Mains and the oscilloscope

[Neukyhm]

I have read all your comments and summarizing, I can measure mains and any other thing with a similar voltage like a three phase motor I have with an inverter board. I have bought a probe in a store here, it says "Max input voltage: 600V CAT I, 300V CAT II (DC+peak AC)", so I think I can't use this probe if I have like 240RMS=340 peak, I will go for a diff. probe then.

[raspberrypi]

Mr Carlsons lab does a great example of how to plug a scope into the mains and how to do it right.

I know in america (not spain) since you have 120VAC you don't know what leg of the 220 your outlet is plugged into, that's why people always say its a 50/50 shot if you blow up your scope.

https://youtu.be/XBsQ3sZ45Fk

[mtdoc]

Nice video but...

I know in america (not spain) since you have 120VAC you don't know what leg of the 220 your outlet is plugged into, that's why people always say its a 50/50 shot if you blow up your scope.

No - The issue has nothing to do with which leg (L1 or L2 in the North American split phase 120/240V system) the particular branch circuit you are plugged into is utilizing.

[Neukyhm]

I know in america (not spain) since you have 120VAC you don't know what leg of the 220 your outlet is plugged into, that's why people always say its a 50/50 shot if you blow up your scope.

I know exactly the cable that is the neutral one, but the thing is, imagine there is a problem and neutral voltage is not 0 volts as it has to be, then I can't connect probe ground to neutral.

[Gyro]

Neutral will normally be NEAR ground, but depending on where they are commoned there will be a voltage between them, caused by Neutral conductor current. This will of course be a very low impedance source. It is not sensible or reasonable to expect a probe ground lead to short this current, regardless of any fault condition or not!

[Neukyhm]

Neutral will normally be NEAR ground, but depending on where they are commoned there will be a voltage between them, caused by Neutral conductor current. This will of course be a very low impedance source. It is not sensible or reasonable to expect a probe ground lead to short this current, regardless of any fault condition or not!

I have like 40 volts between neutral and ground, that's what I'm talking about xD and that's why I want to measure mains, because something is wrong with my electrical installation.

[madires]

A scope is the wrong tool for that kind of measurement. Ask an electrician, he's got the right tool for that.

[tggzzz]

Neutral will normally be NEAR ground, but depending on where they are commoned there will be a voltage between them, caused by Neutral conductor current. This will of course be a very low impedance source. It is not sensible or reasonable to expect a probe ground lead to short this current, regardless of any fault condition or not!

I have like 40 volts between neutral and ground, that's what I'm talking about xD and that's why I want to measure mains, because something is wrong with my electrical installation.

And that, ladies and gentlemen, is an excellent example of why you should determine a potential client's actual problem before discussing their "solution".

First the question was "The question is: is it safe to measure mains 220AC with [a scope]... Again, I know the answer, it is safe.... "
That mutated into "Listen, asking if I can measure mains voltage is my way to ask if I can measure hundreds of volts of anything with the oscilloscope (assuming that voltage is below probe & oscilloscope limits). It's not like I feel fascination for mains."
And we now find that all he wants to do is find a gross fault in his mains installation. Nothing subtle.

A scope is the wrong tool for that problem.

Given what you have said so far, I strongly recommend you get a properly qualified electrician to examine your installation. Don't risk hurting other people.

[mtdoc]

I agree with what madires and tggzzz have said.  An oscilloscope is the wrong tool and it's best to consult an electrician.

But - if you have a DMM (with proper CAT rating) that has a lo-Z setting you can check whether that 40V is really of any consequence. For example, see this nice Fluke application note on ghost voltages.

On the other hand if your neutral is not properly bonded to a proper earth ground, it may be real..

If you're not absolutely sure you can test this safely - call an electrician.

[Neukyhm]

First the question was "The question is: is it safe to measure mains 220AC with [a scope]... Again, I know the answer, it is safe.... "
That mutated into "Listen, asking if I can measure mains voltage is my way to ask if I can measure hundreds of volts of anything with the oscilloscope (assuming that voltage is below probe & oscilloscope limits). It's not like I feel fascination for mains."
And we now find that all he wants to do is find a gross fault in his mains installation. Nothing subtle.

First the question was "measure mains" because that's what I want to measure wheter my installation is good or not.
That mutated into "measure hundreds of volts" because  I want to measure an inverter three phase motor board output.
And then I said that I won't connect probe ground to neutral because a problem in my installation forbids me to do that.

Please, I have not spent 400 euros to just measure mains.

[tggzzz]

If you're not absolutely sure you can test this safely - call an electrician.

Of course, that doesn't work where the Dunning-Kruger syndrome is present.

[Neukyhm]

If you're not absolutely sure you can test this safely - call an electrician.

Of course, that doesn't work where the Dunning-Kruger syndrome is present.

Do you think that just because I want to learn? Because I'm here for that, to Learn (did you learn or you were born knowing electric engineering?), it's very sad that you laugh at unexperienced users that want to learn, denota mucha falta de humildad.

[DimitriP]

If you're not absolutely sure you can test this safely - call an electrician.

Of course, that doesn't work where the Dunning-Kruger syndrome is present.

Do you think that just because I want to learn? Because I'm here for that, to Learn (did you learn or you were born knowing electric engineering?), it's very sad that you laugh at unexperienced users that want to learn, denota mucha falta de humildad.

A quien no pide consejo, darlo es de necios.

You did ask.

[tggzzz]

If you're not absolutely sure you can test this safely - call an electrician.

Of course, that doesn't work where the Dunning-Kruger syndrome is present.

Do you think that just because I want to learn? Because I'm here for that, to Learn (did you learn or you were born knowing electric engineering?), it's very sad that you laugh at unexperienced users that want to learn, denota mucha falta de humildad.

It was a general statement, not specifically aimed at you.

However, in your original post you did write "...Again, I know the answer, it is safe...". I do appreciate English is probably not your first language, just as Spanish is not mine.

[raspberrypi]

Nice video but...

I know in america (not spain) since you have 120VAC you don't know what leg of the 220 your outlet is plugged into, that's why people always say its a 50/50 shot if you blow up your scope.

No - The issue has nothing to do with which leg (L1 or L2 in the North American split phase 120/240V system) the particular branch circuit you are plugged into is utilizing.

Care to elaborate what the issue is? Usually when you tell someone they are wrong you explain why.

[raspberrypi]

Neutral will normally be NEAR ground, but depending on where they are commoned there will be a voltage between them, caused by Neutral conductor current. This will of course be a very low impedance source. It is not sensible or reasonable to expect a probe ground lead to short this current, regardless of any fault condition or not!

I have like 40 volts between neutral and ground, that's what I'm talking about xD and that's why I want to measure mains, because something is wrong with my electrical installation.

My neutral has 40 volts on it too when I put the meter between it and ground. Is this caused by coupling from the wires all running next to each other? How much current roughly would this 40 volts put out? Could I run a 5W light bulb off it or even an LED? Or would it disappear as soon as you put any load on it.

[Neukyhm]

However, in your original post you did write "...Again, I know the answer, it is safe...".

That's because I know the theory, and the theory says that the voltage I want to measure is below probe and oscilloscope limits.
But practice is other story, you guys are more experienced and that's why I'm asking, because I do know that I can measure that IN THEORY, but in practice, anything I have not take care of could go wrong.

A quien no pide consejo, darlo es de necios.

I don't know what you mean with that, I'm here for any advice I can get. I just don't want anyone to laugh at me (an absolut newbie with oscilloscopes) for this, because what if a new  user willing to learn comes here and asks something elementary like how to turn on a led with a 3V battery? I would be sad if I see people laughing at him for that.

I have no problem to say that I know almost nothing compared to you in this topic.

Aaaand talking again about it, what if I build a voltage divider with a few megaohm resistors?

[Brumby]

With mains power, things get a little more dangerous.  It's not about simple voltages any more, it is about serious energy levels.  10KV around a CRT tube will wake you up in a hurry and perhaps fry some types of component, but 230VAC mains can make things explode.  Add a transient or two and things can get really dicey, really quickly and unexpectedly.  Your very first post indicated you were not really aware of this, as you stated:

Hi, I'm new here, I hope you can help me with this. I already know the answer, so the reason I'm asking for this is because I want to make sure that my house won't burn.

....

The question is: is it safe to measure mains 220AC with A-B operation so probes ground clips are not used? (assume the probe is x10)

This line in particular ...

Again, I know the answer, it is safe. I just want to make sure, you guys are more experienced than me.

... demonstrates you didn't know.  This is the sort of thing that makes a lot of experienced people very anxious for your safety (and why some people bring up the D-K syndrome thing).

To your credit, you did have the presence of mind to bring your question here, before sticking probes into a mains environment.  As a result of that, you have, indeed, learned something - and something that has the very distinct potential to save you from injury (or worse) - without exposing yourself to the real risk.

To be fair to those who may have come across as overcritical, nobody here has any real idea of who you are, your background or your capabilities.  You've only been a member for a couple of days - and the only thread you have posted on is this one.  As time goes by and we have the chance to discuss things with you, we will develop a better understanding - but at this point in time, you are somewhat of an unknown to us.

While there are a lot of things we can help you with, sometimes it is better if advice like the following is given:

If you're not absolutely sure you can test this safely - call an electrician.

Not only does an electrician have the qualification for this sort of work, they also have experience.  Sometimes, it is this experience that can interpret the readings and/or follow a testing path that can identify the fault - because faults in the real world are sometimes non-intuitive, until all the necessary information is at hand (which might take a lot of time, reasoning and measurement) ... or it has been encountered previously.


We are certainly in favour of helping and educating here, but we don't want to encourage wrecklessness - and until we get to know a member a bit better, we are likely to err on the side of caution.

One last word .... there is a wide scope of members here - with different backgrounds, different experience, different cultures - and, of course, different native languages.  It's a global community.  There are bound to be communications issues.

[Brumby]

However, in your original post you did write "...Again, I know the answer, it is safe...".

That's because I know the theory, and the theory says that the voltage I want to measure is below probe and oscilloscope limits.
But practice is other story, you guys are more experienced and that's why I'm asking, because I do know that I can measure that IN THEORY, but in practice, anything I have not take care of could go wrong.

You omitted the words "IN THEORY" with your original post.  If you had included those, then a whole lot of the messy part of the discussion would not have happened.

If you thought we should have understood that, then you need to understand the engineering mind - it works on the basis of what is presented to it.

[Brumby]

Aaaand talking again about it, what if I build a voltage divider with a few megaohm resistors?

That idea is how a high voltage probe is designed - but there are a few more considerations than just simple voltage calculations....

The physical layout of the divider is important, as is the substrate on which it is mounted.  If using a PCB, then there has to be proper consideration given to isolation for each point.  Then there is a case which will need to keep the operator safe when transients are encountered.  As well, you will need to consider the voltage rating of the resistors.  There are probably some other considerations that people with better knowledge on this subject could add.

These issues are not trivial, especially if you don't have the practical experience.  Better to buy a commercial HV probe.

[Ian.M]

Neutral will normally be NEAR ground, but depending on where they are commoned there will be a voltage between them, caused by Neutral conductor current. This will of course be a very low impedance source. It is not sensible or reasonable to expect a probe ground lead to short this current, regardless of any fault condition or not!

I have like 40 volts between neutral and ground, that's what I'm talking about xD and that's why I want to measure mains, because something is wrong with my electrical installation.

My neutral has 40 volts on it too when I put the meter between it and ground. Is this caused by coupling from the wires all running next to each other? How much current roughly would this 40 volts put out? Could I run a 5W light bulb off it or even an LED? Or would it disappear as soon as you put any load on it.

Putting a load of more than  a couple of mA between a 'high' neutral and ground will trip any RCD or GFCI on the circuit, unless the ground wire is broken somewhere, and in that case it will put a dangerous voltage on the exterior metalwork of all grounded equipment plugged into the same circuit.

If in doubt, call in a qualified inspecting electrician.

Assuming you have a CATIII or better rated DMM with fused test leads, if you really want to find out what's going on, you'll need to drive a temporary ground rod into moist ground, and bring an insulated wire from it inside to get a true Earth reference to measure against.  If using thin hookup wire, you'd better put an inline fuse holder with a 250mA fuse on the end of it in case you accidentally touch it to a live terminal or other live metalwork.  Do not assume that any pipework is properly grounded.

By testing between the true earth wire and socket ground you can detect if the ground wiring has a problem.  You can also check the voltage between true earth and neutral.

CAUTION: if there is a fault, lethal voltages may exist between the true earth wire and any supposedly grounded metalwork in the room. Treat the earth wire as-if it is live until you are certain no dangerous voltage differences are present.  Do *NOT* attempt this sort of testing with a CATII or lower rated meter, with unfused test leads or if you are unsure what you are doing.

[mtdoc]

Nice video but...

I know in america (not spain) since you have 120VAC you don't know what leg of the 220 your outlet is plugged into, that's why people always say its a 50/50 shot if you blow up your scope.

No - The issue has nothing to do with which leg (L1 or L2 in the North American split phase 120/240V system) the particular branch circuit you are plugged into is utilizing.

Care to elaborate what the issue is? Usually when you tell someone they are wrong you explain why.

*In North American split phase electrical service, there are 2 "hot" legs at the service entrance (L1 and L2) and one neutral which is bonded to earth ground.

*Both L1 and L2 are 120V with respect to neutral/ground. Each individual 120V branch circuit coming off of the main service panel utilizes either L1 or L2 - but not both. (A 240V circuit utilizes both).

*From the point of view of the end user of these 120V circuits at the home electrical outlets, it makes no difference which leg (L1 or L2) is supplying any particular outlet.  It is only relevant to the electrician wiring the service panel and wanting to balance loads between legs.

HERE is a good, concise reference for this. See figure 1.

*From the point of view of oscilloscope use and safety and understanding how not to "blow up your oscilloscope" and use or miss-use of isolation transformers, the distinction or even knowledge of which leg is supplying an outlet is irrelevant and has nothing to do with any supposed "50/50 chance" of blowing up your oscilloscope.

*Please watch Dave's video ("How not to blow up your oscilloscope") and the video you posted again to better understand why and how measuring mains supplied circuits (or measuring mains itself) with an oscilloscope can result in disaster without great care and understanding.  It's not an easy subject to fully grasp and Dave does a much better job in his video of explaining it that I could do in a brief forum post.

[Elf]

My neutral has 40 volts on it too when I put the meter between it and ground. Is this caused by coupling from the wires all running next to each other? How much current roughly would this 40 volts put out? Could I run a 5W light bulb off it or even an LED? Or would it disappear as soon as you put any load on it.

When wired to code, the neutral is bonded to ground at the panel where utility service enters. Neutral typically rises above ground potential due to the current flowing through the resistance of the wire(s) back to where ground and neutral are bonded. For example if that were 2 ohms back and 10 volts N-G then 5 amps would be returning over the neutral. If the load were removed (0 amps draw), then neutral should be at ground potential. You are essentially just measuring the voltage across the length of the neutral wire from you to the bonding point, via the ground wire (as a long extension of your multimeter probe).

40 volts seems very high for a N-G voltage. You may have bad connections (high resistance) or perhaps it is floating entirely and not bonded. I would get the attention of an electrician.

[tggzzz]

In typo veritas

...but we don't want to encourage wrecklessness...

The rest of your post is spot on, too.

[David Hess]

Putting a load of more than  a couple of mA between a 'high' neutral and ground will trip any RCD or GFCI on the circuit, unless the ground wire is broken somewhere, and in that case it will put a dangerous voltage on the exterior metalwork of all grounded equipment plugged into the same circuit.

Putting more than a couple of mA between neutral or hot and any other point will trip a RCD or GFCI whether ground is connected or not.  The RCD or GFCI does not require ground to work.  The other point could be another phase, neutral, hot, or anything which draws a current including a different ground.  The RCD or GFCI is a two wire device.