How to mesure 240V AC with an oscilloscope

[IanB]

Use a differential probe so you aren't referenced to anything, no problems. Dave has some video's using differential probes.

Well surely you are referenced to the chassis ground of the oscilloscope? So if there were a 4 kV spike on one of the probes it might find a path to ground through insufficient insulation or isolation?

[Ribster]

Use a differential probe so you aren't referenced to anything, no problems. Dave has some video's using differential probes.

Well surely you are referenced to the chassis ground of the oscilloscope? So if there were a 4 kV spike on one of the probes it might find a path to ground through insufficient insulation or isolation?

Use a proper isolated one then offcourse. Your CAT rating will tell you how high you can go. The right tool for the job. If your circuit can go up to 4kV potentially, then well, take the differential probe that's rated for 4kV. No ?

[David Hess]

Use a differential probe so you aren't referenced to anything, no problems. Dave has some video's using differential probes.

Well surely you are referenced to the chassis ground of the oscilloscope? So if there were a 4 kV spike on one of the probes it might find a path to ground through insufficient insulation or isolation?

Both probes are referenced to chassis ground through the oscilloscope so an overload will be shunted to chassis ground either through the probes or the oscilloscope input protection circuits.  Note that this means many differential probes will *not* work on a galvanically isolated vertical input depending on how they are configured because their common mode voltage may drift out of range.

If the oscilloscope itself is floated, then breakdown through the insulation of its power supply transformer or EMI filtering networks is possible or even likely.  Old Tektronix application notes which discuss making measurements with the oscilloscope chassis floated mention this danger.

Personally I do not worry about making an occasional single ended measurements using a 300 VAC catagory 2 x10 or x100 passive probe of a power outlet up to 240 VAC while the oscilloscope is properly grounded and the probe ground lead is not used but all of the common mode noise between the oscilloscope ground and the measurement point will be included.  x100 passive probes rated to 2000 volts peak or higher are available and inexpensive if higher levels of protection are warranted.

Good oscilloscope high impedance inputs have no trouble handing an direct connection to 240 VAC at any sensitivity without damage.

[vk6zgo]

When do all these magic 4kV spikes occur?
If they were at all common,we would have equipment being zapped everyday,all around the world.
In fact we do not,& the Power supply Companies take good care that we don't,with protective devices of various types.

Many of these very high voltage ratings are to provide a very large amount of "headroom" over & above the operating voltages of the system.

In the real world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip to anything & rely upon the 'scope Earthing via its own power cord to provide the return path.

And guess what? You can pre-attach the probe & then turn on the power---then,if I & the tens of thousands of other people who have used this method are wrong,all you will do is blow up your probe,or possibly your 'scope.
You will not be touching anything,so will be safe.

[mzzj]

In the real world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip to anything & rely upon the 'scope Earthing via its own power cord to provide the return path.

Many of the X10 probes are rated CAT II 300V, so they should be good for up to 2.5kV anyhows....

Ground clip is the biggest safety risk for beginners and even intermediate hobbyist.
I have seen scope probes with fuse holder on ground clip in one particular company...guess they had some issues 

[David Hess]

In the real world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip to anything & rely upon the 'scope Earthing via its own power cord to provide the return path.

Many of the X10 probes are rated CAT II 300V, so they should be good for up to 2.5kV anyhows....

Ground clip is the biggest safety risk for beginners and even intermediate hobbyist.
I have seen scope probes with fuse holder on ground clip in one particular company...guess they had some issues 

I had a ground clip turn white hot and melt once.  Luckily that was the only damage that occurred.

The safest way to handle the ground clip when making this kind of measurement is to remove it.

[othello]

So as a newbie i think i understand it as follows for a household mains:

- HOT wire carries the current and polarity keeps changing 50 or 60 times a second only on this wire and shows voltage when check against GROUND ?
- NEUTRAL wire is the returnpath and does not carry current and has 0V when checked against GROUND ?

I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

[WarSim]

When do all these magic 4kV spikes occur?
If they were at all common,we would have equipment being zapped everyday,all around the world.
In fact we do not,& the Power supply Companies take good care that we don't,with protective devices of various types.

Many of these very high voltage ratings are to provide a very large amount of "headroom" over & above the operating voltages of the system.

In the real world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip to anything & rely upon the 'scope Earthing via its own power cord to provide the return path.

And guess what? You can pre-attach the probe & then turn on the power---then,if I & the tens of thousands of other people who have used this method are wrong,all you will do is blow up your probe,or possibly your 'scope.
You will not be touching anything,so will be safe.

No you would not the high voltage transients are either filtered or suppressed by the inputs of your appliances.  Scope probes do not have such suppression because it would corrupt the test signal.  They do have input protection but that just tries to clamp the signal at their theoretical maximums. 

+1. For Darwinism. 

[Richard Crowley]

- HOT wire carries the current and polarity keeps changing 50 or 60 times a second only on this wire

Correct

and shows voltage when check against GROUND ?

Probably, not not necessarily.  The actual mains voltage occurs between the "HOT" and the "NEUTRAL" (or "other hot" in places where they don't care!)

- NEUTRAL wire is the return path

Correct

and does not carry current

NO.  If the neutral is the "return path", then, by definition, it carries the return current!

and has 0V when checked against GROUND ?

Again, that depends on the mains wiring convention in your country (or whatever).  Apparently there are several competing standards for this.

I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

So that shows that in your area, you have a neutral that is connected to ground somewhere.

REMEMBER, the green-wire safety GROUND/EARTH wire DOES NOT carry current for that circuit!  It is only there as a safety feature to take any fault current if something goes wrong.  Else it might be going to ground through YOU!

[othello]

- HOT wire carries the current and polarity keeps changing 50 or 60 times a second only on this wire

Correct

and shows voltage when check against GROUND ?

Probably, not not necessarily.  The actual mains voltage occurs between the "HOT" and the "NEUTRAL" (or "other hot" in places where they don't care!)

- NEUTRAL wire is the return path

Correct

and does not carry current

NO.  If the neutral is the "return path", then, by definition, it carries the return current!

and has 0V when checked against GROUND ?

Again, that depends on the mains wiring convention in your country (or whatever).  Apparently there are several competing standards for this.

I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

So that shows that in your area, you have a neutral that is connected to ground somewhere.

REMEMBER, the green-wire safety GROUND/EARTH wire DOES NOT carry current for that circuit!  It is only there as a safety feature to take any fault current if something goes wrong.  Else it might be going to ground through YOU!

Thank you, sounds like it's clearing up for me

So for example, say some one uses a 1000X probe rated to 10kV, he first would identify the NEUTRAL wire, put the ground of the probe on it and then probe the HOT wire to show the wave form on his DSO ?

[Richard Crowley]

Thank you, sounds like it's clearing up for me
So for example, say some one uses a 1000X probe rated to 10kV, he first would identify the NEUTRAL wire, put the ground of the probe on it and then probe the HOT wire to show the wave form on his DSO ?

NO! Just because the wire is called "neutral" (or even "ground") does NOT mean that it is safe to connect the probe/scope ground to it!

If they have ANY voltage on them (more than a few millivolts of noise), relative to scope/probe ground, then it is NOT SAFE to connect the probe/scope ground!  NEVER EVER connect the scope/probe ground to anything that isn't really. truly GROUND!  (or is floating and it doesn't matter)  The power mains neutral and even power mains safety ground are particularly hazardous because of how much current they may have behind them even if you are only measuring a volt or two. And because people just assume they are "ground" when they frequently ARE NOT (as you have shown in your own case!)

Did you not see others saying to NEVER connect the probe ground to any mains terminal?  Did you not read the story of the ground clip turning white from heat?  Did you not read the description of the scope probe that had a fuse in the ground wire?  There are many things here that will trip you up, or blow up your scope or even electrocute you and burn down your house.  This is NOT something to mess with unless you really understand what is happening here.

[David Hess]

- NEUTRAL wire is the return path and does not carry current and has 0V when checked against GROUND ?

I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

Like othello says, the NEUTRAL wire carries the return current from the HOT wire.

When no current is flowing it will be at the same potential as the GROUND wire so it may appear safe however when high current is flowing, its potential will rise above GROUND and this presents a problem for grounded measurements because if the oscilloscope ground lead is attached to NEUTRAL, part of that current will flow through the oscilloscope probe, the oscilloscope, and into the oscilloscope chassis GROUND connection.

Even worse, NEUTRAL may be swapped with HOT.  Almost as bad is when NEUTRAL fails for some reason like if the plug gets pulled out part way.  Then the entire NEUTRAL current may flow through the ground connection of the oscilloscope probe.

In general as far as safety, it is best to treat NEUTRAL and HOT the same and off-line power supplies are not suppose to distinguish them because mistakes sometimes happen.

[othello]

Thank you, sounds like it's clearing up for me
So for example, say some one uses a 1000X probe rated to 10kV, he first would identify the NEUTRAL wire, put the ground of the probe on it and then probe the HOT wire to show the wave form on his DSO ?

NO! Just because the wire is called "neutral" (or even "ground") does NOT mean that it is safe to connect the probe/scope ground to it!

If they have ANY voltage on them (more than a few millivolts of noise), relative to scope/probe ground, then it is NOT SAFE to connect the probe/scope ground!  NEVER EVER connect the scope/probe ground to anything that isn't really. truly GROUND!  (or is floating and it doesn't matter)  The power mains neutral and even power mains safety ground are particularly hazardous because of how much current they may have behind them even if you are only measuring a volt or two. And because people just assume they are "ground" when they frequently ARE NOT (as you have shown in your own case!)

Did you not see others saying to NEVER connect the probe ground to any mains terminal?  Did you not read the story of the ground clip turning white from heat?  Did you not read the description of the scope probe that had a fuse in the ground wire?  There are many things here that will trip you up, or blow up your scope or even electrocute you and burn down your house.  This is NOT something to mess with unless you really understand what is happening here.

Thanks for clarifying that. Also don't worry, i'm not going to try it, just trying to understand where things go wrong when trying something with not enough knowledge / safety precautions.

- NEUTRAL wire is the return path and does not carry current and has 0V when checked against GROUND ?

I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

Like othello says, the NEUTRAL wire carries the return current from the HOT wire.

When no current is flowing it will be at the same potential as the GROUND wire so it may appear safe however when high current is flowing, its potential will rise above GROUND and this presents a problem for grounded measurements because if the oscilloscope ground lead is attached to NEUTRAL, part of that current will flow through the oscilloscope probe, the oscilloscope, and into the oscilloscope chassis GROUND connection.

Even worse, NEUTRAL may be swapped with HOT.  Almost as bad is when NEUTRAL fails for some reason like if the plug gets pulled out part way.  Then the entire NEUTRAL current may flow through the ground connection of the oscilloscope probe.

In general as far as safety, it is best to treat NEUTRAL and HOT the same and off-line power supplies are not suppose to distinguish them because mistakes sometimes happen.

Thank you for the extra info on this !

What would be the official way to display highvoltage A/C waveforms on a DSO ? Not looking to cheat on costs, so just how would one go about it in a professional way ?

[David Hess]

- NEUTRAL wire is the return path and does not carry current and has 0V when checked against GROUND ?

I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

Like othello says, the NEUTRAL wire carries the return current from the HOT wire.

When no current is flowing it will be at the same potential as the GROUND wire so it may appear safe however when high current is flowing, its potential will rise above GROUND and this presents a problem for grounded measurements because if the oscilloscope ground lead is attached to NEUTRAL, part of that current will flow through the oscilloscope probe, the oscilloscope, and into the oscilloscope chassis GROUND connection.

Even worse, NEUTRAL may be swapped with HOT.  Almost as bad is when NEUTRAL fails for some reason like if the plug gets pulled out part way.  Then the entire NEUTRAL current may flow through the ground connection of the oscilloscope probe.

In general as far as safety, it is best to treat NEUTRAL and HOT the same and off-line power supplies are not suppose to distinguish them because mistakes sometimes happen.

Thank you for the extra info on this !

What would be the official way to display highvoltage A/C waveforms on a DSO ? Not looking to cheat on costs, so just how would one go about it in a professional way ?

The best is to use a high voltage differential probe because the ground connection may go to actual ground or not be used at all in some cases.  Two vertical channels may be operated differentially to achieve the same thing although the performance will not be as good.

Second best is to use an oscilloscope which has galvanically isolated vertical inputs.  Some handheld oscilloscopes have this feature as well as the Tektronix TPS2000 series.

Third best is to use an isolation transformer so hot and neutral are isolated and then ground may be attached at the same point that the probe grounds connect to.  In extreme cases I do this while also using a differential probe.

[Richard Crowley]

What would be the official way to display highvoltage A/C waveforms on a DSO ? Not looking to cheat on costs, so just how would one go about it in a professional way ?

As several people have mentioned in this (and similar) discussions, using TWO high-voltage probes into a DIFFERENTIAL input on an oscilloscope would be the "professional" way of looking at the waveform of a signal that isn't really "referenced to ground".  You would connect one probe to the "hot" side, and the other to the "neutral" node, and then the oscilloscope "subtracts" the neutral voltage from the "hot" voltage, yielding exactly what the voltage is between them, but without having to connect EITHER side to ground (because "neutral"/cold is not necessarily "ground" as you have proved with your own tests.

[othello]

Thanks, i've looked up for my Rigol scope the available differential probes and looks like there are a few options. Pricewise they're expensive but better safe than sorry if i ever would need to measure dangerous AC !

One of them seems to be good enough with these specs:

High Voltage Differential Probe, DC-25 MHz, 1400 Vpp

1400Vpp is peak to peak, so half wave would be 700V ? And the 25Mhz is plenty i assume because here the household AC is 50Hz ?

PS: I just saw the reply from Richard, so if my scope would have a differential input then that would be also the good way to go.

[Ribster]

1400Vpp is peak to peak, so half wave would be 700V ? And the 25Mhz is plenty i assume because here the household AC is 50Hz ?

You should really consider an electricity course. Relation between 220VAC  RMS and Vpp is "square root of 2". So in your case if you need to need to go backwards its 1400 / 1,41 -> 992,9VAC RMS.

25Mhz will suffice to check a plain 220V sinewave. Once the waveform starts to deform (eg. motor voltages, triac voltages), the high frequency harmonics start to come into play. Look up harmonics and bandwidth, read up onto this..

Knowledge is power!

[vk6zgo]

When do all these magic 4kV spikes occur?
If they were at all common,we would have equipment being zapped everyday,all around the world.
In fact we do not,& the Power supply Companies take good care that we don't,with protective devices of various types.

Many of these very high voltage ratings are to provide a very large amount of "headroom" over & above the operating voltages of the system.

In the real world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip to anything & rely upon the 'scope Earthing via its own power cord to provide the return path.

And guess what? You can pre-attach the probe & then turn on the power---then,if I & the tens of thousands of other people who have used this method are wrong,all you will do is blow up your probe,or possibly your 'scope.
You will not be touching anything,so will be safe.

No you would not the high voltage transients are either filtered or suppressed by the inputs of your appliances.  Scope probes do not have such suppression because it would corrupt the test signal.  They do have input protection but that just tries to clamp the signal at their theoretical maximums. 

+1. For Darwinism.

Mains supplies have been around for many years.
Only comparatively recently have devices had such protection,& even now,not every piece of equipment has protection---legacy stuff,kitchen appliances,etc.

Huge voltage spikes on the Mains supplies are incredibly rare,& even in the unlikely event of them occurring,most are of such short duration,as to (1) have very little energy, (2)be substantially attenuated by the inline inductance & resistance of the distribution system.

We have to synchronise you looking at the Mains with your 'scope,& the very rare occurrence of a spike capable of damaging your probe.
It's like winning the Lotto every day for 20 years!

For some reason,some people like to think our occupation is incredibly dangerous.
There are dangers,but a little forethought can allow you to work safely for a whole career.
Running around squawking like Chicken Little,on the other hand,is pretty much counter-productive.

[vk6zgo]

1400Vpp is peak to peak, so half wave would be 700V ? And the 25Mhz is plenty i assume because here the household AC is 50Hz ?

You should really consider an electricity course. Relation between 220VAC  RMS and Vpp is "square root of 2". So in your case if you need to need to go backwards its 1400 / 1,41 -> 992,9VAC RMS.

25Mhz will suffice to check a plain 220V sinewave. Once the waveform starts to deform (eg. motor voltages, triac voltages), the high frequency harmonics start to come into play. Look up harmonics and bandwidth, read up onto this..

Knowledge is power!

Of course,you do realise,that when looking at an AC waveform,"peak to peak" is only a "pretend" figure,to make it easier to measure voltages on a 'scope.
(OK,it is also useful for calculating the operating point of amplifiers,ADCs,etc.)

The +ve & -ve half cycles are not present at the same time,so the real peak voltage, Vp of a 1400v p-p waveform is 700v.

The relationship between Vpp & RMS is : Vrms=Vp /root2
From the values given,RMS is 494.97 v,or,near as dammit,500v.

"A little learning is a dangerous thing!"

[WarSim]

When do all these magic 4kV spikes occur?
If they were at all common,we would have equipment being zapped everyday,all around the world.
In fact we do not,& the Power supply Companies take good care that we don't,with protective devices of various types.

Many of these very high voltage ratings are to provide a very large amount of "headroom" over & above the operating voltages of the system.

In the real world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip to anything & rely upon the 'scope Earthing via its own power cord to provide the return path.

And guess what? You can pre-attach the probe & then turn on the power---then,if I & the tens of thousands of other people who have used this method are wrong,all you will do is blow up your probe,or possibly your 'scope.
You will not be touching anything,so will be safe.

No you would not the high voltage transients are either filtered or suppressed by the inputs of your appliances.  Scope probes do not have such suppression because it would corrupt the test signal.  They do have input protection but that just tries to clamp the signal at their theoretical maximums. 

+1. For Darwinism.

Mains supplies have been around for many years.
Only comparatively recently have devices had such protection,& even now,not every piece of equipment has protection---legacy stuff,kitchen appliances,etc.

Huge voltage spikes on the Mains supplies are incredibly rare,& even in the unlikely event of them occurring,most are of such short duration,as to (1) have very little energy, (2)be substantially attenuated by the inline inductance & resistance of the distribution system.

We have to synchronise you looking at the Mains with your 'scope,& the very rare occurrence of a spike capable of damaging your probe.
It's like winning the Lotto every day for 20 years!

For some reason,some people like to think our occupation is incredibly dangerous.
There are dangers,but a little forethought can allow you to work safely for a whole career.
Running around squawking like Chicken Little,on the other hand,is pretty much counter-productive.

That is called a risk assessment, not a guarantee of safety.  If you can't understand the difference between safe and safe enough, say high to Darwin when you meet him. 

Are you using the magnanimous "our", or are you trying to imply we are in the same field?  By you statement I can assure you that we do not have similar work experience.   

We obviously have different opinions.  I state what is possible and you attack the statement with probability.  The question was what could happen not what will happen.  Im not like you, I would personally feel very bad about advising someone to do something that kills them. 

I would say stop but this is hilarious.  You are actually trying to justify the risks you have decided to accept by convincing someone else to take them. I have read about this in school, it was called lemming theory.  It was a study of the Tobacco industry marketing process but I think it applies here too. 

[vk6zgo]

"That is called a risk assessment, not a guarantee of safety.  If you can't understand the difference between safe and safe enough, say high to Darwin when you meet him.  "

Every day,we make risk assessments-----"safe enough" is the best we can ever get.
I guess we could hide under our beds,but we might get bitten by a spider,or choke on dust bunnies.

If Charles Darwin had needed absolute safety,he wouldn't have ventured down into the "very dangerous" South Atlantic in search of Iguanas to study.

Are you using the magnanimous "our", or are you trying to imply we are in the same field?  By you statement I can assure you that we do not have similar work experience.   


40years "hands on" in Electronics,mainly in the field of high power Transmitting equipment.--------Your turn!

I would say stop but this is hilarious.  You are actually trying to justify the risks you have decided to accept by convincing someone else to take them. I have read about this in school, it was called lemming theory.  It was a study of the Tobacco industry marketing process but I think it applies here too.

I think "school" gives you away,sonny!

Cigarette smoking is not necessary,it is a choice ---unfortunately,walking down the street,catching a train ,working,in any capacity,are necessities & all have an element of "risk assessment".
None of my suggestions put anyone at risk,providing they use the basic intelligence that God/Evolution gave them.

[tautech]

Are you using the magnanimous "our", or are you trying to imply we are in the same field?  By you statement I can assure you that we do not have similar work experience.   


40years "hands on" in Electronics,mainly in the field of high power Transmitting equipment.--------Your turn!

I guess you missed this:
https://www.eevblog.com/forum/beginners/how-to-mesure-240v-ac-with-an-oscilloscope/msg487684/#msg487684

As a person that needed to scope up to 145kv regularly the principals are a all the same.

And this:
https://www.eevblog.com/forum/beginners/how-to-mesure-240v-ac-with-an-oscilloscope/msg487729/#msg487729

[nitro2k01]

25Mhz will suffice to check a plain 220V sinewave.

Nitpick time. Ever since the line voltage harmonization, all of EU is required to provide a nominal 230V instead of the 220V that was previously commonplace in most of Europe. I can almost guarantee you that your socket will provide you with 230 V +/-1% in practice, even if the standard allows for +10%/-6%.

[Ribster]

25Mhz will suffice to check a plain 220V sinewave.

Nitpick time. Ever since the line voltage harmonization, all of EU is required to provide a nominal 230V instead of the 220V that was previously commonplace in most of Europe. I can almost guarantee you that your socket will provide you with 230 V +/-1% in practice, even if the standard allows for +10%/-6%.

I'm glad that you don't dispute the technical baggage of what i said. And yes, you are right. It was changed to 230V, a habit of speech i guess.. Allows for more power throughput though, was a nice move from the EU.

[Niels]

I'm glad that you don't dispute the technical baggage of what i said. And yes, you are right. It was changed to 230V, a habit of speech i guess.. Allows for more power throughput though, was a nice move from the EU.

No, as far as I remember/know it was to harmonize the specification around Europe.

UK had 240Vac +-10%, this was changed to 230Vac +10% -6% in a transition period
The rest was 220Vac +-10%, this was changed to 230Vac +6% -10% in a transition period.

And now it is 230Vac +-10% in Europe.