How to mesure 240V AC with an oscilloscope

[Electro Detective]

There are other fun things to aid in the death of the probe ground clips, oscilloscope, operator's eyes, heart rate, and life: 

Mains mis-wires including faulty wall cabling and termination, REVERSALS (everyone's favorite..) DIY extension cords, reversed or cheap suspect IEC leads.. 

all waiting to bite you and your pocket 

Even if you are sure of your abilities and everything checks out, there is still a 50 / 50 chance of it going wrong


Xieos, even if you know what you are doing, the guys here do not want to flip the coin for Heads or Tails at your expense   


and besides x10 may not be enough to see all the waveform at 220v, you may have to back off the vertical gain trim all the way aka vintage 'uncal'


and I don't favor your method above, I would rather play bite me with a poisonous snake 

If you have 2 scope channels and 2 probes, at least go the differential mode way, remove those croc ground death clips

and find out how it's done properly PLEASE   

[onlooker]

I think one of the risks is that the beginner scope users tend to think and treat a scope as just an extended version of multi-meter without fully understanding that, when dealing with lethal voltages, the operator may be exposed to much more danger with a scope than with a multi-meter.

Another point to also realize is that theoretically valid does not equal to practically proper. For using scopes dealing with lethal voltages, forming a good habit with rules that are easy to remember and follow is as important as theoretically understand where the dangers are. This is because people make mistakes.

[Xieos]

@onlooker
"theoretically valid does not equal to practically proper"

Totally Agree ... with you ... but I have seen a lot of fuzz about this here in this thread .... without a single drawing or graphical representation that clearly showing that if you have that earth path open the scope will not blow up .... but you may get electrocuted ....

I used to measure the input/output of Medium and Low Voltage VFDs... but I do it safely with active differential probes and battery powered scope  .....

and I understand the risk of actually plugging the probe into a wall outlet  ... which of course I'll not be doing ..... but the way this discussion was going it is a No No Answer without a single diagram or a circuit that shows a problem ... "A picture is worth a thousand words"
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@tronde

Interesting .... how would we know if the connection in the circuit I have posted will affect the insulation ...?....New Challenge (how to Check Scope Insulation to determine if's going to weaken after the test)




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@Xieos....

I think I understand why people say it is NO NO taboo thing ...maybe if someone got hurt because we took the risk of saying <<yes it's doable >> we will feel guilty

[onlooker]

As others have pointed out, one obvious danger is to have live voltage on the exposed metal parts of the scope. This fact may easily get forgotten since one is so used to touching these  metal parts freely.

Picture is not needed, if you think an anology of sticking a screw driver into the live hole of a mains' socket. It may not always harm someone, but it can be considered a bad habit that adds the chance of harming someone, espetially when there are better alternatives to do it in a safer way.

[Richard Crowley]

@Xieos, you don't seem to be getting the point here. Anybody who says "sure, that circuit will work" is a fool and you shouldn't be taking advice from them.  At least not if you value your gear, your house, and your life.

If you make many assumptions not in evidence in your Happy Path Diagram, and assume that the diagram floated down to us from Planet Ideal, sure, it would work.  But this is NOT "Planet Ideal"

Just the absence of an Earth connection, (either for the 'scope mains power plug, OR for the measured outlet), makes it a Show-Stopping Risk, IMHO.  You are literally betting YOUR LIFE that you know positively 100% which is Line and which is Neutral.  You are further betting that the "Neutral" is some voltage reasonably close to zero.  We have seen and heard too many reports of "Neutral" and even "Earth" not being what they are supposed to be. And people have literally died from those kinds of errors.

The very fact that you are asking that question indicates to us that you are not cautious enough for anyone to tell you "Yes, that will work." without fearing that they are encouraging you to do something that will kill you.

[Richard Crowley]

There is actually a way to do this without a wired connection to a known good Earth reference.  Hold the common lead of your good handheld AC voltmeter in your hand and probe with the volts lead.  Capacitive coupling between you and true earth will provide a reference allowing you to determine if the wired Earth ground, neutral, and hot are wired incorrectly.

This works best with a voltmeter which has a 10 megohm input resistance for AC volts but a 1 megohm meter will work also.

Yes, that is what I suggested in my "gun to your head" scenario in a previous response.

But even that technique (with a meter or with a scope) will NOT tell you if the lower-voltage pin ("Neutral"?) is properly within a few volts of Earth Zero.

[newbrain]

You are literally betting YOUR LIFE that you know positively 100% which is Line and which is Neutral.  You are further betting that the "Neutral" is some voltage reasonably close to zero.  We have seen and heard too many reports of "Neutral" and even "Earth" not being what they are supposed to be. And people have literally died from those kinds of errors.

QFT.

And I'll repeat myself, you know it comes with age  ...

There are places where mains distribution does not use a "neutral" at all: every house I've lived in in Rome (including the one where I am now!) has two live wires, with about 120V to ground from either side. This stems from the conversion from 110V to 220V that happened some decades ago.

Using the connections in the picture will make the scope chassis live, leaving your life in the hands of the GFCI/RCD (called a "differential" or "life-saver" in Italy).

[Electro Detective]

There are places where mains distribution does not use a "neutral" at all: every house I've lived in in Rome (including the one where I am now!) has two live wires, with about 120V to ground from either side. This stems from the conversion from 110V to 220V that happened some decades ago.

Using the connections in the picture will make the scope chassis live, leaving your life in the hands of the GFCI/RCD (called a "differential" or "life-saver" in Italy).

So your local street transformer has a centre tap that is also tied to earth/ground ?

Otherwise the life-saver may disappoint when required     

[newbrain]

So your local street transformer has a centre tap that is also tied to earth/ground ?

Otherwise the life-saver may disappoint when required     

Presumably yes, as the GFCIs work as expected (I might not be writing this otherwise ).

The old distribution was 125V (phase-neutral) for regular sockets, and 220V (phase-phase) for heavy uses (washing machines, boilers etc.) called "industriale".
Everyone called the 125V "110V" 

With the switch to 220V, what was the neutral was "simply" connected to another phase.

I can still measure ~127V phase (any) to ground, and remember, as a boy, to have lit 110V bulbs from phase to "ground" (=heating pipes): no real PE in sockets or GFCI, at the time  .

Newer areas have a more normal (and safer?) phase-neutral distribution (derived from 380-400V triphase).

But the main, in topic, take away is: do not take risks with improvised "solutions", the assumption they rely on might not be valid!

[tronde]

You are literally betting YOUR LIFE that you know positively 100% which is Line and which is Neutral.  You are further betting that the "Neutral" is some voltage reasonably close to zero.  We have seen and heard too many reports of "Neutral" and even "Earth" not being what they are supposed to be. And people have literally died from those kinds of errors.

QFT.

And I'll repeat myself, you know it comes with age  ...

There are places where mains distribution does not use a "neutral" at all: every house I've lived in in Rome (including the one where I am now!) has two live wires, with about 120V to ground from either side. This stems from the conversion from 110V to 220V that happened some decades ago.

Using the connections in the picture will make the scope chassis live, leaving your life in the hands of the GFCI/RCD (called a "differential" or "life-saver" in Italy).

It is typical for people living in certain countries to believe that the entire world is exactly the same as what they have at home... It is just impossible for them to understand that the world is a very diverse place.

It can be that your distribution system is IT (isolated terra). An example of IT can be what is common in Norway. The source transformer is connected as star (wye) with no earth connection from the center. It does have a spark gap in case of lightning, but no permanent connection. The house is connected as delta with only local ground/PE. We have 230V available and if the load is symmetrical on all three phases from the transformer you will measure 230/1.73=133V between one phase in your house and earth. Because of this we do not have "live" or "neutral". We only have "live". If the load is not symmetrical (or if there is a short to ground somewhere) you can measure anything between 0V an 230V between a given phase and PE.

Until the late 1990's it was common to use wall sockets without earth if the room was defined as "dry". Bedrooms and livingrooms will most likely not have any PE in houses built before the late 1990's. I know that this was common practice in some European countries.


Only very few European countries have a defined "live" and "neutral" pin in the wall socket. The schucko socket is not polarised, and the "French" socket that is polarised by design is implemented in different ways in the countries using it.

Because of this, equipment with mains plugs  sold on the European market must be made so it will be safe regardless of which supply wire is "live" or "neutral".

[tronde]

@tronde

Interesting .... how would we know if the connection in the circuit I have posted will affect the insulation ...?....New Challenge (how to Check Scope Insulation to determine if's going to weaken after the test)

Your scope is fed from say 230V mains. Inside your scope you have some kind of a transformer that will insulate the electronics in the scope from the 230V mains so you can use it safely.

The problem I mention is caused by the fact that the 0V input socket (the screen of the probe) is connected to the scopes chassis and therefore also the PE wire. This wire is not meant to be at an elevated voltage during normal operation. If you connect an elevated voltage to the chassis, you will have a voltage difference between the chassis and the transformer providing the insulation (and safety) from the mains voltage. Some manufacturers allow only 50V, while others allow something else.

Because of this you can end up by weakening the insulation when you measure as you indicate in your drawing.

I do really encourage you to find another way to do the measurement. You said your intention was to measure noise feed back to the mains. You also said "and I understand the risk of actually plugging the probe into a wall outlet  ... which of course I'll not be doing ...".

I don't actally understand why you asked about connecting as shown in your drawing if you dont need to connect to mains. You must remember that if you measure anything that is fed from mains without an isolating barrier, you are actually connecting directely to mains safety wise. Many people seems to forget this. It is not only direct connection to the wall socket that is dangerous.

[Xieos]

I'm not actually against safe practices ... but I mean this type of measurement is doable but it's not safe and it's not recommended and whoever will do this has to know what he is doing and proceed at his own risk ...

@tronde .... as long as I have a safer alternative I will not take such a measurement ..... I'm not trying to measure anything like this now but my example was for the sake of this long thread.

[tronde]

@tronde .... as long as I have a safer alternative I will not take such a measurement ..... I'm not trying to measure anything like this now but my example was for the sake of this long thread.

OK.

When somebody ask about a safety related issue on an open forum we have no idea about their actual knowledge or intention. I have met a lot of people that ask a question, and use the answer for something completely different. When this happen in a face-to-face setting, you expect worse on the internet.

People can for instance say they are a certified electrician so they know about these things. The problem is that a certified electrician is not the same in all countries. In some countries they will have a lot of theory in school, while in other countries the education is more about how to follow the rule-book and do the wiring.

Even if you know what level of knowledge is expected in a certain country, this is not the same as the person in question actually has this knowledge.

The education can also change over time. 20 years ago a Norwegian electrician would do basic calculations on three-phase systems and phase compensation in the first year at school. Today you can find pupils leaving school after two years that believe plus and minus is only about which way you connect your voltmeter. This is not a joke.

You will also find certified electricians that don't know how many volts you are expected to measure between the different wires in a 400V TN-system, or why you can get more than 230V in the wall socket if the neutral conductor from outside of the house works loose in the distribution panel.