If someone held a gun to my head, I would use a 10x probe. First I would leave the scope probe ground FLOATING!
Then I would probe green-wire safety ground to see how much difference there is between the oscilloscope ground reference and the mains safety ground. That measurement by itself would be interesting. Then I would probe the "neutral" ("cold") side to see what is the differential between THAT and scope reference ground.
THEN I would consider poking the probe at the hot side of the mains (with the scope set for the expected peak-to-peak voltage we expect based on the nominal voltage. i.e. 679 VAC P-P for a nominal "240V power mains. Even at nominal 120V mains voltage where I live, I would only probe the hot side if you pulled the hammer back on the pistol as an additional threatening gesture.
I STILL don't get the morbid fascination with people here probing the power mains with their oscilloscopes! As far as I can see. it is an adult extension of what small infants do when they poke things into the wall outlets.
At the risk of repeating myself: If I want to know the mains VOLTAGE, then I will use a proper DMM. If I want to look at the WAVEFORM of the power mains, I will grab a discarded wall-wart transformer, and safely probe the low-voltage secondary.
Thank you for the advise ! It's better to at least know than to stay ignorant so i hope you don't mind the questions.
1. I've measured the DSO ground ref with the mains ground (earth i assume), difference is about 1.3V, what does that tell me ?
2. If you talk about "neutral" and "hot", you mean the earth of the mains and one of the hot wires ?
3. I take it that a 100x probe is safer than a 10x probe ? Why if the 10x is rated 300V AC CAT II and DSO INPUT IS 300V AC RMS ?
4. With "FLOATING" does it mean leave the ground wire unconnected of the probe itself ?
5. Say i have a 100X probe, i assume i connect the ground of the probe to the earth wire and then probe one of the hot wires ?
Thank you again, just want to understand more about measuring AC and do's and don'ts without doing it :-)
IF you live in most countries,your Electricity company supplies you with your house power at 240v,220v,or 120V,or whatever,in the form of Live (also called "Active' or "hot") & Neutral conductors.
The Neutral may,or may not be physically grounded elsewhere,depending on the exact system,but it is almost always physically grounded at the entrance to your house.
From the point this is done,a third ground,or "Earth" conductor is extended to your power point.
At the power point (called a "GPO" in Australia,so for convenience I will use that terminology),there are normally two similar socket holes,& a third "different"one.
For instance,an Australian GPO has two slanted flat pins above one vertical one.
At first sight,grounding one side of the Mains seems mad,as there are many incidentally grounded objects in the average house,so you could get between them & Live & be zapped,but there is "method in their madness".
Historically,most Electrical appliances had metal cases,for strength,as plastics were not sufficiently developed.
Imagine with a system where neither Mains conductor was grounded,& two such devices developed short circuits between opposite conductors & the metal case.
Both devices would continues to work OK individually,but there would be,say 220V between the cases.
If you were happily working in the kitchen & touched both devices,you would be "toast",as the Mains fuses wouldn't notice the small increase in current draw needed to cook you!
Enter the three wire "protective Earth' system.
A short develops between the case & the Active conductor----- the Earth conductor is connected back to the Neutral side at the house entry point,so the Power supply sees a dead short,excessive current flows,the fuse in the fusebox blows,& you are protected.
In an ideal world,"Earth" & Neutral" would always be at exactly the same potential.no matter what GPO you looked at,but in the real world,Neutral has to carry the same current as the Live conductor,whereas the Earth does not,so there will be some voltage drop,hence the difference in voltmeter reading. between the two at any given GPO.
Let's look at (2)"If you talk about "neutral" and "hot", you mean the earth of the mains and one of the hot wires ? "
From the above you can see that "NEUTRAL" & "Earth" are
not the same thing.
"Neutral" & "hot" means exactly the same thing as "Neutral" & "Live".
There are two special cases where referring to the two pins on a power outlet as both "hot" is legitimate.-
(a)When you are looking at an old installation where you do not know which output of the GPO is the Live one----for instance,the now mandatory connection of Live as the left socket pin & Neutral as the right one was only a "recommendation" in Australia until the 1970s.
(b) In the USA,some appliances require 240v as distinct from the normal 120v supply.
This is done using a special transformer on the power pole which has a 240v secondary .
The secondary is centre tapped to produce two Live 120v feeds,with the tap as Neutral.
At a 240v socket,there are now
two LIve 120v socket holes & one Neutral.
The device is connected between the two "hots" & hence across the 240v secondary of the transformer.
Re (4):-
Oscilloscopes normally have one side of the 'scope inputs returned to the metal case and/or internal metalwork of the instrument, hence, via the power cord, GPO & internal wiring to where the Earth & Neutral are connected.
If you completely remove the grounding lead from a 'scope probe & probe the "Active" you will ne able to see the Mains waveform,as the return side of the probe's circuit is provided by the instrument Earth as described above.
This is what Richard referred to as "Floating",as he has pointed out in his later posting.