Is there an oscillisocope "what not to do" tutorial?

[tggzzz]

And in most (all?) locations I've heard of, the low/"neutral" side of the mains is connected to ground at some point. Here in North America, they are connected together in the circuit breaker distribution panel.

Importantly that is COMPLETELY WRONG in many locations around the world. And, ISTR, not even the case everywhere in the USA. The OP is in Canada.

Getting that wrong can lead to, um, extreme surprise and disappointment.

I can see four wires outside my house. The lowest (which you are most likely to touch) is the common return and the higher three are each a different phase. I can see my neighbours are on a different phase to me, unsurprisingly.

[newbrain]

Importantly that is COMPLETELY WRONG in many locations around the world.

QFT. In Rome, Italy, most apartments are on two phases of a 127V three-phase system (220V phase to phase), so there's no neutral distributed, only protective earth (with own per building earth, TT system) for residential users.

[Brumby]

How do you get the 127v from just 2 phases without a neutral?

[Renegade]

Thank you so much to everyone who has helped. 

I'll have a fair bit to get through to make sure that I understand these things on a deeper level than I do now.

Going back to your original query - how to NOT damage your scope.  There really are only a handful of things that can really damage the scope that you need to be careful about:

- GROUND: Nearly all scopes have chassis ground referenced inputs.  ALL of the inputs have a COMMON ground connection. This is the biggest difference to consider compared to a DMM.  With a scope, you have to be SURE that the ground connection on the scope is common with the GROUND of your circuit under test.  Dave's video on "how not to blow up your scope" covers this in exhaustive detail.
- OVERLOAD: Always use 10X probes unless you have a specific reason not to, and take care to have a good expectation of the magnitude of the signals you're about to probe.  When in doubt, start at a high V/Div setting, then work your way down.  Obey the maximum voltage ratings on your inputs and on your probes.

If you take care of these two items, it will be EXTREMELY unlikely that you'll damage anything on your scope.

Thanks for those.

Dave's video on "how not to blow up your scope" covers this in exhaustive detail.

I've watched it 3 or 4 times. I watched it a long before I knew jack about squat, and barely understood anything.

But even after watching it a few more times, he gets into extreme detail without giving the "idiot's" version. You summed it up easier there for a newb.

[vk6zgo]

And in most (all?) locations I've heard of, the low/"neutral" side of the mains is connected to ground at some point. Here in North America, they are connected together in the circuit breaker distribution panel.

Importantly that is COMPLETELY WRONG in many locations around the world. And, ISTR, not even the case everywhere in the USA. The OP is in Canada.

Getting that wrong can lead to, um, extreme surprise and disappointment.

I can see four wires outside my house. The lowest (which you are most likely to touch) is the common return and the higher three are each a different phase. I can see my neighbours are on a different phase to me, unsurprisingly.

If I look out the window,I see a big twisted cable which has 3 phases & Neutral coming in from the pole.

My Stove runs off two phases,there is a thumping great 3 ph outlet that I've never used in the carport,& the lights & power are distributed across the phases in ways I don't know.

What I do  know is that Neutral & Protective Earth are connected at the house Power line entry point.

OK,I know that not all systems use the MEN setup like in Oz,but I was pretty sure they all connected Neutral to Earth somewhere in the system.
OK,all bets are off with USA 240v sockets,as both sides are "hot".

[Paul Moir]

Importantly that is COMPLETELY WRONG in many locations around the world. And, ISTR, not even the case everywhere in the USA. The OP is in Canada.

Canada and US are the same in this regard.  The neutral is always directly connected to earth in the panel and out on the pole.  I've worked on some very old installations dating back to the 20s and have not seen otherwise.

[Brumby]

OK,all bets are off with USA 240v sockets,as both sides are "hot".

... but they have a neutral/earth at the mid-point, do they not?

[Richard Crowley]

OK,all bets are off with USA 240v sockets,as both sides are "hot".

... but they have a neutral/earth at the mid-point, do they not?

Yes, they do.  But large loads typically don't even use the "neutral"/center-tap. So supply circuits to those loads typically have two "hot" leads and a green-wire safety ground, but no "neutral" reference.

Note that my original statement about neutral/ground/oscilloscope reference was specifically pointed at the OP from North America where neutral and ground are bonded together at the local distribution point (the circuit breaker panel, etc.)  And as an explanation of why one could appear to measure the mains power voltage without any explicit ground/reference connection.

Yes, from numerous previous discussions, we all realize that power mains distrbution (including neutral/ground protocol) varies widely from place to place on the planet.  Perhaps someone should start a thread discussing this as a primary topic and letting us all see the different practices in our individual jurisdictions.

[zapta]

ElectroBoom had recently a video that touched on this topic

[newbrain]

How do you get the 127v from just 2 phases without a neutral?

You don't...
In the sense that the neutral is not  directly distributed to normal residential users.
PE scheme is usually, AFAIK (and wikipedia seems to agree)  TT (terra-terra=ground-ground), so each building has its own earth electrode.

If there are no major problems, ground potential is close to neutral (which is grounded somewhere in distribution), and you can measure about 127V from each phase to PE and 220V=127*2*sin(60) across.

Older buildings are slowly being brought up to standard wrt PE, the one I used to live had PE distributed to all flats only in 2006. The more recent one I'm in now when in Italy always had PE, but the phases from the meters cabinet are all wired with yellow-green cable .
The two-phase distribution is not so common in other parts of Italy, the certified electrician we called when renovating was quite puzzled, as he had only recently moved to Rome. To me it was the way it's always been, even in several other houses I've lived in.

[Brumby]

So are you saying that they are relying on the conductivity of the dirt/soil/water table from the premises back to the distribution point for the neutral?

[Ian.M]

Europe is 220V 50Hz land, and historically, the further south and east you go, the dodgyer the pre-EU electrical wiring regs were and the slower the uptake/implementation of modern safety standards.  You can blame the Moors, Catholic church and Ottoman empire for many generations of encouraging a culture of fatalism.  If it isn't God's will for you to die today, the wiring wont kill you!

[Brumby]

I was just thinking that if the physical earth under your feet was an active part of the electrical distribution network, it could make for some 'interesting' experiences.

[Ian.M]

Yep, and you've got them in the Outback: https://en.wikipedia.org/wiki/Single-wire_earth_return

[Brumby]

Hadn't thought about it much before - but it makes sense, so long as the ground impedance was low enough.  Which, for the appropriate voltage, it obviously is.

[Richard Crowley]

So are you saying that they are relying on the conductivity of the dirt/soil/water table from the premises back to the distribution point for the neutral?

Yes, in some cases.  Here in the US, it is quite common to see distribution at the residential street level (the final leg of the journey) as a single high-voltage line into the "pole pig" transformer, and a ground wire travelling down the pole to a ground rod which is the return of the circuit.
The secondary of the transformer is typically 220VCT. And the Center-Tap is typically grounded both at the pole, and then again at the final destination termination point (the customer's meter connection).



http://www.utterpower.com/household_power.htm

[newbrain]

So are you saying that they are relying on the conductivity of the dirt/soil/water table from the premises back to the distribution point for the neutral?

In a way, yes. As said, TT, and RCDs ("differentials", as they are called in Italian) are now compulsory.
The conductivity needs only be high enough to trigger the RCD. If it's too low, the network becomes in practice an IT one, and the RCD is useless (the 220V is effectively "floating" so, quite dangerous, but not that much more...).

But, this true only for some localities (Rome and Ostia): I think it happened due to an easier conversion form the preexisting 110V distribution: to convert a line to 220V just attach another phase in stead of the previous neutral and slightly over-volt the street transformer...PE? What PE?

I was just thinking that if the physical earth under your feet was an active part of the electrical distribution network, it could make for some 'interesting' experiences.

And that used to happen, when there were no RCDs and PE was often (the horror!) the water pipe...

[Richard Crowley]

I was just thinking that if the physical earth under your feet was an active part of the electrical distribution network, it could make for some 'interesting' experiences.

There is a legendary tale in telephone circles related to that.  In early days, and especially in rural areas, telephone lines were sometimes ground-return, but almost all of them were "selective ring" party-lines. Many of the "selective ring" schemes used a local ground reference for the ring voltage which approaches 100VAC at different frequencies (ring frequency being one of the selective-ring schemes).

Anyway, there was a customer complaint from a rural customer that there was a strange problem with their telephone ringer.  Apparently when a call came in, the phone never rang for several cycles until their dog barked, and then the phone rang.

They sent a repairman out and he climbed a pole and clipped on his butt-set and dialed the customer's number. Sure enough several ring cycles and nothing happened.  Then their old dog got up and barked a couple times and peed on the metal rod he was chained to.  And then the phone rang.

Apparently the dog was chained to the telephone ground rod which had gradually increased impedance and no longer had enough conductance to the earth.  So when the ring current came down the line, it shocked the dog through his chain, he got up and barked in complaint, and then peed on the ground rod, temporarily lowering the impedance enough to pass sufficient ring current.  Maybe old telco lore, but maybe true.

FYI: Butt-set:

[Richard Crowley]

There have also been cases where people experimented with communicating through ground currents.
The magazine "Popular Electronics" in the January, 1968 featured an article about this history of ground communication and adding a suggested experiment to try CW or voice communication.

http://www.americanradiohistory.com/Popular-Electronics-Guide.htm
1968, January, p.45 "The Mystery of Radio Waves That Travel Along or Beneath the Surface of the Earth"

[Renegade]

Then their old dog got up and barked a couple times and peed on the metal rod he was chained to.

I had no idea that a thread on oscilloscope no-nos would end up in dog pee when I first posted it...

  

[zapta]

There have also been cases where people experimented with communicating through ground currents.

The story I remember is that they used to close telegraph loops using the railroad as return line and one day they realized that it kept working when the rails were disconnected for repair.

[Brumby]

Then their old dog got up and barked a couple times and peed on the metal rod he was chained to.

I had no idea that a thread on oscilloscope no-nos would end up in dog pee when I first posted it...

  

I think this arm of the thread has gone off at somewhat of a tangent to the original topic - but it is an interesting one that deserves it's own thread (as has been suggested somewhere else on the forum, I believe).

I might like to suggest this topic is split from: https://www.eevblog.com/forum/beginners/is-there-an-oscillisocope-'what-not-to-do'-tutorial/msg858591/#msg858591 into it's own topic - something along the lines of "Power distribution and earthing".