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Measuring Mains with Rigol DS1052E

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Kiriakos-GR:

--- Quote from: logicnibble on August 03, 2011, 09:44:31 am ---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?

--- End quote ---

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:

--- Quote from: logicnibble on August 03, 2011, 09:44:31 am ---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?

--- End quote ---

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?

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