Basic Oscilloscope Safety

[electro_pt]

Have been doing basic electronics and electrical work for a while, thought I'd upgrade to an oscilloscope.  Of course of the the first things I'd like to do is visualize the 120V single phase 60Hz AC mains line.  Did a bit of reading on this and you would think doing such a think would cause the second holocaust.

I am hoping someone can explain to me what all the fuss is about.  Unless I am completely missing something, which I obviously must be, seems like as long as I ensure probe is rated for the voltage AND probe ground is connected to the same ground as the o-scope (which would be mains ground) I should be OK.  What am I missing?

[Simon]

You are missing the fact that one of your measurement lines on the scope is indeed grounded, the ground! How are you proposing to take the measurement? If you connect the crocodile clip on your probe to anything but ground there may or may not be an almighty bang and whatever you connect the crocodile clip to every metal part of the scope will also be connected to. So assuming you don't know which is live and neutral in your house if you connect the crocodile clip to live you will create a direct short-circuit to ground and there will be a big bang. Even if you get lucky and connect your crocodile clip to neutral there could still be a significant voltage difference between neutral and ground that could cause problems.

If you want to do anything meaningful you should use 2 probes and use the math function of your oscilloscope to take the difference between the 2. Use the probes to probe the neutral and live. Do not connect the crocodile clips to anything and ensure that the ground of your oscilloscope is indeed at mains ground. The difference signal you will see as a 3rd signal on your oscilloscope will be your mains.

Having said this if you are uncertain it is highly advisable not to do this. Going anywhere near the mains without good reason is an extremely bad idea. I deal with the mains as little as possible. The mains kills it doesn't care how inexperienced you are and how much slack you should be cut. I only ever do this when I really have to.

[rob.manderson]

I only ever do this when I really have to.

And then with one hand in a pocket!

[Simon]

I only ever do this when I really have to.

And then with one hand in a pocket!

Quite

[tautech]

Have been doing basic electronics and electrical work for a while, thought I'd upgrade to an oscilloscope.  Of course of the the first things I'd like to do is visualize the 120V single phase 60Hz AC mains line.  Did a bit of reading on this and you would think doing such a think would cause the second holocaust.

Most of us have done it, however it's generally not recommended especially for a novice. We have no real idea of the electrical experience of yourself or any other when it comes safety procedures when handling HV so the standard advice is leave it alone. That you have 110/120 VAC mains is one thing, many of us have 230VAC so universal advice is again leave it alone.
Most don't have any sort of sig gen and just want to see some squiggly lines like a sinewave on the display and the first thing that comes to mind for many is mains.
Use a transformer to get down to voltages that are less dangerous and a DMM continuity check before powering anything up to ensure you don't have any Gnd loops.
Stay safe.

[Simon]

or just stick the probe to your finger and you will pick up all the mains you like without harm.

[electro_pt]

Perhaps I mispoke.  I was trying to say either the probe's ground clip would be connected to mains ground via the electrical socket's ground plug receiver, or simple don't connect it to anything since the scope would be grounded to the same mains ground.  I believe this is indeed what you said as well.

I could then use the probe to see if there is any residual waveforms/voltage on the neutral leg of the electrical socket or the live leg of the electrical socket.  I believe this is also what you had said.

Seems to me the probe's alligator ground clip is the one that causes all the fuss since its a dead short (on purpose obviously).  Assuming the probe is rated for the voltage once I take head of where the ground alligator clip is connect (or not connected at all as mentioned earlier) than I would be free to probe about anything I wanted.  Correct?

Also curious on the probe to finger post.  I presume this would pick up metabolic electrical noise?  Where would the ground clip go?

Many thanks for the explanation.  I am very comfortable with probing 120V and 220V with a multimeter, assuming appropriate voltage sense mode and not current sense mode.  Just wanted to confirm I am approaching this correct with a new o-scope.

[tautech]

For straight forward simple/basic measurements of Gnd referenced voltages/waveforms indeed the reference (Gnd) lead is not required.

[Ice-Tea]

If you were to put the aligator clip on EARTH or nothing at all and probe L or N with the head (provided it's rated for that) you're not really measuring anything. Remember that you always measure a voltage between two points. In this case you'd be measuring, per example, between L and EARTH. However, the grid works with L and N. So you're measurement is pretty much meaningless.

Yes, in some case N is more or less equal to EARTH, but that's besides the point.

Also:

[rstofer]

You need to take a bigger view of the 120/240V system.  It is driven by a single transformer that is center-tapped with the center tap grounded. This center tap is also your neutral.  Now, whenever there is a short from phase to ground, the current MUST get back to the center tap.  This will take any path it can to get there with current division as you would expect from parallel impedances but one of the paths is through your scope ground to the neutral by way of your ground clip should you have it connected to neutral.  You don't even have to be involved with the ground fault!  It could be a fault at your neighbor's house if you share the transformer.  The fault current (usually just a few thousand amps, generally less than ten thousand) takes ALL parallel paths back to that neutral center tap.  Your scope clip won't be the lowest impedance back to the center tap but it IS a path.

I have been doing electrical all my life and electronics even longer and  I still wouldn't be in a hurry to probe my 120V outlet.  I like systems that are powered by wall warts and batteries.  I have had all of the big power stuff I ever want to see.

[Brumby]

Having said this if you are uncertain it is highly advisable not to do this. Going anywhere near the mains without good reason is an extremely bad idea. I deal with the mains as little as possible. The mains kills it doesn't care how inexperienced you are and how much slack you should be cut. I only ever do this when I really have to.

This is my philosophy too.  I purchased a pair of 100:1 probes for looking at a fault on a mains item, but I thought over the problem a couple of times before attempting a powered investigation.  There were some other tests I could do with it unpowered, even though they required partial disassembly.  I went down this path and found the problem.  As a result, I removed the risk completely - and I feel much happier about that extra effort.

The other thing that has been touched upon here too, is that your measurement may be referenced to Earth - but exactly what point in the earthing path?  If there are any currents flowing, there will be voltage drops that will add to your measured signal and if there are any earth loops, you are going to pick up induced currents which will do the same.

I would have to have a really good reason to probe mains with a scope - and if I did, my first option would be the cheap DSO138 kit I purchased recently running on a battery.

[DimitriP]

Of course of the the first things I'd like to do is visualize the 120V single phase 60Hz AC mains line.

[David Hess]

Unless you control the environment for a device under test on your workbench, just do not connect the probe ground lead.  Even better, temporarily remove it from the probe so that it is not flailing around where it can contact something dangerous.  For most power line work, high frequencies are not involved so the probe ground is not needed especially if the bandwidth is deliberately limited.  Some oscilloscopes have bandwidth limit settings below 20 MHz like 5 MHz or 1 MHz which can be advantageous in this situation.

For the workbench where you would normally connect the probe ground to neutral, an isolation transformer needs to be used on the device under test.  Besides the low impedance difference in neutral and ground potential which can drive considerable current through the oscilloscope ground, a neutral failure would drive all of the current through the oscilloscope ground.  I once watched a probe ground lead turn white hot and melt when this happened; now if I do not have an isolation transformer and need one, I hack one together on the spot.

There are special oscilloscopes intended for this application which have isolated probe grounds like the Tektronix TPS series.  For them, each probe ground has to be connected and it is safe to do so.  An alternative to this is to use high voltage differential probes or sometimes two oscilloscope channels in add and invert mode.

[grouchobyte]

My advice......heed all the suggestions in this thread.

One more thing to consider and a little trick I use in making scope line measurements is to power only  the scope with a battery backup UPS that is UNPLUGGED from the mains. This will isolate it.

This is only something you can do after careful thought and consideration is made so that you dont end up with an "unintentional" connection to mains earth through some other instrument connection via the scope.

If you haven't melted or welded a crock clip or two and/or electrocuted yourself, a friend, co-worker or family member  in your career doing stupid stuff without thinking it through, you will likely learn it sooner or later. Remember, current takes the path of least resistance.

@grouchobyte

[Nuzzler]

you would think doing such a think would cause the second holocaust.

LOL

- Nuzz

[tautech]

My advice......heed all the suggestions in this thread.

One more thing to consider and a little trick I use in making scope line measurements is to power only  the scope with a battery backup UPS that is UNPLUGGED from the mains. This will isolate it.

This is only something you can do after careful thought and consideration is made so that you dont end up with an "unintentional" connection to mains earth through some other instrument connection via the scope.

Doing this will float a scope, a well know dangerous technique and one that should never be followed. 
When the probe Reference (Gnd) lead is connected to the DUT the scope attains the same electrical potential as the reference connection point. Should that be elevated with respect to ground all metal parts of the scope are at this elevated potential and the risk to the user becomes dangerously high.

So knowing these risks well I now advise users not to follow the above advice.

[Kleinstein]

Having the scope powered from battery of UPS can be dangerous. With many scopes the case or at least the connectors is connected to the ground strip of the probe. So if you than connect the scope ground to a live voltage, this level will be all over the scope. One can do this, but one should be very careful when doing it this way. So usually a scope is meant to be used with PE connected.

Usually there is no need to look at the 120 V / 230 V - the waveform is about the same behind a transformer (if not to small, e.g. > 5 VA and rated for sufficient voltage).  Some scopes even have an low voltage AC at one output for the component tester.

[tggzzz]

One more thing to consider and a little trick I use in making scope line measurements is to power only  the scope with a battery backup UPS that is UNPLUGGED from the mains. This will isolate it.

That's called "floating the scope". Bad idea.

Now consider what will happen if a probe's shield is connected to something at a high voltage. The scope's exterior will be connected to that high voltage, leading to the user being exposed to danger. If you are very lucky, the scope's exterior won't be highly conductive, which might mean the shock given to the user is slightly lower - but you would be very unwise to rely on that!

Summary: never "float a scope".

If you haven't melted or welded a crock clip or two and/or electrocuted yourself, a friend, co-worker or family member  in your career doing stupid stuff without thinking it through, you will likely learn it sooner or later. Remember, current takes the path of least resistance.

And if you "float a scope", there is a significant probability that current will flow through the user.

Summary: never "float a scope".

If you want to know how to use scopes safely, follow the relevant references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[grouchobyte]

I forgot to add.....wear rubber gloves and rubber shoes and dont touch your friends! Its a simple tradeoff between smoke and electrocution.

 

My bad....this suggestion was assuming a mature audience. Obviously caution is advised. I guess I mistakingly assumed people know the risks......

I am happy you jumped all over my comments.....you guys have obviously been shocked burned and zapped by this stuff. In the last 40 plus years I certainly avoid it and do these high wire acts with safety in mind

@grouchobyte

[3db]

I forgot to add.....wear rubber gloves and rubber shoes and dont touch your friends! Its a simple tradeoff between smoke and electrocution.

 

My bad....this suggestion was assuming a mature audience. Obviously caution is advised. I guess I mistakingly assumed people know the risks......

I am happy you jumped all over my comments.....you guys have obviously been shocked burned and zapped by this stuff. In the last 40 plus years I certainly avoid it and do these high wire acts with safety in mind

@grouchobyte

You forgot the rubber apron and face shield.      

[tggzzz]

My bad....this suggestion was assuming a mature audience. Obviously caution is advised. I guess I mistakingly assumed people know the risks......

No.
You assumed a knowledgable audience.
But the OP is not knowlegable in this topic area.
But random people reading this thread in the future may or may not be knowlegable. They certainly won't know whether or not you are knowlegable.

Hence your assumptions and your advice were - at the very least - mistaken and are certainly invalid. And invalid in ways that could cause other people to hurt themselves and/or damage their equipment.

I don't think those potential consequences are humourous; do you?
Your attempts at humour in your subsequent posts do you no credit.

[Simon]

its nothing to do with theaudience. Allowing you chasis to be connected to an unknown voltage that is associated with the mains is nuts!

[grouchobyte]

Implying  that I know nothing of this subject is partly valid. Posting to a beginners forum is certainly an error and suggesting that the chassis of the scope float so it can potentially be connected to the mains hot is also dangerous to the beginner or anyone for that matter. No need to over-emphasize the point. It is well said and understood.

I personaly would still do this as an alternative only with several of my lunchbox scopes which have little to no exposed metal parts. Of course, there is always a way to expose yourself to the floating chassis connected to mains hot and that is ill-advised. ( moderator: feel free to delete my post if you think this is off topic or hurtful to those who are outraged by my suggestions )

That said, this is why a seasoned (not a noob/beginner) should not read or heed the advice in my post in the same way a person shouldn't hold a knife by the pointy sharp end or hold a soldering iron on the hot tip or play with explosives. On the other hand, I think making an example of how NOT to do things might be a valuable lesson to bring to the fold.

Call me nefarious and mean-spirited, but I find accidental magic smoke and mild electrocution in the lab a funny and sometimes necessary evil as part of an EEs education.


@grouchobyte

[Simon]

the problem is that if you croc clip ends up at any significant voltage the whole chasis can be live, so that means all the other BNC connectors on the scope or the croc clips on other probes suddenly become live. You just don't do this unless you know the voltage you are connecting to and with the mains that is not a good idea.

[tggzzz]

Call me nefarious and mean-spirited, but I find accidental magic smoke and mild electrocution in the lab a funny and sometimes necessary evil as part of an EEs education.

The magic smoke isn't funny if it comes from flesh, nor if a shock alters the heart's rhythms. There's a (possibly apocryphal) story of a doctor warning potential patients that if they arrive in A&E after a serious HV shock, he will do everything possible to keep them alive until their relatives arrive.

And yes, I do take significant risks in some of my activities, and have encouraged my daughter to do the same. But they are educated risks, and I wouldn't dream of attempting to teach others how to do those activities.