Using an oscilloscope to measure mains

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Hi all.

I have some work I want to do with mains electricity. I need to hook my scope up to it but I cant find any real information on safety and best methods. Anyone got any links?

Thanks

Trev

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Short answer: Don't.

Some will tell you to cut PE if you use a non-isolated oscilloscope. This is stupid, don't. Some will still claim they do it since the beginning of time. This doesn't change that it is stupid, so don't.

You want to read up on isolated oscilloscopes, isolation transformers, isolated differentials probes, CAT safety measurement categories (for oscilloscope and for probes), two channel difference measurement in math mode (with properly rated probes and oscilloscope), and probably local laws and regulations.

Stealing Proteus doesn't make you an engineer.

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You can with care.

Use an isolation transformer to power Your scope or if it runs on batteries let the whole scope float.

Use differential probe method where the probes are designed to handle votage range ( generally these will be attenuated by 10 probes.

If you are right handed stick Your left hand in a pocket and leave it there. Work with one hand only.

This way if anything untoward should happen Your chances of current flowing through the body bulk will be diminished.

TREAT MAINS WITH RESPECT.

If You need to work on mains voltage comstantly see about getting a POWERSCOPE from BWD NSTRUMENTS.
A scope designed for industrial electronics in mind.

Two differential channels and a single ended channel give You a good tool ment just for this kind of work.

Available new from McVan Instruments else usual used equipment sources.

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For AC check your scope specs, and for extra safety use your own isolation capacitors. If you need DC coupling battery powered scopes are safest.

Rubber horse mats are cheap and surprisingly comfortable, and keeping one hand in a pocket is always wise.

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Rent a portable (battery powered) scope.
Via reg at Fluke's website is application note "Measuring power with a Fluke ScopeMeter® 190 Series" by
Home | Support | Application Notes | Case Studies

"Dare to be naïve." - Buckminster Fuller

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Guys, this is great stuff. Thanks.

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Well I actually have a 12v to 240v inverter. I can probably hook up a car battery to that and power my scope from the inverter. That will isolate it.

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Quote:
I have some work I want to do with mains electricity.

I am intrigued what sort of work/measurement you want to do, to determine the best way of obtaining the measurement you want.
I have a healthy respect for mains and apart from looking for some harmonic distortion never needed to look at it with a CRO. It's waveform is fairly predictable.

I have seen a few frazzled CRO's & probes over the years.
About 10 years ago, a young local electrical/electronics know-all, was found dead in his bedroom/radio shack, by his mother after he tried to take some mains measurements!

My recommendation is not to "fiddle" with mains wiring unless you know exactly what you are doing.

So I am intrigued for what purpose you need to any mains measurement with a CRO.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Hi,

Mains voltages are quite dangerous, always be sure to isolate yourself properly.

You always have to keep in mind that grounding is for your protection. It keeps electricity from going through you instead of a known fault circuit.

The difference between a mains powered (grounded) and battery powered handheld (ungrounded) type of test equipment is that the common (or ground) terminal on the mains powered equipment _IS_ a real ground.

So, you need to understand that if you connect a HOT to a real ground, it is going to be VERY BAD.

In non grounded equipment, it is more forgiving because the ground of common is floating and it will tolerate you connecting it up backwards.

People try to get around this by floating their mains type scope which simply removes the protections for them which were designed into the device. Bad idea.

It is perfectly safe to not float the scope and simply connect things properly so that you only connect a ground to the scope ground (real mains ground). You just have to ask yourself before doing it - "Will a massive amount of current flow if I ground this wire?" before connecting it to scope ground.

If you want to use an isolation transformer, use it on the DUT (device under test) instead.

I've probed mains with my Rigol scope many times. Checking out generator output quality, inverter output, mains, etc.

Good luck,

Alan

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Another suggestion is to use safety glasses - when mains powered things go !pop! there tends to be a bit of shrapnel flying around along with arcs and sparks.

I used to design light dimmers for many years and did all that was suggested not to do above. The 'scope probes had many arc scars and I got zapped a few times. I can tell you from bitter experience to think really carefully when working on mains stuff. Think about your surrounds and have a clear area to work on. Use a earth leakage breaker - I've 'tested' these a few times.

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Use an isolation transformer for the DUT. Don't isolate the CRO (otherwise your cro probes can go live).

Make sure everything you use is properly fused and you are running off a properly wired circuit board (ELB and overload breakers).

Another thing worth considering (depending on power consumption) is to wire a incandescent lamp in series with the device under test, that way if there is a short, the lamp turns on rather then the pcb going boom.

You really need to know what you are doing when playing with mains, things go pear shaped very fast. It would be a good idea if you could work alongside someone with experience before going solo. Also make sure someone else is present when you are doing your measurements.

Just picking up on alan2s comments, he is referring to connecting the probe to chassis ground on a non-isolated product, not 0V reference ground after the mains bridge . If you connect it to the 0v mains reference ground things will go bam.

oddbudman

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Quote:
Also make sure someone else is present when you are doing your measurements.

And that they, know their First Aid as far as electrocution rescue and CPR.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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oddbudman wrote:
If you connect it to the 0v mains reference ground things will go bam.

Hi.

I think I did exactly that :oops: (stupid, stupid, stupid....). I had to work on something what was pushed into me. Even at school I had specification on low-current and low-voltage curses. And I love electronics, but going down with the voltages as low as possible and even when some mistakes are done there is not enough energy to make any huge harm.

I had execute first prototype/prof-of-concept of design from a guy I didn't trusted. He told me to put fuse there with same rating as bridge (strange). Then I saw the huge capacitor what he wanted to have inside, so there is no way I will be powering the device without eye protection, and with the caps facing any direction just away from me. Ok so it didn't blowed up, but it didn't worked either. So I had to debug it. Without connecting ground there is no point the mains hum will be stronger that controling singals what I intend to measure so I didn't bothered. I had to ground it to be able to see something, but I was afraid. On the eevblog forums lot of guys broke their leads to learn a lesson that scope is not multimeter and you can't measure between 2 points with any potential, but the ground has to be ground potential.

So I didn't wanted to scope it at all but then I got pushed by the other guy that's ok to connect it. And deadline and device isn't working. His voice got louder than my inner voice and I did it anyway.

So it ended loud bang, somehow it didn't surprised me too much. I tried avoid any damage and kept my rule, if I'm not sure that is safe to do and why is it safe the do it, and now I broke my rule. Breakers down, fuse blown, couple thick traces on the pcb evaporated and the bridge damaged as well.

But what I had expected that I killed my probes or even scope itself. But after getting breakers back and the scope and probes are alive :shock: :o . Somehow I can't understand, force strong enough to evaporate thick traces, trigger strong breakers and the probes are ok.

So my question is what could be damaged in the scope? I gave it quick test and it looks like OK. What else could be sensitive and be damaged inside the scope when probes are OK?

Perhaps I touched and shortened the output of the DC with the alligator clip? And it blown fuse and everything before it could damage the scope?

Could somebody help to understand what happened? Why my scope seams to be ok? Or is it? :roll:

PS: Just FYI every tool and device was mine, even the DUT was paid from my money, so I didn't made any financial harm to anybody else etc...

Last Edited: Thu. Oct 20, 2011 - 08:54 PM
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Quote:
PS: Just FYI every tool and device was mine, even the dc-dc DUT was paid from my money, so I didn't made any financial harm to anybody else etc...

then you where extremely stupid to not listen to your own inner voice.

If you short circuited something in mains then there should be welding marks somewhere.
I have done mains measurements on multiple occasions, but never ever let someone else tell me what to do.
my safety always comes first and if I do not trust something I will not start. you need to have real respect for mains.
I once had someone telling me that I should power on something and I was not convinced that it was safe. he kept on telling to power it on so I walked away. he powered the device and it blew up in his face.

It depends on where you are from, and what scope you have used. in the US mains is 'only' 110Vrms(about 165V peak) on my bench I have a scope that is rated for 150V so it might survive a short over there, but if you run from 230V(europe) then the scope must have suffered somewhere. even though it works now always keep in mind that this has happened. if you even open it or have it send for callibration make a note of it and examine if you can see welding arcs somewhere they might give a clue as to where the arc jumped.

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meslomp wrote:

then you where extremely stupid to not listen to your own inner voice.

If you short circuited something in mains then there should be welding marks somewhere.
I have done mains measurements on multiple occasions, but never ever let someone else tell me what to do.
my safety always comes first and if I do not trust something I will not start. you need to have real respect for mains.
I once had someone telling me that I should power on something and I was not convinced that it was safe. he kept on telling to power it on so I walked away. he powered the device and it blew up in his face.

It depends on where you are from, and what scope you have used. in the US mains is 'only' 110Vrms(about 165V peak) on my bench I have a scope that is rated for 150V so it might survive a short over there, but if you run from 230V(europe) then the scope must have suffered somewhere. even though it works now always keep in mind that this has happened. if you even open it or have it send for callibration make a note of it and examine if you can see welding arcs somewhere they might give a clue as to where the arc jumped.

It's Europe.

When I connected the scope ground to the - pin of the bridge rectifier. It makes difference what voltage the live voltage is even on the - pin after rectifier?

Yeah I know, exactly that why I quit in my previous job, because their were pushing people against law (on paper 40 working hours, in reality around 60), our abilities and will. But then I was judged why I quit a job in recession, I should be thankful for a job etc.... And the companies are misusing the position of recession and are forcing people to more than they should. So it's not so like in the past to walk way from everything that is against me.

About me doing the test, I wouldn't walked away and leave him to break everything. To break all my tools and devices? At least when I should damage something, than it should be me. I protected myself and leaving him alone with my equipment could end much worse and perhaps he wouldn't tell me truth afterwards. So it's easy to say for anybody that I should do this or that. You are not in my position. I know it was stupid but don't tell what I should done, because you are not in my position.

The scope is ds1052E what I got for my self intending to measure just uControllers in my hobby etc... I never expected to use it for something else or in anything else than my personal use. Exactly for this kind of reasons I hate when somebody is wanting to use my personal car for company use.

Last Edited: Thu. Oct 20, 2011 - 08:55 PM
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that scope is rated for 300V RMS, so that does not seem to bad.

I asume you used the scope probe for the - connection and not an external ground pin connection. In that case the current will have traveled through the mantle of the probe to the ground, so effectively shorting the recifier diode.
you could measure a good probe's impedance and check if the probe used for the shortcircuit to see if there is a difference.

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Another thing to watch out for - if you float the scope and connect it's ground to the line, the chassis of the scope, BNCs, and any other metal on it will also be at mains potential. For modern scopes, that also includes the mouse, and I doubt mice are rated with high voltage insulation.

I have seen a 7000-series Tek scope weld itself to a grounded transformer vault when someone was floating it and the dried-out rubber feet cracked.

/mike

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TrevorWhite wrote:
Hi all.

I have some work I want to do with mains electricity. I need to hook my scope up to it but I cant find any real information on safety and best methods. Anyone got any links?

Thanks

Trev

Kind of short on details regarding what you want to do, but that never stopped me. Here in the U.S. if you plug your scope into the mains with the standard 3 prong power cord and plug, your scope's gnd is the same as the mains gnd. If you've got an older scope with a metal cabinet and chassis, like one of those Tek boat anchors, then the cabinet and chassis are at gnd potential too. If your scope is one of those new-fangled digital jobbies in the plastic encloser, then it's just the barrel portion of the BNC connector (all of them) that's at mains gnd potential.

Let's say you are troubleshooting some piece of equipment (DUT) that's also connected to the mains. The problem is knowing what the DUT uses as gnd. In some, generally older, equipment the DUT uses a two wire power cord and maybe not even polarized and not isolated by a transformer, think old time AM radios. In that case, the chassis could be at mains gnd potential or it might be hot, depending on how you plug in the cord. If the chassis's at gnd potential and you connect the ground lead of your scope probe to it, no problem, the scope and the DUT are looking at the same potential. If the DUT chassis is hot though and you do that, sparks will fly, as there's now a large potential difference between the two "gnds."

I think you can do it safely if you know "where your gnds are." To start, you could disconnect the gnd lead from your scope probe. That means only the high impedance scope probe tip will come in contact with any voltages in the DUT. If the DUT does have a connection to gnd "somewhere," then probing with the scope will let you find out where gnd is in the DUT. If what you need to look at is fairly low frequency stuff you can just keep the gnd lead off the probe and still get good data and not worry about shorting the scope gnd to high voltage.

If you need the gnd lead for good freq response then you could put a fuse in series with the gnd lead, that way if you try to connect it where it doesn't belong, you'll just blow the fuse.

The other thing you could do is plug both the scope and DUT into a GFI protected receptacle.

Of course none of this will protect you from putting your body parts in the wrong place (well maybe the GFI). Isolated or not, you still can't just grab stuff and expect it won't bite.

Greg

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Anyone already try to use an AC-opto to measure mains? I know it will modify the phase but depending the app it could be an option..

Regards,

Bruno Muswieck

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meslomp wrote:
that scope is rated for 300V RMS, so that does not seem to bad.

I asume you used the scope probe for the - connection and not an external ground pin connection. In that case the current will have traveled through the mantle of the probe to the ground, so effectively shorting the recifier diode.
you could measure a good probe's impedance and check if the probe used for the shortcircuit to see if there is a difference.

Thanks, the probes have same impedance.

external ground pin? To be clear what I did. The DC unit is connected to the main with 2 wire to Live and Neutral. They go to the bridge rectifier through fuse.
Output of the bridge rectifier goes to DC-DC. And I connected the probe ground to bridge -, that's the same as the metal on the BNC and is the same as the DC - output. And the probe itself (the tip) to the output of the DC converter + output. Traces what evaporated were the bridge - traces. So it is what you meant?

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truhlikl_fredy wrote:
To be clear what I did.

Can be understood what I did? If not I can draw a picture to be sure.

Last Edited: Thu. Oct 20, 2011 - 09:11 AM
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How about using a 240 to 12V or something transformer and measure the secondary? Lots safer, frequency and phase is preserved, voltage is easy to calculate (20:1).

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Tru: what happened is that you shorted your circuit through the probe ground clamp, through the scope chassis, through the earth wire to ground.

The scope chassis is a lot thicker than your traces, so it won't be damaged. Your probe mantle might have taken a small hit, but the weakest link gave, i.e. your PCB trace.

So I wouldn't worry too much about the scope.

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Xantor wrote:
weakest link gave, i.e. your PCB trace.

I wasn't sure if there was more damage, because the trace, the bridge and fuse were blown at once. Fuse was blown as it should be (no explosion or etc...), bridge was internally damaged after I measured him. And the trace was gone, it's almost 3mm (something over 100mil) thick trace and should handle 4amps. So I was worrying what else it could bean damaged when there was guarantee much higher current.

But the bridge and fuse was same rating.

Even when the scope turned on, and it measured everything so I relaxed more but still I was nervous. Plus I didn't understood what happened, somehow I expected to have at least the probe gone. Now I think I will have better sleep :)

Thank you.