely with higher voltage, would it be safe to usethisoscilloscope to take such measurements?Edit: I am not going to mess around with mains voltage. I am aware of how little I know and I would like to not electrocute myself. But I do want to learn about this stuff so that maybe one day I know enough to get started. But that is not in the near future. I am aware.

author: newbrain
author_rank: Super Contributor
timestamp: December 21, 2023, 10:14:14 am
content: Standard forum answer:But my advice is: don't, as if you are asking the question, the conditions in your second question are not fulfilled.

author: Rijsik
author_rank: Contributor
timestamp: December 21, 2023, 10:18:36 am
content: I am absolutely not going to mess around with mains voltage. I am very aware of how little I know and I do not want to electrocute myself. I do however want to learn about stuff like this and the only way to learn is to ask questions. I have already seen that video and it was very insightful. But it doesn't really help me with the question about grounding this scope and whether it makes it safe to measure those voltages withthisscope. Might be because of my lack of knowledge. But that's why I am asking questions here. Thank you for taking the time to reply.If you or anyone can offer more insight it would be much appreciated.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2023, 10:28:53 am
content: Quote from: Rijsik on December 21, 2023, 10:18:36 amThat's a good starting point: being humble and understanding what you do not understandDo not listen to posters who make claims that are equivalent to "I've run into the road without looking many times and I haven't been hurt. Therefore you can too"Can I suggest that you read (and understand!) material made by professionals. I suggest a good starting point is the relevant sources referenced athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/particularly the Textronix documents.Welsome to the forum, and "Have fun, safely".

author: Rijsik
author_rank: Contributor
timestamp: December 21, 2023, 10:42:06 am
content: Thank you tggzzz for your reply, and your advice. I've always been a little wary about electronics since my knowledge is limited. To be honest, I am probably more scared of working with 5V or 12V DC than I should be. But better safe than sorry. So in other words, I am not easily tempted in following others in dangerous behaviour.I've quickly scanned the Textronix document found at the link you provided. It seems like a nice general overview of scope probes. Will definitely look into it this evening when I have the time to read it in peace. As of the question about the protective grounding probe, do you have any idea how these probes are connected? Do I need a special plug for a power socket or something similar? I really have no idea and google doesn't really provide me with any useful results.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 21, 2023, 11:05:24 am
content: All exposed metalwork of the scope including the outer of the BNC sockets is connected to that grounding socket. If you ground the scope the ground clip of a standard probe (x1, x10, x100 or x anything) is ground. You cannot attach that willy-nilly to wherever you fancy.Conversely, if you do not ground the scope then all the exposed metalwork is at the same voltage as the place you have attached the probe's ground clip. The safety implications should be obvious.

author: Rijsik
author_rank: Contributor
timestamp: December 21, 2023, 11:37:55 am
content: Okay, got it. I definitely want to ground the scope. Maybe you can tell me how to do that propperly. So I plugged in one side of the PE lead to the scope, as showed in the attached image. How do I connect the other side of the lead to the ground pin of my power socket. Are there special plugs for that?

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2023, 11:40:54 am
content: Short answersConnecting the scope probe tip to mains voltage = yes, do it with a x10 probe (or x10 probe mode) for extra scope protection since 1/10th of the voltage gets to scope input.Connecting the scopes probe ground clip to mains voltage = NOOOO! you are dead shorting mains voltage.

author: Rijsik
author_rank: Contributor
timestamp: December 21, 2023, 11:50:37 am
content: So for reference, I live in the Netherlands. We have these kinds of wall outlets:https://nlshop.se.com/wcd-compleet-verticaal-2v-kinderbeveiliging-actief-wit-systeem-m. The middle pin at the bottom and top is earth. My lead should connect to that.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 21, 2023, 12:25:17 pm
content: Do what PSI says to do. The scope is already grounded.Only use the 10Xprobe, not the grounding clip. Remove the grounding clip lead from the probe for safety. You can then touch any mains volt line without creating a Big Short.Watch the video...

author: newbrain
author_rank: Super Contributor
timestamp: December 21, 2023, 12:38:40 pm
content: Quote from: Wallace Gasiewicz on December 21, 2023, 12:25:17 pmNo, in this case we do not know.It depends on the USB C power supply that's used with the scope.Only if the OP connects the green lead to PE we can be certain, as described by wasedadoc.That's the risk of assuming things from a post on a forum, the picture is seldom complete.In cases like this I'm always erring on the side of caution.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2023, 01:34:28 pm
content: Quote from: Rijsik on December 21, 2023, 10:42:06 amYou're welcome.Thevoltagefrom a 5V or 12V (or even 24V) won't hurt you. A standard way of testing a small 9V battery was to touch the terminals to your tongue. If youtastedit, it wasn't dead (and neither were you!)However, if the power source allows a highcurrentto flow, then that might hurt you indirectly. For example if you connect a wire across a 12V car battery, then it can supply hundreds of amps - and that would probably vapourise the wiresAn SMPS is intermediate, and requires caution. While it might (or might not) have a current-limited output, not all of the internal wires will be current limited.Safety isn'tonlyabout tools: thinking and practices are also helpful.A non-exhaustive list when working with high-energy equipment is to keep one hand behind your back (to reduce currents through your heart!), and to avoid touching the circuit or probesunlessthe circuit is powered down. The latter gives time to think and check before you blow up your circuit, and reduces the chance that a slipped probe might touch something "embarassing".Be aware that in some circuits voltage/charge can be stored on a capacitor after the power source is removed. And some types of capacitor can "recharge themselves" after being shorted; see "soakage" and "dielectric absorbtion".Final tips: avoid *1/*10 probes because youwillhave them in the wrong setting, don't forget that 240Vac sine wave is 350Vp and that voltage ratings only care about peak voltages.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 21, 2023, 01:45:09 pm
content: OK, Just ASSUME the scope is already grounded. Safer that way. Until you prove otherwise.Also I agree with the problems with 1/10 probes. I do not like them. Besides the possibility that you make a mistake, the switches are not that great on some of them and usually the performance is not as good....

author: Rijsik
author_rank: Contributor
timestamp: December 21, 2023, 06:35:30 pm
content: So if I understand correctly the following is correct.1) The PE socket on the back of the oscilloscope connects to both the case of the scope and the outer ring of the BNC connectors. Not using the PE lead to connect the scope to ground will make it so that the case and the BNC connectors all receive the voltage I am measuring with my ground probe. And this is dangerous so I should not do this. Is this correct?2) So in order to make things safer I should connect the PE lead to the earth pin in one of my power sockets. Is this correct?

author: u666sa
author_rank: Frequent Contributor
timestamp: December 21, 2023, 06:56:53 pm
content: You need main decoupling.

author: Veteran68
author_rank: Frequent Contributor
timestamp: December 22, 2023, 04:34:36 am
content: Quote from: Wallace Gasiewicz on December 21, 2023, 01:45:09 pmThe DHO800/900 scopes are NOT earth grounded by default, as they are USB powered. That's why they provide the separate PE lead the OP is referring to. He's trying to figure out how to connect the other (banana plug) end to ground in order to ground his scope.The simple answer is to connect it to the nearest ground point, but not knowing his environment or his country's electrical system well enough, it's hard to provide specific advice.I definitely concur with the sentiment that given his level of experience, he should probably avoid any high voltage measurements until he's a bit more studied, comfortable, and confident.

author: Rijsik
author_rank: Contributor
timestamp: December 22, 2023, 09:47:52 am
content: You are absolutely right. Thank you for taking the time to reply. I guess I need to get a special plug. One that fits in my power socket and allows a banana plug to connect to the PE pin of that socket.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 23, 2023, 11:21:09 am
content: Quote from: Psi on December 21, 2023, 11:40:54 amNo, bad advice. Cheap x10 probes might not even be cat I rated, but are not normally more than cat I, and personally I'd suggest x100 cat II/III probe might be a better choice - even my humble multimeter is cat III rated. You definitely need cat II or higher for single phase mains - Mains transients are measured in kilovolts, not 100's of volts.The high voltage x100 probes typically keep your hand further from the action which is intrinsically safer.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: December 23, 2023, 01:38:14 pm
content: Quote from: Rijsik on December 22, 2023, 09:47:52 amOn most bench power supplies there is a green/PE banana jack between the black and red outputs, (both mine have it), and that would be a handy place your green PE lead from your scope.

author: Infraviolet
author_rank: Super Contributor
timestamp: December 23, 2023, 07:40:49 pm
content: As a general rule, even at low voltages you want to be careful where you put the grounding clip of the scope probes. Not to protect you, you'll be fine, but to protect the scope. If the thing you are measuring, low voltage as it might be, is relative to mains earth, and not floating, then putting the grounding clip to somewhere on it which isn't at mains earth makes a current flow through the grounding clip, as a dead short to mains earth. This can hurt the scope if the current is large, not sure if there is a figure for what scopes will survive, but whether the thing supplying the current is current limited will be important. Most wall wart supplied power is floating, when power for a circuit comes from a computer's USB port it may or may not have a floating ground, depends on the whether the PC is a desktop (USB ground usually connected to mains earth) or a desktop (whether the USB ground touches mains earth depends on exact model of power brick), bench supplies will always have clearly labelled options for connecting up both a floating ground, to which the DC supply pins are relative, and a mains earth ground. Anything solely battery powered is always floating. When you're new to o-scopes, use only one grounding clip, this prevents the risk of accidentally connecting each clip to different voltages, which even on floating low votage circuits could be bad. You only need both grounding clips when you're measuring fast signals (100s of KHz and above) and need to have reduced capacictance to avoid parasitic ringing in your measurements. Except when you're working on things related to mains, most circuits you are likely to want to build will be floating, so as long as the power supply to them is floating you need not worry about mains earth vs circuit ground, because for a floating circuit, wherever you choose to connect the scope grounding clips to will instantly and harmlessly become mains earth, and all other voltages on the circuit will be relative to that.

author: shapirus
author_rank: Super Contributor
timestamp: December 23, 2023, 09:31:17 pm
content: Quote from: tggzzz on December 21, 2023, 01:34:28 pm...and the input capacitors have  discharged.

author: tggzzz
author_rank: Super Contributor
timestamp: December 23, 2023, 09:41:10 pm
content: Quote from: shapirus on December 23, 2023, 09:31:17 pmOut of curiosity, why did you choose to omit the next paragraph, i.e.Quote

author: shapirus
author_rank: Super Contributor
timestamp: December 23, 2023, 10:01:17 pm
content: Quote from: tggzzz on December 23, 2023, 09:41:10 pmI guess because it's late and I simply overlooked it.Let it be a double reminder then :).

author: tszaboo
author_rank: Super Contributor
timestamp: December 23, 2023, 10:23:07 pm
content: Oh, come on all. Teach the guy to understand what's going on.On the front of the scope, you see a rating. Next to the BNCs. It is saying CAT I, 300V. This is the same CAT rating as multimeters.When you look up CAT I:"Anything from a small circuit board to larger device with high-voltage but low-energy" or"CAT I describes secondary circuits not intended to be connected to the mains electricity supply, such as electronics, including a typical laptop PC, and circuits powered by regulated low voltage sources. "So is it safe? No, it's not rated to work on those circuits.Get a differential probe with the required ratings please, and learn the principles on how to work on circuits like that.

author: shapirus
author_rank: Super Contributor
timestamp: December 23, 2023, 10:26:07 pm
content: Quote from: tszaboo on December 23, 2023, 10:23:07 pmThat's correct, but who said that anyone was going to feed 300V into the scope? There are 100:1 probes for this, with a proper CAT rating.

author: djacobow
author_rank: Super Contributor
timestamp: December 23, 2023, 11:15:45 pm
content: Simple answer: if you can't answer this question yourself, then "no."

author: tggzzz
author_rank: Super Contributor
timestamp: December 24, 2023, 12:22:07 am
content: Quote from: shapirus on December 23, 2023, 10:01:17 pmWhat an excellent recovery!We all make mistak.es. Well, except me: I'm perfectTherefore if never ever done that. Oh no. Never. Not in a million years

author: Jwillis
author_rank: Super Contributor
timestamp: December 24, 2023, 06:09:43 pm
content: Can't for the life of me understand why someone needs to measure mains voltage anyways. Theirs really not much to see other than a sine wave with a peak to peak voltage of your mains supply. If your having problems with the mains power then call your power distributor or a professional electrician. They will have the instruments to locate problems such as noise and other irregularities not associated with consumer products connected to mains voltage in your home or office. If breakers keep tripping then your overloading the circuit. You don't need an oscilloscope to figure that out. If you have noise in the DUT then disconnect other devices from the mains circuit until the problem goes away. You don't need an oscilloscope for that either.

author: Eraldo
author_rank: Regular Contributor
timestamp: December 24, 2023, 06:53:33 pm
content: I'd propably use a variac or another isolation transformer to get the 230V.Also if isolation is your concern then also use a power bank to power the scope so it's floatingIf you don't use an isolation transformer then there is the risk of connecting the live probe to the ground of the scope and if you touch any other ground of the scope then you get shocked.Also don't measure the primary and the secondary of the flyback converter at the same time since the grounds of the scope are connected together and are not isolated from one another.

author: tszaboo
author_rank: Super Contributor
timestamp: December 24, 2023, 10:19:09 pm
content: Quote from: shapirus on December 23, 2023, 10:26:07 pmHmm, let's see."Rigol PVP2350 -- 350 MHz Passive Probe with 1x:10x selector1X: 1MΩ ±1% input resistance 50 pF ±20 pF input capacitance 150 VRMS CAT II max. rated input voltage10X: 10MΩ ±1% input resistance 10pF ±5pF input capacitance 300 VRMS CAT II max. rated input voltage"So the probe has a higher CAT rating than the scope itself. It also has a switch to immediately turn it into unsafe. I have some very bad experience with scopes and mains, frontends overloading and stopping working, even though I didn't do anything obviously wrong. And magically recovering in a few minutes. Since then, I don't recommend anyone to work on mains without a diff probe, touch safe setup and possibly an isolation transformer.

author: shapirus
author_rank: Super Contributor
timestamp: December 24, 2023, 11:03:19 pm
content: Quote from: tszaboo on December 24, 2023, 10:19:09 pm(bold emphasis is mine)Yes this is why this type of probes can't be recommended for working with anything beyond what's generally considered low voltage.The 100:1 probes with proper insulation and CAT rating are much better. But of course, they alone don't render working with mains voltage safe.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 25, 2023, 12:23:26 am
content: Even if it's "CAT Rated" I'd still refrain from using the common freebie 1x/10x switchable probes for anything remotely high voltage. Sure the probe center/tip might be well isolated enough for couple hundred volts, but a cursory look at its ground clip doesn't give much confidence at all.In this case floating the scope only make things worse. Sure it prevents them from blowing up and burning on your hand, but it'll potentially shock you instead.

author: Psi
author_rank: Super Contributor
timestamp: December 26, 2023, 05:50:31 am
content: I've fed 230V mains into my Rigol 1052 on x1 probe setting once, it worked in that it displayed the waveform on screen just fine and survived the experience.But it was still as stupid thing to do. (forgot probe was on x1 and not x10)Was it a good idea = no.Was my scope at risk of damaging if a mains spike happened during the time it was connected = yes.If you accidently do it for a short time is your scope going to be instantly damaged = Na, possible but pretty unlikely.(Unless you have a scope with lower than normal max voltage on the inputs.)Normally its 400V peak to peak so 325V p2p from 230VAC is still within spec, it's really just mains spikes (that can and do happen) that creates the risk of damage.There's no need to take the risk, always use a x10 probe.

author: vsantos90
author_rank: Newbie
timestamp: June 29, 2024, 10:37:17 pm
content: Hi everyone..This is my first week with the same scope, DHO804, it came with a LiteON power supply, seems that the quality is good.I plugged the USB C to the scope and checked that the resistance between the metallic housings of the connectors from the back pane and the ground pin of the power supply AC plug is exactly 1MΩ. For ESD matters, seems to be enough.In such situation, makes any sense to also connect the banana plug to the ground?From my point of view, I can't see any advantages so I would like to share my question, maybe I'm missing something..

author: Psi
author_rank: Super Contributor
timestamp: June 30, 2024, 08:13:28 am
content: Quote from: vsantos90 on June 29, 2024, 10:37:17 pmOdd, that's a USB-C powered scope.Is that thing supposed to be mains isolated or something?Normally you would have a short from exposed metal connectors to mains earth, not 1MΩ.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 30, 2024, 08:35:18 am
content: Quote from: Psi on June 30, 2024, 08:13:28 am1. That scope has the metalwork connected to the rear banana jack.2. The external USB-C PSU (like most) does not have the earth pin of the AC mains plug connected through to the low voltage dc output side.

author: vsantos90
author_rank: Newbie
timestamp: June 30, 2024, 08:44:50 am
content: Just to avoid misunderstanding: as expected, on the scope side, there is no resistance between the metallic housing of the connectors and the banana plug that should be used for grounding purposes.This 1MΩ comes from the power supply, measuring between the metal from the USB C connector and the AC ground pin (the power supply AC cable has 3 pins).I wonder if this banana plug must be used only for another USB C power supplies which are not grounded (I have an Anker USB C charger which does not have the ground pin, for example). I don't have a huge experience with USB C power supplies, but I see the ground connection just in industrial models, or equipment like laptops (this LiteOn power supply which came with the Rigol is exactly the same power supply that came with my Dell laptop).At the end, should I still connect the banana plug to the ground pin in the power outlet or this connection provided by the power supply is enough?

author: madires
author_rank: Super Contributor
timestamp: June 30, 2024, 01:23:38 pm
content: If the BNC/probes's ground isn't connected to the grounding connector this would defeat the purpose of grounding the scope (safety and noise). From your description the power brick has an 1 MOhm resistor between main's earth/PE and the USB plug's shield. And at the DSO the shield of the USB port is connected to the DSO's ground. Therefore you can measure the 1 MOhm resistor between the BNC/probe's ground and the PSU's earth/PE. However, this is a coincidental ESD discharge path, not proper grounding (short to earth/PE). For a beginner I'd recommend to ground the DSO for safety reasons.

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 30, 2024, 02:26:07 pm
content: Even if the modern USB-C powered scopes have no obligatory PE-connection they are definitely not suited to float at dangerous voltages.ALWAYS earth the scope if you work with mains. ALWAYS isolate the device under test instead of the chassis of the instrument.There are -mainly portable- scopes that have galvanically isolated probe inputs. But if the scope does not clearly and directly states that probe-ground is isolated you should not try to use it that way.In the universities lab we had people who used old analogue HAMEG-scopes with insulation tape over the PE to put the instrument on higher electric potential. But for the same reason we had a lot of defect scopes. Luckily no human was injured - at least I know of nobody.Nowadays a high voltage differential probe is affordable enough so that no one should have any reason to play this type of russian roulette.

author: MathWizard
author_rank: Super Contributor
timestamp: July 02, 2024, 11:55:25 am
content: This reminds me I should get some rubber gloves, like I see some guys wear in repair video's. So far I haven't gotten any shocks from working on computer PSU's or stereo's, but yeah there's times some insulated gloves would be nice.

author: madires
author_rank: Super Contributor
timestamp: July 02, 2024, 01:40:45 pm
content: Yep, it's about risk and mitigation. If you're working with high voltages or mains differential probes, rubber gloves and safety procedures help to mitigate the risk.

author: qistoph
author_rank: Newbie
timestamp: March 17, 2025, 07:43:42 pm
content: I have also recently received my DHO804 and am wondering too what would be the right way to ground the scope. The question of HOW to do this, doesn't seem to be answered in this thread or any other.The device has a socket and banana plug for grounding. Where do you put the other end?Online I find "earth plugs with banana connector", but these are for ESD pusposes and have 1MOhm resistence. Not suitable for proper grounding.

author: tggzzz
author_rank: Super Contributor
timestamp: March 17, 2025, 09:32:55 pm
content: Quote from: qistoph on March 17, 2025, 07:43:42 pmTake a three core mains lead and plug it into a mains socket with a functioning protective mains earth (PME). The exposed metal parts on the scope (e.g. case and probe connectors) will then be at PME potential.Never disconnect the connection to PME.

author: tszaboo
author_rank: Super Contributor
timestamp: March 17, 2025, 11:27:01 pm
content: Quote from: qistoph on March 17, 2025, 07:43:42 pmGround output of a lab power supply.Or like this:

author: qistoph
author_rank: Newbie
timestamp: March 18, 2025, 06:34:49 am
content: Thanks for your suggestion. As explained by vsantos90 inhttps://www.eevblog.com/forum/beginners/is-it-safe-to-measure-mains-voltage-with-my-rigol-dho804/msg5558335/#msg5558335, this model is power through a USB-C adapter that does not connect the scope to PME with sufficient grounding. So I am looking for the right way to ground the scope, without the usual three lead power cord into mains.

author: tggzzz
author_rank: Super Contributor
timestamp: March 18, 2025, 11:34:32 am
content: Quote from: qistoph on March 18, 2025, 06:34:49 amIimaginethere is a connection through the USB-C adaptor to PME that will be sufficient for EMI/EMC considerations. Like you, I would not like to rely on that to protect me or the equipment.The manufacturer should tell you what is necessary to use the equipment safely for your use case. If it doesn't, then perhaps you shouldn't use it for those purposes. Not a pleasant conclusion, but there it is.

author: Ranayna
author_rank: Super Contributor
timestamp: March 18, 2025, 02:04:16 pm
content: The majority of USB Power Delivery supplies are using a two pronged plug, thus are not grounded at all.The original power supply that was included in the first versions also isn't grounded:https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/

author: tggzzz
author_rank: Super Contributor
timestamp: March 18, 2025, 02:29:14 pm
content: Quote from: Ranayna on March 18, 2025, 02:04:16 pmShocking - hopefully only figuratively.

author: Enginenear
author_rank: Newbie
timestamp: March 18, 2025, 02:56:52 pm
content: Everything has already been said on some excelent replies by some very knowlegeable users:This is ONE MORE short answer:YES, it is POSSIBLE to connect the scope to the mains (either 220V or 110V AC), either with or without connecting the extra cable to ground.NO, you shouldn’t do it, unless you know exactly what you’re doing and have a TON of experience working with live wires (and, of course, using insulated tools and PPE)RISKS:- Damage to the equipment;- Damage to your life, which, BTW is binary – either you are alive, or you aren’t.

author: Enginenear
author_rank: Newbie
timestamp: March 18, 2025, 03:20:56 pm
content: RISK MITIGATION:ADVICE #1: If you're not 100% sure of what you're doing, DO NOT "play around" with mains voltages.ADVICE #2: If you're not 100% sure of what you're doing, DO NOT "play around" with mains voltages.ADVICE #3: If you're not 100% sure of what you're doing, but decide to ignore #1 and #2, DO USE any type of rubber gloves on both hands and keep other body parts (such as elbows) away from the wires. Rubber and silicon kitchen gloves make pretty good substitutes.ADVICE #4: ALWAYS remember things are not as "innocent" as they seem. For example:- Disconnected circuits may have capacitors charged with high voltages.- Differential circuit breakers sometimes used to shut off power in case someone touches the live circuit will not work if, for instance, you are holding the neutral with one hand and touch the live with the other hand. So... be extra-careful when you use both hands (I've been there)

author: bdunham7
author_rank: Super Contributor
timestamp: March 18, 2025, 03:29:29 pm
content: Quote from: Rijsik on December 21, 2023, 09:42:49 amYou connect the lead to something you know is properly grounded. The previous suggestion of using the ground jack of a bench PSU (if you have one) is a good example. Otherwise you'll need to find or make a good ground from somewhere.Yes, the scope is as safe as any other to use on higher voltages, but extra caution is warranted if those voltages are directly from your electrical mains. This is because the amount of energy available is very high if there's a fault, short or breakdown.With HV differential probes you don't actually have to have the scope grounded for them to work, although noise may be an issue. They have high resistance inputs on both sides that both terminate in the common that is usually referred to ground; the protection they offer is a limitation of the current that can flow (µA) to that common point. So even if the common point--which becomes the ground point on your scope--is not truly grounded and floats up a bit, there's not enough current there for you to notice. I'm not recommending you not ground your scope and I think this arrangement with the PSU is a bit lame, but I'm just explaining how it works. This lack of true galvanic isolation can be a serious issue when using HV differential probes for higher voltages, but for mains it's a nuisance at most.If you use regular 10X or 100X probe, then only one side of the input protects you with a high resistance while the other, the ground side, is passed directly to the scope ground point and all of the BNC shells and exposed metal on the scope. If you connect the probe improperly so that the ground clip is connected to something live, then either you'll have a short and something will blow (if the scope was properly grounded) or the entire scope will be live and waiting to electrocute you if you touch it (if the scope was not grounded). So clearly you want the scope to be firmly and reliably grounded in this case.The best plan is to come up with a way to reliably ground the scope and make sure it stays grounded. Banana plugs are a bit lame for this, IMO, but you can probably make that work somehow. Then if you do measure mains voltages with a probe, just don't use any ground clip. Or perhaps just don't measure high voltages directly at all if you don't feel confident, especially if you are just using the probes that came with the scope.