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

[djacobow]

Simple answer: if you can't answer this question yourself, then "no."

[tggzzz]

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 .

What an excellent recovery!

We all make mistak.es. Well, except me: I'm perfect 

Therefore if never ever done that. Oh no. Never. Not in a million years

[Jwillis]

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.

[Eraldo]

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 floating

If 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.

[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. "

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.

Hmm, let's see.
"Rigol PVP2350 -- 350 MHz Passive Probe with 1x:10x selector
1X: 1MΩ ±1% input resistance 50 pF ±20 pF input capacitance 150 VRMS CAT II max. rated input voltage
10X: 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.

[shapirus]

Hmm, let's see.
"Rigol PVP2350 -- 350 MHz Passive Probe with 1x:10x selector
1X: 1MΩ ±1% input resistance 50 pF ±20 pF input capacitance 150 VRMS CAT II max. rated input voltage
10X: 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.

(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.

[ArdWar]

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.

[Psi]

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.

[vsantos90]

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..

[Psi]

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.

Odd, 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.

[wasedadoc]

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.

Odd, 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.

1. 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.

[vsantos90]

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?

[madires]

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.

[Phil1977]

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.

[MathWizard]

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.

[madires]

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.