Confusion about this Live & Neutral Circuit Problem

[Kevin J]

The left circuit is a simple live&neutral circuit, it is neutral (0V), because it is in fact 'grounded'. If I unplug the wire connection to the ground, makes both wire, live. Now, If I replace the grounding connection with a person, why would he receive the shock, doesn't the man 'neutralize' the wire just like the grounding connection did? Aren't both the grounding wire and the person technically the same?
*
Or does the grounding connection 'shocked' too?

 

[Dale]

The circuit on the left is a standard grounded circuit. The circuit on the right has a floating ground.

https://en.m.wikipedia.org/wiki/Floating_ground

Floating ground can be a good thing or it can cause other problems. The wiki article gives a good overview of the main issues

 

[Nugatory]

Or does the grounding connection 'shocked' too?

Yes. Current will flow through the grounding connection; that's why the wire connected to ground through it is at the same potential as the Earth under your feet (which is what "grounded", "neutral", and "not live" really mean). So in the second picture you're volunteering to be the grounding connection, and whatever current is needed to bring the difference between circuit and Earth is going to flow through you. (And remember with AC the voltage in the circuit is always changing - it's not like a DC circuit which will rapidly come to equilibrium).

We've already mentioned several times that voltages are relative, but I'm not sure that you've fully absorbed the implications of that statement. Those 240V power supplies are NOT holding one side at 0 and the other side at 240V. They are holding the two sides 240V apart, but there's no reason why either side will be anywhere the same potential as the Earth unless there's a connection (either through the ground connector in the first picture or the poor fool in the second) to the earth.

 

[sophiecentaur]

Or does the grounding connection 'shocked' too?

You seem to have started another thread about the same topic. The best approach is to stick with one thread - that's the PF way.
PS Your diagram of the floating situation is not correct. The two Voltages should be marked +/1 120V and -/+ 120V, to denote that there is 240V AC across the terminals.

Your question all hangs on the above quote. A 'Shock' requires enough current to pass through the body and that requires sufficient Volts across the body. If the 'floating arrangement' is used and you touch one side, there will be an initial current flowing through the body. However, once the current starts to flow, the Voltage across the body will rapidly go to very near zero because there is a very high source Impedance. The situation is relatively safe and non-lethal.
However, if someone else touches the other side of the circuit too, you will both find there's a constant 120V across your bodies. 120V is a lot safer than 240V so, in principle, neither of you would die. BUT it isn't as simple as that because, if the path through one of you happens to be very low resistance (or if it's a wire connection to Earth instead of a person), there will be up to 240V across the first person and that will be maintained. A shocking situation.

 

[Kevin J]

You seem to have started another thread about the same topic. The best approach is to stick with one thread - that's the PF way.
PS Your diagram of the floating situation is not correct. The two Voltages should be marked +/1 120V and -/+ 120V, to denote that there is 240V AC across the terminals.

Your question all hangs on the above quote. A 'Shock' requires enough current to pass through the body and that requires sufficient Volts across the body. If the 'floating arrangement' is used and you touch one side, there will be an initial current flowing through the body. However, once the current starts to flow, the Voltage across the body will rapidly go to very near zero because there is a very high source Impedance. The situation is relatively safe and non-lethal.
However, if someone else touches the other side of the circuit too, you will both find there's a constant 120V across your bodies. 120V is a lot safer than 240V so, in principle, neither of you would die. BUT it isn't as simple as that because, if the path through one of you happens to be very low resistance (or if it's a wire connection to Earth instead of a person), there will be up to 240V across the first person and that will be maintained. A shocking situation.

You said once the current starts to flow, voltage/potential difference becomes 0, due to high resistance. How is this possible?

 

[sophiecentaur]

You said once the current starts to flow, voltage/potential difference becomes 0, due to high resistance. How is this possible?

There is no low resistance path from any part of the floating AC circuit. When there is only a (if at all) high resistance path. The voltage across the person will be safely low. Look up Potential Divider Circuits to see why.
Have you heard of a Mains Tester Screwdriver? It works on the principle that it has a high resistance in series between the blade and the button on the end. Enough current can pass to light the Neon lamp but the volts on the tip are too low to harm you.

 

[CWatters]

I'm concerned you are using 240V AC in your example. Are you talking about a 240V AC source in general or specifically about the 240V grid supply you get in your country?

 

[sophiecentaur]

I'm concerned you are using 240V AC in your example. Are you talking about a 240V AC source in general or specifically about the 240V grid supply you get in your country?

HAHA. We have some great shouting matches about this one. It's amazing how the 'rest of the World' seems to choose the wrong system.

 

[Kevin J]

If I connect a live wire to the ground this would be then called 'neutral'. If initially the Live's voltage is 240V and the ground/earth is 0V, would there be an initial current flowing, due to this potential difference? And eventually this two would have the same potential just like what 'neutral' wire means?

 

[sophiecentaur]

If I connect a live wire to the ground

If you do that you will blow a fuse. A wire that's part of a floating system is not strictly "live" at all. If you connect one wire to ground then the others are live.
You need to step backwards, beyond your intuitive view of this, apply some rules and use the correct definitions. The logic will then take you to understanding (or acceptance).

 

[CWatters]

This is why I ask the question.

In the UK you cannot connect a 240V _mains_ live wire to ground and call it neutral. It will cause damage or at the very least a fuse will blow. That's because the grid is three live phases and a neutral already.

However in other countries with a two phase system, or if this was a floating 240 V AC output from a transformer, it might be or is possible.

 

[Kevin J]

If you do that you will blow a fuse. A wire that's part of a floating system is not strictly "live" at all. If you connect one wire to ground then the others are live.
You need to step backwards, beyond your intuitive view of this, apply some rules and use the correct definitions. The logic will then take you to understanding (or acceptance).

Isn't that the way to make a wire neutral, at first you have 2 live wires and one is connected to the ground right? Which makes it neutral

 

[Kevin J]

My problem is that how does connecting a wire to the ground suddenly makes it 0V?Isn't it still connected to the source?

 

[Nugatory]

Isn't that the way to make a wire neutral, at first you have 2 live wires and one is connected to the ground right? Which makes it neutral

You've started with two floating wires, not two live wires. You connect one of them to ground and it becomes grounded/neutral; now the other one is live instead of floating.

 

[Nugatory]

My problem is that how does connecting a wire to the ground suddenly makes it 0V?Isn't it still connected to the source?

I think I've said this before: the source does not hold either wire at any particular voltage - it maintains a 240 volt difference between the two. Before you made the connection, you had two wires that were 240V apart and with no particular relationship to the Earth under your feet. After you make the connection, one of them is at the same voltage as the Earth under your feet and the other is at 240V relative to the first. The difference between the two is 240V, just as it was before you made the connection, and that's all that the power supply does.

 

[sophiecentaur]

at first you have 2 live wires

No. They are not "live"; they are floating. Their potential to ground is irrelevant to the way the circuit works.
The word "live" is specifically used to describe a connection that has a specific potential with respect to ground.
Question for you: Which of the two terminals on a 12V car battery would you call 'live', when the battery is sitting on a bench and not connected to a car?

 

[Kevin J]

So you technically don't get electrocuted by the 'neutral' wire because it has the same potential as the ground?

 

[Nugatory]

So you technically don't get electrocuted by the 'neutral' wire because it has the same potential as the ground?

In ideal conditions (the wires have zero resistance, every connection has been made properly by people who knew what they were doing, the grounding system is good - the US electric code has an entire chapter just on this last part) the neutral wire will be at the same potential as the Earth under your feet so you won't be shocked if you touch the neutral wire.

However, ideal conditions never exist in practice, which is why the answer to the question in your other thread ("Is it safe to touch the neutral?) is emphatically no.

 

[Kevin J]

In ideal conditions (the wires have zero resistance, every connection has been made properly by people who knew what they were doing, the grounding system is good - the US electric code has an entire chapter just on this last part) the neutral wire will be at the same potential as the Earth under your feet so you won't be shocked if you touch the neutral wire.

However, ideal conditions never exist in practice, which is why the answer to the question in your other thread ("Is it safe to touch the neutral?) is emphatically no.

So if the neutral wire is at the same potential with the ground/earth (assume ideal connection), thus there's no current flowing? Am I right?

 

[f95toli]

So if the neutral wire is at the same potential with the ground/earth (assume ideal connection), thus there's no current flowing? Am I right?

No, the neutral is still the return path for the current going "out" via the live wire.
A wire in a circuit can have zero potential and still carry current.

RCDs (residual current devices, should be part of any modern installation) works by measuring the difference in current going out via the live wire and returning via neutral; if there is a fault it means current is going somewhere it shouldn't (e..g through you) and it trips.

Note that the Earth wire should never carry current under normal circumstances; it is purely there for protection (which is why some older systems did not use a separate earth)

 

[Kevin J]

I'm still somehow not getting this, does any current flow from the neutral wire to the grounding wire? If yes, aren't they at the same electric potential, thus no current? If no, why do a person get 'electrocuted' when touching a live wire, doesn't the person neutralize the wire, just what the grounding wire does to make a certain wire 'neutral'?

 

[russ_watters]

I'm still somehow not getting this, does any current flow from the neutral wire to the grounding wire?

No, it shouldn't, but that's not how your previous question was read and answered. That question/answer was about return current in the neutral back to the generator/transformer.

If yes, aren't they at the same electric potential, thus no current? If no, why do a person get 'electrocuted' when touching a live wire, doesn't the person neutralize the wire, just what the grounding wire does to make a certain wire 'neutral'?

A grounding wire, by code, must have a low resistance (25 ohms max, <5 recommended). A person has a high resistance (up to 100,000 dry) and thus there is a large voltage drop across the person.

In basic circuit theory it is typically assumed that wires have zero resistance, but in reality all wires have a finite resistance, and thus a voltage drop across them. One of the goals of the ground is to have a low resistance to dissipate a high current without creating a large voltage drop that could push current in other directions (eg, through a person touching the wire).

 

[sophiecentaur]

There could easily be a high current flowing through the two, if connected. This happens frequently. The voltage difference can be less than 1V but that would be enough for several Amps to be flowing if the cable resistances are a fraction of and Ohm.
I think you need to distinguish between Mathematical results of idealised circuit calculations and the practicalities of Engineering. This applies throughout Engineering, aamof. There are non-ideal effects everywhere.
Firstly, this doesn't compromise safety in any way. Secondly, the paths of the currents can cause mains frequency interference in audio circuits due to the magnetic fields..

 

[CWatters]

My problem is that how does connecting a wire to the ground suddenly makes it 0V?Isn't it still connected to the source?

But the source isn't fixed to anything.

If I was to measure how high your head is and get 405ft 8" above sea level your feet would be at about 400ft right.

If you then walk to the beach your feet would be at 0 feet. How can they suddenly be at 0 feet if they are still connected to you head?

 

[Kevin J]

So let's make things clear here, assume a person having 0 resistance(I know it's impossible but just try to 'assume'). Now, he touches the live wire while standing on the ground, does this makes the 'live' wire the same potential as the ground?
*
My prediction is yes, as the person 'acts' as a grounding wire just like in any other neutral connection. Am I technically right?

 

[CWatters]

Not in my country. You risk death.

You didn't answer my earlier question #7 so i think I'm going to duck out of this thread.

 

[russ_watters]

So let's make things clear here, assume a person having 0 resistance(I know it's impossible but just try to 'assume'). Now, he touches the live wire while standing on the ground, does this makes the 'live' wire the same potential as the ground?
*
My prediction is yes, as the person 'acts' as a grounding wire just like in any other neutral connection. Am I technically right?

No.

I don't know why you are doing this, but proposing a scenario that you know is physically wrong is a terrible way to fix confusion. And using a person inappropriately in a scenario involving danger is making people uncomfortable. You need to stop this or the thread will be closed.

Now, if we short a circuit with a low resistance *wire*, some of the typical assumptions of basic circuit theory are no longer going to be valid. The resistance of the wires can no longer be assumed to be zero and the internal resistance of the current source can't be either (and it can't supply infinite current). So what happens in reality is the current gets very high and the voltage will drop over the wire and be reduced at the source. By how much is tough to predict. And with the amperage high, the wires and circuit components will get hot until the weakest link fails -- which is exactly what the function of a fuse is.

 

[Nugatory]

So let's make things clear here, assume a person having 0 resistance(I know it's impossible but just try to 'assume'). Now, he touches the live wire while standing on the ground, does this makes the 'live' wire the same potential as the ground?
*
My prediction is yes, as the person 'acts' as a grounding wire just like in any other neutral connection. Am I technically right?

No. This is a LIVE wire, not a floating wire, so its voltage won't float down to the same potential as ground.

You've created a short to ground so Ohm's law ($V=IR$ with $V=240$ and $R=0$) says that in theory the current through you to ground should be infinite. In practice, the current is so high that something breaks: if there is no fuse or circuit breaker the wires will melt or catch on fire, or the power supply will fail explosively. You are dead in any case, because the current will kill you long before anything else breaks.

 

[anorlunda]

We have many threads on PF about these topics. Most of them become confused because of multiple definitions of words like live, neutral, and ground, "a lot", or "a little". On one such thread, I found a source that showed more than 100 different electric distribution wiring schemes in use around the world; each with its own nomenclature.

Of course if we stick with is long enough, we might find explanations that satisfy everyone in every country. But in the evolution of that we leave behind a trail of posts in the archive that can be interpreted in isolation that it is safe to touch power mains wires in certain circumstances (with the circumstances dependent on where you live). Those can be quoted in other threads, or copy/pasted into social media.

Because of all this, I am going to close this thread.