How to mesure 240V AC with an oscilloscope

[vk6zgo]

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

Should I genuflect?

[vk6zgo]

WarSim,I withdraw the "sonny" crack,having looked back at your postings over the past few months.---sorry!
No,I'm not stalking you!

It is rather unusual for someone to refer back some 30 years to their schooldays to make a point,though!

[David Hess]

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.

At least where I am in the central US, we have a lot of problems with power.  We have like 2 or 3 significant blackouts per year mostly associated with thunderstorms.  The half life for compact lamps which include electronic ballasts is about 6 months so incandescent lamps are a lot more economical simply because they are less expensive to replace.  All of my computer and test equipment is protected by an online UPS and I have considered doing something similar for the lighting circuits but it would be a lot of work and probably not possible to do to code.

[WarSim]

WarSim,I withdraw the "sonny" crack,having looked back at your postings over the past few months.---sorry!
No,I'm not stalking you!

It is rather unusual for someone to refer back some 30 years to their schooldays to make a point,though!

I can see how it could have suggested that.  I also regret my defensive response.  Mostly because I still can't directly say what I did or know even 25 years latter. 

But I can say my start in electronics was in 1970 back when child labor laws didn't exist.  . Even as a child I repaired cct involving 53kv.  In school I was a student instructor.  After school that is when I can only say I had a very unique careers, that I would not wish on anyone.  This is why I mentioned school because the last 25 years is basically a non subject. 

In the end I just wanted to make sure the possibly dangers where known.  There where plenty of others giving empirical examples of safety.  I agree it is unlikely the tailing will surge or float to 1250v PE referenced.  Even less likely it will contain damaging transients up to 4000v mentioned.  What I am saying is it can and it has, so the author has the right to know. 

[WarSim]

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.

At least where I am in the central US, we have a lot of problems with power.  We have like 2 or 3 significant blackouts per year mostly associated with thunderstorms.  The half life for compact lamps which include electronic ballasts is about 6 months so incandescent lamps are a lot more economical simply because they are less expensive to replace.  All of my computer and test equipment is protected by an online UPS and I have considered doing something similar for the lighting circuits but it would be a lot of work and probably not possible to do to code.

There are whole house UPS systems available in the USA.  Some are even made in the USA.  But the ones that are constant conversion are very expensive and large.  In some ways a bit dangerous, just because you are storing power in huge  sealed lead acid batteries.  My uncle uses 3 heavy duty deep cycle fork lift batteries.  So even his cheeper solutions involves $24,000 in storage alone. 

Just saying the option is there, it is up to you to decide if it is obtainable.

[mzzj]

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.

At least where I am in the central US, we have a lot of problems with power.  We have like 2 or 3 significant blackouts per year mostly associated with thunderstorms.  The half life for compact lamps which include electronic ballasts is about 6 months so incandescent lamps are a lot more economical simply because they are less expensive to replace.  All of my computer and test equipment is protected by an online UPS and I have considered doing something similar for the lighting circuits but it would be a lot of work and probably not possible to do to code.

Same in my parents home in countryside Finland, countless electronic bulbs and 1-2 adsl-boxes per year even if you run around your house like maniac to unplug everything at the first hint of approaching thunderstorm.  In a really bad weather the electric sockets are flashing over and you can see blue flashes and hear pops inside the sockets around the house   

During summer months I wouldn't poke/probe  outlets here with anything "probably good enough"

[David Hess]

At least where I am in the central US, we have a lot of problems with power.  We have like 2 or 3 significant blackouts per year mostly associated with thunderstorms.  The half life for compact lamps which include electronic ballasts is about 6 months so incandescent lamps are a lot more economical simply because they are less expensive to replace.  All of my computer and test equipment is protected by an online UPS and I have considered doing something similar for the lighting circuits but it would be a lot of work and probably not possible to do to code.

There are whole house UPS systems available in the USA.  Some are even made in the USA.  But the ones that are constant conversion are very expensive and large.  In some ways a bit dangerous, just because you are storing power in huge  sealed lead acid batteries.  My uncle uses 3 heavy duty deep cycle fork lift batteries.  So even his cheeper solutions involves $24,000 in storage alone. 

Just saying the option is there, it is up to you to decide if it is obtainable.

It does not make much sense to condition the power for things like the stove, oven, electric dryer, welder, furnace, etc. but expensive light bulbs in fixed circuits which do not draw much power and are expensive are another matter.  Double conversion power conditioning without backup would be sufficient so expensive perishable batteries would not be needed.  I have a couple of smaller online UPSes which are currently operating without batteries but they are not quite large enough.

Even then there is no payoff as long as I have access to inexpensive light bulbs.

It might be fun to leave a DSO running to capture power line transients but all of mine are CRT based and I am not inclined to add that many hours to them.

[othello]

Wow, despite various standards it looks like in the real world things don't always match the theory :-) But i appreciate this topic greatly, i bookmarked it and hope newbies will find it also fruitful.

PS: I found out that the metal sticking out consists of 2 separate hooks, 1 is ground and 1 is according to scope manual the "Compensation Signal Output Terminal". So never mind my previous statement about measuring 1.3V between ground and earth, it was the CSOT and ground i was measuring. So now when i measured the ground of the scope and earth it shows about 4mV average.

[vk6zgo]

WarSim,I withdraw the "sonny" crack,having looked back at your postings over the past few months.---sorry!
No,I'm not stalking you!

It is rather unusual for someone to refer back some 30 years to their schooldays to make a point,though!

I can see how it could have suggested that.  I also regret my defensive response.  Mostly because I still can't directly say what I did or know even 25 years latter. 

But I can say my start in electronics was in 1970 back when child labor laws didn't exist.  . Even as a child I repaired cct involving 53kv.  In school I was a student instructor.  After school that is when I can only say I had a very unique careers, that I would not wish on anyone.  This is why I mentioned school because the last 25 years is basically a non subject. 

In the end I just wanted to make sure the possibly dangers where known.  There where plenty of others giving empirical examples of safety.  I agree it is unlikely the tailing will surge or float to 1250v PE referenced.  Even less likely it will contain damaging transients up to 4000v mentioned.  What I am saying is it can and it has, so the author has the right to know.

Sorry about the "Chicken Little",too mate!
it was uncalled for.

[tautech]

Sorry about the "Chicken Little",too mate!
it was uncalled for.

All this kiss and make up is getting sickly. 
Nice to see though. 
We often omit a quick search of a members posts to gauge their expertise.

[SeanB]

Gettin back to the topic of probing mains, and the nasties on it I have always had a few dozen plugtops with a VDR in them across live and neutral, simply as a plug to occupy a spare outlet, preferably the first in a strip. This has worked well to attenuate spikes for me, and I also have collected a lot of surge supressor plugtops over the years ( thanks to our Telecom supplier who will often simply change them out if they cause tripping of EL breakers due to surges, often replacing with a regular plugtop as well) and as they also have 3 VDR's in them between pins they also work well. Phone lines also have transient suppression as well, and though this may not be the best for ADSL ( adds to loss so I modify them to leave only the spark gap and remove the silicon switch inside, but have a unmodified one after the ADSL filters) It did save all my electronics when lightning hit a building 50m away a few years ago. Neighbours lost TV sets and electronics, I had no damage at all aside from tripped power. I did have some VDR units go short circuit a while later ( they did the job though) and it was a few minutes work to find the faulty one and simply replace the VDR inside.

I did recently buy a Thermaltake PC power supply, and see that they did a very interesting thing to clip spikes, by using instead of a VDR a spark gap. Hope the fusing is good, though I think those will need the warranty voided if it does operate to replace the fuse. I do not think the NTC thermistor will cope well with being used to current limit the mains, they tend to catch fire and burn in that application. I have done that before......., but it survived slightly crispy and still works even though it is now pacman shaped.

[Xieos]

Hi Every One ....

I wonder why there is no circuit diagrams / grapical explnation in all of this long thread ??... ..mmm

so I live where there is a 220VAC 50 Hz, and normally the power plugs doesn'y have an Earth connection

my question here is .... Will the below Circuit is going to work or not and why ? (Check the Photo )

[alm]

No, it could easily shock you when you touch your scope. What does the user's guide of your scope say about operating it without an earth connection? I would not use a non-isolated scope at all near dangerous voltages without an earth connection.

I suggest you go back to the beginning of this thread ask yourself why you desperately need to measure mains and what the proper tools and alternatives might be (e.g. a small transformer).

[Gyro]

@Xieos:

Just to point out another hazard. If you only have 2-pin, non earthed sockets then you can't be sure which way round Live and Neutral are wired. They might be different on every socket in your house too!

Please don't try to scope the mains like that - there really isn't anything interesting to see anyway. Don't try it because you can (or think you can!).

[Richard Crowley]

Please tell us WHY you think you want to put a scope probe directly on the power mains outlet?

Yes, if all the planets align properly and your bio-rhythms have a triple-peak and you hold you tongue just right, you might be able to safely do this.  But there are so many potential problems here that you could just as likely kill yourself AND burn down your house and blow up your oscilloscope (not necessarily in that sequence).

It continues to be a mystery to me WHY people think they want to put their oscilloscope on the power mains!  Nobody has ever offered a cogent justification.  "Just curious." doesn't really cut it.  Now when weighed against the considerable potential consequences.

[djnz]

Please tell us WHY you think you want to put a scope probe directly on the power mains outlet?

Yes, if all the planets align properly and your bio-rhythms have a triple-peak and you hold you tongue just right, you might be able to safely do this.  But there are so many potential problems here that you could just as likely kill yourself AND burn down your house and blow up your oscilloscope (not necessarily in that sequence).

It continues to be a mystery to me WHY people think they want to put their oscilloscope on the power mains!  Nobody has ever offered a cogent justification.  "Just curious." doesn't really cut it.  Now when weighed against the considerable potential consequences.

Exactly my thought. Why would you want to do this? Just to see some sine waves?

[Xieos]

 
,,,
Thank You Guys for the reply ,

the photo I put there was Just for the sake of this discussion and long thread....but ....

lets just say we have a 18 pulse VFD and I want to measure how much noise/harmonics is introduced back to the mains line (not only current harmonic..but voltage too)  ...(I used to do this with a handheld Fluke Scope)...(yes I know it's battery powered)

(and for Netural and Line...can be easly detected with multimeter referenced to Earth)

[Electro Detective]

There are a LOT of reasons why one would want to see the mains waveform on an oscilloscope,

unfortunately none are acceptable to a lot of nay sayers who have no need for such 'eye candy' when diagnosing MULTITUDES of power related issues in the real world.

Either way, do it SAFE guys, or not at all,
the safety earth/ground system in your house is your family's best friend,
but worst enemy for your earth/grounded test gear and DUTs. 

LEARN about it and do it right, if you must.

If you don't do it right, your probe ground leads will go POP and or will take out your oscilloscope,
or it just won't ever work as it should.. if you survive the INSTANTANEOUS fireworks/BBQ     


If safety doesn't interest you, how much will a replacement scope cost ?   

[Richard Crowley]

The premise here is that if someone has a legitimate reason to view the mains power waveform, they are more likely to understand the issues with safety.  It just seems to me that virtually ALL of the "how do I view the mains waveform" questions are from newbies who just got their first 'scope and are itching to look at something with little or no understanding of the safety issues.

[Xieos]

.... .....

what problems will the circuit (I posted above) will have ?

can some explain with a drawing ....please ...

[Richard Crowley]

A significant problem is that innocent-looking ground connection between the probe and the "neutral" socket pin.

How do you KNOW it is "neutral"? Especially if you have no Earth reference?  In many parts of the planet you have a 50/50 chance of it being "Line" 240V.

Anything you clip the ground lead to will essentially connect it to the entire outer enclosure of the oscilloscope.  If you clipped it to Hot instead of Neutral, you have the equivalent of poking your finger into the mains socket.

And if your scope power plug HAS an Earth pin (and assuming the mains outlet is wired properly), then if you clip your probe ground lead to the Line pin, you can hope that your scope probe blows first as an expensive fuse to protect the even more expensive part of your fault circuit (i.e. the scope itself).

The problem with that drawing is that it ASSUMES a 100% "Happy Path" which rarely exists in the Real World.  It is very misleading especially to newbies who don't understand the risks.  If someone is holding a gun to your head and forcing you to do this, at least do NOT connect the ground clip from the probe.  Leave it floating for safety sake.  By probing EACH pin, you can at least get some indication of which side is "hotter" than the other.

[Xieos]

I pointed out up there that the Neural and Line can be determined by a Multimeter referenced to Earth...Before doing the measurement
( a friend of mine actually took a reading for a VFD mains input with a bench scope before with just leaving the earth return path open)

.... let's just say in an Ideal world .... will the above circuit work ... ??
Yes .... or ... No Question

[tautech]

I pointed out up there that the Neural and Line can be determined by a Multimeter referenced to Earth...Before doing the measurement
( a friend of mine actually took a reading for a VFD mains input with a bench scope before with just leaving the earth return path open)

.... let's just say in an Ideal world .... will the above circuit work ... ??
Yes .... or ... No Question

No one here should answer this as we could have a persons demise on our conscience.

It MUST be your and ONLY your choice to undertake such unsafe practices.
Read this and please do not ask again.
https://www.eevblog.com/forum/testgear/floating-scopes/

[tronde]

Xieos has made it perfectly clear that he has NO earth connection. Therefore it does not make any sense to involve the earth/neutral problem into the discussion. It does in fact exist legally installed non-earthed outlets in some countries. My house is full of them for instance.
It is impossible to consider what is normal in one country as normal in another country. Please do remember that.

The danger by measuring as Xieos suggest is that one of the phases will be connected to the chassis of the scope, and therefore you will need to treat the entire scope as "live"; that means you will in fact be connected to mains when you touch it. Even if you can measure which phase is "live" and which is "neutral" you should consider both to be "live".

The circuit you indicate in your drawing will work, given the scope is made for the voltage difference between its chassis and both supply wires. Many scopes are not made for that, and you can weaken the insulation so it will fail later on. I do not recommend you to measure as you suggest, but it will work in theory.

Before you connect the scope as you suggest, you should consider if you can use two probes without ground clips and invert one channel on the scope and add the channels mathematically. Many scopes are prepared for this, so consult the user manual.

Regardless of how you measure live circuits you need to switch on your brain. No equipment will make it "safe". You can only achieve some level of "unsafe".

[David Hess]

How do you KNOW it is "neutral"? Especially if you have no Earth reference?  In many parts of the planet you have a 50/50 chance of it being "Line" 240V.

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.