oscilloscope without wire ground

Simply ensure that your contact information is forwarded to for proper attribution. Thanks!

You shouldn't be touching your ground to line voltage.

but I need measure line (L1) with reference in line(L2).

Trick 1: Use a dual channel scope with two probes and display the difference. This is the regular way to do it.

Trick 2: I have also done the GROUND removal trick, but I am an expert in electrocution. On my scope I mounted an extra connector to disconnect the green/yello wire momentarily.

better you do Trick one I described above.

w.

Back in the 1977-1987 time-frame (OmniComp/GenRad days) I used to have a TEK probe that provided isolation from ground for such measurements, so that I could probe off-line switchers safely. You might want to track one of those down _before_ you electrocute yourself ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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I love to cook with wine.     Sometimes I even put it in the food.

--
Use a 2 channel scope which can invert one channel and add the two 
channels. 

Connect the grounds of both probes to mains ground, the channel which 
doesn't invert to L1, the channel which does invert to L2, invert the 
channel, and add the two channels. 

Also, it would be a good idea to set the sensitivity (volts per box) 
of both channels equal. 

Start off high and work down.

Tobias Gadelha has brought this to us :

The real answer is ___ You do not know enough to be fooling with mains power. IT WILL KILL YOU :-@

--
John G

Use a small transformer to isolate and step down the voltage. Calibrate the ratio with a DVM.

--

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"Tobias Gadelha"

** L1 or L2 will be at the almost same potential as ground - so you have no real problem.

.... Phil

What makes you sure about that?

I'm just learning this stuff myself, but I have a question about your question... Are you talking about the ground in the plug, or the ground for the probe? It would surprise me greatly if they were connected. I would *think* that you could connect the probe's GND to a line with any potential. I would expect only the relative potential to where the other part of the oscilloscope is connected need to be within range for the scope, but not with reference to the earth.

I could very will be extremely wrong about all this, but and would love to be corrected before I end up passing high voltage through my body ;-)

Thanks, Daniel.

Look here:

See number 11.

Use a differential probe, or A-B, if your 'scope has the facility.

NEVER disconnect the supply ground from your instrument. It could be all that stands between you and a day out with the undertaker.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

** About what exactly ?

Make your point or piss off.

How are you sure that "L1 or L2 will be at the almost same potential as ground" is true? I don't understand how you came to that conclusion so I was curious how you did.

My only point was that I wasn't sure, and wondered what your reasoning is. No need to get rude.

My point should have been relatively clear, but I can see it was ambiguous with the "so you have no real problem". So, now that I've clarified, I'd like a real answer. Unless you've realized you were wrong, then a simple note to that effect would also be polite.

Thanks, Daniel.

I see... That makes sense, but is still surprising to me. So it seems likely depending on the OPs application, coupling transformers may be a safer way to handle this situation. Thanks for the lesson ;-)

** The codes L1 and L2 are very often seen on schematics refering to the active and neutral conductors.

( Nothing suggested the OP was dealing with 3 phase power and I hope to hell he is not)

Neutral conductors are always linked to ground at the service box.

So, the voltage seen from active to ground is almost the same as that from to active to neutral - the only difference being due to current in the particular neutral wire due to some load.

Get your DMM out and check it sometime.

... Phil

In the USA neutral and ground are "usually" connected at the service box but not always. L1 and L2 are usually hot wires wrt ground. Confusion with terminology is dangerous. This is a basic group.

Tom

I was not aware of that convention. Thanks.