Measuring Mains with Rigol DS1052E

[Kiriakos-GR]

So, the best way to make those measurements is to use an isolation transformer. Got it.

But if I had to measure it directly, can I use the Rigol DS1052E not connecting its mains plug earth connection?

If the oscilloscope is battery powered, I should no problem, right?

Every device on this planet is safe to use if :  The live phase is not connected with the exterior casing.
But hey wait a minute, those Rigol looks to have plastic casing !! 

[vk6zgo]

So, the best way to make those measurements is to use an isolation transformer. Got it.

But if I had to measure it directly, can I use the Rigol DS1052E not connecting its mains plug earth connection?

If the oscilloscope is battery powered, I should no problem, right?

If you read my posting above,you will see there is a work-around of sorts.

The point I was making was that if you simply want to have a look at the mains,& are not chasing very small high frequency signals riding on the 50/60 Hz mains frequency,you do not have to connect the ground clip to either one of the Active or Neutral lines.

As the Neutral line is,if everything is wired correctly,very close in potential to Earth,connecting the  ground clip to a mains Earth point & probing the Active line will give a result which will be satisfactory in most cases.

Removing the Earth lead from the 'scope  & connecting the probe between the Active & Neutral is an enormous NO NO!!,as,if the Neutral & Active are transposed,any point on the Oscilloscope which is meant to be grounded will be at,in Australia at least,240v above Earth,& can KILL YOU!

I REPEAT! Do NOT connect the probe across Neutral & Active.UNLESS you use an isolation transformer which will 'float" the two sides of the supply with respect to Earth.

If you don't have  an isolation transformer measure between EARTH  & ACTIVE.(ground lead on Earth).It is almost as good,& you will not put yourself in danger.

VK6ZGO

[PetrosA]

Ciccio, from reading some of your other posts, I'm guessing you have a pretty good idea of the dangers your situation presents, so I'll direct this warning to the general public

What's happening in Ciccio's main's (high voltage between neutral and ground) is potentially dangerous to a curious experimenter because if someone were to shunt the two at an outlet with a DMM set to measure current ( or a piece of wire ) or somehow connect the two in a device, that connection would carry quite a large amount of current. There is a high impedance in the neutral serving his home due to an undersized utility neutral wire being overloaded by a poorly balanced load. If the buildings are loaded that far from the transformer and there's a large load on the neutral from imbalancing, any connection made between the ground and neutral in any of the buildings will split that load between the undersized neutral and the grounding conductors  back to the transformer through the house wiring. The RCD device would trip if it's working properly. If it's not or if there weren't an RCD main breaker, the neutral and ground in the house wiring could carry a load many times greater than what it's rated for.

Ultimately it's the utility's problem, as it's their wires that will burn up at some point and when that happens, customers will probably suffer some equipment losses due to a floating neutral and the high voltage that will ensue. Hopefully the utility is insured

[Kiriakos-GR]

I do not know what the Italians do, the Greeks haves 0.09V  at 3V AC manual range ( U1272A) between neutral and earth,
and they snoring peacefully.   

[pplaninsky]

Sorry for sparking old topic, but it seems a hot question among beginners about the earth, neutral and oscilloscope ground clip.
Actually, just after watching Dave's video - How not to blow up your oscilloscope - I was sure I knew what I was doing ....
Yet, I managed to short a bridge rectifier circuit of Switching Power supply, which resulted in:
* Tripped fuse
* Tripped Residual current device in the fuse box
* blown fuse of the power supply under test
* blown (shorted) one of the rectifiers diode.

I promised myself never to  connect a CRO to mains or the primary side of power supply ..... again ...
Thankfully, the CRO and the probe survived - probably due to the fact that all the safety/fuses worked perfectly.

Now, on the topic and this is just in theory, I really don't intend doing it.
I just want to learn more about safety and how things work.
I already read a ton of stuff of grounding and earthing systems in a household, after the accident.

Now, turns out the in most cases the Neutral wire is connected to the Earth wire in the basement of the building.
Actually, there is just one Neutral wire coming into the building as Neutral, which is then split into Neutral and Protective Earth wire.
This is the most popular system TN-C-S or PME.
At least in Germany, where I live.

If that is true, then Neutral and Earth in such a household should be at the same potential.
Actually, I measured the voltage between Neutral and Ground (with a decent multimeter, taking many precautions) and I got 0.2 V difference between them.

In such case - what will happen if you connect the ground clip of the CRO to the Neutral line and probe the Live wire with the tip of the probe?

I see you say it is a big NO, but actually, if your household electric system is OK and there is no currently Lightning storm going on - it should be pretty harmless. Maybe if at the same moment some electric fault occurs - i.e. an appliance goes bad, there is a potential for things to go bad.
I understand that if something goes wrong with the household electric system - it will always be a hazard, not just because you are connecting your CRO to something but in general.

I am interested in theory and if all systems are working well up to standards.
Opposed, to a case where you accidentally connect your CRO grounding clip to the Live wire - obviously not matter what - there will be a big bang.

What are your thoughts on this topic?

[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]

I don't see why anyone would need to measure the mains with an oscilloscope.

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]

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./

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!

[djacobow]

I just wanted to chime in to say that knowing what you are doing is NOT sufficient for safety with live mains equipment. You also have to not make mistakes.

That is harder than it sounds, since we all make mistakes all the time. Discipline and the use of checklists can be assets. Sure, you can look over the isolation xformer and rats nest of power cords on your bench, but did you actually make sure that the DUT you are working on right now is the one plugged into the xformer? Or you could have a policy of always powering the "DUT side" of your bench from isolation, but then you have to remember that when you're working in the field, etc.

Mistakes happen all the time. We've all let the smoke out many times. And yet, with mains AC, the cost of a mistake can be much more exciting.

Just sayin'.

Personally, I prefer to use a HV differential probe. They're not that much more money than an isolation transformer, particularly if your application requires a large one, like 2kVA, and obviously much more convenient if you have to work in the field.

[newbrain]

Just to add one data point:
There are places in Italy where two phases (out of three) are distributed, with no meaningful distinction between Neutral and Phase.

Protective Earth is, electrically, 127V from each phase, and the difference between phases is 220V.

127 x sin(60) x 2 = 220.

I have personally verified this in all the apartments I've been living in (Rome area);
I just checked here in Sweden and I have real Phase and Neutral.

Is this type of distribution common elsewhere? I think it has something to do with the conversion from 110 to 220 that happened some 40 years ago, but I'm not sure.

[Nivr]

What are you trying to measure?

I'm trying to measure a uC circuit which is powered by a capacitive (transformerless) power supply.
This means all circuit is LIVE.
I also would like to measure a dimming circuit, which is LIVE too.

In which case use a mains isolating transformer, and connect the circuit's 0v rail to mains earth so you can ground the scope to it.

Isn't this creating a very dangerous situation, something powered by an isolation transformer, but then earthed nonetheless?
If you touch anything connected to the non-earthed secondary side of the transformer, nothing (no rcd/gfci) would be able to interrupt the current and save your life.

[Zero999]

What are you trying to measure?

I'm trying to measure a uC circuit which is powered by a capacitive (transformerless) power supply.
This means all circuit is LIVE.
I also would like to measure a dimming circuit, which is LIVE too.

In which case use a mains isolating transformer, and connect the circuit's 0v rail to mains earth so you can ground the scope to it.

Isn't this creating a very dangerous situation, something powered by an isolation transformer, but then earthed nonetheless?

Yes, it does increase the risk of getting shocked somewhat, but I wouldn't describe it as a very dangerous situation.

If you touch anything connected to the non-earthed secondary side of the transformer, nothing (no rcd/gfci) would be able to interrupt the current and save your life.

Well no, the secondary of the transformer would be be earthed.

Expanding on my previous comment. It's true that using an isolating transformer, then earthing the secondary, would defeat and RCD/GFCI, thus increase the risk of shock, but that isn't necessarily a very dangerous situation. It's only dangerous if you touch a live part of the circuit, which can be avoided by adhering to safe working practices. People worked on live circuit fine before RCD/GFCIs were invented and didn't get electrocuted.