Hi! I'm having trouble understanding how AC works so i was hoping someone here could help me out.
Lets say i have a normal power outlet, where i live (Sweden) a normal outlet is 230VAC 50hz. How come if i were to touch the phase (correct term?) i get shocked, but not if i were to touch the neutral wire? If the poles keep switching wouldn't that mean that the electrons would switch direction as well, making them go through the neutral wire half of the time?
I have been confused about this for a long time so some help understanding this would be greatly appreciated
Birds can land on bare transmission lines carrying many hundreds of kilovolts without harm.
You could touch the phase conductor in your home installation without harm if you did not touch anything else and wore rubber boots to insulate yourself from the ground.
In a normal domestic electrical system the ground and neutral are interconnected and therefore at the same potential i.e. there is no potential difference between the two. If the electrical system was completely isolated from the ground, you could touch either the "phase" conductor or the "neutral" conductor (but not both) without harm.
If you have a 1.5 battery on your desk and you measure the voltage between the positive and negative terminals with a multimeter, you see a voltage of 1.5 volts. If you have two batteries which are completely isolated from each other and you put the red lead of your multimeter on the positive terminal of one battery and the black lead on the negative terminal of the other battery, you will measure zero volts because there is no common reference between the two. A battery is DC but it is the same for AC.
Does that help ?
Thanks for answering but i think i'm actually a litte more confused.
Let's say i'm very poorly isolated from ground. I'm standing barefoot on a wet metal surface or something. If the neutral wire is also not connected to ground, wouldn't it be equally dangerous to touch either the phase or neutral wire?
In a normal domestic electrical system the ground and neutral are interconnected and therefore at the same potential i.e. there is no potential difference between the two.
That is not quite true in Europe although I believe it is in the states and other countries.
The nutral is the return wire and might get to between 5 to 15V above ground. The Live wire is first 240V above the neutral and then 240V below the neutral.
If the poles keep switching wouldn't that mean that the electrons would switch direction as well, making them go through the neutral wire half of the time?
No the neutral says the same and the live goes positive and negitave with respect to the neutral.
f the neutral wire is also not connected to ground, wouldn't it be equally dangerous to touch either the phase or neutral wire?
No.
On the other hand the ground can be fatal. Most of the shocks I have got is because I touched the ground. ( as well as first touching the live )
Typically in a power distribution system , there is a three phase supply with a neutral connection. You have one phase and the neutral . Back at the supply transformer the neutral is connected to ground . The voltagevof the neutral wire then sits at around 0v ( but may rise a few volts due to imbalance of the theee phase load).
The live , AC line varies in voltage as a sine wave , plus and minus around the neutral line. So the neutral is near to 0v always and the live connection varies in voltage plus/minus around that.
Grumpy_Mike:
No the neutral says the same and the live goes positive and negitave with respect to the neutral.
hammy:
Typically in a power distribution system , there is a three phase supply with a neutral connection. You have one phase and the neutral . Back at the supply transformer the neutral is connected to ground . The voltagevof the neutral wire then sits at around 0v ( but may rise a few volts due to imbalance of the theee phase load).
The live , AC line varies in voltage as a sine wave , plus and minus around the neutral line. So the neutral is near to 0v always and the live connection varies in voltage plus/minus around that.
Ok, so the current doesn't really change direction? I always imagined an AC circuit like a DC circuit where the current change direction periodically. Going from plus to minus and then minus to plus.
But if that is not the case then i guess my confusion is about what happens to the current flow when the live wire goes from positive to negative.
If the question about AC did not imply a domestic electrical system with live, neutral and earth but was simply about AC derived, say, from a 1:1 transformer connected to the AC mains, then you could not speak about one of the output terminals being at zero volts and the other being a 240 volts because these switch at 50/60 Hz.
Edit.
If you scale that model up to a power distribution system you could arbitrarily chose on of the transformer outputs and connect it to the ground and call it neutral.
6v6gt:
If the question about AC did not imply a domestic electrical system with live, neutral and earth but was simply about AC derived, say, from a 1:1 transformer connected to the AC mains, then you could not speak about one ot the output terminals being at zero volts and the other being a 240 volts because these switch at 50/60 Hz.
So in that case touching either wire would be equally dangerous? So in a country where the neutral wire is not connected to ground this would also be the case?
DrJanItor:
So in that case touching either wire would be equally dangerous? So in a country where the neutral wire is not connected to ground this would also be the case?
Touching either output terminal (but not both) of a completely isolated transformer would be perfectly safe. However, in a practical situation with electricity distribution, a supplier could not guarantee that there would not be a connection to ground somewhere in the network so normally force one connection to ground anyway and call it neutral. At least that is how I understand it, but am also willing to learn.
Grumpy_Mike:
Yes it does. Have you not been reading the answers?
The voltage changes polarity and so the current changes direction. BUT only on the live wire.
I have read the answers but i simply don't get it. It's the "only in the live wire" part i don't understand. I mean the neutral wire is part of the circuit, so how can the current change direction only in part of the circuit?
I have read the answers but i simply don't get it. It's the "only in the live wire" part i don't understand. I mean the neutral wire is part of the circuit, so how can the current change direction only in part of the circuit?
Imagine you have two batteries. (Or if you have two batteries you can actually do this experiment.)
Connect your meter between "as usual" with the black-negative meter probe to battery-negative, and the red probe to positive. The meter reads positive voltage as-usual.
Now wire the other battery's + connection to ground. (i.e. The meter's ground is in the middle between two series batteries.)
Now, leave the ground connection as-is but move the meter's red-positive probe to the negative of the other battery. The meter will read negative. From the meter's point-of view you've reversed the voltage (and you've reversed the sight-current through the meter).
You could wire-up a double-pole switch to reverse the voltage & current and you'd have something like AC.
...It's a little harder to visualize, but with a DPDT switch you could reverse the direction with just one battery. And, one of the meter connections could be permanently connected to earth-ground (or neutral) and the voltage and current would still reverse.
It's the "only in the live wire" part i don't understand. I mean the neutral wire is part of the circuit, so how can the current change direction only in part of the circuit?
Think of it like this. When the live is positive and there is some load between live and neutral, it pours current through the load into the neutral wire. When the live wire is negitave it sucks current out of the neutral wire through the load.
The current changes direction and the neutral wire stays at the same potential with respect to ground.
Grumpy_Mike:
Think of it like this. When the live is positive and there is some load between live and neutral, it pours current through the load into the neutral wire. When the live wire is negitave it sucks current out of the neutral wire through the load.
The current changes direction and the neutral wire stays at the same potential with respect to ground.
Thank you! I think i actually finally understand. I felt that if i touched the neutral wire the ground i'm standing on would "suck current" through my body, instead of the live wire. But since the neutral wire is connected to ground the potential difference is 0V so that wont happen. Am i getting this correctly?
But i guess this is also why i feel like it would be dangerous to touch the neutral wire if it is not connected to ground. Becuase then there would be a potential difference?
DVDdoug:
Imagine you have two batteries. (Or if you have two batteries you can actually do this experiment.)
Connect your meter between "as usual" with the black-negative meter probe to battery-negative, and the red probe to positive. The meter reads positive voltage as-usual.
Now wire the other battery's + connection to ground. (i.e. The meter's ground is in the middle between two series batteries.)
Now, leave the ground connection as-is but move the meter's red-positive probe to the negative of the other battery. The meter will read negative. From the meter's point-of view you've reversed the voltage (and you've reversed the sight-current through the meter).
You could wire-up a double-pole switch to reverse the voltage & current and you'd have something like AC.
...It's a little harder to visualize, but with a DPDT switch you could reverse the direction with just one battery. And, one of the meter connections could be permanently connected to earth-ground (or neutral) and the voltage and current would still reverse.
DrJanItor:
Thank you! I think i actually finally understand. I felt that if i touched the neutral wire the ground i'm standing on would "suck current" through my body, instead of the live wire. But since the neutral wire is connected to ground the potential difference is 0V so that wont happen. Am i getting this correctly?
Yes, but see below
But i guess this is also why i feel like it would be dangerous to touch the neutral wire if it is not connected to ground. Becuase then there would be a potential difference?
Thanks! I will try this when i get back home
It is dangerous to touch the neutral, because there is a risk that it carries a high voltage if a wiring
or appliance is faulty. Most of the time it would be at a safe potential, but this is not guaranteed so
for safety purposes you treat neutral as just as dangerous as any of the live phases.
In a single phase system the neutral wire takes all the return current so will be several volts above ground
due to IR losses in the wire, if there is significant load.
In a three-phase system the neutral may not actually connect to anything. Some appliances are delta
wired and neutral is not used. wye-connected appliances do connect to neutral, but the currents are
supposed to balance out and little current will use the neutral wire unless there is a fault.
The basic rule is never come into contact with any mains wire except for protective earth.
But since the neutral wire is connected to ground the potential difference is 0V so that wont happen. Am i getting this correctly?
But I told you in my first post there is about 5 to 15V between neutral and ground. They are joined together somewhere down the line but in the mean time you get induced voltages that affect the real potential. But 5 to 15V is not normally enough to give you a shock. But if you try hard enough it could kill you.
But since the neutral wire is connected to ground the potential difference is 0V so that wont happen. Am i getting this correctly?
Yes, in theory you are correct.
However in real life thing are not ideal, and there is no 0 volts. All voltages are relative to something, usually a grounding rod driven in the ground locally. However if you had a voltmeter with real long leads, connected one at your neutral then connected the other end to a neighbor's neutral it will vary by some significant amount.
Just curious, Sweden uses the type "C" plug which has no polarity. Without measuring, would one know which was "neutral".
DrJanItor:
I have read the answers but i simply don't get it. It's the "only in the live wire" part i don't understand. I mean the neutral wire is part of the circuit, so how can the current change direction only in part of the circuit?
That's probably due to details starting to get jumbled here. To understand the three-phase power system.... it's probably necessary to learn three phase systems..... which requires some electronics theory including reactive components... phasors etc.
Here's a related question I have, and it relates to human resilience and American "exceptionalism".
In America (the United States for the sticklers) mains voltage is 115V ± 5 volts. Under normal circumstances, assuming normal cardio-vascular health, 115 volts is painful, but not lethal. In most of the "rest of the world", mains voltage is twice that, 230 vac., which is often lethal. I teach this is because the y'all don't have the same reverence for life as we Americans do. Is that correct?
