Author Topic: Mains and the oscilloscope  (Read 13342 times)

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Offline Neukyhm

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Mains and the oscilloscope
« on: March 12, 2017, 03:06:15 am »
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.

I'm a physicist, but in my college the didn't teach us how to use a oscilloscope, just how to do some easy measurements.

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)

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

Offline Dubbie

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Mains and the oscilloscope
« Reply #1 on: March 12, 2017, 03:11:25 am »
Not really safe. To be by the book you should use a high voltage cat rated probe.

Edit: across the web you may get very different responses to that given by this forum. Keep in mind that this forum has many experienced professional members and bad advice is very rarely given here. Just because others on other less rigorous forums have gotten away with bad practice does not mean you will.
« Last Edit: March 12, 2017, 08:08:31 am by Dubbie »
 

Online Brumby

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Re: Mains and the oscilloscope
« Reply #2 on: March 12, 2017, 03:14:51 am »
The right probes are critical to safety.

Watching this is a good idea as well....
 

Online Ian.M

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Re: Mains and the oscilloscope
« Reply #3 on: March 12, 2017, 06:24:46 am »
You are under the illusion that your AC supply is a nominally 220V RMS, 50Hz sinusoid, maybe with some flattening of the crests due to loading by SMPSUs without power factor correction.  It is not.   

Firstly the voltage may be at the upper end of its EU harmonised tolerance range as your electricity supplier has no legal obligation to act on overvoltage within that range, so instead of  the 311V peak you expect, you could have up to 358V.  Then you must consider transients - see http://ecmweb.com/contractor/looking-sources-transient-overvoltages - and up to 1KV spikes riding on the sinusoid waveform are common to the point of being expected.   

Connect the tip of an ordinary scope probe to your mains and there is a high probability of it being exposed to spikes well over 1KV and it breaking down catastrophically.   

The situation is better if you have a good quality surge suppressing power filter (with transorbs, delta connected filter caps on input and output and at least one common mode choke) + an isolating transformer between the scope probe and the wall outlet - as the filter and transorbs considerably reduce the maximum spike amplitude, clipping ones on the peaks more heavily, and the transformer adds further low pass filtering as a typical fast transient doesn't have enough energy to significantly change the flux in its core, but I'd still want a CAT II rated probe with an adequate safety margin.  A 500V peak rated probe will only have a 40% margin - barely enough to be safe after such a filter + transformer.
« Last Edit: March 12, 2017, 07:08:51 am by Ian.M »
 

Offline DimitriP

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Re: Mains and the oscilloscope
« Reply #4 on: March 12, 2017, 06:40:39 am »
Quote
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.

When you  "know", "you know". If you have to ask you don't "know".

What I don't know is the reason (or lack thereof) for the fascination with probing the mains voltage.

   If three 100  Ohm resistors are connected in parallel, and in series with a 200 Ohm resistor, how many resistors do you have? 
 

Offline BrianHG

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Re: Mains and the oscilloscope
« Reply #5 on: March 12, 2017, 06:54:08 am »
Only if you must.  Get a normal 240v to 24v ac transformer.  Make sure there is no direct connection to the mains and the transformer's secondary 24v side, use one with safety approvals like a wall-wart which has 24vac, or 12vac out.  Probe the signal on the 24v ac side with 1 probe normally and multiply your readings by 10.  The smaller the transformer, like 2 watts or less, the better the high frequency response.  It may be useful to add a resistor load on the 24v side like a 1k 2 watt resistor.  (The resistor will get warm)  An optional 100nf capacitor in parallel with the 1k resistor if you want to get rid of high frequency noise.

You should see your 50hz sine wave.
« Last Edit: March 12, 2017, 07:01:04 am by BrianHG »
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Offline tggzzz

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Re: Mains and the oscilloscope
« Reply #6 on: March 12, 2017, 09:09:54 am »
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.

I'm a physicist, but in my college the didn't teach us how to use a oscilloscope, just how to do some easy measurements.

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)

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

Engineers consider two things: how X works plus how X can fail. You have only considered how that can work.

The trouble is that there can be many many subtle and surprising ways that things can fail, and it is difficult for anybody (especially a beginner) to think of them all.

Back to your problem. If there isn't a simpler way of achieving your (unstated) goals, you should use the appropriate type of probe. I suggest you have a look at the safety references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline danadak

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Offline David Hess

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Re: Mains and the oscilloscope
« Reply #8 on: March 12, 2017, 10:53:31 am »
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)

Yes, it is generally safe but if it is something that you do regularly, better probes and maybe a better oscilloscope with appropriate safety ratings should be considered.  The peak voltage from 240VAC is lower than many probe and oscilloscope input ratings and with a x10 probe, it helps that the two are in series.  At least my older oscilloscope can handle 400 to 500 volts peak directly connected to their inputs.  Failure do to a severe overload will probably be limited to a blown input.

The common mode range of A-B mode is poor (you will be stuck at 50V/div) so the sensitivity will be limited.  A dedicated differential probe will have much higher sensitivity.
« Last Edit: March 12, 2017, 03:26:04 pm by David Hess »
 

Offline raspberrypi

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Re: Mains and the oscilloscope
« Reply #9 on: March 12, 2017, 12:59:41 pm »
Bend the tines of a two forks and attach them together using a pickle. Stick the tines into your outlet forming a series circuit with the pickle. If the pickle burns evenly at both contact points you have an AC waveform, and at least 100 volts. This is the UL approved way to test this and meets ISO9001 standards. 
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Offline Neukyhm

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Re: Mains and the oscilloscope
« Reply #10 on: March 12, 2017, 03:22:58 pm »
What I don't know is the reason for the fascination with probing the mains voltage.
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.
 

Offline rstofer

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Re: Mains and the oscilloscope
« Reply #11 on: March 12, 2017, 03:47:59 pm »
What I don't know is the reason for the fascination with probing the mains voltage.
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.

I too have an issue with measuring high voltages with a scope.  I don't even own the appropriate PPE (Personal Protective Equipment) like gloves.

Maybe x100 probes are a help.  They will certainly help protect the scope and they will probably have an appropriate safety rating.

A-B measurements have been a scope feature for generations.  It would be worth reading the User Manual for limitations.

It is my understanding that Residual Current Devices are common in Europe and that they protect the entire panel.  That certainly reduces the shock hazard.  In the US, we have Ground Fault Circuit Interrupters on branch circuits to various specified locations but they don't protect the entire panel and they trip at a very low number of mA.  But they are only used for outlets in wet or outdoor locations, not my spare bedroom.

Then there is the 'oops' bit.  How much energy is available on the circuit?  In the US, it will generally be less than 10,000A for residential services and probably lower than 5000A.  This is further limited by the branch circuit wire size but still, there can be a heck of an arc.

All in, I prefer to stay below 50V and more likely below 12V.  I'm especially fond of 5V.  And, no, I have no interest in SMPSs.  If I need one, I'll just buy it.
 

Offline tggzzz

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Re: Mains and the oscilloscope
« Reply #12 on: March 12, 2017, 06:52:22 pm »
What I don't know is the reason for the fascination with probing the mains voltage.
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.

Oh, for heaven's sake! Why don't you ask the question you want answered, rather than encourage everybody to waste their (remaining) life giving useful answers to things that don't interest you.

How would you feel if I asked "is it OK to eat whelks" , and then said "listen, whelks are just an example of seafood I want to eat"?
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline julian1

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Re: Mains and the oscilloscope
« Reply #13 on: March 12, 2017, 08:50:09 pm »
I had a similar sense of trepidation about the A-B thing. Eventually I decided it made no sense not to buy the correct tool for the job - instead of relying on a workaround. A high-voltage differential probe, that's battery powered provides the necessary isolation. About  $200 second-hand on ebay.

Offline raspberrypi

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Re: Mains and the oscilloscope
« Reply #14 on: March 12, 2017, 08:55:43 pm »
What I don't know is the reason for the fascination with probing the mains voltage.
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.

Oh, for heaven's sake! Why don't you ask the question you want answered, rather than encourage everybody to waste their (remaining) life giving useful answers to things that don't interest you.

How would you feel if I asked "is it OK to eat whelks" , and then said "listen, whelks are just an example of seafood I want to eat"?

What if I have a friend who may or may not have access to high voltages and may or may not own an oscilloscope, and say the probes accidentally touch the hot and the neutral...
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Online Ian.M

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Re: Mains and the oscilloscope
« Reply #15 on: March 12, 2017, 09:10:37 pm »
Odds are, in North America, you'll get away with it,  Unless there is a floating neutral,. your domestic mains is under 200V peak (neglecting spikes) so for a typical 500V rated probe, that still leaves you 300V margin to handle spikes.  Its not ideal ore perfectly safe, you should  use a proper Cat II rated probe, and you *REALLY* shouldn't probe before the appliance fuse.

Clipping the probe ground to neutral is another matter - that's just dumb, and can melt the ground clip lead, the probe coax lead or even melt traces off the PCB inside your scope.   A GFCI breaker will trip but probably wont save you because it interrupts the live(s) not the neutral, and the problem is the potential difference between neutral and ground, usually caused by either poor bonding at the panel or heavy loads on the circuit.
« Last Edit: March 13, 2017, 11:04:42 am by Ian.M »
 

Offline Kappes Buur

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Re: Mains and the oscilloscope
« Reply #16 on: March 13, 2017, 09:30:36 am »
See the adventure of ElectroBoom probing an AC circuit  :)

 
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Online Ian.M

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Re: Mains and the oscilloscope
« Reply #17 on: March 13, 2017, 11:29:12 am »
An excellent example of how *NOT* to do it.  e.g. 2:00 - 2:08 - floating a scope with scotch tape!

[highlight]/////////////// CAUTION /////////////// CAUTION //////////////// CAUTION //////////////// CAUTION ///////////////////[/highlight]
« Last Edit: March 13, 2017, 11:36:06 am by Ian.M »
 

Offline grumpydoc

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Re: Mains and the oscilloscope
« Reply #18 on: March 13, 2017, 11:33:24 am »
An excellent example of how *NOT* to do it.  e.g. 2:00 - 2:08 - floating a scope with scotch tape!

[[highlight]]/////////////// CAUTION /////////////// CAUTION //////////////// CAUTION //////////////// CAUTION ///////////////////[/highlight]
I think ElectroBoom specialises in "How NOT to do it" videos. My main worry is that fact might not be obvious to everyone.
 

Online Ian.M

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Re: Mains and the oscilloscope
« Reply #19 on: March 13, 2017, 11:40:46 am »
Yes.  We could make good use of a full width animated gif 'caution' black and yellow hazard tape 'smiley' for use above and below all ElectroBoom and some other videos!

Alternatively, if Dave installs the SMF hilight tag mod: http://custom.simplemachines.org/mods/index.php?mod=3825 we could simply do what I tried to above.
 

Offline BrianHG

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Re: Mains and the oscilloscope
« Reply #20 on: March 13, 2017, 11:57:34 am »
See the adventure of ElectroBoom probing an AC circuit  :)



It would have been funnier if after each time he had an explosion, the scope should have been blanked out, and after the next scene cut, you would see the scope on his desk to the right & a new one in use in front of him until that one is destroyed...
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Offline grumpydoc

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Re: Mains and the oscilloscope
« Reply #21 on: March 13, 2017, 11:58:46 am »
Yes.  We could make good use of a full width animated gif 'caution' black and yellow hazard tape 'smiley' for use above and below all ElectroBoom and some other videos!

Alternatively, if Dave installs the SMF hilight tag mod: http://custom.simplemachines.org/mods/index.php?mod=3825 we could simply do what I tried to above.

[glow] seems to work (in preview, anyway).

/\/\/\/\/\/ CAUTION /\/\/\/\/\/\/
 

Online Ian.M

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Re: Mains and the oscilloscope
« Reply #22 on: March 13, 2017, 12:07:32 pm »
It would have been funnier if after each time he had an explosion, the scope should have been blanked out, and after the next scene cut, you would see the scope on his desk to the right & a new one in use in front of him until that one is destroyed...
Rigol's Legal department should sent him a cease and desist letter requiring him to obscure all Rigol branding and model numbers on any equipment shown in his videos!   >:D  :-BROKE  :-DD

Mehdi Sadaghdar is obviously a graduate of the 'Elect-a-Pope' school of engineering. He reminds me of 'Klaus' the fork-lift driver.
« Last Edit: March 13, 2017, 12:18:49 pm by Ian.M »
 
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Offline rsjsouza

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Re: Mains and the oscilloscope
« Reply #23 on: March 13, 2017, 12:17:34 pm »
What I don't know is the reason for the fascination with probing the mains voltage.
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.
Mains is a different beast when compared to, say, a local high voltage power source whose characteristics such as maximum voltage/current and source impedance are well known.

Mains has a low impedance that can yield currents at the order of kA as mentioned by rstofer, which will be only relevant if a breakdown or an arc happens between the power source and the test equipment (oscilloscope, probes, etc.). The chances of such event occur are defined by either a transient or an accidental mistake.

A transient is extremely hard to predict and can reach spikes at the order of kV which can easily exceed the probe safety specifications. Given the very low impedance of the source, any arc will be quite an event.

Accidental mistakes can be (but not limited to) either shorting the probes or inadvertently switching the probes from 10:1 to 1:1 (several regular oscilloscope probes have this tiny switch on them).

Probing a high voltage power supply is different than mains, given the fact that, even if it has a similar impedance as mains, it still has an advantage as spikes may not exist or be well characterized - thus eliminating the element of surprise.

All in all, kudos to you to double-check your procedure on this forum. In a very distant past I measured the voltage of the three-phase 60A bus bars feeders to my house and no surprises happened, but I was ignorant of all these factors at the time and did not take any precaution other than checking the maximum voltages of the 10:1 probes.
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Offline madires

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Re: Mains and the oscilloscope
« Reply #24 on: March 13, 2017, 01:49:30 pm »
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)

If someone is experienced with scopes and high voltage measurements, knows the limitations of the poor man's differential probe (A-B), and stays within specs of the probes and the scope, the answer would be "not recommended, but it will work at you own risk". In your case I'd strongly recommend proper differential probes rated for the voltages you want to measure plus a generous safety margin. Don't buy cheap knock-offs, buy a well known brand from a well known distributor. Yes, they are expensive, but your life is more valuable.
 


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