Electronics > Beginners
Measuring Mains with Rigol DS1052E
Zero999:
Connecting the neutral and earth will certainly cause the RCD/GFCI to trip.
If there's no earth leakage breaker, then a current will flow from the neutral to earth. How big the current is and whether it will cause any problems depends on the potential difference between neutral and earth and the resistance. If may not cause any problems or it could result in smoke.
I don't see why anyone would need to measure the mains with an oscilloscope. It should be safe to just connect a suitable CAT rating oscilloscope probe to the phase conductor and not connect the chassis clip to anything. That will give you the waveform on the phase conductor which won't be much help.
If I were using an oscilloscope to look at the waveform on the primary of an SMPs, I'd simply power it from an isolating transformer and be aware of the fact that the RCD won't be any use.
IanB:
--- Quote from: Hero999 on September 27, 2015, 06:30:39 pm ---I don't see why anyone would need to measure the mains with an oscilloscope.
--- End quote ---
One case might be to examine the current in a mains conductor, as done by AvE here:
https://youtu.be/IAG61bSTT7k
pplaninsky:
Hi,
thanks for the replies, I was thinking the same - that there will be current flowing from N to PE - just wanted to confirm with you guys.
No, after I blew up the fuses in my previous experiment - I do not intend to measure the mains ... at all.
However, so many people are asking and trying - that I think this topic should be periodically re-answered.
You know, it seems that the beginners are drawn to the mains the same way as moths to the flame :)
There is something magical in trying to fiddle with the mains power, especially with doubtful Chinese instruments :)
One more question, which is more like clarification:
What is causing the potential difference between N and PE? In my building they are connected to each other in the basement. (By the way this is the most common case in Europe.)
Is this potential difference caused by the mere length of the cable and the fact that parts of the Neutral wire are carrying current (people are using things in their homes.) - so there is voltage drop - meaning the Neutral is with lower potential compared to PE, even though they are connected in the basement?
Also the resistance between N and PE is minimal - I think it is about 0.3 ohms - required by law.
If there was bigger resistance I couldn't have been able to blow my power supply - because it is not Earth referenced - it has just 2 prongs plug.
So, when I connected the CRO, the current flowed from N, via the PE, to the Live wire - therefore shorting N and L via the PE and the CRO casing.
If there was big resistance between N and PE - this shouldn't be possible.
I just imagined myself connecting an amp meter between N and PE and turning the power on from the other side of the room.
Just to see what happens...
... as I said moths to the flame :)
/No, I am not doing, but if someone had already done it - and I am sure people have done it - please let us know the results./
mzzj:
--- Quote from: pplaninsky on September 28, 2015, 06:54:11 am ---
One more question, which is more like clarification:
What is causing the potential difference between N and PE? In my building they are connected to each other in the basement. (By the way this is the most common case in Europe.)
Is this potential difference caused by the mere length of the cable and the fact that parts of the Neutral wire are carrying current (people are using things in their homes.) - so there is voltage drop - meaning the Neutral is with lower potential compared to PE, even though they are connected in the basement?
Also the resistance between N and PE is minimal - I think it is about 0.3 ohms - required by law.
If there was bigger resistance I couldn't have been able to blow my power supply - because it is not Earth referenced - it has just 2 prongs plug.
So, when I connected the CRO, the current flowed from N, via the PE, to the Live wire - therefore shorting N and L via the PE and the CRO casing.
If there was big resistance between N and PE - this shouldn't be possible.
I just imagined myself connecting an amp meter between N and PE and turning the power on from the other side of the room.
Just to see what happens...
... as I said moths to the flame :)
/No, I am not doing, but if someone had already done it - and I am sure people have done it - please let us know the results./
--- End quote ---
Yes, the potential difference is caused by resistance (or impedance more exactly, but wire inductance is tiny compared to resistance at 50Hz so we can ignore it)
If you connect amp meter between N and PE you should see about half of the load current.
If the branch of wiring doesn't have load there is no current and no potential difference. But the load can be also somewhere else than the outlet in front of your nose, room lightning and outlets may share same branch of wiring for example.
pplaninsky:
Hey guys,
thanks for all the replies.
I think this is a hugely useful topic and I have a better understanding now about CRO's ground and AC mains ground.
I just measured the voltage between my N and PE and it was 0.025 V AC.
I doubt this measurement has any meaning at all.
I am just putting this info here.
Thanks once again!
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