Measure mains (230V) with osciloscope

[mitrynicolae]

Hello everyone,

I have a simple yet complicated question (at least for me). How do I safely visualize with an oscilloscope the mains voltage? I have this scope https://www.linkinstruments.com/mso19.htm and the 1x/10x probe. Now there are multiple questions for which I still don't have a clear answer.
 - if the osciloscope with the 10x probe can only support 40v then this means that I need a 100x probe in order not broke the scope?
 - in the video #279 Dave Jones presented some ways in which beginner electronics hobbyists can blow up their scopes. Is there any setup, in which I could hook up the probe that could result in destruction of the scope or me being electrocuted?
 - can there be an issue if I put the ground lead to the live wire and the probe tip to the neutral?

To understand what I am trying to achive, I have this stabilizer: https://www.emag.ro/stabilizator-electronic-de-tensiune-electropower-ep-jjw-15kva-cp-jjw-15kva/pd/DW7WSYBBM/ and the producer claims that the stabilizer has a sinusoidal output. More over they claim that the output has 1% accuracy and a response time of 100ms. The issue is that when my home boiler (similar to this one http://www.ariston.com/uk/Wall-Hung_Boilers_Combi/clasnetone) starts up there is a huge voltage spike that eventually leads to an over voltage protection to shut down the mains in the entire home. If I gradually start the appliances in the house then there is no issue and the stabilizer work as it should, but if for some reason the mains was absent and then come back the spike in the stabilizer leads to shut down the mains to the entire home, As you can imagine the whole system enters in a loop from which is hard to recover and that could lead to some devices to blow up.

Thank you for your time!

[george.b]

- if the osciloscope with the 10x probe can only support 40v then this means that I need a 100x probe in order not broke the scope?
 - in the video #279 Dave Jones presented some ways in which beginner electronics hobbyists can blow up their scopes. Is there any setup, in which I could hook up the probe that could result in destruction of the scope or me being electrocuted?
 - can there be an issue if I put the ground lead to the live wire and the probe tip to the neutral?

1. Correct.
2. See next point.
3. Yes. If your scope is grounded (it should be), you'll short out the mains through the oscilloscope.

What I would do if I were you would be to measure the mains waveform through a transformer, say, 230V/12V. Whatever happens on the primary will show up on the secondary, the oscilloscope will be isolated and you won't have to deal with higher voltages.

[Benta]

The correct way of doing this is to use a differential probe, but they are pricey.
George.b's suggestion on using a transformer is cheap, easy and good enough for this test.

[madires]

If you have two 100x probes you could also use the hobbyist's differential probe setup (remove ground clips, CH1 + CH2 inverted). Proper differential probes aren't cheap, but less expensive than a funeral.

[tggzzz]

Is there any setup, in which I could hook up the probe that could result in destruction of the scope or me being electrocuted?

Yes, many many ways, some obvious, some subtle - even when you take precautions. From a post 3 days ago: https://www.eevblog.com/forum/testgear/ground-of-oscilloscope-always-connected-to-earth/msg1967150/#msg1967150

It is good that you know what you don't know, and do ask questions. There are many answers on this forum, or at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[Ian.M]

Even in countries that haven't had the infrastructure problems due to many years of underinvestment and technology embargos during the communist era, you cant count on the power grid delivering anything like a clean 230V RMS 50Hz sinewave on the LINE terminal of a domestic outlet, with the NEUTRAL and Protective Earth terminals at ground potential.   In many countries and most locations within them,  you'll get a somewhat distorted sinewave that's within 10% of that voltage *MOST* of the time, with plenty of high voltage spikes up to about 1KV.  In some locations it can be *MUCH* worse.  However assuming NEUTRAL is at ground potential can get you killed - you are ONE fault away from electrocution, and that fault may not even occur in your building or due to your actions.   Betting your life on Neutral reliably staying anywhere near ground potential in a Romanian (or other former communist block) home, is only an order of magnitude or two less foolish than playing Russian roulette.  I'd rather cross a busy road blindfolded.

If you cant afford a Cat II rated (or better) isolated differential HV probe, this is not a measurement you should even contemplate making.

Also if you are contemplating ways of working around the need for an isolated differential probe, (and assuming you are smart and careful enough to avoid your next of kin buying you a pine box), as that's a PC hosted USB scope, the odds are that a mistake would take out the PC as well as the scope pod, so unless you use a sacrificial PC with no other important programs or data on it, the consequences could have a much higher impact on your life than simply writing off a few hundred dollars of test equipment.

Assuming you've got a sacrificial laptop with a good battery (so it will run with ONLY the single cable to the scope pod plugged into it)  and a wireless keyboard and mouse so you don't have to come anywhere near it once the mains supply you are proposing to test is connected, you can *probably* get away with breaking the safety rules and floating your PC scope so you can use a non-isolated HV probe, but the whole setup needs to be treated with the same respect as an exposed uninsulated live mains busbar!

[mitrynicolae]

@Marires "Proper differential probes aren't cheap, but less expensive than a funeral." That's a good one =)).
@george.b Using a transformer is a great idea, but still I have some questions:
- could the transformer distort the wave form in any way? if yes then it is not a solution
- using a transformer means not only using safer voltages but also isolating the probing points from the mains, correct?
- this also means that I can hook up the scope in any way to the output leads of the transformer, correct?

For the rest, thank you for the help, but using a differential probe to measure the mains is out of question. Paying 600$ for a differential probe to only using it once is not a solution. I really like the transformer solution IF all the above statements can be meet, otherwise I have to think of other ways of viewing the wave form.

Kind Regards,

[Ian.M]

An ordinary mains transformer is likely to contribute some distortion and will filter out  fast HV transients due to its limited bandwidth, but its the only cheap option that doesn't require a deep understanding of electrical safety standards for mains circuits, and of the statistical significance of all the possible risks involved.

It should be rated for a maximum input voltage significantly greater than the nominal supply voltage as if it gets too close to saturation the waveform distortion will become significant.

[george.b]

^ What Ian.M said. Now, whether you need good response at higher frequencies in order to figure out what's going on... 
He also raises a good point about saturation.
You could try it anyway, as it's the cheapest option and not much will be lost if it doesn't suit your needs.

As for your other questions: yes, using a transformer would isolate your probing points from the mains, which means you can hook up the scope whichever way. Just be mindful you don't pick an autotransformer for this, as it's not isolated.

[malagas_on_fire]

Grab some old equpment or older power cords that are not needed anymore and also has some weight to it, open the case, scavange the transformer to get the isolation / measure, for example old cell phone chargers, router power supplies. Mostly can be found in electronic dumpster , but of course check isolation with a multimeter first.

[vk6zgo]

Hello everyone,

I have a simple yet complicated question (at least for me). How do I safely visualize with an oscilloscope the mains voltage? I have this scope https://www.linkinstruments.com/mso19.htm and the 1x/10x probe. Now there are multiple questions for which I still don't have a clear answer.
 - if the osciloscope with the 10x probe can only support 40v then this means that I need a 100x probe in order not broke the scope?
 - in the video #279 Dave Jones presented some ways in which beginner electronics hobbyists can blow up their scopes. Is there any setup, in which I could hook up the probe that could result in destruction of the scope or me being electrocuted?
 - can there be an issue if I put the ground lead to the live wire and the probe tip to the neutral?

To understand what I am trying to achive, I have this stabilizer: https://www.emag.ro/stabilizator-electronic-de-tensiune-electropower-ep-jjw-15kva-cp-jjw-15kva/pd/DW7WSYBBM/ and the producer claims that the stabilizer has a sinusoidal output. More over they claim that the output has 1% accuracy and a response time of 100ms. The issue is that when my home boiler (similar to this one http://www.ariston.com/uk/Wall-Hung_Boilers_Combi/clasnetone) starts up there is a huge voltage spike that eventually leads to an over voltage protection to shut down the mains in the entire home. If I gradually start the appliances in the house then there is no issue and the stabilizer work as it should, but if for some reason the mains was absent and then come back the spike in the stabilizer leads to shut down the mains to the entire home, As you can imagine the whole system enters in a loop from which is hard to recover and that could lead to some devices to blow up.

Thank you for your time!

Why do you need the stabiliser?

I have only seen them in TV & Radio Transmitters & Studios (huge ones in the latter).
They don't seem to be used at all in domestic situations in this country, & I have been to some remote places, where the Mains supply is pretty unstable.
(In one place, whenever they brought the big electrically operated  excavator up to the ore face at the mine, all the lights in town dipped! )

On the other hand, if they are common in your country, maybe you should ask other users.
If they are happy, & only your one has this problem, I suggest you should  complain to the  manufacturer.

[joeqsmith]

I'm not too surprised to hear about your problems with your line conditioner.   

Agree with others as far as possibly using a transformer.  I found one that has pretty good response that I tossed into a box and use to look at the AC line.   I wrote some software for my scope to use it to make some basic measurements but the transformer would be a bit limited for looking at transients. 

I don't do much at home with the AC line.  I did play around building a few differential probes.  Not something I would recommend for a beginner.   It may give you some idea how they work.   You could buy one fairly cheap with more than enough BW for what you are doing.  This one was fairly simple to build and was good for almost a MHz. 
https://youtu.be/_OZ5Xer84eo

I attempted to build something with a bit higher BW.  Somehow I don't think these would pass the IEC standards.
https://youtu.be/0thOfk4I3qs