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Live and Neutral.

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alphacat

New Member
Hello,

I want to clearify something which i'm not sure of.

Why is it said that the Neutral is 0V, and that the Live's voltage alternates?

In every country the Neutral is connected inside the wall to Earth, and therefore it is said that Neutral is 0V?

Thanks.
 

Mike_2545

Super Moderator
Neutral is not connected to Earth in every country or every region of the same country, it is a return path for current with ground being a safety wire.
 

Sceadwian

Banned
The live wire actually provides the fluctuating voltage, the neutral wire sometimes called the return is grounded either at the power company or outside the house on the pole. While the live wire actually brings the power in, if the power does not have a way to go out to ground there is no circuit so no current can flow. In real world electronics devices the current is flowing through both wires when a device is active. The 'ground' wire in many countries is simply a seperate return path, usually for ESD saftey, it's seperate from the neutral wire because if an electrical loops is heavily loaded the resistance in the wire will produce a voltage above ground on the neutral line. The saftey ground is also used for chasis grounding, it should never be used for power grounding.
 

dknguyen

Well-Known Member
Most Helpful Member
You are right, the two lines of AC do alternate and there is no real difference between the two if BOTH are left floating. The problem is that the fact that both AC lines are alternating polarity relative to each other does not mean they cannot have a DC-bias voltage relative to the environment around them (like the ground). The power grid covers such a vast area it can intercept accumulate the minute amount of charge that is moving between the sky/clouds and ground. When this charge gets high enough, the common mode DC-bias of the AC lines can get so high above their environment (like the ground and everything else on it) that lighting bolts will basically jump from wall sockets to balance out the charge. Reaching towards a lightswitch or wall socket only to have a lightning bolt jump out at you is not fun.

So they tie one line to ground to keep the common voltage between the AC lines somewhere near ground so sparks don't fly. In theory the neutral should be at ground voltage, but in reality this is not necessarily the case because of a number of reasons:

1. the accumulated resistance/impedance due to the long length of the wires in the power grid
2. the fact that the ground has a resistance and is not the same voltage everywhere
3. the neutral line is carrying current (see 1)
4. different connections to the ground at different locations are not necessarily the same voltage (see 1 and 2)
5. the long lines in the power grid are still collecting charge from the atmosphere
 
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Sceadwian

Banned
Dk not if you live in the US. Live carry's the voltage, neutral is grounded. If you left them both floating and use a true ground reference on a multimeter you'll find 120 volts (or the local equivlant) on the live wire and 0 volts (maybe a few mv's) on the neutral wire. At least in the US's electrical system, high voltage AC in the US sometimes does anti-phase where the neutral wire carries an AC voltage 180 degrees out of phase, but not all, depends on how things are grounded outside and at the local power station. Depends on your country and exactly how the power is delivered. 3 phase in the US there is no neutral, just 3 AC phases. I'm not sure what is used around the world.
 
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dknguyen

Well-Known Member
Most Helpful Member
Dk not if you live in the US. Live carry's the voltage, neutral is grounded. If you left them both floating and use a true ground reference on a multimeter you'll find 120 volts (or the local equivlant) on the live wire and 0 volts (maybe a few mv's) on the neutral wire. At least in the US's electrical system, high voltage AC in the US sometimes does anti-phase where the neutral wire carries an AC voltage 180 degrees out of phase, but not all, depends on how things are grounded outside and at the local power station. Depends on your country and exactly how the power is delivered. 3 phase in the US there is no neutral, just 3 AC phases. I'm not sure what is used around the world.
When did I say anything differently?

Never thought about the neutral on a 3-phase system though. How do they stop the charge build-up then?
 
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Sceadwian

Banned
You are right, the two lines of AC do alternate and there is no real difference between the two if BOTH are left floating.
Did I mis-read something?
 

Boncuk

New Member
3 phase AC is supplied with five wires in Germany, 3 phases, neutral and protecive earth.

Starting a heavy duty motor requires star-triangle switching. Star switching is done using the three phases + neutral on the motor, thus applying 230V to the motor windings. For triangle only the three phases are used resulting in 440V accross the motor windings.

Electric devices operated without protective earth are prohibited in Germany. Authorities regularly check if newly built production facilities are conformal with the rules. If not they will be closed until the wiring is corrected.

Boncuk
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
I was referring to a alternating current in general since the OP was asking why people say live voltages alternate but neutral is at zero.
Because in most countries the neutral wire is grounded at the sub-station, so is effectively at 'near' zero with regards to earth (I say 'near' because of the small voltage drop across the cable, usually a volt or two AC).
 

Hero999

Banned
It depends on what earth-bonding system you have.

Where I live the neutral and earth conductors actually share a wire until they enter the property. It's like a co-axial cable, with the live in the centre surrounded by the earth/neutral shield. Where it enters the house the earth and neutral split off and become separate connections. This kind of system is pretty common because it saves having two cables for neutral and earth and it's perfectly save since the outer screen is always in contact with the earth.
 

Sceadwian

Banned
As far as I know in my area, neutral is grounded at the sub station and at the pole outside the house, 'ground' connects to neutral outside the house or at the breaker box. Funny that DK brought it up, I can't imagine how much of the US power grid is actually going right to ground from the long distance voltage imbalances.
 
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