Is it safe to measure mains voltage with my Rigol DHO804?

[Rijsik]

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 use this oscilloscope 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.

[newbrain]

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.

[Rijsik]

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 with this scope. 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.

[tggzzz]

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 with this scope. 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.

That's a good starting point: being humble and understanding what you do not understand
Do 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 at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ particularly the Textronix documents.

Welsome to the forum, and "Have fun, safely".

[Rijsik]

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.

[wasedadoc]

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.

[Rijsik]

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?

[Psi]

Short answers

Connecting 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.

[Rijsik]

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.

[Wallace Gasiewicz]

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...

[newbrain]

Do what PSI says to do.  The scope is already grounded. 

No, 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.

[tggzzz]

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.

You're welcome.

The voltage from 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 you tasted it, it wasn't dead (and neither were you!)

However, if the power source allows a high current to 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 wires

An 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't only about 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 probes unless the 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 you will have them in the wrong setting, don't forget that 240Vac sine wave is 350Vp and that voltage ratings only care about peak voltages.

[Wallace Gasiewicz]

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....

[Rijsik]

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?

[u666sa]

You need main decoupling.

[Veteran68]

OK, Just ASSUME the scope is already grounded. Safer that way. Until you prove otherwise.

The 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.

[Rijsik]

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.

[MarkT]

Short answers

Connecting 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.

No, 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.

[BILLPOD]

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.

   On 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.   

[Infraviolet]

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.

[shapirus]

avoid touching the circuit or probes unless the circuit is powered down.

...and the input capacitors have [been] discharged.

[tggzzz]

avoid touching the circuit or probes unless the circuit is powered down.

...and the input capacitors have [been] discharged.

Out of curiosity, why did you choose to omit the next paragraph, i.e.

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".

[shapirus]

Out of curiosity, why did you choose to omit the next paragraph, i.e.

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".

I guess because it's late and I simply overlooked it.

Let it be a double reminder then :).

[tszaboo]

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.

[shapirus]

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. "

That'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.